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Home , Amorphous solid, Kernite, Tincalconite, Ulexite, Zeolite

Transformations in Amorphous

L. Stoch and I. Waclawska

University of Mining and Metallurgy 30-059 Cracow, Poland

ABSTRACT exactly defined, and it comprises substances of different ordering degree of internal structure, varying micro- This paper presents the results of investigations of and macrohomogeneity and varying origin. amorphous substances formed as a result of thermal Amorphous substances are formed as a result of decomposition of inorganic crystalline bodies by different processes. are obtained through preci- way of their structural rearrangement in a solid phase pitation from solutions in which the parent compound as an order-disorder transformation. The investigations is in a state of molecular dispersion. are most were conducted on model substances represented by often obtained by a , but also by hydrated of alkalis and alkaline earths as well means of the deposition from a vapour and by as silica-rich . We examined the transition sputtering in a vacuum system. phenomenon as a mode of cancellation of internal The amorphous state can also be formed through stresses induced by the state of structure randomness, destruction of a crystalline structure under the influence and next the recrystallization process of thermally of energy supplied from the outside in various forms, amorphized substances. Recrystallization processes such as heat, pressure or radiation. proceed usually in several stages with their structural The degree of the randomness and inhomogeneity mechanism changing with temperature. of the internal structure of amorphous substances The formation of the amorphous states and their depends to a high extent on the degree of the dispersion recrystallization represent a cycle of of the primary substance. The highest homogeneity is realised by means of continuous rearrangement of the demonstrated, for example, by glass obtained by con- internal structure. densation of vapours or from solutions (in the sol- These processes may be utilized as an alternate method) when the parent substance was in the state of method of obtaining vitreous and glass- molecular dispersion. materials of various relations between the crystalline Amorphous substances formed through destruction phase and the glassy matrix. of the crystalline structure of the precursor in a solid state, at low temperature, preserve to a considerable degree the elements of the primary structure. An 1. INTRODUCTION example here are the metamictic rendered amorphous by radiation. Solid bodies may occur in two forms differing in Depending on the mode of their formation and their internal structure, i.e. as crystalline substances of chemical composition, the degree of randomness of the ordered internal structure, and as amorphous bodies of internal structure of a solid body may vary. It may random structure. The amorphous state has not yet been overcome only the long range ordering, manifested by

181 Vol. 13, No. 3, 1994 Phase Transformation in Amorphous Solids the geometry of the lattice of the given amorphous state, on the other hand, does not introduce substance, i.e. long range order with preserved medium any essential changes in the nature of chemical bonds, range order, for example, such as the assemblage of or such changes are relatively small. The coordination coordination polyhedra. Examples are the amorphous number of some cations, however, is occasionally substances formed by thermal decomposition of changed. These factors, together with the degree of inorganic (, borates, phosphates) and randomness of the internal structure, determine the certain glasses of complex composition. differences in the properties of the amorphous and In some amorphous substances there occurs only crystalline substances. short-range ordering limited to the particular coordina- The subject of the present study is the chemical and tion polyhedra. Glasses of simple composition are structural transformations occurring in amorphous examples of this. inorganic polymers obtained by heat treatment of The randomness may also extend to a first coordina- crystalline substances. tion zone, i.e. the nearest surroundings of As already shown earlier 111, it is possible to obtain forming the amorphous structure. This takes place in an amorphous substance by heat treatment in cases the case of elements in a glassy form or certain glassy where the destruction rate of the primary crystalline . structure is greater than the recrystallization rate of the It has been assumed in this paper that the internal amorphous matrix which is then formed. structure order in the amorphous substances has been Thermal amorphization of crystalline solids can be distorted or destroyed to a degree excluding the obtained by (1) thermal dissociation of compounds appearance of the X-ray diffraction lines characteristic (dehydration of some hydrous silicates, borates, of their crystalline forms. This means that the phosphates, etc.), (2) destruction or distortion of the disappearance of long-range ordering in the structure is of a solid under prolonged heat a sufficient reason to regard the substance as an treatment (). amorphous one; the state of randomness in X-ray The investigations were carried out on a few amorphous substances may, obviously, have a wider selected borates and zeolites rich in silica. The range of occurrence and include even the first occurring phase transitions, consisting in rearrange- coordination sphere. ment of the internal structure under the influence of The state of randomness in the area of the first temperature, were determined and the observed coordination sphere may change the nature of the inter- phenomena compared with those occurring in other atomic interactions and thus affect the electrical and substances rendered amorphous by heat treatment and magnetic properties of the substance. These states are in glasses. the subject of investigations in solid state physics. The aim of the investigations was to recognize the To investigate the amorphous substances, especially regularities controlling the phase transformations by those in which the short- and medium-range ordering the structural rearrangement of amorphous solids. has been retained, attempts are being made to apply An attempt has also been made to define the mutual methods used in structural chemistry and crystallo- relations between substances rendered amorphous by chemistry. With respect to their chemical and physical heat treatment and glasses. properties, these substances show considerable Thermal amorphization appears to be a promising similarity to the affinated crystalline substances. They method of obtaining some glasses and glass-ceramic contain the same type of coordination polyhedra as the materials, as well as ceramic powders, especially those corresponding crystalline substances. Occasionally, the that cannot be obtained by the sol-gel method. transition into the amorphous state cancels the Moreover, the examined processes correspond to deformations of coordination polyhedra to which they those occurring during vitrification and crystallization are subjected in a crystal structure. This takes place, for of xero-gels in the sol-gel method of obtaining glasses example, in the case of Si04 tetrahedra in vitrified and glass-ceramic materials. framework silicates III. The transition into the

182 L. Stoch and /. Waclawska High Temperature Materials and Processes

2. MATERIALS Zeiss Ur-10 spectrometer and with a Digilab FTS-60V Fourier Transform Infrared Spectrometer. Samples The model substances selected for the examinations were prepared as KBr pellets. were the borates of alkalis and metals of alkaline earths In the case of some substances, of which there were which have different complex anions. They have sufficiently large amounts available, the changes in their equivalents among the borate glasses. Moreover, density and microporosity induced by phase transitions these borates belong to substances of low tem- were also determined. perature. Coarse-crystalline, pure borate minerals from Measurements of the specific surface (BET), pore the collection of the Mineralogical Museum of the volume and size were conducted, taking advantage of Wroclaw University were examined. They included: the phenomena of and desorption of pure

Ca2B608(0H)6-2H20, at the temperature of liquid nitrogen by means

pandermite Ca2B508(0H)3-2H20, of a computer-controlled ASAP 2000 apparatus

kaliborite HKMg2B12Oi6(OH)i0-4H2O), (Micromeritics). Density measurements were performed

NaCaB506(0H)6-5H20, in an Accu-Pyc 1330 helium pycnometer

Na2B40j(0H)4-3H20, and (Micromeritics).

