# Synthesis of Na3WH9 and Na3ReH8 Ternary Hydrides at High Pressures

Tomas Marqueño, Israel Osmond, Mikhail A. Kuzovnikov, Hannah A.

Shuttleworth, Samuel Gallego-Parra, Eugene Gregoryanz, Andreas Hermann,

Ross T. Howie, Miriam Peña-Alvarez

► To cite this version:

Tomas Marqueño, Israel Osmond, Mikhail A. Kuzovnikov, Hannah A. Shuttleworth, Samuel Gallego-Parra, et al.. Synthesis of Na3WH9 and Na3ReH8 Ternary Hydrides at High Pressures. Inorganic Chemistry, 2024, 63 (45), pp.21734-21741. 10.1021/acs.inorgchem.4c02691. hal-05099485

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# Synthesis of Na₃WH₉ and Na₃ReH₈ Ternary Hydrides at High Pressures

Tomas Marqueño, Israel Osmond, Mikhail A. Kuzovnikov, Hannah A. Shuttleworth, Samuel Gallego-Parra, Eugene Gregoryanz, Andreas Hermann, Ross T. Howie, and Miriam Peña-Alvarez*

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Cite This: *Inorg. Chem.* 2024, 63, 21734–21741

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## ACCESS

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Supporting Information

**ABSTRACT:** 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.

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## INTRODUCTION

The search for novel high-temperature superconductors has motivated the study of hydrogen-rich systems under high pressures, leading to the discovery of new binary metal-hydrogen compounds.¹ Some of these novel materials, such as LaH₁₀,² CaH₆,³,⁴ or H₃S⁵,⁶ exhibit superconducting properties with high critical temperature (T_c) above 150 GPa. Despite their promising attributes, such binary hydrides tend to decompose upon decompression, complicating the attainment of superconductive phases under ambient conditions.⁷⁻⁹

The study of ternary hydrides at high pressures has been suggested as an alternative approach toward achieving superconductivity at lower pressures.¹⁰⁻¹² This is because, the addition of a third element is expected to stabilize superconducting phases at lower pressures.¹⁰,¹²⁻¹⁵ In fact, theoretical studies have predicted superconducting properties in compounds combining alkaline/alkaline earth metals with transition metal (TM) and hydrogen (complex transition metal hydrides, (CTMHs)), such as K₂ReH₉ (T_c = 127.1 K at 75 GPa)¹⁶ or Mg₂IrH₆ (T_c = 160 K at ambient pressure).¹⁰ However, experimental research has shown modest T_c values for CTMHs such as BaReH₉¹⁷ and Li₅MoH₁₁¹⁸ (above 100 GPa and below 10 K).

In CTMHs the TM atoms are coordinated by an unusually high number of hydrogen atoms forming homoleptic complex ions, such as [OsH₇]³⁻,¹⁹ [NbH₉]⁴⁻,²⁰ and [NiH₅]³⁻.²¹ Traditionally, the synthesis of CTMHs has been conducted

using autoclave techniques, where a mixture of active (alkali, alkaline earth or rare earth) and transition metals react at room pressure in a hydrogen atmosphere at high temperatures (300–700 °C).¹³,²²

Historically, TMs from groups 3 to 6 were thought incapable of forming semiconducting CTMHs.²³ However, it was later shown that the use of large volume presses to 5 GPa and from 300 to 800 °C with AlH₃ as hydrogen precursor, leads to the synthesis of compounds such as Mg₃CrH₈,²⁴ Li₅(TM)H₁₁ (TM = Mo, W) and Li₆(TM)H₁₁ (TM = Nb, Ta).²⁰ All these compounds contain interstitial H⁻ anions as well as TMHₙ homoleptic ionic complexes.

TM-H hydrides containing cations support high-coordinated TM complexes without interstitial H⁻.¹⁴,²¹ For TMs in groups 8, 9, and 10 these hydrides display a rich polymorphism in phases recovered to ambient conditions, with a *Pnma* structure type for Na₃NiH₅²¹ and Na₃(TM)H₆ (TM = Co, Rh, Ir)²⁵ and a *P4₂/mnm* structure type for Na₃(TM)H₇ (TM = Fe, Ru, Os).¹⁴ In these CTMHs the hydrogen content decreases with the TM group number; therefore, Na-TM-H

Received: June 28, 2024

Revised: October 14, 2024

Accepted: October 17, 2024

Published: October 31, 2024

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with TMs from groups 6 and 7 could yield higher hydrogen-to-metal ratios (H to M ratio), which is beneficial for potential hydrogen storage applications. Under pressure, the Na-based CTMHs take up fcc-like structures with the metal atoms on Heusler sites, and rotationally disordered TMH<sub>n</sub> complexes.

