Reversible Control of the Mn Oxidation State in SrTiO₃ Bulk Powders

Abstract

We demonstrate a low-temperature reduction method for exhibiting fine control over the oxidation state of substitutional Mn ions in strontium titanate (SrTiO₃) bulk powder. We employ NaBH₄ as the chemical reductant that causes significant changes in the oxidation state and oxygen vacancy complexation with Mn²⁺ dopants at temperatures <350°C where lattice reduction is negligible. At higher reduction temperatures, we also observe the formation of Ti³⁺ in the lattice by diffuse-reflectance and low-temperature electron paramagnetic resonance (EPR) spectroscopy. In addition to Mn²⁺, Mn⁴⁺, and the Mn²⁺ complex with an oxygen vacancy, we also observe a sharp resonance in the EPR spectrum of heavily reduced Mn-doped SrTiO₃. This sharp signal is tentatively assigned to surface superoxide ion that is formed by the surface electron transfer reaction between Ti³⁺ and O₂. The ability to control the relative amounts of various paramagnetic defects in SrTiO₃ provides many possibilities to study in a model system the impact of tunable dopant-defect interactions for spin-based electronic applications or visible-light photocatalysis.

Introduction

The oxide SrTiO₃ is a classic perovskite-type member of the valuable ABO₃ semiconductor family. The promising properties such as a large tunable dielectric constant, structural phase transitions, superior charge storage capacity and tunable electronic structure have made SrTiO₃ an exciting candidate for a wide range of multifunctional applications (Weaver, 1959; Faughnan, 1971; Mattheiss, 1972; Müller and Burkard, 1979; Kamalasanan et al., 1993). Although in ambient conditions it exhibits a wide band gap and low electron mobility, introducing impurity dopants and intrinsic defects radically influence the conductivity and optical properties of the host material (Wild et al., 1973; Kozuka et al., 2010). The function of a semiconductor is intimately related to the chemistry and physics of native and targeted defects. The rich defect chemistry enabled by native oxygen vacancies (V_O) in semiconductors such as SrTiO₃, PbTiO₃, and BaTiO₃ has been correlated with numerous functions including ferroelectricity, visible-light photocatalysis, and multiferroics. These V_O defects can donate up to two electrons to the host lattice. Transition metal dopants may also impart additional functionality that result from partially-filled d-orbitals. For example, Cr³⁺ dopants, and Rh³⁺ dopants in SrTiO₃ can reduce protons to generate H₂ gas using visible light that creates an oxidized dopant ion and a conduction band electron, e_cb (Ishii et al., 2004; Sasaki et al., 2009; Kato et al., 2013). However, undesirable defects such as Cr⁶⁺ can form to maintain charge neutrality, but limit the photochemical efficiency by serving as a trap for the e_cb. These types of high-valent defects can be removed by either post-synthetic annealing under reducing atmospheres (Zuo et al., 2010; Tan et al., 2014; Lehuta and Kittilstved, 2016), co-doping the host lattice with additional n-type dopants (Chan et al., 1981; Kato and Kudo, 2002; Wang et al., 2014), irradiating with UV light (Wang et al., 2006), or applying a large electrical bias (La Mattina et al., 2008). Of these, the only potentially “green” reduction source could be UV irradiation from the sun. However, we note that the reported photoreduction step using a 400 W Hg-lamp in Cr:SrTiO₃ powders was of the order of tens of hours. The realization of a fast, low-energy method to modulate the oxidation state of transition-metal dopants in SrTiO₃ and related metal oxide semiconductors could impact various fields such as visible-light photocatalysis, sensing, and spin-based electronics. To this end, recent studies on the photodoping of colloidal Cr:SrTiO₃ nanocrystals show promise (Harrigan and Kittilstved, 2018).

We previously studied the effect of a relatively low-temperature NaBH₄ reduction reaction on the oxidation state of Cr dopants in SrTiO₃ and related Sr₂TiO₄ bulk powders (Lehuta and Kittilstved, 2016; Lehuta et al., 2017). In those studies, we observed an order of magnitude increase in the Cr³⁺ concentration by EPR spectroscopy that we attributed to the reduction of EPR-silent high-valent Cr⁴⁺ and Cr⁶⁺ ions. The increase in the Cr³⁺ concentration in n-type SrTiO₃ presents an interesting scenario where the Cr³⁺ ion has a dual role of being an electron donor and a paramagnetic ion (S = 3/2). In addition, these observed changes are quantitatively reversible upon annealing the powders in air.