Na2B406(0H)2-3H20. Synthetic was also examined. Synthetic zeolites: ZSM-5 of high Si0 2 4. RESULTS content and NaA zeolite containing aluminium in its structure were examined too. They belong to framework A Phase Transformations in Amorphous Borates silicates and are built of Si04 and A104 tetrahedra. They may be regarded as substances corresponding to Results of the thermal analysis (Fig. 1), X-ray

minerals of the Si02 groups and aluminium-doped patterns (Fig. 2) as well as IR spectra (Figs. 3, 4, 5) quartz glass on account of their chemical composition indicate the range of amorphous structure stability and and the kind of short-range order of internal structure. the course of its reciystallization. They depend on the They differ from the above materials in the medium- chemical composition and primary structure of borates and long-range order. The zeolites were in their Na- (Tables 1 and 2). form. Thermal decomposition of chain borate colemanite

Ca2B608(0H)6-2H20 proceeds in two stages at 369°C and 386°C /3/. This process is accompanied by the 3. METHODS gradual destruction of the primary structure of the borate. At 650°C a small endothermic DTA peak To investigate the processes occurring during appears resembling the effect (Tg). At heating of the examined substances, the DTA method 741°C amorphous substance recrystallizes and crystal-

as well as the conventional and constant-rate TG and line 2Ca0-3B203 is formed. It melts at 950°C. DTG methods were used. The measurements were Dehydration and dehydroxylation of layer borate

performed on a Q-1500D derivatograph and ? pandermite Ca2B508(0H)3-2H20 results in the computer-controlled C-derivatograph (MOM formation of an X-ray amorphous substance. At 650°C Budapest). a small endothermic peak of transformation appears. At

In order to examine the structural changes taking 745°C the crystallization of Ca0-B203 starts followed

place during heating, the samples were heated at a rate by Ca0-2B203 formation according to the of 2.5°C min"1 up to the characteristic temperatures, stoichiometry of composition of the amorphous cooled and next subjected to X-ray diffraction and IR substances. They melt at 1100°C (solidus temperature) spectroscopic examinations. X-ray powder patterns

were made with a DRON-3 diffractometer using CuKa Thermal decomposition of chain borate kaliborite

radiation. Absorption IR spectra were obtained with a HKMg2Bj2Oi6(OH)i0-4H2O proceeds in two steps

183 Vol. 13, No. 3, 1994 Phase Transformation in Amorphous Solids

Table 1 Phase transformations of alkaline earth borates colemanite pandermite kaliborite ulexite I I I 400° C 580° C 400°C 260°C I I I I CaAO,, Ca4B,A, KMg2B,A, NaB340J7(0H)3l ^οφΗ. amoφh. βπιοφίι. matrix amoφh. + I I I 741°C 745°C 679°C CaB A + CajBA I I I cryst. cryst. CaAO,, CaB204 + CaB407 MgB407 + Mg.B.O, I cry st. cryst. 3τηοφ1ι. matrix ciyst. cryst. + > 550°C I I I 950°C 800°C KB A, NaBjO, βπιοφίι matrix cryst. I I + melt CaB204 + CaB407 I cryst. cryst. 729°C (Ca2B2O4)05 (Ca2B2Os)02J (BAU I I amorph matrix 1100°C MgB407 + Mg2B205 + KBsO, I cryst. cryst. cryst. > 700°C I + melt I BA NaCaBOj amorph matrix cryst. I 954°C BA I βιηοφίι matrix melt I 854°C I melt Table 2 Phase transformations of borates

borax (tincalconite) kernite

Na2B405(0H)4-8H20 Na2B40c(0H)2-3H20 I 20°-200°C 20°-300°C I Na2B405(OH)4 Na2B406(0H)2 Na2B407 amorph. amorph. amorph.

200°-400°C 540°C I Na2B405(0H)4 + Na2B407 Na2B407 amorph, matrix cryst. cryst. I 575°C |567°C) 735°C

Na2B407 cryst. melt

735°C

melt

184 L. Stock and I. Waclawska High Temperature Materials and Processes

heating at 640°C, the endothermic effect of transformation appears. At 679°C amorphous substance becomes crystalline 151. Excess of B2O3 forms amorphous matrix. Newly formed compounds melt at 954°C. A solid product of dehydration of the island-

structure borate ulexite NaCaB506(0H)6-5H20 is the amorphous matrix containing OH groups in which calcium borates crystallize at once 161. Above 550°C the OH groups are gradually removed and new compounds crystallize. They melt at 854°C. Borax gradually loses its molecular water up to the temperature 200°C. Dehydration is associated with amorphization of the crystalline structure. The newly formed amorphous substance, on the other hand, preserves all OH groups 111. Further increase in temperature causes their gradual removal. At the temperature 400°C, when the number of OH groups

decreases to 20%, the crystallization of Na2B407 begins. When all the OH groups are removed, at the temperature 520°C, there appears the reversible endothermal DTA peak, corresponding to the glassy state transition. It is followed by complete recrystallization of the amorphous matrix. Borax belongs to the island borates. In its structure

there occur the B405(0H)4 polyions consisting of

two B02 (OH) triangles and two B03(0H) tetrahedra. In between the polyions there are sodium cations surrounded by water . X-ray determined radial distribution functions (RDF) allowed it to be established that in the structure of a dehydrated, thermally amorphized borax the distances B-O and O-O are 1.48 Ä and 2.42 Ä, Τ [-C ] respectively. According to /8/, such values are Fig. 1: DTA curves of borates characteristic of the BO4 tetrahedra. It means that in 1) colemanite; 2) pandermite; 3) ulexite; 4) the amorphous structure, free from the OH groups, all

kaliborite; 5) borax; 6) kernite; TA - the atoms change their coordination numbers to temperature of amorphization; - glass 4, and that a tetrahedric configuration is adopted. transition Tincalconite behaves similarly during heating in spite of its slightly different structure in the hydrated (258°C and 272°C). Water molecules and OH groups state. Its structure is more compact on account of a are gradually released up to 400°C. Thermal smaller number of water molecules. Moreover, the OH amorphization of anhydrous kaliborite proceeds in a groups of the polyions combine with the Na+ cations.

step by step mode as all the OH groups are released. As Na2B406(0H)2-3H20 kernite, a member of the a result of the dehydration process, the borate chains of borax family, contains in its structure the same polyions kaliborite structure are destroyed. During the further as borax or tincalconite, but the polyions are

185 Vol. 13, No. 3. 1994 Phase Transformation in Amorphous Solids

(A) 2Ca0-3B203

>> 55 δ -ί- α

50 45 40 35 30 25 20 15 10 2Θ 35 30 25 20 15 10 2Θ

(D) NaB3°5

(C)

MgB^O; NaCaB03 ΚΒ5ΟΘ /WS

MgB^O; JljOTJL-J?

1 ! ! 1 r- 40 35 30 25 20 15 10 2 θ 50 45 40 35 30 25 20 15 10 2Θ

Fig. 2a: X-ray patterns of borates (A) colemanite: 1) , 2) amorphous form, 3) reciystallized (B) pandermite: 1) mineral, 2) amorphous form, 3) recrystallized (C) kaliborite: 1) mineral, 2) amorphous form, 3) and 4) reciystallized (D) ulexite: 1) mineral, 2), 3) and 4) recrystallized.

