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Aug 16, 2016 - Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Phys...
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Crystal Growth, Structure, and Optical Properties of LiGaGe2Se6 Alexander P. Yelisseyev,*,† Ludmila I. Isaenko,†,‡ Pavel Krinitsin,† Fei Liang,§,∥ Alina A. Goloshumova,† Dmitry Yu. Naumov,‡,⊥ and Zheshuai Lin*,§ †

V.S. Sobolev Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences, 43 Russkaya Street, Novosibirsk, 630058, Russia ‡ Novosibirsk State University, 2 Pirogova Street, Novosibirsk, 630090, Russia § Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China ⊥ A.V. Nikolaev Institute of Inorganic Chemistry of Siberian Branch of Russian Academy of Sciences, 3 Acad. Lavrentiev Avenue, Novosibirsk, 630090, Russia S Supporting Information *

ABSTRACT: Large single crystals of LiGaGe2Se6 were grown, and their structure and linear optical properties were studied. According to XRD results there is some disorder because of the Li ion fluctuation and their redistribution along two cationic sites. The shape of the fundamental absorption edge versus temperature was analyzed, and direct band gap values were estimated from the Tauc plots. Raman spectra were recorded and compared with results of ab initio calculations. The high quality of LiGaGe2Se6 crystals is confirmed by signals from free and self-trapped excitons. Photoluminescence in the 696 nm broad band and a set of bands in the 950 to 1100 nm range are related to self-trapped excitons and cation antisite defects, respectively. The luminescence intensity increases two orders as the crystal is cooled to 80 K. Four peaks are observed in the thermoluminescence curves with dominant ones at 218 and 410 K. Pyroelectric luminescence in the 100 to 180 K range confirms the noncentrosymmetric structure of this crystal.

■

INTRODUCTION

analogues, although nonlinear susceptibility of Li compounds is somewhat lower.7−9 During the last years studies have tried to combine these approaches and to create a quaternary compound based on both LiGaS2 and GeS2.10 These authors synthesized a fine-grained powder of Li2 Ga 2GeS 6 and established its orthorhombic structure. Crystal growth and optical properties were described in ref 11. A next step was the synthesis of LiGaGe2Se6 (LGGSe).12 These authors estimated the band gap (2.64 eV), calculated birefringence (Δn = 0.04 for λ ≥ 1 μm) and major second-harmonic generation (SHG) tensor elements for LGGSe (d15 = 18.6 pm/V and d33 = 12.8 pm/V).12 It is important that (1) nonlinear susceptibility of LGGSe is considerably higher in comparison with LiGaSe2 which has d31 = 10 pm/V13 and (2) LGGSe melts congruently at 710 °C, which is considerably lower than that for LGSe (915 °C).12,13 A decrease of the melt temperature lowers the risk for incongruent evaporation during crystal growth. A consequence of such evaporation may be a deviation from the stoichiometric composition and the appearance of inclusions of side phases.

Today the most widespread way to produce tunable coherent radiation is optical parametric oscillation (OPO), and the main part of OPO is a nonlinear crystal. The most widely used nonlinear crystals for the mid-IR are silver thiogallate and selenogallates (AgGaS2 and AgGaSe2),1,2 zinc germanium phosphide, ZnGeP2,3 etc. However, they possess serious drawbacks in their properties. For example, AgGaS2 and AgGaSe2 have a low laser damage threshold, AgGaSe2 is not phase-matchable at 1 μm (Nd:YAG), and ZnGeP2 exhibits strong two-photon absorption of conventional 1 μm (Nd:YAG) or 1.55 μm (Er:YAG) laser-pumping sources. Two ways are used to overcome these drawbacks. One way is to synthesize a quaternary compound. Thus, crystal AgGaGeS4 of the solid solution between the parent AgGaS2 and GeS2 allowed one to improve the laser damage threshold and to extend the range of phase-matching conditions.4−6 It is possible to pump the AgGaGeS4- based OPO using a Nd:YAG laser pumping, which is necessary for many other applications.4−6 Another way is to obtain crystals of the LiBC2 family, where B = In, Ga and C = S, Se, Te with a large band gap up to 4 eV. As a result, their optical damage thresholds exceed the parameters of their Ag-containing © XXXX American Chemical Society

