Metal chalcophosphates are a class of compounds generally composed of medium-and wide-gap semiconductors that boast a propensity for unique crystalline frameworks owing to the broad compositional flexibility of [P x Q y ] z-(Q = S, Se) polyanionic ligands. In the 1990s and 2000s, researchers found that an in situ fusion of A 2 Q (or other chosen alkali salts), P 2 Q 5 , and Q could form and stabilize various [P y Q z ] n-species. These species oxidize metallic elements and function as crystal growth-enhancing mineralizers when reaction temperatures exceed 400 °C. 1 This in turn led to the structural characterization of novel materials with ∞ 1 [PSe
6 -], 2 ∞ 1 [P 2 Se 6 2-
], 3
∞ 1 [P 3 Se 4 -], 4 [PQ 4 ] 3-, [5][6][7] [P 2 Q 6 ] 2-, 8,9 [P 2 Q 6 ] 4-, 10,11 [P 2 S 7 ] 4-, 12,13 [P 2 Se 8 ] 2-, 14 [P 2 Se 9 ] 4-, 15 [P 2 Se 10 ] 4-, 16 [P 3 Se 7 ] 3-, 17 [P 5 Se 12 ] 5-, 18 [P 6 Q 12 ] 4-, 18,19 and [P 8 Q 18 ] 6- species. 20 Additional combinatorial complexity can be brought about by the inclusion of multiple [P x Q y ] z-polyanions within one structure�demonstrated by the isostructural Rb 2 CeP 2 Se 7 and Cs 2 GdP 2 Se 7 , 21 which feature assembly of both [PSe 4 ] 3- and [P 2 Se 6 ] 4-units in their structuresor by coordination to multiple metal centers, as observed in the first quinary selenophosphate Rb 4 Sn 2 Ag 4 (P 2 Se 6 ) 3 . 21,22 Compounds comprising a single [P x Q y ] z-polyanionic unit have also been shown to form unique structures with significant physical proper-ties�[PS 4 ] 3-tetrahedra can flexibly coordinate with trivalent metals and trios of alkali metals to form nanotubular AsPS 4 / SbPS 4 , 5,6 nonlinear optical material InPS 4 , 7 pseudohelical onedimensional (1D) noncentrosymmetric chains of A 3 Bi(PS 4 ) 2 (A = K, Rb), 13 layered Cs 3 Bi 2 (PS 4 ) 2 , 13 and modulated threedimensional (3D) structure K 1.5 Bi 2.5 (PS 4 ) 3 . 23 Historically, the 2D layered M 2 P 2 Q 6 /MP 2 Q 6 /MM′P 2 Q 6 (M = metal; Q = S, Se) materials have drawn the majority of recent attention on metal chalcophosphates. The primary focus has been on CuInP 2 S 6 ; recent studies have advanced its technological applicability by probing its reversibly tunable multistates, 24 testing its use in ferroelectric tunnel junctions, 25 and manipulating its polar alignment to conform to other material interfaces, 26 all contributing toward applications in nanoelectronics and high-density storage. The AgInP 2 S 6 analogue has also gained in popularity, with recent studies highlighting its ultrafast photocarrier dynamics, 27 significant thermal stability and photoswitching reliability, 28 and structural advantages for photoreducing CO 2 in solar fuel applications. 29 Interest has also increased in the lesser-known materials in this family; numerous recent studies have highlighted the suitability of SnP 2 S 6 and SnP 2 Se 6 as dielectric materials for 2D electronics, 30 telecommunication band components for photothermoelectric detectors, 31 and chiral nonlinear optical crystals in high-power laser systems. 32 The 2D layered metal chalcophosphates consist of [P 2 Q 6 ] 4- bipyramid sheets with honeycomb arrangement of metal cations in the octahedral pockets between the ethane-like [P 2 Q 6 ] 4-polyanions. Weak van der Waals forces then stack these sheets upon each other. The first crystallographic report of these materials by Klingen and Hahn in 1965 focused on bivalent transition metals Fe, Co, and Ni, 10 but subsequent exploration of these materials revealed that an exceptionally wide range of cations may slot into the pockets, allowing a correspondingly wide array of functionalities�combinations of cations with an average oxidation state of 2+ over 2 sites are allowed, resulting in MM′P 2 Q 6 materials that can include Li + , Sn 4+ , vacancies, mixed transition metals, and rare earth metals. 