Knowles Kevin M, Vickers Mary E, Sil Anjan, Han Yung Hoe, Jaffrenou Périne
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, England.
Acta Crystallogr B. 2009 Apr;65(Pt 2):160-6. doi: 10.1107/S0108768108042444. Epub 2009 Feb 20.
The phase designated gamma-Zn3(VO4)2 reported as a minor second phase in zinc oxide-based varistor materials doped with vanadium oxide and manganese oxide is shown to be the L phase, (Zn(1-x)Mn(x))2V2O7 (0.188 < x < 0.538), in the pseudo-binary Mn2V2O7-Zn2V2O7 system. Analysis of X-ray powder diffraction patterns and electron diffraction patterns of this phase shows that the previously published a, c and beta values for this thortveitite-related phase are incorrect. Instead, Rietveld refinement of the X-ray powder pattern of the L phase shows that it has a monoclinic C lattice with Z = 6, with a = 10.3791 (1), b = 8.5557 (1), c = 9.3539 (1) A and beta = 98.467 (1) degrees. Although prior convergent-beam electron diffraction work of 'gamma-Zn3(VO4)2' confirmed the C Bravais lattice, the space group was found to be Cm rather than C2/m, the difference perhaps arising from the inability of the X-rays to detect small displacements of oxygen. Attempts to refine the structure in Cm did not produce improved R factors. The relationship between the crystal structure of the L phase and the high-temperature C2/m beta'-Zn2V2O7 thortveitite-type solid solution is discussed.
在掺杂了氧化钒和氧化锰的氧化锌基压敏电阻材料中，被报道为次要第二相的γ-Zn3(VO4)2相被证明是L相，即(Zn(1-x)Mn(x))2V2O7（0.188 < x < 0.538），处于伪二元Mn2V2O7-Zn2V2O7体系中。对该相的X射线粉末衍射图谱和电子衍射图谱的分析表明，此前发表的与钍石相关相的a、c和β值是不正确的。相反，对L相的X射线粉末图谱进行Rietveld精修表明，它具有单斜C晶格，Z = 6，a = 10.3791(1) Å，b = 8.5557(1) Å，c = 9.3539(1) Å，β = 98.467(1)°。尽管之前对“γ-Zn3(VO4)2”的会聚束电子衍射工作证实了C布拉菲晶格，但发现空间群为Cm而非C2/m，这种差异可能是由于X射线无法检测到氧的小位移所致。在Cm中精修结构的尝试并未产生更好的R因子。讨论了L相的晶体结构与高温C2/m β'-Zn2V2O7钍石型固溶体之间的关系。
