Review Thermal Expansion Behavior in the A2M3O12 Family of Materials
Hongfei Liu 1,*, Weikang Sun 1, Zhiping Zhang 1,2, La’Nese Lovings 3 and Cora Lind 3,*
1 School of Physical Science and Technology, Yangzhou University, Yangzhou 225002, China; [email protected] (W.S.); [email protected] (Z.Z.) 2 Guangling College, Yangzhou University, Yangzhou 225002, China 3 Department of Chemistry and Biochemistry, The University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606, USA; [email protected] * Correspondence: [email protected] (H.L.); [email protected] (C.L.); Tel.: +86-514-87975466 (H.L.); +1-419-5301505 (C.L.)
Abstract: Over the past several decades, research on anomalous thermal expansion materials has been rapidly growing, and increasing numbers of compounds exhibiting negative thermal expansion (NTE)
have been reported. In particular, compounds with formula A2M3O12 have attracted considerable attention. A2M3O12 family materials offer a wide range of possible compositions due to the chemical flexibility of the A and M sites. According to published research, more than half of them possess NTE properties. This paper reviews the range of physical properties displayed by materials in
the A2M3O12 family. Research on improving material imperfections and controlling the coefficient of thermal expansion in the A2M3O12 family are systematically summarized. Finally, challenges and questions about the developments of these A2M3O12 NTE compounds in future studies are also discussed.
Keywords: negative thermal expansion; scandium tungstate; hygroscopicity; phase transition; chal- lenges
Citation: Liu, H.; Sun, W.; Zhang, Z.; Lovings, L.; Lind, C. Thermal
Expansion Behavior in the A2M3O12 Family of Materials. Solids 2021, 2, 1. Introduction 87–107. https://doi.org/ In recent years, the fields of microelectronics, photoelectric communications and aerospace have rapidly expanded. Materials with high-precision size are in growing Academic Editor: Guido Kickelbick demand, and dimensional stability and long lifetime of devices at different operating tem- Received: 30 December 2020 peratures is of high importance. A mismatch of thermal expansion coefficients combined Accepted: 11 February 2021 with a temperature variation of the materials’ environment can result in thermal stress, Published: 19 February 2021 thus leading to performance degradation or permanent damage to devices. Therefore, low, and especially near-zero expansion materials, are beneficial to improve the geometrical sta- Publisher’s Note: MDPI stays neutral bilities of these materials and devices. The discovery of negative thermal expansion (NTE) with regard to jurisdictional claims in behavior in compounds provides the possibility of developing materials with controllable published maps and institutional affil- or near-zero thermal expansion coefficients for specific applications. iations. Previous research on NTE materials has focused on the following families: Metal cyanides [1–6], metal fluorides [7–16], Mn3AN (A = Cu, Zn, Ge, Sn, Ag) [17–23], inter- metallics [24–31], metal oxides including AM2O8 (A = Zr, Hf; M = W, Mo) [32–38], AM2O7 (A = Zr, Hf; M = V, P) [39–42], A2O (A = Ag, Cu) [43–47] and A2M3O12 (A = trivalent Copyright: © 2021 by the authors. cation; M = W, Mo) [48–50]. Compared to the other families, the A2M3O12 stoichiometry Licensee MDPI, Basel, Switzerland. has received special attention because of the broad range of metals that can be incorporated This article is an open access article into the structure. The A-site can be fully occupied by a single trivalent metal ranging in distributed under the terms and size from Al3+ to the smaller lanthanides, or partially substituted by most lanthanides and conditions of the Creative Commons many transition metals, whereas the M-site is usually occupied by tungsten or molybde- Attribution (CC BY) license (https:// num. Aliovalent substitution of both metal sites has also been reported. The wide range of creativecommons.org/licenses/by/ compositions accessible in the A2M3O12 family is unique. This paper reviews the thermal 4.0/).
