Growth, Structure, and Optical Properties of Nonlinear LiGa0.55In0

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Growth, structure and optical properties of nonlinear LiGa0.55In0.45Te2 single crystal Pavel Krinitsin, Alexander Yelisseyev, Xingxing Jiang, Lyudmila Isaenko, Maxim Molokeev, Zheshuai Lin, and Alexey Pugachev Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01788 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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Crystal Growth & Design

Growth, structure and optical properties of nonlinear LiGa0.55In0.45Te2 single crystal

Pavel Krinitsin1, Alexander Yelisseyev1,2*, Xingxing Jiang3, Lyudmila Isaenko1,2, Maxim Molokeev4,5.6, Zheshuai Lin3,7, Alexey Pugachev8.

1Sobolev

Institute of Geology and Mineralogy RAS, 3 Ac. Koptyug ave., Novosibirsk 630090,

Russia 2Novosibirsk 3

State University, 2 Pyrogova str., 630090, Novosibirsk, Russia

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190,

PR China 4Laboratory

of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB

RAS, Krasnoyarsk 660036, Russia 5Department 6Siberian

Federal University, Krasnoyarsk 660041, Russia

7University 8 Institute

of Physics, Far Eastern State Transport University, Khabarovsk 680021, Russia

of Chinese Academy of Sciences, Beijing 100190, PR China

of Automation and Electrometry SB RAS, 630090, Novosibirsk, Russia.

*Corresponding to: [email protected], [email protected] (A.P. Yelisseyev)

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Abstracts In search of new nonlinear optical crystals for the mid-IR range, the single crystal of LiGa0.55In0.45Te2 (LGITe) of optical quality was grown, its structure was established and the spectroscopic properties were studied. LGITe crystallizes in tetragonal chalcopyrite-like (I 42 d) structure at 667±5ºС. Crystals are transparent in the 0.76-14.8 μm spectral region with some absorption bands related to OH/H2O and Ge-O vibrations. Analysis of the Tauc plots showed that LGITe is a direct band gap semiconductor with a band gap Eg =1.837 eV at 300 K. Raman spectrum for LGITe is very similar to that for LiGaTe2: the most intense band A1 near 120 cm-1 corresponds to the Li-Te vibration. DFT calculations were carried out for the first time to simulate the energy band structure (band gap~1.67 eV), density of states (DOS), birefringence (0.049 at 2 μm) and nonlinear susceptibility (d14 = d36= -48.73pm/V). For ternary LiInTe2 these parameters are Δn=0.007 and d14 = d36= -61.4 pm/V, respectively. The calculated optical properties indicate that LiGa0.55In0.45Te2 is a promising mid-IR nonlinear optical crystal with nonlinear parameters higher than those of LiGaTe2.


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1. Introduction Currently, there is a great interest in the use of Li-containing ternary chalcogenides of the LiBC2 family where B=In, Ga and C=S, Se, Te in nonlinear optics.1 These crystals are considered promising for direct high-power down conversion from near-IR to deep mid-IR.2 Optical parametric oscillators (OPO) based on LiBC2 crystals usually cover the spectral range from 1.5 to 12 μm when pumping at 0.820 μm. Recently, Japanese authors have expanded this range to 700 μm in the THz region.3 During the last decade particular attention is paid to using of Li-containing compounds for the detection of thermal neutrons.4 It is necessary to take into account that the nuclear reaction 6Li(n, α) 3H involves only the lithium isotope 6Li, the natural content of which is only 7.5 %. Thus, special enrichment with this isotope is necessary when growing crystals.5 The systematization and analysis of the rules of variations in the properties of the LiBC2 chalcogenides were carried out in 1987.6 There is a lot of information about the growth, linear and nonlinear optical properties of sulfides and selenides from this family, crystallized in the wurtzite-type orthorhombic structure. The compounds with B=Ga were found to have a wider band gap and higher optical damage threshold.7-10 Tellurides demonstrate an increase in nonlinear susceptibility dij in a set of LiGaS2 LiGaSe2LiGaTe2: dij values are 10.7; 18.2 and 43 pm/V, respectively.1 The dij values for In counterparts of these compounds are typically 30-50% higher, while tellurides have about 2 times higher birefringence Δn. LiInTe2 compounds have been known since 1985,11-12 but only as powders. Only LiGaTe2 crystals were grown recently.13-16 They were shown to crystallize in the chalcopyrite-type structure the same as well-known AgGaS2 and AgGaSe2, and active research of tellurides is in progress. Tellurides are characterized by a rather low melting point (about 660°C), which allows one to reduce the probability of explosion during growth and to improve the quality of crystals. In nonlinear optics, there is an active interest not only to ternary chalcogenides, but also to all kinds of quaternary compounds. They are expected to implement different variants of



