LOW TEMPERATURE PHASE TRANSITIONS OF THE Cu6PS5Br CRYSTAL

A. Haznar1, A. Pietraszko1, I.P. Studenyak2, M. Wolcyrz1

1Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2 St., 50 950 Wroclaw, Poland
2Uzhhorod State university, 46 Pidhirna St., Uzhhorod 294000, Ukraine
e-mail: haznar@highscreen.int.pan.wroc.pl

Keywords: Cu6PS5Br, superionic conductor, phase transition.

Cu6PS5Br belongs to a family of tetrahedrally close-packed structures, termed the argyrodites. Their general formula is Am+(12-n-x)/m Bn+ X2-6-xYx

(A: Cu, Ag, Cd, Hg; B: Ga, Si, Ge, Sn, P, As; X: S, Se, Te; Y: Cl, Br, I; 0<x<1 ). They have a common cubic (F-43m) high temperature structure, typical feature of which is an anion framework of interpenetrating centred icosahedra.

Some argyrodites (containing Cu+ and Ag+ ions) are efficient ionic conductors. Cu6PS5Br is a mixed, ion - electron conductor and its conductivity (ion) is equal to 1.510-5 (cm)-1 at 295 K and increase up to 1.210-3(cm)-1 at 473 K [1,2]. From structural point of view possible conductivity mechanism can be discussed by considering the positions of the Cu atoms and the directions of their thermal motions. Structure solution and refinement at 295 K and 420 K led to a structure, similar to that presented in [2,3] but with some discrepancies concerning mixed Br-S positions and Cu+ distribution in the crystal. Two sublattices of Cu ions create a three-dimensional, highly defected framework and exhibit large, correlated in direction thermal vibrations. Concerning this hopping mechanism of Cu+ activated by oscillation of S and Br tetrahedra is proposed.

Incomplete occupancy of certain cation sites favours the order-disorder phase transitions. Cu6PS5Br undergoes first-order, superionic phase transition at 166-180 K and second-order, ferroelastic phase transition at about 268 K [4]. Our DSC and Bond's method measurements of temperature dependence of lattice parameters corroborate ferroelastic phase transition and confirm monoclinic deformation of the crystal lattice. The superionic transition is clearly seen at DSC curve but does not show any diffraction anomalies and seems to be isostructural transformation.

Rietveld refinement based on powder data measured at 40 K and 200 K led to a structure of ordered Cu-ion framework confirming monoclinic Cc symmetry of low temperature phase of the crystal [5].

  1. Kuhs W.F., Nitsche R., Scheunemann K., Mat. Res. Bull. Vol. 14, pp. 241-248, 1979.
  2. Kuhs W.F., Heger G.; Fast Ion Transport in Solid 1979, Ed. El. North Holland.
  3. Kuhs W.F., Nitsche R., Scheunemann K., Acta Cryst. B34, pp. 64-70, 1978.
  4. Studenak J.P, Kovach D.S., Oplukas A.S., Kovach E.T.; Izv. AN SSSR. Vol. 56, pp. 86-93, 1992.
  5. Kush W. F., Nitsche R., Scheunemann K.; Mat. Res. Bull. Vol. 11, pp. 1115-1124, 1976.

This work was supported by grant (KBN) No. 7 T08A 027 09.