Solids 2021, 2, 87–107. https://doi.org/10.3390/solids2010005 https://www.mdpi.com/journal/solids Solids 2021, 1, FOR PEER REVIEW 2
Solids 2021, 2 88 Aliovalent substitution of both metal sites has also been reported. The wide range of com- positions accessible in the A2M3O12 family is unique. This paper reviews the thermal ex- pansion properties of the A2M3O12 family with a focus on compounds displaying NTE expansionbehavior. Recent properties advances of the are A2M discussed3O12 family in detail with aas focus well onas promising compounds future displaying prospects. NTE behavior. Recent advances are discussed in detail as well as promising future prospects. 2. Positive Thermal Expansion 2. Positive Thermal Expansion The A2M3O12 family consists of a large number of compounds that can adopt several differentThe Astructure2M3O12 familytypes. consistsThe identity of a largeof th numbere A and ofM compounds site elements that determines can adopt the several pre- differentferred structure structure type types. which The identityis intimately of the rela A andtedM to site the elements thermal determinesexpansion theproperties preferred of structurethe compounds. type which According is intimately to the related systematic to the research thermal conducted expansion propertiesby Nassau of et the al. com-[51], pounds.when the According A-site is occupied to the systematic by the larger research lanthanides conducted ranging by Nassau from et La al.3+ [ 51to], Tb when3+, these the A-site is occupied by the larger lanthanides ranging from La3+ to Tb3+, these tungstates and tungstates and molybdates display positive thermal expansion. Dy2Mo3O12 also belongs molybdates display positive thermal expansion. Dy2Mo3O12 also belongs to this group of to this group of compounds, while Dy2W3O12 does not [52,53]. The crystal structures of compounds, while Dy2W3O12 does not [52,53]. The crystal structures of these positive thermal these positive thermal expansion A2M3O12 compositions are orthorhombic in space group expansion A2M3O12 compositions are orthorhombic in space group Pba2 or monoclinic in Pba2 or monoclinic in space group C2/c, and contain edge-shared AO7-polyhedra, which space group C2/c, and contain edge-shared AO -polyhedra, which are generally considered are generally considered as an unfavorable fa7ctor for NTE behavior. For instance, Figure as an unfavorable factor for NTE behavior. For instance,3+ Figure1 shows the typical structure 1 shows the typical structure3+ of Gd2Mo3O12 [54]. Gd is coordinated by6+ seven oxygen at- of Gd2Mo3O12 [54]. Gd is coordinated by seven oxygen atoms, while Mo is bonded to four oms, while Mo6+ is bonded to four oxygen atoms. The GdO7 units share common edges, oxygen atoms. The GdO units share common edges, causing oxygen atoms to be coordinated causing oxygen atoms 7to be coordinated by three instead of two atoms. This connection by three instead of two atoms. This connection mode hampers transverse vibration of oxygen mode hampers transverse vibration of oxygen atoms, the typical basis of NTE in the atoms, the typical basis of NTE in the A2M3O12 family. A2M3O12 family.
(a) (b)
FigureFigure 1. 1.( a(a)) Crystal Crystal structure structure of of Gd Gd2Mo2Mo33OO1212 and (b) three-fold coordinated oxygen.oxygen.
3.3. NegativeNegative ThermalThermal ExpansionExpansion
InIn thethe fieldfield ofof NTENTE research, research, the the A A2M2M33OO1212 family is alsoalso referredreferred toto asas thethe scandiumscandium tungstatetungstate family.family. Scandium tungstate adopts anan orthorhombicorthorhombic structurestructure inin thethe spacespace groupgroup PncaPnca atat all temperatures (Figure 22))[ [55,55,56].56]. The The crystal crystal struct structureure is is composed composed of of a acorner-sharing corner-sharing framewor frameworkk of octahedral of octahedral ScO ScO6 and6 andtetrahedral tetrahedral WO4 WOunits.4 units.No edge-sharing No edge- sharingis observed, is observed, in contrast in contrast to the toGd the2Mo Gd3O212Mo-type3O12 structure-type structure discussed discussed earlier. earlier. As the As bond the bondlengths lengths of Sc-O of Sc-Oand W-O and W-Oshow show little little change change with with