# Hypervalent hydridosilicate in the Na–Si–H system

Kristina Spektor, Holger Kohlmann, Dmitrii Druzhbin, Wilson A. Crichton, Shrikant Bhat, Sergei I. Simak, Olga Yu Vekilova, Ulrich Häussermann

► To cite this version:

Kristina Spektor, Holger Kohlmann, Dmitrii Druzhbin, Wilson A. Crichton, Shrikant Bhat, et al. Hypervalent hydridosilicate in the Na–Si–H system. Frontiers in Chemistry, 2023, 11, pp.1251774. 10.3389/fchem.2023.1251774. hal-05095248

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EDITED BY
Gunter Heymann,
University of Innsbruck, Austria

REVIEWED BY
Julien Haines,
Centre National de la Recherche
Scientifique (CNRS), France
Christian Tantardini,
UiT The Arctic University of Norway,
Norway

*CORRESPONDENCE

Kristina Spektor,

kristina.spektor@desy.de

Ulrich Häussermann,

ulrich.haussermann@mmk.su.se

RECEIVED 02 July 2023

ACCEPTED 17 August 2023

PUBLISHED 08 September 2023

CITATION

Spektor K, Kohlmann H, Druzhbin D,
Crichton WA, Bhat S, Simak SI,
Vekilova OY and Häussermann U (2023),
Hypervalent hydridosilicate in the
Na–Si–H system.

Front. Chem. 11:1251774.
doi: 10.3389/fchem.2023.1251774

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© 2023 Spektor, Kohlmann, Druzhbin,
Crichton, Bhat, Simak, Vekilova and
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which does not comply with these terms.

Hypervalent hydridosilicate in the
Na–Si–H system

Kristina Spektor<fnref n="1" />,<fnref n="2" />*, Holger Kohlmann<fnref n="1" />, Dmitrii Druzhbin<fnref n="3" />,
Wilson A. Crichton<fnref n="3" />, Shrikant Bhat<fnref n="2" />, Sergei I. Simak⁴,⁵,
Olga Yu Vekilova<fnref n="6" /> and Ulrich Häussermann<fnref n="6" />*

Hydrogenation reactions at gigapascal pressures can yield hydrogen-rich materials with properties relating to superconductivity, ion conductivity, and hydrogen storage. Here, we investigated the ternary Na–Si–H system by computational structure prediction and *in situ* synchrotron diffraction studies of reaction mixtures NaH–Si–H₂ at 5–10 GPa. Structure prediction indicated the existence of various hypervalent hydridosilicate phases with compositions Na<sub>m</sub>SiH<sub>(4+m)</sub> (m = 1–3) at comparatively low pressures, 0–20 GPa. These ternary Na–Si–H phases share, as a common structural feature, octahedral SiH<sub>6</sub><sup>2-</sup> complexes which are condensed into chains for m = 1 and occur as isolated species for m = 2, 3. *In situ* studies demonstrated the formation of the double salt Na<sub>3</sub>[SiH<sub>6</sub>]<sup>H</sup> (Na<sub>3</sub>SiH<sub>7</sub>, m = 3) containing both octahedral SiH<sub>6</sub><sup>2-</sup> moieties and hydridic H<sup>−</sup>. Upon formation at elevated temperatures (>500°C), Na<sub>3</sub>SiH<sub>7</sub> attains a tetragonal structure (P4<sub>1</sub>/mbm, Z = 2) which, during cooling, transforms to an orthorhombic polymorph (Pbam, Z = 4). Upon decompression, Pbam-Na<sub>3</sub>SiH<sub>7</sub> was retained to approx. 4.5 GPa, below which a further transition into a yet unknown polymorph occurred. Na<sub>3</sub>SiH<sub>7</sub> is a new representative of yet elusive hydridosilicate compounds. Its double salt nature and polymorphism are strongly reminiscent of fluorosilicates and germanates.

KEYWORDS

hydridosilicate, gigapascal hydrogenation, multi-anvil techniques, crystal structure prediction, hypervalency

**1 Introduction**

The alkali metal A–Si–H systems (A = Li, Na, and K) have recently attracted attention because computational predictions suggested the existence of hydrogen-rich ternary phases at high pressures with potentially superconducting, superionic, and/or hydrogen storage properties (Liang et al., 2020; Zhang et al., 2020; Liang et al., 2021; Wu et al., 2022; Xie et al., 2022). According to these predictions, at lower pressures (up to 50 GPa), A–Si–H systems commonly possess a stable compound, A<sub>2</sub>SiH<sub>6</sub>, featuring octahedral SiH<sub>6</sub><sup>2-</sup> species in which Si attains a hypervalent bonding situation. Na<sub>2</sub>SiH<sub>6</sub> was suggested being a H<sup>−</sup> superionic conductor in the pressure range 4–10 GPa (Liang et al., 2021) and at temperatures around 1,000°C, and K<sub>2</sub>SiH<sub>6</sub> has been attributed favorable H-storage properties (Xie et al., 2022).

