FERROELECTRICITY AND STRUCTURAL PHASE-TRANSITIONS IN A HEXAGONAL ABX3-TYPE ANTIFERROMAGNETIC COMPOUND - KNICL3

Dielectric properties of a hexagonal ABX3-type antiferromagnet KNiCl3 are studied by measuring dielectric constants and observing D - E hysteresis loops. Dielectric anomalies indicating structural phase transitions are found exclusively along the c-axis at 274 K, 285 K, 561 K and 762 K. Ferroelectric D - E hysteresis loops are observed in the phase above 274 K, though the D - E loop cannot be observed at temperatures higher than 600 K owing to enhancement of electric conductivity of this compound. Within the observed temperature range, the spontaneous polarization P(s) as a function of temperature behaves in a manner wherein as temperature is raised, P(s) appears suddenly at 274 K, decreases gradually towards 285 K after substantial growth and then monotonically increases up to the phase above 561 K. The magnitudes of P(s) at 274.5 K and 533.0 K are estimated as 0.048 muC/cm2 and 0.871 muC/cm2, respectively. Such behavior of P(s) is attributed to a possible ferrielectric alignment of sublattice polarizations inherent to the crystal structure with the space group P6(3)cm.