Effect of sunset yellow dye on morphological, optical, dielectric, thermal and mechanical properties of KDP crystal

2.1 Crystal growth

Pure and dye doped KDP single crystals were grown using commercially available potassium dihydrogen orthophosphate (Merck, 98%) and sunset yellow dye (Sigma Aldrich, 99.9%) as raw materials. KDP salt was dissolved in distilled water (150 mL) until saturation, followed by stirring for 4 h to obtain homogeneous solution. The resulting solution was filtered, kept in beaker with perforated aluminum foil lid and was maintained at fixed temperature of 30 °C (±0.1 °C) using constant temperature bath. Transparent crystals of KDP were harvested after a time period of 20 days. For the growth of 0.05 mol% (SKDP-1) and 0.1 mol% (SKDP-2) SSY stained KDP crystals, appropriate amount of dye was added to the starting solution. Good quality dyed crystals were obtained from the prepared solution in around 15–17 days. A similar growth boosting outcome was observed for several other stained crystals (Yadav et al., 2016a; Goel et al., 2018a). The picture of harvested un-dyed and SSY dyed host (KDP) crystals is displayed in Fig. 1.

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Fig. 1

Photograph of pure & SSY doped KDP crystals.

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2.2 Morphological studies on KDP crystal

The agreement between the observed habit of grown crystal and the theoretical habit derived from various available models in literature becomes important when it comes to the practical usability of dyed crystals in solid state dye lasers and other optoelectronic devices applications (Goel et al., 2017, 2018b; Yadav et al., 2016b, 2016c). How the external appearance of grown crystal is related to its internal crystal structure, has always been the question of interest both for the academicians as well as for the industrialists (Yadav et al., 2016b). To answer this quest, numerous theories are already available in literature to envisage the morphology of the crystal based on its internal crystal configuration (Yadav et al., 2016b; Docherty et al., 1991; Hartman and Perdok, 1955a,b). Fig. 2 displays the high resolution photograph (clearly depicting the crystal planes of actually grown KDP) and simulated indexed morphological drawing of as grown KDP crystal using WinXMorph program (Kaminsky, 2005). The external appearance of a crystal can be understood in terms of its unit cell dimensions and crystal symmetry (space group) using the combined form of law proposed by Bravais (in 1866), Friedel (in 1907), Donnay and Harker (in 1937) (Docherty et al., 1991), named as BFDH law. According to which, the most energetically favorable (slowest growing) faces are those which have largest interplanar spacing i.e., highest density of the intermediate layers (Docherty et al., 1991).

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Fig. 2

Photograph and morphological drawing (indexed) of un-dyed KDP crystal.

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The comparative growths of all the crystallographic facets present in KDP crystal play an inevitable role in the final appearance of the crystal. Based on BFDH theory, Table 1 lists the relative growth rates and morphological importance (M.I.) of facets of KDP crystal. BFDH model predicts the four prismatic (1 0 0), (0 1 0), $(\overline{1}00)$ & $(0\overline{1}0)$ facets with largest appearance or morphological importance. Therefore, we obtain that the crystal habit of a KDP crystal consists fourteen faces in total and the descending order of MI of various facets in KDP crystal can be arranged as: {1 0 0} > {0 0 1} > {1 0 1}. The indexed external appearance simulated using BFDH theory is depicted in last column of Table 1. On clear observation, one could conclude that the theoretically simulated habit by BFDH theory shown in Table 1 does not exactly match with the harvested habit (see Fig. 2(b)) of KDP crystal. The relative growth rate of KDP along [0 0 1]-crystallographic axis actually appears to be more than the predicted one (by BFDH).

Table 1 List of various (hkl) facets along with their morphological importance for KDP crystal based on BFDH theory.

(hkl)Facets	dhkl(Å)	Comparative growth rates	Morphological importance (M.I.)	Morphology predicted using BFDH theory
(0 1 0), (1 0 0), $\left(0\overline{1}0\right)$ , $\left(\overline{1}00\right)$	7.46	1.0	1.0
(0 0 1), $\left(00\overline{1}\right)$	6.98	1.07	0.94
$\left(101\right),$ (0 1 1), $\left(\overline{1}01\right),\left(0\overline{1}1\right),$ $\left(10\overline{1}\right),\left(01\overline{1}\right),\left(0\overline{1}\overline{1}\right),\left(\overline{1}0\overline{1}\right)$	5.09	1.47	0.68