11 This compositional flexibility has sparked recent interest in several M I M III P 2 Se 6 compounds, where M I is a monovalent metal cation (typically one of the coinage metals Cu + or Ag + ) and M III is a trivalent metal cation. Many of the reported M I M III P 2 Q 6 compounds have been observed to undergo ferrielectric, ferroelectric, or antiferroelectric ordering at low temperatures, attributed to second-order Jahn-Teller effects in the d 10 M I cations. 33,34 In 2020, Chica et al. reported a new material LiInP 2 Se 6 and its capabilities as a direct thermal neutron detecting semiconductor, while also noting its suitability for large crystal growth�they reported wide-area crystal growth by iodine-assisted chemical vapor transport and congruent melting behavior between 695 and 717 °C. 35 In 2023, Du et al. followed up with a demonstration that LiInP 2 Se 6 crystals could be grown by the Bridgman-Stockbarger method, then used in the fabrication of nextgeneration solid-state neutron detectors. 36 A later study on LiInP 2 Se 6 flakes by Kim et al. revealed notable mechanical properties resembling hybrid organic-inorganic materials in elasticity and plasticity. 37 These studies opened up interest in the exploration of other Li-containing MM′P 2 Q 6 materials as candidates for new direct thermal neutron semiconductor detectors.
In regards to the trivalent metal selection, the only reported 2D layered chalcophosphate composition with M III = Ga 3+ is AgGaP 2 Se 6 , with two reported phases from Pfeiff and Kniep; attempts to form an analogous CuGaP 2 Se 6 compound failed. 38 α-AgGaP 2 Se 6 , the thermally preferred compound, features P 2 Se 6 octahedra that share edges with GaSe 4 and AgSe 4 tetrahedra. 38 In contrast, β-AgGaP 2 Se 6 maintains the 2D layered chalcophosphate structure with octahedral cations contained in the pockets between P 2 Se 6 polyanionic sheets instead of cleaving along the (001) direction. β-AgGaP 2 Se 6 is a metastable phase, converting to α-AgGaP 2 Se 6 upon annealing at 350 °C for 14 days. In our recent 2023 article, we demonstrated that P 2 Se 5 reactive fluxes could rapidly synthesize crystals of known 2D layered metal selenophosphates, as well as access new multimetal materials of the same structure family. 39 Thus, we took this opportunity to investigate the Li-Ga-P-Se phase space, aiming to extend the P 2 Se 5 reactive flux-mediated synthesis to other LiMP 2 Se 6 materials and further investigate the incorporation of Ga into the 2D layered metal selenophosphate family.
Herein, we describe our studies on the synthesis of LiGaP 2 Se 6 : We begin by describing our challenges in synthesizing pure LiGaP 2 Se 6 by direct combination, then detail a combined strategy of premelting the metals together to form a LiGa alloy (analogous to the LiIn alloy used in the synthesis of LiInP 2 Se 6 ) 35 and dissolving the LiGa alloy in a reactive P 2 Se 5 flux at 800 °C, as described previously. 39 This combined method demonstrates the potential of dissolving an alloy precursor in a reactive P 2 Se 5 flux to unlock new multimetal selenophosphates. We note remarkable air and water stability in this new Li-containing layered selenophosphate. The structure refines best in space group P3̅ 1c and we compare it to similar compositions of the same space group, β-AgGaP 2 Se 6 and LiInP 2 Se 6 , concluding that LiGaP 2 Se 6 features a similar ABAB stacking pattern. The band gap of LiGaP 2 Se 6 is 2.01 eV, nearly identical to that of LiInP 2 Se 6 (2.06 eV), but the energy level of its valence band at 5.44 eV is much lower than that of LiInP 2 Se 6 (5.72 eV).
