Structure, optical, thermal, dielectric, hardness behaviors of l-histidine barium nitrate (LHBN): a metal-organic crystal for optoelectronic applications

3.1 Crystallographic details of LHBN

The X-ray intensity data for l-histidine barium nitrate (LHBN) single crystals were collected using a Bruker APEX II single-crystal X-ray diffractometer equipped with MoKα radiation (λ = 0.71073 Å). The ω/2θ scan method was employed for data acquisition. A crystal with dimensions of 0.18 × 0.14 × 0.12 mm³ was used for the analysis. The unit cell parameters for the as-grown LHBN crystal were determined to be: a = 24.9063(8) Å, b = 4.7226(10) Å, c = 8.3180(3) Å, α = 90°, β = 105.43(10)°, γ = 90°, and the unit cell volume was calculated to be 943.11(5) ų. ²¹ The crystallographic data for the as-grown LHBN crystal are summarized in Table 1. The observed limiting indices were found to be −39 ≤ h ≤ 40, −6 ≤ k ≤ 7, −13 ≤ l ≤ 11 and a total of 2117 unique reflections were recorded out of 6598 measured reflections.

The integrated intensities were corrected for Lorentz, polarization, and decay factors. Absorption corrections were applied using the psi-scan method. For structure refinement all observed reflections were used to refine the lattice parameters. The maximum and minimum transmission factors were determined to be 0.7799 and 0.6950, respectively. The crystal structure was solved by direct methods using the program SIR92 within the WINGX package and refinement was carried out by SHELXL97. ^{22 , 23} The ORTEP plot of LHBN, showing the ellipsoid representation at the 35 % probability level, is depicted in Figure 2.

Figure 2:

Thermal ellipsoidal projection (ORTEP) of LHBN.

3.2 Crystal structure of LHBN

Histidine derivatives play a crucial role in various biochemical and biomolecular interactions. The geometric parameters of the l-histidine moiety in l-histidine barium nitrate (LHBN) are similar to those observed in previously reported structures. ²⁴ The crystallographic analysis reveals a twofold symmetry axis, with the barium atom located at the center and coordinated by ten oxygen atoms. Six of these oxygen atoms are orginated from the carboxylate groups of l-histidine, while the remaining four contributed by two nitrate groups. The Ba–O bond distances range from 2.767(5) to 2.966(2) Å, which is within the expected range for such interactions. The LHBN molecules are linked through oxygen atoms, forming a one-dimensional chain along the [010] direction. These chains are further stabilized by N–H⋯N and N–H⋯O hydrogen bonding interactions, which interconnect the structures.

3.3 Morphological studies of LHBN

The l-histidine barium nitrate (LHBN) crystal exhibits six visible faces. Four of these faces, namely (0 1 0), (0 0 1), (1 0 0), and (2 0 1), are well-defined, as shown in Figure 3. The (1 0 0) face is identified as the largest flat face. The normal to this prominent flat face is oriented along the – c direction. The three crystallographic directions, a, b, and c, are also indicated in Figure 3. The c and −c directions are found to be nearly parallel, make an angle of β = 92.30°. The growth rate along the c-axis is observed to be greater than that along the other axes, while the crystal grows more slowly along the b-axis.

Figure 3:

Morphology of LHBN.

3.4 Spectral studies of LHBN

The infrared (IR) and Raman spectra of l-histidine barium nitrate (LHBN), presented in Figures 4 and 5, respectively, and primarily attributed to the vibrational modes of the l-histidine cation and its associated groups. The observed bands and their corresponding assignments are summarized in Table 2. All spectral data were compared with standard reference data reported in the literature.

Figure 4:

FTIR spectrum of LHBN single crystal.

Figure 5:

FT Raman spectrum of LHBN single crystal.

