Slow evaporation method was used to grow pure and KCl (10 mol%) doped KAP single crystal. The solubility and metastable zone width of aqueous solutions of pure and KCl (10 mol%) doped KAP crystal were evaluated to analyze the crystallization process. Measuring the induction period τ, the critical nucleation parameters like interfacial energy (σ), energy of formation of the critical nucleus (ΔG*) were determined using the classical theory of nucleation. The structural properties and optical constants of the grown crystals have been put to test and observed that the addition of KCl results in an enhancement of properties of the crystal. Grown crystals were characterized by powder X-ray diffraction. FTIR spectra confirmed the presence of KCl in pure KAP crystal. UV- Visible spectroscopic studies revealed that addition of KCl in pure KAP crystal increased transparency from 75% to 80%. The analysis of the optical absorption data revealed the presence of both indirect and direct transitions and both of these band gaps increased with the addition of KCl. The transmittance data was analyzed to calculate the refractive index, oscillator energy, dispersion energy, electric susceptibility, zero-frequency dielectric constant and both the real and imaginary parts of the dielectric permittivity as a function of photon energy. The moments of ε(E) were also determined. The dispersion i.e. spectral dependence of the refractive index was discussed according to the single-effective oscillator model proposed by Wemple and DiDomenico.
Growth of potassium acid phthalate (KAP) crystal of high purity has become an important field of research in a variety of areas. KAP exhibits orthorhombic lattice structure with four molecules per unit cell and the unit cell parameters are a = 6.320 Å, b = 12.343 Å, c = 5.784 Å. On the other hand, in production of optical windows, single crystal of KCl with wide band gap (~8 eV) is widely used [
An effort to investigate the nucleation kinetics and optical constants of KCl doped (10 mol%) KAP crystal is done. Since nucleation is affected by width of the metastable zone, it is essential to measure it for designing products by crystallization processes. It is possible to obtain optimum crystallization processes by tuning the metastable zone width and actual operation point of the crystallizer within this zone [
Analytical reagent grade (AR) and doubled distilled water were used for growing the crystals. At first good quality seed crystals were selected. The seeds were obtained by spontaneous nucleation technique. Later bulk size crystals were harvested by slow evaporation method at room temperature in a span of 60 - 80 days. The as- grown crystals are shown in
In order to observe the dependence on temperature, the solubility of KCl (10 mol%) doped KAP solutions was determined for five different temperatures, namely, 30˚C, 35˚C, 40˚C, 45˚C, and 50˚C. The measurements were carried out in a constant temperature water bath (CTB) with cryostat facility. In our study, polythermal method was used to determine the metastable zone width of pure and KCl (10 mol%) doped KAP solutions [
The density of the crystal was measured experimentally by the floatation method at room temperature (30˚C) using the following expression
where m, m' and ρsolvent are the mass of crystal sample in the air, the mass when the crystal sample was immersed in CCl4 and the density of solvent (CCl4) at measured temperature, respectively. The density of the doped crystal was found to be 1.808 g/cm3.
The induction period τ gives the insight about the process that leads growth from critical nuclei to detectable crystals and is determined experimentally by isothermal method [
then the solution was cooled to the saturation temperature (30◦C). At this stage the solution became supersaturated to the particular level of supersaturation. Once the nucleation occurred, the nucleus grew quickly and formed a bright sparkling speck. The induction period was taken as the difference of the time of observation of the sparkling particle and the time at which the solution reached the saturation temperature [
The crystals were ground using an agate mortar and pestle. The powder X-ray diffraction analysis on pure and KCl doped KAP crystal was recorded using CuKα radiation and has been recorded up to 2θ = 85˚.
By using KBr pellet technique, the FT-IR spectrum of the crystal was recorded at room temperature to identify the functional groups. All the spectra were recorded in transmittance (%) mode in the region of 4000 to 400 cm−1. The characteristic vibrational frequencies were assigned and compared with the doped sample.
The crystals were polished without any antireflection coating and the optical transmission spectrum of 2 mm thick crystal was recorded in the wavelength range of 250 - 750 nm at room temperature in order to derive the absorption coefficient, refractive index and other important optical constants such as oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index, etc.