At higher pressure (50–200 GPa), more varied compositions become stable, with structures where octahedral units are connected and/or H₂ molecules are additionally

<footnote n="1">¹Inorganic Chemistry, Faculty for Chemistry and Mineralogy, Leipzig University, Leipzig, Germany,</footnote>

<footnote n="2">²Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany,</footnote>

<footnote n="3">³ESRF-The European Synchrotron
Radiation Facility, Grenoble, France,
⁴Theoretical Physics Division, Department of Physics, Chemistry and
Biology (IFM), Linköping University, Linköping, Sweden,
⁵Department of Physics and Astronomy, Uppsala</footnote>

<footnote>University, Uppsala, Sweden,</footnote>

<footnote n="6">⁶Department of Materials and Environmental Chemistry, Stockholm
University, Stockholm, Sweden</footnote>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

GRAPHICAL ABSTRACT

incorporated (i.e., KSiH<sub>7</sub>, KSiH<sub>8</sub>, K<sub>2</sub>SiH<sub>8</sub>, Na<sub>2</sub>SiH<sub>14</sub>, and Na<sub>3</sub>SiH<sub>10</sub>)
(Liang et al., 2021; Xie et al., 2022). For A = Li, the structures of the
predicted phases deviate frequently from the octahedral theme
(Liang et al., 2020; Zhang et al., 2020). Also, for Li–Si–H, there
seems to be a greater variety of stable hydrogen-rich structures and
compositions at high pressures (e.g., LiSiH<sub>5</sub>, LiSiH<sub>6</sub>, Li<sub>3</sub>SiH<sub>10</sub>,
Li<sub>2</sub>SiH<sub>10</sub>, and Li<sub>2</sub>SiH<sub>12</sub>). LiSi<sub>2</sub>H<sub>9</sub> and LiSiH<sub>8</sub> were predicted to
become good phonon-mediated superconductors at pressures
above 170 GPa (Liang et al., 2020).

Despite the interesting results from computational structure
prediction, hitherto only polymorphic K₂SiH₆ has been reported
from experimental high-pressure investigations (Puhakainen et al.,
2012; Vekilova et al., 2023). This compound is stable even at ambient
pressure where it adopts the cubic K₂PtCl₆ structure. At pressures
above 8 GPa, K₂SiH₆ crystallizes in a trigonal structure (Vekilova
et al., 2023).

Here, we report on the re-examination of the Na–Si–H system
by crystal structure prediction and *in situ* studies of reactions m
NaH + Si + 4 H₂ (m = 1, 2) at pressures up to 10 GPa. For this, we
employed a large-volume press (LVP) high pressure methodology
which provides well-controlled *p*, *T* environments for high pressure
hydrogenation reactions (Spektor et al., 2020a; Spektor et al., 2020b).
Our initial intention was to obtain the predicted (superionic)
compound Na₂SiH₆ with a simple P3\1m structure (Liang et al.,
2021), which is also the structure of the high pressure polymorph of
K₂SiH₆ (Vekilova et al., 2023). However, in contrast with earlier
reports, we find evidence for the existence of multiple Na–Si–H
phases along the composition line Na_mSiH_(4+m) (m = 1, 2, 3) at
comparatively low pressures up to 20 GPa, among which
polymorphic Na₃SiH₇ was experimentally observed.

**2 Materials and methods**

**2.1 High pressure experiments and data analysis**

All steps of sample preparation and recovery were performed in
a glove box under argon atmosphere. Powdered NaH (Sigma-
Aldrich, 90%) and powdered Si [325 mesh, 99.999% (metals

basis), Thermo Scientific] were mixed at a molar ratio of 1:1 and
2:1 (NaH:Si) and compressed into pellets with an outer diameter
(OD) of 2 mm. Ammonia borane (BH3NH3, Sigma-Aldrich, 90%)
served as a hydrogen source since it has a well-defined
decomposition behavior at high pressures and produces
chemically inert BN as a residue (Nylén et al., 2009). The
amount of BH3NH3 used for each sample corresponded to an
approx. 4× molar excess of H2 with respect to Si. NaH/Si sample
pellets were sandwiched between pelletized BH3NH3 and sealed
inside NaCl capsules with 3.0 mm OD.

