Growth, structural, thermal, and optical characteristics of L-asparagine monohydrate doped magnesium sulphate heptahydrate semiorganic crystals

A novel semi-organic crystal has been grown using slow evaporation technique by doping organic compound L-asparagine monohydrate (C4H8N2O3·H2O) with inorganic material Magnesium sulphate heptahydrate (MgSO4·7H2O). The crystallographic parameters like strain, dislocation density and crystallite size were calculated by powder X-ray diffraction method. Functional groups were identified and bond length, force constants were calculated from FT-IR spectroscopy. Energy dispersive X-ray (EDX) analysis was used to identify the constituent elements of the crystal. Kinetic and thermodynamic parameters, such as, activation energy Ea, change in Gibb's free energy (ΔG) and change in enthalpy (ΔH) have been determined by thermogravimetric analysis (TGA) analysis. Ea, ΔH and ΔG show positive values and change in entropy (ΔS) shows negative ones. The thermal degradation behavior of the crystals has been analyzed by differential scanning calorimetry (DSC) analysis. Various optical constants such as optical band gap, lattice dielectric constant, absorbance, extinction coefficient, the ratio of free charge carrier concentration to the effective mass, Urbach energy, optical and electrical conductivities were estimated from UV–vis transmittance data. High optical conductivity (1010 s−1) justifies the good photo response nature of the semi-organic crystal.


Introduction
Frequency mixing, photonics, second harmonic generation and optoelectronics are the rapidly developing fields of modern era where the use of organic non-centrosymmetric compounds has played a pivotal role [1].It is the π-bonds in the organic materials that cause high nonlinear susceptibility (χ) due to molecular hyperpolarizability (β).However, these organic crystals have some unfavorable features, such as volatility, low thermal stability and poor mechanical strength [2].In comparison, inorganic materials have low nonlinear efficiency but excellent thermal and mechanical stability.Therefore, the growth of a new type of material known as semi-organic crystal has become a recent trend, where the strong physical and chemical features of inorganic crystals are combined with the excellent nonlinear optical characteristics of organic materials [3].Over the past few decades, organic materials based on amino acid and their compounds have attained increasing significance for application in NLO devices.L-asparagine monohydrate is a well-known NLO substance in the amino acid group.Numerous studies have been conducted on L-asparagine doped compounds, notably L-asparagine L-tartaric acid, L-asparaginium picrate, and L-asparaginium L-tartrate [4][5][6].Magnesium sulphate heptahydrate (MgSO 4 ⋅7H 2 O) single crystal has become a significant compound of study for both academic and industrial purposes [7].Accordingly, in the current study, single crystals of magnesium sulphate heptahydrate (MSHH) have been grown in pure form and semi-organic crystals have been grown by doping L-asparagine monohydrate (LAM) with MSHH using the slow evaporation method.The grown crystals have been characterized for thermal, structural, and optical properties.

Growth of the crystals
The crystal was grown in pure form and by adding AR grade LAM with MSHH in double-distilled water.Later the solution was stirred constantly for 4 h with a magnetic stirrer, and finally, Whatmann filter paper was used to filter the solution.Recrystallization was carried out to improve the crystal's quality.Thereafter, the solution was poured into a beaker and covered by a transparent plastic sheet with small pores.Finally, it was placed in a dust-free chamber.After 40 days, transparent and well-shaped crystals were obtained as shown in Fig. 1.

Characterization method
The finely powdered crystals were subjected to powder X-ray diffraction test by Philips X'Pert PW 3040 Powder X-ray Diffractometer over the 2θ range of 10-80 • , with angle step size of 0.02 • with CuK α1 radiation (λ = 0.154 nm).The UV-Vis spectrophotometer (Shimadzu, UV-3100, Japan) was used to measure the optical transmittance spectrum of the crystals in the range of 190-1100 nm.For measuring the transmittance spectra, the crystal was gently polished until a thickness of 2 mm is reached.Simultaneous Thermal Analyzer STA 449 F 3 Jupiter was used to perform the TG/DSC at a heating rate of 10 K/min in nitrogen atmosphere.The FTIR spectra of the powdered samples were recorded in the range of 4000-350 cm − 1 using the KBr pellet technique by STA 449 F 3 Jupiter.

Identification of the crystal system of grown crystals
The narrow, strong and sharp peak of the X-ray diffraction is the evidence of growth of single crystal.The study identifies that the crystal belongs to orthorhombic nature with non-centro symmetric space group P 212121 (Fig. 2).The lattice parameters of the unit cell were estimated from the d-spacing values provided by the instrument and the corresponding ESDs are also listed in Table 1.The unit cell volume is observed to increase from 948.890 (Å) 3 to 969.893 (Å) 3 indicating LAM incorporation into MSHH crystal structure.It is also seen that the calculated and the reported values match quite well.

