Optical Spectroscopy of MgGa2O4:Fe3+

L. P. Sosman et al.: Optical Spectroscopy of MgGa2O4 : Fe3+

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phys. stat. sol. (a) 176, 1085 (1999) Subject classification: 78.55.Hx; S11

Optical Spectroscopy of MgGa2O4 : Fe3+ L. P. Sosman (a), A. Dias Tavares, Jr. (a), P. S. Silva (b), and T. Abritta (b) (a) Instituto de FõÂsica UERJ, Rua SaÄo Francisco Xavier 524, Rio de Janeiro, RJ 20550-013, Brazil (b) Instituto de FõÂsica UFRJ, P.O. Box 68528, Rio de Janeiro, RJ 21945-970, Brazil (Received October 5, 1998; in revised form August 16, 1999) Fluorescence and excitation data for MgGa2O4 : Fe3+ are presented. We have produced by solid state reactions a compound in which a near infrared emission broadband at 77 K as well as at room temperature was observed. The emission is attributed to the spin-forbidden 4T1(4G) ! 6A1(6S) electronic transition of Fe3+ ions in tetrahedral sites.

1. Introduction The Fe3+ ion incorporated as an impurity in host insulating compounds is an attractive system for luminescence studies due to the fact that these materials have a broad emission band in the near-infrared region of the optical spectrum. The emission and excitation spectra of the Fe3+ isolated ion are refereed in several compounds just as: ZnO [1]; LiGaTiO4 [2]; LiGa5O8 [3, 4] and LiAl5O8 [5]. This paper presents luminescence and excitation results for MgGa2O4 : Fe3+ at 77 K and room temperature. In this compound the Fe3+ luminescent band is attributed to the spin-forbidden 4T1(4G) ! 6A1(6S) electronic transition of Fe3+ ions in tetrahedral sites. For this coordination number, the charge transfer band is shifted to higher energy, allowing an easier observation of the high crystal fields electronic transitions. The normal spinel consists of a face-centered cubic close-packed array of oxygen anions with metal cations filling the interstices and the unit cell contains eight AB2O4 molecules, the A2+ ions of which are in the tetrahedral sites and the B3+ ions in the octahedral ones. The MgGa2O4 is a partially inverted spinel, belonging to the space group Fd3m with lattice parameter a0 = (8.286  0.003)  A. In this sample, the eight tetrahedral sites and sixteen octahedral sites are both occupied by Mg2+ and Ga3+ ions. The Mg2+ occupation in octahedral sites is very temperature dependent, varying from 0.9 at 900  C to 0.84 at 1400  C [7, 8]. Since the greater part of Mg2+ ions occupies octahedral sites, Mg2+ remaining ions and Ga3+ ions occupy the tetrahedral sites. The ionic radius of Fe3+ is 0.64  A, while the ionic radius of Ga3+ and Mg2+ are 0.62 and 0.66  A, respectively. These close values could make it possible to replace either Ga3+ or Mg2+ by Fe3+ in the sites. In spite of this, the most important requirement for reactions between the insulate oxides is that the doping ion has the same valence as the substituted ion. Therefore, we believe that it is more suitable to replace Ga3+ ions by a Fe3+ ion in this compound.

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2. Experimental Details The polycrystalline samples used in this study were prepared by a solid-state reaction [7] from high purity raw oxides MgO, b-Ga2O3 and the necessary amount of Fe2O3 by sintering them at 1000  C in normal atmospheric pressure furnace, during 190 h and cooling down to room temperature over 24 h. A Spex model 1702 spectrometer was used to scan the optical spectra. The light source for luminescence and excitation measurements is a 1 kW xenon lamp with its intensity modulated by a PAR 191 variable speed chopper. The detector used was an RCA 7151W photomultiplier and the output signals were fed into an EGG model 5209 lock-in amplifier connected to a chart recorder. The spectra were corrected for the spectral sensitivity of the set-up. The magnesium gallates with Fe3+ concentrations varying from 0.1 to 1% were analyzed through fluorescence and excitation. The room temperature X-ray powder diffraction data [6] corresponds to expected results for this material.

