Ordering by cation replacement in the system Na 2 − x Li x Ga 7 †

Samples of the pseudo-binary system Na 2 − x Li x Ga 7 ( x ≤ 1) were synthesized from the elements at 300 °C in sealed Ta ampoules or by the reaction of Na 2 Ga 7 with LiCl. The peritectic formation temperature decreases with increasing Li content from 501(2) °C ( x = 0) to 489(2) °C ( x = 1). The boundary compositions Na 2 Ga 7 and Na 1 Li 1 Ga 7 crystallize with di ﬀ erent structure types related by a group – subgroup relation. While the Na-rich compositions ( x ≤ 0.5) represent a substitutional solid solution (space group Pnma ), the Li-rich compositions feature an unconventional replacement mechanism in which Li atoms occupying interstitial positions induce vancancies at the Na positions (space group Cmce ). The crystal structure of Na 1 Li 1 Ga 7 ( a = 8.562(1) Å, b = 14.822(2) Å, c = 11.454(2) Å; Z = 8) was determined from X-ray single-crystal di ﬀ raction data, and reveals an anionic framework comprising 12-bonded Ga 12 icosahedra and 4-bonded Ga atoms, with alkali – metal atoms occupying channels and cavities. The arrangement of cations makes NaLiGa 7 a new structure type within the MgB 12 Si 2 structure family. Band structure calculations for the composition NaLiGa 7 predict semiconducting behavior consistent with the balance [Na + ] 2 [Li + ] 2 [(Ga 12 ) 2 − ][Ga – ] 2 , considering closo Wade clusters [(12b)Ga 12 ] 2 − and Zintl anions [(4b)Ga] − . Susceptibility measurements indicate temperature-independent diamagnetic behavior.


Introduction
Inorganic framework compounds composed of p-elements and encapsulated metal cations are frequently studied. 1,2Their crystal structures show the characteristic bonding features of the framework atoms involved.While clathrate-type structures are commonly observed among frameworks based on 4-bonded group 14 elements, 1 frameworks of group 13 elements lack valence electrons to solely form two-center, twoelectron bonds between all framework atoms. 2 They typically adopt polyhedral Wade clusters, such as closo [B 6 ] 2− in CaB 6 , 3 or form multiatomic bonds, like in BaAl 4 , 4 which may be considered as a framework of 2D edge-condensed nido clusters.An example of the first type is the recently discovered Na 2 Ga 7 , 5 which exhibits a framework of 12-bonded [Ga 12 ] 2− closo clusters and 4-bonded [Ga] -Zintl anions.The electronic balance is achieved by the Na cations according to [Na + ] 4 [(Ga 12 ) 2− ][Ga -] 2 .The structure of this family can also be formed with heteroatomic frameworks and varying cation contents, as evidenced by the boride silicides MgB 12 Si 2 6 and Li 2 B 12 Si 2 , 7 which are composed of [B 12 ] 2− closo clusters and 4-bonded Si atoms.In contrast to the boride silicides, the higher number of cations in Na 2 Ga 7 (i.e.Na 4 Ga 12 Ga 2 ) leads to their disorder in the Ga channels.In this study, we investigate the pseudo-binary system Na 2 Ga 7 /NaLiGa 7 to determine the response of the Ga framework on the cation substitution and the possibility to obtain new members of the MgB 12 Si 2 family.

Experimental details
Preparation Reaction from the elements.Na (Chempur, 99.9%), Li (EVOCHEM, 99.9%) and Ga (Chempur, 5 N) were combined in a total amount of 2 g to prepare the compositions Na 2−x Li x Ga 7 (x = 0.2, 0.5, 0.8, 1.0, 1.5) and, with excess of Li, Na 1 Li 1.2 Ga 7 .The mixtures were sealed in Ta tubes under an Ar atmosphere.For homogenization, the sample was heated to 1000 °C for 2 min using an induction furnace and then allowed to cool to room temperature by turning off the furnace.Subsequently, the Ta tube was placed in a quartz vessel and further annealed under vacuum at 300 °C for one week.After annealing, the ampoule was quenched in water.The reaction products consisted of crystalline grains with metallic luster and were sensitive to air and moisture.† Electronic supplementary information (ESI) available.CCDC 2304096 (NaLiGa 7 ).For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt03628f a Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str.40, 01187 Dresden, Germany.E-mail: Michael.Baitinger@cpfs.mpg.deMetathesis reaction.Finely ground powder mixtures of Na 2 Ga 7 5 and LiCl (Chempur, 99.9%) were mixed with molar ratios of 1 : 1 or 1 : 1.5.These mixtures were pressed into pellets (d = 10 mm, h = 2 mm), sealed in a Ta crucible under an Ar atmosphere, and placed in an evacuated glass vessel.The vessel was annealed at 300 °C in a tube furnace for one week and then cooled down to room temperature by switching off the furnace.

