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 T₁ longitudinal relaxation times (20-3000 s). There is a qualitative positive correlation between squared dipolar couplings and 1/T₁, suggesting that these interactions contribute to relaxation. However, two of the compounds, K ₂CdP₂Se₆ and Rb₂CdP ₂Se₆, have T₁ 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 ³¹P 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.