1 Introduction

Non-linear optics is an emerging field as it extends the usefulness of lasers by increasing the original frequency of incident radiation. Non-linear optical (NLO) materials are capable of producing higher values of the original frequency and, hence, this phenomenon can find applications in optical modulation, fiber optic communication, photonics and opto-electronics [1–3]. In recent years, many researchers have tried to find varieties of NLO materials for laser applications. The complexes of organic material with inorganic acids and salts are promising materials for second harmonic generation (SHG) as they tend to combine the features of organic with that of inorganic materials. In general, organic materials are showing a good efficiency for SHG but poor mechanical and thermal properties. It is difficult to grow large size crystals with good optical quality of these materials for device applications [4]. Most of the amino acids and their complexes are the family of organic and semiorganic non-linear optical (NLO) materials that have potential applications in second harmonic generation (SHG), optical storage, optical communication, photonics, electro-optic modulation, optical parametric amplifiers, and optical image processing [5–10]. Also, this kind of crystals are very promising crystals for photo-induced optical and elasto-optical features [11].

Also amino acids are interesting materials for NLO applications due to the fact that the carboxylic acid group donates its proton to the amino group to form a salt of the structure CH3CHCOO−NH3 +. Glycine (NH2–CH2–COOH) is the simplest amino acid. Unlike other amino acids, it has as symmetric carbon atom and is optically inactive. It has three polymeric crystalline forms α, β and γ, in which α-glycine is commonly available. The peculiar physical and chemical properties are exhibiting by the glycine due to the presence of dipolar nature. This will lead the glycine as an ideal candidate for NLO, piezoelectric and pyroelectric applications. And another advantage of glycine is the presence of chromopores namely amino group and carboxyl group which make it as a transparent in the ultraviolet–visible region [12]. Recently, the amino acid group materials were mixed with organic or inorganic salts in order to improve their chemical stability, laser damage threshold, thermal, physical properties and linear and non linear optical properties. Glycine mixed with metal chlorides such as zinc chloride [13], calcium chloride [14], potassium chloride [15], sodium chloride [16], lithium chloride [17] have been reported in the recent years. Interest have been centered on semiorganic crystal which have the combined properties of both inorganic and organic crystals like high damage threshold, wide transparency range, less deliquescence, higher mechanical strength and chemical stability which make them suitable for device fabrication [18]. The advantage of including semiorganic material is to grow from aqueous solution and forms a large three dimensional crystal of excellent physico-chemical properties. Hence, it is necessary to synthesize and grow novel semiorganic crystals which have positive aspects of both organic and inorganic. It is difficult to grow the optically good quality organic single crystals in bulk size using slow evaporation technique. In the present work, the bulk (18 × 10 × 5 mm3) growth of (tri) glycine barium chloride single crystal with higher transparency (around 95 %) is achieved. Also the material has higher thermal stability (169 °C) compared to other organic materials. Hence, in the present investigation we report bulk growth (tri) glycine barium chloride crystal by solution growth technique. The grown crystals were characterized using single crystal XRD and powder X-ray diffraction, Fourier transform infrared (FT-IR) analysis, thermogravimetric analysis (TGA), differential thermal analysis (DTA) and UV–Vis spectroscopy. Optical constants like refractive index, reflectance, extinction coefficient and electric susceptibility and also dielectric constant, dielectric loss and photoconducting nature have been determined for the first time.

2 Experimental procedure

2.1 Synthesis

The compound (tri) glycine barium chloride was synthesized by reacting the glycine and (Merck, GR grade) barium chloride (Merck, GR grade) with stoichiometric ratio of 3:1. A necessary quantity of glycine is taken in a beaker and dissolved in double distilled water at room temperature until it attains a saturated condition. After preparing saturated solution of glycine, the proportionate amount of barium chloride was added with continuous stirring for 4 h to bring a homogenous mixture of solution. The (tri) glycine barium chloride was synthesized on the following chemical reaction.

$$3\left({\text{NH}}_{2}{-}{\text{CH}}_{2}{-}{\text{COOH}} \right) + {\text{ BaCl}}_{2} \to {\text{Ba}}\left( {\text{NH}}_{2}{-}{\text{CH}}_{2}{-}{\text{COO}} \right)_{3} {\text{Cl}}.$$
(1)

2.2 Solubility

In solution growth technique, the size of a crystal depends on the amount of the material available in the solution, which in turn is decided by the solubility of the material in that solvent [19] knowledge of solubility which also helps in selecting the crystal growth method. Hence, the solubility of the grown crystal has been determined.