Received: May 19, 2016

A

DOI: 10.1021/acs.inorgchem.6b01225 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) LiGaGe2Se6 boule of random orientation, 35 mm long and 15 mm in diameter. (b) Polished LGGSe plates 1 mm thick. Then the furnace was switched off. The obtained fine-grained charge was displaced to the crystal growth ampule. Single crystals of LiGaGe2Se6 were grown by the Bridgman− Stockbarger technique. The shape of the ampule was designed to provide nucleation and crystallization in its conical part. In some growth experiments the free volume of the ampule was filled with an inert gas. Typical values of the rate of ampule sinking were about 1 to 2 mm/day, whereas the temperature gradient near the crystallization front was 1.5 to 2 °C/mm. Heater temperature in the furnace was controlled using the chromel−alumel thermocouples. The accuracy of temperature maintenance was ±0.1 °C. Afterthe ampule sinking process, the furnace was cooled at a rate of 25 °C/h. The grown LGGSe single crystals were cones 35 mm long and about 15 mm in diameter at the cone base (Figure 1a). Plates about 1 mm thick were cut for spectroscopic study and polished (Figure 1b). X-ray Structural Analysis. The LiGaGe2Se6 crystal structure was investigated by X-ray diffraction single-crystal analysis. A clean and optically transparent sample 0.15 × 0.10 × 0.06 mm3 in size was used. Single-crystal data were collected at 150 K with Mo Kα radiation (λ = 0.71 Å) using a Bruker APEX DUO diffractometer (graphite monochromator, CCD detector). All calculations were performed using SHELXTL software {Bruker AXS Inc. (2004), APEX (version 1.08), SAINT (version 7.03), SADABS (version 2.11), and SHELXTL (version 6.12) Bruker Advanced X-ray Solutions, Madison, WI, USA, 5226 reflections were collected}. All structures were solved by means of the direct methods and refined by full-matrix least-squares techniques for 1812 independent reflections, R(int) = 0.0324. Goodness-of-fit on F2 is 1.031. Coordination numbers were defined by the Dirichlet polyhedrons method using the Xshell software.19 The graphics visualization of the structures was performed via the BS program (Balls & Sticks ver. 1.42 by Sung J. Kang and Tadashi C. Ozawa). Optical Spectroscopy. Transmission spectra were recorded using a UV-2501 PC Shimadzu spectrometer in the UV to near IR, whereas in the mid-IR we used an Infralum 801 Fourier transform spectrometer. The photoluminescence (PL) spectra were measured using an SDL1 luminescence spectrometer with excitation from a DTL-399QT pulsed Nd:YLF DPSS laser at 526 and 350 nm. To record the PL emission, we used a cooled FEU83 photomultipler, which is sensitive in the 350 to 1200 nm range. The photoluminescence excitation spectra were corrected to a constant number of incident photons using Na salicylate and Rhodamine 640. Raman spectra were measured on cleaved surfaces using a Horiba Jobin Yvon LabRAM HR800 spectrometer with a 1024 pixel LN/CCD detector using the 532 nm Nd:YAG laser. The curves of thermostimulated luminescence were recorded while heating the crystals at the rate β = dT/dt ≈ 20 °C min−1 after UV excitation during 5 min at 80 K. Computational Methods. The first-principles calculations are performed by the plane-wave pseudopotential method implemented in

The latter results in an increase of the light scattering and decrease of the optical damage threshold. Another application of Li-containing compounds is a detection of thermal neutrons. A solid-state semiconductor neutron detector typically consists of two parts: a neutron converter/absorber containing 10B or 6Li isotopes and a charged particle detector such as a Si crystal or diamond film.14−16 However, the semi-insulating crystal LGGSe with 6Li isotopes may act as both the converting and the detecting material. Lithium-6 isotope with a thermal neutron crosssection as high as 938 barns converts thermal neutrons to highly ionizing particles through the nuclear reaction 6Li(n,a)T, while the LGGSe semiconductor could detect electron−hole pairs induced by either tritium ions or α-particles operating as a semiconductor device or a scintillator. To date, attention has been paid mainly to LiInSe: it has a maximum value of the mobility−lifetime product μτ, which is a figure of merit of radiation detectors.17,18 Thus, a study of electrical and luminescence properties of LGGSe is of great importance. In present work large single crystals of LiGaGe2Se6 were grown, and their structure and linear optical properties were studied. Raman spectra were recorded and compared with the results of ab initio calculations. The shape of the fundamental absorption edge was analyzed, and direct band gap values were estimated from the Tauc plots. The high quality of the LGGSe crystals is confirmed by signals from free and self-trapped excitons. These crystals demonstrate an intense photoluminescence, which increases about 2 orders as the crystal is cooled to 80 K.

