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 (P3 m1BAR); the twinning operation T was the twofold axis parallel with th e c axis. After the phase transition (T(trans) = 239 K) into the ferro elastic phase each domain which is present in the high-temperature pha se gives rise to three domains with 120-degrees orientation. Thus, the diffraction pattern of the low-temperature phase can be formally trea ted 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 reflectio ns. 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 ferroelast ic phase transition from a 3m1BAR point group to 2/m and the c axis of the ferroelastic phase which is originally coincident with the trigon al axis doubles its length. On the other hand, contrary to K3Na(SeO4)2 , only twinned single crystals did not crack during this phase transit ion. The phase transition is accompanied by shifts of two crystallogra phically independent K+ cations, as well as by shift and tilting of [C rO4]2-. The phase transition affects mainly the environment of K(2) wh ich is the most loosely bound cation in the structure. The atomic disp lacements from the positions in the high-temperature phase are signifi cantly larger at 200 than at 230 K, thus indicating a continuous phase transition.