In this study, the synthesis of two novel Na–Re–H and Na–W–H compounds under high temperatures and pressures, using diamond anvil cells and infrared laser heating, are reported. Na<sub>3</sub>WH<sub>9</sub> and Na<sub>3</sub>ReH<sub>8</sub> form from NaH, W/Re and H<sub>2</sub> upon laser heating above 7.8 and 10.1 GPa, respectively, remaining stable between at least 30 GPa and down to <0.5 GPa. X-ray diffraction (XRD) and Raman spectroscopy, supported by density functional theory (DFT) calculations are used for the characterization of the new compounds. XRD measurements show that Na<sub>3</sub>WH<sub>9</sub>–II' adopts a distorted fcc Heusler structure<sup>26</sup> above 20 GPa (the first identified rotationally ordered phase), containing [WH<sub>9</sub>]<sup>3−</sup> units. Upon decompression, below 10 GPa, Na<sub>3</sub>WH<sub>9</sub>–II' symmetrizes into an fcc structure (Na<sub>3</sub>WH<sub>9</sub>–II), which then transitions into an hexagonal structure, Na<sub>3</sub>WH<sub>9</sub>–I near ~3 GPa.<sup>26</sup> Na<sub>3</sub>ReH<sub>8</sub>, which also adopts a Heusler structure with a small distortion, was synthesized from NaH + Re + H<sub>2</sub> at 32.5 and 25.5 GPa using laser heating to 1400 K. Na<sub>3</sub>ReH<sub>8</sub> features stable 18-electron [ReH<sub>8</sub>]<sup>3−</sup> complexes. Upon decompression, XRD measurements show that Na<sub>3</sub>ReH<sub>8</sub> also undergoes a series of analogous phase transitions at 17 and 3.5 GPa. The Raman spectra of these phases present broad bands near 1100 and 2000 cm<sup>-1</sup>, associated with TM–H bending and stretching modes of the [TMH<sub>n</sub>]<sup>-3</sup> units, respectively. DFT ab initio calculations were used to evaluate the relative stability of the different structural phases for both ternary hydrides, reproducing the experimentally observed phase transitions. Additionally, molecular dynamics (MD) calculations captured the pseudorotational character of the Na<sub>3</sub>WH<sub>9</sub>–II and Na<sub>3</sub>ReH<sub>8</sub>–II structures, with hydrogen atoms occupying nonisotropic shells around the TM.

## ■ RESULTS

**Powder X-ray Diffraction Experiments.** Approximately two parts sodium hydride (NaH) to one part tungsten (W) by volume were loaded in a diamond anvil cell (DAC) and clamped with excess hydrogen (H<sub>2</sub>) at 0.2 GPa. As shown in Figures 1(a,b) and S1–S3 the XRD patterns of the mixture fully transform during laser heating at 23.5 and 20 GPa (to 1400 K<sup>27</sup>). The new diffraction patterns are compatible with a distorted fcc Heusler arrangement,<sup>26</sup> with the Na and W atoms forming a metallic sublattice that can be described by either the space group *Immm* or *Pmmm* (Figure S2(a,b), respectively) in a proportion of 3:1 (Na<sub>3</sub>WH<sub>n</sub>). These are the highest symmetry subgroups of the fcc structure that are consistent with the experimental data presented here. Nevertheless, the stability of these structures is compromised when hydrogen atoms are incorporated into the DFT calculations. We performed DFT-based search for a possible arrangement of hydrogen atoms in this material, and found a possible candidate with the composition of Na<sub>3</sub>WH<sub>9</sub> with a monoclinic crystal structure and space group *P*2<sub>1</sub>/<c>. Rietveld refinements (Figure 1 (b)) of the crystal structure of the sample after laser heating at 23.5 GPa corroborate that this Na<sub>3</sub>WH<sub>9</sub>–II' phase can be indexed to the monoclinic (*P*2<sub>1</sub>/<c>) structure (a = 4.951 Å, b = 10.488(3) Å, c = 8.570(2) Å and β = 54.4(3) °C; R<sub>wp</sub> = 32.5%). We will refer to this distorted cubic phase as Na<sub>3</sub>WH<sub>9</sub>–II'. Similarly, when the NaH–W–H<sub>2</sub> sample is compressed to 33.5 GPa and laser heated to 1400 K, Na<sub>3</sub>WH<sub>9</sub>–II' is also formed (Figure S4(a,b)).

