3,801,703 United States Patent Office Patented Apr. 2, 1974 1. 2 3,801,703 boracite-type structure in modifications which METAL, BORACTES yield an X-ray diffraction powder pattern that can be Tom Allen Bither, Jr., Woodbrook, Del, assignor to E. I. indexed on the basis of an orthorhombic unit cell. It du Pont de Nemours and Company, Wilmington, Del. is an object of the invention to obtain the boracites in No Drawing. Filed Sept. 17, 1971, Ser. No. 181,605 well-defined , preferably in the form of single Int, C. C01b. 21/20, 35/00 crystals of Sufficient size for the attachment of electrodes U.S. C. 423-277 7 Claims to opposing crystal faces. The recognition of these new compositions as boracites ABSTRACT OF THE DISCLOSURE is readily made by their stoichiometry, MBOINO, de Boracites of the formula MBONO are described O termined by analysis and by their X-ray diffraction wherein M is at least one of Co, Ni, Cu, Zn or Cd. The patterns. The latter bear a characteristic resemblance compounds can be prepared by heating trioxide, to that of the Mg-Cl species, the boracite whose metal borates or other sources of boron with MNOs at structure in both high- and low-temperature forms has 65 kilobars and a temperature of 600 to 1000 C. The been reported by Ito et al., Acta. Cryst. 4, 310 (1951). compounds are crystalline having orthorhombic sym The high-temperature form of the mineral and that of metry, space group Pca21. The compositions are useful all other boracites whose high-temperature structure has as the working elements in piezoelectric devices and are been described give an X-ray powder pattern character also ferroelectric. istic of the space group F43c. For the products of this invention, the X-ray pattern FIELD OF THE INVENTION 20 of the high-temperature form where it has been de This invention relates to novel compounds of boracite termined, is also consistent with this F43c face-centered type and to their use in piezoelectric cubic symmetry. In their low-temperature forms the known boracites are described as having orthorhombic and pyroelectric devices. distortions (space group Pca2) of the prototype cubic PRIOR ART 25 phase. The low-temperature forms of the new boracites Boracites are recognized as a distinct class of com show X-ray diffraction patterns which can similarly be pounds based on the prototype mineral named boracite, indexed by an orthorhombic (with Poa21 extinctions) MgBO3Cl. It is widely known that Mg can be replaced unit cell. by the divalent ions Cr, Mn, Fe, Co, Ni, Cu, Zn, and The NO-boracites are prepared by heating the hy Cd and that Cl can be replaced by Br or I without a 30 drated metal nitrates, M(NO3)2" xH2O, with a source of major change in the structure type or the characteristic boron, conveniently B2O3, at a pressure of 65 kbars at properties of the compounds. Widespread interest in the temperatures ranging from 600-1000 C. using a tetra boracites results from their useful electrical, magnetic hedral anvil device such as that described by Lloyd et and optical properties. These occur in several useful al., J. Res. Nat. Bur. Stands, 65c 59 (1959). High pres crystallographic modifications of the basic structure. For 35 sures are believed to be essential to prevent decomposi example, a cubic, high-temperature form is optically iso tion of the nitrate group at temperatures below those tropic and piezoelectric; slightly distorted orthohombic, necessary to provide a kinetically favorable incorporation and/or trigonal (rhombohedral), modifications exist at of the NO3 group into the boracite lattice. Reduction of lower temperatures and are optically anisotropic and the nitrate ion at the reaction temperature may also pre ferroelectric. The temperatures of transition between the 40 vent the formation of NO-boracites for divalent metals various forms vary widely among the known boracite such as Cr, Mn, and Fe, which are readily oxidized to compounds. the trivalent state. It is preferred to have the nitrate ion Since the low-temperature forms of lower symmetry present in excess of that required to provide the B/NO generally have the more