High-pressure syntheses of alpha-RE2B4O9 (RE = Sm, Ho), with a structure type displaying edge-sharing BO4 tetrahedra.

The compounds alpha-RE2B4O9, with RE = Sm (disamarium tetraborate) and Ho (diholmium tetraborate), were synthesized under conditions of high pressure and high temperature in a Walker-type multianvil apparatus, at 7.5 GPa and 1323 K for alpha-Sm2B4O9 and at 10 GPa and 1323 K for alpha-Ho2B4O9. The crystal structures were determined from single-crystal X-ray diffraction data collected at room temperature. The structures are isotypic with the already known alpha-RE2B4O9 (RE = Eu-Dy) phases, displaying the new structural motif of edge-sharing BO4 tetrahedra next to the known motif of corner-sharing BO4 tetrahedra. As the end members of this isotypic series, the two title compounds mark the borders of the stability field of the appearance of edge-sharing BO4 tetrahedra.