The nature of the intermediate phase in K3Na(SeO4)(2) crystals: three possible transition paths of the trigonal-monoclinic phasetransition

Three different transition paths of the phase transition from the high-symmetry trigonal P3¯¯¯ m1 to the low-symmetry monoclinic phase C2/c in K₃Na(SeO₄)₂ crystals have been analyzed using ab initio calculations based on the density functional theory (DFT) as well as programs and retrieval tools of the Bilbao Crystallographic server (BCS). We conclude that the intermediate state observed in experiments within 346–329 K temperature range consists of a mixture of two monoclinic phases C2/m and C2/c, both with the primitive cell doubled along the z-axis. Both monoclinic phases result from the softening of the double-degenerated zone-edge soft phonon A+2.