It is observed from
The interfacial energy σ takes on a prominent part in the nucleation of crystals [
culated from induction period. The equation of nucleation rate relating induction period can be written as [
or
or
where τ is the induction period of the solution at temperature T, v is the molar crystal volume and A is constant. S is the supersaturation ratio (S = C/C*). At constant temperature, a straight ahead relationship is noticed between lnτ and 1/(lnS)2 (
where m is the slope evaluated from the straight line fit for lnτ against 1/(lnS)2, R is the gas constant, and NA is Avogadro’s number. The energy of formation of a critical nucleus (
The grown crystal was put through the powder XRD which was shown in
The UV-VIS transmittance spectra and reflectance curve (inset) of pure and KCl (10 mol%) doped KAP crystals are shown in
Materials | Unit cell parameters |
---|---|
Pure KAP | a = 9.684 Å, b = 13.442 Å, c = 6.543 Å |
KAP + 10 mol% KCl | a = 9.632 Å, b = 13.456 Å, c = 6.535 Å |
Pure KAP | KAP + 10 mol% KCl | Assignments |
---|---|---|
2485.32 | 2485.32 | -C-H aromatic stretching |
1950.07 | 1950.07 | =C-H out of plane bending |
1673.28 | 1677.13 | Symmetrical C=O stretching |
1572.01 | 1562.37 | -C=O Carboxylate ion =O Asym |
1485.21 | 1485.21 | C=C ring stretching |
1442.78 | 1442.78 | O-H in plane bending |
1383.95 | 1383.95 | -C=O Carboxylate ion =O Symmetric |
1285.58 | 1286.54 | C-COO stretching |
1151.52 | 1151.52 | C-O stretching |
1079.19 | 1079.19 | C-C stretching |
887.27 | 887.27 | C-C-O stretching |
853.52 | 853.52 | =C-H out of plane bending |
811.08 | 811.08 | C-H out of plane bending |
762.86 | 762.86 | C-H out of plane bending |
720.43 | 720.43 | C-C stretching |
677.99 | 677.99 | C-O wagging |
649.06 | 650.02 | C=C-C out of plane ring deformation |
581.55 | 582.51 | C=C-C out of plane ring deformation |
550.69 | 549.72 | C=C-C deformation |
440.74 | 438.81 | C=C out of plane ring bending |
become a fashionable way to interpret the band structure and nature of transition of electrons. The optical energy gap Eg can be expressed with respect to the incident pthoton energy hn by Equation (5) [
where a is the optical absorption coefficient, A is a constant, hν = photon energy, Eg = Energy gap, p is thought to as 2 or 1/2 for a indirect or direct allowed transitions, respectively. The plot of absorption coefficient a on photon energy hn is given in
The rise of the band gap due to doping may be thought of as falling off irregularity and defects in the crystal which is in fact viewed as rise of an electric field by an electrically charged particles within the crystal [
where λ is the wavelength of the incident radiation.
Optical Parameters | Pure KAP | KAP + 10 mol% KCl |
---|---|---|
Egi | 1.5 eV | 2.1 eV |
Egd | 1.2 eV | 1.4 eV |
Eso | 7.9 eV | 7.04 eV |
Ed | 48.49 eV | 29.32 eV |
M-1 | 6.13 | 4.17 |
M-3 | 0.098 | 0.084 |
no | 2.67 | 2.27 |
eo | 7.13 | 5.17 |
Sso | 2.7 × 1014 m−2 | 1.34 × 1014 m−2 |
λso | 1.53 × 10−7 m | 1.76 × 10−7 m |
crystal structure. Atoms easily polarizable (i.e. electron are easily displaced) give rise to a high refractive index. The equations relating transmittance (T), reflectance (R) and refractive index (n) can be expressed with the following equations (considering T + R = 1) [
Hence,
The complex dielectric constant εc can be expressed with real (εr) and imaginary (εi) parts of dielectric constant as
where c is the velocity of light and n is the refractive index. The electrical conductivity can be written as [
Non linear optical (NLO) property is expected for the crystal because
The electrical susceptibility (χc) can be assessed by the relation [
From
Wemple and Di Domenico made use of the single effective oscillator equation and investigated refractive index data lower to the interband absorption edge. The relation between the refractive index and photon energy can be expressed by the equation [
where Eso and Ed are the single oscillator and the dispersion energy, respectively.
The zero-frequency refractive index n0 can be achieved by the expression
The zero-frequency dielectric constant is obtained by using the relation
where
where
Pure and KCl doped KAP crystals were grown by adopting slow evaporation method. The solubility varied proportionately with temperature. Incorporation of KCl resulted in increase of the metastable zone width and interfacial energy with respect to undoped solution. The possible reason of this enhancement might be considered as opposition in chemical activity faced by the metal ions in the mother solution. XRD analysis indicated incorporation of foreign atoms into the KAP crystal matrix. The UV-VIS spectra analysis showed that the transmission capability got better as well as revealed the coexistence of indirect and direct transitions in KCl doped KAP crystals. Optical constants such as the dispersion energy, oscillator strength, oscillator energy and zero-frequ- ency refractive index were evaluated by making use of the Wemple-Di Domenico single-effective-oscillator model and observed to change considerably due to KCl doping.
Authors are grateful to Dr. Abdul Gafur and Dr. Dilip Kumar Saha for their kind permission to perform FTIR and XRD study.