Phase transition and structure of a K{sub 1{minus}x}Li{sub x}MnF{sub 3} single crystal in the range 100--298 K
- Polish Academy of Sciences, Wroclaw (Poland). Inst. of Low Temperature and Structure Research
- Univ. of Silesia, Katowice (Poland). Inst. of Physics
The cubic-to-tetragonal phase transition in the mixed perovskite K{sub 1{minus}x}Li{sub x}MnF{sub 3} (x < 0.036) was investigated by single-crystal X-ray diffraction. Results of the structure refinement at 298 and 145 K are discussed. They confirm the cubic symmetry, the space group Pm3m, of the room temperature phase with strongly anisotropic thermal vibration amplitudes of the F atoms, which were also observed in the parent KMnF{sub 3}. Substitutional lattice imperfections at the K{sup +} sites shift the ferrodistortive phase transition to higher temperature T{sub c1} = 191 K, compared to 186.5 K in KMnF{sub 3}. The structural changes in the tetragonal phase (I4/mcm) are characterized by the displacements of the F atoms from the positions they occupy above T{sub cl}. However, the Li{sup +} ion dopants do not affect the octahedral network, the MnF{sub 6} octahedra remain regular, and no displacements of K{sup +}/Li{sup +} cations along the tetragonal c axis are observed. The thermodynamic character of the transition is illustrated with the critical behavior of the unit cell parameters and the temperature-dependent evolution of the tetragonal deformation and the macroscopic strain. A variation of the integrated intensity of superstructure reflections close to T{sub cl} has been assumed to describe the behavior of the order parameter. As far as the precision of the present experiment allows, all these observations show that 3.6% Li{sup +} substitution at the K{sup +} site does not radically change the nature of the transition in KMnF{sub 3} at T{sub cl}.
- OSTI ID:
- 642150
- Journal Information:
- Journal of Solid State Chemistry, Vol. 137, Issue 1; Other Information: PBD: Apr 1998
- Country of Publication:
- United States
- Language:
- English
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