186 L. Stoch and I. Waclawska High Temperature Materials and Processes

IE) CQO 2 BJOJ IF) CaO • 2 ^03

Na^OglOHl^ '8H20 No2B40g(0H)2 '3H20 JlliIlL· 50 40 30 20 10 2 θ 50 AO 30 20 10 2β Fig. 2b: X-ray patterns of borates: (E) borax: 1) mineral, 2) amorphous form, 3) recrystallized (F) kernite: 1) mineral, 2) amorphous form, 3) recrystallized

A Β

Β A Ν

1600 1400 12D01000 600 TO 600 500 1 1600 1400 1200 1000 8C0 TO 600 500 V [ cm ] V [cm1] Fig. 3: DR. spectra of pandermite: 1) mineral, 2) Fig. 4: IR spectra of ulexite: amorphous form, 3) recrystallized 1) mineral 2) and 3) amorphous form (260° and 550°C), 4) recrystallized 187 Vol. 13. No. 3, 1994 Phase Transformation in Amorphous Solids

R Β A Ν C Ε

sßö ϊοοο is5ö 235ο 2^öö Sbö 3S0 Äöbo [cm1] Fig. 5: IR spectra of borax (tincalconite): 1) mineral, 2) amorphous form, 3) recrystallized polymerized infinite chains. Amorphized borax has pores of a total volume of In spite of small structural differences between 4.5 mm3/g, and their diameters amount to 3.8 nm. Its borax and kernite, with similar chemical composition apparent density is only 1.7655 g/cm3, while glass of after dehydration, the thermal reactions in both the same composition has a density of 2.3697 g/cm3. minerals proceed in a different way. The amorphous This means that it contains a great number of fine, form of kernite exists just at the temperature 300°C. It closed pores, not measurable by the BET method. crystallizes rapidly, which is indicated by the sharp DTA peak, while the crystallization of amorphous borax extends over a wide temperature range. B. Phase Transformations in Silica-Rich Zeolites Crystallization of kernite occurs at a lower temperature NaA zeolite loses water up to 600°C, yet it still (540°C) in comparison with that of borax (575°C) and retains its crystalline structure up to the temperature tincalconite (567°C). 800°C (Fig. 6). Starting from this temperature, the A comparison of the behaviour of these two zeolite lines on an X-ray pattern gradually disappear minerals at elevated temperatures reveals the un- (Fig. 7), which is an indication of amorphization of the doubted effect of structure on the thermal reactions in structure. The amorphization process is accompanied solid bodies. by a characteristic deflection on the DTA curve at Dehydration and dehydroxylation of borates causes 800°C. an increase in porosity. A colemanite sample increases At 860°C there takes place the crystallization of its total pore volume from73.5 mm3/g up to 145.7 cristobalite (S1O2) inside the amorphous alumino- mm3/g at 440°C. The appearing pores have a diameter matrix. This is a slow process, marked by a of 31.6 nm. With increasing temperature the volume of weak DTA peak. the pores gradually decreases, but their diameters At a somewhat higher temperature cristobalite increase, they attain the values of 124 mm/g and 125.4 reacts with the amorphous matrix and there crystallizes nm, respectively. the framework silicate .

188 L. Stoch and I. Waclawska High Temperature Materials and Processes

ZSM-5 zeolite retains an unchanged crystalline structure up to about 840°C. At this temperature the DTA curve shows a weak, irreversible endothermic effect. From that temperature upwards there begins a gradual rearrangement of the open structure of zeolite into a compact structure of cristobalite. This process is completed by the time the temperature reaches 1000°C. It is worth noticing that in both zeolites the recon- struction of the open skeleton framework of zeolite into the compact structure of nepheline or cristobalite proceeds step by step and is not accompanied by any greater thermal effect. As it seems, this rearrangement consists of a change in the orientation of the tetrahedra in the loose structure of the zeolite, without breaking 0 "" 500 r 1000 any greater number of bonds. TCC] Unlike the transformation of the zeolites, the Fig. 6: DTA curves of zeolites transition of the compact structure of quartz into that of 1) NaA zeolite, 2) ZSM-5 zeolite. The cristobalite, which is included in their constructive numbers denote the density of zeolite [g/cm3] polymorphous transformations, consists of the initial at various temperatures formation of an intermediate amorphous phase, within

(Al NaAlSiO/,

50 40 30 20 10 2Θ 10 2 θ Fig. 7: X-ray patterns of zeolites (A) NaA zeolite: 1) , 2) amorphous forms, 3) and 4) recrystallized (B) ZSM-5 zeolite: 1) crystals, 2) reconstructed, 3) recrystallized

189 Vol. 13, No. 3. 1994 Phase Transformation in Amorphous Solids which cristobalite crystallizes afterwards. In view of an with a complete change in the medium-range ordering almost identical composition of the ZSM-5 zeolite and and a transition from an open zeolite structure to a of quartz, it is clearly seen that the primary structure more dense cristobalite structure. Hence this process is has a decisive influence on the mechanism of phase slower and the accompanying thermal effect broadened transition. (Fig. 6). Amorphization of NaA zeolite is connected with In the case of substances of a complex chemical structure condensation. Dehydrated zeolite has a composition, recrystallization proceeds in several specific surface BET of 4.07 m2/g, pore volume of 15.5 stages. The following general regularity can be mm3/g, the diameter of which equals 111.7 Ä. When observed. the irreversible endothermal process has been In the borates the first to crystallize are the borates completed at 800°C, these values are as follows: 2.04 of bivalent cations of simple composition and a low m2/g, 6.80 mm3/g and 133.07 A. This means a twofold ratio of the number of boron atoms to the number of reduction of the volume of pores available for N2, cations in the formula, which means that their structure which points to a radical condensation of the material. contains the borate anions with a low degree of polymerization of BO4 tetrahedra (orthoborates, ring borates, less frequently chain borates) /10/. They are formed in the amorphous matrix enriched 5. DISCUSSION with B2O3. An example here is pandermite. If the compound contains alkalis, they are retained in the A. Multi-Stage Crystallization of Amorphous Solids amorphous matrix (kaliborite, ulexite). Ciystallization of the majority of the considered The next crystallization step occurring at a higher amorphous substances proceeds in several stages. One- temperature comprises the crystallization of the borates stage crystallization takes place when the chemical of alkalis (kaliborite) or the calcium borates formed in composition of the amorphous precursor is similar to the first stage are dissolved, and next the double borates that of the crystalline phase formed from it. of alkalis and alkaline earth metals crystallize, In all the examined cases, crystallization of amor- according to the chemical composition of the given phous substances begins at temperatures considerably substance (ulexite). lower than their melting temperature. When an amorphous substance contains alkali Colemanite and silica-rich zeolite are examples of a cations besides those of alkaline earth, ciystallization single-stage crystallization. In the first case, causes their segregation. The cations of alkalis, as the crystallization is a rapid process and the accompanying more basis ones, remain in the amorphous matrix, exothermal DTA peak is exceptionally sharp, similarly being strongly bound to the borate anions which have to the peak of the Al-silicate metakaolinite an acidic character. Segregation is thus controlled by crystallization. In the opinion of the authors, this is an chemical affinity. The cations of the alkaline earths, indication of the diflusionless mechanism of this phase having greater field strength, form with the borate transition. The crystalline phase is formed through the anions relatively stable elements which, it is assumed, reorientation and topological adjustment of the may even be inheritance from the primary structure of elements of the amorphous structure like in displacive the crystalline substance. The segregation of the polymorphic transformations 191. This is possible due to components makes the amorphous matrix rich in B2O3, structural similarity between the structure of the thus its cross-linking increases accordingly. precursor and that of the crystalline phase being High-temperature phase transformations of layer formed. In the first case ring borate, like its crystalline silicates are very well known processes /11/. precursor, and in the other case^ (Si, Al)-spinel, are Dioctahedral layer silicates transform into X-ray built of the same tetrahedra which form an amorphous amorphous substances after dehydroxylation with the structure. original structure preserved to a considerable degree. Crystallization of amorphized zeolite is associated The structure of these minerals contains layers which

