Electron spin resonance (ESR) and electron spin echo envelope modulation (ESEEM) studies on the ultramarine analogs obtained from zeolite A with various alkaline cations at different temperatures

Abstract

Selected pigment samples prepared from mixture of zeolite A, sulfur and relatively low amount of alkaline carbonates (Na, Li, K, and Cs) by thermal treatment at 500 °C or 800 °C were characterized by means of UV–vis, ESR, ED-ESR and ESEEM spectroscopy in order to explain the influence of various cations on their coloration and properties. The samples maintain the original structure of the parent zeolite. The different colors result from the nature of applied cations as well as from the synthesis temperature. It is reflected in UV–vis spectra showing various intensity of the absorption bands assigned to S 2 - and S 3 - anion-radical chromophores. The ESR spectra recorded at 10 K indicated five components attributed to S 3 - . The best quality single-component rhombic spectrum with g x = 2.0011, g y = 2.0510 and g z = 2.0355, appears in the samples prepared with Na and K cations at 800 °C indicating a local homogeneity of the structure around the radicals. The g-factors are analyzed in terms of molecular orbital theory. The other samples (particularly those prepared at 500 °C) are less ordered which is reflected in a strong ESR line broadening along g-factor z-axis. This disorder is explained as due to a distribution of the trisulfur molecule bend angle. The ESEEM spectra of the pigments containing Li and Cs prepared at 500 °C indicate the peaks assigned to 7Li and 133Cs, while for the analogous samples obtained by heating at 800 °C these spectra lack such peaks. This is explained as an effect of a transition from a static disorder to a dynamic disorder visible as a transformation of static ESR spectrum into broad dynamically averaged homogeneous lines.