Nuclear magnetic resonance investigations of the structural phase transitions of AlPO₄ tridymite

Abstract

²⁷Al and ³¹P NMR spectroscopic data are presented for the tridymite polymorph of AlPO₄ (AlPO₄ ⁻¹) through its structural phase transition at about 80 °C. The RT ²⁷Al and ³¹P spectra of AlPO₄₋t both contain doublets of broad peaks, indicating two well-separated groups of sites in the RT structure with mean Al-O-P bond angles per tetrahedron of ∼ 147.8°and 153.1°(±1). With increasing temperature, the doublets remain the same up to about 74 °C, where the relative intensities of the two peaks start to change. The peak corresponding to smaller Al-O-P bond angles disappears, and above ∼88 °C the ²⁷Al and ³¹P spectra contain single symmetrical peaks, corresponding to a mean Al-O-P bond angle of 153.4°. This bond angle increases gradually with increasing temperature to 153.7°at ∼150 °C and remains constant to about 500 °C. ²⁷Al quadrupole echo experiments suggests that the ²⁷Al nuclear quadrupole coupling constant (QCC) is small and decreases with increasing temperature. QCC remains nonzero in the high temperature phase of AlPO₄₋t, consistent with the previously proposed 3m local symmetry of Al in the high-temperature structure.