Cathodoluminescence of carbonates, 1. Characterization of cathodoluminescence from carbonate solid solutions

Abstract

The purpose of this study was to characterize cathodoluminescence and X-ray induced luminescence in the visible range of the spectrum for Mn 2+ activated carbonate minerals. The cathodoluminescence emission varied from approximately 590 mμ for calcite to approximately 677 mμ for magnesite, illustrating the effect of decreasing bond distance between the alkaline earth cations and the ligands and the resulting increased crystal field splitting parameter, Dq. Although the direction of the wavelength shift was in keeping with crystal field theory, the magnitude of the shift was not. Distortions in the structure as a result of the discrepancy in ionic size between Mg 2+ and Ca 2+ and the increasing degree of covalency of the metal-oxygen bond, as the Mg 2+ concentration was increased, were thought to be the major causes of the deviation from theoretical predictions. Manganese in the aragonite-strontianite series was found to emit from approximately 540 mμ (aragonite) to approximately 590 mμ (strontianite). The direction of wavelength shift was to higher wavelength with apparently longer bond length, in opposition to crystal field theoretical considerations. The similar emission spectra of vaterite and calcite indicated the effective coordination of Ca 2+ (Mn 2+) in vaterite was six rather than eight. Vaterite emits at approximately 595 mμ. Otavite, CdCO 3, on excitation of incorporated Mn 2+ ions, showed an emission band centered at about 594 mμ. Because CdCO 3 and CaCO 3 are almost identical in all structural parameters, the slight difference in emission is attributed to the greater degree of covalency of the CdO bond in otavite. Dolomite showed the emission behavior of calcite and magnesite with Mn 2+ populating both the Ca 2+ and the Mg 2+ sites. Dolomite emitted at about 597 mμ and 675 mμ. No evidence was found to substantiate the belief that the increased polarizability of the ions as one progresses from Ca to Sr to Ba reduces bond length to the extent of creating a shift to higher wavelength with decreasing bond length. This study does not support the hypothesis of transitions from a split T 1g level or from the T 1g and T 2g levels accounting for the emission characteristics of aragonite, strontianite and witherite. Rather, transitions from an unsplit T 1g level to the ground state ( 6S) are believed to be responsible for the 540 mμ emission of aragonite. The theoretical Dq value calculated for an eight-coordinated site in CaCO 3 for a cubic crystal field is very close to the Dq found for aragonite where the coordination number is nine. Similarly, the 590 mμ emission of strontianite and the 677 mμ emission of witherite were attributed to transitions from the T 1g level to ground state. Local distortions arising from the discrepancy in size between the activator ion (Mn 2+) and the host cation in aragonite, strontianite and witherite were thought to be a major consideration accounting for the shift of emission wavelength towards higher wavelength with increasing bond length. In general, intensity of cathodoluminescence decreases with increasing emission wavelength for the aragonite-strontianite-witherite series and for the calcite-magnesite series. Otavite, vaterite and calcite have comparable intensities for similar emission wavelengths.