useful properties, it is desirable ratio of 7.0 required by the formula MBONO3, to obtain boracite compositions whose low-temperature B/NO ratios from 0.6 to 3.0 are most useful to insure forms are stable at ambient conditions. the full accommodation of NO in the boracite struc Boracite compounds, MBOX, wherein M is a variety ture. Atom ratios, B/M in the starting mixture general of divalent cations and X is one of the halogens Cl, ly range from 1.0 to 6.0. Br and I, are disclosed in British Pat. 1,070,834 to 50 Each of the NO-boracites of Co, Ni, Cu, Zn, and Cd, Schmid. Despite intensive studies of the halogen series, as normally prepared by slowly cooling or by quenching, no analogous compound with fluorine, MBO13F, has occurs in a low-temperature orthorhombically distorted been found. form of the prototype cubic structure stable at high tem Elements other than a halogen have been suggested perature. Transformation to the high-temperature form in some cases as occupying the unique anion site, X, in 55 can be followed by differential scanning calorimetry. The the boracite structure; see, for example, Ascher et al., thermal effect is sharp and reversible, endothermic on Solid State Communications 2, 45-49, 1964 for OH heating. High-temperature X-ray diffraction shows the boracites, and Fouassier et al., Z. Anorg. Allg. Chem, transformed material to be of higher symmetry, consistent 375, 202-208, 1970 for S-boracites. No indication has with the face-centered cubic structure of F43c symmetry been given that the NO3 ion might occupy an anion. 60 heretofore reported for the cubic boracites. The trans site in the boracite structure. formation temperatures, however, are generally higher than those reported for the known boracites as can be SUMMARY OF THE INVENTION seen in Table A. The compositions of the present invention are novel TABLE A boracites, M3BO13NO3, wherein M is at least one divalent 65 Transformation temperatures, K., for MBOX) ion of Co, Ni, Cu, Zn or Cd. XIM Co Ni Cu 2n Cd. DETAILED DESCRIPTION OF THE NO3-... 729 700 602 74 786 INVENTION Cll- 623 60 365 723 798 Br. 458 398 224 586 734 The novel products of the invention expressly include 70 I---- 197 61 ------687 67 compounds of formula MaBONO3, having the above As reported by Schmid, J. Phys. Chem., Solids. 26,985 (1965), lower stated limitations on M and having the characteristic values have since been reported for Co-Cl and Zn-Cl. 3,801,703 3 4. NO-boracites containing mixed cations, orthorhombic boracite-type structure as in MgBOCl (space group Paa21 pertains) but with a higher degree of (M,M')BO1NO3, distortion from the high-temperature cubic form or a exhibit transformation temperatures intermediate to those distorted structure related to that of boracite but having of their end members. Transformation of the NO3-bo different symmetry. The powder pattern could be indexed racites to their low-temperature, orthorhombic form on the basis of an orthorhombic unit cell using space generally involves more distortion of the cubic pro group Poa21 extinctions with a-8.509, b=8.761, and totype than does the transformation of the known ortho c=12.213 A. (volume=910.4 A.3). rhombic boracites. This is indicated by the ratio of the two smaller orthorhombic cell dimensions b/a which O is approximately 1.03 for the nitratoboracites as com pared to a value of essentially unity (about 1.001 or less) for the known halogenoboracites. TABLE I Either BO or HBO may be used as a source of (X-ray diffraction powder pattern of CoalbrosNO3) d Spacing, boron, and even BN may be used if water is present in 5 Intensity k A. the system. The M/B atom ratio in the starting materials may range from 1/6 to 1/1. Ratios poorer in metal than 6.031 the stoichiometric 3/7 are generally preferred so that 4,3799 4,3163 excess reactants are easily removed by hot water extrac 3.5579 tion. 20 3.4897 UTILITY 3.0521. Like the boracites well known to the art, the new com 2,762 pounds of this invention have the capacity of producing 2,7303 valuable electrical and optical switching effects which 2, 6974 25 2.566 derive from their special structures. All of the boracites 2.5050 have structures which are noncentrosymmetric and thus 2, 483 are capable of piezoelectric effects. In their low-tempera 2,4690 ture forms, the boracite crystals are believed to be com 2,3553 2,899 prised of domains characterized by distinct polarization 30 2.1586 and strain vectors which may give rise to nonlinear inter 2.1273 actions with an applied electrical field, a mechanical strain 2,0903 or a thermal gradient. 