High pressure structural investigations using single crystal techniques : influence of pressure on Jahn-Teller distorted alkali manganese fluorides

Abstract: The results from high-pressure single-crystal X-ray diffraction studies of some ternary sodium transition metal fluorides are presented. The structures of NaMnF4, Na2MnF5, Na3MnF6 and Na3ScF6 have been investigated at high pressures, and in connection with these studies the Na5Mn3F14 structure has been investigated at ambient pressure. The compressibilities of these compounds are compared in terms of structural features such as atom packing, bulk moduli and Jahn-Teller effect. An introduction to high-pressure diffraction techniques using diamond anvil cells, DAC, is included.Bulk modulus data at pressures lower than 1 GPa show a much higher compressibility for NaMnF4 (B0 = 16 GPa) than for the other compounds (B0 = 46-56 GPa). A successively higher transition pressure for the collapse of the single crystals into polycrystalline specimens is observed in the series NaMnF4 (1.3 GPa), Na2MnF5 (3.0 GPa) and Na3MnF6 (4.0 GPa). This can be explained in terms of the different connectivity between corner-sharing MnF6 octahedra, since NaMnF4 contains two-dimensional layers of corner-connected octahedra, Na2MnF5 one-dimensional strings, and Na3MnF6 isolated octahedra.The transitions in the Mn3+-containing compounds (d4 ion) are shown to be dependent on structural changes due to the Jahn-Teller distortions of the MnF6 octahedra. Both X-ray diffraction and spectroscopic studies of Na3MnF6 show a reversible phase transition at 2.2 GPa, which implies a reorientation of the static prolate Jahn-Teller distortions of the co-ordination around Mn3+. The Na3ScF6 compound, containing Sc3+ (d0 ion), shows no such transitions. Preliminary investigations of NaMnF4, Na2MnF5 and Na3MnF6 by high-pressure powder techniques show that new polycrystalline phases are formed when the pressure is raised above that where the single crystals collapse.