Synthesis and structure of novel Ag₂Ga₂SiSe₆ crystals: promising materials for dynamic holographic image recording

Abstract

Phase diagrams of the AgGaSe₂–SiSe₂ system were explored by differential thermal analysis (DTA) and X-ray diffraction (XRD) analysis methods for the first time. It was demonstrated that the investigated system forms quaternary compounds of compositions Ag₂Ga₂SiSe₆ and AgGaSiSe₄. Ag₂Ga₂SiSe₆ melts at 1042 K and exists in two polymorphous modifications. The crystal structure of the low-temperature modification was determined by the single crystal method (space group I4̄2d (122) and lattice parameters a = 5.9021(1) Å, b = 5.9021(1) Å, and c = 10.4112(10) Å). Additional details (CIF file) regarding the crystal structure investigations are available at the Fachinformationszentrum Karlsruhe. The band gap (E_g) of the Ag₂Ga₂SiSe₆ system was estimated from the fundamental absorption edge and we showed that it decreases with increasing temperature (100–300 K) from 2.13 eV to 1.97 eV. The compound is photosensitive and its spectral dependence on the photoconductivity has two maxima: at λ_max1 = 640 nm and λ_max2 = 900 nm. For the pristine Ag₂Ga₂SiSe₆ crystal surface, X-ray photoelectron core-level and valence-band spectra were obtained. The X-ray photoelectron valence-band spectrum of Ag₂Ga₂SiSe₆ was compared on a common energy scale with the X-ray emission Se Kβ₂ and Ga Kβ₂ bands, representing peculiarities of the energy distribution of the Se 4p and Ga 4p states, respectively. The comparison revealed that the principal contributions of the valence Se p and Ga p states occur in the upper and central parts of the valence band, respectively, with significant contributions to other valence band regions. The illumination by the bicolour coherent pulses of the Er:glass nanosecond lasers at different angles led to the formation of the gratings, which are sensitive to the irradiation time.

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Introduction

The AgGaSe₂ (AGSe) crystal is a preferred non-linear optical material for the frequency doubling of CO₂ laser beams (λ = 10.6 μm) into the mid-IR spectral range, tunable within the 2.5–12 μm wavelength range.¹ However, it has several deficiencies that justify the research into improving the properties of AGSe. These deficiencies include the relatively modest laser damage threshold,² and unlike the sulfide analog AgGaS₂, AGSe lacks phase matching ability under pumping with light having λ of ∼1 μm.³ Thus, their study, with respect to new non-linear optical media, attracts enhanced interest; some interesting results have been obtained during the investigation of these AGSe-based systems. For instance, a new quaternary chalcogenide Ag_xGa_xGe_1−xSe₂ (AGGSe) was found in the AgGaSe₂–GeSe₂ system,^4–6 which melts congruently at 993 K (the melting point maximum corresponds to the composition of AgGaGe₃Se₈ (75 mol% GeSe₂)) and has a wide homogeneity region (64–90 mol% GeSe₂);⁵ we have previously reported on just the crystal and electronic structures of Ag_xGa_xGe_1−xSe₂ in more detail.^7,8 AGGSe crystallizes in the non-centrosymmetric space group Fddd with the lattice parameters a = 12.4423 Å, b = 23.820 Å, c = 7.1403 Å for the AgGaGe₃Se₈ composition.⁷ Petrov et al.⁹ determined that the radiation stability of AGGSe substantially exceeds that of AGSe.

The technology of the single crystal production of the quaternary phase is much simpler as well. They were grown by the Bridgman–Stockbarger method^{1,4,5,7,9–12} or horizontal gradient freeze technique in transparent furnaces.¹³ In addition to promising non-linear optical susceptibilities, the quaternary phase is also a promising piezoelectric¹⁴ and acousto-optical material for the mid-IR spectral range.¹⁵ The wide, homogeneity region of AGGSe permits a certain range of tuning of their parameters, and the peculiarities of the crystal structure lead to the formation of solid solutions by the substitution of any of the components of the compound.^16–18