2,0714 Because of their relatively large distortions from cubic symmetry, the new boracites of this invention provide an 2,0578 opportunity for larger ferroelectric polarization and fer 35 roelastic strain than occurs in the halogenoboracites previ 2.0383 2.0220 ously known to the art. 2. 0087 As an example, opposing faces of a boacite single crys 1.9452 tal may be electroded by conventional techniques of vapor 40 19308 deposition of a conductive metal or by painting with a com 1,8553 mercially available silver paste, to which connecting leads may be attached. If the crystal is then appropriately 1.8467 | 18366 clamped to prevent extraneous movements, the applica 18262 tion of an A.C. voltage can be used to generate Sonic 7799 waves by periodic vibration induced in the direction of 17459 the electrical gradient. , 6938 A pyroelectric device can be made by focusing a pulsed 6543 heat input, e.g., from a laser source, onto one such elec 1,5264 troded face. An electrical output across the electrodes can 50 4808 then be measured as pulses of voltage or current using the 1.4296 appropriate electrical detector. Further embodiments of the invention will be obvious 1.4.192 3939 from the illustrative examples given below. 1,383 EXAMPLE 1. An intensity value of 100 is assigned to the strongestline of the pattern. A 0.178 g. pellet made from a mixture of 0.476 g. of Co(NO3), 6H2O (1.64 mmoles) and 0.114 g. of B2O3 (1.64 mmoles) and contained in a gold capsule was pres sured to 65 kbars in a tetrahedral anvil device and heated 60 Chemical analysis of this phase showed 2.60% nitrogen for two hours at 1000 C., slow-cooled for 4 hours to to be present. This result coupled with the preceding X 400 C., and quenched to room temperature. The resul ray data is indicative of a new class of boracite-type phase tant solids were then extracted with hot water to remove MaBrO13NO3, wherein the halogen X (Cl, Br, or I) in the Soluble impurities. Deep-red polyhedra, some up to 1 mm. known boracites M.B.O.18x is replaced by the monovalent in cross-section, were obtained as the main product of this 65 nitrate anion group NO; Calcd. for CoBONOs: N, reaction. Minor amounts of both pink and white crystals 2.68%. were present as secondary product. The deep-red poly Magnetic susceptibility measurements indicated Curie hedra were then isolated by hand for characterization. Weiss-type behavior for this boracite-type compound from An X-ray diffraction powder pattern was obtained from room temperature to 140 K, with a value of per of 4.99 these deep-red polyhedral crystals with a Hägg-Guinier 70 FB/Co atom based on the formula CosBONO. This ef camera using monochromatic Cu radiation and a KCl in fective moment is compatible with that for high-spin Col-2 ternal standard (a-6.2931 A.). This powder pattern in an octahedral environment, such as pertains in the bora (Table I) was similar to that of the mineral boracite, cite structure. At lower temperatures, magnetic ordering MgBOCl, but appreciable line-splitting was noted, in set in and a magnetic moment us of 2,6 emu./g. was dicating either the same low-temperature form of the 75 observed at 4.2 K, 3,801,703 5 6 EXAMPLE 2 Example 1. This powder pattern could be indexed on the A 0.179 g. pellet made from a mixture of 0.698 g. of basis of an orthorhombic unit cell using space group Co(NO3)2' 6H2O (2.4 mmoles) and 0.101 g of BO (1.45 Pca2 extinctions with a-8.441, b=8.716, and c=12.141 mmoles) was heated at 65 kbars pressure in the manner of A. (volume=893.3 A.3). Analysis of this boracite phase Example 1. Following extraction with hot water, a red showed 47.93% to be present; Calcd. for brown solid comprising intergrown polycrystalline chunks NisBONO3: 49.06%. The measured density of one was obtained. A Debye-Scherrer X-ray diffraction powder piece was 3.71 g/cm3; Calcd. for NiBOINO having pattern of this material was the same as that of the bora four molecules per orthorhombic unit cell, d=3.81 cite-type phase described in Example 1, indicating forma g/cm.3. tion of the compound CoBONO3. 