Crystal structure and polymorphism of PbAlBO 4

Abstract

Two polymorphs of the new borate compound, PbAlBO 4, have been synthesized in microcrystalline form and their crystal structures have been refined from powder neutron diffraction data. The low-temperature form, α-PbAlBO 4, crystallizes in the Pnma space group with a=6.9209(5) Å, b=5.7134(4) Å, c=8.0215(6) Å, Z=4. Its structure is built of straight [010] chains of edge-shared AlO 6 octahedra bridged by BO 3 groups and Pb 2+ cations. The high-temperature form, β-PbAlBO 4, is obtained by heating the α form to 1048 K followed by quenching in air. It crystallizes in the Pbcn space group with a=7.0423(3) Å, b=9.8989(3) Å, c=9.4281(3) Å, Z=8. The structure of β-PbAlBO 4 contains zig-zag chains of AlO 6 octahedra bridged by BO 3 groups and Pb 2+ cations and is related to the structure of α-PbAlBO 4 by unit-cell twinning. The detailed crystal chemical analysis of the α- and β-PbAlBO 4 structures shows that the α→ β transformation is driven by a decrease in density and the relief of bonding and non-bonding interactions around the Pb 2+ cations.