Soft mode mechanism of the transition into the low-temperature ferroelastic phase in K-=SUB=-3-=/SUB=-Na(CrO-=SUB=-4-=/SUB=-)-=SUB=-2-=/SUB=- crystals

Abstract

The sequence of phase transitions P3m1 → C2/m → C2/c in K 3 Na(CrO 4) 2 crystals and existence of the intermediate phase have been studied both experimentally by the Raman scattering method and theoretically by programs and retrieval tools of the Bilbao Crystallographic Server. A zone-edge soft mode responsible for a transition from the intermediate C2/m to a low-symmetry C2/c phase at T = 210 K has been discovered experimentally for the first time. The symmetry of the soft mode has been established theoretically to be A + 2. The temperature interval of the intermediate monoclinic phase has been determined to be 239−210 K.

Screw-Type Motion and Its Impact on Cooperativity in BaNa2Fe[VO4]2

“…17−20 In particular, the sequence of phase transitions from P3̅ m1 to C2/c have been intensively studied for the nonmagnetic compounds, K 3 Na[XO 4 ] 2 with X = Se or Cr, in search of ferroelastic phases. 19 Additionally, it is worthwhile to note that BaNa 2 Fe[VO 4 ] 2 is a metastable compound, since Fe(II) and V(V) oxides have no common existence regime in the thermodynamic limit; see, for example, P. Schmidt. 21 The black luster of the title compound indicates furthermore rather unusual charge-dependent properties such as a small gap insulator possibly due to excitonactivated processes.…”