temperature, temperature, the theglobal global expansion expansion be- behaviorhavior is dominated is dominated by bytransverse transverse motions motions of the of thecorner-sharing corner-sharing oxygens. oxygens. Diffraction Diffraction data datademonstrate demonstrate that thatthese these vibrations vibrations give give rise rise to a to reduction a reduction of ofthe the Sc-O-W Sc-O-W bond bond angle angle and and a ashortening shortening of of the the next-nearest next-nearest neighbor neighbor distances distances on on a microscopic level and macroscopicmacroscopic NTENTE ofof thethe materialmaterial [[55].55]. TheThe resultingresulting rockingrocking motionsmotions ofof thethe polyhedralpolyhedral buildingbuilding blocksblocks areare accompaniedaccompanied byby somesome distortiondistortion ofof thethe polyhedra.polyhedra. LargerLarger A-siteA-site cationscations cancan moremore readilyreadily accompanyaccompany thesethese distortions,distortions, andand thusthus favorfavor moremore significantsignificant NTE NTE [ 57[57].]. SolidsSolids 20212021,, 12, FOR PEER REVIEW 893
Figure 2. Crystal structure of orthorhombic Sc W O : blue: ScO octahedra; green: WO tetrahedra. Figure 2. Crystal structure of orthorhombic Sc22W3O1212: blue: ScO66 octahedra; green: WO44 tetrahedra. When the A-site is occupied by Ho3+,Y3+, Er3+, Tm3+, Yb3+, Lu3+, Sc3+, In3+, Fe3+, 3+When3+ the A-site3+ is occupied by Ho3+, Y3+, Er3+, Tm3+, Yb3+, Lu3+, Sc3+, In3+, Fe3+, Cr3+, Ga3+ Cr , Ga or Al , almost all tungstates and molybdates have been reported to form stable or Al3+, almost all tungstates and molybdates have been reported to form stable com- compounds that adopt the same structure as Sc2W3O12 and display NTE behavior [50,58]. poundsSome compositions that adopt the can same undergo structure a phase as Sc transition2W3O12 and from display the orthorhombicNTE behavior NTE[50,58]. phase Some at compositionshigh temperatures can undergo to a denser a phase monoclinic transition positive from the thermal orthorhombic expansion NTE (PTE) phase structure at high at temperatureslow temperatures to a denser [59]. This monoclinic phase transition positive canthermal also expansion be induced (PTE) by applying structure pressure, at low temperatureswhich will be [59]. discussed This phase later intransition this review. can also be induced by applying pressure, which will be discussed later in this review. Many attempts to synthesize single phase Fe2W3O12 and Cr2W3O12 using different approaches,Many attempts such as to solid-state,synthesize single co-precipitation phase Fe2W and3O12 sol-geland Cr2 methodsW3O12 using failed different to obtain ap- proaches, such as solid-state, co-precipitation and sol-gel methods failed to obtain single single phase Fe2W3O12 and Cr2W3O12 [60,61]. This is mainly because both Fe2W3O12 and phase Fe2W3O12 and Cr2W3O12 [60,61]. This is mainly because both Fe2W3O12 and Cr2W3O12 Cr2W3O12 are metastable phases, which decompose to FeWO4 or CrWO4 and WO3 during areheating. metastable Other phases, researchers which reported decompose successful to FeWO synthesis,4 or CrWO but4 didand not WO display3 during diffraction heating. Otherdata [ 62researchers,63] or displayed reported data successful that cannot synthe excludesis, the but presence did not of andisplay amorphous diffraction component data [62,63]or small or impuritydisplayed phases data that [64, 65cannot]. Recently, exclud Yange the etpresence al. fabricated of an amorphous Mo-doped single-phasecomponent orFe small2W3O impurity12 using rapid phases solid-state [64,65]. Recently, reactions Yang with excesset al. fabricated MoO3 [66 ].Mo-doped Fe2W3O12 single-phasestill showed Fepositive2W3O12 thermalusing rapid expansion solid-state after reactions the monoclinic-to-orthorhombic with excess MoO3 [66]. Fe phase2W3O transition12 still showed with positivea coefficient thermal of thermalexpansion expansion after the monoclinic-to-orthorhombic of 1.35 × 10−6 ◦C−1 (445–600 phase◦C) transition as measured with by a −6 −1 coefficientdilatometry. of Gathermal2Mo3 Oexpansion12 is another of 1.35 difficult × 10 to °C synthesize (445–600 composition, °C) as measured which by can dilatome- only be try.obtained Ga2Mo by3O