The latter feature makes advisable the study of a similar system with silicon diselenide instead of germanium diselenide (no intermediate phase was found with SnSe₂ (ref. 19)). The existence of compounds in the AgGaSe₂–SiSe₂ system was indicated earlier. Goodchild et al.²⁰ reported the formation of the AgGaSiSe₄ compound as one of the representatives of the I-III-IV-VI family. Quite recently, Zhang et al.²¹ obtained AgGaSiSe₄ by the solid state phase growth of equimolar amounts of AGSe and SiSe₂ at 970 K during 24 h (the procedure was repeated with intermediate powdering). The crystal structure determined following the single crystal method is orthorhombic (space group Aea2) with rather large unit cell parameters a = 62.905 Å, b = 7.115 Å, c = 12.376 Å and Z = 32. The compound melts congruently at 1032 K and has the energy band gap equal to 2.63(2) eV, following the reflectance spectra. The growth of Ag₃Ga₃SiSe₈, also melting congruently at 1057 K, was reported in ref. 22. This compound is isostructural with AGSe (space group I4̄2d, a = 0.59041 nm, c = 1.0499 nm, Z = 1) and has the band gap equal to about 2.30 eV. It was additionally reported that its non-linear optical properties are similar to those of AGSe.

Here, we present the results of the investigation of the phase diagram of the AgGaSe₂–SiSe₂ system for the first time, its crystal and electronic structure and some properties of the Ag₂Ga₂SiSe₆ compound.

Experimental

The investigation of the AgGaSe₂–SiSe₂ system was performed over the whole concentration range, with 5 mol% increment steps. Additional alloys in the AGSe-rich region were prepared to ascertain the range of the existence of the quaternary compound. High-purity elements (at least 99.99 wt%) were used for the synthesis. The silicon was crushed into powder in an agate mortar for better co-melting of the elements. The batches of 2 g mass were put in quartz containers that were evacuated to 10⁻² Pa and soldered. The synthesis was performed in a shaft-type furnace using a single temperature technique. The containers with the batches were heated at the rate of 40 K h⁻¹ to 770 K and held at this temperature for 24 h. Then, they were heated more slowly at 20 K h⁻¹ to 1270 K. The melts were kept at this temperature for 12 h and then cooled to 670 K (at the rate of 10 K h⁻¹). The alloys were annealed at this stage for 500 h. The synthesis process was completed by the quenching of the samples in cold water.

All the initial compounds and intermediate alloys were studied by differential thermal analysis (DTA) using the Paulik–Paulik–Erdey derivatograph (Pt/Pt–Rh thermocouple). The heating rate was 10 K min⁻¹. XRPD investigation was performed by DRON 4–13 diffractometer using CuK_α-radiation.

Single crystal data were collected at ambient temperature, using a four-circle diffractometer (Xcalibur Oxford Diffraction diffractometer) with CCD detector (graphite monochromatized Mo-Kα radiation, λ = 0.071073 nm). Scans were taken in the ω mode, the empirical absorption corrections were made by CrystalisRed.²³ The crystal structures of the quaternary compounds investigated in the present work were successfully solved by direct methods and refined using SHELX–97 package programs.²⁴

