Phase transition, crystal water and low thermal expansion behavior of Al2-2x(ZrMg)xW3O12·n(H2O)

Al_2-2x(ZrMg)_xW₃O₁₂ for 0≤x≤1.0 are synthesized to reduce the phase transition temperature of Al₂W₃O₁₂. It is found that the incorporation of (ZrMg)⁶⁺ into the lattice of Al₂W₃O₁₂ not only reduces its orthorhombic-to-monoclinic phase transition temperature but also elevates its softening temperature, broadening its applicable temperature range considerably. Al_2-2x(ZrMg)_xW₃O₁₂ with x<0.5 exhibit low coefficients of thermal expansion (CTEs) and non-hygroscopicity, while those for x≥0.7 are obviously hygroscopic and the CETs decrease with increasing the content of (ZrMg)⁶⁺ so that Al_0.2(ZrMg)_0.9W₃O₁₂ and ZrMgW₃O₁₂ exhibit negative thermal expansion. Temperature-dependent Raman spectroscopic study shows the hardening of W-O bonds above 373 K which is attributed to the release of crystal water. The effect of crystal water on the thermal expansion property is discussed based on the hydrogen bond between H in crystal water and electronegative O in Al(ZrMg)-O-W linkages.