Pressure dependence of the low- temperature crystal structure and phase
transition behaviour of CaFeAsF and SrFeAsF: A synchrotron x-ray diffraction
study
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.Comment: 23 pages, 11 Figure
