The crystal structures of Pb₂ScTaO₆ and Pb₂ScNbO₆ in the paraelectric and ferroelectric states

Abstract

Two systems were studied on high-resolution neutron time-of-flight powder diffraction (HRDP), a totally B-site disordered ceramic disk of Pb²ScNbO⁶, and a partially ordered ceramic block of Pb²ScTaO⁶. We found Pb²ScNbO⁶ to be fully disordered, space group Pm3m with lattice constant 4.08153(1)Å, whereas Pb²ScTaO⁶ was found to be 86(1)% ordered with space group Fm3m and lattice constant 8.14866(1)Å. The high-resolution on HRDP allowed the crystal system in both compounds to be identified as rhombohedral. Two space groups, R3 and R3m were tested. A large asymmetry was found in the 3.0Å co-ordination sphere of Pb, which changes its co-ordination number from 12 to 9 on the ferroelectric transition. The co-ordination of oxygen around the Pb suggests that the phase transition is related to the de-localisation of a stereochemically active lone pair. In both Pb²ScTaO⁶ and Pb²ScNbO⁶, the major displacements occurred for the Pb(s) in the order of 0.25Å, whilst the displacements in the B-site octahedra were much smaller.

Key Words:

• Neutron Diffraction
• Pb²ScTaO⁶
• Pb²ScNbO⁶
• Relaxor Ferroelectric oxides