31P NMR study of the ferroelectric phase transition in PbHPO4

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Abstract
The 31P chemical shift tensors were measured in the paraelectric (P) and ferroelectric (F) phases of PbHPO4. They reflect the approximate C2v and C3 point symmetries of the 31P sites in the P and F phases of the crystal. The transition from C2v to C3 is the result of a proton ordering in double-well potentials below the phase transition temperature. In the P phase the width of the proton decoupled 31P-resonances is larger than in the F phase, and the orientation dependence of the excess width is clearly correlated with that of the 31P line splittings in the F phase. This finding is interpreted as giving evidence that the Pc-space-group symmetry of the F phase persists in the P phase. The authors believe that this is due to the presence of a homogeneous internal biasing field-produced by oriented defects with a nonzero charge and dipole moment-which breaks the symmetry of the high temperature phase in a similar way to an external electric field. The orientation of these defects is due to the fact that the crystal was grown in the ferroelectric phase.