Characterization
Single-crystal X-ray diffraction.A single crystal of NaLiGa 7 was fixed on top of a glass capillary (d = 0.1 mm) with grease and sealed in another capillary (d = 0.3 mm).Room-temperature single-crystal X-ray diffraction data were collected by a Rigaku AFC7 diffractometer (Saturn 724+ CCD detector, Mo Kα radiation, λ = 0.71073 Å).Absorption correction was performed with a multi-scan procedure.The crystal structure was solved and refined by using the WinCSD program. 8Details about the data collection and structure refinement are listed in Table S1; † atomic coordinates and displacement parameters are shown in Table S2; † selected interatomic distances and angles are listed in Tables S3-5.† The program Diamond V4.6.8 was used for structure drawings. 9owder X-ray diffraction (PXRD).All samples were characterized by the Guinier technique (Huber Image Plate Camera G670; germanium monochromator; Cu Kα1 radiation; λ = 1.54056Å; 5.0°< 2θ < 100°; step width = 0.005°).Finely ground specimens were fixed under an Ar atmosphere on the sample holder between two polyimide foils (7.5 μm, Kapton, Chemplex), using a thin film of vacuum grease (Lithelen, Leybold).LaB 6 standard (NIST, SRM 660a) was added to the sample as a reference for the reflection positions.Lattice parameters were obtained from the least-square method, using reflection positions extracted by profile fitting. 8canning electron microscope (SEM).Compositional analysis by EDX failed as polishing of the air-and moisture-sensitive material always led to the formation of a gallium layer on the sample surface.
Thermal analysis.The thermal behaviour of the samples Na 2−x Li x Ga 7 was measured with a heat-flux DTA device (Netzsch DSC 404C).About 50 mg of a pure sample was sealed in a Nb crucible (d = 5 mm, 600 mg).The system was heated from room temperature up to 600 °C then cooled down to 100 °C with heating rate of 5 °C min −1 under an Ar atmosphere.The measurements were calibrated with elemental Al measured under the same conditions.For thermal decomposition analysis, 50 mg of finely ground powder of NaLiGa 7 was filled under an Ar atmosphere in an open Ta crucible.For the heat treatment, the Ta crucible was placed in a glass tube, which was evacuated to 10 −2 mbar and closed.During the heat treatment, the decomposition is evidenced by the precipitation of a sodium mirror at the cold part of the glass vessel.After the measurement, the product was characterized by PXRD.
Magnetic susceptibility.Magnetization was measured in a SQUID magnetometer (MPMS-XL7, Quantum Design) in the temperature range 1.8-400 K. Fields μ 0 H from 2 mT to 7 T were applied, the diamagnetic contribution of the sample tube was subtracted.
Calculation methods.First-principles electronic structure calculations were conducted using the all-electron full-potential local orbital (FPLO) method. 10Exchange-correlation effects were considered via the local density approximation (LDA) within the density functional theory, utilizing the Perdew and Wang parametrization. 11The experimentally obtained crystal structure data were utilized.The Brillouin zone was sampled with a 15 × 15 × 15 mesh.

