New Synthesis and Insight into the Structure of Blue Ultramarine Pigments

A new and easy method for preparing blue sodalite pigments which involves high-temperature calcination of sodalite samples synthesized with aluminum sulfate and an organic template, is presented. Calcination generated the and radicals, and the effects of the Al/Si ratio and the calcination temperature on the nature and amounts of the radicals were examined. The radicals were characterized in detail by continuous wave and pulsed EPR at X- and W-band frequencies (∼9 and 95 GHz, respectively) complemented by UV−vis measurements. The high-field electron-paramagnetic resonance (EPR) measurements allowed us to clearly resolve the g anisotropy of and W-band electron nuclear double resonance (ENDOR) measurements detected strong coupling with extra-framework 23Na cations and weak coupling with framework 27Al. On the basis of the spectroscopic results and density functional theory (DFT) calculations of the g-tensors of and radicals, the EPR signals were attributed to three different radicals, all with the open structure C2v, that are located within the sodalite β cages. While two of these radicals are well isolated, the third one is associated with an exchange-narrowed signal originating from radicals in nearby sodalite cages.