Synthesis and characterization of enstatite and talc doped with zinc and manganese

Abstract

In recent years, particular interest has been addressed by researchers in the synthesis and study of silicates such as enstatite MgSiO3 and talc Mg3Si4O10(OH)2. The first one is useful for several technological applications such as substrates in electronics, high frequency insulators, thermal insulators in high temperatures applications, and as luminescent materials in laser technology. The latter, because of the low cost and good properties (i.e. resistant to heat and acids, hydrophobic, electrical insulating) is widely used in many different products such as ceramics, papers, cosmetics, foods, polymers and filler in composites. The usual presence of foreign ions (e.g., Mn, Ti, Ni, etc.) and their inconstant amounts in natural enstatite and talc hinder the use of these minerals as high-performance materials. For these reasons, in recent years pure and doped enstatite and talc have been grown and characterized in several different ways. Nevertheless, there are still various problems to be solved in order to obtain very high quality crystals and the desired changes in the physical and chemical properties of them when they are doped with metal elements. In this work, Zn-doped enstatite, Mn-doped enstatite, Zn-doped talc and Mn-doped talc have been grown and characterized with different techniques. The starting materials and the final products were characterized and studied by binocular microscope, powder crystal X-ray diffraction (XRPD), scanning electron microscopy with energy-dispersive spectrometry (SEM/EDS), single-crystal X-ray diffraction (XRD), micro-Raman (μ-R), cathodoluminescence (CL), differential scanning calorimetry, thermogravimetric analysis (DSC-TG) and Fourier transform infrared spectroscopy (FT-IR). Zn- and Mn-doped enstatite was successfully produced by slow-cooling flux growth method, using MoO3, V2O5, Li2CO3 as melting agent. Several starting mixtures, with different MnO or ZnO concentrations, were first held at 1350 °C, 1250 °C 1050 °C and 950 °C and then slowly cooled down to 700 °C or 600 °C with different cooling rate (3.75 °C/h, 2.1 °C/h, 1.8 °C/h 1.7 °C/h). Enstatite crystallizes in the orthorhombic and monoclinic systems as revealed by XRD and Raman spectra. Transparent Zn-doped enstatite ( max length of 3.5 mm) and reddish Mn-doped enstatite (max length of 8 mm) single crystals are euhedral in form, not homogeneous in width and inclusion free. Maximum content of Mn-dopant is 14.52 wt %, while the maximum amount of Zn-dopant is 10.49 wt%. Crystals grow under equilibrium conditions only when the dopant content is maintained at low value. When either Zn or Mn is totally substituted for Mg in the starting material, no enstatite is produced. The presence of the dopant in the enstatite structure causes a decrease in unit cell volume respect to the pure one and strongly affects the CL-signal and micro-Raman spectra. CL spectrum of Mn-doped enstatite contains a broad emission located at 677 nm and attributed to the 4T1g(G)→6A1g(S) transition of octahedral Mn2+ centres. The presence of Zn in enstatite induces very remarkable peak broadening by the mode at 133 cm-1 and 343 cm-1 in the Raman spectra; for these modes a strong component of metal ion displacement must be postulated. Raman spectra of Mn-enstatite show: i) a general decrease of Raman intensity due to the increase in surface reflection when the MnO dopant concentration increases; ii) a widening and a down shifting of the peak positions indicating changes in vibrational modes because of the increasing presence of MnO. Zn- and Mn-doped talc was successfully synthesized in hydrothermal conditions at temperatures of 300, 500 and 650 °C, under constant pressure of 2 kbar and reaction time of 160 hours. Talc morphology and content of dopant within the crystals show strong dependence on crystallization temperature. Talc exhibits a cabbage-like morphology, its classical hexagonal tabular morphology and fibrous morphology. The best temperature to obtain the highest abundance of Zn- and Mn-doped talc is 650 °C. A decrease in temperature from 650 to 300 °C: i) worsens the reactions and poorly crystallized Zn- and Mn-doped talc is obtained; ii) increases the content of zinc or manganese dopant. Talc only grows when Mg is not totally substituted by either zinc or manganese in the starting mixture. Zn-doped talc formation is increased by treating the starting mixture with H2O + HCl; conversely, the reactant H2O + CaCl2 inhibits the growth of talc. In order to increase doped talc yield, large amounts of aqueous solution is as crucial as high temperatures. The presence of varying amounts of metal elements replacing Mg in talc influences its temperature decomposition. Indeed, the thermal stability of Zn- and Mn-doped talc decreases with respect to pure one. Zn-dopant in talc mainly affected the hydroxyl stretching fundamental peak (3674 cm-1), splitting itself into as many as four peaks with respect to non-doped talc, which displayed only a sharp band. The splitting appears to be dependent on the degree of substitution of the magnesium in the octahedral layer and related to the electronegativity difference between Zn and Mg. Future studies will be carried out on these materials to have a better knowledge of other physical properties, useful in novel applications.