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phase matching, including noncritical phase matching (NCPM), and to cover the spectral ranges that are not achievable for the ternary tellurides from this family. Examples of promising quaternary compounds in the application to nonlinear optics are AgGa1-x InxSe2

17

and CdxHg1-x Ga2S4 ,18 for which the NCPM mode is implemented. Among the Li-containing compounds, we have recently grown single crystals of Li2Ga2GeS6 ,19 LiGa0.5In0.5Se2,20 LiGaGe2Se6 ,21-22 LiGa0.5In0.5Se2,23 Li2In2GeSe6.24 In this paper, the growth of a single crystal of the quaternary LiGa0.55In0.45Te2 compound of optical quality is reported. Transmission spectra are studied and the bands of impurity absorption are revealed. The Tauc analysis shows that the crystal is a direct band gap semiconductor, the band gap of which is estimated. Raman spectra are obtained. For the first time ab initio simulation of LiGa0.55In0.45Te2 structure was carried out: we calculated the electronic structure and density of states (DOS), refractive index dispersion (birefringence) and nonlinear susceptibility. Crystals LiGa0.55In0.45Te2 are considered as promising for the mid-IR nonlinear optics.

2. Experimental 2.1. Crystal growth Crystals of LiGaxIn1-xTe2 were obtained similarly to LiGaTe2 (LGT).2, 13-16 Weighing and loading of initial elementary substances were carried out in a dry chamber purged with pure argon. The original elementary substances had a purity qualification of O. S. Ch. The basic substance content in the raw materials was 99.9 % for Li and 99.9999 % for In, Ga and Te. Small violations of stoichiometry, arising in the synthesis process due to the high chemical activity of Li and tellurium volatility, were corrected by introducing an excess of appropriate components. After loading the starting materials into a glass graphite crucible, it was placed inside a quartz container. The special geometry of the crucible prevents direct fusion of components, which leads to the release of a large amount of heat and a significant violation of


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stoichiometry in the synthesis process. The quartz container protects the substance from the action of the atmosphere due to degassing or filling of the free volume by inert gas. After loading of the initial substances, the container with the crucible was connected to the vacuum station. The free volume of the container was pumped out or filled with inert gas, after which it was desoldered. In all charges the Ga:In ratio was 1: 1. For the synthesis of LiGaxIn1-xTe2, a quartz container with a glass-graphite crucible placed in it was slowly pushed into a tubular resistance furnace heated to 850 °C degrees. The container was kept at 850 °C during a day, then the temperature was reduced to 700 °C and kept for another day, after which it was cooled to room temperature in the off-furnace mode. As a result, dense dark fine-crystal ingots with a phase content of LiGaxIn1-xTe2 close to 100% were obtained. Splitting and shifting of the ingots into a growth container was carried out in an inert atmosphere. After shifting, the container with the load was connected to a vacuum post and unsealed. Based on the results of X-ray phase analysis and refinement by the Rietveld method, it was found that the synthesized substance has the formula LiGa0.55In0.45Te2. For growing single crystals of LiGa0.55In0.45Te2 we used a modified BridgmanStockbarger method. The growth container was placed in a vertical resistance furnace. The design of the furnace allows one to change the ratio of temperature gradients in the melt and in the growing crystal. This can help to overcome a number of complex moments associated with the thermophysical properties of tellurides.12 The growth rate in all experiments was 4 mm/day, and the average value of axial temperature gradient was 3 °C/mm. After the growth stage, the furnace was cooled at a rate of 10 °C/hour. Melting temperature was estimated to be 667±5ºC for quaternary compound LiGa0.55In0.45Te2. The grown boule was 20 mm in diameter and about 26 mm long (Fig.1a). The sample was opaque, black in color. Two flat, approximately parallel, faces were operated. The images in transmitted light were visualized using a TV camera sensitive in the near IR range (Fig.1b). The study in polarized light showed that a significant part of the boule is single-crystal and has



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a high transparency. For spectroscopic measurements, thin plates with a thickness of 200 µm were produced. The image of four of such transparent plates glued to the glass substrate is shown in Fig.2. The main defects are relatively rare cracks. On the right lower plate there is also a thin microtwin. Such defects are typical of other nonlinear crystals with the chalcopyritetype structure, such as AgGaS2 and AgGaSe2. Atomic-emission spectral analysis was carried out for pieces taken from intitial, middle and final parts of the boule (samples LGIT-1, LGIT-2 and LGIT-3, respectively). The results are given in Table 1. Formula coefficients for LiGa1-xInxTe2 samples were normalized to Li. Confidence interval is specified for each element. It is seen that the results of atomic emission analysis are in good agreement with the data of the X-ray phase analysis and refinement by the Rietveld method.