The solubility was determined by dissolving the two times recrystallized solute in double distilled water in an air tight container maintained at constant temperature with continuous stirring. After attaining the saturation, the equilibrium concentration of the solute was analysed gravimetrically. The solubility of synthesized (tri) glycine barium chloride was determined for six different temperatures namely 30, 35, 40, 45, 50, 55 °C. A constant volume 40 mL of the saturated solution was used in this experiment. The measurement was performed by dissolving the (tri) glycine barium chloride salt in double distilled water at a constant room temperature with continuous stirring. The solution was constantly stirred for 4 h using magnetic stirrer for homogenization. The pH of the solution at super saturation is maintained around 5. From Fig. 1, it is observed that (tri) glycine barium chloride has positive temperature co-efficient solubility and also the solubility is increasing linearly with temperatures, exhibiting a high solubility gradient and hence it is suitable for growth by the slow cooling as well as by the slow evaporation method.

Fig. 1
figure 1

Solubility curve of synthesized (tri) glycine barium chloride

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2.3 Crystal growth

Recrystallization was carried out to eliminate any impurities in the (tri) glycine barium chloride crystal. The recrystallized salt was used for the preparation of saturated solution. The saturated solution was filtered using whattman filter paper to remove impurities. This super saturated solution was tightly covered with polyethylene sheet, to keep out dust before it was allowed to evaporate at room temperature. After 15–20 days good quality seed crystals were obtained. The good quality and defect free seed crystal was selected for bulk growth. The (tri) glycine barium chloride crystal of average dimension 18 × 10 × 5 mm3 has been harvested in the period of 25–35 days and the grown crystals are highly transparent. As grown crystal of (tri) glycine barium chloride is shown in Fig. 2. The optimized growth condition of (tri) glycine barium chloride is given the Table 1.

Fig. 2
figure 2

Photograph of as grown crystal of (tri) glycine barium chloride

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Table 1 Optimized growth condition of tri glycine barium chloride single crystal
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3 Results and discussion

3.1 Single crystal and powder X-ray diffraction

Single crystal X-ray diffraction analysis of (tri) glycine barium chloride was recorded using ENRAF–NONIUS CAD-4 diffractometer. The calculated lattice parameters are a = 8.281 Ǻ, b = 9.410 Ǻ, c = 14.898 Ǻ, α = β = γ = 90º and volume V = 1160.177 Ǻ3 which confirm the orthorhombic crystal system with non-centrosymmetric space group pbcn. Powder sample of (tri) glycine barium chloride was subjected to powder X-ray diffraction studies with Cukα (λ = 1.5406 Ǻ) radiation. The powdered sample was scanned in the range 10–80 °C at a scan rate of 1º per minute. In the powder XRD pattern well defined Bragg’s peaks are observed which reveals that the grown crystal has highly crystalline nature. The recorded indexed powder XRD pattern of the grown (tri) glycine barium chloride is shown in Fig. 3. The (hkl) values are indexed for corresponding intensity value using INDX software.

Fig. 3
figure 3

The powder XRD pattern of (tri) glycine barium chloride

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3.2 Fourier transform infrared (FTIR) spectroscopy study

The infrared spectral analysis is effectively used to understand the chemical bonding and provides information about molecular structure of the synthesized compound. Crushed powder of (tri) glycine barium chloride was pelletized using KBr. The spectrum was recorded using a Thermo Nicolet V-200 FTIR Spectrometer in the range 400–4000 cm−1 wavenumber region. The FTIR spectrum of (tri) glycine barium chloride is shown in Fig. 4. The peaks around 3432 cm−1 is due to NH asymmetric stretching. The peaks obtained at 2981, 2698 cm−1 for CH stretching. The peaks of IR spectrum at 2689, 2589 cm−1 is due to NH3 + stretching vibration. The peaks around 1571 cm−1 is due to NH3 + deformation. A peak at 1478 cm−1 has been assigned to NH2 deformation vibration. A peak 1404 cm−1 is due to COO− symmetric stretching. The peak at 1330 cm−1 is due to C–N–H symmetric bending. The peak around 1116 cm−1 is due to CH2 rocking. A peak at 1031 cm−1 for C–C–N C symmetric stretching. The peaks at 896 and 668 cm−1 are due to C–CN stretching and C–Cl stretching respectively. The band assignments for corresponding wavenumber of FTIR spectrum of (tri) glycine barium chloride are presented in Table 2.