In order to elucidate the peculiarities of the electronic structure and chemical bonding in the synthesized Ag₂Ga₂SiSe₆ specimen, we used the X-ray photoelectron spectroscopy (XPS) in order to perform measurements of the XPS valence-band and core-level spectra. The measurements were made by employing the UHV-Analysis-System designed and assembled by SPECS Surface Nano Analysis Company (Germany). The UHV-Analysis-System is equipped with a PHOIBOS 150 hemispherical analyzer. The XPS spectra were collected using the Ag₂Ga₂SiSe₆ crystal sample. The XPS measurements were made in an ion-pumped chamber having a base pressure less than 6 × 10⁻¹⁰ mbar. The XPS spectra of the Ag₂Ga₂SiSe₆ sample were excited by a Mg Kα source of X-ray radiation (E = 1253.6 eV) and were recorded at a constant pass energy of 35 eV. The spectrometer energy scale was calibrated as described in ref. 25. The C 1s line (284.6 eV) of adventitious carbon was used as a reference to account for the charging effects, as it is suggested for relative quaternary Ag- and/or Ga-bearing chalcogenides.^26–28 Since in the Ag- and/or Ga-bearing ternary and quaternary tetra and sextasulfides (selenides) their electronic structures are determined by significant contributions of the valence S (Se) p and Ga p states throughout the whole valence band region (see ref. 27–31), we have also measured X-ray emission Se(Ga) Kβ₂ bands (K → M_II,III transition) representing the energy distribution of the Se(Ga) 4p states and compared them on a common energy scale with the XPS valence-band spectrum of Ag₂Ga₂SiSe₆. The fluorescent X-ray emission Se(Ga) Kβ₂ bands were recorded using a Johann-type DRS-2M spectrograph equipped with an X-ray BHV-7 tube (gold anode) and a quartz crystal with the (0001) reflecting plane, following the technique described in detail in ref. 32 and 33. The spectrograph energy resolution was determined to be about 0.3 eV in the energy regions corresponding to the position of the measured X-ray emission Se(Ga) Kβ₂ bands.

Results and discussion

Phase diagram of the AgGaSe₂–SiSe₂ system

Phase equilibria of the AgGaSe₂–SiSe₂ system were investigated for 24 samples over the entire concentration range (Fig. 1). The system is characterized by the formation of two compounds: Ag₂Ga₂SiSe₆ and AgGaSiSe₄. The liquidus consists of four fields of the primary crystallization of the compounds or solid solutions of AgGaSe₂, LT-Ag₂Ga₂SiSe₆, AgGaSiSe₄ and SiSe₂. Ag₂Ga₂SiSe₆ melts at 1042 K in a transition point that may be considered as a boundary case of the peritectic reaction L + AgGaSe₂ ↔ Ag₂Ga₂SiSe₆.

Fig. 1 Phase diagram of the AgGaSe₂–SiSe₂ system: (1) – L, (2) – L + α, (3) – γ′, (4) – L + γ′, (5) – γ' + η, (6) – L + η, (7) – η, (8) – L + η, (9) – η + β, (10) – L + β, (11) – β, (12) – α, (13) – α + γ′, (14) – α + γ′, (15) – γ' + γ, (16) – γ, (17) – γ + η.

The polymorphous transition of Ag₂Ga₂SiSe₆ takes place in the subsolidus region in the temperature range of 1009–1021 K, which indicates a certain homogeneity region for the quaternary phase at this temperature. The homogeneity region at the annealing temperature does not exceed 2 mol%. The structure of the low-temperature modification of the quaternary compound is tetragonal (see next chapter). We could not resolve the structure of HT-Ag₂Ga₂SiSe₆, mainly due to the narrow temperature range of its existence. The interaction of HT-Ag₂Ga₂SiSe₆ and AgGaSiSe₄ is eutectic, with the invariant point coordinates of 37 mol% SiSe₂ and 1027 K. AgGaSiSe₄ and SiSe₂ also form a eutectic point at 63 mol% SiSe₂ and 1010 K. We tried to synthesize a high temperature modification of Ag₂Ga₂SiSe₆, however, it was not possible, due to the narrow temperature range of its existence in a non-congruent type of compound formation. We attempted to harden the alloy during 100 hours at 1030 K and additionally, we tried to harden it during 100 hours at temperature 1030 K and liquidus higher than 50 K. Similarly, in the sulfur containing compounds,³⁴ the temperature range for high temperature modification is higher. However, in this case, we obtained a diffractogram containing a mixture of diffraction maxima originating from both modifications. The solid solubility in AgGaSe₂ was determined by the variation of the lattice periods at 670 K (Fig. 2), and reached 9 mol% SiSe₂.

Fig. 2 Variation of the unit cell periods in the solid solution range of AgGaSe₂.

The AgGaSiSe₄ has an open melting maximum at 1047 K, which agrees well with ref. 21. A flat melting maximum of AgGaSiSe₄ indicates partial dissociation of this compound in the melt. The homogeneity region of AgGaSiSe₄ is quite wide at the eutectic temperature of (∼46–58 mol% SiSe₂), but is narrowed considerably at lower temperatures.