Table 1. Formula coefficients for LiGa1-xInxTe2 samples (normalized to Li) Sample

Weight,

Formula coefficient

mG

Li

Ga

Formula

In

Te

LGIT-1

149.9

1

0.55±0.04

0.45±0.03

2.02±0.2

LiGa0.55In0.45Te2

LGIT -2

116.5

1

0.53±0.03

0.47±0.03

2.05±0.15

LiGa0.53In0.47Te2

LGIT -3

135.9

1

0.54±0.04

0.46±0.04

2.01±0.2

LiGa0.54In0.46Te2

.

Fig.1. Appearance of the LiGa0.55In0.45Te2 single crystal boule (a) and its view in transmitted light obtained with a TV camera sensitive in the infrared range.


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Fig.2. Four thin LiGa0.55In0.45Te2 plates 0.2 mm thick glued on a glass substrate for spectroscopic experiments. Images were obtained using a TV camera sensitive in the infrared range. One can see a twin in the right plate.

2.2. X-ray diffraction To determine the phase composition, the XRD analysis was carried out for the LGITe powders and the Rietveld technique was used to refine crystal structure. The powder diffraction data of LiGa0.55In0.45Te2 for Rietveld analysis was collected at room temperature with a Bruker D8 ADVANCE powder diffractometer (Cu-Kα radiation) and linear VANTEC detector (Fig.S1). The step size of 2θ was 0.016°, the 2θ range of 8-120º, and the counting time was 0.6 s per step. Additionally, XRD patterns were measured in the temperature range of 303-463 K. Rietveld refinement was performed by using TOPAS 4.2.25

2.3. Atomic-emission spectral analysis Atomic-emission spectral analysis was carried out using “Thermo Scientific”, iCAP6500 spectrometer to determine the composition of grown crystals.

2.4. Optical spectroscopy ACS Paragon Plus Environment


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Transmission spectra in the UV to near IR spectral range were recorded using a UV2501 PC Shimadzu spectrometer. Transmission spectra in the mid-IR were obtained on a Fourier transform spectrometer Infralum 801. The shape of the shortwave absorption edge was analyzed: we constructed the Tauc plots 26 to establish the type of the band-to-band electronic transitions for LiGa0.55In0.45Te2. Band gap values were calculated for 80 and 300 K. Raman spectra were measured at 300 K using a Bruker Vertex80+RAM III spectrometer at the 1.06 μm excitation and a confocal LabRAM HR microRaman spectrometer at the 0.532 μm excitation.

2.5. Ab initio calculations We performed the first-principles calculations using the plane-wave pseudopotential density-functional theory (DFT)27-28 implemented by CASTEP package.29 Calculation details can be found elsewhere.24 To calculate the second harmonic generation (SHG) coefficients we used the formula based on the gauge formalism method.30-31 The absorption, i.e., the imaginary part of the dielectric function, was calculated by the electronic transition across the forbidden band. The refractive index, i.e. the real part,

was determined using the Kramers-Kronig

transformation.32

3. Results and discussion 3.1. Structure When the charge was prepared correctly (without breaking the stoichiometric composition) and the thermal conditions were optimal, we obtained single crystals of LiGa0.55In0.45Te2 composition 20 mm in diameter and 26 mm long (Fig.1). The sample was not decomposed during the experiments. All peaks were indexed by tetragonal cell (I 42 d) with parameters close to LiInTe2,12 Therefore, this crystal structure was taken as a starting model for refinement. The site of In ion was occupied by Ga and In ions, and their occupancies were


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refined with linear restraint occ(In)+occ(Ga)=1 (Table 2, Fig.3). Refinement was stable and gave low R-factors (Rwp - 6.74 %, Rp - 5.27 %, Rexp - 5.20 %, χ2 - 1.30, RB - 1.36 %). Coordinates of atoms and main bond lengths are in Table 3 and Table 4, respectively.