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EXPERIMENTAL SECTION

Crystal Synthesis and Growth. The LiGaGe2Se6 compound was synthesized from elementary starting components of high purity: lithium (99.99% pure) and gallium, germanium, and selenium (99.999% pure). Components were weighed and loaded inside a waterproof box filled with pure argon. The total mass of the substance was within 10−15 g, whereas Ge and Se contents were preset above the stoichiometric one in order to provide a 3 to 10 wt % excess of GeSe2. Starting components were loaded into a glassy-carbon crucible, which was placed inside a silica ampule. This ampule was joined to the vacuum system and evacuated to 10−3 Torr residual pressure. Afterward the ampule was sealed. Synthesis was carried out in a vertical dual-zone resistance furnace. The ampule with the load was placed in an area with a temperature gradient of about 5−6 °C/mm and heated to 750 °C during 2 days. B

DOI: 10.1021/acs.inorgchem.6b01225 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the CASTEP package.20 The generalized gradient approximation with the Perdew−Burke−Ernzerhof functional21 is adopted, and the ion− electron interactions are modeled by the optimized norm-conserving pseudopotentials22 for all elements. The kinetic energy cutoffs of 1000 eV and Monkhorst−Pack k-point meshes23 with a span of less than 0.07/Å3 in the Brillouin zone are chosen. Both the lattice constants and atom sites are fully optimized until the maximal Hellmann− Feynman force acting on each atom is less than 0.01 eV/Å. The linear response method24 is employed to obtain the phonon dispersion and Raman spectrum. The Raman spectrum is calculated with 0.5 cm−1 resolution under 532 nm laser excitation at 300 K temperature. Our tests reveal that the above computational parameters are sufficiently accurate for the present purposes. For the LiGaGe2Se6 crystal calculations, in order to simplify the situation of disorder occupation of atoms in the lattices, a disorder-free structural model is adopted by directly removing some Li atoms so that the vacancy and the Li atoms are alternatively located in an ordered manner. The volume of the structure is the same as the volume determined experimentally.

Figure 2. Crystal structures: (a) LGGSe; (b) LGSe; (c) Li−Se polyhedrons; (d) Li2Se4 chains in LGGSe.

Table 2. Cation−Anion Bond Lengths in LiGaSe225 and LiGaGe2Se6 Crystal Structures

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LiGaGe2Se6

RESULTS AND DISCUSSION Crystals. The LiGaGe2Se6 boules 15 mm in diameter and about 35 mm long were grown (Figure 1a). In Figure 1b the polished plates of optical quality are shown: one can see good transparency and the absence of defects such as cracks or inclusions. Crystals are orange in color. Structure. LiGaGe2Se6 crystals have a non-centrosymmetrical orthorhombic structure with the Fdd2 space group and a C192v positional symmetry (Table 1). Obtained data are in

bond

space group a, Å b, Å c, Å V, Å3 Dcalc, g/cm3 Z

LiGaGe2Se6

Pna21 6.832(1) 8.237(1) 6.535(1) 367.7(1) 4.2369(15) 4

Fdd2 12.5864(14) 23.746(3) 7.1304(7) 2131.1(4) 4.384(2) 8

distance, Å

bond

distance, Å

Li1−Se1 Li1−Se1 Li1−Se2 Li1−Se2

2.541 2.565 2.529 2.493

Ga−Se1 Ga−Se1 Ga−Se2 Ga−Se2

2.403 2.4 2.389 2.405

Li−Se

Table 1. Unit Cell Parameters in LiGaSe225 and LiGaGe2Se6 Crystal Structures LiGaSe2

LiGaSe2

Li1−Se3 Li1−Se1 Li1−Se1 Li1−Se2 Li1−Se2 Li2−Se1 Li2−Se1 Li2−Se3 Li2−Se2

2.56(4) 2.60(2) 2.72(9) 3.10(7) 3.11(8) 2.611(16) 2.68(3) 2.69(2) 2.81(3)

Ga−Se2 Ga−Se2 Ga−Se1 Ga−Se1

2.3855(6) 2.3855(6) 2.3935(5) 2.3935(5)

Ge−Se1 Ge−Se3 Ge−Se2 Ge−Se3

2.3597(6) 2.3618(5) 2.3663(6) 2.3707(6)

Ga−Se

Ge−Se

a good agreement with those in ref 12. The LGGSe structure is a frame built of GeSe4 and GaSe4 tetrahedrons. Li ions are situated in its interstices, forming distorted lithium polyhedrons: LiSe4 and LiSe5. All cation polyhedrons are connected via corner sharing. Chains of GeSe4 tetrahedrons, connected by LiSe 4, form layers parallel to (010), whereas gallium tetrahedrons join these layers with each other. Lithium tetrahedrons form curved chains along the c-axis (Figure 2c,d). There are no through channels in the quaternary selenide structure, in contrast to LiGaSe2, where such channels are formed by empty tetrahedral interstices. Calculated density is higher for Ge-containing crystals than for a ternary compound (Table 1). These features, together with more dense polyhedron packing in the LiGaGe2Se6 structure, influence its stability and determine mechanisms of impurity incorporation. Thus, ion substitution is preferable in these crystals, whereas interstitials are rather unlikely. According to XRD results, there is some disordering in the structure associated with the fluctuation of Li ions and redistribution along two cation sites (Li1 and Li2). This may be explained by a rather large size of the cavity, occupied by Li+. The deviation between Li1 and Li2 sites is 0.498 Å. Lithium may occupy one or another position in the unit cell (Table 2).