Therefore, during the investigation of the AgGaSe₂–SiSe₂ phase diagram, we ascertained the composition of the known compound Ag₃Ga₃SiSe₈ (25 mol% SiSe₂). According to our results, it is described by the formula Ag₂Ga₂SiSe₆ (33.3 mol% SiSe₂). Diffraction patterns for the alloys of the system in the 0–50 mol% SiSe₂ region are shown in Fig. 3. Three sets of diffraction reflections are observed in this range. Diffraction pattern (1) corresponds to the AgGaSe₂ compound, which crystallizes in the tetragonal chalcopyrite structure (SG I4̄2d), the pattern (6) corresponds to the Ag₂Ga₂SiSe₆ compound, which crystallizes in the same space group, and the pattern (8) corresponds to AgGaSiSe₄. Despite having the same space group, their solid solubilities are rather modest (0–8 mol% SiSe₂). Although the diffraction patterns of the compounds are similar, the diffraction patterns of the intermediate alloys clearly exhibit the superposition of the reflections of both phases, with the gradual growth of the intensity of the reflections of the quaternary phases with the increase of the SiSe₂ content. It should also be emphasized that the 2-2-1-6 compounds are often found in the systems of this type. The first representative was Ag₂In₂GeSe₆ (SG Cc).³⁵ Later, they were found in other silver-containing systems, Ag₂In₂Si(Ge)S₆ (ref. 36) and Ag₂In₂SiSe₆ (ref. 37) (all SG Cc), as well as copper- (Cu₂In₂SiS₆ (SG Cc)³⁸), lithium- (Li₂In₂MX₆ (M = Si,Ge; X = S, Se) (SG Cc)^38,39) and sodium-containing systems (Na₂M₂M′S₆ (M = Ga, In, M′ = Si, Ge, Sn) (SG Fdd2 or Cc)^40,41).

Fig. 3 Diffraction patterns of the alloys of the AgGaSе₂–SiSе₂ system quenched from 670 K (all in mol% SiSe₂): (1) 0, (2) 10, (3) 15, (4) 25, (5) 30, (6) 33.3, (7) 40, (8) 50.

Crystal structure of LT-Ag₂Ga₂SiSe₆

Structural data, refinements and data collection are presented in Table 1. The structure was solved by the direct method after the analytical absorption correction. On the first stage of the structure solution, the positions of Ag, Ga, and Se atoms were obtained. The refinement of occupancies indicates that Ga and Si atoms form a statistical mixture in the 4a site and 4b site, partially occupied by Ag atoms. The one 8d site is fully occupied by Se atoms.

Table 1 Crystal data collection and refinement data for Ag₂Ga₂SiSe₆

Crystal data
Ag2.85Ga2.78Si1.22Se8 (or Ag2.13Ga2.08Si0.92Se6)	V = 362.67(4) Å^3
Mr = 1167.20	Z = 1
Tetragonal, I4̄2d (122)	F(000) = 509
a = 5.9021(1) Å	Dx = 5.344 g cm^−3
b = 5.9021(1) Å	Mo Kα radiation, λ = 0.71073 Å
c = 10.4112(10) Å	μ = 29.00 mm^−1
α = 90°	T = 293 K
β = 90°	0.06 × 0.05 × 0.01 mm
γ = 90°
Data collection
Radiation source: fine-focus sealed tube	Rint = 0.083
Graphite	θmax = 29.3°, θmin = 4.0°
3035 measured reflections	h = −8 → 7
242 independent reflections	k = −8 → 8
217 reflections with I > 2σ(I)	l = −13 → 14
Refinement
Refinement on F^2	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.026	w = 1/[σ^2(F0^2) + (0.018P)^2 + 0.438P] where P = (F0^2 + 2Fc^2)/3
wR(F^2) = 0.061	(Δ/σ)max < 0.001
S = 1.13	Δ>max = 0.83 e Å^−3
242 reflections	Δ>min = −0.76 e Å^−3
13 parameters	Absolute structure: Flack H D (1983), Acta cryst. A39, 876–881
0 restraints	Flack parameter: 0.04 (3)