Results and discussion
Preparation of Na 2−x Li x Ga 7 (x ≤ 1) When a stoichiometric mixture of the elements at composition NaLiGa 7 was melted and cooled down to room temperature within a few minutes, the resulting product predominantly consisted of NaLiGa 7 , along with smaller amounts of Li 3 Ga 14 12 and an unidentified phase, as determined by PXRD analysis.This behaviour notably differs from the preparation of Na 2 Ga 7 , which does not form directly from the melt. 5owever, the presence of foreign phases indicates that NaLiGa 7 forms incongruently as well.Subsequent annealing at 300 °C for one week led to the formation of single-phase products for all compositions Na 2−x Li x Ga 7 , with x = 0.2, 0.5, 0.8 and 1 as confirmed by PXRD analysis (Fig. 1).For the sample with x = 1.5, the majority phase was Na 1 Li 1 Ga 7 , accompanied by foreign phases (Table 1).During annealing, the sample melted partially, indicating that the identified phases, Na 1 Li 1 Ga 7 , Li 3 Ga 14 , LiGa 2 and LiGa 6 , 13 were not in equilibrium upon cooling.Therefore, we conclude that the upper limit for Fig. 1 PXRD patterns of Na 2−x Li x Ga 7 (x = 0, 0.5, 1) after annealing at 300 °C (Cu Kα1).The calculated patterns of Na 2 Ga 7 (space group Pnma, red lines) and NaLiGa 7 (space group Cmce, blue lines) are based on single-crystal diffraction data.For Na 1.5 Li 0.5 Ga 7 and Na 2 Ga 7 , some characteristic reflections breaking the C-centered lattice are indexed.In NaLiGa 7 , symmetry inequivalent reflections overlap due to the hexagonal axis ratio b/a (Table 1).

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This journal is © The Royal Society of Chemistry 2024 substitution of Na atoms is reached at x = 1.Furthermore, a homogeneity range Na 1 Li 1+δ Ga 7 with a higher Li content was not detected by refinement of the lattice parameters.The product with nominal composition Na 1 Li 1.2 Ga 7 consisted of Na 1 Li 1 Ga 7 and exhibited weak reflections of an unknown phase.
Other than from direct synthesis, Na 1 Li 1 Ga 7 can be formed through a metathesis reaction of Na 2 Ga 7 and LiCl (eqn ( 1)).The reaction is supported by the higher stability of NaCl compared to LiCl increasing with temperature. 14When Na 2 Ga 7 and LiCl were reacted in a 1 : 1 molar ratio at 300 °C, the product consisted of Na 1 Li 1 Ga 7 and NaCl as confirmed by PXRD analysis (Table 2).
No residual LiCl was detected, and the refined lattice parameter of NaCl (a = 5.6400(7) Å) matched the reported value. 15his indicates that the cation substitution was quantitative.
When Na 2 Ga 7 was reacted with 1.5 equivalents of LiCl at 300 °C, a mixture of Na 2−x Li x Ga 7 , Li 3 Ga 14 and NaCl was observed in PXRD.The overlapping reflections lowered the precision of the lattice parameter refinement, but the obtained values are consistent with the composition NaLiGa 7 .This finding confirms that Li can substitute up to one equivalent of Na in Na 2 Ga 7 .With an excess amount of LiCl, Na 1 Li 1 Ga 7 was not formed.Reacting an equimolar mixture of NaLiGa 7 and LiCl resulted in the formation of binary gallides Li 3 Ga 14 , LiGa 2 and NaGa 4 . 16ermal stability of the composition range Na 2−x Li x Ga 7 The binary phase Na 2 Ga 7 forms peritectically at 501(2) °C from Na 7 Ga 13 17 and the melt. 5Regarding the ternary system  S6 †).
The thermal decomposition of Na 1 Li 1 Ga 7 was investigated on bulk material to examine the possibility of the formation of a new ternary phase through the evaporation of Na.However, no such phase was observed.Under static vacuum conditions at 300 °C, no reaction was observed, and the initial material remained unchanged for a period of 1 week.At 400 °C, after 1 day, sodium droplets were observed in the cooler region of the glass reactor.PXRD patterns of the solid residue consisted of NaLiGa 7 , minor amounts of Li 3 Ga 14 and amorphous Ga, as indicated by its characteristic background.When the reaction time was prolonged to 3 days, only Li 3 Ga 14 was detected along with contributions of amorphous Ga (eqn (2)).
Crystal structure of Na 2−x Li x Ga 7 Structure determination.The crystal structure of NaLiGa 7 was determined using single-crystal XRD data, revealing an orthorhombic unit cell with the space group Cmce (no.64).The direct structure solution revealed five independent Ga positions, forming a framework consisting of icosahedral cluster units (Ga1-Ga4) and 4-bonded atoms (Ga5).A mixed occupancy of Ga and Li as it was recently detected in covalent Ge networks [18][19][20] was not observed.After refining the Ga posi-Table 1 Lattice parameters of samples with the nominal compositions Na 2−x Li x Ga 7 (x = 0, 0.2, 0.5, 0.8, 1, 1.5) and NaLi 1.2 Ga 7 obtained by annealing at 300 °C.PXRD patterns of samples with x ≤ 0.5 show primitive reflections breaking the Cmce symmetry.For a better comparison of the lattice parameters, samples with x = 0, 0.  tions, the cation positions were determined from the difference Fourier map.Two distinct peaks were identified, with the larger one located within the channels of the Ga framework, and a smaller peak situated in a cavity between three Ga 12 icosahedra.Assigning the larger peak at Wyckoff position 8e to Na and the weaker one at Wyckoff position 8f to Li, subsequent refinement cycles were performed with an anisotropic approximation of the atomic displacement parameters (ADPs).These refinement cycles resulted in full occupancy for all positions and yielded a residual value of R F = 0.040.The ADPs of all Ga atoms and Li exhibited nearly isotropic behavior with regular U eq values.Only for the Na atom, the ADPs showed a distinct ellipsoidal shape with a larger U eq value (Fig. S1 231), ( 503) and ( 611), which violate the reflection conditions for space group Cmce (Fig. 1).Also, for another reason, the powder diffraction pattern of Na 2 Ga 7 is more line-rich than that of NaLiGa 7 .The lattice parameters a and b in NaLiGa 7 exhibit the almost ideal hexagonal ratio of √3, whereas in Na 2 Ga 7 , this value is not reached (Table 1).Therefore, reflections that overlap in NaLiGa 7 , separate in Na 2 Ga 7 .The crystal structure of samples with compositions Na 2−x Li x Ga 7 (x = 0.2, 0.5, 0.8) was determined by the Rietveld method (Tables S7-S9 †).In the initial structure model, the Ga positions were adopted from Na 2 Ga 7 5 and refined with isotropic ADPs.Subsequently, the positions of cations were determined.The partial occupancy of Na and Li positions could only be approximated.
Structure description.6][7] The framework is built up of a face-centered arrangement of icosahedra, bonded together by exohedral bonds and 4-bonded atoms (Fig. S3 †).While the overall framework topology is characteristic for all representatives, the cation positions vary depending on number and size of the cations in relation to the framework.While the anionic framework can always be described with space group Cmce, the cation arrangement in turn can lower the space group symmetry.Among the known representatives, MgB 12 Si 2 and Na 2 Ga 7 crystallize in the space group Pnma, while Li 2 B 12 Si 2 and Na 1 Li 1 Ga 7 crystallize in the space group Cmce (Table 3).The framework offers three distinct cation positions (Fig. 2), which are located along channels oriented in the [100] direction in the Cmce setting and in the [010] direction in the Pnma setting (Fig. 3).The potential cation positions can be visualized on a plane obtained by sectioning the three-dimensional framework perpendicular to the [001] direction (Fig. 4).The most favorable cation position

