Spectroscopic Study of the Reversible Chemical Reduction and

Article pubs.acs.org/IC

Spectroscopic Study of the Reversible Chemical Reduction and Reoxidation of Substitutional Cr Ions in Sr2TiO4 Keith A. Lehuta, Anubhab Haldar, Dongming Zhou, and Kevin R. Kittilstved* Department of Chemistry, University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: The solid-state synthesis and controllable speciation of Cr dopants in the layered perovskite Sr2TiO4 is reported. We employed a chemical reduction procedure with NaBH4 at relatively mild temperatures ( 375 °C, the lattice is reduced, and localized Ti3+ defects are detected by both diffuse-reflectance and EPR spectroscopy. These results are qualitatively similar to our previous results20 of Cr in SrTiO3 under similar chemical reduction conditions. The observed changes are also quantitatively reversible upon aerobic annealing when Treox ≈ T; however, for Cr:SrTiO3 the Treox > Tred in order to revert the material back to a similar Cr3+ concentration as the as-prepared sample. These results are consistent with enhanced charge transport in the 2D layered perovskite (Sr2TiO4) compared to a three-dimensional (3D) ordered perovskite (SrTiO3).

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described above were mixed in a 1:1 mol ratio with NaBH4 by mortar and pestle. The mixture was placed in a porcelain crucible that was inserted into a 1-in. diameter evacuable quartz insert in the tube furnace. The samples were heated under a static vacuum (10−4 mbar pressure) at different temperatures up to 425 °C for 30 min. After being cooled under a vacuum, the samples were alternately washed with deionized water and ethanol and then dried at 100 °C for 2 h. Physical Characterization. Powder X-ray diffraction patterns were collected using a PANalytical X’Pert Material Research diffractometer, and phase purities were estimated from manual fitting of well-separated reflections for the SrO, TiO2, and Sr−Ti−O phases that are possibly present in the samples. Diffuse reflectance spectra were collected using an integrating sphere (Ocean Optics ISP-REF) coupled by fiber optic cables to a CCD-based spectrophotometer (Ocean Optics USB2000+ UV−vis). Signal responses were transformed to absorption using the Kubelka−Munk relation.25 Semiquantitative comparisons of the diffuse reflectance spectra were achieved by diluting the samples with enough MgO so that the Kubelka−Munk response at 310 nm was equivalent. Room temperature X-band EPR spectra were measured on powders in the perpendicular mode of a dual resonator cavity (Bruker Elexsys E-500 with ER-4116 cavity). EPR spectra were analyzed using the simplified S = 3/2 spin Hamiltonian that describes the 4A2 ground term of pseudo-octahedral Cr3+ given by eq 1. 2 Ĥ = D[Sẑ − 1/3(S(S + 1))] + A(I · S) + gμB B

(1)

where D is the axial component to the zero-field splitting parameter, A is the hyperfine splitting coefficient, I is the nuclear spin (9.5% relative abundance 53Cr, I = 3/2), g is the Lande g-factor, μB is the bohr magneton, and B is the external magnetic field strength. The rhombic component to the ZFS is not observed and therefore omitted from eq 1. The tetragonal compression of the B-site of Sr2TiO4 combined with out-of-state spin−orbit coupling results in a nonzero D parameter and g-anisotropy (g⊥ ≠ g∥). We estimated the sign of D by ligand field theoretical calculation of the EPR parameters within the angular overlap model using LIGFIELD and the magnetic moment operator, μ̂ = (L̂ + 2Ŝ) μBH.26 The magnetic field dependence of the lowest spin− orbit states of the 4A2 ground state was estimated from eq 1 using a custom computer program written by Tregenna-Piggott.27−29 The ligand field parameters used for the calculations were Dq = 16350 cm−1 (eσ = 10Dq/3 = 5450 cm−1), B = 650 cm−1, and C = 4.3B = 2795 cm−1, and ζ = 275 cm−1 (the free ion spin−orbit coupling constant). These values were estimated numerically using the Tanabe-Sugano energy matrices30 and the experimental energies of the 4A2 → 4T2, 4 T1(F), and 2E(G) transitions for Cr3+ in SrTiO3 colloidal nanocrystals where all three transitions have recently been observed at 16350 cm−1, 23200 cm−1, and 12600 cm−1, respectively.31

