Growth and characterization of pure and doped potassium pentaborate (KB5) single crystals

Abstract

Single crystals of pure and Mg⁺, Ba⁺, Ca⁺ and Cu⁺ doped potassium pentaborate (KB5) have been grown by low temperature solution growth technique. The growth conditions and surface morphology of pure and doped single crystals of KB5 are optimized and the grown crystals are confirmed by XRD. The pure and doped crystals of KB5 are subjected to TGA and DSC studies. Using the TGA and DSC curves, the enthalpies, decomposition temperature (T_d) and weight loss are measured and the results are analysed and discussed. The influence of the presence of added dopants on the microhardness behaviour of Mg⁺, Ba⁺, Ca⁺ and Cu⁺ doped KB5 crystals are also studied and discussed. The SHG of the pure and doped samples of KB5 is confirmed by Nd:YAG pulsed laser employing the Kurtz powder technique.

Introduction

Inorganic borates exist in numerous structural types and some crystals such as KB5 and BBO are excellent non-linear optical (NLO) materials, particularly in the UV region. These borate crystals generally possess chemical stability, high damage threshold and high optical quality, as well as wide range of transparency far into the ultraviolet on account of the rather large difference in the electronegativities of B and O atoms [1]. The properties determining an effective NLO material particularly, the borate compound materials have been discussed and reviewed by several researchers [1], [2], [3], [4]. By mixing the fundamental and fifth harmonic at room temperature, Umemura and Kato achieved sixth harmonic generation of the Nd:YAG laser frequency at 1.0642 μm in KB5 crystals [5]. The superiority of KB5 crystals over the popular NLO materials like ADP and KDP has been proved by the experiments conducted by Kato, it was necessary to refrigerate the sum frequency generator crystals to about 70°C for KDP and −55°C for ADP to obtain phase matching, but in the case of KB5 it can be achieved at room temperature itself [6], [7]. Potassium pentaborate KB₅O₈·4H₂O belongs to the space group (Aba C_2v¹⁷) and contains B₅O₁₀ groups in its structure [8]. KB5 crystal is uncolored, optically biaxial positive with optic plane 010 [9]. Using the Bond–Valence theory of complex crystals, the origin of non-linearity of the KB5 crystals has been investigated by Xue Dongfeng and Zhang Siyuan and its larger NLO coefficients are also estimated [10]. The growth of single crystals and twinned crystals of KB5 by low temperature solution growth is reported by several workers [8], [11]. Kato attributed the primary advantage of KB5 over KDP and ADP to its slightly large birefringence, which permitted generation of shorter wavelengths without refrigeration of crystal [6], [7]. The absolute coefficients of two photon absorption (TPA) of KB5 at 270 and 216 nm were measured by Gurzadyan and Ispiryan and the presence of crystal dopes found to increase non-linear losses [12]. Studies of pyroelectric properties, the influence of hydrostatic pressure on spontaneous polarization, electro-optic effect and spontaneous birefringence of potassium pentaborate tetrahydrate crystal are reported by Poprawski et al. [9]. The experiments conducted by Ramachandra Raja et al. confirmed the anisotropic nature of dielectric constant values of KB5 crystals in the frequency range 0.1–100 KHz [13]. Frequencies of the lattice longitudinal optical and transverse optical modes, their intensities, dampings as well as oscillator strengths and induced dipole moments have been determined for KB5 at 90 and 300 K temperatures [14]. Fedorava et al. investigated the Raman polarization spectra of KB5 and deutero analog (DKB5) in the region of stretching vibrations of OH(OD) groups [15]. In this paper, we report the results of the growth of pure and doped (Mg⁺, Ba⁺, Ca⁺ and Cu⁺) crystals of KB5 along with the effect of doping on the thermal and microhardness behaviour of the grown crystals.