Paramagnetic Mn Antisite Defects in Nanoceramics of Aluminum–Magnesium Spinel

Abstract

The effect of structure and size parameters on the formation of intrinsic and impurity paramagnetic centers in nanoceramics of aluminum–magnesium spinel is studied. The studied samples (grain size ~30 nm) are obtained by thermobaric synthesis. Microcrystalline ceramics and MgAl₂O₄ single crystal are used as the reference samples. Characteristic paramagnetic centers of Mn²⁺ (hyperfine structure constant (HFS) A = 82 G) are present in both single crystal and microceramics. In the studied samples of nanoceramics in the initial state, both impurity Mn²⁺ and intrinsic F⁺ centers exist. Unlike the nanoceramics, the centers of F⁺ type in the reference sample appear only after the irradiation with accelerated electrons (130 keV). The parameters of Mn²⁺ centers in nanoceramics significantly differ on that in microceramics and single crystal. EPR signal of Mn²⁺ centers in nanoceramics is characterized by two anomalous constant HFS (A₁ = 91.21 G, A₂ = 87.83 G) caused by two types of octahedrally coordinated manganese ions ([Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ antisite defects). The features of spectral parameters of manganese centers correlate with a decrease in the cell parameter of MgAl₂O₄ in the nanostructural state. The observed effects are interpreted based on the assumed scheme of [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ charge compensation by the aluminum antisite defect and F⁺ center.

INTRODUCTION

Intense development of photonics and electronics requires the creation of new functional materials with specific properties such as high radioactive resistance, transparency in a wide spectral range, and thermal stability. The ceramics of aluminum–magnesium spinel is one of the most radiation-resistant materials, which is proved in many works on the irradiation of spinel with electrons, neutrons, and ions having different energies [1–7]. The spinel is also an attractive material for the doping with transition 3d-elements and rare-earth elements to obtain desired optical properties (absorption, luminescence) [8–11].

As a result of high-energy impact on the crystal structure of spinel (ion implantation, accelerated electrons, fast neutrons, thermobaric effects), both intrinsic and impurity defects with a local disturbance of the crystal field form. Clear representation of the effect of intrinsic and extrinsic defects on the matrix of aluminum-magnesium spinel is important for the understanding of the mechanisms of the formation of defects and their microscopic parameters.

As it is well known, the neutron or ionic bombardment of aluminum–magnesium spinel (MgAl₂O₄) stimulates the formation of anionic sublattice defects (VO) (V_O) [1–3]. Such anionic defects can be induced as a result of thermochemical reducing [12]. Anionic vacancies act as the effective electron traps. When one or two electrons are trapped by an anionic vacancy, partial or complete charge compensation occurs, and it is accompanied by the formation of F⁺ and F centers, respectively. The trapping of one unpaired electron is characterized by band with a g-factor of ~2.003 in the electron paramagnetic resonance spectra [1]. The features of change in the EPR signal in a single crystal with the contribution of one F⁺ center are described in details elsewhere [13]. However, the behavior of F⁺ centers in the synthesized ceramics is poorly studied and needs a detailed consideration.

The cationic sublattice of spinel consists of two types of cations, which leads to the formation of two types of cationic sublattices: aluminum and magnesium. Aluminum and magnesium cations are in octahedral and tetrahedral oxygen surrounding, respectively. Under a high-energy influence (neutron, electron, ionic radiation, thermobaric impact), the formation of the defects of cationic mixing (antisite defects, ADs) is possible between the two types of sublattices. Such defects are an aluminum ion in the manganese position [Al³⁺]$_{{{\text{M}}{{{\text{g}}}^{{{\text{2 + }}}}}}}$ and vice versa. In this case, the charge compensation is carried out by the formation of a pair of such defects [Al³⁺]$_{{{\text{M}}{{{\text{g}}}^{{{\text{2 + }}}}}}}$ and [Mg²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ so that the neutrality of the crystal lattice is, in general, kept [14]. In ceramics obtained by the thermobaric treatment, a great amount of antisite defects exist due to the quenching of the nonequilibrium state of the system under high pressure as a result of fast removal of the pressure and temperature [15, 16]. The studies of paramagnetic features of single-crystal aluminum-magnesium spinel (AMS) and ADs showed that such centers do not have a resonance absorption of electromagnetic energy in a magnetic field, but can significantly distort the signal of paramagnetic centers present nearby (for example, F⁺ centers) [13]. The question about the role of intrinsic defects of anionic and cationic sublattices or impurity centers in the EPR spectrum of nanoceramics is still opened.