10 An infrared spectrum on these NiBONO crystals By use of a differential scanning calorimetry technique was very similar to that obtained on the CoBONO (DSC), a sharp and reversible thermal transition was de boracite-type compound of Example 1 and confirmed the tected in this compound around a temperature of 456 C., presence of trigonally- and tetrahedrally-coordinated in the manner observed in the known boracites MBOX, boron, both of which are present in the boron-oxygen net wherein X is Cl, Br, or I. X-ray diffraction powder pat 5 work of the boracite structure. Differential scanning cal terns made on this CoBONOs boracite at room tem orimetry showed the presence of a sharp and reversible perature following heatings through the transition to tem thermal transition in this NiBONOs boracite around peratures as high as 770° C. were identical to that of the a temperature of 427 C. compound prior to the heating, confirming the reversi A second sample of NiBONO was prepared in the 20 same manner as that described previously in this example. bility of the transition and also indicating the thermal sta This material was observed to give a positive test for pi bility of this compound. eZoeletricity when examined with a transmission-type EXAMPLE 3 piezoelectricity detector. The capacitance of this com Pellets weighing 0.174-0.176 g. made from a mixture of pound, when measured as a function of temperature, was 0.666 g. of Co(NO3)2·6H2O (2.3 mmoles) and 0.160 g. 25 observed to rise sharply in the range 410-430 C, while of BO (2.3 mmoles) and contained in gold capsules passing through the region of the reversible transition de were pressured to 65 kbars in a tetrahedral anvil device, scribed previously. heated for 6 hours at temperatures of 800 C. or 600 C., EXAMPLE 6 and then quenched to room temperature. The resultant Pellets weighing 0.176 g. made from a mixture of 0.698 products were extracted with hot water. From the reaction 30 g. of Co(NO3)2·6H2O (2.4 mmoles), 0.698 g. of carried out at a temperature of 800° C., irregularly Ni(NO)a 6H2O (2.4 mmoles), and 0.334 g. of BO shaped, deep-red shards were isolated having a Debye (4.8 mmoles) and contained in gold capsules were pres Scherrer X-ray diffraction powder pattern that was the sured in duplicate runs to 65 kbars in a tetrahedral anvil same as that of the boracite-type phase described in Ex device and heated for 6 hours at 800° C., slow-cooled for ample 1. Analysis for oxygen gave 50.2%; Calcd, for 2 hours to 400 C, and quenched to room temperature. CoBONO3: 49.0% The resultant solids were combined and extracted with From the reaction carried out at a temperature of hot water to remove soluble impurities. A mixture of ir 600 C., red polyhedral crystals similar in appearance to regularly shaped red crystallites plus some tan platelets those of Example 1 and having the same X-ray diffraction was obtained, and the red phase was isolated for char powder pattern were isolated mixed with some white 40 acterization. A Debye-Scherrer X-ray diffraction powder crystals of HBO. An infrared spectrum on these red pattern obtained on this red phase (Table II) was ob CoBONO crystals confirmed the presence of trigon served to be isotypic with those of the boracite-type com ally- and tetrahedrally-coordinated boron, both of which pounds CoBONO (Example 1) and NiBONO are present in the boron-oxygen network of the boracite (Example 5). This pattern could be indexed on the basis structure. The infrared spectrum was also compatible 45 of an orthorhombic unit cell using space group Pea2. with the presence of a nitrate group in this compound. extinctions with as 8.476, b=8.729, and c=12.177 A. EXAMPLE 4 This unit cell had a volume (901.0 