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Polymorphism in the negative thermal expansion material magnesium hafnium tungstate

Published online by Cambridge University Press:  31 January 2011

Amy M. Gindhart ,
Cora Lind  and
Mark Green
Amy M. Gindhart
Affiliation:
Department of Chemistry, The University of Toledo, Toledo, Ohio 43606
Cora Lind*
Affiliation:
Department of Chemistry, The University of Toledo, Toledo, Ohio 43606
Mark Green
Affiliation:
National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102; and Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115
*
a)Address all correspondence to this author. e-mail: [email protected]
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Abstract

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 diffraction. 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 αl = −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.

Keywords

Crystallographic structureNeutron scatteringX-ray diffraction (XRD)
Type
Articles
Information
Journal of Materials Research , Volume 23 , Issue 1 , January 2008 , pp. 210 - 213
Copyright
Copyright © Materials Research Society 2008

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