One should not forget that while considering the BFDH law, the directions of strong and weak bonds present in KDP crystal system were overlooked. Therefore, a proper consideration needs to be given to various intermolecular interactions there among the crystallizing units of KDP crystal. In year 1955, two scientists namely, Hartman and Perdok successfully defined the connection of external crystal shape with the interaction bonds present between crystallizing units, which were called as periodic boundary chain (PBC) vectors (Hartman and Perdok, 1955b; Kaminsky, 2005; Hartman, 1956). KDP crystallizes in the I $\overline{4}$ 2d space group with a = 7.46 Å and c = 6.98 Å. In KDP unit cell, K⁺ and H₂PO₄⁻ serves as the basic building units which are arranged in a tetragonal lattice and results in crystallization of KDP bulk crystal. Fig. 5(b) depicts the unit cell packing diagram of KDP crystal view along ab plane (K⁺ cations have been omitted for the reason of clarity). Each phosphate H₂PO₄⁻ anion is hydrogen bonded to four other H₂PO₄⁻ anions which lie approximately perpendicular to the ferroelectric [0 0 1]-axis. These hydrogen bonds are the shortest (strongest) hydrogen bond, while the K⁺ cations form much weaker ionic interaction with the H₂PO₄⁻ anions. Presence of the tetragonal prisms on both sides of the tetragonal pyramid with {0 0 1} planes being least visible can be attributed to large number of periodic boundary chain vectors present parallel to [0 0 1] direction.

2.3 SSY dye inclusion in different growth sectors of KDP crystal

No change in the morphology of pure and SSY stained KDP crystal was observed except the incorporation of dye in selected growth sectors for dyed crystal. Due to anisotropic behavior of KDP crystal, various crystallographic faces have different affinities for SSY dye molecules in the aqueous dye solution (Hartman, 1956; Goel et al., 2018a; Goel et al., 2020). Fig. 3(a) displays the snap of as harvested SSY stained KDP crystal clicked under day light. The distribution of SSY dye into different crystallographic planes of KDP was studied by drawing the morphology of SSY-dyed KDP using ‘WinXMorph’ software. Fig. 3(b) depicts the indexed morphology and selective dye inclusion in KDP crystal. The coloring of KDP crystal was found to be biased and was limited to only to the pyramidal ({1 0 1} & {0 0 1}) growth sectors. The inclusion of SSY dye in ({1 0 1} & {0 0 1}) growth sectors of the KDP crystal was further confirmed by recording the image of SSY dyed crystal under long UV and short UV illumination (Fig. 3(c & d)). Similar dyeing patterns have been observed for KDP crystals doped with other organic azo sulfonated dyes (Chandran et al., 2017; Arumugam et al., 2019; Kanimozhi et al., 2019; Velikhov et al., 2007).

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Fig. 3

(a) Photograph of SSY stained KDP crystal clicked in day light. (b) Indexed morphological drawing of selective colouring in pyramidal ( $\{001\}$ & {1 0 1}) sectors of host KDP crystal. Fluorescence from the same SSY doped KDP crystal under (c) long UV, and (d) short UV illumination.

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As found in above studies, SSY molecules are introduced selectively into the pyramidal ( $\left\{001\right\}$ & {1 0 1}) growth sectors of the growing KDP crystal. Moreover, the prismatic growth sector did not attract any SSY dye molecules and remained colorless. Therefore, the dye inclusion selectivity of SSY dye in different growth sectors of KDP crystal can be ordered as: ({0 0 1} = {1 0 1}) > ({1 1 0} = 0). This selective nature of SSY dye molecules towards different crystallographic facets of host KDP crystal can be understood on the basis of positive charge at the ten faces {1 0 1} & {0 0 1} and the neutral charge of the four prismatic faces ({1 0 0}). Fig. 4 depicts the ball and stick representation of a single sunset yellow dye (disodium 6-hydroxy-5-[(4-sulfophenyl)azo]-2-napthalenesulfonate) molecule. It can clearly be seen that each dye molecule contains two negatively charged anionic, namely, SO₃⁻ functional groups. As a result of which, SSY dye molecule will show a strong tendency to get adsorbed on the positively charged facets of the KDP crystal, which can be ordered as ({0 0 1} = {1 0 1}) > ({1 1 0}), hence explaining the obtained pattern of dye inclusion. The understanding developed on selective SSY dye inclusion is summarized in Fig. 5. However, this model remains interim at this time (though suggestive) and accounts fordetailed analysis in near future. Further, conformity on the presence of SSY dye molecules was obtained by recording the solid state absorption characteristics of SSY-dyed KDP crystal in experimental section (see Section 3.4).