Determination of crystallite size, strain, and dislocation density
The crystallite size (d) has been obtained using Scherrer's Eq. (1).
where the value of constant k is 0.9 (only for spherically shaped particle [8]), the wavelength of X-rays (λ) is 0.154 nm, and full width at half maximum (FWHM) of the peak is represented by β.The strain (ε) was determined from the slope of the curve, plotted between βcosθ and 4sinθ representing Williamson Hall Eq.(2).The dislocation density has been determined using Eq.(3) as follows: Crystallite size, strain, dislocation density are given in Table 2.The crystallite size is observed to increase from 99.039 nm to 154.060 nm and strain from 0.3 to 1.4, which reveals incorporation of LAM into MSHH crystal matrix.The dislocation density is seen to decrease from 0.1020 × 10 − 3 nm − 2 to 0.0421 × 10 − 3 nm − 2 due to the addition of LAM, which is a clear indication of good crystallinity.

FTIR spectroscopy analysis
The FTIR spectra of a pure and LAM-doped MSHH crystal are shown in Fig. 3.The broad band at 3271.27 cm − 1 with the   stretching vibration.MgO group is reported to cause the peak at 661.85 cm − 1 [9].At 451 cm − 1 , the T d symmetry of the SO − 2 ion can be seen [10].The additional peak in spectra of LAM doped MSHH crystals at 564 cm − 1 is caused by torsional oscillation of NH 3

+
, which confirms the presence of LAM content in the doped sample.
Eq. ( 4) is used to calculate the bond length of the functional groups.The unit of the force constant k is measured in dynes per centimeter.
Hook's law is applied in eq. ( 5) to determine the force constant (k) which is an indicator of the stiffness of the bond [11].Bonds having shorter lengths are thought to be stronger and have a greater force constant (k).
where μ is the reduced mass (g) of the atoms being bonded and derived using Eq. ( 6) [12]: where the atomic masses of the atoms are m 1 and m 2 .Table 3 lists bond lengths, force constants of pure and LAM doped MSHH crystals.

Energy dispersive spectral analysis
Energy dispersive X-ray spectra shown in Fig. 4 provide evidence that LAM molecule has been absorbed into MSHH crystal structure.Additionally, the stoichiometries distribution of the chemical composition in the pure and LAM doped MSHH crystal is validated by the use of EDX spectra.The carbon content of LAM is observed to increase with doping concentration (Fig. 4).

Thermal analysis
Thermal analysis of the grown crystals has been conducted using thermogravimetric (TG) and differential scanning calorimetric (DSC) techniques.While DSC calculates heat (enthalpy) changes with temperature or time and TG analysis shows the mass change with    the same [13].Numerous kinetic and thermodynamic parameters are computed from TG curves.The TG profile in Fig. 5 shows that the mass loss happens in five stages and a respective shift in DSC curve is also evident.The first stage occurs in 60-80 • C temperature range where 1.5 % mass loss is observed, which indicates the release of one water molecule.According to Fig. 6, an endothermic reaction occurs in the initial dehydration stage, which is confirmed by the negative DSC peak [14].
The weight loss of 30 % appears as a steep fall in the second stage with a notable peak between 82 • C and 145 • C. A loss of roughly four water molecules of MSHH crystal and five water molecules for LAM doped MSHH samples may be responsible for the rapid decrease in mass [14,15].The third stage, which occurred between 145 and 203 • C, represents 12 % of mass loss showing a steady decrease in weight loss.In total, six water molecules are lost in the second and third phases representing a mass loss of about 42 % [14].An endothermic transition is evident by the negative DSC peak in the second and third dehydration steps.The fourth stage occurs between 204 and 221 • C. Between 291 and 365 • C, fifth stage is observed displaying a rate of 5 % mass loss which is slower than the previous stages.These phases feature the decomposition of MSHH molecules.
This indicates that MgSO 4 ⋅6H 2 O is changed to anhydrous MgSO 4 during these fourth dehydration steps [14].It is believed that successive decompositions are driven by the removal of SO 4 − 2 ions, which changes into SO 2 and O 2 and finally leave MgO as residue [10].
In Fig. 6, all DSC curves are observed to have endothermic peaks below 230 • C, confirming the dehydration step where energy is needed to eliminate moisture.The degradation of the crystal is indicated by succeeding exothermic peaks.Phase change is attributed to the small peak at about 340 • C.