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This journal is © The Royal Society of Chemistry 2024 seems to be the one denoted C1, which is located in the channels and the only occupied position in the compounds MgB 12 Si 2 and Li 2 B 12 Si 2 .At C1, the cations occupy hollows formed by three condensed six-membered rings (Fig. 2b).
In the boride silicide Li 2 B 12 Si 2 , all C1 positions are filled by Li atoms (Fig. 3a, 4a), while in MgB 12 Si 2 only half of them are occupied by Mg atoms (Fig. 3b, 4b).The partial occupancy manifests in such a way that in each channel half positions are removed.In this way, the partial occupancy leads to a reduction in symmetry from Cmce in Li 2 B 12 Si 2 to Pnma in MgB 12 Si 2 .The crystal structure of Na 2 Ga 7 (vNa 4 Ga 12 Ga 2 ) contains twice as many cations as Li 2 B 12 Si 2 , so that the occupation of all positions C1 is not sufficient to accommodate all Na atoms.Consequently, two additional cation positions C2 and C3 are found, which have not been observed in the boride silicides (Fig. 3c, 4c).Atoms at position C2 are also located in the channels (Fig. 2b), which creates a conflict with the spatial requirements of the atoms at C1.In Na 2 Ga 7 , full occupancy of C1 and C2 in the channels by Na atoms is impossible for steric reasons.The maximum filling of the channels is achieved, when 50% of the C1 positions and 100% of C2 are occupied.The replaced C1 atoms occupy C3 positions instead, located in small cavities between three Ga 12 icosahedra (Fig. 2b).However, only half of the available C3 positions are occupied by Na atoms.Always two C3 positions are arranged in close proximity (Fig. 5), resulting in a distance d(C3-C3) = 2.78 Å being too short if both neighboring positions were filled by Na.The half occupancy of C1 and C3 in Na 2 Ga 7 along with the ordered arrangement of Na results in a symmetry reduction from Cmce to Pnma.
In the crystal structure of NaLiGa 7 , the smaller Li atoms fill up all C3 positions.Simultaneous occupation of the adjacent positions C1 and C3 is excluded for their short distance of ≈1.60 Å requiring the C1 positions to remain empty.The C2 positions in the channels of NaLiGa 7 are fully occupied by Na atoms (Fig. 3d, 4d), as in Na 2 Ga 7 .With full occupancy of C3, there is no symmetry reduction observed in NaLiGa 7 and the space group remains Cmce (Table 3).The arrangement of cations in NaLiGa 7 introduces a new structure type within the MgB 12 Si 2 structure family.
The topology of the Ga framework in NaLiGa 7 is very similar to the one in Na 2 Ga 7 .The Ga 12 icosahedra are distorted with endohedral bond distances ranging from 2.601(1) Å to 2.817(1) Å.The distances between atoms in transposition vary from d(Ga3-Ga3) = 4.902 Å to d(Ga2-Ga2) = 5.338 Å (Fig. S4 and S5 †).As expected, the exohedral two-center, two-electron bonds (d ˉexo = 2.54 Å) are shorter than the endohedral multicenter bonds (d ˉendo = 2.67 Å).The longest Ga-Ga bond d(Ga4-Ga4) = 2.817 Å is found at the cavity position C3, which shows that the occupation of this position creates distortion in the framework.For the boride silicides, where C3 is not occupied, this distortion of the icosahedra does not occur.The 4-bonded atoms Ga5 deviate from ideal tetrahedral coordination with bond angles ranging from 97.59(1) to 123.83(4) degrees and bond distances ranging from 2.517 Å to 2.591 Å. Comparing the bond spectra of Na 1 Li 1 Ga 7 and Na 2 Ga 7 , the framework in Na 1 Li 1 Ga 7 shows a smaller dispersion of Ga-Ga bond lengths (Fig. 6).The dispersion of bond lengths in Na 2 Ga 7 is mainly caused by the accommodation of the larger Na atoms in 50% of the small C3 cavities.In agreement with the reduced cell volume, the mean Ga-Ga bond distance in Na 1 Li 1 Ga 7 of 2.62 Å is shorter than 2.66 Å in Na 2 Ga 7 .