Figure 3. Crystal structure of LiGa0.55In0.45Te2. Table 2. Main parameters of processing and refinement of LiGaTe2, LiGa0.55In0.45Te2 and LiInTe2. Compound

LiGaTe2, 13

LiGa0.55In0.45Te2

LiInTe2. 11,12

Space group

I 42 d

I 42 d

I 42 d

a, Å

6.3295 (6)

6.38124 (8)

6.398

c, Å

11.682 (1)

12.1108 (2)

12.460

V, Å3

468.01

493.16 (2)

510.0

Z

4

4

4

The crystal structure of LiGa0.55In0.45Te2 retains the structure of double tellurides and is a package of atoms with tetrahedral and octahedral voids. Half of the tetrahedral voids are occupied by Li+ and Ga3+/In3+ ions, whereas the remaining tetrahedral and octahedral voids remain free. In this case, the anionic sublattice is a face-centered cubic package. It was found that in the series LiGaTe2- LiGa0.55In0.45Te2-LiInTe2 the unit cell parameters increase, and for mixed compositions the parameters are close to LiInTe2 (Table 2).12 On the other hand, not the dependence of the "cell parameter on x (content In)”, but the dependence "cell volume on x” is



the most indicative for LiInxGa1-xTe2

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(Fig.4). You can see here that the point for our

quaternary compound LiGa0.55In0.45Te2 is just in the middle between parental LiInTe2 and LiGaTe2 compounds. Since the ionic radii for Ga3+, In3+ are 0.47 and 0.62 A33 and the electronegativity values are close (1.81 and 1.78, respectively),

34

the probability of formation

of continuous solid solutions in this system is high.

Figure.4. The cell volume versus (x) for LiInxGa1-xTe2.

Table 3. Fractional atomic coordinates and isotropic displacement parameters (Å2) of LiGa0.55In0.45Te2 Atom

x

y

z

Biso

Occ.

Ga

0

0

0.5

1.3 (1)

0.55 (1)

In

0

0

0.5

1.3 (1)

0.45 (1)

Li

0

0

0

5 (2)

1

Te

0.2572 (3)

0.75

0.125

1.3 (1)

1

Table 4. Main geometric parameters (Å, º) of LiGaTe2, LiGa0.55In0.45Te2 and LiInTe2. LGIT LGT LIT

2.6901 (12) Ga—Tei

2.611

2.7443 (13) Li—Teii

2.7693


2.736 2.725

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LGIT LGT

Tei—Ga—Teiii Tei—Ga—Teiv

LIT

108.46 (5)

113.04 (6)

111.51 (6)

107.72 (5)

111.137

Teii—Li—Tev

115.498

Teii—Li—Tevi

106.544

108.46

110.28

111.52

109.07

Symmetry codes: (i) -x+1/2, -y+1/2, z+1/2; (ii) x, y-1, z; (iii) -y+1, -x+1/2, z+1/4; (iv) x-1/2, y+1, -z+3/4; (v) -x, y-1/2, -z+1/4; (vi) -y+1, x, -z. The Rietveld refinement of structure under heating also gave low R-factors, but GaLi and Te phases were added. The dependencies of cell parameters (Figure 5) showed an increase in a(T), V(T) and a decrease in c(T) under heating, similarly to LiGaTe2 compound for the same reasons.35 The difference in the signs for a(T) and c(T) is typical of the crystals with chalcopyrite structure .1 It should be noted that the thermal coefficients αa are similar: 19.1×10-6 K-1 and 18.9×10-6 K-1 for LiGaTe2 and LiGa0.55In0.45Te2, respectively. However, αc of LiGaTe2 -8.6×10-6 K-1 has a bigger module in comparison with αc of LiGa0.55In0.45Te2 -5.7×10-6 K-1. Therefore, the replacement of gallium ions with larger indium ions is accompanied by a weakening of the effect of negative expansion of c(T) and one can predict that LiInTe2 will have nearly zero thermal expansion which is important for a wide class of devices. Further details concerning the crystal structure may be obtained from Fachinformationszentrum Karlsruhe, Germany.36



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Figure 5. Cell parameters a, c and V for LiGa0.55In0.45Te2 versus temperature: blocks (a), (b) and (c), respectively.

