Preparation of Zr(Mo,W)(2)O(8) with a larger negative thermal expansion by controlling the thermal decomposition of Zr(Mo,W)(2)(OH,Cl)(2)∙2H(2)O

Solid solutions of Zr(Mo,W)(2)O(7)(OH,Cl)(2)∙2H(2)O with a preset ratio of components were prepared by a hydrothermal method. The chemical composition of the solutions was determined by energy dispersive X-ray spectroscopy (EDX). For all the samples of ZrMo(x)W(2−x)O(7)(OH,Cl)(2)∙2H(2)O (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0), TGA and in situ powder X-ray diffraction (PXRD) studies (300–1100 K) were conducted. For each case, the boundaries of the transformations were determined: Zr(Mo,W)(2)O(7)(OH,Cl)(2)∙2H(2)O → orthorhombic-ZrMo(x)W(2−x)O(8) (425–525 K), orthorhombic-ZrMo(x)W(2−x)O(8) → cubic-ZrMo(x)W(2−x)O(8) (700–850 K), cubic-ZrMo(x)W(2−x)O(8) → trigonal-ZrMo(x)W(2−x)O(8) (800–1050 K for x > 1) and cubic-ZrMo(x)W(2−x)O(8) → oxides (1000–1075 K for x ≤ 1). The cell parameters of the disordered cubic-ZrMo(x)W(2−x)O(8) (space group Pa-3) were measured within 300–900 K, and the thermal expansion coefficients were calculated: −3.5∙10(−6) – −4.5∙10(−6) K(−1). For the ordered ZrMo(1.8)W(0.2)O(8) (space group P2(1)3), a negative thermal expansion (NTE) coefficient −9.6∙10(−6) K(−1) (300-400 K) was calculated. Orthorhombic-ZrW2O(8) is formed upon the decomposition of ZrW(2)O(7)(OH,Cl)(2)∙2H(2)O within 500–800 K.