Fig. 4
figure 4

The FTIR spectrum of (tri) glycine barium chloride

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Table 2 Band assignments of FTIR spectrum of (tri) glycine barium chloride
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3.3 Optical transmission study

The optical transmission spectrum was recorded using DOUBLE BEAM UV–Vis Spectrophotometer:2202 in the region 200–1000 nm and the optical transmission spectrum of (tri) glycine barium chloride is shown in Fig. 5. The transmission is maximum in the entire visible region and infrared region. In (tri) glycine barium chloride, the UV transparency cut-off wavelength lies at 234 nm and the percentage of transmission is high in the entire visible region from 234 to 1000 nm. The absence of absorption in the entire visible region makes the (tri) glycine barium chloride crystal as a potential candidate for second harmonic generation and various applications [20].

Fig. 5
figure 5

Optical transmission spectrum of (tri) glycine barium chloride crystal

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3.4 Determination of optical bandgap and optical constants

The measured transmittance (T) using optical transmittance spectrum was used to calculate the absorption coefficient (α) using the formula.

$$\alpha = \frac{{2.3026\log \left( {\frac{1}{\text{T}}} \right)}}{t}$$
(2)

where, T is the transmittance and t is the thickness of the crystal.

Optical band gap (Eg) has been evaluated from the transmission spectrum and the optical absorption coefficient (α) near the absorption edge is given by

$${\text{h}}\upupsilon = {\text{A}}\left( {{\text{h}}\upupsilon - {\text{E}}_{\text{g}} } \right)^{1/2}$$
(3)

where A is a constant, Eg the optical bandgap, h the planck’s constant and υ the frequency of incident photons. The band gap of the (tri) glycine barium chloride crystal was estimated by plotting (αhυ)2 versus hυ as shown in the Fig. 6. The value of bandgap was found to be 5.4 eV. The wide band gap of the (tri) glycine barium chloride crystal confirms the large transmittance in the visible region [21] and this crystal could be suitable for the optoelectronic devices like laser diode [22].

Fig. 6
figure 6

Tauc’s plot of (tri) glycine barium chloride crystal

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Extinction coefficient (K) can be obtained from the following equation:

$$K = \frac{\lambda \alpha }{ 4\pi }$$
(4)

The extinction coefficient as a function of absorption coefficient (α) is shown in Fig. 7. The transmittance (T) is given by [23].

Fig. 7
figure 7

A plot of extinction coefficient versus absorption coefficient

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$$\frac{{\left( {1 - R} \right)^{2} { \exp }\left( { - \alpha t} \right)}}{{1 - R^{2} { \exp }\left( { - 2\alpha t} \right)}}.$$
(5)

The reflectance (R) in terms of the absorption coefficient can be obtained from the above equation. Hence,

$$R = \frac{{{ \exp }\left( { - \alpha t} \right) \pm \sqrt {{ \exp }\left( { - \alpha t} \right)T} - { \exp }\left( { - 3\alpha t} \right) + { \exp }\left( { - 2\alpha t} \right)T^{2} }}{{{ \exp }\left( { - \alpha t} \right) + { \exp }\left( { - 2\alpha t} \right)T}}$$
(6)

The refractive index (n) can be determined from reflectance data using the equation.

$$n = \frac{{ - \left( {R + 1} \right) \pm 2\sqrt R }}{{\left( {R - 1} \right)}}$$
(7)

The absorption coefficient versus reflectance is shown in the Fig. 8. Figure 9 represents the variation of refractive index as a function of wavelength. The refractive index (n) decreases with increase in wavelength indicates that the grown sample absorbs at lower wavelength region. The variation of n and K values with respect to wavelength reveals the interaction of photon with electron. The refractive index ‘n’ is 1.5038 at 1000 nm and the refractive index is strongly dependent on wavelength.

