Phase transition and proton disordering in some ferroelectric hydrogen-bonded lead salts

Abstract

The phase behavior of two lead acid salts, PbHPO 4 (PHP) and PbHAsO 4 (PHA), is examined in the solid state, by differential scanning calorimetry and high-resolution ac calorimetry in the 200–650 K temperature range. For both samples, a first-order phase transition was detected at T 1=618 K (for PHP) and 503 K (for PHA), only when freshly prepared samples with a well-characterized single phase (orthorhombic for PHP and monoclinic for PHA) were heated, while it was not exhibited in the subsequent cooling of the samples. This shows that the high-temperature phase is metastable below T 1. However, for PHA, an additional reversible first-order phase transition was also observed at higher temperature ( T 2=574 K). Large changes of the ac impedance were observed at all phase transitions, indicating an appreciable disordering process in the hydrogen-bonded framework. The PHA phase above T 2 displays a fast-ion conducting behavior. The ac specific heat data taken on the PHA samples below 340 K showed a step-like anomaly near 320 K attributable to ferroelectric ordering with decreasing temperature.