Nature of the High-Temperature Dielectric Anomaly in TlH_<b>2</b>PO_<b>4</b>

Abstract

The dielectric constant of TlH 2 PO 4 along the b axis, ε b ′ , was measured as a function of temperature between 290 and 443 K. The ε b ′ displayed a new high-temperature anomaly at about 425 K ( T P ) on heating above the ferroelastic-paraelastic transition at 356 K ( T U ), while it showed a quasi-irreversibility on subsequent cooling to room temperature. After prolonged heat treatment at and above T P , microscopic spots, which grew into multigrains, were observed, indicating an additional tendency towards the incipient partial decomposition at temperatures above T P . The appearance of spots and the persistence of the high-temperature state on cooling back to room temperature suggest that the high-temperature anomaly near T P is topochemical in nature and that polymerization due to dehydration can take place on tempering near T P . The quasi-irreversibility of ε b ′ seems to result from the nonstoichiometry at the surface.