In situ synchrotron diffraction high-pressure experiments were
performed at beamline ID06-LVP at the ESRF and employed 14/
7 multi-anvil assemblies, which are described in detail elsewhere
(Vekilova et al., 2023). Amorphous SiBCN rods and either MgO or
amorphous BCN epoxy were used as X-ray windows in the
octahedra and gaskets, respectively, along the beam direction.
Assemblies were compressed to target pressures ≈5 and ≈9 GPa
and heated in a Voggenreiter-built modified-cubic press (Guignard
and Crichton, 2015). Pressure was estimated in situ from PXRD
diffraction patterns using the equation of state of NaCl by Matsui
et al. (2012). The temperature was evaluated from power-T
calibration curves. Angle-dispersive powder X-ray diffraction
patterns were collected in the 1.27°–15.26° 2θ range at a constant
wavelength (λ = 0.233933 Å). Data were acquired using the
Pilatus3X-900 kW CdTe high-resolution 2D detector. The in situ
data were integrated, visualized, and manipulated using Fit2D
software (Hammersley, 2016). Indexing of the powder patterns
was performed using DICVOL and TAUP algorithms within the
CRYSFIRE package (Shirley, 2004). Le Bail fitting (Le Bail et al.,
1988) and Rietveld refinement (Rietveld, 1969) against the in situ
data were performed in Jana 2006 (Petříček et al., 2014). A detailed
description of the high-pressure experiments and data analysis is
given in Supplementary Material.

**2.2 Theoretical calculations**

The Na–Si–H system was studied with a crystal structure
prediction methodology using Ab initio random structure
searching implemented in the code AIRSS (Pickard and Needs,
2006; Pickard and Needs, 2011) and an evolutionary algorithm implemented in the Universal Structure Predictor: Evolutionary Xtallography (USPEX) code (Oganov and Glass, 2006; Oganov et al., 2011; Lyakhov et al., 2013), both coupled with the Vienna Ab Initio Simulation Package (VASP) (Kresse and Hafner, 1993; Kresse and Furthmüller, 1996). VASP calculations were based on a first-principles projector-augmented wave (PAW) method (Blöchl, 1994) within the density functional theory (DFT) (Hohenberg and Kohn, 1964; Kohn and Sham, 1965). The generalized gradient approximation for exchange and correlation potential and energy was used in its Perdew-Burke-Erzernhof (PBE) (Perdew et al., 1996; Perdew et al., 1997) flavor. H at zero pressure was calculated as a molecule (H₂) and at higher pressures as a solid with an *I4/mmm* structure (according to the work of Pickard and Needs (2007)).

All Na<sub>m</sub>SiH<sub>(4+m)</sub> (m = 1, 2, 3) model structures were fully relaxed; i.e., volume, shape, and internal atomic positions were adjusted to get nearly zero forces and stresses (stress components of order 0.1 GPa and forces of order 0.01 eV/Å maximum). The considered pressures were 0, 10, and 20 GPa. The Monkhorst-Pack (Monkhorst and Pack, 1976) k-point density for integrations over the Brillouin zone was set with 0.2. The energy cutoff for plane waves was set to 320 eV. All the static calculations were conducted at temperature T = 0 K.

Phonons were calculated from the force constants in real space obtained by VASP using density functional perturbation theory (DFPT) used as an input for the PHONOPY program (Togo and Tanaka, 2015). A plane-wave energy cutoff of 500 eV was used in all the corresponding calculations. A 2 × 2 × 2 supercell of each structure was used. To investigate the dynamical stability of P4/mbm Na<sub>3</sub>SiH<sub>7</sub> at experimentally relevant temperature of 600 K and pressure 10 GPa, ab initio molecular dynamics (AIMD) calculations, as implemented in VASP, were conducted and then post-processed by the temperature-dependent effective potential method (TDEP) (Hellman et al., 2011; Hellman et al., 2013). TDEP maps the AIMD data onto a model Hamiltonian and provides the best harmonic fit to the system of anharmonic vibrating atoms at a particular temperature. Therefore, effective temperature-dependent phonon dispersions are obtained, and in particular, dynamical stability due to anharmonic vibrations can be studied. AIMD simulation were performed in 2 × 2 × 2 supercells (176 atoms) for P4/mbm Na<sub>3</sub>SiH<sub>7</sub>, with 2 × 2 × 2 k points, and 500 eV energy cutoff for the plane waves, with the same PAW potentials and exchange-correlation as in the static calculations. The canonical NVT ensemble using a Nosé-Hoover thermostat with the default Nosé mass as set by VASP and a 0.5 fs time step was applied. The data from 600 time steps after equilibration were used for the extraction of temperature-dependent force constants.

