Syntheses of Hafnium–Zirconium Silicate Solid Solutions and a Simple Method to Efficiently Evaluate the Homogeneity of Zr and Hf Distribution

Abstract

Solid solutions of the microporous mixed tetrahedra− octahedra framework with the structures AV-3 Na 2+z [(Zr 1−x Hf x )-Si 3 O 9 (Cl,OH) z ]•H 2 O (x = 0, 0.33, 0.67, and 1), AV-13 Na 2+z [(Zr 1−x Hf x )Si 3 O 9 Cl z ]•2.5H 2 O (x = 0, 0.33, and 1), and AV-14 Na 2 [(Zr 1−x Hf x )Si 4 O 11 ]•2H 2 O (x = 0, 0.33, and 1) have been prepared under hydrothermal conditions. This was the first time that the hydrothermal synthesis of Hf-AV-3 was reported. Wadeite K 2 [(Zr 1−x Hf x )Si 3 O 9 ] (x = 0, 0.33, 0.67, and 1) has been prepared using high-temperature phase transformation. The desired phase and phase purity were studied by powder X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectrometry. The framework substitution of Zr by Hf has been investigated by 29 Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and Raman spectroscopy. In the selected region, Raman shifts depend on the hafnium content in these samples. 29 Si MAS NMR is a unique technique, which provides clear evidence for the framework substitution of Zr by Hf. Therefore, the hafnium content is able to be estimated from Raman shifts of bands in the selected region and the intensity of the 29 Si MAS NMR signals. 29 Si MAS NMR spectroscopy is also proposed to be an efficient technique to evaluate the homogeneity of Zr and Hf distribution in the hafnium−zirconium silicate solid solution.