Reagents. Lithium pellets (99.9%, Sigma-Aldrich), gallium buttons (99.99%, Aldrich), red phosphorus lump (99.999+%, Thermo Scientific Chemicals), and selenium shot (99.99%, American Elements) were used as purchased without additional purification.
Synthesis of LiGa Alloy Precursor. In an argon-filled glovebox, lithium pellets were removed from mineral oil, wiped down using a Kimwipe, and temporarily transferred to a nitrogen-filled glovebox. In the nitrogen-filled glovebox, 0.094 g (13.54 mmol) of lithium pellets and 0.9057 g (12.99 mmol) of gallium pieces were inserted into a boron nitride crucible (MTI Corporation, 16 mm outer diameter, 12 mm inner diameter), which was in turn inserted into an 8-in.-long fused silica tube (18 mm outer diameter, 16 mm inner diameter). The tube was then removed from the glovebox, evacuated to a pressure of 3.2 × 10 -3 mbar and flame-sealed with an oxygen/natural gas torch to a length of ∼6 in.. The tube was then loaded into a vertical tube furnace and heated using the following temperature profile: a 16-h ramp from room temperature to 800 °C, a 2-h dwell at 800 °C, and then the furnace cooling to room temperature (Figure 1a).
Synthesis of P 2 Se 5 /P 2 Se 6 Precursor. Red phosphorus lump and selenium shot were layered into 8-in.-long fused silica tubes (outer diameter of 18.0 mm and inner diameter of 14.0 mm) lined with Al foil to minimize powder coating the inside walls of the tube� selenium shot was loaded first, then red phosphorus on top. The tube was then evacuated to a pressure of 4.5 × 10 -3 mbar and flame-sealed with an oxygen/natural gas torch to a length of ∼6 in. After the tube had cooled, the reagents were physically mixed by shaking the tube for 1 min. The tube was then loaded into a tube furnace, heated using the following temperature profile: a 12-h ramp from room temperature to 500 °C, a 72-h dwell at 500 °C, and then the furnace cooling to room temperature (Figure 1b). The tubes were opened in an ambient atmosphere, but the P 2 Se 5 was stored in a nitrogen-filled glovebox. For a 20-g batch of P 2 Se 5 , 2.7128 g (87.58 mmol) of red phosphorus powder and 17.2885 g (218.92 mmol) of selenium shot were used.
Synthesis of LiGaP 2 Se 6 Crystals. Crystals of LiGaP 2 Se 6 were synthesized using a P 2 Se 5 reactive flux. In an N 2 -filled glovebox, 0.098 g (1.28 mmol) of LiGa alloy pieces were inserted into an 8-in.-long carbon-coated fused silica tube (outer diameter of 18.0 mm and inner diameter of 14.0 mm) lined with Al foil to minimize reagent contact with the inside walls of the tube. 2.903 g (6.36 mmol) of P 2 Se 5 pieces were then layered on top of the LiGa alloy. The tube was then removed from the N 2 -filled glovebox and evacuated to a pressure of 3.2 × 10 -3 mbar, then flame-sealed with an oxygen/natural gas torch to a length of ∼6 in. The tube was then loaded into a computercontrolled tube furnace, using the heating profile shown in Figure 1c.