Fig. 8
figure 8

Absorption coefficient versus reflectance (tri) glycine barium chloride crystal

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Fig. 9
figure 9

Plot of refractive index versus photon energy for (tri) glycine barium chloride crystal

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The electrical susceptibility (\(\upchi_{\text{c}}\)) can be calculated using the following relation,

$$\begin{aligned}\upchi_{\text{c}} & =\upvarepsilon_{\text{r}} - 1\quad ({\text{or}}) \\\upchi_{\text{c}} & = {\text{n}}^{2} - 1 \quad \left( {{\text{i}}.{\text{e}}, \,\upvarepsilon_{\text{r}} = {\text{n}}^{2} } \right) \\ \end{aligned}$$
(8)

Hence, Susceptibility = 1.25.

Since electrical susceptibility is greater than 1, the material can be easily polarised when the incident light is more intense.

3.5 TGA/DTA annalysis

Thermal properties of the material was studied by thermogravimetric (TGA) and differential thermal analysis (DTA) using STA 409 C instrument between the temperature 50 and 800 °C at a heating rate of 20 °C per min in the nitrogen atmosphere. (tri) glycine barium chloride sample weighing 4.237 mg was taken for the measurement. TGA and DTA curve of (tri) glycine barium chloride crystal is shown in the Fig. 10. DTA curve shows a sharp endothermic peak at 169.3 °C which corresponds to the melting point of the compound. Hence the thermal stability of (tri) glycine barium chloride is around 169 °C. The absence of water of crystallization in the molecular structure is indicated by the absence of weight loss around 100 °C. The material decomposes at 321.8 °C, which is represented by the sudden loss of the mass. The weight loss is due to the decomposition of glycine. Above 321.8 °C, the material undergoes irreversible endothermic transition around at 500 °C. From the TG curve, the mass loss takes place after the temperature of 169.3 °C. The mass lost from 169 to 321 °C is found to be 43 % which is the sublimation of the Cl. There is further mass loss of 7 % occuring in the temperature limit of 321–500 °C which involve the evolution of NH3. The actual residual amount of mass is 50 % which may be considered to be the compound of barium. From the above analysis, the melting point of the (tri) glycine barium chloride is 169 °C which is higher than the other semiorganic materials like bis-glycine hydrogen chloride (146.8 °C), tetra glycine barium chloride (160 °C), α-glycine sulpho-nitrate (143 °C) [24, 25].

Fig. 10
figure 10

TG/DTA curve of (tri) glycine barium chloride crystal

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3.6 Kurtz powder SHG test

In order to confirm the non-linear optical property of powdered sample of (tri) glycine barium chloride was subjected to KURTZ and PERRY techniques, which remains powerful tool for initial screening of materials for SHG efficiency [26]. A Q-switched Nd: YAG laser emitting 1.06 µm with power density up to 1 GW/cm2 was used as a source of illuminating the powder sample. The sample was prepared by sandwiching the graded crystalline powder with average particle size of about 90 µm between two glass slides using copper spices of 0.4 mm thickness. A laser was produced a continuous laser pulses repetition rate of 10 Hz. The experimental setup uses a mirror and 50/50 beam splitter. Here well known NLO crystal KDP is taken as a reference material.

The fundamental beam was spitted into two beams by the beam splitter (BS); one of them was used to illuminate the powder under study and the other constituted the reference beam of power Pω. Half-wave plate (HW) placed between two parallel polarizers (P) and was used to pump the beam power. The input power was fixed at 0.68 J and the output power was measured as 4.4 mJ, which was compared to output 8.8 mJ of standard KDP. The diffusion of bright green radiation of wave length λ = 532 nm (P2ω) by the sample confirms second harmonic generation (SHG). The powder SHG efficiency of (tri) glycine barium chloride crystal was about 0.5 times of KDP. The good second harmonic generation efficiency indicates that the (tri) glycine barium chloride crystals can be used as a suitable material for non-linear optical devices.