A.3) intermediate be tween those of the two end members, indicating a mixed A 0.171 g. pellet made from a mixture of 0.350 g. of (Co.,Ni)BONO phase. Analyses on this boracite-type Co(NO)6HO (1.20 mmoles) and 0.250 g. of B2O3 50 phase showed 48.97% oxygen and 2.50, 2.39% nitrogen (3.60 mmoles) was heated at 600° C. in the manner of to be present; Calcd. for CosN5BONO: O, 49.03%; Example 3. Deep-red polyhedral crystals were isolated N, 2.68%. following extraction of the reaction product with hot Differential scanning calorimetry showed the presence Water. A 0.184 g. pellet made from a mixture of 2.911 g, of 55 of a sharp and reversible thermal transition in this Co(NO)6HO (10 mmoles) and 0.496 g. of BN (20 immoles) was heated in the manner of Example 1 in a Pt (Co.,Ni) BONO3 capsule. Irregularly shaped reddish crystallites plus a few boracite around a temperature of 442 C. By use of a black crystals (COO) were isolated following extraction high temperature X-ray diffraction powder camera, the of the reaction product with hot water. 60 pattern of (Co,Ni)BO1sNO at 300° C. appeared to be The red phase isolated from each of these reactions the same as that at room temperature (except for ther gave the same Debye-Scherrer X-ray diffraction powder mal displacement of all reflections). At the higher tem pattern as that of CoBONO3 as described in Example 1. perature of 500 C., a much simpler pattern was obtained. This was difficult to read but face-centered cubic sym EXAMPLE 5 65 metry was suggested with a cell dimension a ~12.2 A. A 0.180 g. pellet made from a mixture of 0.476 g. of Upon cool-down, the lower-symmetry pattern was again Ni(NO)6H2O (1.64 mmoles) and 0.114 g. of BOs present at room temperature. These data are consistent (1.64 mmoles) was heated in the manner of Example 1. with the behavior of the known halogenoboracites, where Following extraction with hot water to remove soluble in F43c symmetry pertains in their high-temperature form impurities, a mixture of irregularly shaped, orange-brown 70 above their respective reversible transitions. crystallites plus some green to yellow-green platelets was A positive test for piezoeelectricity was obtained in a obtained. The orange-brown phase was isolated for char Second sample of (Co,Ni)BONO prepared in the acterization and a Debye-Scherrer X-ray diffraction pow same manner as that described previously in this exam der pattern obtained thereon was observed to be isotypic ple. The capacitance of this material, when measured as with that of the boracite-type compound CosBONO of 75 a function of temperature was observed to rise sharply 8,801,703 7 8 while passing through the region of the reversible tran Example 3. Straw-yellow crystallites in the form of ir sition described previously. Magnetic susceptibility meas regular pieces were obtained following extraction with urements indicated Curie-Weiss type behavior for this hot water to remove soluble impurities. A Debye-Sch boracite-type compound from room temperature to about errer X-ray diffraction powder pattern obtained on this 100 K., with a value pet of 4.4 uVCo,Ni atom. This 5 material was observed to be isotypic with those of the effective moment is compatible with that for a mixture boracite-type compounds CoBO13NO3 (Example 1) and of high spin Cot?--Nit-2 in an octahedral environment NisbO3NO (Example 5) and could be indexed on the such as pertains in the boracite structure. In contrast to basis of an orthorhombic unit cell using space group CosBO13NO (Example 1), essentially no magnetic or Pca2 extinctions with a-8.445, b=8.663, and c=12.102 dering was observed at 4.2 K. in this mixed (Co,Ni) O A. (volume=885.4 A.3). nitrato-boracite. Analyses on this boracite-type phase showed 48.45% oxygen and 2.40, 2.34% nitrogen to be present; Calcd. for CuBONO, O, 47.74%, N, 2.61%. Differential TABLE II scanning calorimetry showed the presence of a sharp and (X-ray diffraction powder pattern of Cois Niis BO 13 NO3) 15 reversible thermal transition in this CusBO13NOaboracite d Spacing, around a temperature of 329 C. Intensity i l A. EXAMPLE 8