Structural evolution in the composition range Na 2−x Li x Ga 7 .When Na atoms are substituted with Li atoms, the unit-cell volume decreases with increasing Li content (Fig. 7a).However, for compositions with x ≤ 0.5 (space group Pnma), the volume reduction is more pronounced than for compositions with x > 0.5 (space group Cmce).Hence, different substitution mechanisms are at work.This can be understood by considering the occupancies of the cation positions (Table S10 †): • In Na 2 Ga 7 , the positions C1 and C3 are half-occupied by Na.
• In NaLiGa 7 , C1 is empty, while C3 is fully occupied by Li.
• In both phases, position C2 is entirely filled with Na atoms  To determine the cation site occupancies for the compositions between 0 < x 1, only PXRD data were available (Table The accuracy of the derived structure models was further constrained by the low scattering power of Li located in proximity to the heavier Ga atoms.Despite of these experimental limitations, the substitution mechanism can be inferred: For 0 ≤ x ≤ 0.5, the Na occupancy at C1 remains constant, while the Na occupancy at C3 clearly decreases (Fig. 7b).
Consequently, starting from Na 2 Ga 7 , the Li atoms first replace Na atoms in the small cavities at C3.This behavior corresponds to a conventional substitution alloy (Fig. 8a).Finally, at x = 0.5, all Na atoms at C3 are substituted, so that C3 is half occupied by Li.Now, for 0.5 < x ≤ 1, the Li atoms fill up the remaining vacant C3 positions, while Na atoms at C1 evanish (Fig. 7b).This exchange mechanism is unconventional as it represents an intermediate between an interstitial and a substitution alloy.The Li atoms occupy interstitial positions but simultaneously replace Na atoms located at other positions (Fig. 8b).At x = 1, C3 is completely filled by Li and C1 is completely emptied.Deviations of the experimental results from the substitution model only occur for low Na occupancies and Li positions, as expected.Seemingly, the occupancy of the narrow C3 cavities has a stronger impact on the cell volume than the occupancy of C1, sharing the large channels with C2.This is also reflected in the distortion of the six-membered ring planes (Fig. 4).While the lattice parameters a and b in NaLiGa 7 nearly exhibit the ideal hexagonal ratio (Table 1), the value deviates significantly in Na 2 Ga 7 .When the Na atoms in the small C3 cavities are replaced by Li in the range of 0 ≤ x ≤ 0.5, the ratio approaches the hexagonal value (Fig. 7c).Conversely, the occupation of the C1 position by Na in the range of 0.8 ≤ x ≤ 1 has no influence on the axis ratio.
The proposed substitution mechanism finds further support in the interatomic distances of the Na atoms at C2, located within the channels of the Ga framework.In NaLiGa 7 , the Na atoms at C2 show uniform distances from each other (Fig. 4d).Conversely, in Na 2 Ga 7 , the channels are shared by Na  atoms at C2 and C1, resulting in mutual that leads the Na atoms at C2 form dumbbells (Fig. 4c).Therefore, the refined distance between C2 atoms is a signature of the Na occupancy at position C1.Starting from Na 2 Ga 7 , the experimentally determined Na-Na distances at C2 remain constant for 0 ≤ x ≤ 0.5, indicating the occupation of position C1 (Fig. 7d).For x > 0.5 the distances converge to a common value, signifying vacancies at C1.Moreover, the occupancy of C1 by Na affects the adjacent Ga atoms.The Na atoms at C1 are situated in a hollow formed by three Ga six-membered rings (Fig. 2b).As C1 becomes vacant, the depth of the hollows decreases (Fig. 7e and Fig. S6 †).The decrease clearly starts for x > 0.5, in agreement with the proposed substitution model.The X-ray results in this work suggest that the two boundary phases, Na 2 Ga 7 and Na 1 Li 1 Ga 7 , each have a homogeneity range.However, the two-phase region between both solid solutions was not detected in this study.In general, the ternary phase diagram Na-Li-Ga is unknown so far.A key challenge in determining the phase equilibria in future work will be the metallographic treatment of the air and moisture sensitive samples.

