CsMnF4 under hydrostatic pressure: Structural phase transitions and influence of the pressure calibrant

The structural behaviour of the AMnF4 (A = Cs, Rb and K) layered perovskites has been studied as a function of the external pressure using energy dispersive powder diffraction. Two structural phase transitions, a sharp one at 1.4±0.2 GPa accompaigned by a large discontinuity in the unit cell volume and a smooth one at 6.3±1 GPa showing no discontinuity in the unit cell volume, have been detected for CsMnF4 when silver is used as pressure calibrant. The crystal symetries of the high pressure phases are, respectively, orthorhombic Pmab and monoclinic P21/a. These symmetries nicely correspond to those observed at ambient pressure for RbMnF4 and KMnF4, respectively. Strong differences are observed in the structural behaviour of CsMnF4 when NaCl is used as pressure calibrant. Changes in crystal symmetry and discontinuity in the unit cell volume are not observed. However, for pressures higher than 1.25±0.2 GPa anisotropic broadening of Bragg peaks is strongly noticeable. These results are interpreted in terms of a pressure induced substitution reaction of Cs ions by Na ones within the CsMnF4 lattice.