High-temperature23Na MAS NMR data for albite; comparison to chemical-shift models

Abstract

Recently, correlations of 23Na chemical shifts with structural parameters such as bond length, coordination number, and bond strength in inorganic compounds have been proposed. When considering only silicate materials we have found that some of these models lead to conflicting predictions. To test the applicability of these correlations to silicates, we collected 23Na MAS NMR spectra of a pure, ordered albite (NaAISi30g) from 25 to 352°C. Over this temperature range, the Na-O bond lengths increase, and the measured isotropic shifts relative to 1 M NaCI decrease linearly from -7.1 ppm at room temperature to -7.8 ppm at 352°C. This agrees with the observed trend in silicates of increasing bond length leading to more negative values of 0 but is not consistent with the bond-valence model, which predicts a change of several parts per million in the opposite direction over this temperature range. Quadrupolar coupling constants were also seen to change, decreasing linearly with temperature from 2.69 MHz at 20°C to 2.30 MHz at 352°C. Static spectra measured from 22 to 1020 °C yielded values consistent with this in the low-temperature range and continued to decrease linearly to 1.50 MHz at the highest temperature. The 23Na spin-lattice relaxation times (T1) were collected between 25 and 1140 °C as well. T[ decreases gradually up to about 500°C and then drops rapidly, indicating that there is a change in the relaxation mechanism at this temperature.