High-Pressure Synthesis, Crystal Chemistry, and Ionic Conductivity of a Structural Polymorph of Li₃BP₂O₈

A new structural polymorph of Li₃BP₂O₈ has been successfully synthesized via a solid-state reaction between Li₃PO₄ and BPO₄ at 4 GPa and 600 °C. The high-pressure phase of Li₃BP₂O₈ (HP-Li₃BP₂O₈) was found to crystallize in monoclinic symmetry with the cell parameters of a = 8.57010(4) Å, b = 11.11812(5) Å, c = 5.55380(3) Å, and β = 97.7269(3)° [space group P2₁/a (No. 14)]. HP-Li₃BP₂O₈ has a new crystal structure that has not been reported so far. The total ionic conductivities measured for the polycrystalline sample by alternating-currrent impedance were 3.4 × 10^–7 and 2.1 × 10^–6 S/cm at 399 and 456 K, respectively. The lithium ionic conductivity of HP-Li₃BP₂O₈ was higher than that of the low-pressure phase Li₃BP₂O₈ in the temperature range of 375–456 K. This is caused by the difference in the dimensions of the lithium arrangements between LP- and HP-Li₃BP₂O₈.