Polymorphism in the negative thermal expansion material magnesium hafnium tungstate

Magnesium hafnium tungstate [MgHf(WO4)3] was synthesized by high-energy ball milling followed by calcination. The material was characterized by variable-temperature neutron and x-ray difftaction. It crystallized in space group P21/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 A2(MO4)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 αa = -5.2 � 10-6 K-1, αb = 4.4 � 10-6 K-1, αc = -2.9 � 10-6 K-1, αV = -3.7 � 10-6 K-1, and α1, = -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. © 2008 Materials Research Society.