The impurity defects in spinel are most frequently presented by the ions of 3d elements, whose specific spectral features are caused by the unshielded 3d shell. Under the influence of different types of radiation, strong reducing agents, or large mechanical loads, the distortion of the local surrounding of 3d ion occurs in the matrix. As a result, the spectral characteristics of 3d ions change, which allows them to be used as crystal field indicators. For example, the introduction of impurity Mn²⁺ ions into the AMS matrix provides a set of characteristic lines of hyperfine splitting (HFS) in EPR spectra due to the interaction of the magnetic moment of an unpaired electron with a magnetic moment of the nucleus. Since the spin of the Mn²⁺ nucleus I = 5/2, the nucleus magnetic moment will have 2I + 1 = 6 values under the effect of a magnetic field H. Thus, the electron magnetic moment is affected by the six different values of H. In such a field, the magnetic moment of the electron μ_S will have six possible values of energy. As a result, each electronic Zeeman-sublevel splits into six sublevels of hyperfine structure, (2S + 1). Based on (2I + 1) × (2S + 1), the resulting amount of energy levels is 36. Using corresponding magnetic quantum numbers for electron spin M and nuclear spin m (–S ≤ M ≤ S, –I ≤ m ≤ I), there are five allowed electron spin transition for ΔM_S = ±1; Δm_I = 0: |5/2m〉 ↔ |3/2m〉 ↔ |1/2m〉, |‒1/2m〉 ↔ |–3/2m〉, |–3/2m〉 ↔ |–5/2m〉, and |1/2m〉 ↔ |–1/2m〉. However, due to a strong angle dependence of the first four transitions in both powder samples and ceramics, only the |1/2m〉 ↔ |–1/2m〉 transition can be registered [17].

It is well known that the constant of HFS lines of manganese is extremely sensitive to the lattice parameters [18]. One of the possible factors significantly affecting the AMS lattice parameters is the method of its preparation and the size of crystallites. Therefore, the study of Mn²⁺ EPR spectra in an HFS range in the matrix of spinel obtained by various synthesis methods allows a deeper understanding of fundamental features of the interaction of an impurity manganese ion with the matrix.

This work is aimed at the study of paramagnetic centers of intrinsic and impurity defects in micro- and nanomodifications of the AMS ceramics. The following issues have been solved: the obtaining of EPR spectra of nanoceramics and standard samples before and after irradiation by accelerated electrons (130 keV), studying of the characteristics of saturation of different EPR signals, and the analysis of the obtained results.

MATERIALS AND METHODS

The nanoceramics samples were obtained by the thermobaric quenching of the AMS nanopowder in the Institute of Solid State Chemistry UB RAS. The content of impurities of transition elements in the studied ceramics is within standards regulated by respective documents on magnesium and aluminum nitrates (GOST 11088-75 and GOST 3757-75, respectively). The data concerning synthesis regimes and sample characterization are described [19]. The microceramics with a trace amount of Mn²⁺ impurities are provided by Peter the Great St. Petersburg Polytechnic University. The MgAl₂O₄ natural single crystal with a Mn²⁺ impurity is provided by the Zavaritsky Institute of Geology and Geochemistry UB RAS.

The characterization of micro- and nanoceramics was performed using the X-ray diffraction (XRD) of powder in transmitted light. The measurements were carried out at room temperature using a STADI-P diffractometer with a linear mini CDD detector in 2θ range of 5°–120° (step 0.05°) with Cu$K_{\alpha }^{1}$ radiation. Polycrystalline silicon (a = 5.43075 (5) Å) was used as the standard of measurements. The search and identification of impurity phases were carried out using an ICDD (USA, Release 2016) database. The lattice constant and values of microstresses of the spinel are specified by the Rietveld method based on XRD data and CSAS software [20, 21]. The results of the XRD analysis of micro- and nanoceramics are listed in Table 1. According to literature data [22], the value of the lattice constant for the sample of natural single-crystal is 8.089 Å.

