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Acta Cryst. (2014). A70, C156
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SnMo2O8 has been shown to exhibit very different phase behavior and thermal expansion from previously studied members of the AM2O8 family.1 At high temperatures, SnMo2O8, ZrW2O8, and ZrMo2O8 assume cubic structures with orientationally disordered MO4 tetrahedra; however, their behavior is widely divergent at lower temperatures. ZrMo2O8 maintains its disordered structure and continues to display negative thermal expansion (NTE). While cubic symmetry is retained when cooling ZrW2O8, its WO4 tetrahedra become ordered, and its NTE increases in magnitude. Rapid cooling of SnMo2O8 leads to a cubic structure that only minimally differs from its high temperature form.1 Slowly heating this cubic phase results in a transformation to a rhombohedral (γ) structure with ordered MoO4 tetrahedra that is not isostructural to any known phases of ZrW2O8 and ZrMo2O8.1 In stark contrast to ZrW2O8, and ZrMo2O8, all SnMo2O8 phases exhibit positive thermal expansion.1 In the current work, the phase behavior and thermoelastic properties of cubic SnMo2O8 under hydrostatic conditions were investigated via in situ synchrotron x-ray powder diffraction in a recently designed sample environment.2 Previous studies of ZrW2O8 and ZrMo2O8 in this environment have shown that pressure-induced disordering of MO4 tetrahedra, which only occurred in the orientationally ordered low temperature ZrW2O8 phase, was linked to both elastic softening on heating and enhancement of NTE.3 At 298K, cubic SnMo2O8 is significantly softer (κT =30GPa) than ZrW2O8 (64GPa) and ZrMo2O8 (43GPa).3 Unlike ZrW2O8, which softens upon heating to 516K (ΔκT = -9GPa), SnMo2O8 stiffens (+5GPa) more than ZrMo2O8 (+2GPa).3 The phase behavior of SnMo2O8 under pressure also differs from that of ZrW2O8 and ZrMo2O8. Compression elevated the γ->cubic transition temperature significantly: at ambient temperature, this transition occurs at ~435K; at 310MPa, it occurs at ~490K.
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