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.

In this work we explore the synthesis of Na₃WH₉ and Na₃ReH₈ at pressures over 20 GPa using diamond anvil cells and laser heating techniques. These compounds host 18-electron [WH₉]³⁻ and [ReH₈]³⁻ homoleptic anions, respectively. The samples were analyzed using X-ray diffraction and Raman spectroscopy. At the highest studied pressures, these compounds crystallize in a distorted fcc structure. Upon decompression, the distortion disappears and below 3–4 GPa they transform into variants of a hexagonal phase.