CaZrF6 has recently been shown to combine strong negative thermal expansion (NTE) over a very wide temperature range (at least 10–1000 K) with optical transparency from mid-IR into the UV range. Variable-temperature and high-pressure diffraction has been used to determine how the replacement of calcium by magnesium and zirconium by niobium­(IV) modifies the phase behavior and physical properties of the compound. Similar to CaZrF6, CaNbF6 retains a cubic ReO3-type structure down to 10 K and displays NTE up until at least 900 K. It undergoes a reconstructive phase transition upon compression to ∼400 MPa at room temperature and pressure-induced amorphization above ∼4 GPa. Prior to the first transition, it displays very strong pressure-induced softening. MgZrF6 adopts a cubic (Fm3̅m) structure at 300 K and undergoes a symmetry-lowering phase transition involving octahedral tilts at ∼100 K. Immediately above this transition, it shows modest NTE. Its’ thermal expansion increases upon heating, crossing through zero at ∼500 K. Unlike CaZrF6 and CaNbF6, it undergoes an octahedral tilting transition upon compression (∼370 MPa) prior to a reconstructive transition at ∼1 GPa. Cubic MgZrF6 displays both pressure-induced softening and stiffening upon heating. MgNbF6 is cubic (Fm3̅m) at room temperature, but it undergoes a symmetry-lowering octahedral tilting transition at ∼280 K. It does not display NTE within the investigated temperature range (100–950 K). Although the replacement of Zr­(IV) by Nb­(IV) leads to minor changes in phase behavior and properties, the replacement of the calcium by the smaller and more polarizing magnesium leads to large changes in both phase behavior and thermal expansion.

Heat treatment of cubic YbZrF7, after quenching from 1000 °C, leads to a material displaying precisely zero thermal expansion at ∼300 K and negative thermal expansion at lower temperatures. The zero thermal expansion is associated with a minimum in the lattice constant at ∼300 K. X-ray total scattering measurements are consistent with a previously proposed model in which the incorporation of interstitial fluoride into the ReO3-related structure leads to both edge and corner sharing coordination polyhedra. The temperature dependence of the experimental pair correlation functions suggests that the expansions of edge and corner sharing links partly compensate for one another, supporting the hypothesis that the deliberate incorporation of excess fluoride into ReO3 structure materials can be used as a design strategy for controlling thermal expansion. Cubic YbZrF7 has a bulk modulus, K 0, of 55.4(7) GPa and displays pronounced pressure-induced softening [K 0′ = −27.7(6)] prior to an abrupt amorphization on compression above 0.95 GPa. The resulting glass shows a single sharp scattering maximum at Q ∼ 1.6 Å–1.

Cubic ReO 3 -type fluorides often display negative or very low thermal expansion. However, they also typically undergo phase transitions upon cooling and/or modest compression, which are undesirable from the perspective of potential applications. Density measurements and total scattering data for Mg 1−x Zr 1+x F 6+2x , x = 0.15, 0.30, 0.40, and 0.50, indicate that the introduction of excess fluoride into cubic MgZrF 6 is accompanied by the population of interstitial fluoride sites and the conversion of corner to edge shared coordination polyhedra. Unlike MgZrF 6 , no phase transitions are seen upon cooling these materials to 10 K, and the first high pressure phase transition in these compositions occurs at pressures much higher than that previously reported for MgZrF 6 (0.37 GPa). The introduction of excess fluoride also provides for control of thermal expansion. For all of the compositions studied, negative thermal expansion is seen at the lowest temperature examined, and positive thermal expansion is observed at the highest temperature. The temperature at which zero thermal expansion occurs varies from ∼150 K for x = 0.50 to ∼500 K for x = 0.00. High pressure diffraction also indicates that increasing the amount of excess fluoride elastically stiffens the cubic ReO 3 related structure and reduces the extent of pressure induced softening.

Defect perovskites (He□)(CaZr)F can be prepared by inserting helium into CaZrF at high pressure. They can be recovered to ambient pressure at low temperature. There are no prior examples of perovskites with noble gases on the A-sites. The insertion of helium gas into CaZrF both elastically stiffens the material and reduces the magnitude of its negative thermal expansion. It also suppresses the onset of structural disorder, which is seen on compression in other media. Measurements of the gas released on warming to room temperature and Rietveld analyses of neutron diffraction data at low temperature indicate that exposure to helium gas at 500 MPa leads to a stoichiometry close to (He□)(CaZr)F. Helium has a much higher solubility in CaZrF than silica glass or crystobalite. An analogue with composition (H)(CaZr)F would have a volumetric hydrogen storage capacity greater than current US DOE targets. We anticipate that other hybrid perovskites with small neutral molecules on the A-site can also be prepared and that they will display a rich structural chemistry.

Strong volume negative thermal expansion over a wide temperature range typically only occurs in ReO-type fluorides that retain an ideal cubic structure to very low temperatures, such as ScF, CaZrF, CaHfF, and CaNbF. CaTiF was examined in an effort to expand this small family of materials. However, it undergoes a cubic ( Fm3̅ m) to rhombohedral ( R3̅) transition on cooling to ∼120 K, with a minimum volume coefficient of thermal expansion (CTE) close to -42 ppm K at 180 K and a CTE of about -32 ppm K at room temperature. On compression at ambient temperature, the material remains cubic to ∼0.25 GPa with K = 29(1) GPa and K' = -50(5). Cubic CaTiF is elastically softer and shows more pronounced pressure induced softening, than both CaZrF and CaNbF. In sharp contrast to both CaZrF and CaNbF, CaTiF undergoes a first-order pressure induced octahedral tilting transition to a rhombohedral phase ( R3̅) on compression above 0.25 GPa, which is closely related to that seen in ScF. Just above the transition pressure, this phase is elastically very soft with a bulk modulus of only ∼4 GPa as octahedral tilting associated with a reduction in the Ca-F-Ti angles provides a low energy pathway for volume reduction. This volume reduction mechanism leads to highly anisotropic elastic properties, with the rhombohedral phase displaying both a low bulk modulus and negative linear compressibility parallel to the crystallographic c-axis for pressures below ∼2.5 GPa. At ∼3 GPa, a further phase transition to a poorly ordered phase occurs.