3.2. Optical absorption. The transmission spectrum for a 5 mm thick LGITe plate is shown by curve 1 in Figure 6. The transparency range of LiGa0.55In0.45Te2 is 0.76 -14.8 µm, whereas wide absorption bands near 2.92, 6.24, 6.96, 8.07 µm and a shoulder of about 10.9 µm are observed. For a thin 200 µm thick plate, the fundamental absorption edge is shifted to 0.66 µm and the transmittance increases faster with increasing wavelength. The 2.9 µm band is related to the OH - group and H2O extrinsic bands. The bands near 2.87 and 3.05 µm are ν3 and ν1 modes, corresponding to asymmetric and symmetric stretch vibrations, respectively. A band near 6 µm is related to ν2 bend vibration.37 Indeed, these bands increased when storing in air: the powder LGITe is the fastest to degrade. The 8 µm band is also widespread in Li ternary chalcogenides and in chalcogenide glasses, in which this band is related to Ge-O vibrations with participation of Ge and O impurity atoms.38 It can be seen that 200 μm thick plates are transparent enough in the range up to 20 μm and even further and can be used in various optical devices, including OPO. The 17 μm band refers to the two-phonon absorption in LGITe: In fact, a corresponding wide band of about 290 cm-1 (34.5 μm) is present in the IR absorption spectra of this material.


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Fig.6. Transmission spectra for 5 and 0.2 mm thick LiGa0.55In0.45Te2 plates (1 and 2, respectively). T=300 K.

The Tauc plot for LiGa0.55In0.45Te2, constructed for the case of 200 μm thick plate following,26 is given in Fig.7. The product (α* hν)2 versus photon energy is given. Here α is the absorption coefficient and hν is the photon energy. The constructed plot fits well the straight line: This means that the direct allowed electronic band-to-band transitions are responsible for the fundamental absorption edge as in the case of LiGaTe2.15 The cross point between this line and x-axis gives the band gap Eg value. It is 1.837 eV at 300 K. For comparison: Eg values are 4.15 eV, 3.48 eV39 and 2.41 eV15 at 300 K for LiGaS2, LiGaSe2 and LiGaTe2, respectively. As it is known, the Eg values for indium analogues are known to be slightly lower compared to gallium compounds: for example, the Eg values are 2.83 and 3.13 eV for LISe and LGSe, respectively.40 Experimental data on LITe are not available in the literature: there are only calculated data Eg= 1.313 / 1.513 eV41). It should be noted that the calculated values are usually underestimated in comparison with the experimental ones. Such an understatement is typical of the DFT-based methods.

41

Thus, the obtained value Eg= 1.837 eV, intermediate

between the values for LiGaTe2 and LiInTe2, is quite expected.

Fig.7. The Tauc plot for the case of direct band-to-band electronic transitions for LiGa0.55In0.45Te2. ACS Paragon Plus Environment


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3.3. Raman spectra Raman spectra for LiGa0.55In0.45Te2 and also LGS, LGSe, LiGaTe2 taken for comparison are given in Fig.8. The main components observed in Raman spectra of these four compounds are listed in Table 5. Intense lines are given bold and the most intense are additionally underlined. LGS and LGSe crystallize in orthorhombic structure with the C2V space group14, 40 whereas both tellurides have the tetrahedral chalcopyrite-type (I 42 d) or D212d (Nr.122) structure .12 The group analysis for LGS and LGSe shows that, in the centre of the Brillouin zone, the 48 normal modes are distributed in the following ways among various irreducible representations :40, 12 Γvib (C2V ) = 12 A1 +12A2 +12 B1 +12B2

For LiGaS2, there are two main groups of vibrations: one group is located in the 250 to 450 cm-1 range with a dominating line at 280 cm-1 (A1 mode) and the other one covers the range below 200 cm-1. They are associated with Li-S and Ga-S vibrations, respectively.40 The lowfrequency modes are highly covalent, whereas the high-frequency modes are mainly ionic.40 For LiGaSe2, these groups shift to 150-350 cm-1 and 60-130 cm-1, respectively, with A1 mode near 166 cm-1.41 The A1 shift, when S is replaced by Se, is in good agreement with the estimations based on ion masses. The ratio between experimentally measured phonon energies in the A1 line is hω(LiGaS2)/hω(LiGaSe2) =1.68 and the estimated value is (MSe/MS)1/2=1.87. For LiGaTe2 and LiGa0.55In0.45Te2 tellurides with a chalcopyrite-type I 4 2 d structure the distribution of optical vibrational modes among various irreducible representations is the following:42