non-hydrolytic12 is another difficult sol-gel to chemistry synthesize [67 composition,], and remains which monoclinic can only with be obtained positive byexpansion non-hydrolytic until its sol-gel decomposition chemistry temperature [67], and remains around monoclinic 650 ◦C. with positive expansion until itsIt isdecomposition generally accepted temperature that the coefficientaround 650 of °C. thermal expansion (CTE) is related to the sizeIt of is the generally A-site trivalent accepted cation, that andthe coefficien that it decreasest of thermal with increasingexpansion size (CTE) ofthe is related A-site ion. to theTo date,size of Y3+ thehas A-site the largest trivalent ionic cation, radius and (0.90 that Å) it of decreases the materials with adopting increasing the size orthorhombic of the A- sitePnca ion. structure, To date, and Y3+ Yhas2Mo the3O 12largestand Yionic2W3 Oradius12 display (0.90 intrinsicÅ) of the linear materials thermal adopting expansion the −6 ◦ −1 −6 ◦ −1 orthorhombiccoefficients of Pnca−9.36 structure,× 10 Cand Y[682Mo] and3O12− and7.0 toY2−W7.43O12× display10 C intrinsic[69,70 linear], respectively, thermal ◦ expansionin the temperature coefficients range of − of9.36 200 × to10 800−6 °CC.−1 [68] Only and Ho −27.0Mo 3toO 12−7.4has × been10−6 °C reported−1 [69,70], to respec- possess −6 ◦ −1 tively,an even in larger the temperature negative intrinsic range αofl value200 to of 800−11.56 °C. Only× 10 Ho2CMo3[O7112], has although been reported several rare to possessearth molybdates an even larger and negative tungstates intrinsic show similar αl value magnitudes of −11.56 × of10− linear6 °C−1 [71], NTE although [52,68,72 ].several These 3+ rareA-site earth cations molybdates are very and similar tungstates in size toshow Y .similar However, magnitudes many A2 ofM 3linearO12 compounds NTE [52,68,72]. with Theselarge A-site ionscations possess are very the similar ability toin absorbsize to water,Y3+. However, which inhibits many A the2M NTE3O12 compounds behavior of withthe materials. large A-site ions possess the ability to absorb water, which inhibits the NTE behavior of the materials. 3.1. Mechanism of Negative Thermal Expansion in the A2M3O12 Family
3.1. MechanismSeveral excellent of Negative reviews Thermal discussing Expansion the in originsthe A2M of3O NTE12 Family in different classes of com- pounds have recently been published [73,74]. Both intrinsic structural features, and struc- tural or magnetic phase transitions, can contribute to the observed behavior. The negative Solids 2021, 2 90
thermal expansion observed in Pnca-A2M3O12 compounds is categorized as intrinsic struc- tural NTE. It arises from transverse vibrational motions of the 2-coordinated oxygens. In materials that contain mostly 2-coordinate linkers in approximately linear coordination, these vibrations result in concerted bending and torsional motions that reduce the dis- tances between next-nearest neighbor metal centers. When these reductions outweigh the bond expansion due to longitudinal vibrations, NTE results. This visual concept for explaining NTE behavior was introduced early on for ZrW2O8 [34], and has since been applied to many other NTE materials that possess corner-sharing polyhedral networks. Other authors refer to it as the “tension effect” to indicate that the vibrations result in forces that cause rocking motions of the polyhedral building blocks [75,76]. In physics terms, these lattice vibrations are described as phonon modes, and both optical and acoustic transverse phonon modes with negative Grüneisen parameters can contribute to NTE, which is observed when the overall Grüneisen parameter is negative over an extended temperature range. If these modes occur without distortion of the polyhedra, they are referred to as rigid unit modes (RUMs), whereas modes in solids that show some changes within the polyhedra are often called quasi rigid unit modes (QRUMs) [37,77]. Tao and Sleight showed that the A2M3O12 family can only support QRUMs [78]. Evidence for transverse phonon modes in NTE materials was obtained by heat capacity and phonon density of states measurements [38,79]. Theoretical calculations have also been used to elucidate NTE mechanisms [80,81].
