9.6 GHz and 34 GHz electron paramagnetic resonance studies of chromium-doped forsterite

Budil, David E.; Park, Dong Gon; Burlitch, James M.; Geray, Roland F.; Dieckmann, Rüdiger; Freed, Jack H.
September 1994
Journal of Chemical Physics;9/1/1994, Vol. 101 Issue 5, p3538
Academic Journal
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 chromium-doped forsterite powder synthesized via a sol–gel method. Three crystals with chromium concentrations of 110, 300, and 390 ppm were studied. At 34 GHz, transitions are observed for the laser-active tetrahedral 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 at 34 GHz that were not observed at 9.6 GHz. These peaks are not analyzed in detail, but are tentatively ascribed to Cr4+ in the octahedral substitution sites of the crystal. At the highest chromium concentration, the Cr3+ spectra show evidence of direct interaction with Cr4+. A global least-squares fit of the combined 9.6 and 34 GHz data for the 300 ppm crystal gives D=64.26±0.18 GHz, E=-4.619±0.009 GHz, gx=1.955±0.009, gy=2.005±0.040, gz=1.965±0.006, and places the magnetic z axis in the ab plane at an angle of 43.8±0.3° from the b crystallographic axis (in Pbnm). A method for accurately measuring the Cr4+/Cr4+ ratio using EPR line intensities is given. The EPR linewidth of the Cr4+ center exhibits a strong orientation dependence that is well-modeled by including site variations in the D and E zero-field splittings and in the orientation of the z magnetic axis. The linewidth analysis reveals a high degree of correlation between the distributions 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 the tetrahedral cage along the a crystallographic axis. The Cr4+ EPR linewidths...


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