Toward the Identification of Point Defects Formation and Solution Growth Mechanisms of Herbertsmithite ZnxCu4–x(OH)6Cl2 Crystals

Abstract

Zn_xCu_4–x(OH)₆Cl₂ single crystals were grown under varying experimental conditions such as dissolution and growth temperatures, pH, and initial ZnCl₂ concentration of the solution. The crystals were characterized to determine their crystallographic phase purity and lattice parameters, chemical composition, and surface morphology. Two spatially resolved mapping techniques, laser-induced breakdown spectroscopy (LIBS) and micro-X-ray fluorescence spectroscopy (μ-XRF), were used to firmly establish the Cu and Zn concentrations of the crystals obtained in different growth conditions. Once correctly calibrated, LIBS allowed for mapping of Cu and Zn contents with very good statistics and at various depths from the crystal surface. An interesting correlation between the c-lattice parameter and the x-value was observed in the range of x = 0.75–1.04. Using the microdiffraction multimodal station (μ-Laue, μ-XRF) at the BM32 beamline of the European Synchrotron Radiation Facility (ESRF), we were able to orient the most salient features of the (101) facets’ morphology, including the ubiquitous macrosteps. Combining these data with an exhaustive thermochemical investigation of the growth solutions, aimed at identifying the most concentrated Cu- and Zn-based species as a function of the growth conditions (T, pH, Zn, Cu, Cl, and O concentrations), we proposed plausible growth and point defect disorder formation reactions. This analysis was partially supported for Zn-based species by in situ Raman spectroscopy. Further, through a systematic analysis of the height-difference correlation function obtained by atomic force microscopy (AFM) images of large terraces and macrosteps, we concluded that surface diffusion and related thermal noise are the kinetically limiting mechanisms in the growth process.