The laser damage threshold measurement was carried out on (tri) glycine barium chloride single crystals using single shot mode setup. The observed damage threshold value of (tri) glycine barium chloride single crystal was found to be about 8.21 GW/cm2 and it is greater than that of standard KDP and other well known crystals like urea, benzimidazole and BBO [4].

3.7 Dielectric studies

Dielectric properties are correlated with the electro-optic property of the crystals.

Figures 11 and 12 shows the variations of dielectric constant and dielectric loss of (tri) glycine barium chloride crystal at different temperatures as a function of frequency. The dielectric constant decreases with increasing frequency and becomes almost saturated beyond 2 kHz for all temperatures. The decrease in dielectric constant of (tri) glycine barium chloride crystal at low frequencies may be attributed to the dependence of electronic, ionic, orientational and space charge polarizations. The space charge contribution will depend on the purity and perfection of the material and it has noticeable influence in the low frequency region.

Fig. 11
figure 11

Variation of dielectric constant with log frequency at different temperatures of (tri) glycine barium chloride crystal

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Fig. 12
figure 12

Variation of dielectric loss with log frequency at different temperatures of (tri) glycine barium chloride crystal

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Hence, the larger values of dielectric constant exhibited by sample at low frequencies may be attributed to space charge polarization arising due to the crystal defects at grain boundary interfaces. At low frequencies, the charge on the defects can be rapidly redistributed so that defects closer to the positive side of the applied field becomes negatively charged, while defects closer to the negative side of the applied field becomes positively charged. This leads to a screening of the field and an overall reduction in the electric field. As capacitance is inversely proportional to the field, this reduction in the field for a given voltage results in the increased value of capacitance when the frequency is lowered. However, at high frequency, the defects no longer have enough time to rearrange in response to the applied voltage, and so the capacitance decreases.

The variations of dielectric loss (tan δ) with frequency are shown in Fig. 12. It is observed that the dielectric loss decreases with increasing frequency. The low value of dielectric loss indicates that the grown crystals are of moderately good quality. The larger values of dielectric loss (tan δ) at lower frequencies may be attributed to space charge polarization owing to charged lattice defects [27].

3.8 Photoconductivity studies

Photoconductivity measurements are carried out on a polished sample of the grown crystal by fixing it onto a microscope slide. The sample is connected in series with a DC power supply and KEITHLEY 485 picoammeter. The sample is covered with a black cloth and the voltage applied is increased from 0 to 160 V in steps of 20 V. The dark current is recorded. The sample is illuminated by the radiation from 100 W halogen lamp containing iodine vapour and tungsten filament. The photocurrent is recorded for the same values of applied voltage. The photocurrent is found to be less than the dark current at every applied field. This phenomenon is known as negative photoconductivity. Figure 13 shows the field dependent of dark and photo currents in (tri) glycine barium chloride crystal. The negative photoconductivity exhibited by the sample may be due to the reduction in the number of charge carriers in the presence of radiation [28]. The decrease in mobile charge carriers during negative photoconductivity can be explained by the Stockmann model [29].

Fig. 13
figure 13

Field dependent conductivity of (tri) glycine barium chloride crystal

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4 Conclusion

Optically good quality (tri) glycine barium chloride crystals have been grown successfully by slow evaporation technique at room temperature. Unit cell parameters and crystal system were determined by single crystal X-ray diffraction technique. Powder XRD shows good crystalline nature of the grown crystal. The FT-IR spectrum reveals the various functional groups present in the grown crystal. The UV cut off wavelength of (tri) glycine barium chloride crystal was found to be around 234 nm. From the UV spectrum the optical band gap was found to be 5.4 eV and also the absorption coefficient (α), extinction coefficient (K) were calculated and reported. The thermal analysis confirms the crystal is thermally stable up to 169 °C. The powder SHG efficiency analysis shows that the efficiency of crystal is 0.5 times than that of KDP. The dielectric studies show that the dielectric constant and dielectric loss of the crystal decreases exponentially with increase in frequency for different temperatures. The grown crystal exhibited negative photoconducting nature. The material has good optical transmittance in the entrie visible region and it has high thermal stability makes it an attractive candidate for second and third harmonics generation and device applications.