Antiferroelectric instability in double perovskite systems La(2-x)/3LixTiO3, La(1-x)/3LixTaO3 and La(1-x)/3LixNbO3.

Anomalous softening of the in-plane, out-of-phase vibration of Ti, Nb or Ta cations in these double perovskite systems has been observed by Raman spectroscopy upon decreasing temperature. In the titanates softening ceases below approximate to 60 K, while in Ta and Nb systems a broad minimum in the frequency of the soft-mode at about 140 K has been found, suggesting that a real phase transition takes place in these cases. In general, an increase in lithium content results in the gradual disappearance of the soft-mode behavior. This effect has been interpreted as due to the enhancement of the random field at the Ti site due to increasing structural disorder, a behavior that is intimately related to the cation ordering in the A site of the perovskite cell. Since the behavior in the titanates resembles that of incipient ferroelectrics such as SrTiO3 and KTaO3 the phenomenon in double perovskites may be termed as incipient antiferroelectricity, though the explanation for the low temperature frequency stabilization is not the same.