Ressonância paramagnética eletrônica de titanato de estrôncio dopado com manganês

ce Cerâmica Cerâmica 0366-6913 1678-4553 Associação Brasileira de Cerâmica Resumo Espectros de ressonância paramagnética eletrônica de titanato de estrôncio dopado com manganês foram investigados para várias concentrações de Mn. Os espectros dos íons de Mn2+ e Mn4+ foram observados e atribuídos, respectivamente, a íons de Mn ocupando sítios do Sr2+ e do Ti4+. A intensidade relativa dos dois espectros sugeriu que os íons de manganês ocupam preferencialmente os sítios do Ti4+. Os resultados mostraram que a largura de linha pico a pico do espectro do Mn4+ aumenta com a concentração de manganês de acordo com a equação teórica ΔHpp=0,45+210.f.(1-f)80 (mT). Isto sugeriu que a interação de câmbio entre íons de manganês tetravalente tem um alcance aproximado de 0,96 nm, comparável ao do Gd3+ no mesmo composto. INTRODUCTION Strontium titanate (SrTiO3) is a ceramic material with several industrial applications 1)- (5 whose properties can be improved by doping 6)-(10. In this work, we investigate the influence of the degree of Mn doping on the electron paramagnetic resonance (EPR) linewidth of Mn4+ in polycrystalline SrTiO3. In this way, one can use the EPR results to measure, rapidly and nondestructively, small concentrations of Mn in commercial SrTiO3, as it has been done for other ions and other ceramic materials 11)- (16. In addition, the extent of the interaction of tetravalent manganese ions in manganese-doped strontium titanate may be helpful for the investigation of the magnetic behavior of manganese-doped barium titanate, which could be used as a multiferroic material, since, in these materials, magnetic and electric properties coexist and manganese is a dopant with a strong influence on both electric and magnetic properties 9), (10), (17. A previous investigation by EPR of manganese-doped strontium titanate 17 has shown that Mn ions may occupy either Sr2+ sites or Ti4+ sites. The Mn2+ spectrum is described by a spin Hamiltonian with parameters g=2.0032 and A=82.8x10-4 cm-1, while the Mn4+ spectrum is described by a spin Hamiltonian with parameters g=1.9920 and A=71.2x10-4 cm-1. EPR of paramagnetic impurities in solids: according to previous works 18), (19, the peak-to-peak first derivative linewidth is given by: ΔH pp = ΔH o + ΔH d = ΔH o + c . f e (A) where ∆Ho is the intrinsic linewidth, ∆Hd is the dipolar broadening, c is a constant, and fe is the concentration of substitutional ions of the paramagnetic impurity not coupled by the exchange interaction, which can be expressed as: f e = f . ( 1 - f ) z ( r c ) (B) where f is the impurity concentration, z(rc) the number of cation sites included in a sphere of radius rc, and rc the effective range of the exchange interaction. EXPERIMENTAL PROCEDURE Sample preparation: the sample preparation method was the same as in 20. The starting materials were reagent grade SrTiO3 (Aldrich, <5 μm, 99%) and MnO2 (Carlo Erba, 99%). The powders of SrTiO3 and 0.1 to 3.0 mol% of MnO2 were ground together, and then the mixtures were fired for 24 h at 1200 °C in air. Measurements: X-ray diffraction measurements were performed in a Panalytical X’Pert Pro diffractometer with CuKα radiation (0.154 nm). All magnetic resonance measurements were performed at room temperature and 9.50 GHz using an electron paramagnetic resonance (EPR) spectrometer (Varian, E-12) with 100 kHz field modulation. The microwave power was 200 mW, and the modulation amplitude was 0.1 mT. The magnetic field was calibrated with an NMR gaussmeter. EXPERIMENTAL RESULTS X-ray diffraction: all diffractograms (a typical one is shown in Fig. 1) were indistinguishable from the diffractogram of pure SrTiO3 (JCPDS 86-0179). This was expected, since the Mn doping, up to only 3.00 mol%, was not enough to change either the lattice constant or the amplitude of the diffraction peaks as long as the Mn ions remained in solid solution, as attested by the fact that no other lines than those attributed to SrTiO3 were observed. Moreover, the lines were very narrow, as expected from micron-sized particles. Figure 1: X-ray diffraction pattern of a SrTiO3 sample doped with 0.6 mol% Mn. The indices were taken from the file JCPDS 86-0179. Figura 1: Difratograma de raios X de uma amostra de SrTiO3 dopada com 0,6 mol% de Mn. Os índices foram obtidos do arquivo JCPDS 86-0179. EPR spectra: the spectrum of a typical sample is displayed in Fig. 2. Two sextets were seen, one with g=2.00 and another with g=1.99. Due to the similarity of the measured g-values and hyperfine constants to those of previously reported spectra 14, the first sextet was attributed to Mn2+ ions occupying Sr2+ sites and the second to Mn4+ ions occupying Ti4+ sites. The intensity of the second sextet was much larger than that of the first for all concentrations and increased with Mn concentration, while