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EXPERIMENTAL METHODS

RESULTS AND DISCUSSION Phase Purity and Characterization of Cr-Doped Sr2TiO4 As-Prepared Powders. The powder XRD pattern of 0.1% Cr:Sr2TiO4 after 6 h via the solid state method at 1100 °C is shown in Figure 1, and the many of the diffraction peaks match well to the known reflections of Sr2TiO4;32 however, there are many minor peaks between 34 and 43° that match with SrO, SrTiO3, and TiO2 (rutile, TiO2-R) impurities in the sample.33−35 After additional regrinding of the powder and heating at 1100 °C for 15 h both SrTiO3 and TiO2-R phases have disappeared (within the limit of detection in the diffractometer), but the relative amount of SrO increased. Phase refinement of the 21 h annealed sample reveals that the product is indeed composed of primarily Sr2TiO4 (92.8 ± 3.8%) with minority phases of SrO (5.7 ± 0.3%) and SrTiO3 (1.4 ± 0.7%). For comparison, the initial 6 h intermediate was only ∼86% Sr2TiO4 with many other phases and is shown in the Supporting Information. Previous work on the sol−gel

Materials. TiO2 (99.9%, nanopowder, anatase, 32 nm APS powder, Alfa Aesar), SrO (Sigma-Aldrich, 99.9%, trace metals basis), Cr(NO3)3·9H2O (crystalline, certified, Fisher Scientific), NaBH4 (≥98%, white powder, MP Biomedical), MgO (Fisher Science Education), and ethanol (190 proof, ACS/USP grade, PharmcoAaper) were used without further purification. Synthesis of Cr Doped Sr2TiO4. Sr2Ti1−xCrxO4−δ powders were made using the solid state method, where x is the nominal concentration of Cr (x = 0.001 to 0.1) and δ is the concentration of oxygen vacancies (VO). In a typical synthesis, SrO, TiO2, and Cr(NO3)3·9H2O were mixed in the desired stoichiometry and ground using a mortar and pestle until uniform consistency was achieved (∼5 min). The mixture was transferred to a crucible and heated for 6 h at 1100 °C in a tube furnace (Mini-Mite Lindberg BlueM). The crucible was removed from the tube furnace, and the product was ground thoroughly again for ∼5 min after cooling and then annealed again for an additional 15 h at 1100 °C. NaBH4 Reductions. Chemical reductions of Cr:Sr2TiO4 were performed using the method previously described for Cr:SrTiO3 powders.20 Briefly, Cr:Sr2TiO4 powders after annealing for 21 h as 9178

