Phase transitions in triammonium hydrogen diselenate, (NH₄)₃H(SeO₄)₂

Abstract

Abstract The Raman and infrared spectra of triammonium hydrogen diselenate (NH4)3H(SeO4)2 (NHSe) have been recorded between 100 and 400 K. The present spectroscopic investigation cofirms the existence of five phases previously observed by other techniques. The analysis shows a progressive ordering with decreasing temperature. The ferroelectric IV → V tansition in this compound is also believed to be of the order-disorder type. The vibrational modes have been assigned by comparison with the sulphate compound (NH4)3H(SO4)2 (NHS).