the intensity of the first sextet remained the same and was thus discarded from our calculations. On the other hand, the data showed that the intensity of a broad line that was seen in all spectra and was attributed to Mn-rich clusters 21), (22 increased at the same rate as the spectrum of Mn4+ ions. Even if a significant fraction of the doping Mn is incorporated to Mn-rich clusters, this does not invalidate our analysis, as long as the intensity of this line increases at the same rate as the intensity of the Mn4+ spectrum with Mn concentration, since this lack of correspondence between the nominal doping and the effective concentration of Mn in Mn4+ sites leads only to a small decrease in the value of c in Eq. A, but does not affect the concentration dependence of the linewidth of the Mn4+ ions. The linewidths of the hyperfine lines of the sextet with g=1.99 appear in Table I for several manganese concentrations. We concentrated our attention on the line pointed out by an arrow in Fig. 3 because it is the one with less superposition with the lines of the other sextet. Figure 2: EPR spectrum of an SrTiO3 sample doped with 0.8 mol% Mn. Figura 2: Espectro de RSE de uma amostra de SrTiO3 dopada com 0,8 mol% Mn. Table I Experimental results for the Mn4+ peak-to-peak linewidth in SrTiO3 (T=300 K, ν=9.50 GHz). Tabela I Resultados experimentais para a largura de linha pico a pico de Mn4+ em SrTiO3 (T=300 K, =9,50 GHz). f (mol%) 0.10 0.20 0.40 0.60 0.80 1.00 1.50 2.00 2.50 3.00 ΔHpp (mT) 0.63 0.84 1.10 1.20 1.30 1.40 1.35 1.24 1.24 1.06 Figure 3: Concentration dependence of the peak-to-peak linewidth, ΔHpp, in Mn-doped SrTiO3. The circles are experimental points; the curves represent the results of theoretical calculations for 8 different ranges of the exchange interaction. Figura 3: Variação da largura de linha pico a pico, ΔHpp, com a concentração em amostras de SrTiO3 dopadas com Mn. Os círculos são pontos experimentais; as curvas mostram o resultado de cálculos teóricos para 8 alcances diferentes da interação de câmbio. DISCUSSION In the discussion that follows, the contribution of Mn2+ in Sr2+ sites is ignored, since, according to intensity data, the fraction of ions in these sites was negligibly small. The theoretical functions for ∆Hpp, given by Eq. A, are shown in Fig. 4 for ∆H0=0.45 mT and 8 different values of rc, computed from the lattice constant of SrTiO317, ao=0.3901 nm. The values of z(rc), given in Table II for n= 1 to 8, are those consistent with the crystal lattice of SrTiO3. The experimental results, also displayed in Fig. 2, followed closely the theoretical function for n=7, given by Eq. C, which, as can be seen in Table II, corresponded to rc=0.96 nm. Figure 4: Concentration dependence of the peak-to-peak linewidth, ΔHpp, in Gd-doped and Mn-doped SrTiO3. The circles are experimental points; the curves are theoretical: ΔHpp=0.60+600.f.(1-f)80 for Gd-doped SrTiO3 [20], and ΔHpp=0.45+210.f.(1-f)80 for Mn-doped SrTiO3 (this study). Figura 4: Variação da largura de linha pico a pico, ΔHpp, com a concentração em SrTiO3 dopado com Gd e Mn. Os círculos são pontos experimentais; as curvas são teóricas: ΔHpp=0,60+600.f.(1-f)80 para SrTiO3 dopado com Gd [18] e ΔHpp=0,45+210.f.(1-f)80 para SrTiO3 dopado com Mn (este estudo). Table II Values of rc and z(rc) for SrTiO3. Tabela II Valores de rc e z(rc) para SrTiO3. n 1 2 3 4 5 6 7 8 rc (nm) 0.00 0.39 0.55 0.68 0.78 0.87 0.96 1.11 z(rc) 0 6 18 26 32 56 80 92 ∆ H pp = 0 . 45 + 210 . f . ( 1 - f ) 80 (C) The peak-to-peak linewidth of Mn2+ in SrTiO3 is compared in Fig. 4 with that of Gd3+ in the same host lattice, which can be approximated by the equation ∆Hpp=0.60+600.f.(1-f)80 (mT) 18. Although rc is the same for both ions, the value of c in Eq. A is much larger for Gd3+, i.e., the linewidth rises much faster with the Gd3+ concentration than with the Mn4+ concentration. A probable explanation is that, according to Table III, the difference in ionic radii is much higher between Sr2+ and Gd3+ than between Ti4+ and Mn4+, since, as discussed in 24, the coefficient c in Eq. A increases with the difference Δr between the ions of the dopant and the replaced element. Moreover, as previously discussed, the presence of a broad line showed that not all Mn ions were incorporated as independent Mn4+ ions in Ti4+ sites and this led to a decrease in the value of c, although this decrease seems to be not sufficient to explain the large difference between the c coefficients in Gd3+ and Mn4+ doped SrTiO3. Table III Ionic radii 23 and their differences. Tabela III Raios iônicos 23 e suas diferenças. Ion Coordination Radius (nm) Difference (nm) Sr2+ 12 0.144 0.044 Gd3+ 12 0.100 Ti4+ 6 0.061 0.008 Mn4+ 6 0.053 CONCLUSIONS The EPR spectra of Mn4+ and Mn2+ were observed in SrTiO3 powders doped with different concentrations of manganese. 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