Structures and Phase Transition of Vaterite-Type Rare Earth Orthoborates: A Neutron Diffraction Study

The structure of vaterite-type rare earth orthoborate (LnBO 3 ) has long been a subject of interest and controversy. In the present work, the crystal structures of two polymorphs of the vaterite-type rare earth orthoborates, i.e., the low- and high-temperature modifications of (Y 0.92 Er 0.08 )BO 3, were solved and refined from neutron powder diffraction data. The low-temperature polymorph crystallizes in a C-centered monoclinic cell with C 2/ c space symmetry, the unit cell parameters being a = 11.3138(3) Å, b = 6.5403(2) Å, c = 9.5499(2) Å, and β = 112.902(1)°. The boron atoms in the structure are all tetrahedrally coordinated and form the three-membered ring borate B 3 O 9 groups. The high-temperature form crystallizes in a new structure type in a monoclinic cell with C 2/ c space symmetry, and the unit cell constants a = 12.2019(3) Å, b = 7.0671(2) Å, c = 9.3424(2) Å, and β = 115.347(1)°. The borate groups in the high-temperature structure are all isolated flat BO 3 triangles. As far as the structural chemistry is concerned, both structures are different from the typical CaCO 3 vaterite. However, they do share some common features, particularly the packing fashion of the cations, which results in similarly looking X-ray diffraction patterns as that of the typical vaterite.