Abstract
The defect double perovskite [He2-x□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, ∼60% 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□x][CaNb]F6 is estimated to be ∼+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 ∼150 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 (Fm3̅m) to rhombohedral (R3̅) 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 ∼0.4 GPa.
Original language | English |
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Pages (from-to) | 11006-11013 |
Number of pages | 8 |
Journal | Journal of Physical Chemistry C |
Volume | 128 |
Issue number | 26 |
DOIs | |
State | Published - Jul 4 2024 |
Funding
A.P.W. is grateful for a very helpful discussion with Prof. Ryan Lively at Georgia Tech. The authors are grateful to Breaunna R. Wright for preparing the sample that was used in the high-pressure X-ray study. The work at Georgia Tech was partially supported under NSF DMR-1607316 and NSF DMR-2002739. The authors are grateful for technical assistance from the high-pressure sample environment team, Mark Loguillo and Matt Rucker, at the Spallation Neutron Source, Oak Ridge National Laboratory. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The authors are grateful for experimental assistance from the staff of beamline 17- BM at the Advanced Photon Source. Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF Cooperative Agreement EAR-1661511 and by GSECARS through NSF grant EAR-1634415. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.