190 L. Stoch and I. Waclawska High Temperature Materials and Processes consist of sheets of - tetrahedra and structure to exist. aluminium-oxygen-hydroxyl octahedra. The mechan- Montmorillonite (Cheto modification) starts to ism of thermal transformation of most of these minerals dehydroxylate at 450°C. Interlayer spaces are a is well known. Al4Si40io(C)H)8 and its convenient way of escape of water molecules. Due to polymorphs (1:1 layer Al-silicates) are built up of pairs this, the dehydroxylation process is completed at about of tetrahedral and octahedral sheets. They dehydroxyl- 650°C. Residual OH groups are slowly removed up to ate at 400-700°C and dehydroxylation products contain 840°C (endothermal peak). At this temperature, the about 10% of preserved OH. Their structure has a structure is completely amorphized. Hydroxyl removal distorted silicon-oxygen tetrahedral sheet. An results in the formation of 5-coordinated aluminium octahedral sheet is reconstituted and transformed in with smaller amounts of 4- and 6-coordinated regions of distorted Al-0 tetrahedra, containing aluminium. The dehydroxylation degree necessary for randomly distributed isolated residual hydroxyls the amorphous structure to be formed is reached at a associated with Al-0 coordination polyhedra of lower temperature than in pyrophyllite, in spite of the octahedral and tetrahedral symmetry /12/. At 970°C similarity in structure of these minerals. Open hydroxyl groups are removed and at the same moment interlayer spaces of montmorillonite structure, small the amorphous structure is transformed into defective particle size and crystal defects make it permeable to Al rich (Al, Si)-spinel and mullite in amorphous S1O2 the water molecules, which are obtained from the matrix /11/. dehydroxylation, to escape at a relatively low Thermal transformation of 2:1 layer silicates, made temperature /16/. up of layers consisting of two tetrahedral and one Substances which are the dehydroxylation products octahedral sheet inside, is more complicated. De- of layered silicates retain pieces of the structure of the hydroxylation starts at a higher temperature. It explains primary material up to about 1000°C. Afterwards they well the "sealed box" model of thermal decomposition undergo internal rearrangement which consists of the /13/. The 2:1 structure is less permeable to water formation of new crystalline phases. This process molecules than the 1:1 structure. The internal pressure proceeds in several stages. of water vapour necessary to disrupt it must be higher, In kaolinite the first transition at the temperature thus a high temperature is required for this reaction. 980°C is very rapid (extremely sharp DTA peak) and it Consequently, the range of existence of X-ray consists of a small, difiusionless reorientation of AIO4 amorphous structure is much narrower. and S1O4 tetrahedra with the formation of (Al, Si)- The major dehydroxylation of pyrophyllite takes spinel and mullite in amorphous silica matrix. At about place in the range of 500-800°C. Pyrophyllite can 1200°C there follows an exchange of components tolerate the loss of at least 30% of hydroxyl water between the matrix and the earlier crystallized phases, without destruction of its crystal structure. The loss of and mullite Alg03.5(AlSi30i6) and cristobalite Si02 more water results in the formation of an X-ray are formed. It is a thermodynamically stable phase amorphous structure with five coordinated aluminium. composition. It contains about 10% of OH groups; removal of the In the first-stage crystallization of internal hydroxyls causes the gradual crystallization of mullite character, ribbon-like crystals of spinel and mullite are in amorphous silica matrix /14/. Dehydroxylation of formed within the kaolinite plates, and their orientation muscovite KAl2[AlSi3O10](OH)8 begins at 700°C remains in a topological relation to the primary (more dense structure, less permeable to H2O structure. During the second stage, mullite of prismatic molecules). On the completion of dehydroxylation, (Si, habit, typical of this mineral, is formed. Its ciystals are Al)-spinel crystallizes and interlayer K+ with tetra- variously oriented and they reach far beyond the plate hedral sheet form a -like phase related to area/11,12/. leucite, next highly aluminous mullite at the expense of The dehydroxylated pyrophyllite structure contains spinel is formed /15/. The temperature of dehydroxyla- 10% of the residual OH groups combined with Al five- tion completion is too high for a real X-ray amorphous coordinated aluminium. Above 900°C, when they are

191 Vol. 13, No. 3, 1994 Phase Transformation in Amorphous Solids removed, there begins the step-like crystallization of as crystallization proceeds. The densification processes mullite which occurs at about 1100°C, and next the of the amorphous structures of borates and silicates of silicate matrix recrystallizes into cristobalite /14/. microporous structure, on account of the pore On account of its chemical composition, dehydro- dimensions and the character of the internal structure xylated muscovite recrystallizes with the formation first of the substance, resemble the sintering of ceramic of (Al, Si)-spinel which later recrystallizes into mullite, powders of nanometric dimensions. and from the amorphous matrix a leucite-like phase is crystallized /15/. B. Components Mobility Control Magnesium-rich montmorillonite, Cheto-type, after a loss of 10% of the residual OH groups, recrystallizes Multistage recrystallization of the amorphous solids with the formation first of a solid solution of a quartz considered here belongs to the internal structure structure and enstatite MgSi03. Aluminium is initially reconstitution processes. The mechanism of these included in a high-cordierite phase (Mg2Al4Si50ig), processes is specific, according to the particular which at higher temperatures is replaced by sapphirine properties of the primary structure as a medium of these