<figure><img src="image_11.png" /><figcaption>**Figure 1.** Rietveld refinements of powder XRD patterns at different pressures in two different experiments (λ = 0.2904 and 0.2914 Å). (a, b) Show the sample before and after laser heating, respectively. (c, d) Correspond to different phases observed upon decompression. The reflection labeled with an asterisk is associated with the rhenium gasket. Experimental data is depicted with black dots. The difference between the experimental and calculated pattern is depicted with a pink line. The calculated pattern for each individual phase is represented with its corresponding colored line, which is indicated on the legend. The March–Dollase model was used to account for preferred orientations. Pressures and both R<sub>wp</sub> and R<sub>p</sub> factors are shown on each plot. The calculated positions for the Bragg reflections of each phase are depicted with vertical ticks. The refined parameters for the Na<sub>3</sub>WH<sub>9</sub> phases shown in (b–d) are shown in Tables S1–S3, respectively.</figcaption></figure>

Upon compression up to 42 GPa, Na<sub>3</sub>WH<sub>9</sub>–II' remains stable and no features of other Na–W–H phases are observed (Figure S5(c)).

Upon decompression of Na<sub>3</sub>WH<sub>9</sub>–II', we observe the merging of some of its Bragg reflections at 6.4 GPa (Figures 1(c) and S1 and S5), indicating that the structure undergoes a phase transition to fcc Na<sub>3</sub>WH<sub>9</sub>–II, a more symmetric structure. At 5.5 GPa the XRD patterns are dominated by Na<sub>3</sub>WH<sub>9</sub>–II. When Na<sub>3</sub>WH<sub>9</sub>–II is further decompressed to 3 GPa, it undergoes a phase transition to Na<sub>3</sub>WH<sub>9</sub>–I, adopting a hexagonal structure (Figure 1(d)).<sup>26</sup> The patterns of Na<sub>3</sub>WH<sub>9</sub>–I can be indexed to a 3Na:W metallic sublattice with space groups *P*6<sub>3</sub>/<mmc<sup>26</sup> or *P*6<sub>3</sub>/<cm<sup>28</sup> similar to those reported for Na<sub>3</sub>As at low pressures. After considering hydrogen occupancies, DFT calculations reduce the total symmetry of the crystal to *P*6<sub>3</sub>/<m>. A Rietveld refinement using a structure with
space group *P6<sub>3</sub>/<sub>m</sub> shows good compatibility with the experimental data (*a* = 5.590(2), *c* = 10.059(2) Å; *R*<sub>wp</sub> = 37.7%), **Figure 1** (d). All the phases found for Na<sub>3</sub>WH<sub>9</sub>−II' (42−6.4 GPa), −II (6.4−3 GPa) and −I (below 3 GPa) are characterized by [WH<sub>9</sub>]<sup>3−</sup> anionic units. These accommodate 18 valence electrons, with tungsten achieving its highest (and most common) oxidation state (+6).

In a different set of experiments, we loaded NaH, Re and H₂ gas at 0.2 GPa in a DAC. The sample was compressed to 32.5 GPa and laser-heated using the rhenium metal as a coupler. XRD patterns of the sample before and after heating are shown in Figure 2(a,b). As in Na₃WH₉−II', the diffraction peak

<figure><img src="image_12.png" /><figcaption>**Figure 2.** XRD patterns and corresponding Rietveld refinements of the Na−Re−H sample before laser heating (a), after laser heating (b) and subsequently decompressed down to 14.6 (c) and 1.8 GPa (d). Experimental data is represented with black dots. The difference between these and the refinement is depicted with a pink solid line. The rest of the colored solid lines correspond to the calculated partial contributions the phases indicated on the legends. Vertical ticks denote the position of the Bragg reflections. The refined parameters for the Na<sub>3</sub>ReH<sub>8</sub> phases shown in (b–d) are presented in Tables S4–S6, respectively.</figcaption></figure>