**3 Results and discussion**

**3.1 Revisiting structure prediction for the**
**Na–Si–H system**

A recent exploration of the Na–Si–H system for stable ternary
high-pressure phases yielded only one compound, trigonal
P3m1 Na2SiH6, up to 50 GPa (Liang et al., 2021). At pressures
below 3 GPa, the proposed Na2SiH6 is not dynamically stable, i.e., its

phonon dispersion has branches with imaginary frequencies. This
indicates lattice instability, and as a consequence, there should be a
more stable structure with this composition, or decomposition into a
different composition. We re-examined structure predictions (and
also introduced a number of trial structures from chemical intuition)
allowing for Z = 4 (Z = number of formula units) and focused on the
low-pressure region up to 20 GPa which would be accessible in LVP
hydrogenations. The dominant feature of SiH6<sup>2-</sup> octahedral units in
low-pressure structures, as established from previous A–Si–H
structure prediction work (Liang et al., 2020; Liang et al., 2021;
Zhang et al., 2020; Wu et al., 2022; Xie et al., 2022), together with
chemical reasoning, suggested that compositions Na<sub>m</sub>SiH<sub>(4+m)</sub> (m =
1, 2, 3) are most plausible, and we restricted our search to these
compositions. Phase diagrams were established for 0, 10, and
20 GPa. Compared to earlier work, a radically different picture
evolved.

Figure 1 shows the phase diagrams for ambient pressure and 10 GPa (the phase diagram for 20 GPa is virtually identical to that for 10 GPa). In *Supplementary Figure S5*, phonon dispersions of some of the predicted structures are depicted. At ambient pressure (Figure 1A), a compound Na<sub>3</sub>SiH<sub>7</sub> with an orthorhombic *Pbam* structure appeared to be stable with respect to decomposition into NaH, NaSi, Si, and H<sub>2</sub> but was not dynamically stable. This suggests that the composition Na<sub>3</sub>SiH<sub>7</sub> is stable at ambient pressure but should occur in a different, most likely more complex, structure. As a matter of fact, identifying Na<sub>3</sub>SiH<sub>7</sub> as a potential stable composition at ambient pressure was rather inferred from the experimental results than initially obtained from computational structure prediction. We will discuss the structures for Na<sub>3</sub>SiH<sub>7</sub> and their stability in detail later. Furthermore, at ambient pressure, we found a hexagonal form of Na<sub>2</sub>SiH<sub>6</sub> with *P6<sub>3</sub>mc* symmetry slightly (by ≈0.05 eV/Z) more stable than P3m1-Na<sub>2</sub>SiH<sub>6</sub>. However, the enthalpy of Na<sub>2</sub>SiH<sub>6</sub> is still above the convex hull, and both polymorphs are not dynamically stable at ambient pressure (cf. *Supplementary Figures 5D, E*). The Na<sub>2</sub>SiH<sub>6</sub> structures are depicted in Figure 2.

