THERMAL HYSTERESIS AT THE LOW-TEMPERATURE PHASE-TRANSITION OF (NH4)H2PO4, (NH4)H2ASO4, THEIR DEUTERATED ANALOGS, AND OF KH2PO4 AND THE EFFECT OF CR IMPURITY IONS ON THE TRANSITION-TEMPERATURE

Paraelectric-antiferroelectric phase transition temperatures, determined by dielectric, NMR, EPR, and DSC techniques for ammonium dihydrogen phosphate, ammonium dihydrogen arsenate, and their deuterated analogues, show, in contrast to some previously published reports of others, the thermal hysteresis of the transition to be close to 2° in all cases. The hysteresis is found to be practically independent of technique, crystal orientation, anion type, and deuteration and impurity dopant levels. The influence of Cr impurity ions on the transition appears to be only to lower the transition temperatures by ≃3.5°/% impurity. For potassium dihydrogen phosphate, thermal hysteresis is negligible as is the effect on the transition of Cr doping to the 1% level. The nature of the phase transitions and the relevance of the impurity effects to central peak phenomena are discussed. © 1979 American Chemical Society.