distribution of the product, Na₃ReH₈−II', resembles those of an fcc lattice, the Bragg peaks of the phase synthesized after laser heating at 32.5 GPa are not symmetric and can be more accurately described by an orthorhombic distortion of the structure. In fact, our DFT calculations reduce the fcc Heusler of Na₃ReH₈ to a slightly distorted variant, which shows a reasonably good compatibility with our data, as shown in the Rietveld refinement depicted in Figure 2 (b) (*Pca*2₁; *a* = 9.711(1) Å, *b* = 6.898(1) Å and *c* = 9.607(2) Å; *R*<sub>wp</sub> = 33.3%). We suggest the formula Na₃ReH₈, as only in the [ReH₈]<sup>3−</sup> ions can accommodate 18 valence electrons. Hence, whereas W is in its highest oxidation state +6, Re can only achieve its +5 oxidation state in Na−TM−H compounds.

As in Na₃WH₉, Na₃ReH₈−II' transitions to fcc Na₃ReH₈−II (Figures 2(c) and S6(a,b)): upon decompression at around 17

GPa, the (020) and (202) reflections of Na₃ReH₈−II' merge into a single (200) reflection characteristic of an fcc phase). At 1.8 GPa, the XRD patterns of Na₃ReH₈ present a different peak distribution that corresponds to a new low-pressure phase Na₃ReH₈−I, **Figure 2**(d). Similarly to Na₃WH₉−I, we initially considered a highly symmetric hexagonal lattice for Na₃ReH₈−I, which our DFT calculations reduced to a Pbca symmetry. Rietveld refinements in **Figure 2**(d) show a reasonably good compatibility with this structural model (*a* = 9.511(7) Å, *b* = 9.393(2) Å, *c* = 11.002(9) Å; *R*<sub>wp</sub> = 32.6%). In this structure, the Re atoms lay on a quasi-hexagonal sublattice, similar to that of Na₃WH₉−I. Laser heating the reactants at 25.5 GPa yields identical products and phase transformations under decompression (see **Figure S6**(c)).

**Figure 3**(a,b) present lattice parameters and unit-cell volumes per formula unit as a function of pressure for the Na₃WH₉ and

<figure><img src="image_13.png" /><figcaption>**Figure 3.** Pressure dependence of the lattice parameters and volumes per formula unit of (a) Na₃WH₉ and (b) Na₃ReH₈. Circles, squares and diamonds correspond to the data collected during different sets of experiments. The II', II, and I phases are indicated with red, green and blue symbols, respectively. Our calculated DFT values are depicted with black solid lines. Colored dashed lines correspond to the sum of the reactant volumes (NaH, TM, H₂)<sup>29−32</sup> for different quantities of hydrogen (values of *n*). The discontinuity in these lines correspond to the NaH-I to NaH-II phase transition at ~29 GPa.<sup>29</sup></figcaption></figure>

Na₃ReH₈ phases. The unit-cell volumes are compared with the sum of the individual volumes of the reactants, considering different hydrogen amounts.<sup>29−32</sup> In all cases, the experimental volumes for both Na−W−H and Na−Re−H compounds are smaller than the sums of the individual volumes of the reagents i.e., all the suggested stoichiometries fulfill Le Châtelier's principle regarding volume change upon reaction. Despite hydrogen disorder is expected in the fcc phases, the lattice parameters of the H-ordered phases are in reasonably good agreement with the experimental results (Figure 3). The subgroup axis transformation relating the *P*2₁/*c* space group to the Heusler structure is (*a*, *b*, *c* sin β) = (*a*<sub>fcc</sub>/√2, *a*<sub>fcc</sub>/√2, *a*<sub>fcc</sub>). For *Pca*2₁, this relationship is (*a*, *b*, *c*) = (*a*<sub>fcc</sub>/√2, *a*<sub>fcc</sub>, *a*<sub>fcc</sub>/√2). Random preferred orientations in our patterns lead to some deviations between the refined (Tables S1–S11) and DFT calculated (Tables S12–S15) atomic coordinates at similar pressures.