Removal of Excess P 2 Se 5 Flux from Reaction Products. For reactions in P 2 Se 5 reactive flux, after the initial heating, the ampules were opened, and the ingots were loaded into new 12-in.-long fused silica tubes with outer diameters of 18.0 mm and inner diameters of 16.0 mm. No Al foil liners were used for this step. The tubes were then evacuated again to a pressure of 3.2 × 10 -3 mbar and flamesealed to a length of ∼10 in. For the second heating step, the tubes were placed in a tube furnace with the ingot-containing end adjacent to the thermocouple in the center of the tube furnace and the other end sticking out of the furnace, exposed to air. Insulation was placed only to the edge of the tube furnace. A picture of the setup is available in Figure S1. The tubes were then heated under the P 2 Se 5 removal conditions shown in Figure 1d. This setup generated a temperature gradient between 400 °C and room temperature, causing volatilization of excess P 2 Se 5 to condense on the colder end of the tube sticking out of the furnace. This distillation of P 2 Se 5 was repeated 1-3 times until all the P 2 Se 5 red-black glass was visibly separated.
SEM-EDS Analysis of Bimetallic LiGaP 2 Se 6 Crystals. SEM-EDS was used to confirm the elemental composition of the LiGaP 2 Se 6 samples. Data were obtained with a Hitachi S-3400 VP-SEM with an accelerating voltage of 30 kV, a probe current of 50 μA on flakes affixed onto an SEM stub with carbon tape. An image of the flakes used are available in Figure S2. Spectra were taken from multiple areas on each flake, with output count rates around 2000 cps, for 10 s each. The energy-dispersive X-ray spectra were collected using an Oxford INCAx-act EDS system. The spectra were analyzed using Oxford Instruments AZtec software. The signal from carbon was excluded. The atomic percentage of Li in LiGaP 2 Se 6 was unable to be confirmed, as the characteristic Li K X-ray emissions are weak and easily absorbed before detection, resulting in an insufficient signal.
Single-Crystal X-ray Diffraction. Suitable plate-shaped single crystals were mounted on a glass fiber with super glue and diffracted on a STOE IPDS 2T diffractometer at 293 K. The STOE IPDS 2T diffractometer was equipped with an Mo Kα (λ = 0.71073 Å) sealed X-ray source with X-ray fiber optics and an image plate detector. Data reduction was performed with the STOE X-Area ver. 1.90 software package. A numerical absorption correction was applied by using STOE X-Red ver. 1.65.2 and STOE X-Shape ver. 2.21 followed by scaling and outlier rejection with STOE LANA ver. 1.83. 8 The structure was solved with the ShelXT intrinsic phasing solution method 40 and was refined with ShelXL full-matrix least-squares minimization on the F 2 method. 41 Olex2 was used as the graphical interface. 42 Full crystallographic details are provided in Tables 1, 2, 3 and 4. ■ RESULTS AND DISCUSSION Synthesis of LiGaP 2 Se 6 Crystals. LiGaP 2 Se 6 was first observed as a product of a stoichiometric combination of Li 2 Se, Ga, P 2 Se 5 , and Se, heated using the heating profile in Figure 1c. Powder X-ray diffraction (PXRD) analysis of the resultant products revealed primary products of LiGaSe 2 and Ga 2 Se 3 , with an unidentified minor product with lattice parameters similar to those of LiInP 2 Se 6 (Figure 2a). It was hypothesized that the LiGaSe 2 and Ga 2 Se 3 were thermal degradation products of LiGaP 2 Se 6 , so follow-up stoichiometric combination reactions at lower dwell temperatures of 350, 450, and 550 °C for 24 h were conducted. All three lowtemperature reactions had Ga spherical beads, showing that Ga did not incorporate in the overall reaction mixture. The very low surface area of the gallium metal spheres, in concert with the insufficient dwell temperature, precluded further reaction with the lithium phosphorus selenide flux.
