Exploring the milarite minerals, Na2Mg5- xMxSi12O30 (M = Co2+, Zn2+, Ni2+, Cu2+) and Na2Mg2.5Ca0.5- xLnxZn2Si12O30 (Ln = Eu3+, Tm3+, Tb3+), towards new colored compounds, white light emission, and oxygen evolution reaction (OER) properties

Abstract

New transition metal substituted milarite silicates of the general formula A₂B₂C[T(2)₃T(1)₁₂O₃₀] were prepared by a conventional solid state technique and their structures determined by powder X-ray diffraction (PXRD) methods. Raman spectroscopic studies indicated expected Raman bands. The oxidation states of the transition elements were confirmed by XPS studies. The optical absorption bands were rationalized based on the ligand-centered emission and allowed d-d transitions. The white compounds exhibited good deep UV cut off values, suggesting their possible use as optical filters. The white compounds also showed good near-infrared (NIR) reflectivity comparable to that of the commercial TiO₂. The rare earth substituted compounds, Na₂Mg_2.5Ca_0.5-xLn_xZn₂Si₁₂O₃₀, exhibited intense red (Eu³⁺), green (Tb³⁺) and blue (Tm³⁺) emissions. An optimal composition having all the three lanthanide ion substitutions resulted in white light emissions in Na₂Mg_2.5Ca_0.48Tm_0.01Tb_0.02Eu_0.01Zn₂Si₁₂O₃₀. The presence of Co²⁺ in the Na₂Mg₃Co₂Si₁₂O₃₀ compound facilitated the study of the oxygen evolution reaction (OER) with good values that are comparable to those of RuO₂. The white compounds have reasonably good dielectric constant values with low dielectric loss, which indicates their possible use in communication devices. The milarite based compounds exhibited considerable potential towards new colored compounds, white light emission and OER properties. The present study suggests that it is profitable to explore mineral structures towards new and known material properties.