New phase transition with increasing symmetry on cooling in barium sodium niobate

Abstract

A new and unsual transition is reported in Ba 2NaNb 5O 15: the crystal, which was previously known to undergo at 573 K a ferroelastic transition 4 mm → mm2 ( C 4 v → C 2 v ), reverts back at 110 ± 5 K to a low temperature phase of 4 mm ( C 4 v ) symmetry. Thus this crystal presents a unique example of a ferroelastic ( mm2) phase stable in a wide temperature range (470 K) and sandwiched between two phases possessing the same higher point symmetry.