The occupancy of Li1 and Li2 sites is 24% and 76%, respectively. Li−Se bond distances are given in Table 2. Depending on the occupied site lithium has different coordination numbers. Li1 is surrounded by five Se2− ions (N = 5): its coordination polyhedron is a distorted trigonal dipyramid where two of five bonds are considerably longer. Li2 is located inside the LiSe4 tetrahedron (N = 4). It should be noted that data for Li2 are very close to previously reported ones.12 Li polyhedrons in the quaternary selenide structure are more asymmetric in comparison with LiSe4 in LiGaSe2. Thus, in the LiGaGe2Se6 structure the spreads of cation−anion bond distances are Δl = 0.55 and 0.202 Å for the Li1−Se polyhedron and Li2Se4 tetrahedron, respectively, whereas in LiGaSe2 Δl = 0.072 Å. The Li−Se bonds are much longer in LiGaGe2Se6 as compared with the ternary compound: in LiGaSe2 the average Li−Se distance is lav = 2.532 Å, whereas in LiGaGe2Se6 it is lav = 2.700 Å for Li2Se4 and lav = 2.818 Å for the Li1−Se polyhedron (Table 2). Ga−Se bonds in Ge-containing selenide become shorter than those in LiGaSe2: the average Ga−Se distance decreases from 2.399 Å in LiGaSe2 to 2.384 Å in LiGaGe2Se6. Optical Transmission/Absorption. The transmission spectrum for LGGSe is given in Figure 3 in comparison with C

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Inorganic Chemistry

Figure 3. Transmission spectra for LGGSe (1, 1a) and LGSe (2) plates 1 mm thick. Spectra were recorded at 300 K (1, 2) and 80 K (1a).

with results of ab initio calculations.12 A similar situation takes place for Li-containing ternary chalcogenides such as LiInS2, LiInSe2, LiGaS2, and LiGaSe2.25 A Tauc plot for room and liquid nitrogen temperatures is shown in Figure 4. The cross point between fitting a straight line and the x-axis corresponds to the band gap value Eg: at 80 and 300 K these values are 2.603 and 2.383 eV, respectively. Earlier, the authors of ref 12 established an Eg = 2.64 eV value at 300 K for LGGSe from the diffuse reflection. Obtained Eg values are larger than that of AgGaSe2 (1.8 eV) but smaller than that of LiGaSe2 (3.34 eV). The full set of Eg values at different temperatures in the 80 to 290 K range is given in Figure 5.

that for LGSe. One can see that the transparency range for LGGSe is red-shifted relative to its ternary analogue LGSe. At room temperature, for a 1 mm thick LGSe and LGGSe plate the transparency ranges at the 5% transmission level are 0.506− 13.5 and 0.555−17.1 μm for LGSe and LGGSe, respectively. Particularly important is the shift almost to 18 μm in the midIR. An intense absorption line near 14.8 μm in LGGSe is similar to those observed in ternary nonlinear crystals such as AgGaS2 and AgGaSe2: this line may be associated with the Fano resonance. A possible reason for this resonance may be submicrometer metallic inclusions as well as plasmonic effects.26 As the temperature decreases to 80 K, the fundamental absorption edge in LGGSe shifts to 0.475 μm at the 5% level. To determine the type of optical transitions responsible for the fundamental absorption edge, we studied a polished plate about 60 μm thick and recorded transmission spectra at 15 different temperatures in the 80 to 290 K range (at 80, 90, 120, 165, 181, 195, 211, 229, 234, 242, 248, 256, 267, 273, and 290 K). For such a thin plate the transmission edge is blue-shifted to 0.476 and 0.521 μm at 80 and 300 K, respectively. The fundamental absorption gives a direct line in coordinates (α × hν)2 = f(hν) (Figure 4), which corresponds to the case of direct allowed electronic transition between the (parabolic) valence and conduction bands. A direct allowed band gap agrees well

Figure 5. Temperature dependence of the band gap in LGGSe.

