Sol-Gel Synthesis of Chromium-Doped Forsterite

Park, Dong Gon Burlitch, James M. Geray, Roland F. Diechmann, Rudiger Barber, Duane B. Pollock, Clifford R. 숙명여자대학교 자연과학연구소 1994 자연과학논문집 Vol.- No.5

Pale purple, polycrystalline, chromium-doped forsterite powder was prepared at low temperature from a magnesium silicate sol. The chromium dopant, introduced as either chromium(Ⅱ) acetate or chromium(Ⅳ) oxide, was combined in solution to maximize homogeneity. Electronic absorption spectroscopy indicated dopantions, CrIII and CrIV, were intimately bound to the magnesium silicate network. At a Cr/Si mole ratio of 0.04 or less, the dried xerogel formed only a single phase, forsterite, after calcination in humid air to 560℃, and was fully crystalline above 800℃. The residual carbon content of the xerogel was decreased by the use of humidified air during calcination. FTIR spectroscopy in dicated that the dopant occupied both octahedral and tetrahedral sites. In the first attempts to grow single crystals of chromium-doped forsterite, coarse-grained samples were obtained by melting and solidification of the polycrystalline powder under an oxygen atmosphere by the crucible-free, floating zone method. Observation of a broad ESR peak for Cr4+ at room temperature and of typical near IR emission spectra indicated that the Cr4+/Cr3+ ratio in the melt grown samples was higher than that of commercial single crystals, grown under a more reducing atmosphere.

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, Rudiger Freed,Jack H. 숙명여자대학교 자연과학연구소 1994 자연과학논문집 Vol.- No.5

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 power 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, g3= 2.005±0.040, g2=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 increase significantly at higher chromium concentration, but maintain the same qualitative orientation dependence. The EPR data indicate that the major contribution to inhomogeneity in the tetrahedral site, which may be related to the tunable range of the Cr4+ laser center, is distortion induced by chromium substitution into the crystal lattice rather than direct chromium-chromium interactions.

Sol-Gel Synthesis of Chromium-Doped Forsterite

Park, Dong Gon,Burlitch, James M.,Geray, Roland F.,Diechmann, Rudiger,Barber, Duane B.,Pollock, Clifford R. 숙명여자대학교 자연과학연구소 1994 자연과학논문집 Vol.- No.5

Pale purple, polycrystalline, chromium-doped forsterite powder was prepared at low temperature from a magnesium silicate sol. The chromium dopant, introduced as either chromium(Ⅱ) acetate or chromium(Ⅳ) oxide, was combined in solution to maximize homogeneity. Electronic absorption spectroscopy indicated dopantions, CrIII and CrIV, were intimately bound to the magnesium silicate network. At a Cr/Si mole ratio of 0.04 or less, the dried xerogel formed only a single phase, forsterite, after calcination in humid air to 560℃, and was fully crystalline above 800℃. The residual carbon content of the xerogel was decreased by the use of humidified air during calcination. FTIR spectroscopy in dicated that the dopant occupied both octahedral and tetrahedral sites. In the first attempts to grow single crystals of chromium-doped forsterite, coarse-grained samples were obtained by melting and solidification of the polycrystalline powder under an oxygen atmosphere by the crucible-free, floating zone method. Observation of a broad ESR peak for Cr4+ at room temperature and of typical near IR emission spectra indicated that the Cr4+/Cr3+ ratio in the melt grown samples was higher than that of commercial single crystals, grown under a more reducing atmosphere.

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, Rudiger,Freed,Jack H. 숙명여자대학교 자연과학연구소 1994 자연과학논문집 Vol.- No.5

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 power 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, g3= 2.005±0.040, g2=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 increase significantly at higher chromium concentration, but maintain the same qualitative orientation dependence. The EPR data indicate that the major contribution to inhomogeneity in the tetrahedral site, which may be related to the tunable range of the Cr4+ laser center, is distortion induced by chromium substitution into the crystal lattice rather than direct chromium-chromium interactions.

Crystallization of Precursors to Forsterite and Chromium-Doped Forsterite

Park, Dong Gon Martin, M. Hogan E. Ober, Christopher K. Burlitch, James M. Cavin, O. Burl Porter, Wallace D. Hubbard, Camden R. 숙명여자대학교 자연과학연구소 1994 자연과학논문집 Vol.- No.5

The pyrolysis and crystallization of poly(mathacrylate) precursors and xerogels of forsterite and chromiumdoped forsterite were studied by in situ high-temperature, dynamic X-ray diffraction and thermal analysis. For both types of precursor, crystallization of forsterite occurred at lower temperature when doped with chromium. Also, exotherms above 700℃ occurred 50℃ lower when chromium was present. When residual carbon in the xerogels was more than ∼1%, an unidentified crystalline intermediate phase formed at ∼800℃. Conversion of the intermediate phase to forsterite was faster than amorphous material. Thus, full crystallinity was attained at a lower temperature when the xerogels had some residual carbon.

Crystallization of Precursors to Forsterite and Chromium-Doped Forsterite

Park, Dong Gon,Martin, M. Hogan E.,Ober, Christopher K.,Burlitch, James M.,Cavin, O. Burl,Porter, Wallace D.,Hubbard, Camden R. 숙명여자대학교 자연과학연구소 1994 자연과학논문집 Vol.- No.5

The pyrolysis and crystallization of poly(mathacrylate) precursors and xerogels of forsterite and chromiumdoped forsterite were studied by in situ high-temperature, dynamic X-ray diffraction and thermal analysis. For both types of precursor, crystallization of forsterite occurred at lower temperature when doped with chromium. Also, exotherms above 700℃ occurred 50℃ lower when chromium was present. When residual carbon in the xerogels was more than ∼1%, an unidentified crystalline intermediate phase formed at ∼800℃. Conversion of the intermediate phase to forsterite was faster than amorphous material. Thus, full crystallinity was attained at a lower temperature when the xerogels had some residual carbon.