Polymorphism in the negative thermal expansion material magnesium hafnium tungstate

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.

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Acknowledgments

This research was funded by the National Science Foundation under Grant DMR-0545517. We acknowledge the support of the National Institute of Standards and Technology, United States Department of Commerce, in providing the neutron research facilities used in this work. The authors would like to thank J. Stalick and J. Leao for their help with the experiments.

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Correspondence to Cora Lind.

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Gindhart, A.M., Lind, C. & Green, M. Polymorphism in the negative thermal expansion material magnesium hafnium tungstate. Journal of Materials Research 23, 210–213 (2008). https://doi.org/10.1557/JMR.2008.0013

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