The energy band gap of semiconductors tends to decrease as the temperature is increased. This behavior can be better understood if one considers that the interatomic spacing increases when the amplitude of the atomic vibrations increases due to the increased thermal energy. This effect is quantified by the linear expansion coefficient of a material. An increased interatomic spacing decreases the potential seen by the electrons in the material, which in turn reduces the size of the energy band gap. The energies were fit with the Varshni equation,27,28 which is a common empirical fitting for the temperature dependence of semiconductor band gap energy and is given as follows: Eg (T ) = E0 − αT 2/(T + β)

Figure 4. Tauc plot: Absorption spectra for LGGSe near the fundamental absorption edge in coordinates (α·hν)2 = f(hν) at 80 (1) and 300 K (2). It is the case of direct allowed transitions between the parabolic bands. The Eg values are given.

(1)

where Eg(0) is the band gap value at 0 K, and α and β are the Vashni fitting parameters characteristic of a given material. In D

DOI: 10.1021/acs.inorgchem.6b01225 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Parameter Values for Modeling the Band Gap Energies Eg(0), eV α, eV/K β, K

Ge29

Si29

GaAs29

LISe

LGGSe

0.7437 4.77 × 10−4 235

1.166 4.73 × 10−4 636

1.519 5.41 × 10−4 204

3.07 0.99 × 10−3 245

2.64 1.9 × 10−3 280

Table 3 these fitting parameters are listed for germanium, silicon, gallium arsenide, LISe, and LGGSe. For LGGSe Eg(0) is 2.64 eV, whereas the parameters are α = 1.9 × 10−3 eV/K and β = 280 K. These values are very close to those for another Licontaining chalcogenide, LiInSe2. On the other hand, the band gap value at room temperature is 2.35 eV for LGGSe single crystal, which is lower than the 2.64 eV value obtained for the LGGSe powder by diffuse reflectance spectroscopy.12 Vibrational Spectra. We will now discuss the vibrational properties of LGGSe. First of all it is necessary to note that all ternary compounds of the family LiBC2 (where B = In, Ga and C = S, Se) with tetrahedral coordination of the atoms crystallize in the orthorhombic β-NaFeO2 structure with space group Pna21 (group of positional symmetry C92v).25 The primitive unit cell contains four formula units. According to the group theory, there are 48 normal modes at the center of the Brillouin zone, which can be described by the irreducible representations of the point group C2v as follows:30 Γ = 12A1 + 12A 2 + 12B1 + 12B2

(2)

where the three acoustic modes are A1, B1, and B2 and the remaining 45 optical modes are Raman-active modes. In the Raman spectra for LiBC2 (where B = In, Ga and C = S, Se) there are two groups of lines: one group is in the 250−350 and 150−250 cm−1 ranges for sulfides and selenides, respectively. The second group contains modes in the 350−450 cm−1 range for sulfides and in the 280−380 cm−1 range for selenides. The structure of the Raman spectra remains the same at In for Ga substitution, although there is some shift in position of first group lines. As expected from reduced mass considerations, the optical mode frequencies of selenides are shifted to lower frequencies due to the selenium substitution for sulfur. The total symmetrical A1 mode lies at 280 cm−1 for LGS and at 166 cm−1 for LGSe. The ratio of 1.66 of the frequencies of these lines corresponds to the square root (1.57) of the ratio of the atomic masses of selenium and sulfur. The high-energy group with frequencies above 250 cm−1 in LGS can be referred to as Li−Se vibrations.31 Experimental and simulated Raman spectra for LiGaSe2 are given in Figure 6a and b, respectively. A comparison of the experimental Raman spectrum and that calculated from first-principles shows that the gross features of these spectra are very similar: this indicates that our calculation is accurate enough. Each spectrum is a combination of many lines/modes; thus, the structure is well-ordered. The LGGSe compound crystallizes also in the orthorhombic structure with an mm2 point group, but the space group is different: Fdd2 (C192v). The primitive unit cell contains eight formula units. The factor group analysis gives the following irreducible representation: Γ = 29A1 + 29A 2 + 31B1 + 31B2

Figure 6. Raman spectra for LGSe (a, b) and LGGSe (c, d): experimental spectra (a, c) and those calculated from first-principles (b, d). In panel (c) the experimental spectrum for LGGSe is shown also with 5× magnification.