(Mg,Fe)4Al8Si208. At 1200°C the quartz-like com- reactions. ponent is completely transformed into cristobalite /16/. Internal structure reconstitution processes or The course of phase transition of layered silicate internal processes occur through the displacement of shows many similar features to that of phase transitions the parent structure components which rate changes in in borates. In either case the first new compounds a different manner with temperature. The result is the crystallize in the amorphous matrix in two stages. In formation of intermediate metastable phases and a the case of silicates, the matrix is rich with S1O2. The multistage course of many internal processes and step- type of the first phase being formed is determined by by-step mode of the thermodynamic stability state the cations. In silicates, the groups of cations, forming reached /19/. the preserved fragments of the octahedral layer, are the The crystallization of many glasses takes a multi- starting material for new compounds (crystallization of stage course and may be an example. Al,Si-spinel or high cordierite and quartz solid In silicate glasses the first crystallizing phase is solution). often a quartz-like solid solution or another compound At higher temperatures the second stage of of the structure and chemical composition close to the crystallization takes place; this is connected with an parent glass. At higher temperatures their structure exchange of chemical components between the becomes rebuilt with the formation of crystalline phases previously formed compounds and the matrix. New whose chemical composition is increasingly closer to compounds are then formed which, with respect to the stable thermodynamic phase (cordierite, spodu- composition and structure, correspond to the state of mene, mullite, etc.). Many examples of multistage the thermodynamic equilibrium of the system. crystallization of glasses can be found in recent The dehydroxylation products of layer silicates are scientific literature /20-22/. Complex reaction series in porous. Their dehydroxylation, similarly to that of amorphous borates and silicates, considered here, can borates, is an internal process of crystals and water be included among them. vapour is formed in their entire volume /17/. As a result Multistage recrystallization can be explained as the the products of dehydroxylation have a microporous consequence of limited mobility of the components- structure. Dehydroxylation of kaolinite causes a reagents which changes with temperature. In the decrease of density from 2.55 to 2.34 g/cm3 /18/. Crys- formation of new phases the participation is permitted tallization of spinel and that of mullite is accompanied only of those chemical components which at the given by a rapid diminution of the specific volume and temperature have a possibility of rearrangement or densification of the structure. In a similar way, displacement determined by their diffusion coefficient. dehydroxylation of muscovite brings about an increase The diffusion coefficient of the components of solid in the specific volume /17/. It subsequently diminishes structure increases with increasing temperature, but

192 L. Stoch and I. fVaclawska High Temperature Materials and Processes this increase is different for different chemical com- With increasing temperature and mobility of the ponents. It may happen that at various temperatures primary structure components, the proportion of long- different components become capable of forming new distance diffusive displacements may increase, causing compounds. Then, compounds differing in their a corresponding change in the mechanism of formation chemical composition successively crystallize. of new phases. The increase in the mobility of the structural It changes into the diflusional mechanism, which elements with temperature can be easily observed in enables exchange of the components between the grains glasses. Up to the temperature of the glass transition, of phases formed earlier, as well as between the which corresponds to the 1013-3dPa-s, glass amorphous matrix. This permits the decomposition of behaves like a rigid and brittle body. A little above this the metastable phases and crystallization of new temperature it demonstrates viscoelastic properties with thermodynamic stable ones. This takes place in alkalis rearrangement of large fragments of the network and/or and alkaline earth borates. structural units. When the temperature increases, At this stage, due to sufficient mobility of all 6 3 causing a reduction of viscosity to 10 -10 dPa-s, the chemical components, the formation of new compounds glass shows the ability of viscous deformation is determined by their chemical affinity. According to connected with the orientable flow of the structural unit Prigogine 1251, the free energy of the proper reaction is with a geometry of chains, spheroids and disc-like a measure of this affinity. Accordingly, the compounds particles. With further increase in the temperature, the corresponding to the thermodynamic equilibrium state dimensions of these units diminish and accordingly the of the system are formed at that stage of the phase viscosity of the glass decreases /23/. transitions. This scheme of change of mobility of the structural In the first, low temperature stage of the phase elements with temperature can be extended to the transitions, due to the selectively limited mobility of amorphous borates and other glass-like amorphous components, this mobility becomes a governing factor. substances. Compounds containing the most mobile components At low temperatures, when the mobility and are then formed or difiusionless transformations take possibility of rearrangement of structural elements is place. limited, only those phases may be formed whose chemical composition and structure are close to the C. OH Groups in Amorphous Solids primary structure. Recrystallization of amorphized colemanite or the first stage of amorphized layer Amorphous solid bodies retain a number of OH silicate ciystallization are suitable examples. groups. They remain in their structures to veiy high In that case, the formation of new structure is temperatures. The data collected here indicate that the possible, as it seems, through short distance OH groups have an essential influence on the stability rearrangements of structural elements, similar to those of the amorphous structure, although they occasionally occurring in the so-called displacive or distortional occur in a small number. polymorphic transitions according to Buerger's The OH groups retain the amorphized island classification 191. The formation of another inter- borates, containing Na, in the structure of which there mediate phase requires a more complex mechanism, occur isolated polyions made up of triangles and when a change in the anion subnetwork, usually less coordination polyhedra in which the OH groups exist, mobile, has also a difiusionless character, but the i.e., ulexite, borax, tincalconite and kernite. mobile cations become displaced at considerable dis- The borate-sodium matrix with the OH groups tances in a diflusional mode. Crystallization of calcium retains its amorphous state up to above 500°C. Calcium and magnesium borates in amorphized borates of the borates (ulexite) crystallize in it. It becomes fully complex composition are an example. recrystallized, forming sodium borates, not until the This is an analogy to the disorder-order OH groups have been completely removed (Table 1). transformations in and /24/. The amorphized dioctahedral layer silicates also

193 Vol. 13. No. 3. 1994 Phase Transformation in Amorphous Solids retain part of the OH groups (10%) which are 2Θ, i.e. within the range of angles in which the associated with the Al-0 polyhedra. Removal of the strongest line of cristobalite appears. Amorphization residual OH groups from kaolinite is connected with brings about total collapse of the open structure of rapid crystallization of (Al, Si)-spinel. Pyrophyllite also zeolite, as already mentioned. retains about 10% of the OH groups which are released The phenomena described above indicate the at a temperature of about 1000°C followed by the existence of some relations between the structure of the crystallization of mullite /14/. In dehydroxylated amorphous phase and the crystalline phases derived montmorillonite, removal of the residual 10% OH from it. On the other hand, the middle-range ordering groups starts crystallization 1161. of the amorphous phase appears also be inherited, at These examples indicate that the OH groups least in part, from the primary structure. stabilize the existence of the amorphous forms of some The existence of such structural dependences is also substances. When these groups are removed, there is a indicated by the course of the crystallization of amor- rapid rearrangement of the amorphous structure and phous phases, as discussed in detail earlier in this crystallization. paper. Amorphization cancels the splitting of the absorp- tion bands in infrared, induced by the influence of the D. Structure Inheritance crystal field. It is for this reason that the amorphized X-ray patterns of amorphized borates and silicates substances exhibit broad, smooth bands of the IR do not show sharp diffraction lines of the crystal lattice. spectrum (Figs. 3, 4, 5, 8). Positions of the bands do not Instead, there appear broad, diffused bands of change, which is an indication that the transition into amorphous structure (Fig. 2). They are evidence of the the amorphous state does not change the chemical occurrence of some kind of middle-range order in the bonds and does not disturb the near order of the structure. Similar bands can be observed in X-ray elements of the anion framework. It is not changed by patterns of glasses. the subsequent recrystallization, either. They frequently occur within the same range of angles in which the strongest lines of the primary crystalline phase are found. Examples are colemanite £. Structure Relaxation Phenomena and pandermite, as well as ulexite. In the latter case, Inorganic as well as organic glasses exhibit a the X-ray pattern of an amorphized sample shows two structure strain relaxation phenomenon termed the bands, about 30° and 45° 2Θ, with superimposed lines glass transition effect. According to 1261, the glass of calcium borates which have started to crystallize. transition process distinguishes glass from other solid In the case of kaliborite, the diffused band appears bodies. at about 20° 2Θ, i.e. beyond the range of the diffraction Glass transition is defined as that phenomenon in lines of the primary structure. This may be attributed to which a substance exhibits a sudden change in its the formation, in the course of amorphization, of a properties, such as heat capacity, expansion coefficient middle-range order other than the primary one. This and viscosity, from crystal-like to liquid-like values. band is transformed next into diffraction lines of At the transition temperature, the viscosity of glass magnesium borates. decreases to about 1013-3dPas. This means that the The sodium borates, borax and kernite, in the relaxation of stresses is connected with loosening part amorphous state, show three bands in the range of of the chemical bonds in the glass structure. Up to the angles, 45° or 50°, and 30° and 18°. Two of them cor- transition temperature glass behaves like a brittle body respond to the most intensive lines of the primary of rigid structure. A little above this temperature it has phase. All three become transformed into an intensive viscoelastic properties. line of the crystalline phases formed from the The transformation effect is induced by the amorphous phase. relaxation of internal stresses occurring in the glass Amorphized zeolite NaA yields a band at about 25° structure at temperatures lower than the glass transition