3.2. Hygroscopicity in the A2M3O12 Family When the A-site is occupied by the smaller rare earth elements or pseudo-lanthanides, 3+ 3+ 3+ 3+ 3+ 3+ more specifically, Ho ,Y , Er , Tm , Yb and Lu , the A2M3O12 materials display strong NTE and remain orthorhombic to the lowest temperatures studied [68,69,71,72,82–84]. The large negative CTEs are attributed to the large ionic radii. However, the large ionic radii also cause hygroscopicity, as this gives rise to the emergence of microchannels accessible to water in the crystal structure [82,84,85]. Moisture from the atmosphere results in formation of trihydrates, with the water molecules interacting with the structure via strong hydrogen bonds. The presence of water and hydrogen bonding networks weakens or in most cases eliminates NTE behavior [86], and removal of the water is necessary to reclaim the NTE properties. Marinkovic et al. researched NTE and hygroscopicity of Y2Mo3O12 in detail, and found that the microchannels in the lattice are large enough for water molecules to enter freely and play a part in causing partial amorphization of Y2Mo3O12 upon hydration. A summary of CTEs and the corresponding temperature ranges for these hygroscopic materials can be found in Table1.
Table 1. Intrinsic coefficients of thermal expansion (CTEs) of hygroscopic materials in the A2M3O12 family based on variable temperature diffraction data.
−6 ◦ −1 ◦ Compound αl (×10 C ) T Range ( C) Ref. 1 Y2Mo3O12 −9.36 25–800 [68] 1 Y2W3O12 −7.34 200–800 [72] Dy2W3O12 −8.60 150–500 [52] Ho2Mo3O12 −11.56 200–700 [71] Ho2W3O12 −6.97 200–600 [83] 1 Er2Mo3O12 −7.56 25–800 [68] 1 Er2W3O12 −6.74 200–800 [72] Tm2Mo3O12 −4.04 200–800 [71] Tm2W3O12 −3.99 200–800 [71] 1 Lu2Mo3O12 −6.02 25–800 [68] 1 Lu2W3O12 −6.18 200–800 [72] 1 Yb2Mo3O12 −6.04 25–800 [68] 1 Yb2W3O12 −6.38 200–800 [72] 1 Collected under vacuum. Solids 2021, 2 91
Ample research has been performed with the materials in Table1 to synthesize con- trollable thermal expansion materials that are less hygroscopic by chemical modification. Substitution of small amounts of Al, Cr, Fe or Sc generally did not eliminate water up- take [87–93], while lanthanide poor compositions showed less tendency to absorb moisture. For Al, Cr and Fe, hygroscopicity was overcome for compositions that resulted in formation of the denser monoclinic PTE phase, which is less favorable for water incorporation. For Sc, a non-hygroscopic material was obtained for Sc1.75Y0.25W3O12. A length change of −7.13 × 10−6 ◦C−1 was detected by dilatometry. Substitution of a larger rare earth ion that favors edge-shared structures has also been explored [88,94–97]. In this context, we previously prepared Y2−xCexW3O12 solid solutions to adjust the magnitude of the CTE and to overcome the hygroscopicity of Y2W3O12 [97]. Results showed that moisture ab- sorption was no longer observed for x ≥ 1.5 at room temperature, however, compositions with x ≥ 0.5 also showed the presence of the denser monoclinic C2/c structure, with coexistence of the orthorhombic and monoclinic phases for x = 0.5. Dilatometer measure- −6 ◦ −1 ments still showed NTE for compositions up to Y0.25Ce1.75W3O12 (−0.82 × 10 C ), suggesting that such monoclinic ceramics could be promising near-zero thermal expan- sion materials. Double ion substitution such as (LiMg)3+ has also been implemented to reduce hydrophilicity in Y2Mo3O12 by Cheng et al. [98]. The hygroscopicity of the sample was eventually eliminated, but the thermal expansion of the material increased with the 3+ addition of (LiMg) . In addition to the method of ion-doping, Liu et al. chose C3N4 as a coating material to protect Y2Mo3O12 from water exposure [99]. This successfully prevented hydration, and retained similar expansion coefficients.
