Electron paramagnetic resonance of manganese-doped strontium titanate

Ressonância paramagnética eletrônica de titanato de estrôncio dopado com manganês

Abstract

Electron paramagnetic resonance (EPR) spectra of manganese-doped strontium titanate were investigated for several Mn concentrations. The spectra of Mn²⁺ and Mn⁴⁺ ions were observed and attributed, respectively, to Mn ions occupying Sr²⁺ and Ti⁴⁺ sites. The relative intensity of the spectra suggested that the manganese ions occupy preferentially Ti⁴⁺ sites. The results showed that the EPR peak-to-peak linewidth of the Mn⁴⁺ spectrum increases with manganese concentration according to the theoretical equation DH_pp=0.45+210.f.(1-f)⁸⁰ (mT). This suggested that the exchange interaction between tetravalent manganese ions in strontium titanate has an approximate range of 0.96 nm, comparable to that of Gd³⁺ in the same compound.

Keywords:
ceramics; electron paramagnetic resonance; strontium titanate; manganese

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 ΔH_pp=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.

Palavras-chave:
cerâmicas; ressonância paramagnética eletrônica; titanato de estrôncio; manganês

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:

${\mathrm{\Delta H}}_{\mathrm{pp}}={\mathrm{\Delta H}}_{\mathrm{o}}+{\mathrm{\Delta H}}_{\mathrm{d}}={\mathrm{\Delta H}}_{\mathrm{o}}+\mathrm{c}.{\mathrm{f}}_{\mathrm{e}}$ (A)

where ∆H_o is the intrinsic linewidth, ∆H_d is the dipolar broadening, c is a constant, and f_e is the concentration of substitutional ions of the paramagnetic impurity not coupled by the exchange interaction, which can be expressed as:

${\mathrm{f}}_{\mathrm{e}}=\mathrm{f}.{(1-\mathrm{f})}^{\mathrm{z}\left({\mathrm{r}}_{\mathrm{c}}\right)}$ (B)

where f is the impurity concentration, z(r_c) the number of cation sites included in a sphere of radius r_c, and r_c 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, ΔH_pp, 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, ΔH_pp, 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 ∆H_pp, given by Eq. A, are shown in Fig. 4 for ∆H₀=0.45 mT and 8 different values of r_c, computed from the lattice constant of SrTiO₃¹⁷, a_o=0.3901 nm. The values of z(r_c), 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 r_c=0.96 nm.

Figure 4:
Concentration dependence of the peak-to-peak linewidth, ΔH_pp, in Gd-doped and Mn-doped SrTiO₃. The circles are experimental points; the curves are theoretical: ΔH_pp=0.60+600.f.(1-f)⁸⁰ for Gd-doped SrTiO₃ [20], and ΔH_pp=0.45+210.f.(1-f)⁸⁰ for Mn-doped SrTiO₃ (this study).
Figura 4:
Variação da largura de linha pico a pico, ΔH_pp, com a concentração em SrTiO₃ dopado com Gd e Mn. Os círculos são pontos experimentais; as curvas são teóricas: ΔH_pp=0,60+600.f.(1-f)⁸⁰ para SrTiO₃ dopado com Gd [18] e ΔH_pp=0,45+210.f.(1-f)⁸⁰ para SrTiO₃ dopado com Mn (este estudo).

Table II
Values of r_c and z(r_c) for SrTiO₃.
Tabela II
Valores de r_c e z(r_c) 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

$∆{\mathrm{H}}_{\mathrm{pp}}=0.45+210.\mathrm{f}.{(1-\mathrm{f})}^{80}$ (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 ∆H_pp=0.60+600.f.(1-f)⁸⁰ (mT) ¹⁸. Although r_c 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. The measurements showed that Mn ions occupy preferentially Ti⁴⁺ sites. It was also found that the EPR peak-to-peak linewidth of Mn⁴⁺ increased predictably with Mn concentration and that the range of the exchange interaction between Mn⁴⁺ ions was about 0.96 nm. These results may contribute to the study of the magnetic properties of manganese-doped strontium titanate.

ACKNOWLEDGMENTS

The authors thank CNPq and CAPES for financial support.

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Publication Dates

• Publication in this collection
  08 May 2020
• Date of issue
  Apr-Jun 2020

History

• Received
  04 Apr 2019
• Reviewed
  04 June 2019
• Reviewed
  01 July 2019
• Reviewed
  12 Nov 2019
• Accepted
  15 Nov 2019