One-Pot Hydrothermal Synthesis of Tb<sup>III</sup><sub>13</sub>(GeO<sub>4</sub>)<sub>6</sub>O<sub>7</sub>(OH) and K<sub>2</sub>Tb<sup>IV</sup>Ge<sub>2</sub>O<sub>7</sub>: Preparation of a Stable Terbium(4+) Complex

Two terbium germanates have been synthesized via high-temperature and high-pressure hydrothermal synthesis with 20 M KOH as a mineralizer using Tb<sub>4</sub>O<sub>7</sub> as a starting material. Tb<sub>13</sub>(GeO<sub>4</sub>)<sub>6</sub>O<sub>7</sub>(OH) crystallizes in trigonal space group <i>R</i>3̅, is built up of isolated GeO<sub>4</sub> units, and contains a complex arrangement of terbium oxide polyhedra. K<sub>2</sub>TbGe<sub>2</sub>O<sub>7</sub> is a terbium­(4+) pyrogermanate that is isostructural with K<sub>2</sub>ZrGe<sub>2</sub>O<sub>7</sub> and displays a rare stable Tb<sup>4+</sup> oxidation state in the solid state.