At 10 GPa, we find that the complete sequence of compositions Na<sub>m</sub>SiH<sub>(4+m)</sub> (m = 1, 2, 3) is stable, both thermodynamically and dynamically. *Pbam*-Na<sub>3</sub>SiH<sub>7</sub>, which was found only enthalpically stable at ambient pressure, is now also dynamically stable. With pressure, the trigonal Na<sub>2</sub>SiH<sub>6</sub> polymorph stabilizes over the hexagonal one and also becomes dynamically stable. NaSiH<sub>5</sub> structures are not expected to realize separated SiH<sub>6</sub><sup>2-</sup> octahedral units but chains of corner-condensed or dimers of edge-condensed octahedra, [SiH<sub>4</sub>H<sub>2/2</sub><sup>-</sup>]<sub>∞</sub> and Si<sub>2</sub>H<sub>10</sub><sup>2-</sup>, respectively. Indeed, *Pbcm*-NaSiH<sub>5</sub>, which is isostructural to BaAlH<sub>5</sub> (Figure 2) (Zhang et al., 2002), was found on the convex hull. At the same time, a different octahedral chain structure, Pna<sub>2</sub>1-NaSiH<sub>5</sub> [isostructural to SrAlH<sub>5</sub> (Sato et al., 2018)], is only about 0.15 eV/Z less stable (Figure 2). It can be speculated that the composition NaSiH<sub>5</sub> gives rise to a manifold of enthalpically close-lying polymorphic structures (Weidenthaler et al., 2006; Shlyapnikov et al., 2018) and that, most likely, the most stable one has yet to be identified. *Supplementary Tables S3–S5* in *Supplementary Material* list the parameters of some Na<sub>m</sub>SiH<sub>(4+m)</sub> structures. It is also easy to understand that computational structure prediction meets severe difficulties when tackling ternary compositions with complex structural features (Kvashnin et al., 2020).
<figure><img src="image_5.png" /><figcaption>**FIGURE 1**
Na–Si–H phase diagram at (A) 1 atm and (B) 10 GPa. Large green circles represent compounds along the composition line mNaH + SiH₄ (m = 1, 2, 3) which are located on the convex hull. Note that Na₃SiH₇ in the here-established *Pbam* structure is not dynamically stable at 0 GPa.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**FIGURE 2**
Compilation of predicted structures for Na<sub>m</sub>SiH<sub>(4+m)</sub> (m = 1, 2) compositions. Na, Si, and H atoms are depicted as gray, yellow, and green circles, respectively. SiH<sub>6</sub><sup>2-</sup> octahedra are shown in yellow.</figcaption></figure>

## 3.2 *In situ* experiments and identification of Na₃SiH₇

Reactions m NaH + Si + 4 H₂ (m = 1, 2), targeting originally proposed Na₂SiH₆, were performed at around 5 and 9 GPa. After compressing to target pressure, the reaction mixtures were initially heated to a temperature around 400°C at which the H-source BH₃NH₃ is expected to be completely decomposed into h-BN and hydrogen fluid (Nylén et al., 2009). The samples were then equilibrated for about 15–20 min. The reaction mixtures behaved

very similar, independent of starting composition (m = 1 or 2) or pressure (5 or 9 GPa). In the following, reported results refer to m = 2 (2NaH:1Si) mixtures.

Figure 3 shows the evolution of diffraction patterns for the *p* ≈ 9 GPa run. In this experiment, one can recognize the growth of an intermediate phase above ≈380°C, which manifests as broad low-intensity diffraction peaks. The peaks could be indexed to a hexagonal unit cell (a ≈ 4.75 Å, c ≈ 7.65 Å), yet this phase remains uncharacterized due to the diffuse character and poor intensity of its reflections. Above ≈560°C, the peaks of the
<figure><img src="image_7.png" /><figcaption>FIGURE 3</figcaption></figure>

Compilation of PXRD patterns (λ = 0.233933 Å) acquired during the *p* ≈ 9 GPa experiment (2NaH:1Si starting mixture) (observed intensities are shown on the logarithmic scale). Overlain is the simulated pattern of P4/mbm-Na₃SiH₇ (shown in red, linear scale) based on the DFT optimized model structure (Supplementary Table S5D). Blue arrows mark the appearance of an intermediate hexagonal phase (a ≈ 4.75 Å, c ≈ 7.65 Å). The blue asterisk marks a peak belonging to an additional unidentified phase, growing in parallel with Na₃SiH₇ and disappearing upon heating above ≈830°C. Splitting of the tetragonal reflections upon transition to the *Pbam* phase is visible on cooling below 130°C (at the very top of the figure).

<figure><img src="image_8.png" /><figcaption>FIGURE 4</figcaption></figure>

Compilation of PXRD patterns (λ = 0.233933 Å) acquired during the *p* ≈ 5 GPa experiment (2NaH:1Si starting mixture) during cooling and decompression. Observed intensities are shown on the logarithmic scale. Data collected upon decompression (1 pattern/min) are highlighted in green. The transition of P4/mbm-Na₃SiH₇ to a *Pbam* polymorph occurs below 120°C, and a further transition to an unidentified low-pressure polymorph is visible below ≈4.3 GPa (both events are marked with red dashed lines). Overlain is the simulated pattern of P4/mbm-Na₃SiH₇ (shown in red, linear scale) based on the refined structure model (Supplementary Tables S1, S2). Mismatch between observed and calculated intensities is due to texture which developed in Na₃SiH₇ above ≈770°C (cf. Supplementary Figure S4).

