Heats of Formation for Several Crystalline Polymorphs and Pressure-Induced Amorphous Forms of AMo2O8 (A = Zr, Hf) and ZrW2O8

The enthalpies of drop solution (ΔHds) for several polymorphs of ZrW2O8 (cubic, orthorhombic, trigonal, and amorphous), ZrMo2O8 (cubic, trigonal, monoclinic, and amorphous), and HfMo2O8 (cubic, trigonal, and amorphous) have been measured in molten 3Na2O·4MoO3 at 975 K. Using the values of ΔHds for the binary oxides, we calculated enthalpies of formation from the oxides at 298 K. Monoclinic ZrMo2O8 is the only polymorph examined that is enthalpically stable relative to the binary oxides (ΔHf,ox = −5.1 ± 3.5 kJ/mol), and even that stability is marginal. The ΔHf,ox values for cubic and trigonal ZrMo2O8 are 45.2 ± 5.3 and 32.2 ± 4.5 kJ/mol, respectively; for cubic and trigonal HfMo2O8, 55.8 ± 3.1, and 46.5 ± 3.6 kJ/mol, respectively; and for cubic, orthorhombic, and trigonal ZrW2O8, 64.8 ± 2.8, 50.6 ± 3.0, and 49.8 ± 4.1 kJ/mol, respectively. Therefore, these phases are either entropically stabilized and/or their formation/persistence is kinetically controlled. The enthalpies of formation from the oxides, at ambient pressure, for the amorphous phases recovered after compression of the cubic polymorphs in a multianvil press are more positive than those of the crystalline polymorphs: 98.9 ± 5.0 kJ/mol for ZrMo2O8, 102.0 ± 6.3 kJ/mol for HfMo2O8, and 127.8 ± 5.5 kJ/mol for ZrW2O8. The entropies of the amorphous phases are probably greater than those for the cubic ones and thus the pressure-induced amorphous phase can be entropically (as well as volumetrically) favored, and the pressure-induced amorphization boundary probably has a negative P−T slope. Nevertheless, these amorphous phases could be metastable with respect to more dense crystalline polymorphs, either in the P−T range where they are observed or at higher pressure.