RESUMEN
Strong volume negative thermal expansion over a wide temperature range typically only occurs in ReO3-type fluorides that retain an ideal cubic structure to very low temperatures, such as ScF3, CaZrF6, CaHfF6, and CaNbF6. CaTiF6 was examined in an effort to expand this small family of materials. However, it undergoes a cubic ( Fm3Ì m) to rhombohedral ( R3Ì ) transition on cooling to â¼120 K, with a minimum volume coefficient of thermal expansion (CTE) close to -42 ppm K-1 at 180 K and a CTE of about -32 ppm K-1 at room temperature. On compression at ambient temperature, the material remains cubic to â¼0.25 GPa with K0 = 29(1) GPa and K'0 = -50(5). Cubic CaTiF6 is elastically softer and shows more pronounced pressure induced softening, than both CaZrF6 and CaNbF6. In sharp contrast to both CaZrF6 and CaNbF6, CaTiF6 undergoes a first-order pressure induced octahedral tilting transition to a rhombohedral phase ( R3Ì ) on compression above 0.25 GPa, which is closely related to that seen in ScF3. Just above the transition pressure, this phase is elastically very soft with a bulk modulus of only â¼4 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 â¼2.5 GPa. At â¼3 GPa, a further phase transition to a poorly ordered phase occurs.
RESUMEN
Defect perovskites (He2-xâ¡x)(CaZr)F6 can be prepared by inserting helium into CaZrF6 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 CaZrF6 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 (He1â¡1)(CaZr)F6. Helium has a much higher solubility in CaZrF6 than silica glass or crystobalite. An analogue with composition (H2)2(CaZr)F6 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.
RESUMEN
The defect double perovskite [He2-x â¡ x ][CaNb]F6, with helium on its A-site, can be prepared by the insertion of helium into ReO3-type CaNbF6 at high pressure. Upon cooling from 300 to 100 K under 0.4 GPa helium, â¼60% of the A-sites become occupied. Helium uptake was quantified by both neutron powder diffraction and gas insertion and release measurements. After the conversion of gauge pressure to fugacity, the uptake of helium by CaNbF6 can be described by a Langmuir isotherm. The enthalpy of absorption for helium in [He2-x â¡ x ][CaNb]F6 is estimated to be â¼+3(1) kJ mol-1, implying that its formation is entropically favored. Helium is able to diffuse through the material on a time scale of minutes at temperatures down to â¼150 K but is trapped at 100 K and below. The insertion of helium into CaNbF6 reduces the magnitude of its negative thermal expansion, increases the bulk modulus, and modifies its phase behavior. On compressing pristine CaNbF6, at 50 and 100 K, a cubic (Fm3Ì m) to rhombohedral (R3Ì ) phase transition was observed at <0.20 GPa. However, a helium-containing sample remained cubic at 0.4 GPa and 50 K. CaNbF6, compressed in helium at room temperature, remained cubic to >3.7 GPa, the limit of our X-ray diffraction measurements, in contrast to prior reports that upon compression in a nonpenetrating medium, a phase transition is detected at â¼0.4 GPa.