A newer approach of growing methyl-p-hydroxybenzoate (p-MHB) single crystals from a melt without a polymorphic change in their form
As a newer approach, single crystals of methyl-p-hydroxybenzoate (p-MHB), an efficient organic non-linear optical material, were grown from a melt by employing the Czochralski pulling technique for the first time. A specially designed mini-Czochralski crystal puller and a locally designed muffle furnace having a temperature controlling accuracy of ±0.1 °C were used for the experiment. Different types of seeding mechanisms were employed using: (i) micro tubes, (ii) portion of melt grown crystal boules and (iii) previously grown smaller dimensional single crystals from solution as seeds. Crystal growth conditions were optimized on a trial and error basis and bell shaped single crystal boules were successfully grown. The unit cell parameters of the grown crystals were determined by a powder X-ray diffraction (PXRD) study and the internal crystallographic structure was confirmed by single crystal X-ray diffraction (SCXRD) analysis. Comparison of the XRD results with that of the solution grown crystal reveals that no change in the crystal form except small variation in the lattice parameters. Fourier transform infrared (FT-IR) spectral analysis reveals the presence of various functional groups in the grown crystals. Differential scanning calorimetry (DSC) analysis confirms the thermal stability of the grown crystals in the temperature range 30–120 °C. Second harmonic generation (SHG) efficiency of the solution crystals and the melt grown crystals were studied by the Kurtz Perry powder method with a 1064 nm Nd:YAG laser beam as a fundamental source and were found to be nearly equal and about 1.64 and 1.58 times higher, respectively, than that of the inorganic standard potassium dihydrogen phosphate (KDP).