Pressure-induced structural phase transitions in theAMnF4series (A=Cs, Rb, K) studied by synchrotron x-ray powder diffraction: Correlation between hydrostatic and chemical pressure
Physical Review B (Condensed Matter)
The effect of applying hydrostatic pressure in the layered-perovskite AMnF 4 ͑AϭCs, Rb, K͒ series has been studied using energy-dispersive synchrotron x-ray powder diffraction at pressures between ambient and 20 GPa. At ambient pressure CsMnF 4 is tetragonal with space group P4/n, RbMnF 4 is orthorhombic with space group Pmab and KMnF 4 is monoclinic with space group P2 1 /a. CsMnF 4 was found to undergo a first-order structural phase transition, from tetragonal to orthorhombic symmetry at P c
... ϭ1.4Ϯ0.2 GPa. At pressures in excess of P c 2 ϭ6.3Ϯ1 GPa, for the Cs derivative, and P c 3 ϭ4.5Ϯ1 GPa, for the Rb derivative, the symmetry appears to be monoclinic. Moreover, the critical unit-cell volumes associated with P c 1 , P c 2 , and P c 3 are slightly higher than the ambient pressure unit-cell volumes of RbMnF 4 for P c 1 and KMnF 4 for P c 2 and P c 3 . Hydrostatic pressure has been found to have a similar effect on the crystal symmetry of the series as the decreasing of the radius of the alkaline ion from Cs to Rb and K. A correlation between hydrostatic and chemical pressure can therefore be established from the structural point of view for the AMnF 4 series. The tetragonal to orthorhombic transition of CsMnF 4 has been found to be inhibited when NaCl is used as an internal pressure calibrant. The partial substitution of Cs by Na in CsMnF 4 at P c 1 has been shown to be a likely explanation for this behavior. The anisotropic broadening of the Bragg peaks for pressures higher than P c 1 has been analyzed in terms of microstrain affecting the CsMnF 4 lattice due to Na incorporation. A substitutional reaction has been shown to be a competitive process, versus a structural phase transition, that enables the system to return to equilibrium after applying pressure on it. Finally, the equation of state associated with the different high-pressure phases has been calculated including compressibilities.