DOI: 10.1021/acs.inorgchem.7b01210 Inorg. Chem. 2017, 56, 9177−9184

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theoretical calculations.38 Previous studies on the electronic structure of Sr2TiO4 confirm a larger band gap energy compared to SrTiO3 primarily from an increase in the conduction band energy.39 The transition in the near-IR in Cr:SrTiO4 in Figure 2a is consistent with the origin of this band as a MLCT-type transition. The room-temperature EPR spectra of 0.1% Cr:Sr2TiO4 after the two annealing stages is shown in Figure 2b. The EPR spectrum of the sample after 6 h of heating at 1100 °C resembles the previously reported EPR spectra11 of the phase impure Cr:Sr2TiO4 powder with higher nominal Cr level of 1%. The increase in the phase purity after the additional heating step to 21 h results in significant, but expected changes in the EPR spectrum. First, all the features in Figure 2 centered at ∼347 mT are assigned to EPR-allowed transitions between the |+1/2⟩ and |−1/2⟩ components of the 4A2 ground state. The asymmetry of this EPR signal is primarily the result of a axial component to the zero-field splitting (D) caused by an axial compression of the B-site.32 In addition, the intensity and line width of the central transition decreases and narrows with increasing Sr2TiO4 phase purity, which is expected from the simulated spectrum. However, the Cr3+ content was too low to observe the |±3/2⟩ ↔ |±1/2⟩ transitions at room temperature as previously reported.11 A fraction of the signal intensity in the 21 h spectrum at ∼347 mT is likely from Cr3+ in a minority phase of SrTiO3 that displays a single line with an isotropic gfactor of giso = 1.978. Contrary to some reports on Cr:SrTiO3, the absence of an EPR signal at g ≈ 1.95 at room temperature suggests that the amount of Cr5+ (d1, S = 1/2) dopant ions is negligible in the as-prepared product.40 Characterization of Samples after Low-Temperature NaBH4 Reduction. The diffuse reflectance spectra of 1% Cr:Sr2TiO4 at reduction temperatures below Tred < 350 °C display minimal changes from the as-prepared sample (spectra not shown). However, at Tred ≥ 350 °C drastic changes are observed up to Tred = 400 °C as shown in Figure 2b. At Tred = 350 °C, the diffuse reflectance spectrum displays a broad, featureless transition that starts at ∼750 nm and increases in intensity slowly to the band edge transition. Upon increasing to Tred = 375 °C, the absorption changes dramatically and a new transition centered in the near-UV (∼400−450 nm) dominates the visible absorption. On an expanded y-scale, there is a small increase in the near-IR region. The spectra of the sample after Tred = 400 °C is dominated by a new feature in the near-IR that is assigned to a metal-to-metal charge transfer (MMCT) transition from reduced lattice Ti3+ to the empty Ti4+ conduction band.20,23 The near-edge absorption band is also visible in the highest reduction temperature, but is less pronounced due to the large visible light absorption. The changes in the diffuse reflectance spectra under different reduction temperatures of the 1% Cr:Sr2TiO4 are very similar to those of reduced Cr:SrTiO3, with decreased absorption in the visible region prior to the formation of large Ti3+-related MMCT absorption at longer wavelengths. Because of the reduction of the lower energy absorption peak initially and the similarity to Cr:SrTiO3, this broadened absorption in the asprepared sample can be attributed to a charge transfer transition between the valence band and a neutrally charged VO defect.20 One mechanism for these changes in the charge state of the VO defect is based on our previous work on reduced Cr:SrTiO3 where the Cr3+ concentration is observed to increase with increasing Tred by EPR spectroscopy.20 The increasing Cr3+ concentration in SrTiO3 or Sr2TiO4 requires additional

Figure 1. Powder X-ray diffraction patterns of the Sr2TiO4 reaction intermediates and products after the initial aerobic heating at 1100 °C for 6 h (black) and after an additional 21 h (red). The reference diffraction patterns of SrO (S), TiO2-R (R), SrTiO3 (∞), Sr2TiO4 (1), and Sr3Ti2O7 (2) are from the literature.15,32,34,36,37

synthesis of Cr:SrTiO3 powders revealed presence of Cr3+ in Sr2TiO4 as an intermediate phase.20 Attempts to synthesize phase pure Cr:Sr2TiO4 using a modified sol−gel method with higher Sr precursor concentrations yielded a maximum purity of only ∼68% Sr2TiO4 (data not shown). The as-prepared Sr2TiO4 powders containing 1% Cr appear purple under ambient lighting compared to the reddish-brown appearance of 1% Cr:SrTiO3. The corresponding diffuse reflectance spectra of the as-prepared sample shown in Figure 2 displays a broad absorption band with a slightly higher transition threshold than Cr:SrTiO3.20 The transitions in the spectrum of Cr:SrTiO3 is assigned to combination of a chargetransfer transitions from Cr3+ to the conduction band (a metalto-ligand charge transfer, MLCT-type excitation) and valence band to oxygen vacancy defect states (VO), and is supported by

Figure 2. (a) Normalized diffuse reflectance spectra of 1% Cr:Sr2TiO4 and 1% Cr:SrTiO3 as-prepared powders. (b) Room temperature EPR spectra of the central |−1/2⟩ ↔ |+1/2⟩ transitions of 0.1% Cr:Sr2TiO4 powder after annealing at 1100 °C for 6 h (gray) and 21 h (red). The simulated spectra for Cr3+ in Sr2TiO4 and SrTiO3 (shown in black) were calculated with the refined spin-Hamiltonian parameters except for the weak hyperfine splitting from 53Cr (see text). 9179

