Identification and structures of the X-ray induced luminescence centers in the zeolites Zr,X,Cs,Na-LTA, X = Cl, Br, and I

Abstract

Abstract Zr4+ and halide anions were introduced into zeolite A (LTA) by treating Cs,Na-A with ZrX4(g), X = Br and I, under anhydrous conditions. The crystal structures of Zr,Br,Cs,Na-A and Zr,I,Cs,Na-A were determined by single-crystal crystallography with compositional confirmation by scanning electron microscopy energy-dispersive X-ray (SEM-EDX) analysis. Their X-ray induced luminescence (XIL) properties were throughly studied. These structures and XIL properties were compared with those of previously reported Zr,Cl,Cs,Na-A. In both structures, most Na+ and Cs+ ions occupy positions similar to those found in Cs,Na-A and Zr,Cl,Cs,Na-A. About 9% of the large cavities of Zr,Br,Cs,Na-A are centered by ZrBr62− ions. In Zr,I,Cs,Na-A, about 6% are centered by ZrI62−. These octahedral ZrX62− ions are each held in place and stabilized in their large cavities by 24 bonds between all six of its halide ions and eight surrounding Cs+ ions. They are further bridged by 8-ring Cs+ ions to form a cubic 3-dimensional Cs11ZrX69+ continuum in the surface volume of the crystal. Their XIL domain peaks have red-shifted from 495 nm for Zr,Cl,Cs,Na-A, to 520 nm for Zr,Br,Cs,Na-A, and to 640 nm for Zr,I,Cs,Na-A. This may be because the self-trapped excitons (STE) of ZrBr62− and ZrI62− have smaller band gaps than that of ZrCl62−, a result of the valence band maximum 4p energy level in Br− being higher (5p in I− even higher) than the 3p energy level in Cl−.