<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>Crystals of Th(As<sub>8</sub>O<sub>18</sub>) are obtained along with an amorphous phase and CsAs<sub>3</sub>O<sub>8</sub> by high‐temperature/high‐pressure reaction of Th(NO<sub>3</sub>)<sub>4</sub>·5H<sub>2</sub>O, As<sub>2</sub>O<sub>3</sub>, and CsNO<sub>3</sub> in a ratio of 1:8:2 (Pt capsule, 3 GPa, 1200 °C; 0.2 °C/min cooling rate).</p> </abstract>