3.3. Non-Hygroscopic A2M3O12 Compositions with Corner-Shared Networks 3+ 3+ 3+ 3+ 3+ 3+ When the A-site is occupied by Sc , In , Fe , Cr , Ga and Al ,A2M3O12 compo- sitions do not absorb water molecules from air. Most of these materials undergo a reversible temperature-induced phase transition from a low temperature monoclinic structure in the space group P21/a to the orthorhombic Sc2W3O12 structure in the space group Pnca at higher temperatures. Exceptions are Sc2W3O12, which remains orthorhombic to the lowest temperatures investigated [55], and Ga2Mo3O12, which decomposes to the binary oxides before transforming to the orthorhombic phase [67]. Only Pnca-A2M3O12 materials exhibit notable negative thermal expansion. In general, the phase transition temperature increases with increasing electronegativity of the A-site ion. This can be attributed to the fact that A-site cations with higher electronegativity reduce the partial negative charge of the oxygen atoms and weaken the repulsive forces between them, which stabilizes the denser mono- clinic structure to higher temperatures. The transition temperature can be determined by refinement of diffraction data, Raman spectroscopy, dilatometry or thermal analysis. The temperatures of the monoclinic to orthorhombic phase change are summarized in Table2. The exact relationship between the monoclinic and orthorhombic structures was demonstrated for Sc2Mo3O12 by Evans et al. [59]. This material adopts a monoclinic struc- ture below −93 ◦C that displays PTE with an expansion coefficient of 2.19 × 10−5 ◦C−1. ◦ Above −93 C, orthorhombic Sc2Mo3O12 shows NTE with a linear expansion coefficient −6 ◦ −1 of −6.3 × 10 C . Figure3 shows the two crystal structures of Sc 2Mo3O12 and their crystallographic relationship, which is given by am ≈ −bo + co, bm ≈ −ao, and cm ≈ −2co. Scandium tungstate is the only non-hygroscopic compound that does not undergo a phase transition to the lowest temperatures investigated [55]. It shows stable NTE perfor- mance over a wide temperature range from −263 ◦C to 800 ◦C and is non-hygroscopic in air. Because of this, it is considered to be one of the most promising NTE compounds in the A2M3O12 family. Sc2Mo3O12 [59] and Al2W3O12 [101] possess phase transition tempera- tures that are below room temperature, while the rest of the non-hygroscopic phase change materials undergo the transition at temperatures between 200 and 500 ◦C [66,100–104]. High phase transition temperatures are considered undesirable for fabricating devices, and approaches that allow lowering of the transition temperatures would be beneficial. To date, this has mainly been attempted by doping with different A-site elements, which Solids 2021, 1, FOR PEER REVIEW 6
Cr2Mo3O12 403 0.67 420–740 [100] Fe2Mo3O12 512 1.72 550–740 [100] Fe2W3O12 414–445 1.35 1 445–600 [66] Solids 2021, 2 Ga2W3O12 NR −5 1 NR [58] 92 ln2Mo3O12 335 −1.85 370–760 [102] In2W3O12 250 −3.00 1 277–700 [103] 1 Dilatometer data. showed that controlling the phase transition temperature to meet specific environments is promisingThe exact and feasiblerelationship [100, 103between,104]. the monoclinic and orthorhombic structures was demonstrated for Sc2Mo3O12 by Evans et al. [59]. This material adopts a monoclinic struc- Tableture below 2. Phase −93 transition °C that displays temperatures PTE with and an CTEs expansion of orthorhombic coefficient phasesof 2.19 of× 10 compounds−5 °C−1. Above in the 3+ 3+ 3+ 3+ 3+ 3+ A−293M 3°C,O12 orthorhombic(A = Al , Sc Sc, Cr2Mo,3O Fe12 shows, Ga NTEand Inwith) family.a linear Intrinsic expansionαl values coefficient based of on −6.3 variable × temperature10−6 °C−1. Figure diffraction 3 shows data the are two reported crystal unless structures indicated of Sc otherwise.2Mo3O12 and NR their = not crystallographic reported. relationship, which is given by am ≈ −bo + co, bm ≈ −ao, and cm ≈ −2co. ◦ −6 ◦ −1 CompoundScandium tungstate TPT is (theC) only non-hygrosαl (×10 copicC ) compound T Range that does not undergo Ref. a phaseAl2 Motransition3O12 to the lowest200 temperatures 2.32investigated [55]. 