1/2 and w = 1/(σ 2 (I) + 0.0001I 2 ). Inorganic Chemistry pubs.acs.org/IC Article https://doi.org/10.1021/acs.inorgchem.5c03190 Inorg. Chem. XXXX, XXX, XXX-XXX
To address the apparent incomplete incorporation of Ga in the reaction mixture, Li and Ga pieces were pre-alloyed using the heating profile shown in Figure 1a to form a homogeneous LiGa metal alloy precursor. This alloy was then used in two sets of experimental reactions: (1) in stoichiometric direct combination with P 2 Se 6 (LiGa + P 2 Se 6 → LiGaP 2 Se 6 ) and (2) in combination with excess P 2 Se 5 for reactive flux investigation (LiGa + 5 P 2 Se 5 → LiGaP 2 Se 6 + P x Se y ). These reactions were conducted at 500, 650, and 800 °C dwell temperatures for 24 h. PXRD analysis of the stoichiometric direct combination reactions show primary LiGaP 2 Se 6 product in the 500 °C reaction but revealed LiGaSe 2 and Ga 2 Se 3 products at higher temperatures of 650 and 800 °C (Figure 2b). This suggests that LiGaP 2 Se 6 is less readily formed than LiInP 2 Se 6 , which can be synthesized by direct combination to yield a flaky polycrystalline ingot with minimal impurities. PXRD analysis of the three flux reactions shows LiGaP 2 Se 6 is the major product for all three reaction temperatures, albeit with increased impurities at higher temperatures (Figure 2c). Visually, all three flux reactions produced polycrystalline LiGaP 2 Se 6 ingots, with small crystals suitable for single crystal XRD exclusively found on the side of the glass of the 800 °C flux reaction (Figure S3). This also contrasts with LiInP 2 Se 6 syntheses in P 2 Se 5 flux, which readily produce large, highquality LiInP 2 Se 6 flakes. 43 2.1929(5) P(1)-P(1) 1 2.206(3) Ga(1)-Se(1)-Li (1) 85.023( 14) P(1)-Se(1)-Ga (1) 103.84(3) P(1)-Se(1)-Li (1) 99.67(2) Se(1) 2 -Ga(1)-Se (1) 83.075( 16) Se(1) 3 -Ga(1)-Se (1) 178.274( 12) Se(1) 3 -Ga(1)-Se(1) 2 95.787( 16) Se(1) 4 -Ga(1)-Se (1) 98.093( 19) Se( 1) 5 -P(1)-Se (1) 113.17(3) Se(1)-P(1)-P(1) 1 105.46(3) Se(1) 4 -Li(1)-Se(1) 6 95.266(15) Se(1) 7 -Li(1)-Se(1) 6 177.517( 13) Se(1)-Li(1)-Se(1) 6 86.465( 14) Se(1) 4 -Li(1)-Se (1) 91.860 (18) a Symmetry transformations used to generate equivalent atoms:
Table 3. Fractional Atomic Coordinates (×10 4 ) and Equivalent Isotropic Displacement Parameters (Å 2 × 10 3 ) for LiGaP 2 Se 6 a Atom x y z U (eq) Se(1) 6775.8(6) 9927.8(5) 6232.2(3) 21.36(15) Ga(1) 10000 10000 7500 25.1(2) P(1) 3333.33 6666.67 6671.6(9) 18.4(2) Li(1) 6666.67 13333.33 7500 35(4) a U eq is defined as 1/3 of the trace of the orthogonalized U IJ tensor. The Anisotropic displacement factor exponent takes the form:
Comparing the thermal behavior of the reported AgGaP 2 Se 6 phases proved insightful in understanding the Li-Ga-P-Se system. 38 α-AgGaP 2 Se 6 is the thermally preferred compound� stoichiometrically combining the elements, quenching the melt, and then annealing at 400 °C for 21 days results in the α-AgGaP 2 Se 6 phase. The β-AgGaP 2 Se 6 phase, which has the 2D layered selenophosphate structure, is a metastable phase formed from a stoichiometric melt cooled from 750 °C to room temperature over 24 h. The β phase can be converted to the α phase by annealing the crystals at 350 °C for 14 days. Similarly, LiGaP 2 Se 6 could be a metastable phase at higher temperatures (and/or stabilized by reactive P 2 Se 5 flux) that thermodynamically prefers degradation into LiGaSe 2 and Ga 2 Se 3 .