The (N–H⋯O) hydrogen bonding network, formed between the amino hydrogen atoms of the cation and the oxygen atoms of adjacent anions, plays a crucial role in achieving non-centrosymmetry within the crystal structure. In the IR spectrum, the stretching vibration of the O–H bond is observed at 3,362 cm⁻¹ but is absent in the Raman spectrum. The band at 3,127 cm⁻¹ in the IR spectrum is assigned to the stretching vibrations of the C–H bonds in the imidazole ring, ¹³ with a corresponding Raman line detected at 3,145 cm⁻¹. In the spectra of LHBN, these bands appear slightly shifted: 3,127 and 3,003 cm⁻¹ in the IR, spectrum and 3,145 and 3,123 cm⁻¹ in the Raman spectrum. The presence of such sharp and well-defined bands in this region is characteristic of histidine salts. ²⁰

The stretching vibrations of aliphatic CH and CH2 groups are observed at 2,957 cm⁻¹ and 2,940 cm⁻¹, respectively, in the Raman spectrum. In contrast, the corresponding stretching vibrations of the C–H bonds in CH and CH2 groups are not detected in the IR spectrum. A band at 2,852 cm⁻¹ in the IR spectrum is likely attributed to the stretching of N–H bonds in the imidazole ring, with this vibration absent in the Raman spectrum. ^{12 , 13} The band observed at 1,746 cm⁻¹ in the IR spectrum is associated with the C=O stretching vibrations of the carboxyl group. This characteristic band is absent in the Raman spectrum. The asymmetric stretching vibrations of the carboxylate group are observed at 1,602 cm⁻¹ and 1,609 cm⁻¹ in the IR and Raman spectra, respectively.

The bending vibration of the amino group is observed at 1,488 cm⁻¹ and 1,489 cm⁻¹ in IR and Raman spectra, respectively. The bands at 1,079 cm⁻¹ and 1,083 cm⁻¹ in both spectra correspond to the symmetric stretching vibrations of the nitrate ion. A band at 967 cm⁻¹ is detected in the Raman spectrum correspondence to the bending of the NH2 group and is absent in the IR spectrum. A band at 854 cm⁻¹ in IR and 853 cm⁻¹in Raman spectra are attributed to the wagging of the NH group. Additionally a broad band associated with the CNH group is observed at 787 cm⁻¹ in the IR spectrum.

A pronounced band at 677 cm⁻¹ in the FTIR spectrum corresponds to the bending vibration of the CH group, while its counterpart does not appear in the Raman spectrum. The bending vibration of the carboxylate group is indicated by peaks at 655 cm⁻¹ in the FTIR and 656 cm⁻¹ in the Raman spectrum. The rocking vibration of the amino group is observed at 487 cm⁻¹ in the FTIR spectrum and 455 cm⁻¹ in the Raman spectrum. The band positions in both the FTIR and Raman spectra exhibit good agreement, and the observed results are consistent with those reported for other l-histidine complexes. ^{25 , 26}

3.5 UV–visible absorption spectrum of LHBN

The absorption spectrum of l-histidine barium nitrate (LHBN) single crystal was recorded in the wavelength range of 200–800 nm using a Varian Cary 5E UV–visible Spectrometer, and the results are displayed in Figure 6. The lower cutoff wavelength for LHBN was found to be 226 nm. The crystal exhibits optical transparency in the wavelength range of 250–800 nm, with absorbance values remaining below one unit between 300 and 800 nm. This optical behavior is particularly beneficial for potential applications in optoelectronic device fabrication and higher-order harmonic generation. ¹⁸

Figure 6:

UV–visible spectrum of LHBN.

3.6 Thermal analysis of LHBN

Thermal analysis of the grown l-histidine barium nitrate (LHBN) crystals was performed to assess their thermal stability and melting point. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out using a NETZSCH STA 409C thermal analyzer in the temperature range of 26–800 °C at a heating rate of 10 °C/min, as shown in Figure 7. An alumina crucible was employed for heating the sample, and the analysis was carried out in a nitrogen atmosphere.

Figure 7:

TG-DTA curve of LHBN.

From the TGA curve, it is evident that LHBN is thermally stable upto 240 °C, indicating greater thermal stability compared to other l-histidine compounds. ^{27 , 28 , 29} The thermal stability is attributed to the strong bonding between the NH₃+ group and the barium metal ion, which stabilizes the crystal structure The endothermic peak observed at 305 °C in the DTA curve corresponds to the melting point of LHBN. Thermal degradation of LHBN noticed between 240 and 360 °C in diferent stages. In this first stage a weight loss of 11.82 % correspondence by the release of NH3. During the second stage (330–450 °C), 12.16 % loss is due to the decomposition of the volatile substance like CO and CO2. During the last stage at 360–580 °C, almost 18 % of weight is eliminated. In this stage most of organic materials are oxidized and NO2 gas. An additional endothermic peak at 580 °C indicates complete decomposition of the LHBN crystal. At 800 °C (22.36 %), leaving behind inorganic barium residues of BaO.