Table 1. Structural characteristics of the studied samples

EPR spectra were recorded using an ELEXSYS 580 spectrometer (Bruker) with a resonance frequency of 9.87 GHz (X-band) at room temperature. The signal of quartz test-tube (Bruker) was taken into consideration. The EPR signal was normalized by the sample mass.

The irradiation with accelerated electrons was performed using a RADAN-EKSPERT pulse linear accelerator with the electron energy of 130 keV and a current density of 60 A/cm². The pulse duration was 1 ns; the fluence was ~2.4 × 10¹¹ m^–3.

RESULTS

The EPR signal of nanoceramics and reference samples is given in Fig. 1. In all samples, the signal of hyperfine structure (HFS) of Mn²⁺ ions is registered. The highest amplitude of the HFS signal is observed for the natural crystal, which is explained by a higher concentration of impurity manganese and low defectively of the sample.

Fig. 1.

EPR spectra of single-crystal sample (Mono), microceramics (Micro), and nanoceramics (Nano) of AMS. The dashed lines show the bands characteristic of the HFS of the Mn²⁺ ion in a single-crystal.

The position of HFS lines of the impurity Mn²⁺ in the sample of microceramics coincides with the position of these lines in the natural single-crystal. The couples forbidden transition lines are observed between the lines of hyperfine structure [17]. Additional bands are not observed in the EPR spectra of reference samples in the studied range. However, additional absorption lines with g = 2.005 and g = 1.981 are observed in nanoceramics spectra; their positions are characteristic for the F⁺ centers and Ti³⁺ impurities [2, 23]. As it is seen from the EPR spectrum of nanoceramics, the impurity Mn²⁺ ions have the HFS constant increased in 10 G.

Figure 2 presents a detailed analysis of the edge HFS band. It is seen that the HFS signal is the superposition of at least two signals of [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (1) and [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (2). An average value of the HFS constant (parameter A) is 91.21 and 87.83 G for A₁ and A₂, respectively. Since the HFS bands located closer to the center have a lower difference of overlapping signals, their separation is possible with a large error. Only the first and sixth components of the hyperfine structure were used for the calculation on A₁ and A₂.

Fig. 2.

The first component of the HFS of Mn²⁺ impurity in nanoceramics. The presence of two signals forming a band of a hyperfine structure is shown.

The effect of the electron irradiation with the energy of 130 keV on the EPR signal is given in Fig. 3.

Fig. 3.

EPR spectra of a single-crystal sample (Mono), microceramics (Micro), and nanoceramics (Nano) of AMS after irradiation with accelerated electrons. Dashed lines show the characteristic HFS bands of the Mn²⁺ ion.

The appearance of an additional signal with g-factors of 2.007, 2.007, and 2.005 for the single crystal, micro- and nanoceramics, respectively, is registered in the samples. Decreasing the amplitude of HFS bands of Mn²⁺ is observed in all samples. The appearance of a broad signal with g = 2.023 is an additional feature of the EPR spectra of micro- and nanoceramics. The HFS parameters of reference samples and AMS nanoceramics before and after the irradiation with electrons are given in Table 2. It is seen that the irradiation with accelerated electrons leads to the decline of the amplitude of the HFS signal of Mn²⁺; the sample of natural single-crystal is more sensitive to the irradiation. The change in g-factor and a slight change in the A parameter are also registered. No changes in the half-widths of the HFS lines (ΔH) were observed.

Table 2. HFS parameters of Mn²⁺ ion in single-crystal, micro- and nanoceramics of AMS

In order to estimate the observed changes in the absorption band of Mn²⁺ ion between single crystal sample and nanoceramics, the EPR HFS spectra on Mn²⁺ were recorded at different power of electromagnetic radiation. Figure 4 demonstrates the curves of the dependence of the intensity of Mn²⁺ HFS components of the single crystal on the power of microwave radiation. The saturation of the allowed HFS transitions of Mn²⁺ in the single crystal occurs at 15 mW. Due to the fact that in the nanoceramics the HFS band of Mn²⁺ ion is formed by two signals, two characteristic power of saturation were determined for [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (1) and [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (2); their values were 32 and 2.6 mW, respectively (Fig. 5). This allows us to conclude that the observed signals relate to different centers. The character of the dependence of the saturation curve of the band with g = 1.981 in nanoceramics coincides with the Mn²⁺ band in the single-crystal, which indicates that a part of the signal is connected with the Mn²⁺ hyperfine structure. Apparently, the signal overlapping occurs in the nanoceramics at the mentioned g-factor. According to our estimates, the saturation of the signal of F⁺ centers occurs at 100 mW. The bands of forbidden transitions of the single crystal sample demonstrate a minimum on a curve of the dependence of intensity on the power of the electromagnetic field at 5 mW. Further increasing of power leads to linear growth of the intensity of spin-forbidden transitions.