0 ( I 4 2 d ) = A1+2A2+3B1 +3B2 +6E


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Here B2 and E are the polar Raman active modes, A1 and B1 are the nonpolar Raman active modes, and A2 is the optically inactive mode. For both tellurides, the most intense line is A1 and it is located near 120 cm-1. The ratio between experimentally measured phonon energies in the A1 line is hω(LiGaS2)/hω(LiGaTe2) =2.29 and the estimated value is (MTe/MS)1/2=1.99. A1 is totally symmetrical irreducible representation and only the anions participate in the A1 vibrations. Thus, the position of the A1 line is weakly sensitive to the mass of the second cation. At a fixed first cation (Li), there is a very weak shift (about 3 cm-1) towards large wave numbers when replacing the second cation in the AlGaIn series.41 This weak offset of the A1 line is the result of an increase in the size of a unit cell and a decrease in the force constants due to shifting of cations and anions from each other. Both In-containing tellurides (LiGa0.55In0.45Te2 and LiInTe2) have greater lattice parameters in comparison with LiGaTe2 because of the large size of the In atom than that of Ga. Raman spectra for LiGa0.55In0.45Te2, recorded at different excitation wavelengths are given in blocks (c) and (d) in Fig.8. Spectra (d) were obtained by excitation 1.06 µm, which corresponds to the region of transmittance of the crystal LiGa0.55In0.45Te2. On the other hand, Raman spectra in block (c) were recorded under the excitation of 0.532 µm. Since the transparency range for this crystal is 0.76 -14.8 µm, the 0.532 µm excitation corresponds to the band-to-band electronic transitions. Thus, the Raman spectra (d) carry information about the crystal lattice in the volume of the sample LiGa0.55In0.45Te2, whereas the spectra (c) correspond to different points in the near-surface layer of the crystal. It is seen that spectra (c) demonstrate significantly broadened lines: this indicates the deformation of the crystal lattice. Overall, Raman spectrum for LiGa0.55In0.45Te2 covers a range of 50 to 350 cm-1. Thus, two-phonon absorption determines the long-wave edge of the transparency range of about 14 µm.



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Fig. 8. Raman spectra for LGS and LGSe (a, spectra 1 and 2, respectively), LiGaTe2 (b) and LiGa0.55In0.45Te2 (c,d). Spectra (a, b, c) were recorded at 0.532 μm excitation whereas (d) corresponds to 1.06 μm excitation. Curve 2 is shifted upwards for clarity (a). Curves 4 and 8 are given with ×10 and ×5 magnifications, respectively. Curves 5 and 6 show Raman spectra, corresponding to different points on the surface.

Table 5. Components in Raman spectra for LiGaS2, LiGaSe2, LiGaTe2 and LiGa0.55In0.45Te2. LiGaS2 43

LiGaSe2 44

LiGaTe2 13

LiGa0.55In0.45Te2

LiGa0.55In0.45Te2

Exc. 0.532 μm

Exc. 0.532 μm

Exc. 0.532 μm

Exc. 0.532 μm

Exc. 1.06 μm 44.8 68.1

81.6

86.8

93.9 102

111.1

115.8 119.6

126.3 138.7

128.7

122.0

118.3

141.0

140.5

128.4 141.7

149.0 158.4 ACS Paragon Plus Environment

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174.5

168.9 199.5

166.0

171

196.0

198

305.0

305

207.6 211.0 220.4 244.7 255.7 262.3

270

283.5 306 313.1

322.3

323.5

334.5 343.4 351.5 360.5 372.6 379.4 443.6

3.4. Calculation results The electronic structure calculated from the first principles revealed

that

LiGa0.55In0.45Te2 is a direct transition semiconductor with both the valence band (VB) maximum and conduction band (CB) minimum located at Γ(0,0,0) point (Figure 9) in good agreement with the result obtained from analysis of the absorption edge shape (section 3.2). The smaller theoretical band gap (1.67 eV) than the value estimated from the absorption spectra (1.837 eV) is attributed to the discontinuity of the localized form of PBE functionals. The partial densities of states (PDOS) projected onto the constituent atoms are shown in Fig. 10 and accordingly some characteristics can be deduced:



Page 18 of 29

Fig.9. The calculated band structure along a highly symmetric path in the Brillouin zone for LiGa0.55In0.45Te2. Band gap at direct electronic transition at Γ point is Eg ~ 1.67 eV.

