A REVERSIBLE, ISOSYMMETRIC, HIGH-PRESSURE PHASE-TRANSITION IN NA3MNF6

The crystal structure of Na3MnF6 has been investigated at high pressur es by means of single-crystal x-ray diffraction, and its Mn(III) coord ination environment has been studied by means of single-crystal optica l absorption spectroscopy using diamond anvil techniques. Compressibil ity data (unit cell parameters) were collected in the pressure range f rom ambient to 4.06 GPa, and structural refinements based on single-cr ystal diffraction data were performed at 0.12, 0.91, 2.27, and 2.79 GP a. The monoclinic space group symmetry (P2(1)/n) is retained in the en tire pressure range, but, at increasing pressure, a discontinuous phas e transition is observed at similar to 2.2 GPa. This is interpreted as an effect of a reversible, isosymmetric phase transition with a hyste resis width of 0.5 GI)a, observed when the pressure is successively lo wered. The structure refinements show that the phase transition involv es a reorientation of the static prolate distortion of the coordinatio n around manganese(III). The angle between the elongation axis (z) Of the MnF63- octahedron with [001] flips from similar to 20 degrees at a mbient pressures to similar to 70 degrees at 2.79 GPa, Polarized singl e-crystal absorption spectra of Na3MnF6 show drastic changes of the po larization of bands due to spin-allowed d-d transitions in Mn(III) whe n passing the transition pressure, which confirm the results of the si ngle-crystal structure refinements. A possible explanation for this tr ansition is discussed in terms of structure packing arguments. The iso thermal bulk modulus at ambient pressure and its pressure derivative w ere determined to B-0 = 47.8(1) GPa and B-0' = 1.2(1), respectively.