A Cr3+ luminescence study of spodumene at high pressures: Effects of site geometry, a phase transition, and a level-crossing

摘要

Cr3+ luminescence of the green Cr-bearing variety of spodumene (LiAlSi2O6) has been studied under hydrostatic conditions up to similar to 15 GPa. R-line luminescence is a particularly sensitive site-specific probe of the Al-site, and high-pressure phase transitions that affect the symmetry or electron density at this site should produce obvious changes in the luminescence spectra. Thus, the nature of Cr3+ luminescence is probed across known and possible phase transitions in spodumene. Discontinuous shifts of the R-lines and their sidebands to higher energy at 3.2 GPa are associated with the C2/c to P2(1)/c phase transition. Both R-lines and sidebands shift to lower energy after the 3.2 GPa transition up to similar to 15 GPa. The C2/c to P2(1)/c phase transition is confirmed to be first order in nature based on its observed hysteresis on decompression, and R-line and sideband measurements give no evidence of a second proposed transition up to similar to 15 GPa. The splitting between the R-1 and R-2 bands is dramatically enhanced by pressure, with the split decreasing at the phase transition. These trends correspond to pressure-induced shifts in the distortion of the M1 site, and a likely shift in off-centeredness of the Cr3+ ion. Pressure-induced decreases in line widths are consistent with the R-lines shifting at slower rates than the phonons to which they are most closely coupled, as demonstrated by large pressure shifts of vibronic peaks. Observations of a pressure-induced cross-over between the T-4(2) and E-2 levels of the Cr3+ ion indicate that spodumene undergoes a shift from an intermediate strength crystal field environment to a high strength crystal field environment at pressures between ambient and 3.2 GPa.