**Figure 4** shows ΔΗ(*P*), the relative enthalpy curves between 0 and 40 GPa for the Na−W−H and Na−Re−H systems calculated with DFT. The formation enthalpies of the reaction
<figure><img src="image_14.png" /><figcaption>**Figure 4.** $\Delta H$ vs $P$ curves of the (a) Na-W-H and (b) Na-Re-H systems. The color corresponding to each phase is indicated in the legend. The difference in enthalpy was calculated with respect to the enthalpy of the separate reactants i.e., $H(\text{NaH}) + H(\text{TM}) + H(\text{H}_2)$. Enthalpy units are given in eV per formula unit (f.u.).</figcaption></figure>

products were calculated with respect to the sum of the enthalpies of NaH, TM (W, Re) and H₂. The dynamic stability of the II' and I phases of Na₃WH₉ and Na₃ReH₈ was confirmed by DFT (see calculated phonon bands in Figure S7). The fcc structures Na₃WH₉-II and Na₃ReH₈-II can only be stabilized

dynamically, i.e., through MD simulations, therefore DFT calculations at T = 0 K reduce the Heusler fcc structure to the P2₁/c (Pca2₁) symmetry. For the Na-W-H system, Na₃WH₉ is predicted to have lower enthalpy than its separate reactants across the considered pressure range (P < 40 GPa). Below 8 GPa, the hexagonal Na₃WH₉-I (P6₃/m) phase becomes more stable than the Na₃WH₉-II' (distorted cubic P2₁/c) phase, in good agreement with the experimental transition between 5.5 and 3 GPa during decompression. In the Na-Re-H system, (Figure 4(b)) both Na₃ReH₈-I (Pbca) and Na₃ReH₈-II' (Pca2₁) show negative values of $\Delta H$ in the 0 to 40 GPa range. DFT predicts the transition from the high- to the low-pressure phase below 9 GPa, supporting our experimental observations. Interestingly, Na₃WH₉-I and Na₃ReH₈-I both exhibit negative values of $\Delta H$ at low pressures, indicating their stability at ambient conditions. However, relative Gibbs free energies $\Delta G$ (not calculated here), particularly when considering hydrogen, might favor their decomposition. Unfortunately, the product reacts with the moisture in the air when the DAC is opened, so its stability at ambient conditions was not explored.

The DFT calculated crystal structures of the high pressure phases, Na₃WH₉-II' (P2₁/c) and Na₃ReH₈-II' (Pca2₁), (assuming static positions for hydrogen atoms i.e., full occupancies) is depicted in Figure 5(a,b), respectively. Na₃WH₉-II' consists of an arrangement of Na⁺ cations and [WH₉]³⁻ anionic units, in which the Na and W atoms form a slightly distorted Heusler fcc structure. Crystallographic data for the high-pressure phases Na₃WH₉-II' and Na₃ReH₈-II' obtained from DFT calculations are given in Tables S12 and S13, respectively.

Figure 5(c) shows the unit-cell corresponding to the Na₃WH₉-II structure (fcc) obtained after MD calculations in which H atoms have 1/2 partial occupations. This Heusler arrangement (fcc) has been reported for Na₃FeH₇, Na₃CoH₆, Na₃NiH₅ and K₃ReH₆.¹⁴,²¹,³³ As shown in Figure S8(a), hydrogen atoms are delocalized around the transition metal atoms, occupying spherical shells due to rotation of the [WH₉]³⁻ anions. The anion complex' rotations are not fully free, and hydrogen angular distribution is uneven. The calculated time

<figure><img src="image_15.png" /><figcaption>**Figure 5.** Crystal structures of Na₃WH₉ and Na₃ReH₈ obtained in calculations: (a) Na₃WH₉-II' (P2₁/c, DFT), (b) Na₃ReH₈ (Pca2₁, DFT), (c) Na₃WH₉-II (fcc, MD), (d) Na₃WH₉-I (P6₃/m, DFT) and (e) Na₃ReH₈-I (Pbca, DFT). Yellow, gray, purple and white spheres represent Na, W, Re, and H atoms, respectively. Gray and purple polyhedra represent [WH₉]³⁻ and [ReH₈]³⁻.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Figure 6.** Raman spectra of the (a) Na₃WH₉ (decompression) and (b) Na₃ReH₈ (compression) phases. The spectra corresponding to the II', II, and I phases are depicted with red, green and blue lines, respectively. The bands indicated with the labels "D1" and "D2" correspond to the first and second Raman modes of diamond. Vertical ticks stand for the calculated frequencies of the phases indicated in the figure.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 7.** Pressure dependence of the individual Raman bands composing the [TMHₙ]³⁻ stretching modes of (a) Na₃WH₉ and (b) Na₃ReH₈. Dashed lines are a guide to the eye.</figcaption></figure>

evolution of the lattice parameters for this phase is depicted in Figure S8(b,c), showing how the structure reduces into an fcc lattice with time. Our MD calculations show the equivalent time evolution of the lattice parameters of Na₃ReH₈−II at 0, 4, 10, and 20 GPa in Figure S9. These results indicate that pressure reduces hydrogen mobility, hindering rotational degrees of freedom, eventually causing the transition from the rotationally disordered fcc (II) to the distorted high-pressure phases (II') in both Na₃WH₉ and Na₃ReH₈.

