Heats of Formation for Several Crystalline Polymorphs and Pressure-Induced Amorphous Forms of AMo2O8 (A = Zr, Hf) and ZrW2O8
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- 作者：Tamas Varga ; Cora Lind ; Angus P. Wilkinson ; Hongwu Xu ; Charles E. Lesher ; Alexandra Navrotsky
- 刊名：Chemistry of Materials
- 出版年：2007
- 出版时间：February 6, 2007
- 年：2007
- 卷：19
- 期：3
- 页码：468 - 476
- 全文大小：74K
- 年卷期：v.19,no.3(February 6, 2007)
- ISSN：1520-5002
文摘
The enthalpies of drop solution (
Hds) 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 Hds forthe binary oxides, we calculated enthalpies of formation from the oxides at 298 K. Monoclinic ZrMo2O8is the only polymorph examined that is enthalpically stable relative to the binary oxides (Hf,ox = -5.1± 3.5 kJ/mol), and even that stability is marginal. The Hf,ox values for cubic and trigonal ZrMo2O8 are45.2 ± 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/ortheir formation/persistence is kinetically controlled. The enthalpies of formation from the oxides, at ambientpressure, for the amorphous phases recovered after compression of the cubic polymorphs in a multianvilpress 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 phasesare probably greater than those for the cubic ones and thus the pressure-induced amorphous phase canbe entropically (as well as volumetrically) favored, and the pressure-induced amorphization boundaryprobably has a negative P-T slope. Nevertheless, these amorphous phases could be metastable withrespect to more dense crystalline polymorphs, either in the P-T range where they are observed or athigher pressure.
Hds) 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 Hds forthe binary oxides, we calculated enthalpies of formation from the oxides at 298 K. Monoclinic ZrMo2O8is the only polymorph examined that is enthalpically stable relative to the binary oxides (Hf,ox = -5.1± 3.5 kJ/mol), and even that stability is marginal. The Hf,ox values for cubic and trigonal ZrMo2O8 are45.2 ± 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/ortheir formation/persistence is kinetically controlled. The enthalpies of formation from the oxides, at ambientpressure, for the amorphous phases recovered after compression of the cubic polymorphs in a multianvilpress 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 phasesare probably greater than those for the cubic ones and thus the pressure-induced amorphous phase canbe entropically (as well as volumetrically) favored, and the pressure-induced amorphization boundaryprobably has a negative P-T slope. Nevertheless, these amorphous phases could be metastable withrespect to more dense crystalline polymorphs, either in the P-T range where they are observed or athigher pressure.