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interactions, especially after reductions ≥375 °C (see Supporting Information). Chemical reductions at temperatures greater than 375 °C could not be compared quantitatively to lower reduction temperatures due to the increased absorption of the microwaves by the sample. Reduction of the sample at 400 °C results in a significantly broadened EPR spectrum (see Supporting Information) and the appearance of a broad absorption in the near-IR region by diffuse-reflectance. Similar spectral signatures were observed previously in reduced Cr:SrTiO3 and are consistent with the creation of Ti3+ lattice defects and increased Cr3+ concentration.20 Structural investigation of Tred = 375 °C samples by powder diffraction also display little change in the relative purities of the Sr2TiO4 and SrTiO3. There is a noticeable decrease in the amount of SrO impurity phase (see Supporting Information). The impact of the chemical reduction and reoxidation on the structure of pure and doped Sr2TiO4 and SrTiO3, and in particular the creation and removal of oxygen vacancies, are currently under investigation. Characterization of Reduced Cr:Sr2TiO4 upon Reoxidation at Various Temperatures. Reoxidation of reduced Cr:SrTiO3 powders reported recently requires higher oxidation temperatures to revert the EPR intensities back to their asprepared values. In contrast, the Cr3+ EPR signal after reduction at 375 °C reverts quantitatively back to the as-prepared values after reoxidation of the sample in air at 375 °C (see Figure 4). This result suggests very similar activation energies for the reoxidation process of Sr2TiO4 compared to SrTiO3.

charge compensation to maintain neutrality. The decrease in the intensity of the diffuse-reflectance between 450 and 600 nm in the samples reduced between 325 and 375 °C also displays two features: (1) an increase of the sub-bandgap absorption centered at ∼420 nm and (2) an increase in Cr3+ content by EPR spectroscopy. Finally, at Tred ≥ 400 °C there are two observable changes to the diffuse reflectance: (1) an intense near-IR transition appears that is assigned to trapped electrons on Ti4+ sites thus creating Ti3+ centers, and (2) transitions to both singly and doubly ionized VO decrease in intensity in the near-UV sub-bandgap region. Figure 3b shows the quantitative EPR spectra of 0.1% Cr:Sr2TiO4 before (“ap”) and after NaBH4 reduction at

Figure 3. (a) Diffuse reflectance spectra of 1% Cr:Sr2TiO4 before and after NaBH4 reductions at temperatures between 350 and 400 °C. (b) Quantitative EPR spectra of 0.1% Cr:Sr2TiO4 before (as-prepared, ap; black) and after NaBH4 reduction at 300 °C (blue), 325 °C (green), and 375 °C (red) for 30 min under static vacuum at 4 × 10−4 mbar. The right and left panels show the experimental (spectra) and simulated (angular-dependent resonance curves) of the |±3/2⟩ ↔ |±1/2⟩ transitions, respectively, that have been scaled by 50 for clarity and placed on larger field ranges than the narrow |−1/2⟩ ↔ |+1/2⟩ transition in the central panel. Figure 4. Quantitative EPR intensities (filled circles) of the axial Cr3+ signal for a 0.1% Cr:Sr2TiO4 sample reduced at 375 °C as a function of reoxidation temperature (T′reox). The reoxidation step consisted of heating the sample in air for 30 min at the given T′reox. Sample intensities are defined as the difference between the peak maximum and minimum (Δχpp″) as shown in the inset of the figure. The raw data have been normalized to the relative to the Δχpp″ value of the asprepared sample (relative intensity ≡ 1; green-dashed horizontal line) before Tred = 375 °C (relative intensity = 11.9; red-dashed horizontal line).