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The obtained single crystal data show that the Ag₂Ga₂SiSe₆ compound crystallizes with the tetragonal space group I4̄2d as a disordered derivative structure from the CuFeS₂ type. The refined fractional atomic coordinates and displacement parameters are given in Table 2. The projections of the unit cell and coordination polyhedra of the atoms are shown in Fig. 4. The inter-atomic distances are listed in Table 3. All the atoms are enclosed in tetrahedrons. The packing architecture of tetrahedrons in the unit cell is shown in Fig. 5.

Table 2 Fractional atomic coordinates and thermal displacement parameters (Ų) for Ag₂Ga₂SiSe₆

Atom	Site	x	y	z	Uiso*/Ueq	Occ
Se1	8d	0.22293 (10)	0.2500	0.1250	0.0294 (3)	1.000
Ag2	4b	0.0000	0.0000	0.5000	0.0329 (4)	0.713 (3)
Ga3	4a	0.0000	0.0000	0.0000	0.0158 (4)	0.695 (4)
Si3	4a	0.0000	0.0000	0.0000	0.0158 (4)	0.305(4)
	U^11	U^22	U^33	U^12	U^13	U^23
Se1	0.0133 (3)	0.0266 (4)	0.0483 (5)	0.000	0.000	-0.0182 (3)
Ag2	0.0313 (5)	0.0313 (5)	0.0360 (6)	0.000	0.000	0.000
Ga3/Si3	0.0142 (5)	0.0142 (5)	0.0189 (6)	0.000	0.000	0.000

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Fig. 4 The projection of the unit cell and coordination polyhedra of the atoms for Ag₂Ga₂SiSe₆.

Table 3 Interatomic distances (Å) for Ag₂Ga₂SiSe₆

Se1—Ga3	2.3669 (3)	Ag2–Se1^v	2.5583 (4)
Se1–Si3^i	2.3669 (3)	Ag2–Se1^vi	2.5583 (4)
Se1—Ga3^i	2.3669 (3)	Ag2–Se1^vii	2.5583 (4)
Se1–Ag2^ii	2.5583 (4)	Ga3—Se1^viii	2.3669 (3)
Se1–Ag2^iii	2.5583 (4)	Ga3—Se1^ix	2.3669 (3)
Ag2–Se1^iv	2.5583 (4)	Ga3—Se1^x	2.3669 (3)

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Fig. 5 Packing of tetrahedrons in the unit cell for Ag₂Ga₂SiSe₆.

In order to avoid statistical mixture of Ga/Si atoms, the structure was transformed into the tetragonal space group I4̄ (a = 5.9021 Å, b = 5.9021 Å, c = 10.4112 Å; Se in 8g: x = 0.2247, y = 1/4, z = 1/8; Ag1 in 2b: x = 0, y = 0, z = 1/2; Ag2 in 2d: x = 0, y = 1/2, z = 3/4; Ga in 2a: x = 0, y = 0, z = 0; Si in 2c: x = 0, y = 1/2, z = 1/4) in which gallium and silicon atoms occupy separate sites, 2a and 2c, respectively.

However, attempts to refine the Ag₂Ga₂SiSe₆ structure in the possible I4̄ space group were unsuccessful. The better results were obtained for the tetragonal disordered structure model in the I4̄2d space group. Further details of the AgGaSiSe₄ crystal structure investigations (also CIF file) can be obtained from the Fachinformationszentrum (FIZ) Karlsruhe.†