The Na₃WH₉−I phase (P6₃/m) shown in Figure 5(d) can be described as an arrangement of two types of linear structures disposed along the c-axis direction: pure Na chains and (−Na−Na−[WH₉]³⁻)− chains. The WH₉ complexes form a hexagonal close packed structure, with the Na atoms from the pure Na chains occupying centers of the in-planar triangles located in between the two octa-sites; and the remaining Na atoms occupying tetrahedral interstitial sites. The Na₃ReH₈−I phase (Pbca), whose unit-cell is depicted in Figure 5(e), can be seen as a 4-fold supercell of the hexagonal structure, for instance as (a₀,

b₀, c₀) = (a₀ + 2b₀, c₀ + 2a₀). DFT calculated crystallographic data for the two phases is given in Tables S14 and S15. The symmetry-lowering in Na₃ReH₈−I manifests in the metal lattice as in-plane displacement of the tetrahedral site Na atoms. We performed DFT-MD simulations of Na₃ReH₈−I at 2 GPa and noticed that this displacement disappeared upon heating when (simultaneously) the [ReH₈]³⁻ units became rotationally disordered (Figure S10). Hence, we expect that at low pressure but high temperature, Na₃ReH₈ also takes up a hexagonal phase; at low temperature, a disorder-order transition occurs as the [ReH₈]³⁻ complex is not compatible with hexagonal symmetry.

**Raman Spectroscopy Experiments.** As illustrated in Figures 6(a) and S11(a), the Raman spectra of Na₃WH₉−II' exhibit prominent bands in the 1000−1300 and 1900−2500 cm⁻¹ ranges, which correspond to the bending and stretching modes of the [TMHₙ]³⁻ anionic units.¹⁷,²⁰,³⁴−³⁶ Particular attention is given to the 1900−2500 cm⁻¹ region, indicative of the TM−H stretching mode. Density functional theory (DFT) predicts that the Raman spectra are composed of numerous
closely spaced contributions. By analyzing the second derivative of the experimentally obtained spectra at 41 GPa, we identified three distinct components for Na₃WH₉-II' (see Figure S12(a)). Upon decompression from 16.5 to 6.5 GPa, significant spectral changes are observed in the TM-H stretching modes, transitioning from three distinct components to one intense peak with a low-frequency shoulder. This is related with the Na₃WH₉-II' to Na₃WH₉-II phase transition. Further decompression to 3.5 GPa results in an additional splitting of these TM-H stretching modes into five components, Figure S12(c), which is attributed to the transition to the Na₃WH₉-I phase, which aligns well with the DFT-predicted modes illustrated in Figure 6(a).

Analogous to the Na-W-H system, the Raman spectra of Na₃ReH₈-II' exhibit two principal Raman active regions around 1100 and 2100 cm⁻¹, corresponding to the transition metal-hydrogen (TM-H) bending and stretching modes, respectively (Figures S11(b) and 6(b)). Similar to Na₃WH₉-II', the Raman spectra of Na₃ReH₈-II' at approximately 30 GPa in the TM-H stretching region are characterized by multiple contributions predicted by DFT. Through second derivative analysis of the experimental spectrum, we resolved three contributions (Figure S12(d)). Additionally, between 20 and 16 GPa, coinciding with the phase transition to Na₃ReH₈-II, the spectra simplify, particularly in the TM-H modes region, which reduce from three to two contributions (Figure S12(e)). During compression from 5.8 to 10.5 GPa (Figure 6(b)) and decompression from 3.7 to 3.2 GPa (Figure S12(b)), coinciding with the phase transition to Na₃ReH₈-I, the spectra undergo notable changes. These changes are especially relevant in the TM-H stretching region, which becomes more complex with over ten contributions predicted by DFT, experimentally resolved as three (Figure S12(f)).

