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

Abstract

K 3 Na(CrO 4 ) 2 , M r =327.27, 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 (P3m1); 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 o orientation. Thus, the diffraction pattern of the low-temperature phase can be formally treated as if it were composed of six domains with 60 o orientation