A Reversible, Isosymmetric, High-Pressure Phase Transition in Na3MnF6

Abstract

The crystal structure of Na3MnF6 has been investigated at high pressures by means of single-crystal x-ray diffraction, and its Mn(III) coordination environment has been studied by means of single-crystal optical absorption spectroscopy using diamond anvil techniques. Compressibility data (unit cell parameters) were collected in the pressure range from ambient to 4.06 GPa, and structural refinements based on single-crystal diffraction data were performed at 0.12, 0.91, 2.27, and 2.79 GPa. The monoclinic space group symmetry (P21/n) is retained in the entire pressure range, but, at increasing pressure, a discontinuous phase transition is observed at ∼2.2 GPa. This is interpreted as an effect of a reversible, isosymmetric phase transition with a hysteresis width of 0.5 GPa, observed when the pressure is successively lowered. The structure refinements show that the phase transition involves a reorientation of the static prolate distortion of the coordination around manganese(III). The angle between the elongation axis (z) of the MnF63- octahedron with [0 0 1] flips from ∼20° at ambient pressures to ∼70° at 2.79 GPa. Polarized single-crystal absorption spectra of Na3MnF6 show drastic changes of the polarization of bands due to spin-allowed d−d transitions in Mn(III) when passing the transition pressure, which confirm the results of the single-crystal structure refinements. A possible explanation for this transition is discussed in terms of structure packing arguments. The isothermal bulk modulus at ambient pressure and its pressure derivative were determined to B0 = 47.8(1) GPa and B0‘ = 1.2(1), respectively.