Unconventional Electronic Properties of Mg2Si Thermoelectrics

Article pubs.acs.org/JPCC

Unconventional Electronic Properties of Mg2Si Thermoelectrics Revealed by Fast-Neutron-Irradiation Doping Alexander E. Karkin,† Vladimir I. Voronin,† Natalia V. Morozova,† Sergey V. Ovsyannikov,*,‡,§ Ken-ichi Takarabe,∥ Yoshihisa Mori,∥ Shigeyuki Nakamura,⊥ and Vladimir V. Shchennikov† †

M. N. Miheev Institute of Metal Physics of Russian Academy of Sciences, Urals Division, 18 Sofia Kovalevskaya Street, Yekaterinburg 620041, Russia ‡ Bayerisches Geoinstitut, Universität Bayreuth, Universitätsstrasse 30, Bayreuth D-95447, Germany § Institute for Solid State Chemistry of Russian Academy of Sciences, Urals Division, 91 Pervomayskaya Street, Yekaterinburg 620990, Russia ∥ Okayama University of Science, Ridai-cho 1-1, Kita-ku, Okayama 700-0005, Japan ⊥ Tsuyama National College of Technology, 624-1 Numa, Tsuyama 708-8509, Japan ABSTRACT: In this work, we systematically investigated the electrical resistivity, Hall, and magnetoresistance effects of Aldoped Mg2Si thermoelectrics, irradiated with a fluence of fast neutrons and consequently isochronally annealed at moderate high temperatures up to 500 °C. We found that the fastneutron bombardment itself only slightly modified the electronic properties. Meanwhile, a series of the postirradiation high-temperature anneals affected the properties dramatically. Thus, after annealing at temperatures of 275−325 °C, Mg2Si:Al showed pronounced jumps in both the electrical resistivity and the Hall constant values by several orders of magnitude. This unexpected electronic transition corresponded to a significant variation in the carrier concentration. We proposed that this unusual electronic transition may be related to temperature-assisted chemical bonding of the radiation defects that can involve free charge carriers in the sample, thereby tuning dramatically its transport properties. We proposed a simple model for Mg2Si:Al thermoelectrics, which links the chemical bonding of the interstitial Mg ions with the electronic properties of this material. This finding suggests a new avenue to tuning of electronic properties and unconventional electronic transitions in materials with a high concentration of defects.

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INTRODUCTION Magnesium silicide, Mg2Si, is a semiconducting material with a narrow band gap of about 0.65−0.8 eV and interesting physical properties.1−6 At ambient conditions, Mg2Si crystallizes in a cubic antifluorite (CaF2)-type structure (space group no. 225 − Fm3̅m, Z = 4) (Figure 1a). Although several classes of thermoelectric materials, including tellurides,7−9 half-Heusler alloys,10,11 and higher manganese silicides,12 had been previously investigated, recently, it has been revealed that Mg2Si comprising two earth abundant and nontoxic elements has a significant potential for thermoelectric applications in the range of elevated temperatures above 500 K.13−17 Plenty of works were dedicated to investigations of various strategies of improvement of the thermoelectric performance of this material, which included combinations of different traditional optimization methods, such as doping (e.g., by Al, Bi), strong chemical substitution (e.g., Mg2Si1−x−ySnxGey), and mesostructuring.18−53 In addition, it has been established that application of moderate stress/pressure can dramatically tune the relevant physical properties of narrow-band gap thermoelectrics,54−57 © XXXX American Chemical Society

and can also enhance the thermoelectric performance, for instance, in Al-doped Mg2Si.58,59 Doping with Al atoms, those prefer to occupy the Mg sites in the crystal structure (Figure 1a),17,60 has been established to be a simple and effective method of fabrication of n-type thermoelectrics with high thermoelectric performance at elevated temperatures. Furthermore, Mg2Si:Al thermoelectrics demonstrated rather intriguing properties at low temperatures; those included a negative magnetoresistance effect and a Kondo-like hump in the electrical resistivity.58 Both features might be related to magnetic scattering of charge carriers, although this material is free from any magnetically active ions. It is worth mentioning here that another thermoelectrics, Bi2Te3 containing no magnetic ions as well, in its nanometric state showed a pronounced ferromagnetic signal, which was proposed to originate from distortion of density of states Received: March 22, 2016 Revised: April 16, 2016

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DOI: 10.1021/acs.jpcc.6b02980 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Crystal structure of Mg2Si at ambient conditions. (a) A unit cell of the original cubic antifluorite (CaF2)-type structure. (b) A model of the unit cell with one Mg ion shifted from its regular position toward the central interstitial site. Supposedly, this is the most common radiation defect the fast-neutron bombardment has led to. (c) A model of the unit cell with a fully bonded Mg ion sitting at the central interstitial site. Supposedly, the annealing at intermediate high temperatures of 275−325 °C could lead to completing the chemical bonding of the interstitial Mg ions.

phenomena, which are not observed in conventional “parent” compounds.

