Effect of pressure on site-symmetry distortions of Mn5+ and Cr4+ in Y2SiO5

High pressure is used to study the effects of nontetrahedral site distortio ns on luminescence properties of Cr4+ and Mn5+ in Y2SiO5. Luminescence spec tra and lifetime measurements of Mn5+:Y2SiO5 as a function of pressure up t o 120 kbar indicated that the local Mn5+ site distortion increased strongly up to similar to 50 kbar and increased only slightly at higher pressure. A crystal field energy level calculation that included spin-orbit coupling e ffects was completed to quantitatively model the luminescence and lifetime data in the context of a D-2d distortion model. The model provided excellen t agreement with the spectral and lifetime data and showed that the T-3(1) State has a significant influence on the lifetime of E-1 luminescence. Lumi nescence studies of Cr4+:Y2SiO5 revealed a redshift up to similar to 40 kba r followed by a blue shift at higher pressure. These results are consistent with a pronounced increase in the Cr4+ site distortion up to similar to 40 kbar that levels off at higher pressures. The Cr4+:Y2SiO5 results also sup port assignment of the ambient-pressure luminescence spectrum to a transiti on from orbital components of the T-3(2) Sate to the (3)A(2) ground state. No evidence of a T-3(2)-E-1 electronic crossover in Cr4+:Y2SiO5 was observe d up to 120 kbar. The results indicate more generally that the luminescence properties of 3d(2) systems are controlled by competing cubic crystal fiel d and distortion effects. The ability of modest pressures to alter the rela tive importance of these effects suggests that cubic crystal field and dist ortions vary widely from host lattice to host lattice at ambient pressure a nd are responsible for the complicated optical properties of 3d(2) ions.