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 Figure 1.

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 [4] [5] . First of all the solutions were kept in a CTB with cryostat facility. Using a magnetic stirrer the solutions were stirred continuously for a period of 7 hours and it was slowly cooled at a rate of 3 K/h. At the appearance of the first crystal, the temperature was noted there and then. The metastable zone width of the solution was measured as the change in saturation and nucleation temperature [5] . The method was replicated for rest of the saturation temperatures; 35˚C, 40˚C, 45˚C, and 50˚C, and the respective metastable zone widths were obtained.

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 ρ_solvent 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 [5] [6] . In order to get the required degree of supersaturation, at first the required amount of KAP (with and without KCl) was dissolved in the solvent and

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 [5] . Experiments were repeated to get the following degrees of supersaturation (C/C^*); 1.03, 1.06, 1.09, 1.13 and 1.16, (C and C^* are the concentration of solute in supersaturated solution and the saturated concentration, respectively).

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 Figure 2(a) that the metastable zone width has increased due to the addition of KCl (10 mol%). Trivalent metals like Cr³⁺, Fe³⁺ and Al³⁺ can significantly affect the growth of the crystals. Even after repeated recrystallization these impurities cannot be completely removed and doping can do what recrystallization cannot do, i.e. it can reduce the effect of these impurities. In order to enhance the metastable zone width as well as to achieve optimum growth rate of the crystals, KCl is incorporated in the solution. As it is believed that the harmful effect of the metal ion impurities can be moderated by adding KCl [7] .

The interfacial energy σ takes on a prominent part in the nucleation of crystals [8] . This parameter has been cal-

culated from induction period. The equation of nucleation rate relating induction period can be written as [9]

or

or (2)

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 (Figure 2(b)), due to independence of lnA on temperature. Many researchers considered two different straight lines: one standing for homogeneous nucleation and the other heterogeneous nucleation [10] . The interfacial energy σ (Figure 3(a)) has been determined using the following expression [11]

where m is the slope evaluated from the straight line fit for lnτ against 1/(lnS)², R is the gas constant, and N_A is Avogadro’s number. The energy of formation of a critical nucleus (Figure 3(b)) has been evaluated using the following equation

The grown crystal was put through the powder XRD which was shown in Figure 4. The unit cell parameters are given in Table 1. The well-defined Bragg’s peaks at specific 2θ angles give the evidence of high crystallinity of the crystal. It is observed from the XRD data that there is a slight change in the peak position and unit cell parameters which indicates that KCl might have entered into KAP molecular structure.

Figure 5 shows the FTIR spectra of the pure and KCl (10 mol%) doped KAP crystal. The peak assignment is given in Table 2. The data indicate shifting of symmetrical C=O stretching of KAP to higher energy for KCl doping. This budge to higher energy indicates interaction of KAP with KCl [12] . The characteristic C-COO stretching and C=C-C at 1285.58 and 581.55 cm⁻¹ are shifted to 1286.54 and 582.51 cm⁻¹, indicating substitution. The asymmetric stretching vibration of the carboxylate ion is shifted to lower energy (1562.37 cm⁻¹) compared with pure KAP (1572.01 cm⁻¹).

The UV-VIS transmittance spectra and reflectance curve (inset) of pure and KCl (10 mol%) doped KAP crystals are shown in Figure 6. A cut off wavelength is noticed near about 240 nm. There is no striking absorption in the entire region of the spectrum. The investigation of the optical absorption coefficient on the photon energy has

become a fashionable way to interpret the band structure and nature of transition of electrons. The optical energy gap E_g can be expressed with respect to the incident pthoton energy hn by Equation (5) [13] ,

where a is the optical absorption coefficient, A is a constant, hν = photon energy, E_g = 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 Figure 7. Direct and indirect band gap E_gd and E_gi are evaluated by the extrapolations of the linear part down to and respectively [14] . The values are tabulated in Table 3.

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 [15] . The extinction coefficient (K) can be written as

where λ is the wavelength of the incident radiation.

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) [16] .

Hence, (7)

The complex dielectric constant ε_c can be expressed with real (ε_r) and imaginary (ε_i) parts of dielectric constant as, where and. As K is very small, it can be considered [16] . The optical conductivity σ_op of the crystal is associated with the absorption coefficient as [16]

where c is the velocity of light and n is the refractive index. The electrical conductivity can be written as [16]

Non linear optical (NLO) property is expected for the crystal because Figure 8 reveals the lower value of complex dielectric constant along the transmission range which in turn indicates induced polarization. Lower electrical conductivity at higher photon energy (Figure 9(a)) specifies the dielectric nature of the material. On the other hand, the higher value of optical conductivity at higher photon energy (Figure 9(b)) brings to light superior conversion capability for second harmonics generation devices.

The electrical susceptibility (χ_c) can be assessed by the relation [15]

From Figure 10(a), it is clear that electrical susceptibility is larger than 1 and the material is polarizable if the light is made highly intense.

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 [17]

where E_so and E_d are the single oscillator and the dispersion energy, respectively. Figure 10(b) plots the change of the refractive index with wavelength. In Figure 11(a), the oscillator parameters are figured out from (n² − 1)⁻¹ versus (hν)² plot by measuring the slope and intersection of the straight line with y-axis. The above-men- tioned model establishes a connection between the single oscillator parameters E_so and E_d and imaginary part ε_i of the complex dielectric constant. The M₋₁ and M₋₃ moments of the ε(E) optical spectrum can be formulated as the following expression

The zero-frequency refractive index n₀ can be achieved by the expression

The zero-frequency dielectric constant is obtained by using the relation. The oscillator energy E_so represents mean gap energy and can be expressed in terms of the lowest direct band gap E_gd by the equation E_so = 2E_gd on experimental basis [18] . The oscillator strength S_so can be obtained from the refractive index n which is expressed by single Sellmeier oscillator equation as (in low energy range) [19]

where is the oscillator wavelength. From Equation (18) we can get the following equation [19]

where. The values of M₋₁, M₋₃, n_o, e_o, S_so and λ_so evaluated from versus λ⁻² plot can be seen in Figure 11(b) and are given in Table 3.