structure. In fact, Li atoms occupy two crystallographically nonequivalent Li1 and Li2 in the LiSe5 polyhedron and LiSe4 tetrahedron, respectively. The length of the Li−Se bond varies in the range of Δl = 0.55 and 0.202 Å in Li1Se5 and Li2Se4 polyhedrons, respectively. In the LiGaSe2 lattice the Li atoms occupy the only site and the Δl value is 0.072 Å only. This indicates the much stronger distortions in the LiGaGe2Se6 structure. A disorder-free structural model is adopted by directly removing some Li atoms so that the vacancy and the Li atoms are alternatively located in an ordered manner. Again experimental and simulated spectra were found to be similar. One can see two main groups of peaks in the 100−150 and 180−220 cm−1 ranges. There are three well-pronounced peaks near 76, 90, and 134 cm−1 in the first range, whereas their simulated analogues are at about 77, 89, and 138 cm−1. The most intense peaks in the second range are a 189 cm−1 and a 198/209 cm−1 doublet. Corresponding modes in the simulated spectra are at 182 cm−1 and 206/212 cm−1. The former peak at 189 cm−1 is comparable with the A1 mode of GaSe4 units observed in AgGaSe232 and Ba4CuGa5Se12.33 The doublet at 198/209 cm−1 can be assigned to the local modes of cornersharing GeSe4 units analogous to those in GeSe2.34 Photoluminescence. In Figure 7 photoluminescence spectra for LGGSe recorded at 350 and 526 nm excitations

(3)

The experimental and simulated Raman spectra of LiGaGe2Se6 are given in Figure 6 (in panels c and d, respectively). One can notice a broadband background in the Raman spectrum of LiGaGe2Se6, in contrast to the case of LiGaSe2. This may indicate some disorder in the LiGaGe2Se6 E

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Figure 7. PL spectra for LGGSe at 80 K, at 350 nm (1) and 526 nm (2) excitations.

Figure 8. Photoluminescence excitation spectra for emissions 760 (1) and 960 nm (2) at 80 K in LGGSe. Resolution is 2.0 nm. The arrow shows the band gap Eg = 2.603 eV at 80 K.

at 80 K are given. At 350 nm excitation, corresponding to a band-to-band transition, two broad bands with maxima near 696 nm (1.78 eV) and 960 nm (1.29 eV) are observed. At 526 nm excitation from the LGGSe transparency range there is a broad PL band near 1024 nm (1.21 eV). Thin lines in Figure 7 show the results of PL spectra decomposition into Gaussian components; their parameters are given in Table 4. Variation of position of the low-energy component indicates that there are at least two PL bands related to different defects. The photoluminescence excitation spectra recorded at 80 K for emissions at 760 and 960 nm are given in Figure 8. Here the arrow shows the Eg = 2.603 eV value at 80 K taken from Figures 4 and 5. One can see that the 696 nm band can be excited only in the 460 nm (2.7 eV) band corresponding to band-to-band transitions. PL in low-energy bands is excited not only at band-to-band transitions but also in two bands at 513 nm (2.416 eV) and 940 nm (1.318 eV). A large Stokes shift (∼1 eV) and impossibility to excite 696 nm emission in the LGGSe transparency range allow us to relate it to recombination of self-trapped excitons.35 A narrow line near 477 nm (2.60 eV) with a photon energy close to Eg is associated with absorption of free excitons. We did not find any PL bands in LGGSe that could be associated with selenium vacances VSe. Chalcogen vacancies in ternary chalcogenides of the LiBC2 family are known to emit at relatively short wavelengths, near the fundamental absorption edge. Thus, in LiInS2 and LiInSe2 the broad PL bands are observed at 600 and 720 nm:36 photon energies corresponding to maximum of these bands are approximately 0.8 × Eg. For LiGaS2 two PL bands near 425 and 450 nm are associated with sulfur vacances VS in the S1 and S2 sites.37 Features related to chalcogen vacances are easily identified when analyzing the changes in PL spectra at high-temperature annealing under vacuum or in the chalcogen vapor.36,37 High transmission in the LGGSe transparency range allows us to suppose that the

composition of our LGGSe crystals is close to the stoichiometric one and the VSe concentration is rather low. Maybe there is some deficit of Li that interacts actively with the ampule walls during the growth. In such case, the cation antisite defects when Ga and Ge occupy the Li sites are likely present. Indeed, the size of Ga3+ and Ge4+ ions is 0.47 and 0.40 Å, respectively: the Li+ ion is considerably larger (0.59 Å).38 The low-energy PL bands in the 950 to 1050 nm range can be associated with cation antisite defects, whereas variation in position of PL bands is due to both the type of B ion (Ga or Ge) and the Li site (Li1 or Li2). PL Temperature Dependence. In Figure 9 temperature dependences for dominating PL emissions in LGGSe are given. One can see that PL quenches quickly in the 80 to 300 K range as the temperature increases. In most cases temperature quenching of the intracenter PL fits Mott’s law suggested for a model with two recombination channels: one of them is radiative, whereas the other is a nonradiative transition from the relaxed excited state.39 Temperature dependence of the quantum yield η is described by the following expression: η(T ) ≈ [1 + τR ν0 exp( −ΔE /kT )]−1

