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 31P T(1) 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, K(2)CdP(2)Se(6) and Rb(2)CdP(2)Se(6), 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 31P relaxation. The importance of relaxation in application of (31)P NMR to these systems is demonstrated in analysis of the mixture of crystalline products formed in a Ag(4)P(2)Se(6) synthesis. At short relaxation delays, the NMR intensities are non-quantitative and overestimate the concentration of an Ag(7)PSe(6) impurity.