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

The structure of vaterite-type rare earth orthoborate (LnBO3) 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 (Y0.92Er0.08)BO3, were solved and refined from neutron powder diffraction data. The low-temperature polymorph crystallizes in a C-centered monoclinic cell with C2/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 B3O9 groups. The high-temperature form crystallizes in a new structure type in a monoclinic cell with C2/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 BO3 triangles. As far as the structural chemistry is concerned, both structures are different from the typical CaCO3 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.