HIGH PRESSURE SINGLE CRYSTAL AND POWDER X-RAY DIFFRACTION STUDIES OF SODIUM MANGANESE FLUORIDES

Stefan Carlson, Yiqiu Xu & Rolf Norrestam

Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden

Keywords: high pressure, X-ray diffraction, sodium manganese(III) fluoride

The crystal structures of some sodium manganese fluorides have been investigated using high pressure powder and single crystal X-ray diffraction techniques. The single crystal diffraction experiments were performed with a diamond anvil cell mounted on a four-circle diffractometer (MoK), while the powder experiments was performed using a image plate system on the same rotating anode X-ray generator.

Pressure induced phase transformations has been observed in studies of NaMnF₄ [1], Na₂MnF₅ [2] and Na₃MnF₆ [3]. As comparison to these manganese containing compounds, the structures of Na₃ScF₆ [4] and NaScF₄ are also being investigated. The series consisting of NaMnF₄, Na₂MnF₅ and Na₃MnF₆ display destructive single crystal to polycrystalline transitions at successively higher pressures; 1.3, 3 and 4 GPa, respectively. This can be anticipated by considering the different connectivities (corner-sharing) of the MnF₆ octahedra in the structures. NaMnF₄ is layered, Na₂MnF₅ contain strings of octahedra and Na₃MnF₆ has isolated octahedra. Furthermore, powder diffraction measurements have shown that these pressure induced transitions are reversible in the polycrystalline state. High pressure single crystal X-ray diffraction and optical absorption studies of Na₃MnF₆ have shown a reversible phase transition at ~2.2 GPa, due to a remarkable ~90^o switch of the Jahn-Teller distorted (elongated) MnF₆ octahedron.

A discussion of the observed transformations in connection with both the Jahn-Teller affected Mn³⁺ ion, and general atom packing arguments will be given.

1. S. Carlson, Y. Xu & R. Norrestam; Submitted to J. Solid State Chem.
2. S. Carlson, Y. Xu & R. Norrestam; Submitted to Z. Kristallogr.
3. S. Carlson, Y. Xu, U. Halenius & R. Norrestam; Inorg. Chem. 7 (1998) 1486-1492.
4. S. Carlson, Y. Xu & R. Norrestam; J. Solid State Chem. 1 (1998) 116-120.