Radiowave and microwave frequency dielectric relaxations at the superionic, incommensurate and ferroelectric phase transitions in NH4HSeO4 and ND4DSeO4

Abstract

Ammonium hydrogen selenates NH4HSeO4 (AHSe) and ND4DSeO4 (ADSe) have been studied in the radiowave and microwave frequency ranges by network and impedance analysers between 200 and 410 K. Three relaxations of about 5*10-8, 5*10-10 and 5*10-11 s have been observed and assigned to reorientations of ammonium ions, proton jumps and reorientations of selenate ions, respectively. The corresponding dielectric susceptibilities have been studied and mechanisms for the different phase transitions from the superionic phase to the paraelectric, incommensurate and ferroelectric phases have been proposed. The origin of metastable phases and of memory effect is discussed.