because of the presence of certain antisite defects.61 Other nonmagnetic Mg-bearing systems, like a simple cubic MgO, were also predicted and experimentally verified to have an intrinsic ferromagnetism.62,63 Yet the origin of this ferromagnetism remained vague, and was tentatively addressed either to vacancies at the Mg sites in the crystal lattice of MgO, which could induce local moments linked to the 2p orbitals of oxygen atoms,62 or, on the contrary, to vacancies at the oxygen sites in the crystal structure.63 In application to thermoelectrics, the introduction of defects in crystal structure can affect both the electronic transport properties (electrical resistivity and Seebeck coefficient) and the thermal transport properties (thermal conductivity), but in different manners. Thus, introduction of defects can potentially lead to a certain optimization of the thermoelectric performance. In this regard, we can mention a recent theoretical work predicting that point defects in Mg2Si can improve the thermoelectric performance because of a considerable decrease in its thermal conductivity.64 Other theoretical works on Mg2Si explored energetically favorable native defects and linked them to different types of electrical conduction (p- or n-) that can be realized in this thermoelectrics.65−67 Thus, both the roles of different point defects and their contributions to the physical properties of Aldoped Mg2Si seem to be substantial. However, to date, only a limited number of studies were dedicated to this topic.64−67 In this work, we irradiated Al-doped Mg2Si thermoelectrics with a fluence of fast neutrons to create a very high concentration of point defects in the bulk of this material. Fast-neutron bombardment is known to be a powerful tool for generation of radiation defects, which can then be fully or partly annealed upon a series of high-temperature treatments. We systematically investigated the electrical and galvanomagnetic properties of the irradiated samples of Mg2Si:Al at different stages of their annealing. We established that after the annealing at moderate high temperatures of 275−325 °C, Mg2Si:Al showed a jump in the electrical resistivity value by several orders of magnitude. We analyzed the most probable point defects in the cubic antifluorite structure of Mg2Si and proposed a simple model, linking a temperature-assisted chemical bonding of these radiation defects to this dramatic variation in the electronic transport properties. The discovery of this elegant electronic transition in Mg2Si:Al demonstrates that defect structure and its tuning can lead to new remarkable

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EXPERIMENT The polycrystalline samples of Mg2Si doped with 1 at. % of Al were prepared by the spark plasma sintering technique at 1113 K under a uniaxial stress of 30 MPa using a SPS-820S equipment at the Okayama Ceramics Research Center.68 Chemical composition and morphology of the samples were controlled by conventional methods, including the scanning electron microscopy (SEM) and microprobe analysis. X-ray diffraction studies of both initial as-grown samples and after their fast-neutron bombardment were carried out on a conventional laboratory UM-1 M diffractometer with Cu Kα radiation, λ = 1.5418 Å, in the range of 10−140°. To reduce an effect of the texture, the samples were rotated inside the focal plane during acquisition of the X-ray images. The X-ray diffraction patterns were analyzed with a use of the common Rietveld refinement method.69 These structural investigations showed that the as-grown samples of Mg2Si:Al crystallize in the cubic anti-CaF2-type structure, but also contain a few weight percent of MgO precipitates, which are hardly avoidable at high synthesis temperatures (Figure 2). The electronic transport properties of the samples were investigated by measurements of temperature dependencies of the electrical resistivity, Hall effect, and the magnetoresistance effect using a conventional four-probe method.70 These measurements were carried out on an Oxford Instruments setup, following the same procedures as reported in previous works.71−75 We performed the comparative investigations of the electronic properties of the same samples, before the fastneutron bombardment, after that, and after each temperature step of the postirradiation isochronous annealing. These anneals were accomplished in vacuum conditions during 30 min at different temperatures, varying from 70 to 500 °C with a 25 °C step. In total, we measured two bulk samples, labeled as #1 and #2, cut from the same big ingot. Sample #1 with the size of 2 × 1.5 × 7 mm3 was used mainly for the systematic measurements of the electrical resistivity and magnetoresistance effect. At selected temperature steps of the annealing, we also measured the Hall constant of this sample #1. Sample #2 with size of 2 × 2 × 0.7 mm3 was used primarily for the systematic B


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Figure 3. Temperature dependencies of the electrical resistivity (a) and Hall constant (b) of Mg2Si:Al, measured on the initial as-grown samples #1 (a) and #2 (b), and after their fast-neutron irradiation.

Figure 2. X-ray diffraction patterns of Mg2Si:Al, collected from the initial as-grown sample and after the fast-neutron irradiation. Both Xray diffraction patterns (points) were fitted by Rietveld refinement (solid lines). Dashes below the lower pattern indicate the expected reflection positions for the cubic anti-CaF2-type structure. Insets show two magnified parts of these patterns, including the strongest (220) reflection and the high-angle peaks. Asterisks mark the reflections of MgO impurities.

