3D correlation NMR spectrum between three distinct heteronuclei for the characterization of inorganic samples: Application on sodium alumino-phosphate materials.

We report here an original NMR sequence allowing the acquisition of 3D correlation NMR spectra between three distinct heteronuclei, among which two are half-integer spin quadrupolar nuclei. Furthermore, as two of them exhibit close Larmor frequency, this experiment was acquired using a standard triple-resonance probe equipped with a commercial frequency splitter. This NMR technique was tested and applied to sodium alumino-phosphate compounds with P as the spin-1/2 nucleus and Na and Al as the close Larmor frequencies isotopes. To the best of our knowledge, such experiment with direct P and indirect Al and Na detection is the first example of 3D NMR experiment in solids involving three distinct heteronuclei. This sequence has first been demonstrated on a mixture of Al(PO) and NaAlPO crystalline phases, for which a selective observation of NaAlPO is possible through the 3D map edition. This 3D correlation experiment is then applied to characterize mixing and phase segregation in a partially devitrified glass that has been proposed as a material for the sequestration of radioactive waste. The P-{Na,Al} 3D experiment conducted on the partially devitrified glass material conclusively demonstrates that the amorphous component of the material does not contain aluminum. The as-synthesized material thus presents a poor resistance against water, which is a severe limitation for its application in the radioactive waste encapsulation domain.