Probing the local structural environment of calcium by natural-abundance solid-state 43 Ca NMR

New natural-abundance Ca-43 magic angle spinning (MAS) NMR data measured at high magnetic field (14.1 T) is presented for a range of crystalline calcium-containing binary and ternary inorganic compounds. The combination of high field, moderate MAS (up to 4.5 kHz), and large sample volume (a 9.5 mm diameter MAS rotor) means that a good signal-to-noise ratio can generally be obtained in a time (similar to12 h) that makes Ca-43 NMR a feasible approach for determining information about calcium siting in a wide range of materials of physical interest. This study greatly increases the number of Ca-43 NMR parameters determined for solid materials in the literature, extending reports to local nearest-neighbor coordinations to other than oxygen. These data show that the isotropic chemical shift range is >250 ppm and typically that the quadrupole interaction is <4 MHz. In ternary compounds where Ca is coordinated in the nearest-neighbor shell by only oxygen, the isotropic Ca-43 chemical shift correlates well to the mean Ca-O distance, consistent with the only previous study. In binary compounds the isotropic Ca-43 chemical shift does not appear to be correlated with the mean Ca-X bond length. The extension of natural-abundance Ca-43 MAS NMR studies to amorphous materials are reported by examining sol-gel prepared calcium silicate materials. The data show that in the initial amorphous mixture at lower temperatures (120-350degreesC) the calcium environment is more like that in the parent calcium nitrate than a silicate, and that further heat treatment causes very significant broadening of the calcium resonance. The implications of this observation for the use of natural-abundance Ca-43 MAS NMR structural studies of amorphous materials are examined.