Pressure dependence of the low- temperature crystal structure and phase transition behaviour of CaFeAsF and SrFeAsF: A synchrotron x-ray diffraction study

TL;DR

This study investigates how pressure affects the crystal structures and phase transitions of CaFeAsF and SrFeAsF at low temperatures using synchrotron x-ray diffraction, revealing distinct transition behaviors and their implications for superconductivity.

Contribution

It provides the first detailed high-pressure phase diagram of CaFeAsF and SrFeAsF, highlighting differences from 122 compounds and their potential link to superconductivity.

Findings

CaFeAsF transitions from orthorhombic to monoclinic at 13.7 GPa.

SrFeAsF shows coexistence of orthorhombic and monoclinic phases from 9 to 39 GPa.

No collapse tetragonal phase observed in these 1111 compounds.

Abstract

We report systematic investigation of high pressure crystal structures and structural phase transition upto 46 GPa in CaFeAsF and 40 GPa in SrFeAsF at 40 K using powder synchrotron x-ray diffraction experiments and Rietveld analysis of the diffraction data. We find that CaFeAsF undergoes orthorhombic to monoclinic phase transition at Pc = 13.7 GPa while increasing pressure. SrFeAsF exhibits coexistence of orthorhombic and monoclinic phases over a large pressure range from 9 to 39 GPa. The coexistence of the two phases indicates that the transition is of first order in nature. Unlike in the 122 compounds (BaFe2As2 & CaFe2As2) we do not find any collapse tetragonal transition. The transition to a lower symmetry phase (orthorhombic to monoclinic) in 1111 compounds under pressure is in contrast with the transition to a high symmetry phase (orthorhombic to tetragonal) in 122 type compounds. On heating from 40 K at high pressure, CaFeAsF undergoes monoclinic to tetragonal phase transition around 25 GPa and 200 K. Further, it does not show any post-tetragonal phase transition and remains in the tetragonal phase upto 25 GPa at 300 K. The dPc/dT is found to be positive for the CaFeAsF & CaFe2As2, however the same was not found in case of BaFe2As2. We discuss observations of structural evolution in the context of superconductivity in these and other Fe-based compounds. It appears that the closeness of the Fe-As-Fe bond angle to its ideal tetrahedral value of 109.470 might be associated with occurrence of superconductivity at low temperature.