Electronic density of states
The computed electronic density of states (DOS) for NaLiGa 7 reveals a band gap of 0.47 eV (Fig. 9).This result is in line with the expectation of an electronically balanced composition for [(12b)Ga 12 ] 2− icosahedral closo clusters and [(4b)Ga] − atoms.The Ga contributions are classified into two groups: (i) sum of the partial DOS of the Ga atoms forming the icosahedra Ga 12 ; (ii) DOS contributions of the 4-bonded Ga.These two chemically different Ga species are expected to be distinguishable in their local DOS projections.
The [(4b)Ga] − species do not significantly participate in the lowest-lying cluster orbital below −10 eV, containing 2 e − per Ga 12 cluster, and the computed energetical center of gravity of the (5 + 1)-fold coordinated cluster Ga species lies energetically lower than the one for the 4-fold coordinated one (Fig. 9).All these results are in good agreement with the results previously found for Na 2 Ga 7 (calculated band gap = 0.29 eV). 5 As the DOS reveals (Fig. 9), there is no fundamental difference in the contribution of Na and Li atoms to the electronic structure of NaLiGa 7 .We attribute the differences in the band gap between both compounds mainly to the larger distortions of the Ga icosahedra in Na 2 Ga 7 (Fig. 6).The significant variation in the nearest neighbor distances is expected to result in a smaller intra-icosahedral band gap, which is of the bonding-antibonding type. 26,27