Fig. 4.

Dependence of the EPR signals of single-crystal HFS on the power of microwave radiation.

Fig. 5.

Dependence of the EPR signals of nanoceramics on the power of microwave radiation.

DISCUSSION

We showed earlier [24, 25] that the bands characteristic for F and F⁺ centers are registered in the spectra of optical absorption of electronically bombed transparent micro- and nanoceramics. Microceramics was obtained by the method of uniaxial hot pressing in a graphite mold, which likely caused the appearance of anion vacancies according to the mechanism of thermochemical coloring [26]. As a result of the following irradiation with electrons (130 keV), their localization on the anionic vacancies takes place. The EPR signal of F⁺ centers is registered in the initial samples of nanoceramics. The existence of anionic defects in nanoceramics is explained by a small size of crystallites and, as a consequence, a large number of grain boundaries. Such boundaries are the structure defects with the broken bonds including oxygen vacancies. High pressures stimulate the plastic deformation of grains, which leads to the breaking of chemical boundaries followed by the electron emitting. The localization of electron on anionic vacancy results in the formation of an F⁺ center in the spinel nanoceramics. Thus, the high-energy impact on the spinel matrix can be simulated by the thermobaric synthesis.

Decreasing the HFS amplitude in the EPR spectrum of Mn²⁺ after the irradiation with accelerated electrons correlates with the growth of the signal of F⁺ centers. The authors of [13] showed that the formation of complex defects (with the participation of both an F⁺ center and antisite defects) is possible in the spinel. However, the mechanism of the reaction of F⁺ center with an impurity Mn²⁺ is unclear. It is possible that at a close location of F⁺ centers to the Mn²⁺, the wave functions of the electron localized on F⁺ center and Mn²⁺ unpaired electron could overlap.

The saturation curves allow us not only to resolve the overlapping EPR signals, but estimate the time of spin–lattice (T₁) and spin–spin (T₂) relaxation. The method of T₁ and T₂ calculation is given in [27]. According to the calculations, the relaxation times of F⁺ center in nanoceramics are T₁ = 4.41 × 10^–4 s and T₂ = 1.09 × 10^–8 s.

It is seen from Table 2 that a slight shift of the position of g-factor of the HFS occurs depending on the modification of the crystalline lattice (nano, micro, or mono). The change in the g-factor of Mn²⁺ HFS between the samples with the different structural state is connected with different concentration of intrinsic defects in the anionic and cationic sublattices. It is shown in [28] that the shrinkage of the spinel matrix leads to the growth of the defects of cationic sublattices (antisite defects). In our case, a strong nonequilibrium state of the system with a large amount of the antisite defects keeps in the nanoceramics sample due to the harding (a sharp decrease of pressure and temperature). Thus, in our opinion, the observed shift of the g‑factor of the EPR signal of HFS of Mn²⁺ impurity is caused by a large concentration of the intrinsic defect of the matrix.

It is known that the HFS constant in the EPR spectrum of the impurity Mn²⁺ depends on the fractions of covalent bonding of manganese, i.e., on the degree of localization of the electron on the nucleus. When the fraction of the covalent bond decreases, the A parameter increases, for example, CdTe: A = 55 × 10⁴ cm^–1; ZnS: A = 64 × 10⁴ сm^–1; MgO: A = 81 × 10⁴ cm^–1; CaF₂: A = 96 × 10⁴ cm^–1 [18]. Figure 6 demonstrates the given in [18] dependence of A parameter on c/n ratio, where c is a covalence degree taken from the expression of the difference of electronegativity of the transition metal impurity and its ligand and n is a number of ligands. Based on this dependence, the graphical estimation of the covalence parameter of the [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (1) and [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (2) paramagnetic centers in nanoceramics was conducted, and the values were 6.4 and 8.0%, respectively. Using the found values of the covalency parameter, the number of ligands can be refined by the empirical formula (1) [30]:

$$c = 1 - 0.16({{X}_{A}} - {{X}_{B}}) - 0.035{{({{X}_{A}} - {{X}_{B}})}^{2}},$$

((1))

where X_A and X_B are electronegativities of the anion and cation, respectively. In the case of the MgAl₂O₄ ideal single-crystal (without impurities or structural defects [18]), the amount of ligands is 4 (with a small error). For the [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (1) and [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (2) centers the n value is ~6. According to [30], the increase in the A parameter could be caused by the growth of the coordination number of impurity manganese.