intermediate phase are superseded by another set of reflections, which were indexed to a primitive tetragonal unit cell with *a* ≈ 6.59 and *c* ≈ 4.78 Å (at ≈7.2 GPa and 770°C, as observed in the *p* ≈ 5 GPa run). *P4/mbm* was derived as the highest applicable space group from the extinction symbol *P*–*b*–. The symmetry and lattice parameter ratio (c/a ≈ 0.725) suggested an isostructural relation to *K*₃*SiF*₇ (c/a = 0.719 at ambient pressure) (Deadmore and Bradley, 1962; Hofmann and Hoppe, 1979) and, thus, a composition *Na*₃*SiH*₇. Calculated reflection intensities for *K*₃*SiF*₇-type *Na*₃*SiH*₇ matched closely with the experiment. Furthermore, with the knowledge of composition and formula units, *P4/mbm-Na*₃*SiH*₇ could be obtained as a low-enthalpy phase from the computational structure prediction methodology. The DFT optimized structure (Supplementary Tables S5D, E) was then used in Rietveld analysis of the *in situ* PXRD data (≈7.2 GPa, 770°C), yielding a reasonable fit (Robs≈5.5%) despite the low phase fraction of *Na*₃*SiH*₇ (Supplementary Figure S3). Details of the refinement process as well as the corresponding plot and extracted structural data are provided in Supplementary Material (Supplementary Tables S1, S2).

The formation of *P4/mbm-Na*₃*SiH*₇ during hydrogenations of NaH–Si mixtures requires pressures above 5 GPa (cf. Supplementary Figures S2, S4). In the *p* ≈ 5 GPa hydrogenation experiment, the intermediate hexagonal phase was not seen. Diffraction peaks from *P4/mbm-Na*₃*SiH*₇ become noticeable above ≈490°C. Irrespective of pressure, the *P4/mbm-Na*₃*SiH*₇ phase grows very sluggishly even when raising

temperatures up to 850°C. This indicates a rather high thermal stability of *P4/mbm-Na*₃*SiH*₇ at high-pressure conditions and shows at the same time that elemental Si represents a rather unreactive precursor.

Upon cooling from (high) synthesis temperatures to below 120°C, the tetragonal diffraction pattern showed a clear splitting into an orthorhombic one, which appeared continuous and was accompanied with a doubling of unit cell (*a*₀ ≈ *b*₀ = √2*a*₀, *c*₀ ≈ *c*₀). For the *p* ≈ 9 GPa run, this feature is included in Figure 3. For the *p* ≈ 5 GPa experiment, this tetragonal-to-orthorhombic transition is shown in Figure 4. The reflections were indexed to a unit cell *a* ≈ 9.23 Å, *b* ≈ 9.36 Å, and *c* ≈ 4.76 Å (at *p* ≈ 5.2 GPa, ≈RT). Again, suspecting analogy with fluorides, the (NH₄)₃GeF₇ structure (*Pbam* space group symmetry, Z = 4) (Mel'nikova et al., 2016; Mel'nikova et al., 2017; Bogdanov et al., 2019) was assigned to the orthorhombic phase.

In both 5 and 9 GPa runs, a further transition was noticeable upon decompression below ≈4.5 GPa, see again Figure 4 (and Supplementary Material for details). Indexing of the diffraction peaks arising at ≈4 GPa suggested as best-fitting candidates two primitive tetragonal unit cells (parameters are given relative to the initial tetragonal phase): a) *a* ≈ √2*a*₀, *c* ≈ 2*c*₀ (a ≈ 9.42 Å, *c* ≈ 9.61 Å, Z = 8); b) *a* ≈ *a*₀, *c* ≈ 2*c*₀ (a ≈ 6.66 Å, *c* ≈ 9.61 Å, Z = 4). The structure was difficult to resolve due to peak overlap as well as the occurrence of sudden pressure drops during decompression which obscured analysis. Further structure prediction work would be required to establish the structure and corresponding space group. Also, it remains unclear whether Na₃SiH₇ is recoverable to ambient pressure. The *ex situ* analysis of the only partially reacted run products was inconclusive.
<figure><img src="image_9.png" /><figcaption>FIGURE 5</figcaption></figure>

Structures of P4/mbm-Na₃SiH₇ (left) and Pbam-Na₃SiH₇ (right). Their group-subgroup relation is depicted at the top, and the experimentally observed pressure stability ranges are shown at the bottom. Na, Si, and H atoms are shown as gray, yellow, and green circles, respectively. SiH₆ 2⁻ and HNa₆ octahedra are shown in yellow and green, respectively.