Air/Water Stability. Finely ground powder from the resultant polycrystalline LiGaP 2 Se 6 ingot produced from the LiGa + 5 P 2 Se 5 reactive flux 500 °C reaction was used to gauge air and water stability. PXRD patterns were collected from LiGaP 2 Se 6 powder exposed to the ambient atmosphere for 1 day, then 2 weeks (Figure 3a). Comparison to the initial PXRD pattern reveals no discernible changes, displaying remarkable resilience to ambient atmosphere as compared to LiInP 2 Se 6 powder, which rapidly changes in color from bright orange to dark brown within minutes. 35 To test its water tolerance, finely ground LiGaP 2 Se 6 powder was soaked in a 50/50 DI water/ ethanol mixture, then was rinsed with acetone, and allowed to dry in air as the acetone evaporated. A comparison of the initial PXRD pattern also revealed no changes (Figure 3b). This also contrasted with LiInP 2 S 6 and LiInP 2 Se 6 , which both rapidly exfoliate in water. 35,44 Thermal Behavior of LiGaP 2 Se 6 . In order to probe the thermal behavior of LiGaP 2 Se 6 , differential thermal analysis (DTA) was conducted. The DTA plot shows reproducible thermal events in both heating and cooling cycles: an endothermic event at 458 °C during the heating segments and exothermic events at 424 and 546 °C during the cooling segments (Figure 4a). Pre-and post-DTA PXRD comparisons show no differences between the loaded and resulting materials, aside from the reduction of LiGaSe 2 signal after the DTA cycles (Figure 4b). The observation of repeated thermal events on heating and cooling, coupled with the recovery of the same compound after the experiment, points toward congruent melting behavior with a melting point at 458 °C and a freezing point at 424 °C. The 546 °C exothermic event is suspected to be a decomposition to LiGaSe 2 or Ga 2 Se 3 , at too slow of a rate to be detected by the post-DTA PXRD analysis. These thermal events occur at much lower temperatures than LiInP 2 Se 6 , which has a melting point at 717 °C and a freezing point at 695 °C, about 250 °C greater than those of LiGaP 2 Se 6 . 35 Structure Refinement. A visualization of the LiGaP 2 Se 6 crystal structure is shown in Figure 5. The room temperature structure of LiGaP 2 Se 6 is isostructural to those of β-AgGaP 2 Se 6 and LiInP 2 Se 6 , with no apparent off-centering of either metal cation in the octahedral positions�all three crystallize in space group P3̅ 1c with ABAB stacking sequences. 35,38 Synthetic precession images of the LiGaP 2 Se 6 data set used for the single crystal refinement are given in Figures S4-S6. Given the octahedral radii of Li + (0.76 Å) compared to Ag + (1.15 Å), and Ga 3+ (0.62 Å) to In 3+ (0.92 Å), we would expect the unit cell volume of LiGaP 2 Se 6 to be the smallest. 45 A comparison of the unit cell dimensions for all three compounds is shown in Table 5�as hypothesized, the unit cell of LiGaP 2 Se 6 is the smallest at 457.33 Å 3 . This is consistent with observations between average ionic radii and unit cell volumes across the entire family of materials. 11 The P-P bond lengths of LiInP 2 Se 6 and LiGaP 2 Se 6 are 2.212(3) and 2.206(3) Å, respectively�these are within the typical range of 2.15 and 2.30 Å. 11,35 However, the P-P bond length of β-AgGaP 2 Se 6 is much shorter, at 2.08(7) Å. This is only marginally compensated by longer P-Se bond lengths: β-AgGaP 2 Se 6 has a P-Se bond length of 2.211(11) Å, barely longer than the 2.1959(5) Å P-Se bond length in LiGaP 2 Se 6 and 2.1759(6) Å P-Se bond length in LiInP 2 Se 6 . Comparing the M 3+ -Se bond lengths reveals that the Ga-Se bond lengths of 2.6232(5) and 2.626(6) Å are remarkably close, for LiGaP 2 Se 6 and β-AgGaP 2 Se 6 , respectively; the In-Se bond length in LiInP 2 Se 6 is longer, at 2.7576(4) Å. The Li-Se bond length of 2.7570(5) Å in LiGaP 2 Se 6 is also remarkably similar to that of LiInP 2 Se 6 , which is 2.7650(4) Å; the analogous Ag-Se bond length in β-AgGaP 2 Se 6 is 2.862(6) Å.
