Wall-related Raman scattering in ferroelastic lead phosphate Pb3(PO4)2

Micro-Raman spectroscopy is applied to study structural changes in the vicinity of the domain walls in ferroelastic lead phosphate Pb-3(PO4)(2). The Raman spectra measured in untwinned and in heavily twinned regions of monoclinic Pb-3(PO4)(2) are studied and the observed differences are analysed on the basis of calculated vibrational modes of monoclinic structural units with different atomic arrangements. The wall-related Raman signals observed near 537, 65 and 51 cm(-1) result from static displacements of the Pb-2 atoms along the monoclinic binary axis. PO4 tetrahedra of rhombohedral geometry can also contribute to the extra Raman scattering near 537 cm(-1). The deviations of the Pb-2 atoms from their monoclinic positions couple preferably within the Pb-2-O sheets parallel to the cleavage plane. Rearrangements of the Pb-2 atoms occurring across the domain walls affect the intensity ratios between out-of-plane and in-plane components of nu (4)(PO4) bending, PO4 rotational and PO4 translational modes.