Synthesis of Na₃WH₉ and Na₃ReH₈ Ternary Hydrides at High Pressures

The Na-W-H and Na-Re-H ternary systems were studied in a diamond anvil cell through X-ray diffraction and Raman spectroscopy, supported by density functional theory and molecular dynamics calculations. Na₃WH₉ can be synthesized above 7.8 GPa and 1400 K, remaining stable between at least 0.1 and 42.1 GPa. The rhenium analogue Na₃ReH₈ can form at 10.1 GPa upon laser heating, being stable between at least 0.3 and 32.5 GPa. Na₃WH₉ and Na₃ReH₈ host [WH₉]^3- and [ReH₈]^3- anions, respectively, forming homoleptic 18-electron complexes in both cases. Both ternary hydrides show similar structural types and pressure dependent phase transitions. At the highest pressures they adopt a distorted fcc Heusler structure (Na₃WH₉-II' and Na₃ReH₈-II') while upon decompression the structure symmetrizes becoming fcc between ∼6.4 and 10 GPa for Na₃WH₉-II and at 17 GPa for Na₃ReH₈-II. On further pressure release, the fcc phases transform into variants of a (quasi-) hexagonal structure at ∼3 GPa, Na₃WH₉-I and Na₃ReH₈-I.