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Fig. 4

The ball-stick representation of the Sunset Yellow (disodium 6-hydroxy-5-[(4-sulfophenyl)azo]-2-napthalenesulfonate) dye molecule with atomic numbering scheme. Oxygen, sulfur, carbon and nitrogen atoms are represented as yellow, red, dark grey and blue color balls, respectively. The hydrogen atoms are shown by green capped sticks for clear view.

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Fig. 5

(a) KDP crystal containing luminescent SSY dye in the ( $\{001\}$ & {1 0 1}) growth sectors.(b) Molecular packing diagram of KDP crystal when viewed along ab plane. (c) The habit of KDP crystal bounded by SSY dye molecule that stains the pyramidal ( $\{001\}$ & {1 0 1}) growth sectors. Conformations of guest dye molecules in pyramidal sectors using ball- & stick diagrams.

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2.4 Characterization techniques

The crystallographic structure of the grown crystals was examined by Rigaku Ultima IV X-ray diffractrometer with CuK_α (λ = 1.5405 Å) radiation operated at 40 kV and 40 mA. FTIR spectra of pure and SSY dye doped KDP crystals were recorded by Perkin Elmer Frontier FTIR spectrometer between the wavenumber 4000–400 cm⁻¹ using KBr pellets. Raman spectra of the crystals powder were recorded by Via-Renishaw Laser Raman Spectrometer having laser photon of wavelength 514 nm. The UV–Vis analysis of pure and dye doped KDP crystals was recorded using High Performance Perkin-Elmer Lambda 950 UV–Vis spectrometer. PL emission spectrum of both the crystals was measured with the help of Model QM-8450-11, Quanta master, up conversion and down conversion fluorescence spectrometer. For dielectric characterization, the crystals were polished and the electrodes were prepared using silver paste. The dielectric properties were then investigated for all the samples using Agilent E4980ALCR meter. Thermo gravimetric (TG) and differential thermal (DT) analysis were carried out on a Perkin-Elmer diamond TG/DTA analyzer (in N₂ atmosphere; heating rate @ 1 °C/min). Micro hardness number estimation was performed on KDP, SKDP-1 and SKDP-2 samples using Vickers micro-hardness tester (MVH-I) equipped with pyramidal diamond indenter.

3.1 XRD analysis

The XRD patterns of powdered samples of pure and SSY dye doped KDP crystals have been obtained in order to analyze the phase purity and crystallinity of the crystals and are shown in Fig. 6. No peaks other than tetragonal phased KDP were observed, suggesting that high-purity KDP crystals were obtained. However, variation in the intensity of Bragg peaks was observed as an effect of dye doping which may be due to varied electronic density along stained crystallographic planes (Hussain et al., 2020). All the peaks of undoped KDP in the XRD pattern were indexed using standard JCPDS card no. 035-0807. The same tetragonal crystal structure was also observed for the stained crystals. In order to analyze the effect of dye in the crystal structure of KDP, the lattice parameters were calculated by Chekcell software and are tabulated in Table 2. The cell parameters of SKDP-1 and SKDP-2 were slightly increased when compared with the pure KDP crystal due to the strain produced in the crystal lattice as a result of SSY dye inclusion. Therefore, the expansion of the unit cell along with variation in the relative intensity of the Bragg peaks confirmed successful inclusion of SSY dye molecules in the crystal structure of KDP.

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Fig. 6

XRD patterns of pure and SSY-dye doped KDP crystals.

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3.2 Functional group analysis using FTIR spectroscopy