temperatures of Tred = 300 and 375 °C. As shown in Figures 3a and S2, there is a negligible concentration of Ti3+ defects at Tred ≤ 375 °C. At Tred ≥ 400 °C, the Cr3+ EPR signal broadens significantly indicating the likely presence of Ti3+ defects based on similar observations20 for chemically reduced Cr:SrTiO3. The central axial Cr3+ EPR signal increases dramatically with increasing reduction temperature with minimal line broadening. After the 375 °C reduction the Cr3+ EPR signal increases by a factor of 11.9. This intensity gain also revealed the weaker transitions between the |± 3/2⟩ to | ± 1/2⟩ spin−orbit states. The observed increase in the Cr3+ signal intensity suggests that higher oxidation states of Cr, such as the EPR silent Cr4+ or Cr6+ ions, are present in the as-prepared sample and subsequently reduced through this low-temperature chemical reduction by NaBH4. The spectra also display minimal broadening or change in line shape after the reduction that we attribute to the low doping level (0.1% nominal) that reduces dipole−dipole interactions by Cr3+ dimers and the highly anisotropic nature of the dipole−dipole interactions in Sr2TiO4. The line width broadens significantly in 1% nominally doped samples as the result of increased dipole−dipole

The increase in the Cr3+ EPR signal intensity observed here for Sr2TiO4 low-temperature chemical reduction is similar to the increase in Cr3+ signal in 0.1% Cr:SrTiO3 after the identical reduction treatment.20 The increase in the Cr3+ signal was also a factor of ∼12 increase in SrTiO3 after reduction at 350 °C. For Sr2TiO4 the increase in Cr3+ signal is similar, but requires a slightly higher reduction temperature, Tred = 375 °C. However, upon reoxidation, there is a clear difference between the two lattices with regards to the T′reox values required to fully reoxidize the sample to the as-prepared intensity. For Sr2TiO4, this reoxidation temperature is equal to Tred, whereas for 9180

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change in D that is governed primarily by the magnitude of the tetragonal distortion of the B-site as shown by the angular overlap calculations. In a tetragonal field, the perturbation expression for D depends on the spin−orbit coupling parameter (λ) and the energies of the 4B2 and 4E states from the split 4T2 excited state according to eq 2,

SrTiO3 the temperature required to fully oxidize the sample was typically much higher than Tred and also dependent on the Tred value.20 For instance, we reported spectroscopic evidence that the temperature required to fully revert the EPR and diffuse-reflectance spectra of reduced Cr:SrTiO3 at Tred = 350 °C back to the as-prepared spectra was T′reox ≥ 400 °C. Furthermore, Cr:SrTiO3 samples reduced at high enough temperatures to produce Ti3+ required T′reox ≈ Tred + 50 °C to reoxidize just the Cr3+ dopants, but typically significantly greater temperatures such as Tred + 150 °C to fully reoxidize the lattice Ti3+ defects back to Cr:SrTiO3 as-prepared values. We recently attributed this modulation of the Cr 3+ concentration in Cr:SrTiO3 via chemical reduction and reoxidation to (1) the facile reduction of defects easily reduced Cr4+ or Cr6+ dopants by decomposed NaBH4 that is limited by diffusion and occurs initially at the crystal surface, and (2) a larger activation barrier associated with oxidation. One plausible hypothesis to explain the reduction/reoxidation behavior of Cr in Sr2TiO4 is that the Cr oxidation state is controlled by defects that diffuse faster or have lower energy barriers to formation in the layered Sr2TiO4 lattice compared to SrTiO3. This scenario would therefore increase the probability of those defects encountering and reducing a high-valent Cr dopant or oxidizing Cr3+ dopants. Studies to address the origin of the defect(s) responsible for this behavior are currently underway. Temperature-Dependence of the Axial Component to the Zero-Field-Splitting of Cr3+ in Sr2TiO4. The increased EPR signal after chemical reduction allowed for a more thorough investigation of the lattice dynamics as a function of temperature as shown in Figure 5. EPR spectra of the |+1/2⟩ ↔

⎡ 1 1 ⎤ D = 8λ 2⎢ − ⎥ Δ2 ⎦ ⎣ Δ1 ⎡ ⎤ 1 1 = 8λ 2⎢ − ⎥ 10Dq + δ /3 ⎦ ⎣ 10Dq − 2δ /3

(2)

where Δ1 and Δ2 are the energies of the B2 and E excited states the 4T2 term (Oh symmetry), respectively.28 The relation can be rewritten to depend on the ligand field splitting parameter 10Dq and the energy splitting of the 4T2 state by the tetragonal distortion, δ = E(4E − 4B2). The dependence of D on these two variables (10Dq and δ) with ζ = 3λ = 275 cm−1 is shown in Figure 6 as a contour plot. 4