Calculating the rate constant and half-life of the reaction by the TGA graph
Eq. ( 7) is a first-order reaction rate equation that has been applied to derive the activation energy along with other thermodynamic parameters [16].
Where, = wi− wt wi− w f , w t is the sample's weight at a specific time t, w i and w f are starting and final weights.From Eq. ( 7), we can write If a graph is plotted using Eq. ( 8), then it can be seen that each phase traces a straight line, indicating that the changes are reactions of the first order.The slope of every line was used to calculate the rate constant (k) for a specific phase, and Eq. ( 9) was applied to calculate the half-life (t 1/2 ).Table 4 provides the k and t 1/2 values.
where E a stands for activation energy, T indicates temperature in Kelvin, R is the gas constant (8.3143J mol -1 K − 1 ), A is the preexponential factor, and the heating rate β is 20 • C/min.The basic thermodynamic equations [24] were applied to calculate the remaining parameters.The activation energy for each phase is measured using graphs that are plotted with ln [ln (1 -x)] as y-axis and 1000/T as x-axis (Fig. 7).According to the values in Table 4, every phase indicates non-spontaneous endothermic process.

Estimation of thermodynamic parameters
Eqs. 11-13 were used to calculate different thermodynamic parameters for each phase, including change in enthalpy (ΔH), change in Gibbs free energy (ΔG), and change in entropy (ΔS).
where A is the Arrhenius constant and k b is the Boltzmann constant [24].Table 4 lists the estimated values for the thermodynamic parameters k, t 1/2 , ΔH, ΔG, and E a , ΔS are displayed in Fig. 8.The change in enthalpy (ΔH) in a chemical reaction symbolizes the energy change between reactants and products.It has been found that there is a very low energy barrier (~3 kJ/mol) between activation energy and change in enthalpy, indicating that the chemical reaction may begin easily [25].

Linear optical studies and relevant constants
Since the optical quality of a crystal is closely related to its electrical properties, atomic structure, and electronic band structure, it is very essential to know the optical characteristics of the crystal.In order to understand the optical behavior of a crystal, it is required to measure its optical transparency, absorption coefficient, extinction coefficient, band gap, and refractive index.

Optical band gap
Fig. 9 displays the optical transmittance of the grown crystals.Around at 257 nm, the cut-off wavelength is detected.The amine group having low scattering and low absorption nature is believed to eliminate defect centers making the doped crystals to have higher optical transmittance [26,27].The absorption coefficient (α) was calculated by applying the equation: where t and T represent the sample's thickness and transmittance (%).The relation between photon energy (hυ) and absorption coefficient (α) is given by where A is a constant and E g is the optical band gap [28][29][30], n is 1/2 and 2 for direct band gap and indirect band gap respectively.Table 5 lists the direct optical band gaps E gd which are calculated from Tauc's plot (Fig. 10).An important attribute for photonics and optoelectronic devices is good dielectric nature, which is clearly indicated by an increase in band gap values of the doped crystals.
Likewise, the doped crystal can be employed in solar panels as an antireflection coating that requires low reflectance, good band gap, low absorption, and high transmission.In a different way, the material's optical band gap E th was also assessed by applying Plank's equation given below.
where λ is cut-off wavelength.The values of E th matched reasonably with that E gd as shown in Table 5.The high transmittance in the visible range is confirmed by the crystal's large band gap, and this crystal may be ideal for optoelectronic devices such as laser diodes [31].

Extinction coefficient
The extinction coefficient estimates the amplitude decay of the incident magnetic and electric fields which is represented by the following equation [32].
where the wavelength of the incident beam is λ.Fig. 11 depicts how the extinction coefficient changes with wavelength.It is observed that the extinction coefficient (k) increases with wavelength until the cut-off wavelength is met.The lower value of k is thought to be caused by a weak interaction between the electrons and photons in the material.

Optical and electrical conductivity
The frequency response of the crystal to incident light, also known as optical conductivity (σ op ) was determined by applying the equation shown in Ref. [33].
where c is the speed of light, n is the refractive index, and α is the absorption coefficient.Fig. 11 shows how optical conductivity changes with photon energy of the incident light.The optical conductivity is observed to reduce with the increase in molar concentration.The high photo-responsive behavior of the crystal is confirmed by a higher optical conductivity value (10 10 s − 1 ), which validates as an appropriate material for use in computing and information processing [34].The transit of a large quantity of charge carriers from the valence to the conduction band is indicated by exponential growth at high energy values.A sudden increase in optical conductivity is observed at 5.2 eV confirming the exactness of energy band gap estimations.The electrical conductivity (σ e ) can be written in terms of the optical conductivity and absorption coefficient as [35].
Variations of electrical conductivity with photon energy are presented in Fig. 11.The lower range of electrical conductivity values reveals the dielectric nature of the crystal.

