ns2 type luminescence in Pb-bearing carbonates using cathodoluminescence

We conducted to clarify an emission mechanism of ns2 type activation in Pb-bearing carbonates crystal structure with calcite and aragonite structures by cathodoluminescence (CL) analysis. Single crystals of tarnowitzite (Namibia and Morocco) and plumbocalcite (Namibia) were employed for CL measurements. Their color CL image were obtained with a cold-cathodo microscopy of the Luminoscope. CL spectra were measured by using a SEM-CL comprised of SEM combined with a grating monochromator at accelerating voltage of 15 kV and beam current of 0.1 nA in the wavelength-range of 300-800 nm. The sample temperature was controlled in the range from -192 to 25 °C with a cryo-stage. Color CL imaging shows a blue emission in tarnowitzite and a red emission in plumbocalcite. CL spectra of tarnowitzite have an intense broad band emission at around 390 nm in a blue region, whereas plumbocalcite exhibits two broad band emissions at around 320 nm in a UV region and at 620 nm in a red region. A blue CL is derived from Pb2+ activator as ns2 emission center from s2-sp transition in tarnowitzite. Its intensity depends on Pb content, which suggests a concentration quenching of CL above PbO 10 wt%. Tarnowitzite with aragonite structure shows the ns2 emission caused by Pb2+, whereas plumbocalcite gives no emission related to ns2 type. Pb ions in plumbocalcite behave as a sensitizer to transfer the energy to Mn activator for a red emission. Blue CL intensities at various temperatures of tarnowizite shows almost unchanged between 25 to -70 °C, and subsequent sudden reduction below -90 °C, and gradual sensitizing above -150 °C. The change in CL intensity below -90 °C might be due to the Jahn-Teller effect, which alters the symmetry of the electron configuration responsible for stability of a crystal field at low temperature. Conclusively, blue Pb activation in tarnowitzite should be characteristic in ns2 type emission, which of process might be related to electron transfer among two or more energy levels under the influence of sample temperature.