ESEEM and multi-frequency EPR study on Mn2+ luminescence centres in leucophanite

The location and coordination of Mn²⁺ paramagnetic luminescent centres in three natural samples of leucophanite (ideally, NaCaBeSi₂O₆F) have been investigated by electron paramagnetic resonance (EPR) and electron spin-echo envelope modulation (ESEEM) spectroscopy. Continuous wave (cw)-EPR was carried out at multiple frequencies (9, 94 and 188 GHz) which allowed the precise determination of the spin Hamiltonian parameters. The simulation of the cw-EPR spectra shows that the signal originates from one manganese located in a single distorted site. This distortion was close to axial symmetry with a slight rhombicity and was consistent with an eight coordinated Mn²⁺ substituting for Ca²⁺. Spectral differences between the samples could be attributed mainly to D-strain effect. Three-pulse ESEEM spectroscopy was also used to investigate the coordination environment of the manganese and showed weak coupling between the manganese and beryllium (⁹Be), sodium (²³Na) and fluorine (¹⁹F) nuclei in the second and subsequent coordination spheres. The Mn ligand distances estimated from the ESEEM data are in good agreement with bond distances estimated from X-ray diffraction data. To our best knowledge, only few ESEEM studies have been reported so far in Earth science and this is believed to be the first detailed ESEEM study of Mn²⁺ in a natural mineral system.