XPS and X-ray emission spectroscopy features of Ag₂Ga₂SiSe₆

The survey XPS spectrum of the pristine Ag₂Ga₂SiSe₆ crystal surface is shown in Fig. 6. It is obvious that all the XPS survey spectrum features, except C(O) 1s levels and C(O) KLL Auger lines, may be assigned to core-levels or Auger lines of atoms constituting the quaternary Ag₂Ga₂SiSe₆ compound. It should be mentioned that the XPS C 1s core-level line (not shown here) for the pristine Ag₂Ga₂SiSe₆ crystal surface was established to be quite narrow, with its maximum fixed at 284.6 eV. The XPS C 1s core-level line was found to be without any shoulders at its higher binding energy side, which could be attributed to carbonate formation. This fact indicates that the C 1s line visible on the survey XPS spectrum of the pristine Ag₂Ga₂SiSe₆ crystal surface is evidently due to the hydrocarbons adsorbed from air. Furthermore, the XPS O 1s core-level line (also not presented here) with its maximum at about 532.3 eV is also because of oxygen-containing species adsorbed from air on the Ag₂Ga₂SiSe₆ crystal surface.

Fig. 6 Survey XPS spectrum of the Ag₂Ga₂SiSe₆ crystal.

Data from measurements of the binding energies for constituent element core-level electrons of the Ag₂Ga₂SiSe₆ crystal surface are listed in Table 4, whereas Fig. 5 displays the principal core-level spectra of the atoms constituting the quaternary Ag₂Ga₂SiSe₆ compound. These results indicate that the binding energies of the Ag 3d_5/2, Ge 2p_3/2 and Se 3p_3/2 (3d) core-level electrons correspond to Ag⁺, Ga³⁺ and Se²⁻, respectively.⁴² The effective charge state of the Si atoms in Ag₂Ga₂SiSe₆ is difficult to evaluate because the XPS Si 2p line is superimposed by the Ga 3p core-level spectrum, while the Si 2s line – by the Se 3p terms (see Fig. 7). Following Fig. 8, the XPS valence-band spectrum of the Ag₂Ga₂SiSe₆ crystal reveals two fine-structure peculiarities, namely A and B. In order to clarify the peculiarities of the occupation of the valence band by the Se 4p and Ga 4p states in the Ag₂Ga₂SiSe₆ compound, we have made the comparison of its X-ray emission Se Kβ₂ and Ga Kβ₂ bands and the XPS valence-band spectrum, using a common energy scale. The result of such a comparison is plotted in Fig. 9. It is worth mentioning that the zero energy of the X-ray emission Se Kβ₂ and Ga Kβ₂ bands and the XPS valence-band spectrum shown in Fig. 9, correspond to the position of the Fermi level of the PHOIBOS 150 hemispherical energy analyzer of the UHV-Analysis-System. Following Fig. 9, the energy position of the fine-structure peculiarity A of the XPS valence-band spectrum of Ag₂Ga₂SiSe₆ corresponds to the spectral position of the maximum of the X-ray emission Se Kβ₂ band, while the maximum of the X-ray emission Ga Kβ₂ band is positioned in the central portion of the XPS valence band spectrum. Furthermore, the above experimental results reveal that the Se 4p and Ga 4p states contribute significantly in other parts of the valence band region of Ag₂Ga₂SiSe₆.

Table 4 Binding energies^a of the constituent element core levels of the pristine surface of the Ag₂Ga₂SiSe₆ crystal

Core-level line/valence band spectrum	Binding energy
a Uncertainty of the measurements is ±0.1 eV.
Valence band region (features A and B)	3.5, 5.9
Ga 3d	20.2
Se 3d	54.6
Si 2p	104.7
Se 3p3/2	160.9
Se 3p1/2	166.1
Ag 3d5/2	367.9
Ag 3d3/2	374.0
Ga 3p3/2	1117.8

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Fig. 7 Detailed XPS core-level spectra of the Ag₂Ga₂SiSe₆ crystal: (a) Ga 3p_3/2, (b) Ag 3d, (c) Se 3p and Si 2s, (d) Si 2p and Ga 3p, and (e) Se 3d.

Fig. 8 Detailed XPS valence-band spectrum (including upper Ga 3d core-level) of the of the Ag₂Ga₂SiSe₆ crystal.

Fig. 9 Comparison, on a common energy scale, of the XPS valence-band spectrum and the X-ray emission Se Kβ₂ and Ga Kβ₂ bands of the Ag₂Ga₂SiSe₆ crystal.

