ESR AND ELECTRON SPIN-ECHO STUDIES OF MNALPO5

The immediate environment of Mn(II) in MnAlPO5 after synthesis, calcination, hydration, and dehydration has been investigated by both conventional ESR spectroscopy and by several electron spin-echo (ESE) methods. Several samples with Mn(II) contents between Mn/P = 0.05 and 0.5 atom % were synthesized, and samples of impregnated Mn-AlPO5 and exchanged Mn-SAPO5 were used as references. The X-band and Q-band ESR spectra of all as-synthesized and calcined MnAlPO5 samples and of impregnated Mn-AlPO5 are characteristic of Mn(II) in a slightly distorted octahedral symmetry with an hyperfine coupling constant of 90 G and a zero-field splitting parameter of 140 G. The spatial distribution of the Mn(II) cations was investigated by the "2 + 1" ESE experiment. A random homogeneous distribution of isolated Mn(II) cations was found only in the sample with the lowest Mn(II) content. In all other samples regions with enriched Mn(II) concentrations, where most of the Mn(II) are located, were found to exist. Dehydration causes the migration of the Mn(II) cations toward each other, and consequently the spin exchange interaction increases, resulting in the averaging of the hyperfine interaction. Electron spin-echo envelope modulation (ESEEM) experiments, which detect only the isolated Mn(II) species, showed that the isolated Mn(II) cations interact through weak dipolar interactions with an average of 6 P-31 nuclei at a distance of 5 angstrom. This does not agree with a model of Mn(II) substituting for framework Al. Significant changes in the Al modulation after calcination and hydration indicated that this treatment results in a change in the Mn(II) location. Mn(II) in calcined and hydrated MnAlPO5 and in impregnated Mn-AlPO5 is coordinated to three or four framework oxygens, and the remaining ligands are water molecules and/or hydroxyl groups. All the above observations lead to the conclusion that the majority of the Mn(II) in MnAlPO5 does not occupy framework sites but is probably coordinated to the external surface.