250–650 It shows stable NTE [100 perfor-] manceAl2 Wover3O12 a wide temperature−6 range from 1.51 −263 °C to 800 °C 20–800 and is non-hygroscopic [69,101] in air.Sc Because2Mo3O12 of this, it is considered−93 to be one−2.11 of the most promising−73–27 NTE compounds [59] in − − the ScA2MW33OO1212 family. Sc2MoNR3O12 [59] and Al2W2.203O12 [101] possess263–177 phase transition tempera- [55] turesCr 2Mothat3 Oare12 below room403 temperature, while 0.67 the rest of 420–740the non-hygroscopic [100 phase] Fe2Mo3O12 512 1.72 550–740 [100] change materials undergo the transition at temperatures1 between 200 and 500 °C [66,100– Fe2W3O12 414–445 1.35 445–600 [66] 104]. High phase transition temperatures are considered1 undesirable for fabricating de- Ga2W3O12 NR −5 NR [58] vices, and approaches that allow lowering of the transition temperatures would be bene- ln2Mo3O12 335 −1.85 370–760 [102] ficial. To date, this has mainly been attempted by1 doping with different A-site elements, In2W3O12 250 −3.00 277–700 [103] which showed that controlling the phase transition temperature to meet specific environ- 1 Dilatometer data. ments is promising and feasible [100,103,104].
(a) (b)
Figure 3. Crystal structures of (a) monoclinic and (b) orthorhombic Sc2Mo3O12.
3.3.1. Single Ion Substitution
Single ion substitution can be used to improve performance in A2M3O12 NTE com- pounds that display high phase transition temperatures (A = Al, In, Cr, Fe; M = W and Mo). This has been applied to solid solutions to tune phase transition temperatures in the 200 to 500 ◦C range [100,104]. However, from an applications point of view, suppressing the phase transition temperature to room temperature or below is desirable. This can mainly be achieved by incorporation of A-site cations with low electronegativity, which is known Solids 2021, 1, FOR PEER REVIEW 7
Figure 3. Crystal structures of (a) monoclinic and (b) orthorhombic Sc2Mo3O12.
3.3.1. Single Ion Substitution
Single ion substitution can be used to improve performance in A2M3O12 NTE com- pounds that display high phase transition temperatures (A = Al, In, Cr, Fe; M = W and Mo). This has been applied to solid solutions to tune phase transition temperatures in the Solids 2021, 2 200 to 500 °C range [100,104]. However, from an applications point of view, suppressing93 the phase transition temperature to room temperature or below is desirable. This can mainly be achieved by incorporation of A-site cations with low electronegativity, which is known to lower the phase transition temperature [101]. In our previous work, we pre- to lower the phase transition temperature [101]. In our previous work, we prepared solid pared solid solutions of In2−xScxW3O12 (0 ≤ x ≤ 2) using solid state reaction methods to re- solutions of In Sc W O (0 ≤ x ≤ 2) using solid state reaction methods to reduce the 2−x x 3 12 3+ duce the phase transition temperature of In2W3O12[103].3+ Sc was shown to effectively re- phase transition temperature of In2W3O12 [103]. Sc was shown to effectively replace place In3+, and the monoclinic-to-orthorhombic phase transition temperature was shifted In3+, and the monoclinic-to-orthorhombic phase transition temperature was shifted from from ◦248 °C to◦ 47 °C for x = 1. The average linear thermal expansion coefficient of In- 248 C to 47 C for x = 1. The average linear thermal expansion coefficient of InScW3O12 −6 −1 ScWdetermined3O12 determined by dilatometry by dilatometry is −7.13 is ×−7.1310− ×6 10◦C− °C1 in thein the temperature temperature range range 58 58 to to 700 700◦C. °C.The The obtained obtained thermal thermal expansion expansion curve curve for for this this sample sample is shownis shown in Figurein Figure4a. We4a. We have have also also prepared AlScMo3O12 by non-hydrolytic sol-gel routes [105], and found that it re- prepared AlScMo3O12 by non-hydrolytic sol-gel routes [105], and found that it remains mainsorthorhombic orthorhombic to −173 to ◦−C,173 but °C, approaches but approaches the phase the transitionphase transition to the monoclinic to the monoclinic structure structureat that temperature. at that temperature. The thermal The expansionthermal expansion curve for curve this sample for this is shownsample inis Figureshown4 b.in Figure 4b.