Fourier Transverse Infrared (FTIR) Spectroscopy is probably the fast technique to reveal molecular structure with primary functional groups. Atoms of molecules of any substance vibrate continuously with natural frequency in the range of 10¹³ to 10¹⁴ Hz, which is the frequency range of mid-infrared radiation. In this spectroscopic technique, the frequencies of infrared signal corresponding to vibrational frequencies of functional groups or bonds get absorbed. Fig. 7 shows the recorded FTIR spectra in transmission mode of three different pellets of pure KDP, SKDP-1 and SKDP-2 using KBr pellet method, in the spectral range of 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹. These spectra mainly show the absorbance peaks of stretching and bending vibration of bonds in free (H₂PO₄)⁻ ions of KDP crystal. Each PO₄ group is centered by four adjacent PO₄ groups linked with four hydrogen bonds lying almost in same plane in KDP crystal structure (Ettoumi et al., 2013). The SSY dye doped crystals show slight shift in some of the characteristics vibration frequencies and variation in peak intensity. In FTIR spectrum, there is a broad band in the higher frequency region due to O-H stretching vibration of (H₂PO₄)⁻ ions. The absorption at 3441 cm⁻¹ in both doped crystals SKDP-1 and SKDP-2 may be attributed to hydrogen bonded O-H stretching which is actually at 3447 cm⁻¹ in pure KDP (Zhou et al., 2019). There are some other absorption peaks at 2925, 2856, 2352 and 2344 cm⁻¹ in pure KDP which may due to different frequencies of O-H stretching in H₂PO₄ ions and these are slightly shifted in SKDP-1 and SKDP-2 due to dye doping. The absorption peaks at 1635 cm⁻¹ are due to O=P-OH stretching in all three crystals (Dizaji and Rousta, 2016; Krishnamurthy et al., 2013). The absorption peaks below 1400 cm⁻¹ are mainly due to different kinds of PO₄ vibration. The peaks at 1302, 1106 and 918 cm⁻¹ in pure KDP, peaks at 1301, 1102 and 912 cm⁻¹ in SKDP-1, and peaks at 1301, 1102 and 910 cm⁻¹ in SKDP-2 are due to P=O asymmetrical stretching, P=O symmetrical stretching and P-OH symmetrical stretching, respectively (Zhou et al., 2019; Krishnamurthy et al., 2013). A strong absorption peak at 536 cm⁻¹ in pure KDP represents the HO-P-OH bending which is shifted at 538 cm⁻¹ in both doped crystals (SKDP-1 and SKDP-2) (Zhou et al., 2019; Dizaji and Rousta, 2016; Krishnamurthy et al., 2013). A weak strength of absorption peak at 450 cm⁻¹ in pure KDP is due to HO-P-OH bending vibration, which is shifted at low frequency 445 cm⁻¹ in both doped crystal (SKDP-1 and SKDP-2) (Ettoumi et al., 2013; Dizaji and Rousta, 2016). From comparison of the FTIR spectra of these three crystals, it will be easy to say that both SSY dye doped crystals contain all functional groups occurred in pure KDP crystal with a slight shift in frequency. Peaks in all three crystals and their functional group assignments are listed in Table 3.

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Fig. 7

FTIR spectrum of pure KDP, SKDP-1 and SKDP-2 crystals.

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3.3 Raman analysis

Raman spectra help in identification of involved vibrational states of the molecules in a substance. This yields by a molecular spectroscopic technique, Raman spectroscopy which works on principle of inelastic scattering of monochromatic light. Raman spectroscopy provides an important tool for molecular fingerprinting and monitors the changes in bond structure, state changes and crystalline form. Fig. 8 shows the recorded Raman spectra of crystal powder of pure KDP, SKDP-1 and SKDP-2 in the spectral range 50–1250 cm⁻¹ using the laser photon (wavelength 514 nm). Raman spectra can be divided into two frequency region viz. low-frequency region (below 300 cm⁻¹) and high-frequency region (300–1200 cm⁻¹). The low-frequency region, attributed to external vibrations, includes the vibration of the crystal lattice consists of K⁺ and H₂PO₄⁻ ion whereas the high-frequency region represents the internal vibrations of H₂PO₄⁻ ion (Som et al., 2001; Subramonyet al., 1999; Srivastava et al., 1974; Babrouj and Malekfar, 2007; Lu and Sun, 2002). External modes of vibrations are effect of translational and vibrational motion of anions and cations of KDP crystal. The translational and vibrational motion of anions is form of stretching and bending of hydrogen bond inside the layers of crystal. These are observed below 300 cm⁻¹ for KDP crystal families (Ettoumi et al., 2013). In Raman spectra (Fig. 8), the peaks observed at 112 and 152 cm⁻¹ in pure KDP, 113 and 153 cm⁻¹ in SKDP-1 and 114 and 154 cm⁻¹ in SKDP-2 may be assigned to K-PO₄ translatory vibrations. The peaks at 96 cm⁻¹ in both pure KDP and SKDP-1 and 95 cm⁻¹ in SKDP-2 show the cluster external vibrational band. The band observed at 189 cm⁻¹ from pure KDP, 191 cm⁻¹ from SKDP-1 and 194 cm⁻¹ from SKDP-2 can be attributed to the stretching (O-H···O) bridge vibration (Ettoumi et al., 2013; Som et al., 2001). The in-plane bending of P(OH)₂ in pure KDP occurred at 357 cm⁻¹ while it is shifted at 359 cm⁻¹ in both SKDP-1 and SKDP-2. The out-of-plane twisting of P(OH)₂ in pure KDP and SKDP-1 occurred at 390 cm⁻¹ while it is shifted at 391 cm⁻¹ in SKDP-2. The peaks at 474 and 530 cm⁻¹ in all three crystals are of PO₂ bending vibration (Yin et al., 2016; Liu et al., 2013). The main peak with high intensity at 915 cm⁻¹ in the Raman spectrum of the pure KDP and SKDP-1 crystals is due to the asymmetrical P(OH)₂ stretching mode which is the ‘fingerprint' part of spectra for this crystal material (Pritula et al., 2016). It is the main characteristics of the internal vibration mode and the obtained results matched with previous research papers on KDP crystals. In Raman spectra, all peaks that occurred in pure KDP are also present in SKDP-1 and SKDP-2, with a slight shift in frequency and intensity. The intensity of the most important peak at 913 cm⁻¹ in SKDP-2 is nearly one and half times higher than in pure KDP. The intensity of same peak in SKDP-1 is also higher than pure KDP. Peak positions for pure as well as doped crystals and their vibrational assignments are listed in Table 4.