(4)

where τR is the radiative lifetime at low temperature, ΔE is the energy barrier to the nonradiative decay route, and ν0 is a jump frequency. The proportional sign is used because other nonradiative decay channels must be available before the system reaches the relaxed excited state. However, sometimes such monotonic quenching is complicated by superposition of one or several peaks.39 In Figure 9 one can see such maxima near 140, 230, and 300 K. Such effects are due to centers ionization by exciting light and generation of free charge carriers, which may be captured by the traps. If one studies the PL temperature dependence on heating a sample from 80 K,

Table 4. Parameters of the PL Bands in LGGSe at 80 K (Results of Decomposition of PL Spectra into Individual Gaussian Components): Peak Position E, λ, and Full Width at Half-Maximum (fwhm) excitation

emission

photon energy (eV)

wavelength (nm)

Emax1 (eV)

λmax1, (nm)

fwhm1 (eV)

Emax2 (eV)

λmax1 (nm)

fwhm2 (eV)

3.541 2.356

350 526

1.287 1.21

963 1024

0.17 0.159

1.78

696

0.542

F

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term afterglow is typical for solids with traps of charge carriers. Free charge carriers are generated in LGGSe at UV excitation, which corresponds to the band-to-band electronic transitions. Some of these carriers are captured on the traps and may be released afterward as afterglow at fixed temperature or as a set of TSL peaks if the sample is heated. At first we subtracted the afterglow (curve 2) and then decomposed the resulting TSL curve into four components: peaks centered near 164, 228, 307, and 410 K. These components are shown as dotted curves 4a− 4d in Figure 10. Analysis of the peak shape shows that they are rather symmetric, and the half-width at half-maximum (hwhm) values at rise and at decay (δ1 and δ2, respectively) are close enough and sometimes δ2 > δ1. This indicates a second-order TSL kinetics when the probability of recombination of free carriers is negligible compared with the probability of their retrapping.40 The shape of the TSL peak is described by the following expression:40

Figure 9. Temperature dependence of PL at 350 nm (1−3) and 526 nm (4) excitation for emissions in the 760 (1, 3) and 940 nm (2−4) bands in LGGSe. Arrows show the direction of the temperature change (heating, curves 1−2, 4 and cooling, 3). Dotted curve 3a shows the results of the fitting of PL temperature dependence following Mott’s law (expression 4).

I(T ) = n02s exp( −ET /kT ) /[1 + (n0s /β) ×

∫T

T

exp( −E T /kT )dT ]2

(5)

0

where n0 is the initial concentration of electrons, s is the escape factor, β is the rate of linear heating (K/min), k is the Boltzmann’s constant, ET is the thermal activation energy (eV), and T0 is the initial temperature.40 Parameters ET and s, calculated for components 4a−4d following 2 are given in Table 5. There is a good coincidence between the sum of these

the charge carriers are accumulated on the traps and released at a specific temperature with further radiative recombination. Near this temperature a maximum is observed on the I = f(T) curve. A set of these maxima is similar to those in the curve of thermostimulated luminescence (TSL). This effect is much weaker if the sample is cooled (not heated). Thus, curve 3 recorded at cooling fits eq 3 with ΔE = 0.12 eV and preexponential factors (τR ν0) = 1.8 × 103 (dashed curve 3a in Figure 9). Nonmonotonic PL temperature dependence with maxima and quenching delay indicates the ionization of point defects in LGGSe both at 350 nm (band-to-band) excitation and at 527 nm illumination in the transparency range. To get information concerning the traps of charge carriers in LGGSe, we studied the TSL curves. Thermostimulated Luminescence. In Figure 10 one can see a TSL curve for LGGSe after 350 nm excitation (curve 1) and the results of its decomposition into individual components. One can see an intense emission at low temperature whose intensity decays slowly following a hyperbolic law, I(t) = I(0) × 1/(1 + 0.12t)0.8. Such long-

Table 5. Parameters of the TSL Components in LGGSe (Results of TSL Curve Decomposition) NN

TSL peak position, K

thermal activation energy ET, eV

1 2 3 4

164 228 307 410

0.13 0.27 0.37 0.71

escape factor s, s−1 1 1 1 5

× × × ×

102 104 104 105

components and the experimental curve (Figure 10). Positions of the TSL maxima agree rather well with those in the PL temperature dependence (Figure 9). Pyroluminescence. If the LGGSe sample is heated or cooled at a rate exceeding 30 K/min, one observes a spontaneous emission as a set of short light pulses in the 95−180 K range. In Figure 11 an intensity of such emission

Figure 10. TSL (experimental, 1) and hyperbolic PL decay at 80 K (2) in LGGSe. A corrected TSL curve after PL decay subtracting 3 and results of TSL curve decomposition into four individual components (dotted lines 4a−4d). Curve 2 shows a hyperbolic decay following I(t) = I(0) × 1/(1 + 0.12t)0.8.