(1) the energy levels around -10eV are mainly formed of Ga 4s/4p, In 5s/5p and Te 5s orbitals. It is difficult to excite these orbitals by the perturbation of the external photoelectric field. Moreover, these orbitals strongly hybridize with each other, manifesting a strong covalent interaction within GaTe4 and InTe4 tetrahedrons. (2) The electron states at the top of VB and at the bottom of CB are formed by Ga 4s/4p, In 5s/5p and Te 5s/5p orbitals. It is necessary to note that the orbitals of lithium almost contribute nothing to any states, highlighting its strong iconicity. This means that the optical properties of LiGa0.55In0.45Te2 are predominantly determined by the electronic stimulation within GaTe4 and InTe4 tetrahedrons. On the other hand, the contribution from the orbitals of Li+ cations is negligibly small.


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Fig. 10. Density of states (DOS) and partial density of states (PDOS) projected onto the constituent atoms in LiGa0.55In0.45Te2. The band gap (1.67 eV) is highlighted by pink shadow.

Previous studies of Ag- and Li-containing ternary chalcogenides showed certain regularities in the replacement of the second cation Ga with a larger In ion: this increases the lattice parameters a and c, tetragonal distortion is attenuated (c/2a), the band gap is reduced significantly, almost twice.13 Such replacement is known to reduce birefringence by an order of magnitude, while the nonlinear susceptibility increases slightly.13 A factor determining high anisotropy of dense gallates is a strong distortion of the GaX4 tetrahedra, whereas the InX4 tetrahedra in a loose lattice of indates are distorted much weaker. Thus, when Ga is replaced



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with In simultaneously with the improvement of nonlinear properties, there is a negative effect of narrowing the range of transparency and reducing birefringence: the latter significantly limits the available range of phase matching. The optimal is some intermediate state with partial replacement of Ga with In. It was for this purpose that the present work was carried out. The most important optical parameters such as band gap, nonlinear susceptibility and birefringence for quaternary LiGa0.55In0.45Te2, as well as parental LiGaTe2 and LiInTe2 compounds are given in Table 6 and Fig.11. In each of the sections (a-d) in this figure, the midpoint corresponds to the quaternary compound, while the points on the left and right refer to LiGaTe2 and LiInTe2, respectively. We calculated from the first principles the second order nonlinear susceptibility values for the grown LiGa0.55In0.45Te2, and also for LiInTe2, for comparison. For d14=d25=d36, the calculated values of -48.73 pm/V and -61.4 pm/V, respectively, are obtained. However, close values d36=48.73 pm/V for LiGa0.55In0.45Te2 and d36 = -49.6 pm/V for LiGaTe2 surprise.38 The value of d36 =- 43 pm/V obtained experimentally48 and shown by a round dot in Fig.11c seems to be more appropriate.

Table 6. The most important optical parameters (experimental and calculated values) for tellurides LiInTe2, LiGa0.55In0.45Te2 and LiGaTe2 . Crystal LiInTe2

Tetragonal distortion, c/2a 0.973

Band gap Eg, eV (exp./calc.) 1.513/1.0

LiGa0.55In0.45Te2

0.949

1.837/1.67

LiGaTe2

0.923

2.4115/1.8549

Birefringence Nonlinear ∆n susceptibility, pm/V (exp./calc.) (exp./calc.) /0.007 /61.4 /0.042 0.09713/0.09


/48.73 39.348/49.638

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Fig.11. Dependence on X, the In contribution for the main parameters in LiGa1-xInxTe2 crystals: tetragonal distortion (a), band gap (b), second-order nonlinear susceptibility(c), and birefringence (d). Tetragonal distortion (a) is estimated based on XRD data whereas band gap was determined experimentally (b). Nonlinear susceptibility and birefringence were calculated from the first principles (square points). A round point in (c) shows the experimental value given in article48.