The pressure evolution of the TM-H stretching modes contributions is depicted in Figure 7(a),(b) for Na₃WH₉ and Na₃ReH₈, respectively. There is no significant effect of the transition metal (TM) on the Raman shift of the TM-H stretching modes, as both Re and W are located in the same spectral region. Consistent with other ternary transition metal polyhydrides, regardless of the phase, all spectral contributions in both Na₃WH₉ and Na₃ReH₈ upshift with pressure, which is related to the shortening of the TM-H bond under pressure.<sup>12,20</sup> The lower frequency of the TM-H stretching modes associated with the low-pressure hexagonal phases, Na₃WH₉-I and Na₃ReH₈-I, is related to a decrease in the electronic density of the TM-H bond, leading to an expansion of the [TMH<sub>n</sub>]<sup>3-</sup> unit. A similar analysis was carried out for the H-TM-H bending modes of Na₃WH₉ and Na₃ReH₈, shown in Figure S13(a,b), respectively.

To explore the stability of Na₃WH₉, a NaH-W-H₂ sample was laser heated at different pressures. Laser heating at 1 GPa and 1400 K did not lead to any spectral changes, indicating no reaction between the precursors. However, laser heating to approximately 1400 K at 7.8 GPa induced spectral changes related to the formation of Na₃WH₉-II, as shown in Figure S14(a). Analogous experiments were conducted for the NaH-Re-H₂ system. Laser heating at 1 GPa and 1400 K did not result in the formation of Na₃ReH₈. It was only at pressures above 10 GPa and temperatures of 1400 K that the spectral features of Na₃ReH₈-II were identifiable upon quenching, Figure S14(b).

**Discussion.** The synthesis of Na₃WH₉ and Na₃ReH₈ demonstrates the feasibility of forming Na-TM-H compounds with TM of groups 6 and 7 using high pressures and laser heating

techniques. Stable novel Na-TM-H compounds featuring d⁰ [WH₉]<sup>3-</sup> and d² [ReH₈]<sup>3-</sup> homoleptic anions are proven. These anionic units accommodate 18 valence electrons and may be common to other Na-TM-H compounds with TMs of groups 6 and 7. [ReH₈]<sup>3-</sup> represents an interesting case, as this coordination is highly unusual in CTMHs, in which the highest coordination of TM elements is usually either 7 or 9.<sup>13</sup> In fact, only few systems are known to host 8-coordinated TM atoms, such as Cs₃OsH₉ and Rb₃OsH₉.<sup>19,37,38</sup> Other systems with similar stoichiometries to Na₃ReH₈, such as Mg₃CrH₈, host [CrH₇]<sup>5-</sup> anions and interstitial H<sup>-</sup>.<sup>24,25</sup>

As mentioned in Na₃WH₉-II and Na₃ReH₈-II, the [WH₉]<sup>3-</sup> and [ReH₈]<sup>3-</sup> anions are in freely rotating states. Low temperatures and high pressures can induce similar distortions, as both restrict dynamic effects. For instance, it has been observed that in Na₃NiH₅ the fcc structure observed at 5 GPa and 700 K becomes a quasi-cubic orthorhombic structure when quenched at room temperature. Likewise, reversible transitions from rotational to nonrotational states are the foundation of thermal energy storage in solid-solid phase change materials (ss-PCM's),<sup>39,40</sup> which in some cases exhibit crystallographic transformations similar to those seen in the hydride systems in this study.<sup>41</sup>

Both, Na₃WH₉ and Na₃ReH₈ remain transparent within the whole experimental pressure range. To check for possible metallic phases, we calculated the pressure evolution of the electronic band gap of Na₃WH₉-II' and Na₃ReH₈-II', (Figure S15). The band gap of Na₃WH₉-II' (P2₁/c) is predicted to progressively close with increasing pressure, becoming metallic above 180 GPa. Na₃ReH₈-II' is expected to have a smaller band gap than Na₃WH₉-II' below 100 GPa, but it would only decrease substantially above 350 GPa, suggesting that metallicity can only be achieved at extremely high pressures (over 500 GPa). The main reason for the lack of metallicity at more attainable pressures is that [TMH<sub>x</sub>]<sup>3-</sup> anions are disconnected and considerably separated from each other inside the reported structures, which prevents the formation of a monatomic hydrogen framework.<sup>2</sup>