There are minimal differences in the bond angles between the three structures, as well. β-AgGaP 2 Se 6 has a P-P-Se bond angle of 107.9(0)°and a Se-P-Se bond angle of 111.0(7)°. LiInP 2 Se 6 has a slightly narrower P-P-Se bond angle of 106.61(5)°and a slightly wider Se-P-Se angle of 112.17(4)°. LiGaP 2 Se 6 narrows the P-P-Se bond angle even further with an angle of 105.46(3)°and widens the Se-P-Se angle at 113.17(3)°.
Optical and Electronic Properties. A polycrystalline flake of LiGaP 2 Se 6 was repeatedly exfoliated by the scotch tape method, then investigated with UV-vis diffuse reflectance spectroscopy and photoemission yield spectroscopy in air (PYSA) measurements, as shown in Figure 6. The UV-vis data were used to determine the bandgap by extrapolating the linear onset of the absorption edges. The band gap of LiGaP 2 Se 6 was experimentally determined as 2.01 eV (616.8 nm), consistent with the typical wide-gap semiconducting character of these materials (Figure 6a) and similar to the experimental band gap of LiInP 2 Se 6 (2.06 eV). 35 PYSA measurements were also conducted to obtain the work functions by extrapolating the linear onset of the data. For semiconductors, the work function is equal to the energy of the valence band maximum (VBM) with respect to the energy of a free electron. Thus, in combination with the bandgap energy, the energy level of the conduction band minimum (CBM) can also be estimated. The work function was calculated to be 5.44 eV (Figure 6b), within the 5.25-5.72 eV range of other layered metal selenophosphates�specifically, closest to the measured 5.43 eV of CuCrP 2 Se 6 �but lower than the reported work function of LiInP 2 Se 6 (5.72 eV). 39 Therefore, LiGaP 2 Se 6 and LiInP 2 Se 6 are ideal candidates for a type II heterojunction due to their similar band gaps but differing work functions. LiGaP 2 Se 6 has a lower work function compared to LiInP 2 Se 6 , resulting in a staggered band alignment. This means the conduction band minimum of LiGaP 2 Se 6 is lower, while its valence band maximum is higher. Such an arrangement facilitates spatial separation of electrons and holes, enhancing charge transfer and making this combination well-suited for applications like photovoltaics, light emitting diodes, photodetectors and photocatalysis.
