Controllable solute-diffusion gel-growth of BCHT: an effective approach towards large functional material single crystal synthesis

Abstract

An underutilized, but highly advantageous, method of growing functional single crystals, the gel-growth technique, is reported here. This method enables precise control of solute-diffusion, allows crystals to be grown at ambient temperatures and facilitates a lower concentration of material defects. Furthermore, the gel-growth method creates preferable stable growth conditions where the effects of gravity are well-balanced by buoyancy effects making it analogous to growing crystals under microgravity conditions. Thus, the gel-growth method is a promising technique for engineering hard-to-synthesize and function-by-design single crystals. In this study, large (cm), nonlinear optical crystals of bismuth chloride hydrate thiourea (BCHT), were successfully grown as a model material system using the double-diffusion gel-growth method. BCHT is one of the four phases of the bismuth thiourea chloride (BTC) compound. BTC is an emerging nonlinear optical material which posesses non-centrosymmetric structure, a high second-harmonic-generation (SHG) relative efficiency, and unique nonlinear optical (NLO) properties which find utility in lasers and optical systems. Its promising physical properties may also lend it to ionizing radiation detection. Systematic atmosphere-dependent and temperature-dependent in situ X-ray diffraction scans were performed to study the phase change of the BCHT crystals, revealing a step-by-step dehydration process towards the anhydrous β-BTC phase when exposed to air. The dehydration phase transformation from BCHT to β-BTC was also observed when the temperature of the sample was raised above 80 °C. The elemental and structural properties of the stable anhydrous β-BTC phase were further investigated using time of flight secondary ion mass spectroscopy (TOF-SIMS) and micro-computed tomography (micro-CT) techniques. TOF-SIMS revealed that the BCHT phase could still be present near the surface of the sample even when the bulk of the material had transitioned to β-BTC, and that no Si inclusions form from the gel matrix. The micro-CT results demonstrate that, after the BCHT dehydrates to β-BTC, a homogeneous sample volume with an abundant, complex network of low-density defects is present. These defects may be sub-grain boundaries, micro-cracks or voids formed when the sample dehydrated as part of its transition from BCHT to β-BTC.