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

Abstract

The crystal structure of Na 3 MnF 6 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 (P2 1 /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 MnF 6 3octahedron with [0 0 1] flips from ∼20°at ambient pressures to ∼70°at 2.79 GPa. Polarized single-crystal absorption spectra of Na 3 MnF 6 show drastic changes of the polarization of bands due to spinallowed d-d transitions in Mn(III) when passing the transition pressure, which confirm the results of the singlecrystal 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 B 0 ) 47.8(1) GPa and B 0 ′ ) 1.2(1), respectively.

High-Pressure Phase Transition in Cs2KMnF6

“…The rigid three-dimensional ReO type of octahedral network in Cs KMnF , formed by alternating corner-sharing KF and MnF octahedra, o!ers an obvious explanation for the lower compressibility. The compressibility of Cs KMnF is similar to that observed for Na MnF (K "47.8(6) GPa, Carlson et al (14)) with its related ReO network of NaF and MnF octahedra.…”

Theory and Practice – Thermodynamics, Equations of State, Elasticity, and Phase Transitions of Minerals at High Pressures and Temperatures

“…Both Ce(I) and Ce(IV) have the f.c.c. At 2.2 GPa this compound undergoes a first-order isosymmetric phase transformation, which is associated with a change of orientation of the Jahn-Teller elongation of MnF 6 octahedra (Carlson et al, 1998). The volume change at the transition is very large (13%) at room temperature, but it rapidly decreases along the Ce(I)-Ce(IV) equilibrium line until it disappears at the critical point (2.15 GPa and 613 K).…”

Single-Crystal High-Pressure Studies of Na3ScF6

“…The octahedral coordination around the d ion in the manganese(III) compound is considerably Jahn-Teller distorted. Any effects induced by varying the pressure can be expected to be more normal for Na ScF than for Na MnF , which exhibits inter alia a phase transition at elevated pressures (1). The latter is likely to be due to a spatial change of the uneven electron distribution around the Mn> ions.…”