High-Pressure Synthesis, Crystal Chemistry, and Ionic Conductivity of a Structural Polymorph of Li<sub>3</sub>BP<sub>2</sub>O<sub>8</sub>

A new structural polymorph of Li<sub>3</sub>BP<sub>2</sub>O<sub>8</sub> has been successfully synthesized via a solid-state reaction between Li<sub>3</sub>PO<sub>4</sub> and BPO<sub>4</sub> at 4 GPa and 600 °C. The high-pressure phase of Li<sub>3</sub>BP<sub>2</sub>O<sub>8</sub> (HP-Li<sub>3</sub>BP<sub>2</sub>O<sub>8</sub>) was found to crystallize in monoclinic symmetry with the cell parameters of <i>a</i> = 8.57010(4) Å, <i>b</i> = 11.11812(5) Å, <i>c</i> = 5.55380(3) Å, and β = 97.7269(3)° [space group <i>P</i>2<sub>1</sub>/<i>a</i> (No. 14)]. HP-Li<sub>3</sub>BP<sub>2</sub>O<sub>8</sub> 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<sup>–7</sup> and 2.1 × 10<sup>–6</sup> S/cm at 399 and 456 K, respectively. The lithium ionic conductivity of HP-Li<sub>3</sub>BP<sub>2</sub>O<sub>8</sub> was higher than that of the low-pressure phase Li<sub>3</sub>BP<sub>2</sub>O<sub>8</sub> 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-Li<sub>3</sub>BP<sub>2</sub>O<sub>8</sub>.