Investigation of longitudinal 31P relaxation in metal selenophosphate compounds

Molten salt syntheses yield a rich variety of metal selenophosphate compounds which have a wide range of P-31 T, longitudinal relaxation times (20-3000 s). There is a qualitative positive correlation between squared dipolar couplings and 1/T-1, suggesting that these interactions contribute to relaxation. However, two of the compounds, K2CdP2Se6 and Rb2CdP2Se6, have T-1 which are significantly shorter than what is expected from dipolar couplings. The ESR spectra of these compounds show the presence of unpaired electrons which may accelerate the rate of P-31 relaxation. The importance of relaxation in application of P-31 NMR to these systems is demonstrated in analysis of the mixture of crystalline products formed in a Ag4P2Se6 synthesis. At short relaxation delays, the NMR intensities are non-quantitative and overestimate the concentration of an Ag7PSe6 impurity. (C) 2003 Elsevier Inc. All rights reserved.