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Fig. 8

Raman spectra of pure KDP, SKDP-1 and SKDP-2 crystals.

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3.4 UV–Vis-NIR analysis

The UV–Vis-NIR analysis was performed on solution grown pure and SSY doped KDP crystals in the range from 190 to 1100 nm. Fig. 9(a) depicts the transmittance spectrum of the pure and SSY doped KDP single crystals. The materials with lower cut off wavelength from 200 nm to 300 nm with high transmittance are suitable and potential candidate for UV tunable lasers (Shaikh et al., 2015, Vivek and Murugakoothan, 2013; Sinha et al., 2009). The value of lower cut off wavelength for pure KDP and dye doped KDP crystal was observed at ∼199 nm, which indicates KDP is a good material for photo-electronic applications. The transparency of pure KDP single crystal was found to be (see Fig. 9(a)) 85% which is slightly higher (1%) than the SSY doped KDP crystal in the UV–Vis region. The high transmittance of pure KDP shows that it has high degree of optical quality as compared to SSY doped KDP crystal. Moreover, transmittance of KDP crystal further decreased with further increase in dye concentration. The positively charged face of KDP lattice site adsorbed the organic dye molecules and results in decreased transmittance (Velikhov et al., 2007).

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Fig. 9

(a) UV–Vis-NIR transmittance spectrum of pure KDP and SSY dyed KDP crystals. (b) SSY dye transmittance spectrum recorded in aqueous solution.

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The absorption dips at 230 nm, 311 nm and 477 nm in the transmittance spectrum (Fig. 9(a)) confirmed that the dye molecules are incorporated in the matrix of pure KDP single crystal. A small dip in the wavelength range 255–270 nm was observed for all the samples as a consequence of change in optics of photometer. The transmittance spectrum of SSY dye in aqueous solution shows four characteristic peaks at 233 nm, 316 nm, 403 nm and 483 nm (Fig. 9(b)). Also, these blue shifts in absorption maximas further show the successful inclusion of dye molecules in the lattice of pure KDP crystal. The absorption coefficient was calculated by the relation (Bhukkal et al., 2018):

In the above relation, ‘A’ is a constant, ‘h’ is the Plank’s constant, ‘E_g’ represents the optical energy band gap and ‘n’ determine the optical transition nature. The value of ‘n = 1/2’ for indirect transition and ‘n = 2’ is selected for direct transition (Karuppasamy et al., 2018). For pure and SSY dyed KDP crystal the value of ‘n’ was observed to be ‘2’, which shows that KDP crystal is an optical direct band gap material. Fig. 10 display the Tauc’s plot of pure and SSY dyed KDP single crystal. The Tauc’s plot (Fig. 10) analysis shows that the value of ‘E_g’ was observed to be 6.422 eV, 6.392 eV and 6.354 eV for pure KDP, 0.05 mol% and 0.1 mol% SSY dyed KDP crystal, respectively. The decrease in the value of E_g with dye doping may be accredited to increase in defect concentration, which is apparent from reduced transmittance in SSY dyed crystals (Chandran et al., 2015; Stapper et al., 1999).

