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Large negative thermal expansion and anomalous behavior on compression in cubic ReO-type AB F: CaZrF and CaHfF 3
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Justin C. Hancock, Karena W Chapman, Gregory J Halder, Cody R. Morelock, Benjamin S. Kaplan, Leighanne C. Gallington, Angelo Bongiorno, Chu Han, Si Zhou, and Angus P. Wilkinson Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00662 • Publication Date (Web): 06 May 2015 Downloaded from http://pubs.acs.org on May 7, 2015
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Chemistry of Materials
Large negative thermal expansion and anomalous behavior on compression in cubic ReO3-type AIIBIVF6: CaZrF6 and CaHfF6 Justin C. Hancock,† Karena W. Chapman,‡ Gregory J. Halder,‡ Cody R. Morelock,† Benjamin S. Kaplan,† Leighanne C. Gallington,† Angelo Bongiorno,†§ Chu Han,† Si Zhou§ and Angus P. Wilkinson† #* †
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400
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X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, IL 60439
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School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430
#
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245
ABSTRACT: CaZrF6 and CaHfF6 display much stronger negative thermal expansion (NTE) (αL100K ~ -18 and -22 ppm∙K-1, respectively) than ZrW2O8 and other corner-shared framework structures. Their NTE is comparable to that reported for framework solids containing multi-atom bridges, such as metal cyanides and metal-organic frameworks. However, they are formable as ceramics, transparent over a wide wavelength range and can be handled in air; these characteristics can be beneficial for applications. The NTE of CaZrF6 is strongly temperature-dependent, and first-principles calculations show that it is largely driven by vibrational modes below ~150 cm-1. CaZrF6 is elastically soft with a bulk modulus (K300K) of 37 GPa and, upon compression, starts to disorder at ~400 MPa. The strong NTE of CaZrF6, which remains cubic to < 10 K, contrasts with cubic CoZrF6, which only displays modest NTE above its rhombohedral to cubic phase transition at ~270 K. CaZrF6 and CaHfF6 belong to a large and compositionally diverse family of materials, AIIBIVF6, providing for a detailed exploration of the chemical and structural factors controlling NTE and many opportunities for the design of controlled thermal expansion materials.
1. INTRODUCTION Few materials display strong volume negative thermal expansion (NTE) over a large temperature range. The long-time leading NTE exemplar was ZrW2O8,1 in which NTE arises from low-frequency vibrations involving the correlated motion of corner-shared framework polyhedra.2,3 ZrW2O8 and other materials displaying strong NTE have attracted considerable attention due to the rarity of the phenomenon and the possibility of using them to compensate for the positive thermal expansion of other materials either in expansion-controlled composites or as separate components.4 To move significantly beyond the expansion characteristics of ZrW2O81 and other oxide frameworks, solids with spatially extended links between structural units or alternative mechanisms for their NTE have been pursued. This has resulted in the discovery of very strong NTE due to vibrations in metal-organic frameworks (MOFs) with carboxylate links5,6 and cyanidebridged frameworks7-9 and materials where magnetostrictive10,11 or electronic12,13 mechanisms operate. For potential application, NTE materials need to be stable under the conditions of use and amenable to the fabrication of composites or bulk components. Both MOFs and cyanidebridged frameworks are not readily processed into ceramic bodies, and MOFs only display NTE when carefully
evacuated to remove guests, such as atmospheric moisture, from their pore systems. Metal fluorides are known to adopt structures analogous to those of oxides. However, the more ionic character of the M-F bonding can lead to very different thermophysical properties. In an early study of NTE in fluorides, we reported that ScF3, which has a very simple ReO3-type structure and remains cubic to
