Thermochemistry of A 2 M 3 O 12 negative thermal expansion materials
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The enthalpies of the monoclinic to orthorhombic transition for a series of A2M3O12 (A ⳱ Al, Cr, Fe, In, and Sc; M ⳱ Mo or W) compounds were measured by differential scanning calorimetry, and entropies of transition were estimated. The enthalpies of formation from the binary oxides at 25 °C for several A2M3O12 samples were obtained from drop solution calorimetry in molten 3Na2O·4MoO3 at 702 °C. The monoclinic and orthorhombic phases of Sc2Mo3O12 and Sc2W3O12 are the only phases that are enthalpically stable under ambient conditions. The enthalpies of formation from the oxides (⌬Hf,ox) for orthorhombic Sc2Mo3O12 and Sc2W3O12 are −47.2 ± 2.1 kJ/mol and −8.5 ± 2.7 kJ/mol, respectively. For Fe2Mo3O12, In2Mo3O12, and In2W3O12, ⌬Hf,ox values are 51.5 ± 4.5, 7.4 ± 2.9, and 44.5 ± 2.3 kJ/mol, respectively. These phases are entropically stabilized and/or metastable. Enthalpies of formation for phases that could not be measured by calorimetry have been estimated from the enthalpies of transition or trends in the enthalpies of formation. In general, the monoclinic phase is slightly enthalpically stabilized over the orthorhombic phase, while transition to the orthorhombic phase is entropically favored. This confirms that the orthorhombic phase is stable at high temperatures, the monoclinic is stable at low temperatures, and the monoclinic to orthorhombic transition is reversible.
I. INTRODUCTION
Double tungstates and molybdates with the general formula A2(MO4)3 (A ⳱ trivalent cations Al, Sc, Cr, Fe, In, or the smaller rare earths from Lu to Ho; M ⳱ W, Mo) are known to exhibit negative thermal expansion (NTE) in their orthorhombic form (space group Pnca).1–9 A2(MO4)3 materials are also known for their trivalent cation conductivity.10–13 Sc2W3O12 is the first solid material in which trivalent cation conduction has been unambiguously established.14 Sc2(MoO4)3 and Sc2(WO4)3 showed the highest conductivity among molybdates and tungstates.11,12 A2(WO4)3 ditungstates have also been considered as Raman shifters for solid state laser applications.15 Orthorhombic Sc2W3O12 can be regarded as the structural prototype for orthorhombic A2M3O12 materials showing volume NTE (Sc2W3O12 family). Therefore, it has been the most studied in this respect5,6,16–23 along with orthorhombic Sc 2 Mo 3 O 12 5,22,24–29 and
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Address all correspondence to this author. e-mail: [email protected] Present address: Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 DOI: 10.1557/JMR.2007.0311 2512 J. Mater. Res., Vol. 22, No. 9, Sep 2007 http://journals.cambridge.org Downloaded: 14 Mar 2015
Al2W3O12.5,8,27,30–35 Orthorhombic A2M3O12 materials can be considered a flexible framework of corner-sharing AO6 octahedra and MO4 tetrahedra, and they display anisotropic thermal expansion with two axes contracting and one expanding on heating.8,24 The contraction can be viewed as a consequence of dynamic cooperative tilting of the quasi-rigid polyhedral units that make up the framework.6 Many members of the Sc2W3O12 family undergo a reversib
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