Physical properties
The magnetization M(T ) of the sample of NaLiGa 7 was measured in the temperature range from T = 1.8K to 400 K in magnetic fields µ 0 H = 0.002 T, 0.1 T, 3.5 T and 7 T.The magnetic moment of the sample holder was subtracted from the measured data and the magnetic susceptibility χ = M/H was calculated.The magnetic susceptibility χ(T ) is temperatureindependent and diamagnetic (Fig. S7 †) with value χ = -1.70 × 10 −4 emu mol −1 per formula unit, agreeing within 1% with the calculated value (−1.59 × 10 −4 emu mol −1 ) from diamagnetic increments for Na + , Li + and elemental Ga. 28The superconductivity observed below T ≈ 8 K can be attributed to the presence of elemental Ga impurities since the Meissner fraction of the signal is low (≈ 0.09%).From the reported data, the allotropes β-, γand δ-Ga show superconducting transitions at 5.9 K-6.2 K, 6.9 K-7.6 K and 7.85 K, respectively. 29The resistivity measurements on Na 2−x Li x Ga 7 were challenging because of   the tendency to form elemental Ga segregation on the surface of sample the measurements caused by oxidation.
The pseudo-binary system Na

Na 2 −
x Li x Ga 7 , the thermal stability decreases with increasing Li content.A single-phase sample of Na 1.8 Li 0.2 Ga 7 revealed a slightly lower decomposition temperature of T = 498(2) °C.At the maximum Li-content Na 1 Li 1 Ga 7 , the formation temperature is further reduced to T = 489(2) °C.At all compositions of Na 2−x Li x Ga 7 , the presence of two signals in the DTA experiments indicates that the samples undergo incongruent melting (Table †).The orientation of the Na ellipsoids aligns to the walls of the channels, which run in a zig-zag pattern along the [100] direction (Fig. S2 †).The disorder can be modeled by splitting the Na1 position (8e) into a 0.5-occupied Na1a position (8e) and a 0.25occupied Na1b position (16g).In the final refinement cycle, the restraint occ(Na1a) + 2 × occ(Na1b) = 1 was applied, leading to a further reduction of the residual value R F to 0.034.The refined unit-cell composition Na 8 Li 8 Ga 56 (Na 1 Li 1 Ga 7 with Z = 8) is electronically balanced according to [Na + ] 8 [Li + ] 8 [(Ga 12 ) 2− ] 4 [Ga -] 8 , assuming the presence of closo Wade clusters 21,22 [(12b)Ga 12 ] 2− and Zintl anions [(4b) Ga] -.Unlike NaLiGa 7 , all other compounds in the series Na 2−x Li x Ga 7 with x ≤ 0.5 crystallize with the subgroup Pnma.The lower symmetry is evident from the presence of additional reflections indexed in the PXRD patterns, e.g.(303), (

Fig. 5
Fig. 5 Pair of Li atoms situated in adjacent C3 cavities having a distance of 2.78 Å.

Fig. 7
Fig. 7 (a) Cell volume of Na 2−x Li x Ga 7 (x = 0, 0.2, 0.5, 0.8, 1) against Li content.(b) Substitution model (lines) and refined occupancies (red triangles: Na at C1; blue circles: Na at C3; black circles: Li at C3). (c) b/a ratio of the hexagon planes.(d) Na-Na distances at position C2.(e) Depths of the hollow at C1. Shaded areas show the assumed homogeneity ranges.Error bars for all figures are smaller than the symbols used.

Fig. 8
Fig. 8 (a) Scheme of a substitution alloy.(b) The unconventional intermediate of an interstitial and a substitution alloy.Green lines symbolize the Ga framework, blue spheres Na and red spheres Li.

Table 2
Lattice parameters of NaLiGa 7 obtained from different preparation methods.Parameters were refined from the same set of non-overlapping reflections 910 | Dalton Trans., 2024, 53, 908-916 This journal is © The Royal Society of Chemistry 2024

Table 3
Cation arrangement in compounds of the MgB 12 Si 2 structure family.Three cation positions are available in the B/Si or Ga framework (M = Mg, Li, Na; F = B, Si, Ga) Open Access Article.Published on 15 December 2023.Downloaded on 6/5/2024 8:23:17 AM.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Dalton Trans., 2024, 53, 908-916 | 911 2−x Li x Ga 7 is terminated by the compositions Na 2 Ga 7 and Na 1 Li 1 Ga 7 .Both phases show a homogeneity range with different substitution mechanisms.The crystal structure of Na 1 Li 1 Ga 7 belongs to the MgB 12 Si 2 structure family, characterized by a framework of closo Ga 12 icosahedra and 4-bonded Ga atoms.Within the Ga framework, the alkali metal atoms occupy distinct channels and cavities.For Na 1 Li 1 Ga 7 , the arrangement of cations is fully ordered, representing a new variant within the MgB 12 Si 2 structure family.Na 1 Li 1 Ga 7 was prepared by either annealing the elements or by the quantitative metathesis reaction of Na 2 Ga 7 with LiCl.Considering the electron balance [Na + ] 8 [Li + ] 8 [(12b) Ga 12 2− ] 4 [(4b)Ga − ] 8 , the compound can be classified as a Zintl-Wade phase.The balance is consistent with the diamagnetic susceptibility and an electronic band gap, calculated to be 0.47 eV.