Polymorphism in the negative thermal expansion material magnesium hafnium tungstate

摘要

Magnesium hafnium tungstate [MgHf(WO_4)_3] was synthesized by high-energy ball milling followed by calcination. The material was characterized by variable-temperature neutron and x-ray diffraction. It crystallized in space group P2_1/a below 400 K and transformed to an orthorhombic structure at higher temperatures. The orthorhombic polymorph adopted space group Pnma, instead of the Pnca structure commonly observed for other A_2(MO_4)_3 materials (A = trivalent metal, M = Mo, W). In contrast, the monoclinic polymorphs appeared to be isostructural. Negative thermal expansion was observed in the orthorhombic phase with α= -5.2 × 10~(-6) K~(-1) α_b = 4.4 × 10~(-6) K~1, α_c = -2.9 × 10~(-6) K~(-1), α_v = -3.7 × 10~(-6) K-~1 and α_1 = -1.2 × 10~(-6) K~(-1). The monoclinic to orthorhombic phase transition was accompanied by a smooth change in unit-cell volume, indicative of a second-order phase transition.