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Fig. 10

Shows the Tauc's plot for calculation of band gap of pure KDP and SSY dyed KDP.

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3.5 Photoluminescence analysis

Photoluminescence spectroscopy is a non-destructive technique and widely used to determine the presence of defects in a material. Fig. 11 depicts the PL spectrum of the pure and SSY dyed KDP single crystal. The spectrum shown in Fig. 11 was recorded for the excitation wavelength 225 nm in the range from 380 nm to 425 nm. The PL spectra of both pure and doped KDP crystal show a maximum emission peak centered at 409 nm with two low intensity peaks centered at 392 nm and 428 nm. These PL peaks are originated due to relaxation of excited electrons from higher to lower energy states of specific color centers present in KDP crystal with a corresponding emission of photons under certain conditions. The low intensity peaks in the PL spectra are due the intrinsic defects present in the forbidden band region of KDP single crystal (Goel et al., 2013). In Fig. 11 (PL spectra) the increase in peak intensity due to doping in KDP crystal is clarified with the help of Rashba effect (Meletov and Shchanov, 1986). Doping of SSY dye in the KDP crystal creates the defect levels in KDP crystal system, these defect levels is the main reason of increase in the PL yield. This makes the SSY dyed KDP more promising material for photonic applications (Karuppasamy et al., 2018).

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Fig. 11

PL spectra of pure and SSY doped KDP crystal.

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3.6 Dielectric study

The knowledge about the dielectric behavior of a material is important as it helps in understanding the utility of the material in various applications such as patch antenna, filters, charge storage devices etc. (Yadav et al., 2016a; Hatton et al., 2006; Ramteke et al., 2018). Fig. 12(a) depicts the frequency dispersion in room temperature dielectric constant (ε') values of grown KDP crystals along (1 0 0) plane in the frequency range 20 Hz to 2 MHz. The value of ε' was found to decrease rapidly at low frequencies (upto 1 kHz), slowly in the frequency range 1 kHz to 10 kHz and attained saturation at high frequencies. The higher value of ε' at low frequencies originate from the combined effect of different polarizations (i.e., dipolar, ionic, space charge and electronic). With increasing frequency, the effect of space charge polarization deceases leaving ε' frequency independent in high frequency regime.

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Fig. 12

(a) Frequency and (b) temperature dependence of dielectric constant (ɛ́) for KDP, SKDP-1 and SKDP-2 crystals. (c) Plot of tangent dielectric loss (tan δ) vs. frequency for pure and dyed KDP crystals.

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Fig. 12(b) shows the temperature dependence of ε' for the grown crystals measured over the temperature range 30 °C–100 °C at 1 kHz. The increment in the value of ε' due to SSY dye inclusion into the host KDP lattice may be attributed to the creation of additional electric dipoles. Also, in the selected temperature range (30 °C−100 °C), no phase transition was observed in any sample. Fig. 12(c) depicts the plot of dielectric loss (tan δ) values, which represents the energy dissipated in a dielectric system, versus frequency. As can be seen, tan δ exhibits similar trend as that of ε' with increasing frequency. The dielectric loss for dyed KDP crystals was observed to be smaller compared to that obtained for pure KDP, suggesting the improved optical quality of dyed crystals with lesser defects (Rajesh et al., 2014). A reasonably low value of tan δ (less than 0.05) was achieved for the frequencies above 5 kHz, suitable for practical applications. Enhanced dielectric constant and reduced dielectric loss observed for SSY dye doped KDP crystals implies that dyed crystals are better candidates for charge storage and non-linear optical devices.