An isoelectronic analog of Cr³⁺ is Mn⁴⁺, which is known to also occupy the Ti⁴⁺-site in SrTiO₃. Although additional defects are required to maintain charge neutrality in Cr³⁺:SrTiO₃, Mn⁴⁺ in the B-site of SrTiO₃ is an isovalent dopant. Amongst the transition-metal doped oxides, Mn:SrTiO₃ has recently received extensive attention due to its complex and unique behavior than intrinsic SrTiO_3. The concurrent doping of Mn and oxygen vacancies in SrTi_1−xMn_xO_3−δ is reported to promote ferromagnetic ordering, dielectric permittivity and possible metallic behavior (Savinov et al., 2008; Choudhury et al., 2011, 2013; Middey et al., 2012; Thanh et al., 2014). These observations make nonstoichiometric Mn:SrTiO₃ a highly attractive candidate for spin-based electronics applications. Although not completely understood, the results are attributed to the interplay of redox-active Mn ions and the intrinsic charge compensating defects. In this regard, quantitative research is challenging due to a lack of experimental control over the interactions, and the complexity of Mn ions present in multiple oxidation states. Herein we report on the nature of the oxidation state of Mn ions and associated defect centers in bulk Mn:SrTiO₃ powders. We utilized various dopant-specific spectroscopic probes to elucidate the Mn oxidation state including EPR and diffuse-reflectance spectroscopies. We extend the use of NaBH₄ as a solid-state reductant to monitor changes in the three, unique Mn-related species as well as oxygen-related defects and “self-doped” Ti³⁺ ions. Comparison to other studies of reduced Mn:SrTiO₃ and noticeable absences of certain EPR-active Mn-centers is also discussed. We also observe a new signal in reduced samples that we attribute to superoxide anions, .

Materials and Methods

Chemicals

TiO₂ (>99.5%, Aeroxide P25 powder, Acros Organics), Sr(NO₃)₂ (>99%, Acros Organics), Mn(NO₃)₂·4H₂O (analytical grade, Acros Organics), NaBH₄ (≥98%, white powder, MP Biomedical), MgO (Fisher Science Education), and ethanol (200 proof, ACS/USP grade, Pharmco-Aaper) were used as received.

Synthesis of Bulk Mn-Doped SrTiO₃

Bulk powders of SrTi_1−xMn_xO_3−δ (abbreviated Mn:SrTiO₃) were synthesized by a conventional solid-state reaction method, where x is the nominal concentration of Mn (x = 0.001) and δ is the concentration of oxygen vacancies. Briefly, Sr(NO₃)₂, Mn(NO₃)₂·4H₂O, and TiO₂ were mixed in the desired stoichiometry and ground with a mortar and pestle for about 10 min. The mixture was then transferred to a porcelain combustion boat and placed in the center of a tube furnace inside a quartz insert. The reaction mixture was heated in air for 6 h at 1,000°C, reground for 10 min, then heated again for an additional 16 h at 1,000°C.

NaBH₄ Reductions and Reoxidation

Chemical reductions of the bulk powders were carried out using a modified version of reduction previously described by our group for Cr:SrTiO₃ (Lehuta and Kittilstved, 2016). For each reduction, an amount of powder was mixed in a 1:1 mole ratio with NaBH₄ using a mortar and pestle for 5 min and then placed in a porcelain combustion boat in the middle of a quartz insert in a tube furnace. The atmosphere in the quartz insert was continuously purged by a controlled flow of Ar gas monitored by a rotameter (Matheson 7300). The samples were heated at temperatures ranging from 300 to 425°C in 25°C increments under Ar flow for 30 min. After reducing, samples were cooled under Ar to room temperature, washed and centrifuged alternately with deionized water and ethanol to ensure complete removal of NaBH₄. After washing, samples were dried in an oven at 100°C for 2 h.

Reoxidation was performed by aerobically annealing the reduced samples until the physical color of the sample reversed to Mn:SrTiO₃ as-prepared sample.