Unfortunately, the available facilities do not allow the measurement of the energy distribution of the valence Si (s, p) and Ag (d) states, which are expected to be the other significant contributors to the Ag₂Ga₂SiSe₆ valence-band region.

Optical and photoelectric properties of LT-Ag₂Ga₂SiSe₆

The spectral distribution of the absorption coefficient in the region of the fundamental absorption edge of Ag₂Ga₂SiSe₆ crystals at 300 K is presented in Fig. 10. The band gap estimated by the energy quanta at the edge of the fundamental absorption band (for α = 300 cm⁻¹) is equal to E_g = 1.97 eV. The temperature dependence of the band gap energy at T > 77 K is linear and decreases with higher T (Fig. 11), demonstrating a clear shift of the absorption edge to the lower-energy region. Generally, the band energy gap is direct. With the enhanced temperature, one can expect a possible change to indirect band energy gap; however, it is not technically possible, as indicated above.

Fig. 10 Spectral distribution of the absorption coefficient of the Ag₂Ga₂SiSe₆ crystals at 300 K.

Fig. 11 Temperature coefficient of the bandgap energy variation in Ag₂Ga₂SiSe₆ crystals.

The temperature coefficient of the band gap variation (E_g (T) = E_g77 − β(T − 77)) was calculated as β = 7×10⁻³ eV K⁻¹. The obtained value is close to that of the majority of disordered chalcogenide semiconductors.⁴³

The Ag₂Ga₂SiSe₆ crystals are photosensitive semiconductors, with the conductivity multiple of 1.4 (300 K) upon illumination of 100 lux white light. The spectral distribution of the photoconductivity (SDP) of the studied compound is shown in Fig. 12. The low photosensitivity of the crystals is likely caused by the high concentration of rapid recombination centers that are usually related to the intrinsic defects of the crystal lattice. This is confirmed by the large half-width (∼200 nm) of the peak of the intrinsic photoconductivity. A characteristic feature of SDP is the presence of two maxima: I (λ_max1 = 640 nm) and II (λ_max2 = 900 nm). The first SDP maximum (λ_max1) of all investigated samples lies in the region of the edge of the fundamental absorption band, and is unambiguously caused by the intrinsic photoconductivity of the semiconductor. No substantial changes were observed in SDP at 77 K (compared to ambient temperature), except a moderate shift of the intrinsic photoconductivity maximum to the higher energy region, due to an increased E_g of the compound at lower temperature.

Fig. 12 Spectral distribution of the photoconductivity of the Ag₂Ga₂SiSe₆ crystals.

Laser induced gratings

The title crystals were studied with respect to the formation of the all-optical gratings. For this reason the titled single crystals were illuminated by two coherent beams originating from the pulsed Er:glass lasers. The 20 ns laser beams were formed by a splitting of the incident beam into the two channels. The first one possessed the fundamental laser wavelength of 1540 nm and the second one was formed by frequency doubled crystal KTP, cut under the angle of phase matching conditions. Registration was performed by CCD. Generally, the set-up is similar to that described in the literature.^43,44

After 1 min of the treatment, there occurred some interferograms, as shown in Fig. 13. The gratings occurred in a very narrow angle range, within 20–21°, with respect to the sample surface. Additionally, the pictures are clearly seen at some fixed ratios of intensities between the fundamental and the doubled frequency beam, varying between 4 : 1 up to 6 : 1. The sketches are presented for the power densities of the photoinducing beams, about 0.5 GW cm⁻². After 2–3 min, the process was saturated with the higher space frequencies (see Fig. 13). The origin of the effect is caused by the coexistence of the third and second order susceptibility tensors. The gratings thus formed were relaxed after several seconds, without any irreversible changes.

Fig. 13 General sketch of the grating plots at two different times of illuminations at power density of 400 MW cm⁻²: (a) at time of about 1 min; (b) at time of illumination of 3 min.

For the formation of such gratings, the occurrence of photoconductivity carriers that will favor the occurrence of the gratings is very crucial.

The proper second order optical effects, i.e., second harmonic generation, are relatively weak (about 0.5 pm V⁻¹ at 1064 nm). However, additional bicolour optical treatment leads to the formation of the stable gratings. The latter is determined by the intrinsic defect trapping states situated within the energy gap.