Complex dielectric constant
Complex dielectric constant (ε c ) can be determined by the following equation [36].
where the real and imaginary parts of the dielectric constant are ε r and ε i respectively and their relation with the refractive index n and extinction coefficient k are as follows [37].
The parameters ε r and ε i are depicted as a function of photon energy in Fig. 12.The low value of the imaginary part of the dielectric constant is a crucial condition for optoelectronic device applications because it shows an impediment in the transmission of electromagnetic (EM) energy.

Lattice dielectric constant and carrier concentration
The following equation can be used to determine the lattice dielectric constant from the refractive index (n) [38].
where e is the electronic charge, c is the speed of light, and (N/m*) is the ratio of the carrier concentration to the effective mass.The plot of n 2 versus λ 2 , is shown in Fig. 13 (a).From the slope of the linear part and from the intercept of the straight line to the n 2 -axis, the lattice dielectric constant ε L and ratio N/m × are calculated respectively and shown in Table 5.Table 5 also lists Urbach energy E u , which is determined from the plot of lnα versus photon energy as shown in Fig. 13 (b).The Urbach energy is found to decrease due to LAM doping, and this fall of Urbach energy is a clear indication of the decrease of defects in the doped crystals [39,40].

Conclusions
LAM doped MSHH semi-organic crystals with excellent optical quality have been grown by slow evaporation process from aqueous solutions and compared its properties with pure MSHH crystal.The XRD data showed that the grown crystals were orthorhombic.Additionally, the strong, sharp peaks and decrease of dislocation density show that LAM doped MSHH crystals have excellent crystalline structure.Increase of the unit cell volume, crystallite size and strain indicate the incorporation of LAM into MSHH crystal structure.Thermal investigation using TG and DSC showed that the doped crystals had improved thermal stability.All thermodynamic parameters, including a, ΔH, and ΔG values, are clearly positive while ΔS values are clearly negative, specifying a non-spontaneous chemical process.The fluctuation in activation energy is indicative that multi-reaction mechanisms exist during the process.It is evident from the DSC study that both endothermic and exothermic chemical processes are involved in the degradation process.The entrance of LAM molecule into the MSHH crystal lattice is confirmed by the energy dispersive X-ray and Fourier transform infrared spectroscopy.Furthermore, EDX analysis indicates the stoichiometric distribution of the C, Mg, S, and O components in the semiorganic crystal.LAM-doped MSHH semi-organic crystals have excellent transparency throughout the visible spectrum and can be applied to photonic and electro-optic devices.The band gap was determined based on UV-visible spectroscopy data and was found to increase with increasing doping concentration, making it suitable for applications in laser diodes.The estimated extinction coefficient confirmed its suitability for use in optical devices.For applications in optoelectronic devices, the low value of the imaginary part of the dielectric constant is a crucial requirement.The decrease in the Urbach energy and the increase in the optical band gap indicate that

Fig. 11 .
Fig. 11.(a) Extinction coefficient of pure and doped MSHH crystals.(b) Relations of Electrical conductivity (σ e ) and Optical conductivity (σ o ) with photon energy for pure and LAM MSHH crystals.

Fig. 12 .
Fig. 12.(a) Real and (b) imaginary part of dielectric constant with photon energy for pure and LAM doped MSHH crystal.

Table 1
Lattice parameters and calculated ESDs of pure and LAM doped MSHH crystals.

Table 2
Crystallite size, strain, dislocation density of pure and LAM doped MSHH crystals.OH stretching is caused by water molecules.At 1647.21 cm − 1 , the HOH bending vibrational band is observed.The peak for SO 4 − 2 bending vibration can be seen at 981.77 cm − 1 , while the peak at 1064.71 cm − 1 is the absorption band caused by SO 4 Strain ε ( × 10 − 3 ) Dislocation density δ ( × 10 − 3 ) (nm − 2 ) M.A. Rahman et al. characteristic

Table 3
Bonds and their lengths, force constants of pure and LAM doped MSHH crystals.

Table 4
Kinetic and thermodynamic parameters of all phases during thermogravimetric analysis.

Table 5
Optical parameters of pure and LAM doped MSHH crystals.