Structure determination of the ferroelastic phase of K3Na(CrO4)2 at 200 and 230 K and the redetermination of its parent phase at 290 K

K3Na(CrO4)2, Mr = 372.27, λ(Mo Kα) 0.71073 A, 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 (P anti 3m1); the twinning operation T was the twofold axis parallel with the c axis. After the phase transition (Ttrans = 239 K) into the ferroelastic phase each domain which is present in the high-temperature phase gives rise to three domains with 120 orientation. Thus, the diffraction pattern of the low-temperature phase can be formally treated as if it were composed of six domains with 60 orientation. 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 anti 3m1 point group to 2/m and the c axis of the ferroelastic phase which is originally coincident with the trigonal axis doubles its length. 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. (orig.)