## 3.3 Phase relations in Na₃SiH₇

Figure 5 shows the structures of polymorphic Na₃SiH₇ along with the *p*, *T* conditions for the observed phases. As mentioned, Na₃SiH₇ represents a double salt Na₃[SiH₆]H containing both octahedral SiH₆ 2⁻ moieties and hydridic H. Hydridic H is octahedrally coordinated by six Na atoms, and HNa₆ octahedra build up a corner-connected framework similar to the BO₃ framework in perovskites ABO₃. SiH₆ 2⁻ octahedra are located in the voids accommodating the (larger-sized) A constituent of perovskites. Thus, the Na₃SiH₇ structures may be considered as an anti-perovskite arrangement (SiH₆ 2⁻)[HNa₃]²⁺, and as for perovskites, there is inherent structural flexibility from rotations and tilts of octahedra. Pbam is a subgroup of P4/mbm. The group-subgroup relationship is indicated in Figure 5. Analogous and also more extended sequences of phase transitions have been reported for fluorosilicate and fluorogermanate double salts [e.g., (NH₄)₃SiF₇ and (NH₄)₃GeF₇] as a function of temperature (Mel'nikova et al., 2014; Molokeev et al., 2014; Pogoreltsev et al., 2014; Mel'nikova et al., 2016; Mel'nikova et al., 2017; Bogdanov et al., 2019). These may potentially also include the yet unknown Na₃SiH₇ structure at room temperature and low pressures.

As previously mentioned, Pbam-Na₃SiH₇ was found enthalpically stable but dynamically unstable at ambient pressure. According to total energy calculations, this polymorph is by 0.4 eV/Z more stable than the P4/mbm structure. At 10 GPa, Pbam-Na₃SiH₇ is by about 0.24 eV more stable than the tetragonal form and dynamical stable, whereas P4/mbm-Na₃SiH₇ appears to be dynamically unstable at all pressures (cf. Supplementary Figures S5G, H).

To investigate the dynamical stability of the P4/mbm-Na₃SiH₇ phase at experimentally relevant conditions, phonon dispersions were calculated in the framework of the TDEP method (Hellman et al., 2011; Hellman et al., 2013) at 10 GPa and 600 K. Figures 6A, B compare the phonon dispersions for 10 GPa at zero and 600 K. Si-H stretching and bending modes of the entities SiH₆ 2⁻ (i.e., internal modes) and the (translational) modes of the hydridic H within Na₆ octahedra are above around 500 cm⁻¹, whereas SiH₆ 2⁻ librational modes (i.e., rotation of octahedral units against each other) and optic translation modes of Na⁺ and SiH₆ 2⁻ units (i.e., external modes) are below 500 cm⁻¹.

At 0 K, P4/mbm-Na₃SiH₇ has two imaginary branches (Figure 6A). These originate from librational and acoustic modes which, according to the atom-decomposed phonon density of states (shown in Supplementary Figure S6), involve Na1 atoms (which are situated on the 4-fold axes 0,0,z and ½,½,z, cf. Figure 5) and the H atoms being part of SiH₆ octahedra. The imaginary modes stabilize with temperature, which is attributed to the anharmonicity of atomic vibrations, causing renormalizing of the phonon modes due to phonon-phonon interaction (Figure 6B). Thus, P4/mbm-Na₃SiH₇ is stable at high-pressure and high-temperature conditions. Conversely, lowering temperature introduces dynamical instability, driving the distortion to the orthorhombic Pbam structure (a₀ ≈ √2a_t, b₀ ≈ √2a_t) which is dynamically stable at 10 GPa (Figure 6D). Reducing pressure to about 5 GPa introduces dynamical instability for the Pbam polymorph (Figure 6C), indicating a transition to yet another polymorph.
<figure><img src="image_10.png" /><figcaption>FIGURE 6</figcaption></figure>

Phonon dispersion relations for P4/mbm and Pbam Na₃SiH₇ at different *p*, *T* conditions. (A) P4/mbm-Na₃SiH₇ at 10 GPa and 0 K. (B) P4/mbm-Na₃SiH₇ at 10 GPa and 600 K. (C) Pbam-Na₃SiH₇ at 5 GPa and 0 K. (D) Pbam-Na₃SiH₇ at 10 GPa and 0 K.