3.7 Thermal analysis

Thermal analysis of the grown crystal has been carried out using thermo gravimetric (TG) and differential thermal analysis (DTA); these are the important tools for getting information about different stage of phase transition as well as decomposition temperature of the grown crystals. Fig. 13 depicts the TG/DTA spectra of un-dyed KDP and SSY stained KDP crystals. It is clear from the plot that no weight loss till the temperature 193 °C, which means un-dyed KDP is thermally stable from room temperature to 193 °C. For SSY stained KDP crystals, no weight decay was recorded up till 200 °C (for SKDP-1) and 207 °C (for SKDP-2). Moreover, the DTA traces display an endothermic dip (decomposition temperature) at 222 °C, 226 °C and 234 °C for un-dyed, SKDP-1 & SKDP-2 crystal, respectively. In comparison to un-dyed KDP, the decomposition temperature got enhanced as a consequence of SSY dyeing. Dye molecules make bonding with ionic structure of KDP crystal. These bonding are a cause of increase in melting point. The thermal stability of a material is sensitive to the melting point of dopant. There are examples of decreased thermal stability (Rao et al., 2014) as well as increased thermal stability (Sinha et al., 2015; Goel et al. 2018a) depending on the relative decomposition temperature of dopant as compared to host material. The increase in the thermal stability of SSY dyed KDP single crystals in the present case is due to the higher decomposition temperature of SSY (>300 °C, Sigma-Aldrich safety data sheet) as compared to host KDP.

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Fig. 13

TG/DTA traces for (a) un-dyed KDP crystal, and (b& c) SSY dyed KDP crystals.

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3.8 Mechanical properties

Material strength is determined as amount of resistance presented by the crystal to permanent distortion under applied load and it plays an important role in fabrication of devices (Ray et al., 2017; Justin et al., 2019; Joseph et al., 2020). Mechanical strength was estimated by using Vickers micro-hardness tester, which has a pyramid shape diamond indenter and indents were made on both pure and dye doped KDP crystal. Defect free, smooth and flat crystalswere selected for micro-hardness measurement. Impressions were made on pure and dyed KDP crystals for a constant dwell time of 5 s at varying applied load of 5 g, 10, 20, 30, 40, 50 and 100 g. Beyond an applied load of 100 g, multiple cracks were developed at the surface of crystals, which may occur due to release of internal stress.

Fig. 14 shows the impressions on surface of all three KDP crystals at varying applied loads. It is obvious from Fig. 14 that the size of the indentation marks increases with increasing applied indenter loads. The Vickers micro-hardness value (H_V) of the material was calculated from the following relation (Goel et al., 2016; Hussain et al., 2020):

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Fig. 14

Indentation marks on surface of (a-c) pure KDP, (d-f) SKDP-1, (g-i) SKDP-2 crystals at various applied indenter loads.

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Fig.15

Vickers hardness number vs. applied load for pure KDP, SKDP-1 & SKDP-2 crystals.

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Elastic stiffness constant (C₁₁) provides information about bond strength of molecules and rigidity of as grown crystals. Value of stiffness constant (C₁₁) is evaluated at different loads for all the three samples using Wooster relation (Batra et al., 2019): C₁₁ = H_V^7/4. The values of stiffness constant for pure, SKDP-1 and SKDP-2 crystals at various applied loads are shown in Table 5. It is clear from the table that stiffness constant decrease with increasing concentration of SSY dye.

A material can be classified as hard or soft using the Meyer's relation which relates indentation diagonal size (d) and load (P) as (Anbukumar et al., 1986): P = kdⁿ, which can be rewritten as:

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Fig. 16

Plot of ln Pvs ln d for undoped and SSY dyed KDP crystals.

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Fig. 17 shows graph between average diagonal indentation length (d, in μm) and average indentation depth (h, in μm) versus applied indenter load for KDP, SKDP-1 and SKDP-2 crystals. Average diagonal indentation length was nonlinearly increasing with applied load. Average indentation depth (h) depends on indentation length (d) for a diamond pyramid indenter of angle 136 ^ͦ between faces follow the relation: h= $\frac{(\sqrt{2}\mathrm{c}\mathrm{o}\mathrm{t}({68}^{°})\mathrm{d})}{4}$ (Goel et al., 2016; Joseph et al., 2020). Indentation depth ‘h’ follows the same nonlinear increasing tendency as of d. This kind of nonlinear nature can be understood on the basis of applied indenter load as: when applied indenter load value is low, only tiny layer of crystal surface were pierced by indenter load thus impressing was easy by load. However, as increasing indenter load indentation depth got increased, then inner layer of crystals played significant role, hence penetration on crystal surfaces were slightly difficult. Similar results were observed by several other authors as a result of dye inclusion in various host substance (Batra et al., 2019; Goel et al., 2016).

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Fig. 17

Graph of average diagonal indentation length (d, in µm) and average indentation depth (h, in µm) versus applied indenter load for (a) KDP, (b) 0.05 mol% SSY doped KDP and (c) 0.1 mol% SSY doped KDP.

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