Based on the above, these crystals could be promising materials for dynamic holographic image recording, and they may be comparable to other chalcogenide crystals.⁴⁵

Comparison of the nonlinear optical properties of similar compounds, AgGaSe₂, Ag₃SiSe₈ and GaSe–AgGaSe₂,^46–48 has shown that the Ga(Si)–Se₄ structural clusters should play the principal role. Following these reasons and considering the crystallochemistry presented in Fig. 4 and 5, we performed B3LYP DFT quantum chemical calculations for the principal structural fragments by a method described in ref. 18. We discovered that the ground state dipole moment for the Si–Se₄ cluster is about 9.8 D, and 7.8 D for the Ga–Se₄ cluster, which is almost half order higher with respect to other clusters.

Conclusions

In the present work, we report a detailed study of the phase diagram of the AgGaSe₂–SiSe₂ system for the first time, using DTA and X-ray phase analysis methods. It was established that quaternary compounds, of the composition Ag₂Ga₂SiSe₆ and AgGaSiSe₄, form in the system. We have determined that the compound Ag₂Ga₂SiSe₆, melting at 1042 K, exists in two polymorphous modifications. The crystal structure of the low-temperature modification of Ag₂Ga₂SiSe₆ was determined to belong to the tetragonal space group I4̄2d (122), with the unit–cell parameters as follows: a = 5.9021(1) Å, b = 5.9021(1) Å, and c = 10.4112(10) Å. We have measured X-ray photoelectron core-level and valence-band spectra for the pristine Ag₂Ga₂SiSe₆ crystal surface. In addition, we have compared the X-ray photoelectron valence-band spectrum of Ag₂Ga₂SiSe₆ with the X-ray emission Se Kβ₂ and Ga Kβ₂ bands, representing the peculiarities of the energy distribution of the Se 4p and Ga 4p states, respectively, provided that a common energy scale is used. The comparison of the X-ray emission and photoelectron spectra indicate that the main contributions of the Se 4p and Ga 4p states occur in the upper and central portions of the valence band of Ag₂Ga₂SiSe₆, respectively, with their significant contributions in other valence-band regions. Measurements of the band gap for Ag₂Ga₂SiSe₆ reveal that the E_g value decreases from 2.13 to 1.97 eV when the temperature increases from 100 K to 300 K. The Ag₂Ga₂SiSe₆ compound is photosensitive. Its spectral dependence on the photoconductivity manifests the existence of two maxima at λ_max1 = 640 nm and λ_max2 = 900 nm. Regrettably, we could not resolve the structure of the high-temperature modification of Ag₂Ga₂SiSe₆, mainly due to the small temperature range of its existence.

To understand the origin of the optical and photoinduced effects observed, we performed B3LYP DFT quantum chemical calculations for the principal structural fragments (clusters) by a method described in the literature¹⁸. We discovered that the dipole moments of the Si–Se₄ cluster and the Ga–Se₄ cluster are equal to about 9.8 D and 7.8 D, respectively, which is almost half order higher with respect to other clusters, including those with silver. Therefore, all further designs of such kinds of materials with improved optical features should be concentrated on optimization of the Ga(Si)–Se4 clusters. Additionally, these clusters possess higher polarization, which may have potential for use in optical treatment.

The title crystals were explored with respect to the formation of all-optical gratings and the single crystals were illuminated by two coherent beams originating from pulsed Er:glass lasers. The gratings occurred in a very narrow angle range, within 20–21°, with respect to the sample surface. Additionally, the pictures are clearly seen at some fixed ratios of intensities between the fundamental and the doubled frequency beam, varying from 4 : 1 up to 6 : 1.

Acknowledgements

The project was financially supported by King Saud University, Vice Deanship of research chairs.

Notes and references

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Footnote

† Details of the AgGaSiSe₄ crystal structure investigations (including the CIF file) can be obtained from the Fachinformationszentrum (FIZ) Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +(49) 7247- 808-666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the appropriate CSD number: 431487.

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