194 L. Stoch and I. Waclawska High Temperature Materials and Processes

500 1000 1500 2000 2SOO 3000 3500 4000 500 1000 1500 2000 2500 3000 3930 «00 V [cm- 1 1 N) [cm1]

Fig. 8: IR spectra of zeolites: (A) NaA zeolite: 1) crystals, 2) amorphous form (B) ZSM-5 zeolite: 1) crystals, 2) and 3) reconstructed (890° and 950°C)

point, i.e. when it has the properties of a brittle body. achieve a fully ordered state. There are certain crystals These stresses are, among other things, the for which the remaining disorder is frozen which consequence of the disordered arrangement of elements becomes the reason for the above phenomenon. forming the glass structure. This transformation has been observed i.a. in Relaxation of stresses in the structure at the glass B(OH>3, (290K), in some hydrates and it may be attri- transition temperature is coupled with a change in its buted to the randomness of the proton arrangement. It properties as mentioned above. For glasses of simple is exhibited by SnCl2-2H20 (150K), hexagonal ice (105 chemical composition the transformation temperature is K) and a number of organic compounds /29/. approximately equal to 2/3 of the melting temperature The described phenomena occur at low temperatures Ι2ΊΙ. and the anomalous change of the heat capacity is As demonstrated by Suga et al. /28,29/, the associated with the of the orientational phenomena of structure relaxation occur in some freedom or other relatively small displacements of the crystalline substances. They are also manifested by the elements of the crystalline structure which as a whole anomaly of the curve of heat capacity as a function of exhibits a high degree of ordering. For this reason a temperature, which is reversible and depends on the change in the heat capacity accompanying the thermal history of the substance in the same way as the transition is very small and can be measured only by transition of glass. Suga has termed the substances means of extremely sensitive calorimeters. exhibiting this phenomenon "glassy crystals". Suga's findings essentially modify our views on the These are crystalline substances in which at glass transition phenomenon as a feature distinguishing elevated temperatures the molecules forming them glasses from other solid bodies. It has been visualised show orientational disorder (orientationally disordered that a glass transition effect occurs in solid bodies, both crystals). The disorder generally disappears dis- in amorphous and crystalline ones, when a certain state continuously through a solid-solid phase transition to of disordered structure has become frozen in.

195 Vol. 13, No. 3, 1994 Phase Transformation in Amorphous Solids

Thermally amorphized borates show a reversible Table 3 endothermic effect which corresponds to that of a glass Phase transformations of zeolites transition (Fig. 9). The temperatures of this effect are NaA zeolite ZSM-5 zeolite listed in Table 4. They are higher for calcium borates

(650°C) than for sodium borates (520°C). NaAlSi04-2,25H20 Ν ao,03 A 1U,03S i0,97O4 · 0,3 Η 2 Ο Applying the 2/3 rule to the examined borates and I I calculating their transformation temperatures from 20°-600°C 20°-400°C their melting temperatures indicated by DTA curves, I I NaAlSiO« Nao,o3Alo,o3Sio,9704 we will obtain temperature values very close to the zeolite zeolite measured ones. For calcium borates the calculated I value amounts to 640°C, and for sodium borates - 600°-800°C 840°C 534°C. This may be interpreted as a confirmation of the NaAlSi04 -> NaAlSiO« Nao.oaAlo.osSio.srO., -> Si02 hypothesis that the reversible endothermic effect of zeolite amorph. zeolite cristobälite amorphous borates is produced by a heat change, that is I induced by the glass transition effect. This effect has 800°-860°C 1000°C not been recorded on the DTA curve of ulexite, since at I NaAlSi04 Si02 the temperature at which it should occur there is a amorph. cristobälite crystallization of the amorphous matrix, which cancels I the frozen randomness. In the case of kernite, the effect 860°-960°C of glass transition coincides with the beginning of xSi0 + NaAlSii_x0 crystallization. 2 4 cristobälite amorph, matrix Unlike borates, the zeolites do not demonstrate the I glass transition effect. Their melting temperature is so 960°C high that this effect should appear at 1020°C in the NaALi04 nepheline

Table 4

Glass transition temperature (Tg) of amorphized borates

Mineral Amorphous Τ Tm Substance

Colemanite Ca2BeOn 650° C 950° C

Pandermite CaiBjoOiä 650° C 1100°C

Kaliborite KMg2B12021 640° C 954° C

Ulexite NaB3,403,7(0H)3.8 - 854° C

Borax Na2B4Os(OH)4 520° C 755° C (tincalconite)

Fig. 9: DTA curve of (A) colemanite and (B) glass Kernite Na2B406(0H)2 520° C 735° C of colemanite composition

