The role of lone pairs in the ferroelastic phase transition in the palmierite-type lead phosphate-arsenate solid solution

The structural changes associated with the R (3) over barm to C2/c ferroelastic phase transition in the solid solution Pb-3(P1-xAsxO4)(2) have been followed by Raman spectroscopy on samples with x = 0.00, 0.08, 0.37, 0.57, 0.65, 0.80, 1.00 over the temperature range 80-850 K. The high-temperature structural state of four intermediate samples with x = 0.37, 0.57, 0.65 and 0.80 was also studied by synchrotron single-crystal X-ray diffraction. The new results, in combination with previous data, reveal that Pb-3(P1-xAsxO4)(2) undergoes a complex sequence of transformation processes with several characteristic temperatures that can be clearly identified in addition to that of the ferroelastic phase transition at T-c. These structural changes are mainly driven by two phonon modes: vibrations of the Pb2 atoms (stereochemically active in the C2/c phase perpendicular to the layers characteristic of the palmierite-type structure), and rotational modes of the PO4 and AsO4 units within the plane of the layers. The former ensures coupling of the atomic displacements both within and between the layers, while the latter favours the coupling of the monoclinic species within the layers. With decreasing temperature, at T-4 >> T-c the Pb2 atoms are displaced from their special positions on the triad axis of the R (3) over barm structure. At T-3 the size of the nanoclusters composed of coupled displacements of Pb2 atoms on the monoclinic pattern is sufficient to produce both macroscopic effects in all compositions (diffuse super-lattice intensities in the diffraction patterns, non-symmetry-breaking strain in the unit-cell parameters, a step in the C-p curve) and microscopic effects in the broadening of the vibrational modes of the Pb2 atoms. The temperature interval between T-3 and T-4 is considerably smaller for the intermediate compositions than for the end-members indicating that the chemical disorder on the tetrahedral site facilitates the condensation of monoclinic species, probably by smearing the potential barrier between different monoclinic orientational states. At T-2 < T-3, but still above T-c, the AsO4 tetrahedra undergo monoclinic deformation in order to better coordinate the adjacent displaced Pb2 atoms, while the geometry of the PO4 tetrahedra remains unchanged down to T-c. (and for As-rich compositions, slightly below T-c). At T-c the ordered Pb2 displacements and rotated tetrahedra drive a displacement of the Pb1 atoms from their trigonal (paraelastic) positions and long-range-ordered ferroelastic domains are formed. However, for all compounds, residual disorder of the Pb2 displacements persists below T-c a further characteristic temperature T-1 at which the displacements of all types of atoms become completely ordered on all length scales.