PRESSURE-INDUCED STRUCTURAL PHASE-TRANSITIONS IN THE AMNF(4) SERIES (A=CS,RB,K) STUDIED BY SYNCHROTRON X-RAY-POWDER DIFFRACTION - CORRELATION BETWEEN HYDROSTATIC AND CHEMICAL PRESSURE

The effect of applying hydrostatic pressure in the layered-perovskite AMnF(4) (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 P2(1)/a. CsMnF4 was found to undergo a first-order s tructural phase transition, from tetragonal to orthorhombic symmetry a t P-c1 = 1.4+/-0.2 GPa. At pressures in excess of P-c2 = 6.3+/-1 GPa, for the Cs derivative, and P-c3 = 4.5+/-1 GPa, for the Rb derivative, the symmetry appears to be monoclinic. Moreover, the critical unit-cel l volumes associated with P-c1, P-c2, and P-c3 are slightly higher tha n the ambient pressure unit-cell volumes of RbMnF4 for P-c1 and KMnF4 for P-c2, and P-c3. Hydrostatic pressure has been found to have a simi lar 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 betw een hydrostatic and chemical pressure can therefore be established fro m the structural point of view for the AMnF(4) series. The tetragonal to orthorhombic transition of CsMnF4 has been found to be inhibited wh en NaCl is used as an internal pressure calibrant. The partial substit ution of Cs by Na in CsMnF4 at P-c1 has been shown to be a likely expl anation for this behavior. The anisotropic broadening of the Bragg pea ks for pressures higher than P-c1 has been analyzed in terms of micros train affecting the CsMnF4 lattice due to Na incorporation. A substitu tional reaction has been shown to be a competitive process, versus a s tructural phase transition, that enables the system to return to equil ibrium after applying pressure on it. Finally, the equation of state a ssociated with the different high-pressure phases has been calculated including compressibilities.