31P NMR study of the influence of defects on the symmetry of ferroelectric crystals of PbHPO4 above the phase transition

Abstract

The width delta v of the 31P NMR in PbHPO4 is found to be larger in the paraelectric than in the ferroelectric phase. The authors interpret this increase of delta v as an unresolved splitting due to the persistence of the low temperature ferroelectric space group Pc in the high temperature paraelectric phase, where a centrosymmetric space group is expected. A comparison of delta v in a crystal grown at ambient temperature, i.e. in the ferroelectric phase, and in a crystal growth at 313 K, i.e. in the paraelectric phase, shows that delta v is smaller in the latter crystal. Annealing of this crystal resulted in a further decrease of delta v. This leads us to the conclusion that polar defects in real crystals conserve, locally, the symmetry of the ferroelectric phase above the transition temperature Tc and that a crystal free of defects should change its symmetry at Tc as required by Landau's theory of structural phase transitions.