Pressure-induced structural phase transitions in the AMnF4 series (A=Cs, Rb, K) studied by synchrotron x-ray powder diffraction: Correlation between hydrostatic and chemical pressure

Abstract

The effect of applying hydrostatic pressure in the layered-perovskite AMnF4 (A=Cs, Rb, K) series has been studied using energy-dispersive synchrotron x-ray powder diffraction at pressures between ambient and 20 GPa. At ambient pressure CsMnF4 is tetragonal with space group P4/n, RbMnF4 is orthorhombic with space group Pmab and KMnF4 is monoclinic with space group P21/a. CsMnF4 was found to undergo a first-order structural phase transition, from tetragonal to orthorhombic symmetry at Pc1=1.4±0.2 GPa. At pressures in excess of Pc2=6.3±1 GPa, for the Cs derivative, and Pc3.=4.5±1 GPa, for the Rb derivative, the symmetry appears to be monoclinic. Moreover, the critical unit-cell volumes associated with Pc1, Pc2, and Pc3 are slightly higher than the ambient pressure unit-cell volumes of RbMnF4 for Pc1 and KMnF4 for Pc2 and Pc3. 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 AMnF4 series. The tetragonal to orthorhombic transition of CsMnF4 has been found to be inhibited when NaCl is used as an internal pressure calibrant. The partial substitution of Cs by Na in CsMnF4 at Pc1 has been shown to be a likely explanation for this behavior. The anisotropic broadening of the Bragg peaks for pressures higher than Pc1 has been analyzed in terms of microstrain affecting the CsMnF4 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.