4

Figure 6. (a) Abbreviated energy level diagram of the lowest energy spin-allowed ligand field transition of Cr3+ in Oh and D4h crystal fields with definitions for 10Dq and δ. (b) Contour plot of the D parameter as a function of 10Dq and δ with fixed at the free ion spin−orbit coupling parameter (ζ = 3λ = 275 cm−1). Blue shading denotes a negative D and occurs whenever δ > 0. Red shading denotes the exact opposite trend in D. The white line define the isoenergetic D values from +0.3 cm−1 to −0.3 cm−1. Figure 5. Variable-temperature (120−295 K) EPR spectra of 0.1% Cr:SrTiO4 in the region of the central |+1/2⟩ ↔ |−1/2⟩ lines. The sample was chemically reduced at Tred = 375 °C and collected at Xband (hν = 9.62 GHz). Panels (a) and (b) are the same spectra but either as a stack plot (120−295 K) or contour plot (120−210 K), respectively. The inset to panel (a) shows the peak-to-peak separation (ΔBpp, mT) as a function of temperature. The dashed line in the inset is a linear fit to the ΔBpp vs temperature data.

The vanishing of D suggests that the B-site becomes more isotropic with decreasing temperature. Recent neutron diffraction studies41 show that the thermal expansion for the axial and equatorial Ti−O bonds are equivalent in Sr2TiO4. In other words, the ratio of the bond lengths is constant between 298 and 1273 K (Ti−Oax/Ti−Oeq = 0.9856(2) Å). Therefore, when the temperature decreases, the shortening of the Ti−O bond lengths will increase in 10Dq. Therefore, if the bond length effects 10Dq to a greater extent than λ or δ, then even in a lattice undergoing isotropic thermal contraction should result in an increase in the D parameter based on the increasing magnitude of 10Dq according to the above perturbation expression. Spin-Hamiltonian Parameters for Cr3+ in Sr2TiO4 Revisited and Comparison to Other Titanium Oxide Lattices. The increase in the substitutional Cr3+ EPR signal in the lattice after NaBH4 reduction of 0.1% Cr:Sr2TiO4 at 375 °C

|−1/2⟩ transition of Cr3+ collected at various temperatures shows a clear reduction of the peak-to-peak separations (ΔBpp) with decreasing temperature (full spectra shown in Supporting Information). The temperature dependence of ΔBpp is linear over the range from 120 to 295 K with r-squared = 0.997. The EPR-allowed transitions between the |±3/2⟩ ↔ |±1/2⟩ spinors also display a linear temperature dependence albeit with a higher slope (see Supporting Information). The origin of the temperature dependence of ΔBpp suggests an anisotropic 9181

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Table 1. Room Temperature Bond Lengths (R, Å), Bond Angles (∠, °) of the Ti(IV)-Site of Various Titanium Oxides Taken from Crystallographic Data,a and Zero-Field Splitting Parameters D and E (10−4 cm−1) for Substitutional Cr3+ Dopants Determined from Experimental EPR Spectra lattice A-TiO2 R-TiO2 SrTiO3 Sr2TiO4

B-site symm b

D2d D2hc Oh D4h

R(Ti−O∥)

R(Ti−O⊥)

∠(O∥−Ti−O⊥A,⊥B)

∠(O⊥A−Ti−O⊥B,⊥B′)

D

E

1.874 1.981 1.9526 1.981

1.923 1.947 1.9526 1.943

75.9, 104.1 90, 90 90, 90 90, 90

93.4, 151.8 81.17, 180 90, 180 90, 180

374 −6858 0 −201

n/a −1352 0 0

refs 35, 36, 42, 41,

43 44 45 d

a The oxide ions are defined as O∥ which defines the z axis of the point symmetry, and the four equatorial oxide ions as O⊥A and O⊥B. The prime modifiers on some of the equatorial ligands, O⊥A′ and O⊥B′, are used to designate the ligands that are related by a C2 rotation about the z axis. The A and B subscripts are arbitrary. bThe equatorial oxides are distorted out of the equatorial plane in anatase TiO2. cThe equatorial oxides are distorted in the equatorial plane in rutile TiO2. dThis work.

