9.6-GHZ AND 34-GHZ ELECTRON-PARAMAGNETIC-RESONANCE STUDIES CHROMIUM-DOPED FORSTERITE

Chromium-doped forsterite single crystals grown under conditions that produce a high Cr4+/Cr3+ ratio were examined by electron paramagnetic resonance (EPR) at 9.6 and 34 GHz. The crystals were grown in 2-3 atm of oxygen by the floating-zone method starting from polycrystalline ch romium-doped forsterite powder synthesized via a sol-gel method. Three crystals with chromium concentrations of 110, 300, and 390 ppm were s tudied. At 34 GHz, transitions are observed for the laser-active tetra hedral Cr4+ species that are not observable at 9.6 GHz, which improve the resolution and accuracy with which the magnetic parameters can be measured by EPR. In addition, peaks for a non-Kramers species appear a t 34 GHz that were not observed at 9.6 GHz. These peaks are not analyz ed in detail, but are tentatively ascribed to Cr4+ in the octahedral s ubstitution sites of the crystal. At the highest chromium concentratio n, the Cr3+ spectra show evidence of direct interaction with Cr4+. A g lobal least-squares fit of the combined 9.6 and 34 GHz data for the 30 0 ppm crystal gives D=64.26+/-0.18 GHz, E=-4.619+/-0.009 GHz, g(x)=1.9 55+/-0.009, g(y)=2.005+/-0.040, g(z) =1.965+/-0.006, and places the ma gnetic z axis in the ab plane at an angle of 43.8+/-0.3 degrees from t he b crystallographic axis (in P-bnm) A method for accurately measurin g the Cr4+/Cr4+ ratio using EPR line intensities is given. The EPR lin ewidth of the Cr4+ center exhibits a strong orientation dependence tha t is well-modeled by including site variations in the D and E zero-fie ld splittings and in the orientation of the z magnetic axis. The linew idth analysis reveals a high degree of correlation between the distrib utions in D and E, and a somewhat weaker correlation between E and the z axis orientation. These results are interpreted to suggest that the tetrahedral Cr4+ sites vary mainly in the degree of compression of th e tetrahedral cage along the a crystallographic axis. The Cr4+ EPR lin ewidths increase significantly at higher chromium concentration, but m aintain the same qualitative orientation dependence. The EPR data indi cate that the major contribution to inhomogeneity in the tetrahedral s ite, which may be related to the tunable range of the Cr4+ laser cente r, is distortion induced by chromium substitution into the crystal lat tice rather than direct chromium-chromium interactions.