STRUCTURE DETERMINATION OF THE FERROELASTIC PHASE OF K3NA(CRO4)2 AT 200-K AND 230-K AND THE REDETERMINATION OF ITS PARENT PHASE AT 290-K

K3Na(CrO4)2, M(r) = 372.27, lambda(Mo Kalpha) = 0.71073 angstrom, two crystals were measured; crystal A at 290, 230 and 200 K and crystal B at 200 K. Both crystals were twinned in the room-temperature phase (P3m1BAR); the twinning operation T was the twofold axis parallel with the c axis. After the phase transition (T(trans) = 239 K) into the ferroelastic phase each domain which is present in the high-temperature phase gives rise to three domains with 120-degrees orientation. Thus, the diffraction pattern of the low-temperature phase can be formally treated as if it were composed of six domains with 60-degrees orientation. Crystal data: crystal A: T = 290 K, trigonal, P3m1BAR, a = 5.857(3), c = 7.521(2) angstrom, V = 223.4 angstrom3, mu = 38.33 cm-1, Z = 1, F(000) = 180, D(x) = 2.766 g cm-3, R = 0.0145 for 238 observed reflections. T = 230 K, monoclinic, C2/c, a = 10. 128 (3), b = 5.8437 (5), c = 15.022 (2) angstrom, beta = 89.97 (2)degrees, V = 889.1 angstrom3, mu = 38.53 cm-1, Z = 4, F(000) = 720, D(x) = 2.780 g cm-3, R = 0.0331 for 2078 observed reflections. T = 200 K, monoclinic, C2/c, a = 10.117 (1), b = 5.843 (2), c = 15.024 (2) angstrom, beta = 89.967 (7)degrees, V = 888.1 angstrom3, mu = 38.57 cm-1, Z = 4, F(000) = 720, D(x) = 2.783 g cm-3. Crystal A: R = 0.0315 for 2149 observed reflections; crystal B: R = 0.0288 for 2045 observed reflections. The respective high- and low-temperature phases of K3Na(CrO4)2 and K3Na(SeO4)2 are isomorphous. By analogy with K3Na(SeO4)2 the title compound undergoes a ferroelastic phase transition from a 3m1BAR point group to 2/m and the c axis of the ferroelastic phase which is originally coincident with the trigonal axis doubles its length. On the other hand, contrary to K3Na(SeO4)2, only twinned single crystals did not crack during this phase transition. The phase transition is accompanied by shifts of two crystallographically independent K+ cations, as well as by shift and tilting of [CrO4]2-. The phase transition affects mainly the environment of K(2) which is the most loosely bound cation in the structure. The atomic displacements from the positions in the high-temperature phase are significantly larger at 200 than at 230 K, thus indicating a continuous phase transition.