Characterization

Powder X-ray diffraction (XRD) patterns were collected at room temperature using a Bragg-Brentano configuration with Cu K-α source (Rigaku SmartLab SE Diffraction System with cross-beam optics and D/Tex 250 Ultra 1D Si strip detector). X-band quantitative EPR spectra were collected at room temperature in 4 mm quartz EPR tubes (Wilmad-Glass) in a double rectangular resonator cavity (Bruker Elexsys E-500 with ER 4105DR cavity). Room temperature quantitative EPR spectra were collected consecutively on chemically perturbed samples (either oxidized or reduced) and an as-prepared sample using identical sample placement and instrument settings (Eaton et al., 2010). The resonance field positions in the EPR spectra for each paramagnetic Mn center were simulated using the “resfields” function in EasySpin using the reported EPR parameters from literature and referenced below (Stoll and Schweiger, 2006). Low-temperature X-band EPR spectra were measured at 77 K on powders using the perpendicular mode of a dual-mode resonator cavity with a quartz finger dewar insert (Bruker Elexsys E-500 with ER-4116 cavity) ensuring the sample height exceeded the cavity height for quantitative analysis. Diffuse-reflectance spectra were collected with an integrating sphere (Ocean Optics ISP-REF) coupled by fiber optics to a CCD-based spectrophotometer (Ocean Optics USB2000+ VIS-NIR). The optical density between the absorption minimum and the absorption at 320 nm was adjusted by diluting the powders with MgO.

Results and Discussion

The room temperature powder XRD patterns of as-prepared and reduced (T_red = 425°C) Mn:SrTiO₃ are shown in Figure 1. All samples designated Mn:SrTiO₃ contain nominally 0.1% Mn content. Both the as-prepared and reduced samples indicate the presence of the cubic phase of SrTiO₃ (Mitchell et al., 2000). However, a clear increase in the lattice parameter is observed after reduction. This result is consistent with other observations and has been attributed to both changes in ionic size and electronic effects after reduction of the lattice (Janotti et al., 2012). For example, the reduction of both Mn ions (Mn⁴⁺ → Mn²⁺) and lattice cations (Ti⁴⁺ → Ti³⁺) would result in larger ions leading to lattice expansion (Shannon, 1976). No appreciable secondary phases were observed after low-temperature chemical reduction despite clear spectroscopic changes in the samples (vide infra).

Figure 1

2002). Inset: expanded region of the (110) diffraction peak clearly showing a shift to lower 2θ after T_red = 425°C.

The electronic structure of Mn:SrTiO₃ is dependent on the nature of the Mn-ion speciation (i.e., oxidation state(s) and first-coordination sphere). Mn⁴⁺ has a d³ electronic configuration yielding a ⁴A_2g ground state when substituted at the Ti⁴⁺-site in SrTiO₃. The physical appearance of the Mn:SrTiO₃ as-prepared (oxidized) powders is off-white and gradually turns to black with increased reduction temperature as shown in Figure 2A. The black appearance of SrTiO₃ has been previously observed and indicates reduction in the SrTiO₃ lattice resulting in self-trapped electrons localized at Ti³⁺ sites (Tan et al., 2014; Lehuta and Kittilstved, 2016). The diffuse-reflectance spectra corroborates the assignment of the black color to excitations from Ti³⁺ to conduction band states also referred to as a metal-to-metal charge transfer (MMCT) transition in the near-IR region (Khomenko et al., 1998). In the Mn:SrTiO₃ powder this MMCT appears at T_red between 350 and 375°C. The sub-bandgap tailing absorption at ~2.9 eV has been assigned to excitations from the valence band to V_O's with different charge states (Mitra et al., 2012). With increasing T_red, the relative intensity of the V_O-related transitions decreases and disappears at T_red = 375°C and is consistent with electron accumulation in the V_O states. Both spectral changes observed here for Mn:SrTiO₃ with increasing T_red are similar to our recent study on chemically-reduced Cr:SrTiO₃ (Lehuta and Kittilstved, 2016).

Figure 2

We also do not observe Mn-centered transitions from the diffuse-reflectance spectra which we attribute to either (1) low concentrations of Mn³⁺ or Mn⁴⁺, which have spin-allowed transitions in the visible, or (2) the Mn ions are primarily in their +2 oxidation state, which only has spin-forbidden transitions when the d-electrons order in the high-spin configuration (S = 5/2, ⁶A_1g ground state).

