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Solid solutions of Zr(Mo,W)2O7(OH,Cl)2∙2H2O 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 ZrMoxW2−xO7(OH,Cl)2∙2H2O (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)2O7(OH,Cl)2∙2H2O → orthorhombic-ZrMoxW2−xO8 (425–525 K), orthorhombic-ZrMoxW2−xO8 → cubic-ZrMoxW2−xO8 (700–850 K), cubic-ZrMoxW2−xO8 → trigonal-ZrMoxW2−xO8 (800–1050 K for x > 1) and cubic-ZrMoxW2−xO8 → oxides (1000–1075 K for x ≤ 1). The cell parameters of the disordered cubic-ZrMoxW2−xO8 (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 ZrMo1.8W0.2O8 (space group P213), a negative thermal expansion (NTE) coefficient −9.6∙10−6 K−1 (300-400 K) was calculated. Orthorhombic-ZrW2O8 is formed upon the decomposition of ZrW2O7(OH,Cl)2∙2H2O within 500–800 K.

Single phase ZrW 2 O 8 was prepared by hydrothermal synthesis via the decomposition of ZrW 2 O 7 (OH 1.5 , Cl 0.5 ) ⋅ 2H 2 O at 570°C. Transmission electron microscopy, in situ high temperature X ray diffraction, and thermogravimetric-differential thermal analyses of synthesized ZrW 2 O 8 were performed. The morphology of the material was represented as rod shaped particles having a block structure. The stabil ity fields of ZrW 2 O 8 were determined. ZrW 2 O 8 demonstrated a negative thermal expansion behavior from 25 to 750°C.

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