Fig. 6.

Hyperfine splitting constant A in impurity Mn²⁺ as a function of the covalency parameter c/n [18]. Asterisks and triangles show the values of the Mn²⁺ centers in nanoceramics and MgAl₂O₄ single-crystal, respectively [18].

Apparently, the additional shrinking of the lattice and existence of the significant fraction of antisite defects in the nanoceramics leads to the location of impurity Mn²⁺ primary in octahedral positions. As a result, the unpaired electron is mostly localized on the Mn²⁺ nucleus.

The presence of two paramagnetic signals of [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (1) and [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (2) indicates two manganese ions with the different local surrounding. Since the number of ligands of such centers is a multiple of six, it is reasonable to assume the compensation of the missing positive charge in the node through the second coordination sphere. One of the more possible mechanisms of the charge compensation is the localization of the antisite defect (Fig. 7a). In this case, the charge of [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ ion is compensated by the [Al³⁺]$_{{{\text{M}}{{{\text{g}}}^{{{\text{2 + }}}}}}}$ one. We consider the participation of F⁺ centers as an alternative mechanism of the charge compensation of the impurity Mn²⁺ in the nanoceramics. Such a center could act as the charge compensator of the octahedral Mn²⁺ in the second coordination sphere (Fig. 7b).

Fig. 7.

Scheme of charge compensation of impurity Mn²⁺ with the participation of antisite defect (a) and F⁺ center (b). The direction of interaction is indicated by the gray color.

Thus, the observed Mn²⁺ (1) and Mn²⁺ (2) signals (Fig. 2) are the results of the existence of two octahedral Mn²⁺ ions with the different surrounding. Moreover, because the value of the A parameter is higher for [Mn²⁺]$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$ (1), the charge compensation is possibly realized with the participation of the antisite defect. In this regard, in low concentrations the impurity Mn²⁺ acts as an indicator of the structural state of the regular and irregular (antisite) positions of Mg²⁺ cation in the nanoceramics.

CONCLUSIONS

The study of the paramagnetic centers of aluminum–magnesium spinel in the nanoceramics and its microceramic analog shows that the parameters of the impurity and intrinsic centers of the microceramics are close to the single crystal. It allows us to conclude that, in this case, the grain with a large size can be considered as individual microscopic single-crystals. In nanoceramics, additional lattice distortions appear due to small grain size and synthesis features (thermobaric synthesis) which leads to the anomalous parameters of intrinsic and impurity centers such as F⁺ and Mn²⁺ centers. The following specific examples can be highlighted:

1. It is established that the initial nanoceramics contains a significant part of F⁺ centers with g = 2.005. After the electron irradiation, the amplitude of the specified signal increases and the appearance of a broad signal of unknown nature with g = 2.023 is observed. The characteristic times of spin–spin and spin–lattice relaxation of the F⁺ center are 1.09 × 10^‒8 and 4.41 × 10^–4 s, respectively.

2. An anomalous constant of the HFS of impurity Mn²⁺ was found, associated with six-fold coordination with respect to oxygen. Features of spectroscopic parameters of HFS of manganese centers correlate with a decrease in the lattice parameter in the nanostructured state.

3. The HFS signals with A₁ = 91.21 G and A₂ = 87.83 G arise due to the charge compensation of Mn²⁺ impurity with the participation of the antisite defect [Al³⁺]$_{{{\text{M}}{{{\text{g}}}^{{{\text{2 + }}}}}}}$ and the F⁺ center, respectively.

The set of the obtained results allows us to conclude that the method of ceramics synthesis significantly affects the structural properties giving the opportunity to manage those at the microlevel. The results of the study of the EPR spectra in the range of HFS of Mn²⁺ in the spinel matrix obtained by various synthesis methods are of interest for the simulation of local interactions of impurity Mn²⁺ with the matrix.