Thus, total energy and phonon calculations support the experimental findings described in the previous section. The structural chemistry and polymorphism of Na₃SiH₇ reminds of fluorosilicates and germanates A₃Si/GeF₇ (A = K, Rb, Cs, NH₄). These materials have wide band gaps in the UV region (≈6 eV) in combination with interesting birefringent optical properties (Mel'nikova et al., 2016; Huang et al., 2020). In contrast, the calculated band structure of Na₃SiH₇ (Supplementary Figure S7) suggests a semiconductor with a direct band gap of around 2 eV. An interesting aspect, which has not been investigated in the course of this work, is a potentially superionic behavior of Na₃SiH₇. Superionicity of H⁻ was proposed in (yet) hypothetical Na₂SiH₆ where extraordinary diffusive H seems to be promoted by the hypervalent nature of SiH₆²⁻ units (Liang et al., 2021). Against this background, it appears worthwhile to study the dynamical properties of Na₃SiH₇ over a wider temperature range by dedicated AIMD simulations.

## 4 Conclusion

In situ studies of reactions m NaH + Si + 2.5 H₂ (m = 1, 2) at pressures up to 10 GPa revealed a new hypervalent hydridosilicate Na₃SiH₇ which corresponds to a double salt Na₃[SiH₆]H, featuring

isolated octahedral SiH₆²⁻ complexes and H⁻ anions. In the pressure range 5–10 GPa, Na₃SiH₇ occurs in a tetragonal high-temperature and orthorhombic low-temperature form. It is not yet clear whether and in which polymorphic form Na₃SiH₇ is recoverable to ambient pressure. Computation suggests the accessibility of further sodium hydridosilicate along the composition line NaₘSiH(₄+m) (m = 1–3), i.e., Na₂SiH₆ and NaSiH₅. The structures of phases NaₘSiH(₄+m) constitute octahedral SiH₆²⁻ complexes which are condensed for m = 1 and occur as isolated species for m = 2, 3. Interestingly, previously predicted superionic Na₂SiH₆ with a P3m1 structure has not been observed during our investigations. It may be speculated that this composition disappears from the convex hull once the most stable structure for NaSiH₅ has been identified. The same may hold true for Na₃SiH₇ at ambient pressure. The composition NaSiH₅ is expected to yield a manifold of enthalpically close-lying polymorphic structures based on corner- and edge-condensed SiH₆²⁻ octahedra. In order to access NaSiH₅ structures experimentally, the rather inert elemental Si should be replaced with a more active reactant, such as the Zintl phase NaSi. In addition to a higher reactivity, calculated reaction enthalpies with respect to NaSi (i.e., NaSi + 5/2H₂) are about 0.4 eV/Z lower than with respect to NaH + Si + 2H₂.
## Data availability statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

## Author contributions

KS, HK, and UH contributed to the conception and design of the study. OV and SS performed the computational part. SB, DD, and WC assisted in the implementation of experiments and the evaluation of data. KS and UH wrote the first draft of the manuscript. OV and SS wrote sections of the manuscript. All authors contributed to the article and approved the submitted version.

## Funding

This research was supported by the Swedish Research Council (VR) through project 2019-06063 and the Bundesministerium fuer Bildung und Forschung (BMBF)—German Federal Ministry of Education and Research (Grant No. 05K20OLA awarded to HK) and the Deutsche Forschungsgemeinschaft (Grant No. 277832266 awarded to HK). The computations were enabled by resources provided by the National Academic Infrastructure for Supercomputing in Sweden (NAISS) at the National Supercomputer Center (NSC) and Center for High Performance Computing (PDC), partially funded by VR through Grant Agreement No. 2022-06725. SS acknowledges the support from VR (Project No. 2019-05551), the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009-00971), the Knut and Alice Wallenberg Foundation, and the ERC (synergy grant FASTCORR project 854843).

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## Acknowledgments

The ESRF is thanked for allocating the beamtime CH-5986 and the provision of experimental facilities. The authors also thank Harald Müller for assistance with the Chemistry Laboratory facilities at ESRF. They are grateful to Per Mistenius for skillfully manufacturing the miniature press dies used for sample preparation. Stefan Sonntag and Robert Farla are thanked for assistance and resources needed to perform calibration experiments at P61B-LVP, PETRA III, DESY.

## Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

## Publisher's note

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## Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2023.1251774/full#supplementary-material

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