Raman spectrum of ferroelectric PbHPO4

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Abstract
The Raman spectrum of ferroelectric PbHPO4 (TC∼310 K) has been measured at temperatures between 10 and 320 K in the frequency region 10–3200 cm−1. Mode assignments have been made, which are consistent with the m(Cs) point-group symmetry of the ferroelectric phase. From examination of the correlation field splittings, it is concluded that the distortion of the PO4 tetrahedron is relatively small. The Raman study confirms the acentric structure of the paraelectric phase as proposed from neutron and x-ray diffraction data. No drastic change in the Raman spectrum occurs at TC. A soft mode with A′(Bu)-type symmetry is observed, which softens from 77 cm−1 at 10 K to 50 cm−1 at 300 K. The A″(Au)-type tunneling mode is tentatively assigned to a line at 164 cm−1. A low-frequency wing appears near 200 K and its intensity grows with increasing temperature reaching a maximum around 290 K. The presence of the wing feature indicates the phase transition is of an order–disorder type.