EPR-active species involving Mn ions in the +2, +3, and +4 oxidation states in SrTiO₃ have previously been reported (Müller, 1959; Serway et al., 1977; Azamat et al., 2012). However, Mn³⁺ exhibits large zero-field splitting due to the S = 2 electronic spin state and thus, it is EPR-silent at conventional X-band frequencies (Azamat et al., 2012). The room temperature quantitative X-band EPR spectra of the Mn-doped SrTiO₃ samples are shown in Figure 3 as a function of reduction temperatures ranging from T_red = 300–425°C. The as-prepared sample consists of two sets of sextet peaks. In accordance with the reported g-value and hyperfine splitting constant (A) of Mn⁴⁺ in SrTiO₃, the main sextet in the as-prepared sample is assigned to Mn⁴⁺ substituting for Ti⁴⁺ with an isotropic g = 1.996 and |A| = 69.4 × 10⁻⁴ cm⁻¹ (Müller, 1959). The second and much weaker set of sextets is somewhat occluded by the Mn⁴⁺ transitions, but the low-field resonances agree well with substitutional Mn²⁺ at the Ti⁴⁺ site in SrTiO₃ with g = 2.004 and |A| = 82.30 × 10⁻⁴ cm⁻¹ (Azzoni et al., 2000; Choudhury et al., 2013). Despite reports of both axial Mn²⁺- and Mn³⁺- complexes in Mn:SrTiO₃, we do not observe these complexes in the as-prepared Mn:SrTiO₃ sample. Hence, only the substitutional Mn⁴⁺ and Mn²⁺ species in an octahedral oxide crystal field co-exist in the as-prepared Mn:SrTiO₃ powders.

Figure 3

After chemical reduction with NaBH₄ under Ar(g) at T_red = 300°C, a new, third set of transitions are detected near the Mn²⁺ lines. Concomitant with the appearance of this new set of peaks is a decrease in the intensity of Mn⁴⁺ lines and an increase in the relative intensity of Mn²⁺. The new set of lines agree well with the report of a substitutional Mn²⁺ center at the Ti⁴⁺-site coupled to a doubly ionized oxygen vacancy (Serway et al., 1977). The reported EPR parameters of this Mn²⁺- complex includes a large axial component to the zero-field splitting (D = 0.544 cm⁻¹), g_|| = 2.003, and |A| = 76 × 10⁻⁴ cm⁻¹. We were unable to detect any transitions at lower or higher magnetic fields likely from the low relative concentration and low nominal concentration of Mn in the lattice. The Mn²⁺- complex forms at low temperatures before reduction of the SrTiO₃ lattice at T_red < 375°C (see Figure 2). One possible mechanism to explain the formation of this complex could be that oxygen vacancies may diffuse through the lattice and localize in the vicinity of Mn⁴⁺ substitutional sites at low temperatures. This work demonstrates that mild reduction at only T_red = 300–325°C is sufficient to form the Mn²⁺- complex in bulk powders. This result contrasts with the high temperature reductions above 825°C previously used to create these centers in Mn:SrTiO₃ (Blazey et al., 1983; Kutty et al., 1986). In addition, we observe the co-existence of Mn⁴⁺ and the Mn²⁺- complex in the same EPR spectra at every T_red. This does not agree with previous single crystal studies where the Mn²⁺- complex was only observed when the Mn⁴⁺ lines were fully removed upon reduction in 5% hydrogen for 3 h at 1,000°C (Serway et al., 1977).

A new single feature centered at B₀ ~ 350 mT (g ~ 2.003) with no associated hyperfine structure was also observed in the EPR spectra after reduction. This feature increases in spectral intensity and also narrows with increasing T_red. This feature is similar to a feature observed in Cr:SrTiO₃ at T_red > 375°C (Lehuta and Kittilstved, 2016), but has a significantly larger relative intensity compared to the dopant EPR signal in the Mn:SrTiO₃ sample with the same nominal dopant concentration. This feature is tentatively assigned as the EPR-active superoxide anion () adsorbed on the surface of SrTiO₃ and could form via a surface reaction between Ti³⁺ and oxygen (Bykov et al., 2013; Harrigan et al., 2016). Table 1 below summarizes the EPR spectral parameters for previously reported Mn species in SrTiO₃ in the as-prepared and reduced Mn:SrTiO₃.

The quantitative EPR spectra measured using the double resonator cavity shown in Figure 3 was analyzed and the relative intensity of the observed EPR centers is shown in Figure 4 as a function of T_red. Compared to the as-prepared Mn⁴⁺ signal intensity (defined as 1), a gradual decrease in the Mn⁴⁺ and Mn²⁺ EPR signals is observed with similar correlations in their temperature dependences. The signal for Mn²⁺ is not detected for T_red ≥ 375°C. In contrast, the EPR intensity of the Mn²⁺- complex shows little change and is more intense than the Mn⁴⁺ and Mn²⁺ EPR signals for T_red ≥ 375°C. The intensity of the Mn²⁺- complex drops by an order of magnitude after increasing T_red from 375 to 400°C. At the highest temperature, T_red = 425°C, the EPR intensity of the ion is nearly 3 orders of magnitude more intense than the Mn²⁺- complex, indicative of substantial surface defects. Studies to identify the nature of this defect center are currently underway.