(a) (b)
FigureFigure 4. 4. (a(a) )Expansion Expansion curve curve of of InScW InScW3O3O1212 obtainedobtained by by dilatometry dilatometry and and (b ()b unit) unit cell cell volume volume of of AlScMo AlScMo3O3O12.12 .
WuWu et et al. al. synthesized synthesized Ln Ln2−2x−CrxCrxMoxMo3O312O (Ln12 (Ln = Er = Erand and Y)Y) and and showed showed via via differential differential scanningscanning calorimetry calorimetry that that the the phase phase transition transition of of Cr Cr2Mo2Mo3O3O1212 couldcould be be suppressed suppressed from from 400 400 ◦ 3+ toto 197 197 °CC by by substitution substitution of of 10% Er [[93].93]. Similarly,Similarly, Li Li et et al. al. showed showed that that with with increasing increasing Y Ycontent3+ content in in the the solid solid solution solution Fe Fe2−2x−xYYxxMo3OO1212, ,the the phase phase transition transition temperature temperature could could be be ◦ decreaseddecreased significantly, significantly, with with Fe Fe1.51.5YY0.50.5MoMo3O312O 12showingshowing the the transition transition below below −170−170 °C C[87].[87 ]. SubstitutionSubstitution of of Fe Fe2−x2Ln−xxLnMoxMo3O123 Oby12 otherby other lanthanides lanthanides (Ln = (Ln Er,= Lu, Er, Yb) Lu, resulted Yb) resulted in ortho- in or- rhombicthorhombic materials materials at or at below or below room room temper temperatureature [87,90,91]. [87,90,91 These]. These reports reports demonstrate demonstrate thatthat single single ion ion substitution substitution can can successfully successfully broaden broaden the thetemperature temperature range range in which in which the orthorhombicthe orthorhombic NTE phase NTE phase is stable. is stable. However, However, these compounds these compounds become become hygroscopic hygroscopic even 3+ aftereven incorporation after incorporation of small of quantities small quantities of lanthanides, of lanthanides, making making substitution substitution by Sc3+by most Sc attractive.most attractive. Work on Work single on ion single doping ion on doping the M-site on the to M-sitereduce to the reduce phase the transition phase transitiontemper- temperature is rare. Shen et al. prepared Al Mo − W O solid solutions to adjust the ature is rare. Shen et al. prepared Al2Mo3−xWxO212 solid3 xsolutionsx 12 to adjust the coefficient of coefficient of thermal expansion of Al Mo O and found a less significant reduction of 2 3 12 2 3 12 thermal expansion of Al Mo O and found a less significant reduction of the6+ phase tran- the phase transition temperature of Al2Mo3−xWxO12 with increasing W content [106]. Liu et al. later suggested that this could be explained by the lower electronegativity of W6+ compared to Mo6+, which is consistent with the effect of the electronegativity of the A3+ cation on the transition temperatures [107]. Table3 gives an overview of selected mono-substituted compounds for which phase transition temperatures and expansion coefficients have been reported. Solids 2021, 2 94
Table 3. Variation in phase transition temperature (TPT) by single A-site doping and corresponding intrinsic linear CTEs determined from variable temperature diffraction data, unless otherwise indicated. Elements for each site are listed in alphabetical order.
α α Compound T (◦C) l Ref. Compound T (◦C) l Ref. PT (×10−6 ◦C−1) PT (×10−6 ◦C−1)
Al2Mo3O12 200 2.32 [100] Cr1.8Er0.2Mo3O12 197 0.47 [93] Al1.8Cr0.2Mo3O12 214 NR [100] Cr0.2Er1.8Mo3O12
