Response to Pressure of Negative Thermal Expansion Rhenium-Trioxide-Derivative Fluorides

Abstract

A variety of ReO_3-type and II-IV double-ReO_3-type metal fluorides have been examined for their potential as negative and low thermal expansion materials. This dissertation investigates derivatives developed to influence behavior on compression in these compounds. NaNbF_6 and NaTaF_6 were synthesized as examples of I-V double-ReO_3-types. These compounds exhibited the familiar, undesirable cubic-to-rhombohedral phase transition on cooling and on compression. Both I-V double-ReO_3-type fluorides were determined to be elastically softer than their II-IV counterparts and displayed low-magnitude thermal expansion. Compression of NaNbF_6 in helium at room temperature and below provides no evidence for helium penetration into the structure. The excess-fluoride, ReO_3 derivative, LuZrF_7, was also studied and determined to undergo amorphization on compression instead of the cubic-to-rhombohedral phase transition. In-situ high-pressure X-ray total scattering data indicate that the local structure of cubic LuZrF_7 is largely preserved out to about 7 Å on amorphization, suggesting no major changes in bonding and providing an explanation for why the amorphization is reversible. It is likely the amorphization is associated with the reorientation and distortion of coordination polyhedra in an uncorrelated fashion. Lastly, insertion into the A-site of CaZrF_6 on compression in helium creates the noble gas perovskite, He_2 CaZrF_6, and increases the amorphization pressure from 0.5 GPa in the parent material to above 3 GPa in the daughter compound. The insertion of helium leads to a reduction in the magnitude of NTE when compared to CaZrF_6 while contributing to a considerable stiffening of the structure. At 15 K and elevated pressure, cubic He_2 CaZrF_6 undergoes a structural phase transition, that likely involves a cooperative tilting of framework octahedra to give a lower-symmetry phase, which is tentatively assigned as tetragonal.