Dielectric Study on the Phase Transitions in RbMgBr3 and Cs2MgCl4

Single crystals of RbMgBr3 and Cs2MgBr4 were prepared, and phase transitions have been studied by dielectric measurements. Three phase transitions were found in RbMgBr3 at 226 K, 238 K, and 456 K. The phase between 226 K and 238 K (phase III) becomes narrower with increasing hydrostatic pressure and disappears at about 800 MPa. The dielectric anomalies at the phase transitions and the pressure-temperature phase diagram are quite similar to those KNiCl3-family ferroelectrics. This would imply that phase III of RbMgBr3 is ferroelectric. Phase transitions accompanied with rather small dielectric anomalies are found in Cs2MgBr4 at 51 K, 84 K, and 123 K. Ferroelectric hysteresis loop has not been observed for Cs2MgBr4 along any of the crystallographic axes.