enabled by the 2D nature of this Ruddlesden−Popper phase and therefore potentially optimized in this “mono-layered” perovskite in contrast to SrTiO3. Spin-Hamiltonian parameters for Cr3+ in Sr2TiO4 were determined by simulation of the experimental EPR spectrum of a 0.1% Cr-doped sample after NaBH4 reduction at 375 °C for 30 min. The resulting best-fit parameters at room temperature were obtained with D = −201 × 10−4 cm−1, g⊥ = 1.980, g∥ = 1.978, and A = −16.2 × 10−4 cm−1. The D-parameter was revealed to decrease linearly with decreasing temperature as expected from perturbation expressions for the ligand field splittings of the lowest quartet states.

allowed us to refine the spin Hamiltonian parameters that were previously estimated for Cr 3+ -doped “impure” Sr 2 TiO 4 prepared by a sol−gel method.11 The experimental and simulated spectra shown in Figure 2 are in good agreement and confirm that Cr3+ ion has a slight g-anisotropy (g⊥ = 1.980 and g∥ = 1.978) and axial component to the zero-field splitting (D = −201 × 10−4 cm−1). The g-anisotropy and nonzero D are consistent with the Cr3+ being in the axially compressed B-site of Sr2TiO4 (for structural parameters see Table 1).41 Additional sets of peaks associated with the hyperfine coupling due to 53Cr ions (I = 3/2, 9.5% relative abundance) are also visible in the spectrum of 0.1% Cr:Sr2TiO4 after reduction at 375 °C and agree with the hyperfine splitting for Cr3+ in SrTiO3 of A = −16.2 × 10−4 cm−1 (see Figure S4).42 These new values are similar to our previously reported values, however, we have assigned D as a negative parameter based on ligand field calculations within the angular overlap model with the axially compressed D4h point symmetry similar to the B-site in Sr2TiO4 (see Supporting Information).27,26 The magnitude of D for Cr3+ in Sr2TiO4 is almost half of the value observed for Cr3+ in TiO2 (anatase),35 which also displays an axial compression at the Ti site, but has overall D2d point symmetry due to an asymmetric out-of-plane distortion in the equatorial plane that removes the C2′ symmetry operation (|D| = 374 × 10−4 cm−1).43 Contrary to the other phases discussed, the rutile phase36 of TiO2 has an axially elongated Ti site with D2h symmetry caused by in-plane distortion of the equatorial oxide bond angles from 90°. This results in both significant axial and rhombic contribution to the zero-field splitting, D = −6858 × 10−4 cm−1 and E = −1352 × 10−4 cm−1, respectively.44

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01210. Additional powder X-ray diffraction patterns, diffuse reflectance and EPR spectra of different nominal doping content (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anubhab Haldar: 0000-0002-2308-7415 Kevin R. Kittilstved: 0000-0002-9852-7454 Funding

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This work was supported by startup funds provided by the University of Massachusetts Amherst.

CONCLUSIONS In this work, we presented a spectroscopic investigation of Cr:Sr2TiO4 after solid state chemical reduction and reoxidation. We monitored the electronic structure of both the dopant ion and host lattice and demonstrated the ability to control both the charge state of lattice VO and relative amount of Cr3+ without the introduction of Ti3+ defects as shown by the combination of diffuse-reflectance and EPR spectroscopies. These changes are similar to that recently observed in Cr:SrTiO3; however, the formation of Ti3+ defects requires ∼25 °C higher reduction temperature in Sr2TiO4 due to the higher energy associated with the primarily Ti 3d composed conduction band in the axially compressed material. Reoxidation of the reduced material to nearly equivalent Cr3+ content in Sr2TiO4 was achieved at roughly the same temperature for reduction, T′reox ≈ Tred. The relatively equal energy barrier for reduction or reoxidation of Cr3+ in Sr2TiO4 is

Notes

The authors declare no competing financial interest.

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REFERENCES

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01210 Inorg. Chem. 2017, 56, 9177−9184

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DOI: 10.1021/acs.inorgchem.7b01210 Inorg. Chem. 2017, 56, 9177−9184