196 L. Stoch and I. Waclawska High Temperature Materials and Processes case of NaA zeolite and at 1150°C in the case of ZSM- the amorphous structure is too rigid for stress 5 zeolite, i.e., when they have already undergone com- relaxation to occur without disturbing it. In this case plete recrystallization. the disturbance of the amorphous structure induces The DTA curves for zeolites record small irrever- crystallization. sible endothermic peaks or deflections at the As demonstrated here, it may also occur that further temperature 810°C for NaA zeolite and at 820°C for destruction of the amorphous structure takes place, but ZSM-5 zeolite. the crystallization process is slow and it attains a These transitions are accompanied by evident measurable rate at a higher temperature than this condensation of the structure, manifested by a change transition. It is marked by a small DTA peak, thus it is in microporosity or density. This fact allows us to accompanied by a change of the molar heat or enthalpy. assume that the considered effect is induced by collapse This is an irreversible transition contrary to the glass or reorganisation of the amorphous structure, which transition. Montmorillonite and zeolites examined here precedes the crystallization process. Collapse of zeolite supply the appropriate examples. structure causes an increase in its density above the In elastic structures, where the temperature increase small endothermic temperature (Fig. 6). It is worth brought about sufficient loosening of a part of the pointing out that this rebuilt structure obtains a density chemical bonds, a reversible relaxation transition, such proper for cristobalite structure (2,364 g/cm3) before as the glass transition, may take place. This is possible the beginning of the exothermic peak of its especially in glasses and amorphous substances of crystallization. In the case of crystallization processes relatively low melting temperature. occurring in the solid phase and consisting in the Even so, the glass transition effect indicates that the rearrangement of the primary structure, it may be amorphous solid obtained flexibility and uniformity of assumed that the formation of a new crystal structure its structure corresponding to the glass. consists in fitting in the elements of the amorphous structure. F. Thermal Amorphized Solids and Glass Relation A similar endothermic effect, not definitely explained as yet, is demonstrated by the A complete definition of glass which would montmorillonite Cheto modification. This is a characterize all types of glass known so far has not relatively intensive effect occurring at a temperature of been formulated until now. Neither have the criteria about 840°C, when the residual OH groups are being indispensable for the formation of the glassy state been removed. It precedes the crystallization of mullite and defined. Hence, a very general definition of glass of spinel (exothermal DTA peak). This is an indication formulated by the U.S. National Research Council /30/ that the irreversible processes of the rearrangement of is frequently used, according to which glass is an X-ray amorphous structures preceding crystallization are amorphous material exhibiting the glass transition more frequent. They occur, most probably, when the effect. structure of the amorphous phase is rigid and the The glass structure is usually described by a release from stresses in the structure proceeds through "random network" model, and considering the its continuous rearrangement. existence of microinhomogeneities in the glass From the above data it follows that the occurrence structure resulting from the existence of the elements of of the so-called glass transition effect is not restricted to medium-range order and/or non-uniform distribution of glasses. This alone cannot be regarded as an immanent the components - the "random array" picture of glass feature of the glassy state. Randomness of structure is structure is viewed as being more adequate. According an indispensable parallel criterion. It appears in various to this model, in glass no unit of structure is repeated at types of amorphous substances as well as in crystalline regular intervals in three dimensions. ones when a certain state of disordered structure has These models are derived from the traditional been frozen in. Certain amorphous substances do not Zachariasen's model of silicate glass as a three- exhibit this effect, since in the given temperature range dimensional, continuous, but geometrically disordered

197 Vol. 13, No. 3, 1994 Phase Transformation in Amorphous Solids network in which part of the oxygen bridges combining This refers, among other things, to Goldschmidt the S1O2 tetrahedra is broken by cations of uni- or and Zachariasen^ crystallochemical criterion of the bivalent metals (modifiers). necessary existence of a three-dimensional network It is known now that besides glasses of polymeric built of tetrahedra or triangles joined with each other in homogeneous continuous network as mentioned above, the corners, which in fact is indispensable if the net- one can distinguish glasses of a polymeric, continuous work is to be flexible. Of equal importance is Smekal's but inhomogeneous framework. Among these are criterion of the necessary existence of mixed chemical glasses of mixed network, built of coordination bonds, i.e. the directed (covalent bonds) but also polyhedra of various kinds (alumino-silicate, boro- misoriented ones ( and Van der Waals bonds), i.e. silicate, alumino-silicate-phosphate glasses, etc.). The those that permit the necessary displacement of the structure of a polymeric, discontinuous framework is structural elements. In Sun's criterion the requirement made up of chains or rings of tetrahedra linked by that the glass forming substances possess high bond surrounding cations (phosphate and borate glasses). A energy (90 kcal/mole) corresponds to the strong spatial number of glasses have non-polymeric structure. Some network in polymeric glasses as a factor inhibiting the of them are made up of oxides or halides with small ordering of the glass structure. interspersed islands of coordination polyhedra of other It follows from considerations presented here that components. the flexible structure as a characteristic feature of the The structural diversity of glasses known so far, in glassy state and a criterion of its existence are valid which different coordination polyhedra assemble and also with reference to other amorphous bodies. all kinds of chemical bonds are encountered, allows one It may be assumed that all amorphous substances to conclude that factors regarded so far as indispensable differ from each other in the degree of elasticity of their to the formation of the glassy have structure and its micro- and macrohomogeneity. proved not to be essential or to play a limited role. The amorphous borates discussed here are distin- The following factors can be indicated as essential guished by the elasticity of their structure which is as for glass formation /31/: high as that of glass, and accordingly they exhibit a 1. The glass structure must possess enough elasticity to glass transition effect similar to that of glass. Thus with enable its elements to occupy varying positions with respect to the structure they represent an equivalent of respect to each other and to adjust each other to a glass of polymeric structure, but of low polymerization degree sufficient to give their arrangement the degree (rings, chains), hence an inhomogeneous character of a random array. structure. 2. Agents must operate to stabilize the random On account of the degree of randomness of structure arrangement and counteract their ordering. These and its homogeneity, their state may be defined as a are, among others, strong bonds in the polymeric glassy one. They differ from glasses in the lack of structure or admixtures and inhomogeneities in uniformity, being microporous, whereas glasses should non-polymeric glasses. be uniform. However, by means of a sufficiently long According to this glass is a solid body with a thermal treatment, above the transformation flexible random structure. The construction and temperature, they may be transformed into a uniformity of the structure depend on the chemical homogeneous body. composition and the origin as well as the history of the On the other hand, layered silicates in an glass (flexible framework model of glass /31/). amorphous state exhibit a structure with a high degree The criterion of the structure flexibility and the of inhomogeneity manifested by the preservation of factors preventing ordering of the structure as large elements of their primary layered structure. Thus indispensable to the existence of glasses and other the long-range ordering becomes retained to a great inorganic amorphous substances comprise all the extent. This structure is not elastic enough for stress particular criteria which have so far been considered as relaxation in the form of glass transition to occur prior indispensable for the existence of this state. to crystallization. Stresses may induce, on the other