## ■ CONCLUSIONS

By combining both XRD and Raman scattering techniques we explore the Na-W(Re)-H ternary systems under pressure. We observed the formation of distinct high-hydrogen content structures that we investigated upon decompression. Na₃WH₉ can be synthesized above 7.8 GPa after laser heating, featuring 18-electron [WH₉]<sup>3-</sup> homoleptic units. In the Na-Re-H system, Na₃ReH₈ was synthesized at 32.5 and 10.1 GPa, with [ReH₈]<sup>3-</sup> units. Upon decompression from about 40 GPa, these structures undergo analogous phase transitions: II'(distorted fcc) → II (Heusler fcc) → I (distorted hcp). The specific distortion scenarios and transition pressures depend on the transition metal. Experiments and theoretical calculations indicate that these ternary compounds are stable to pressures below 1 GPa, making them good candidates for hydrogen storage materials, however, Na₃WH₉ and Na₃ReH₈ are electrical insulators with wide bandgaps. The thermodynamic stability of the different observed polymorphs was studied via DFT and MD calculations, showing excellent compatibility with the experimentally reported structural transformations.
■ ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c02691.

Description of the experimental and computational
details, (ii) additional XRD and Raman data from
different samples, (iii) results on MD calculations, (iv)
calculated pressure evolution of the band gap of Na₃WH₉
and (v) DFT calculated atomic positions for different
phases (PDF)

Accession Codes

Deposition Numbers 2367343–2367344 and 2383663–2383664 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.

■ AUTHOR INFORMATION

Corresponding Author

Miriam Peña-Alvarez – Center for Science at Extreme
Conditions (CSEC) and the School of Physics and Astronomy,
The University of Edinburgh, Edinburgh EH3 9FD, U.K.;

● orcid.org/0000-0001-7056-7158;

Email: miriam.pena.alvarez@ed.ac.uk

Authors

Tomas Marqueño – Center for Science at Extreme Conditions (CSEC) and the School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH3 9FD, U.K.;

● orcid.org/0000-0001-5371-1082

Israel Osmond – Center for Science at Extreme Conditions (CSEC) and the School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH3 9FD, U.K.;

● orcid.org/0000-0001-8756-8501

Mikhail A. Kuzovnikov – Center for Science at Extreme Conditions (CSEC) and the School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH3 9FD, U.K.;

● orcid.org/0000-0003-4030-4861

Hannah A. Shuttleworth – Center for Science at Extreme Conditions (CSEC) and the School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH3 9FD, U.K.;

● orcid.org/0000-0002-5113-1575

Samuel Gallego-Parra – European Synchrotron Radiation
Facility (ESRF), Grenoble 38000, France

Eugene Gregoryanz – Center for Science at Extreme Conditions (CSEC) and the School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH3 9FD, U.K.; Key Laboratory of Materials Physics, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China; Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai 201203, China

Andreas Hermann – Center for Science at Extreme Conditions (CSEC) and the School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH3 9FD, U.K.;

● orcid.org/0000-0002-8971-3933

Ross T. Howie – Center for Science at Extreme Conditions (CSEC) and the School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH3 9FD, U.K.; Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai 201203, China; ● orcid.org/0000-0002-7013-8211

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.4c02691

Notes

The authors declare no competing financial interest.

For the purpose of open access, the authors have applied a
Creative Commons Attribution (CC BY) license to any Author
Accepted Manuscript version arising from this submission.

■ ACKNOWLEDGMENTS

The work was supported by the UKRI Future Leaders fellowship
Mrc-Mr/T043733/1 and by the European Research Council
(ERC) under the European Union's Horizon 2020 research and
innovation programme (Grant agreement no. 948895, MetE-
One). The authors thank Prof. M. McMahon for allowing us to
use the glovebox of his laboratory to load the samples. We
acknowledge DESY (Hamburg, Germany), a member of the
Helmholtz Association HGF, for the provision of beamline
P02.2 at PETRA-III, allocated under proposals I-20220830, I-
20221042, I-20221366 and I-20230232. We would like to thank
beamline scientists Nico Giordano, Konstantin Glazyrin and
Hanns-Peter Lierman for their help during the experiments. The
ESRF (Grenoble, France) is acknowledged for providing access
to beamline ID15B for the experiment HC-5457. We would like
to thank beamline scientists Michael Hanfland, Gaston
Garbarino, Mohamed Mezouar and Samuel Gallego-Parra.
Computational resources provided by the UK's National
Supercomputer Service through the UK Car-Parrinello HEC
consortium (EP/X035891/1) and by the UK Materials and
Molecular Modelling Hub (EP/P020194) are gratefully
acknowledged.

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