3.7 Dielectric measurements of LHBN

The capacitance and dissipation factor were restrained for the (0 1 0) plane of the LHBN crystal in the frequency series of 100 Hz to 5 MHz at 313, 323, 373, and 423 K. Dielectric constant (ε _r ), dielectric loss (tan δ) were considered by means of the equation

The values of ε _r and tan δ are shown in Figures 8 and 9 as a function of frequency at different temperatures. The dielectric polarization in total of the material is from the contribution of dipolar, electronic, space and ionic, charge polarization at inferior frequencies and the value of ε _r rises primarily due to the low frequency range between 100 Hz and 5 MHz which is produced due to the orientation of the dipoles.

Figure 8:

Log f versus dielectric constant of LHBN.

Figure 9:

Log f versus dielectric loss of LHBN.

The lower rate of dielectric value at high frequency is a suitable constraint for enhancing SHG competence. The decreasing dielectric constant ε _r with increasing frequency is mostly attributed to the space charge polarizations of the grown LHBN crystal. ³⁰ The grown crystal with high optical quality encompasses fewer defects due to detected lower values of dielectric loss at higher frequencies moreover ε _r and tan δ increase with rise in temperature. This is due to the enhanced dipolar orientation and space charge polarization at higher temperatures. At higher frequencies, the dielectric constant becomes relatively stable, as dipolar contributions lag behind the oscillating frequency. The low dielectric loss at high frequency shows that the crystal shows low energy dissipation, which makes the crystal suitable for nonlinear optical applications. ^{31 , 32 , 33}

The AC conductivity (σ_ac) studies have been measured for the LHBN crystal using σ_ac = ε_oε_rωtanδ where ω is the angular frequency (ω = 2πν) of the applied field, ε _r is the relative dielectric constant ε_o is the vacuum dielectric constant (8.85 × 10⁻¹² farad/m). Figure 10 shows the deviance of ac conductivity with various temperatures and frequencies. It is perceived that the value of ac conductivity rises with a rise in frequency. AC conductivity of grown crystal increases with frequency, exhibiting the strength of hydrogen bonds in molecules and making it more suitable for optoelectronic applications. ^{33 , 34}

Figure 10:

Log f versus AC conductivity of LHBN.

3.8 Microhardness measurements of LHBN

The grown LHBN crystals were tested using Vicker’s micro hardness tester along the (0 1 0) crystallographic plane. A constant indentation time of 15 s was deployed, and the applied load was varied from 5 to 50 g. The indentations impression along the diagonals of the sample was measured using Leitz Metallax II microscope. The H_ν (Vickers micro hardness) of the crystal was calculated using the following relation

where, d is the mean diagonal length of the indenter impression, P is the applied load, various loads of 10, 20, 30, 40, and 50 g were applied. Micro cracks were observed around the indentation site at the loads above 50 g and therefore, readings were not collected at higher loads above 50 g. A graph is drawn amongst H_ν and applied load P (Figure 11). It is witnessed from the graph that the hardness value initially surges with the surge of the applied load by obeying the indentation size effect (ISE). ³⁵

Figure 11:

Microhardness of LHBN

3.9 Second harmonic generation (SHG) studies of LHBN

The second-harmonic generation (SHG) of l-histidine barium nitrate (LHBN) was investigated using the Kurtz and Perry powder method, using Nd:YAG laser, operating at a wavelength of 1,064 nm. The as-grown LHBN crystal was finely ground into powder and loaded into a microcapillary with a uniform bore. A high-intensity laser beam was directed onto the sample and the SHG was confirmed by the emission of green light at 532 nm. The SHG efficiency of LHBN was compared to that of potassium dihydrogen phosphate (KDP). The SHG efficiency of LHBN was found to be 2.2 times that of KDP, indicating that the LHBN crystal exhibits a higher SHG efficiency compared to other l-histidine derivative crystals. ³⁶