198 L. Stoch and I. Waclawska High Temperature Materials and Processes

hand, farther destruction of the primary structure, may come into existence when its structure exhibits which is an irreversible process preceding sufficiently high elasticity, indispensable for the crystallization. Zeolites examined here behave in a randomly oriented components to fit each other. similar way. Amorphized NaA zeolite structure Some of the amorphous substances reach the state of resembles the mixed network alumino-silicate glass. structural elasticity which permits relaxation of the As we can see, the elasticity of structure is a internal stresses induced by the state of randomness criterion determining the properties and behaviour of below their crystallization temperature. This is mani- many amorphous substances at different temperatures. fested as the effect of glass transition. The temperature The structural similarity between the given amorphous of this effect equals about 2/3 of the melting tempera- substance and glass of the same composition exists ture of these substances, similarly to glasses. This according to this feature. means that as regards the state of their structure they have the nature of glass. They differ occasionally from glasses in their microporosity which is a remainder of 6. FINAL REMARKS these gaseous components removed during decomposi- tion. The porosity disappears quickly near the Many crystalline substances, the inorganic polymers temperature of glass transition. Vitreous substances of in particular, can be directly converted into the this kind are formed as a result of thermal amorphous state by way of heat treatment. Some of amorphization of borates. Some data suggest that they them will retain the amorphous state within a may be formed also by phosphates. This is an instance temperature interval long enough for the phenomenon of the formation of a glassy state from a crystalline one of thermal amorphization in a solid state to find by way of internal structure rearrangement of the order- practical application in the production of new disorder type under the influence of thermal vibrations. materials. Perspectives exist to utilize this process for the At high temperatures these substances undergo production of glassy materials including those of mixed recrystallization which often proceeds in several stages. network, as well as non-polymeric glasses such as the It occurs through rearrangement of the amorphous halide-oxide ones. Through appropriate crystallization structure and next through reconstitution of the it is also possible to obtain from them materials with successive metastable intermediate crystalline phases. different ratios between the crystalline phase and the Some of these stages are accompanied by an exchange glassy matrix. The number of possible variants of this of chemical compositions between the coexisting type of materials seems to be very high. phases. Another type of amorphous substance is formed as a The amorphous state formation by thermal result of thermal amorphization of . decomposition of a crystalline precursor represents one These substances are formed due to a certain elasticity of the many stages of changes on the way from the state of the structure which results, on the specific to the precursor substance at low temperatures one hand, from the possibility of the relation and a to a state characteristic of it at high temperatures. This change of the angle between the tetrahedra in their process is realized through permanent reconstruction of framework, and, on the other hand, from the possibility the internal structure. that aluminium will form various coordination poly- The amorphous structures formed through thermal hedra (Al coordination number with respect to oxygen decomposition of solid bodies preserve to a greater or may be 4, 5 and 6). Magnesium does not exhibit such a smaller extent the medium-range ordering inherited property, and, consequently, magnesium silicates do from the structure of the primary substance. This not form amorphous intermediate structures. affects their properties as well as their thermal stability The elasticity of their structure, however, is not and changes which occur in these structures at higher high enough, and the stresses induced by the state of temperatures. randomness are canceled through crystallization Similarly to the glassy state, the amorphous state connected with segregation of the components. This

199 Vol. 13, No. 3, 1994 Phase Transformation in Amorphous Solids process takes place at a temperature considerably lower 7. I. Waclawska, J. Thermal Anal., in press. than the temperature at which the glass transition effect 8. P. Medda, A. Musinn, G. Paschins and G. would appear. The structures of this group of Piccaluga, J. of Non-Cryst. Solids, 150, 76 substances preserve to a great extent fragments of the (1992). primary structure. This stabilizes the amorphous state 9. M.J. Buerger, Fortschr. Mineral., 29, 9 (1961). but, at the same time, makes the structure more rigid. 10. H.E. Patch, K.S. Pennington and J.D. Cuthbert, In spite of distinct differences in structure and Amer. Min., 47 (1962). behaviour with increasing temperature, the groups of 11. G.W. Brindlay and J. Lamaitre, in: Chemistry, amorphous substances have many features in common. Clays and Clay Minerals, A.C.D. Newman (ed.), One of them is the presence of the OH groups Mineralogical Soc. Monograph, London, p. 319 (about 10% of their initial number) performing the role (1987). of a factor that stabilizes the amorphous state. 12. K.J.D. MacKenzie, I.W.M. Brown, R.H. Another phenomenon common to both groups of Meinhold and M.E. Bowden, J. Amer. Ceram. substances is the segregation of the chemical Soc., 68, 293 (1985). substances, accompanying crystallization, the effect of 13. L. Stoch, Thermochim. Acta, 203, 259 (1992). which is the formation of amorphous matrix in which 14. K.J.D. MacKenzie, I.W.M. Brown, R.H. the first crystallization stages take place. The structural Meinhold and M.E. Bowden, J. Amer. Ceram. mechanism of this crystallization changes in the same Soc., 68, 266 (1985). way with increasing temperature. It is manifested by an 15. K.J. MacKenzie, I.W.M. Brown, C.M. Cardis and increasing number of components participating in the R.H. Meinhold, J. Mat. Sei., 22, 2645 (1987). diffusive displacements at long distances, leading to a 16. W.M. Brown, K.J.D. MacKenzie and R.H.J. transition from difiusionless to diffusive Meinhold, J. Mat. Sei., 22, 3265 (1987). transformations. 17. L. Stoch, J. Thermal Anal., 37, 1415 (1991). The phenomena and regularities described here 18. F. Freund, Proc. 1972 Int. Clay Conference, appear to have a sufficiently general character to Pergamon Press, London, 13 (1973). include vitrification and, subsequently, crystallization 19. L. Stoch, J. Thermal Anal., 40, 107 (1993). of xerogels formed in the sol-gel method. This 20. S.M. Salman and S.N. Salmana, Ceramic Int., 12, suggestion will be the subject of examination in further 221 (1986). studies. 21. Z. Strnad, Glass-Ceramic Materials, Elsevier, Amsterdam (1989). 22. A. Marotta, P. Pernice, A. Aronne and M. REFERENCES Catauro, J. Thermal Anal., 40, 181 (1993). 23. A H. Babcck and R Briickner,in:Fundamentals of 1. M. Handke and W. Mozgawa, , Papers Glass Science and Technology, Proceedings of of the Commission on Ceramic Science, Polish the Second Conference of the European Society Academy of Science, Cracow, 42, 49 (1993). of Glass and Technology, Stazione Sperimentale 2. L. Stoch and I. Waclawska, J. Thermal Anal., 40 del Vetro (publisher), Venice, Italy, 1993, p. 169. (1993) in press. 24. A.B. Thompson and E.H. Perkins, in: 3. I. Waclawska, L. Stoch, J. Paulik and F. Paulik, Thermodynamics in Minerals and Melts, RC. Thermochim. Acta, 126, 307 (1988). Newton, A. Navrotsky and B.J. Wood (eds.), 4. L. Stoch and I. Waclawska, Thermochim. Acta, Springer Verlag, New York, Heidelberg, Paris, 57 215, 255 (1993). (1977). 5. L. Stoch and I. Waclawska, Thermochim. Acta, 25. I. Prigogine and R. Defay, Chemical Thermo- 215, 265 (1993). dynamics, Longmans Green and Co., London 6. L. Stoch and I. Waclawska, J. Thermal Anal., 36, (1954). 2045 (1990). 26. J. Zarzycki, Glass and Vitreous State, Pergamon Press, Oxford (1991). 200 L. Stoch and I. Waclawska High Temperature Materials and Processes

27. S. Sakka and J.D. MacKenzie, J. Non-Crystalline 30. J. Wong and C.A. Angell, Glass Structure by Solids, 6, 145 (1971). Spectroscopy, Dekker, New York (1976). 28. M. Oguni, T. Matuso, H. Suga and S. Seki, Bull. 31. L. Stoch, High Temp. Materials and Processes, Chem. Soc. Jpn., 50, 825 (1977). 10, 245 (1992). 29. H. Suga, J. Chem. Thermodynamics, 25, 463 (1993).

201