Influence of pH on the growth and characteristics of nonlinear optical zinc thiourea chloride (ZTC) single crystals

Abstract

Growth of bulk zinc thiourea chloride (ZTC) single crystals from aqueous solution by low temperature solution growth technique has been reported. In this report we bring out the influence of pH on the structural, optical, electrical and mechanical properties of the grown crystals. At pH 5.3, the crystals are elongated along a-axis. Bulk crystals have been grown from the optimized pH value of 3.0. The single crystal XRD analysis confirms the change in lattice parameters for the crystals grown at these different solution pH values. The change in the crystal morphology at pH 5.3, and at 3.0 and 4.0 is accounted based on the difference in the nature of the species transported to the crystal. At pH 5.3, it appears that Zn 2+ is preferentially transported from solution to the crystal surface, and this is followed by 2Cl − ions and two thiourea molecules to give enhanced growth rate along the a-axis. Previously reported unit cell structure is used to support this view. At pH 3.0 and 4.0, protonated thiourea is preferentially transported to the crystal surface leading to increase in growth rate along b-axis. Cl − ions already bounded to Zn 2+ in the crystal are expected to offer the necessary driving force and this is followed by the transport of other species. This idea is based on the fact that free Zn 2+ is expected to be less at low pH due to formation of ZnCl 4 2− FTIR and laser Raman spectra confirm the presence of all functional groups. The dielectric loss of ZTC grown from the optimized pH value is low indicating that the crystals have fewer defects. Crystals grown at an optimized pH value possess higher transmittance. The second harmonic generation conversion efficiency measured using Nd:YAG laser was found to be comparable with KDP crystals. Hardness property increases for crystals grown from lower pH values.