3. Results and Discussion The luminescence spectrum of MgGa2O4 with 1% of Fe3+ at 300 K (Fig. 1, curve a) obtained with 400 nm excitation light, presents a broad band centered at 13 889 cm ±±1. This band has been interpreted as being due to the symmetry and spin-forbidden 4 T1(4G) ! 6A1(6S) electronic transition of Fe3+ ions in tetrahedral sites. The lifetime emission measure was 4 ms and this value is in agreement with the low oscillator strength for this transition. The half peak energy difference for this band is 1315 cm-1. The luminescence for this sample at 77 K is show in Fig. 1, curve b. This spectrum also is attributed to the 4T1(4G) ! 6A1(6S) electronic transition of Fe3+ ions in tetrahedral sites [1 to 5]. The full width at half maximum for this band is 1031 cm ±±1, showing the band is larger at room temperature than at 77 K. This fact is interesting if the system would be investigated as a possible source of tunable radiation at room temperature. The effect of iron concentration on luminescent band size is not observed, indicating that over the interval studied no significant couplings between Fe3+ ions are present. No luminescent band center shift is detected for these

Fig. 1. Luminescence spectrum of MgGa2O4 with 1% of Fe3+ (a) at 300 and (b) 77 K

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Optical Spectroscopy of MgGa2O4 : Fe3+

Fig. 2. Excitation spectrum of MgGa2O4 with 1% of Fe3+ at 77 K

samples at 77 or 300 K and no band shape modifications occur. Moreover, the constant fluorescence lifetimes for 77 and 300 K indicate that radiationless processes are not competitive for these temperatures. The obtained results are not conclusive about the intensity dependence on temperature. The importance of these measurements is to provide information on the 4 T1(4G) state fine structure. Unfortunately, the best data lie beyond our instrumental capabilities. A luminescence excitation spectrum for the sample at 77 K was determined and is shown in Fig. 2. The spectrum shows sharp features associated with Fe3+ tetrahedrally coordinated [1 to 5]. For d5 configuration ions the electric dipole transitions are forbidden, thus a weak absorption band is expected for this sample. The complete assignment is listed in Table 1 and we can observe that the assignments for transitions lead to a satisfactory accordance between predicted and experimental data. Using the Tanabe-Sugano energy matrices [9] for a d5 eletronic configuration the experimental parameters Dq = 863 cm ±±1, B = 531 cm ±±1 and C = 3091 cm ±±1 were obtained. The energy transition 6 A1(6S) ! 4T1(4G) for tetrahedral Fe3+ was calculated to be at 14 493 cm ±±1 and is not observed in the excitation spectra. This fact is attributed to the low oscillator strength of the near-infrared absorption, leading to a weak signal not detectable with our instrumental sensitivity. The Stokes shift between the observed emission and predicted absorption was 599 cm ±±1. The 4T1(e3t2) and 6A1(e2t3) energy states belong to different electronic configurations, and this occurs because the transition 4 T1 $ 6A1 is strongly lattice-coupled Ta b l e 1 Optical absorption data for MgGa2O4 : Fe3+ transitions 6A1(6S) !

4

T1(4P)

4

E(4D)

4

T2(4D)

4

E + 4A1(4G)

4

T2(4G)

experimental data (cm ±±1) theoretical data (cm ±±1) observed wavelength (nm)

29 412

25 000

22 222

20 408

17 242

29 820

24 474

22831

20 760

17 483

340

400

450

490

580

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L. P. Sosman et al.: Optical Spectroscopy of MgGa2O4 : Fe3+

giving broad bands with large Stokes shift between emission and absorption when compared with those for Fe3+ doping another structures [2]. Moreover, the 4T1(4G), 4 T2(4D), 4T2(4G) and 4T1(4P) energy levels depend on crystal-field intensity justifying band broadening between the ground state and these levels. The possibility that the excitation spectrum was ascribed to Fe3+ in octahedral sites was investigated. No experimental evidence was found to support this hypothesis that then it was rejected. The low value of the Racah parameter B suggests that the impurity±ligand bond, is predominantly covalent instead of ionic. The mixture between Fe3+ and oxygen levels was responsible for the breaking of the spin multiplicity selection rule leading to the observed spectra. The crystal-field and Racah parameters are close to those obtained for Fe3+ in tetrahedral sites in other oxide compounds [1 to 5]. Previous studies [10 to 11] showed that Fe3+ has a greater preference for octahedral sites than Ga3+ and consequently we believe that Fe3+ occupies octahedral sites. In this case, the luminescence is expected to occur at 10 000 cm ±±1 [1 to 5]. We found no evidence of optical transitions associated with this site occupancy for MgGa2O4 : Fe3+.

4. Conclusions Luminescence and visible excitation of Fe3+ in MgGa2O4 were completed. The emission is identified as the 4T1(4G) ! 6A1(6S) electronic transition of isolated tetrahedral Fe3+ ions in Ga3+ lattice sites. No modifications of fluorescent band shape or lifetime transitions were observed as a function of iron concentration, indicating that over the interval studied no significant couplings between Fe3+ ions are present. No changes of emission lifetime with temperature occur and thus indicate that radiative processes predominates on the spectra and the value of 4 ms for the decay time is in agreement with the low value of oscillator strength value for the fluorescent transition. Acknowledgements The authors are grateful to M. R. Amaral, Jr. for the X-ray measurements and to FAPERJ, FINEP and CNPq for the financial support.

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