Synthesis\nof Na<sub>3</sub>WH<sub>9</sub> and Na<sub>3</sub>ReH<sub>8</sub> Ternary\nHydrides at High Pressures

The Na–W–H\nand Na–Re–H ternary systems\nwere studied in a diamond anvil cell through X-ray diffraction and\nRaman spectroscopy, supported by density functional theory and molecular\ndynamics calculations. Na<sub>3</sub>WH<sub>9</sub> can be synthesized\nabove 7.8 GPa and 1400 K, remaining stable between at least 0.1 and\n42.1 GPa. The rhenium analogue Na<sub>3</sub>ReH<sub>8</sub> can form\nat 10.1 GPa upon laser heating, being stable between at least 0.3\nand 32.5 GPa. Na<sub>3</sub>WH<sub>9</sub> and Na<sub>3</sub>ReH<sub>8</sub> host [WH<sub>9</sub>]<sup>3–</sup> and [ReH<sub>8</sub>]<sup>3–</sup> anions, respectively, forming homoleptic 18-electron\ncomplexes in both cases. Both ternary hydrides show similar structural\ntypes and pressure dependent phase transitions. At the highest pressures\nthey adopt a distorted fcc Heusler structure (Na<sub>3</sub>WH<sub>9</sub>–II′ and Na<sub>3</sub>ReH<sub>8</sub>–II′)\nwhile upon decompression the structure symmetrizes becoming fcc between\n∼6.4 and 10 GPa for Na<sub>3</sub>WH<sub>9</sub>–II\nand at 17 GPa for Na<sub>3</sub>ReH<sub>8</sub>–II. On further\npressure release, the fcc phases transform into variants of a (quasi-)\nhexagonal structure at ∼3 GPa, Na<sub>3</sub>WH<sub>9</sub>–I and Na<sub>3</sub>ReH<sub>8</sub>–I.