Low temperature behaviour of barium sodium niobate

Abstract
Ba2NaNb5O15, a ferroelectric crystal with the tungsten bronze structure, undergoes the sequence of phase transitions 4/mmm ← 4mmmm2 ← 4mm on cooling. Low temperature birefringence and dielectric measurements down to 1.5 K have been achieved on crystals from different origins. The existence of the mm2 ← 4mm transitions appears extremely sample dependent. Moreover a newly discovered sample-independent dielectric anomaly at 12 K is interpreted as the indication of a new phase transition which can possibly be related to the occurrence of ferroelectricity in some other lead containing tungsten bronze compounds.