Electronic Structure Calculations. The electronic band structure and partial density of states (PDOS) were calculated for LiGaP 2 Se 6 . Density Functional Theory (DFT) calculations show that LiGaP 2 Se 6 has an indirect bandgap of 0.99 eV (Figure 7a) with a flat valence band maximum between H-K and a conduction band minimum at Γ. This behavior is remarkably similar to that of LiInP 2 Se 6 , which has a calculated band gap of 0.94 eV and an experimental band gap of 2.06 eV. 35 The total density of states in Figure 7a (subdivided into partial density of states in Figure S7) show that the valence band is dominated by Se p-states. The two lowest conduction bands comprise the 4 Ga 4s-states hybridized with Se 4p states separated by a small gap of 72 meV from the higher energy conduction bands. Corresponding to their 3+ formal charge, the remaining depopulated Ga 4p-states are hybridized with the higher energy P sp-states hybridized with Se 4p-states, which dominate the higher energy conduction bands. The [P 2 Se 6 ] 4+ units contain quaternary P in tetrahedral coordination corresponding to a 1+ formal charge. The surrounding anionic Se atoms are nonbridging, with their 1-formal charge due to the filing of the Se 4p-states by lone pairs. This can be seen by the lack of hybridization of Se 4p-states in the PDOS in Figure 7a and by the low and nonexistent electron density between Ga and Li atoms, respectively, and the Se atoms in Figure 7b. Furthermore, the absence of electron density between the layers confirms that LiGaP 2 Se 6 is a layered 2D material. Inorganic Chemistry pubs.acs.org/IC Article https://doi.org/10.1021/acs.inorgchem.5c03190 Inorg. Chem. XXXX, XXX, XXX-XXX
The conduction band minimum has high curvature, with a low effective electron mass m e * of 0.18 m e and 0.29 m e for the in-plane and out-of-plane directions, respectively. These values are close to the effective electron mass values calculated for the same directions in the isoelectronic material, LiInP 2 Se 6 , with effective masses 0.16 m e and 0.30 m e , respectively. 35 The valence band maximum also similarly lies along a flat band in the out-of-plane direction in reciprocal space (H to K), resulting in heavy hole effective mass values for the out-ofplane direction of 15.16 m e (VBM to K) and 13.01 m e (VBM to H). In-plane, the holes are much lighter with effective masses of 0.56 m e (K to) and -13.01 m e (VBM to H). These directions in the Brillouin zone are visualized in Figure 7c.
Crystalline flakes of LiGaP 2 Se 6 can be synthesized by using a reactive P 2 Se 5 flux, further demonstrating the formation of new bimetallic layered selenophosphates using Li + and a 3+ metal cation. Despite containing Li, this new material demonstrated remarkable stability in both air and water. The structure of LiGaP 2 Se 6 was determined using single crystal X-ray diffraction and refined in the space group P3̅ 1c, similar to other M I M III P 2 Q 6 compounds with ordered cation sublattices. LiGaP 2 Se 6 flakes were determined to congruently melt at 458 °C, with notable stability in both ambient atmospheric and hydrated conditions. UV-vis spectroscopy on an exfoliated flake showed a band gap of 2.01 eV, similar to the direct thermal neutron semiconductor material LiInP 2 Se 6 . Photoemission yield spectroscopy in air measurements showed a work function of 5.44 eV. Earlier, we noted that the layered metal chalcophosphate family had a notable lack of Ga 3+containing bimetallic members, previously limited to β-AgGaP 2 Se 6 . Specifically, we have previously demonstrated two potential Li sources for the synthesis of LiInP 2 Se 6 : Li 2 Se and LiIn. 35 Given the hazards involved in synthesizing Li 2 Se, the use of LiIn alloy poses a more accessible option for other exploratory chemists. By using a LiGa alloy to synthesize LiGaP 2 Se 6 , we further demonstrate that LiMP 2 Q 6 materials can be synthesized using an LiM alloy (and potentially even pure Li) instead of Li 2 Q binaries.
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c03190.
Additional experimental details for PXRD, DTA, UVvis spectroscopy, PYSA analyses, and ab initio calculations; SEM-EDS tables of the atomic percentages collected from each point scan; a photo of the P 2 Se 5 distillation setup; an SEM-EDS microscope image of the LiGaP 2 Se 6 flakes used for analysis; images of reaction products from P 2 Se 5 reactive flux experiments; synthetic precession images of LiGaP 2 Se 6 data used for single crystal XRD along the 0kl, h0l, and hk0 planes; and the partial density of states of Li, Ga, P, and Se in LiGaP 2 Se 6 (PDF)
Deposition Number 2368788 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Center (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.
https://doi.org/10.1021/acs.inorgchem.5c03190Inorg. Chem. XXXX, XXX, XXX-XXX B
https://doi.org/10.1021/acs.inorgchem.5c03190Inorg. Chem. XXXX, XXX, XXX-XXX