Thus, the second order nonlinear susceptibility of LiGa0.55In0.45Te2 is about a quarter higher than that for commercially available AgGaSe2 single crystal with d36 (at 9.2714 µm) = 39 pm/V.1 On the other hand, the nonlinear parameters of LiGa0.55In0.45Te2 are considerably larger than the parameters of other ternary Li-containing chalcogenides (d31=7.2 pm/V, d32=5.7pm/V for LiInS2 and d31=10.4 pm/V, d32=7.8 pm/V for LiInSe2 1). Additionally, we calculated dispersive refractive index for LiGa0.55In0.45Te2 and LiInTe2. The birefringence (Δn=no-ne) of LiGa0.55In0.45Te2 is as high as 0.049 near 2.0 μm whereas for LiInTe2 this parameter is 7 times smaller: Δn = 0.007 (Figure S2). The value for LiGa0.55In0.45Te2 exceeds that of AgGaSe2 (Δn~0.02) by two and a half times1 and indicates a ACS Paragon Plus Environment


Page 22 of 29

strong phase-matching ability of LiGa0.5In0.5Te2. On the other hand, birefringence of LiGa0.5In0.5Te2 is about 2 times smaller in comparison with LiGaTe2 (Δn~0.09).2 All abovementioned optical properties indicate that LiGa0.55In0.45Te2 is a potential NLO crystal used in the mid-IR region. On the other hand, multicomponent tellurides with a smaller band gap are expected to be preferable to selenides also in neutron detection. The experience gained so far on the ternary compounds of the LiBC2 family with C=S, Se shows that of tellurides the In-containing quaternary compounds of type LiGa0.55In0.45Te2 with a smaller band gap will also be preferable over LiGaTe2.

Conclusions Large single LiGa0.55In0.45Te2 crystals of optical quality were grown by the Bridgeman Stockbarger technique, and their crystal structure was established. The thermal expansion parameters for LiGa0.55In0.45Te2 were determined: αa and αc values are of opposite sign, but αc is 1.5 times smaller than that for LiGaTe2. As-grown LiGa0.55In0.45Te2 crystals are transparent in the 0.76-14.8 μm spectral region whereas some absorption bands are associated with impurities. The Tauc analysis showed direct band-to-band electronic transitions and band gap values Eg= 1.837 eV at 300 K. For LiGa0.55In0.45Te2 tellurides with a chalcopyrite-type I 4 2 d structure, the optical vibrational modes are decomposed into 15 normal modes A1+2A2+3B1 +3B2 +6E. Raman spectrum covers the 50 to 350 cm-1 range and A1 line near 120 cm-1 is the most intense. Twophonon absorption determines the long-wavelength edge of the transparency range near 14 μm. Electronic structure, density of states, and some optical properties were calculated from the first-principles for LiGa0.55In0.45Te2 and LiInTe2. Substitution of half of Ga ions by In allowed to improve nonlinear properties of LiGa0.55In0.45Te2 compared to LiGaTe2 (48.73 and


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39 pm/V, respectively), while maintaining the possibility of broad band tuning in optical parametric oscillators.

Acknowledgements This work was supported by state assignment project # 0330-2016-0008 and partly by the Russian Foundation of Basic Research (Grants No. 15-02-03408a, 17-45-540775r_a and 17-52-53031), national scientific foundations of China (Grants 11474292, 51702330, 11611530680, 91622118, and 91622124), the special foundation of the director of Technical Institute of Physics and Chemistry (TIPC), the China “863” project (No. 2015AA034203) and the Youth Innovation Promotion Association, CAS (outstanding member for Z. Lin and Grant 2017035 for X. Jiang). Some part of the experiments was performed in the multiple-access center “High-Resolution Spectroscopy of Gases and Condensed Matter” in IA&E SBRAS (Novosibirsk, Russia). Authors are grateful to N.F. Beisel from the Analitical laboratory of the Nikolaev Institute of Inorganic Chemistry SB RAS for atomic-emission spectral analysis of the studied crystals.

Supporting information X-ray crystallographic data of LiGa0.55(1)In0.45(1)Te2 (CIF). Figure S1. Difference Rietveld plot of LiGa0.55(1)In0.45(1)Te2. Figure S2. Calculated dispersion curves for LiGa0.55In0.45Te2 5Te2 (a) and LiInTe2 (b).

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For Table of Contents Use Only

Growth and optical properties of nonlinear LiGa0.55In0.45Te2 single crystal Pavel Krinitsin, Alexander Yelisseyev, Xingxing Jiang, Lyudmila Isaenko, Maxim Molokeev, Zheshuai Lin, Alexey Pugachev

Sinopsis The single crystal of LiGa0.55In0.45Te2 of optical quality was grown, its structure was established and the spectroscopic properties were studied. Electronic structure, density of states, as well as nonlinear susceptibility and birefringence are calculated from the first principles. LiGa0.55In0.45Te2 was found promising for mid-IR nonlinear applications.


Growth, Structure, and Optical Properties of Nonlinear LiGa0.55In0

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