Figure 4

The EPR spectra of the Mn:SrTiO₃ powders after preparation, after T_red = 400°C, and after aerobic reoxidation at T_air = 500°C for ~1 h are shown in Figure 5. The observed changes in the EPR spectra of the reduced samples revert to the as-prepared EPR spectrum by aerobically annealing the sample. The process of forming Mn²⁺- complex in the reduced samples is thus reversible. However, elevated temperatures and longer reoxidation times were required in contrast with the chemical reductions. Since the Mn²⁺- complex is a charge-neutral complex in the lattice, it is expected to be at least metastable. The apparently slower reoxidation kinetics compared to reduction kinetics suggest a metastable complex.

Figure 5

The 300 K (room temperature) and 77 K (liquid N₂) EPR spectra of the Mn centers in the as-prepared and T_red = 300°C powders are shown in Figure 6. Two things are revealed from the EPR spectra of both as-prepared and lightly-reduced Mn:SrTiO₃ samples: (1) there is no evidence of self-trapped electrons at Ti³⁺ sites in the lattice based on the 77 K spectra, and (2) the EPR intensity of Mn²⁺ completely disappears at 77 K. At low temperature, the intensity of Mn⁴⁺ is pronounced following the typical Boltzmann statistics. In contrast, the Mn²⁺ EPR signal completely disappears at 77 K in these two samples. These results agree with a previous magnetic susceptibility and EPR study of the Mn²⁺ signal vanishing, where the behavior was attributed to increased antiferromagnetic interactions between adjacent Mn²⁺ ions with decreasing temperature (Azzoni et al., 2000). This explanation cannot be extended to describe the EPR signal of the Mn²⁺- complex, which does not disappear at 77 K in the T_red = 300°C sample. To confirm the behavior of EPR signals as a function of temperature, the EPR spectrum of the reduced sample at 300 K was repeated after cooling it to 77 K and the entire spectrum is nearly identical.

Figure 6

The cryogenic EPR measurements were also collected for samples reduced above 350°C to reveal the effect of Ti³⁺ defects on the EPR spectra that are observed in the diffuse-reflectance spectra shown in Figure 2. Figure 7 shows the 300 K and 77 K EPR spectra of T_red = 375°C. The paramagnetic Ti³⁺ defects are not observed in the EPR performed at 300 K due to fast spin-lattice relaxation but are promptly observed at 77 K (Lehuta and Kittilstved, 2016; Harrigan and Kittilstved, 2018). At 77 K, the T_red = 375°C sample is dominated by a broad and intense asymmetric Ti³⁺ lattice defect centered at g = 1.94. The appearance of this fast-relaxing defect, however, has no apparent effect on the linewidth of the Mn-centers nor the single line that we tentatively assign to surface-adsorbed ions. We recently showed that linewidth and relaxation-dynamics of substitutional Cr³⁺ ions in SrTiO₃ powders and colloidal nanocrystals can be significantly altered when Ti³⁺ defects are present in the lattice through a near-resonant cross-relaxation process (Lehuta and Kittilstved, 2016; Harrigan and Kittilstved, 2018). This same behavior is not observed for any of the Mn-centers in the reduced SrTiO₃ powder.

Figure 7

Conclusions

A low-temperature chemical reduction technique has been implemented for tunability of the Mn dopant oxidation states and the related intrinsic defects in bulk Mn:SrTiO₃. We employed a myriad of structural and spectroscopic techniques on samples subjected to a systematic chemical reduction. Both isotropic Mn⁴⁺ and Mn²⁺ species were identified in the as-prepared powders. Following the thermal reduction, the samples exhibited a continuous decrease in Mn⁴⁺ EPR signal and an increase in the Mn²⁺ intensity, accompanied by the introduction of a Mn²⁺- complex. We demonstrate that our chemical treatment at merely T_red = 300–325°C generates sufficient driving force to significantly reduce the intensity of the octahedral Mn⁴⁺ and Mn²⁺ dopants and form the Mn²⁺- complex. All the Mn peaks showed distinctive changes at low-temperature in the EPR that are readily reversible upon warming back the samples. Reductions at 375°C and above generated significant concentrations of Ti³⁺ defects that were confirmed by diffuse-reflectance and low-temperature EPR spectroscopy. All the observed perturbations in the reduced samples are entirely reversible by aerobic annealing at elevated temperatures. We also observe an intense spectral feature in the EPR spectrum in heavily-reduced Mn:SrTiO₃ powders that we attribute to ions at the surface. This fast and effective strategy offers a general low-temperature reduction process that allows tunability and control over the rich dopant-defect chemistry in transition-metal doped SrTiO₃ materials.

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