0.694 A 0.179 g. pellet made from a mixture of 0.892 g. of 5.436 4.361 20 Zn(NO)6HO (3.0 mmoles) and 0.209 g, of B2O3 (3.0 4,302 mmoles) was heated for 8 hours at 800° C. in the manner 3,547 of Example 3. Following extraction with hot water, a mix 3.479 3,379 ture of irregularly shaped beige and colorless crystals was 3,042 obtained. The beige crystals were isolated and a Debye 2.869 25 Scherrer X-ray diffraction powder pattern obtained there 2,747 on was observed to be isotypic with those of the Co, Ni, 2,721 and Cu-nitratoboracite phases of Examples 1, 5, and 7 in dicating formation of ZnaBONO3. This powder pattern, 2,688 after deletion of a few lines belonging to the second phase 2,826 2.504 30 above, could be indexed on the basis of an orthorhombic 2.460 unit cell using space group PCa21 extinctions with a = 2.353 2.258 8.510, b=8.775, and c=12.222A. (volume=910.6 A.3). 2, 182 A second sample of ZnBONO prepared from the 2.51 2.118 same starting materials at 1000 C. and in a manner simi 35 lar to that of Example 1 was found by differential scan 2,082 ning calorimetry to have a sharp and reversible thermal 2.064 transition around a temperature of 468 C. 2.050 EXAMPLE 9 2,03 40 In a manner similar to the preceding example, a 0.219 g. pellet made from a mixture of 2.000 g. of 2,002 1923 Cd(NO) 42O 1849 (6.5 mmoles) and 0.451 g. of B2O3 (6.5 mmoles) and 1.84 1830 45 contained in a platinum capsule with internal boron nitride 819 end plugs inserted for crystallite nucleating sites was pres 1,774 sured to 65 kbars in a tetrahedral anvil device and heated 1,739 for 5 hours at 800° C., slow-cooled for 4 hours to 400 1688 C., and quenched to room temperature. The resultant 1,650 product was treated with hot water, both to extract out 1539 50 soluble impurities and to disperse out unreacted BN from 1,52 the end plugs. Hazy, colorless crystals were isolated that 1.490 had an X-ray diffraction powder pattern that was ob 47 served to be isotypic with that of the Zn-nitratoboracite 1.441 described above. This powder pattern could be indexed on 55 the basis of an orthorhombic unit cell using space group 1.424 Pca2 extinctions with a-8.806, b=8.981, and c=12.603 A. (volume=996.8 A.3. Analysis on this boracite-type .416 phase showed 2.4% nitrogen to be present; Calcd. for CdsBONO: N, 2.1%. Differential scanning calorimetry 1.403 60 showed the presence of a sharp and reversible thermal 1.390 transition in a second sample of this 1.377 CdaBONO3 boracite around a temperature of 513 C. 1.36 65 1.345 EXAMPLE 10 A 0.190 g. pellet of Zn(NO3)2.6H2O (0.64 mmole) iAnintensity value of 100 is assigned to the strongest line of the pattern contained in a nongas-tight gold capsule and cold-pres sured to 65 kbars was reacted therein with BO, furnished 70 by a slow transport mechanism from an external packing comprising a mixture of boron nitride//hydrous EXAMPLE 7 aluminum silicate, at a temperature of 1000 C. for 2 A 0.192 g. pellet made from a mixture of 0.966 g. of hours followed by a cool to 400° C. in 6 hours and a sub Cu(NO3)2·3H2O (4.0 mmoles) and 0.279 g. of BO sequent quench to room temperature. Following removal (40 mmoles) was heated at 800° C. in the manner of 75 of water-soluble impurities, off-white crystals of cubic habit 3,801,703 9 10 were isolated that had a Debye-Scherrer X-ray diffraction 3. Composition of claim 1 wherein M is Co. powder pattern the same as that of ZnBONO3 of Ex- 4. Composition of claim 1 wherein M is Ni. ample 8. Analysis for nitrogen confirmed the formation of 5. Composition of claim wherein M is Cu. this boracite by way of a transport process; Found: N, 6. Composition of claim 1 wherein M is Zn. 2.26%; Calcd. for ZnBONO: N, 2.58%. 7. Composition of claim 1 wherein M is Cd. The embodiments of the invention in which an exclu- 5 sive property or privilege is claimed are defined as follows: References Cited 1. Metal boracites having the formula UNITED STATES PATENTS MBONO O 3,384,447 5/1968 Schmid ------423-277 y:M is at least one divalent metalion of Co, Ni, Cu, MILTON WEISSMAN, Primary Examiner 2. Composition of claim it having the formula U.S. Cl. X.R. CosNisBONO3 252-62.9