Figure 11. PEL temperature dependence in LGGSe, recorded during the sample heating at a rate of 30 K/min. In the inset a PEL pattern during the LGGSe heating is given. G

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Inorganic Chemistry versus temperature for the case of LGGSe heating is shown. A pattern of such spontaneous emission is shown in the inset. Pink emission in the central part of the LGGSe sample is similar to the PL pattern of the same sample at UV excitation. One can see also an intense whitish-blue emission on the sample periphery and around it. Such emission occurs in the darkness, without any excitation: it is typical only of pyroelectrics and is a result of the modification in atomic polarization in crystal volume as the temperature changes.41 The strength of the accumulated pyroelectric field can reach many tens of kV. The occurrence of such pyroelectric luminescence (PEL) in LGGSe indicates that this crystal is related to pyroelectrics. All pyroelectrics have no symmetry center in their structure, and thus they can be used for nonlinear frequency conversion. This agrees with the orthorhombic cell Fdd2, as it has been established above and in SHG results in ref 12. Spontaneous light flashes are a result of the electric breakdown, and they happen both on the crystal surface and inside it.22 We have observed the PEL effect in many typical nonlinear optical crystals such as LiIO3, β-BaB2O4 (BBO), LiB3O5 (LBO), Ag3AsS3, LiNbO3, LiGaS2, KTP,22 and LGGS.11 On the other hand PEL is absent in AgGaS2 and AgGaSe2 with a chalcopyrite structure with space group I4̅2d, which are not pyroelectrics. PEL disappearance at high temperature is a result of electrical conductivity increase as the temperature grows: increased conductivity promotes leakage of the pyroelectric charge, and the breakdown probability becomes smaller. The PEL effect shows a low electrical conductivity of LGGSe samples and, in turn, a low concentration of electrically active defects. An increase of defect concentration and sample illumination before or during the heating may result in the increase of the electrical conductivity and in a leak of pyroelectric charge; thus, PEL quenching occurs. The PEL intensity weakens down to complete disappearance if the heating/cooling rate decreases: This is a result of a decrease in the strength of the pyroelectric field. Whitish-blue PEL emission in the inset of Figure 11 has no analogue in the PL spectrum. This emission corresponds to an electric discharge on the sample surface or in the gas around the sample. A pink emission in this pattern is considered to be an electroluminesce and partly a secondary luminescence inside the sample excited by the shortwave discharge emission.

crystals is confirmed by signals from free and self-trapped excitons. Pyroelectric luminescence in the 100 to 180 K range confirms the non-centrosymmetric structure of this crystal. Our experimental results indicate that LiGaGe2Se6 is promising as an IR NLO material for many practical applications. Further research is in progress.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01225. Crystallographic file for LiGaGe2Se6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (A. Yelisseyev): [email protected]. *E-mail (Z. Lin): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Foundation of Basic Research (Grant No. 15-02-03408a), state assignment project No. VIII.67.3.2, and the China “863” project (No. 2015AA034203).

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REFERENCES

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CONCLUSIONS Large single crystals of LiGaGe2Se6 are a new IR nonlinear optical material grown by the Bridgman−Stockbarger technique. It melts congruently at a rather low temperature of 710 °C. LiGaGe2Se6 crystallizes in the non-centrosymmetrical space group Fdd2. The structure is a three-dimensional framework composed of GaSe4 and GeSe4 tetrahedrons via corner sharing. Li ions are situated in its interstices, forming distorted lithium polyhedrons (Li1Se5 and Li2Se4). XRD shows some disorder in the structure because of the Li ions’ fluctuation and their redistribution along two cationic sites. Analysis of Tauc plots showed that LiGaGe2Se6 has a relatively large band gap (2.35 eV at 300 K). Main details of experimental and simulated Raman spectra agree rather well for LGGSe. The background in the experimental spectra is associated with some disorder in the structure. These crystals demonstrate a photoluminescence in the 696 nm broad band and a set of bands in the 950 to 1100 nm range related to self-trapped excitons and cation antisite defects, respectively. The emission intensity increases 2 orders as the crystal is cooled to 80 K. The high quality of LiGaGe2Se6 H

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DOI: 10.1021/acs.inorgchem.6b01225 Inorg. Chem. XXXX, XXX, XXX−XXX