made a minor accumulative contribution to the transport properties. At room temperature, the initial samples of Mg2Si:Al had the n-type conduction and were characterized by the electron concentration as n ≈ 6.2 × 1019 cm−3 and by the Hall mobility value as μH ≈ 96 cm2/(V·s). Hence, both the initial and the irradiated samples demonstrated a semimetallic electrical conduction, for which the weak temperature dependencies of the electrical resistivity and Hall constant are typical. To investigate a contribution of the radiation defects to the electronic transport properties in more detail, we subjected both irradiated samples #1 and #2 to a series of isochronous anneals at temperatures, varying from 70 to 500 °C, and examined the evolution of their properties after each step of the annealing. We noted that this series of the anneals led to dramatic and nonmonotonic changes in all of the properties measured, including the electrical resistivity, Hall constant, and magnetoresistance effect (Figures 4 and 5). After the annealing at average temperatures of 275−325 °C, the electronic properties of this originally semimetallic material became more semiconducting around room temperature (Figure 4). After the annealing at higher temperatures, Mg2Si:Al then turned back to a semimetal (Figure 4). A temperature profile of this “semimetal−semiconductor−semimetal”-type electronic transition may be better seen in Figure 6, in which we plotted the electrical resistivity and Hall constant values for both samples #1 and #2, at two temperatures, 4.2 and 300 K, as functions of the annealing temperature, Tann. All of these dependencies demonstrate an extremum near 325 °C (Figure 6). We also noted that these extrema in the curves of resistivity and Hall effect values look rather asymmetric (Figure 6). Although the temperature dependence of the electrical resistivity after the annealing at 325 °C (Figure 4) tended to a finite value upon approaching zero temperature, and hence, strictly speaking, it is not semiconducting, around room temperature (between 200 and 380 K), this resistivity curve showed an apparent activation behavior. We estimated an activation energy for this 325 °C curve as Ea ≈ 50 meV. This value is much smaller than a semiconductor band gap of 0.65− 0.8 eV in undoped Mg2Si.1−6

investigations of the Hall coefficient, but its electrical resistivity was also measured for control. Both samples #1 and #2 were simultaneously irradiated with fast neutrons with a fluence of Φ = 2 × 1019 cm−2 (for neutron energies exceeding En > 0.1 MeV) at the irradiation temperature of Tirr = 50 ± 10 °C. The fast-neutron bombardment was performed on the Beloyarsk Nuclear Power Station (Zarechny, Sverdlovsk region, Russia).

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RESULTS The comparative X-ray diffraction studies on the initial samples and after their fast-neutron irradiation revealed no apparent difference between them (Figure 2). All of the X-ray diffraction patterns corresponded to the original cubic antifluorite (CaF2)type structure with a minor impurity of MgO. The X-ray diffraction patterns of the irradiated samples showed no traces of amorphization or strong structural disordering (Figure 2). The comparative Rietveld refinement of the X-ray diffraction patterns collected from the initial and irradiated samples found no detectable changes in the site occupancies of both the Mg and the Si atoms after the irradiation. Meanwhile, this refinement established a minor detectable swelling of the crystal lattice after the irradiation (right inset in Figure 2). Thus, the lattice parameter was changed from a = 6.348(12) Å in the initial sample to a = 6.351(12) Å in the irradiated one; that is, it was increased by ∼0.05%. Hence, we can infer that the fast-neutron bombardment led to the formation of stable radiation defects of which the concentration should be less than 1% of the total number of the atoms. The electrical resistivity and Hall constant measurements on the initial samples and after their fast-neutron bombardment also did not reveal any dramatic difference between them (Figure 3). Both the electrical resistivity and the absolute value of the Hall constant only increased a bit after the irradiation (Figure 3). This result would suggest that the radiation defects C


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Figure 4. Electrical and galvanomagnetic properties of Mg2Si:Al, measured on the initial as-grown samples, and after their fast-neutron irradiation and the consequent postirradiation annealing at different temperatures, from 100 to 500 °C. The annealing temperatures are given near the curves. The points are experimental data, and the lines are fitting curves based on eqs 1 and 2. (a) Selected temperature dependencies of the electrical resistivity of sample #1. (b) Selected temperature dependencies of the Hall constant measured on sample #2 in a magnetic field of 13 T.

higher temperature range of these curves, that is, between 150 and 300 K, would better correspond to a quadratic dependence, like ρ ∼ T2 (Figure 4a). The former dependence is typical for magnetic scattering of the Kondo type, while the latter one is characteristic for conventional electron−electron scattering. Hence, the total electrical resistivity may be considered as a superposition of these two contributions, as follows: ρ = ρ0 − a ln T + a2T2, where ρ0 is the residual resistivity value. However, increasing in the electrical resistivity with temperature decreasing below 100 K (it is seen in Figure 7a for the initial sample) seems to deviate from the above logarithm dependence. In addition, for temperatures above 300 K, the resistivity curves looked weaker than a quadratic function. Thus, for fitting of the electrical resistivity curves (Figure 4a), we should also consider one more conduction band of which resistivity has a different power dependence, as follows: ρ = ρ0 + anTn. The total electrical resistivity then is determined, as follows:76,77

The magnetic-field dependencies of the magnetoresistance (MR) demonstrated the negative values of this effect in the low magnetic fields, and the positive slopes of the curves in the high magnetic fields (Figure 5). The largest positive and negative magnitudes of the magnetoresistance effect at 13 T, MR ≈ +20% and −7% (Figure 5), were observed after the annealing at temperatures of 325 and 350 °C, respectively. The observation of the largest values of the MR effect after the annealing at 325 and 350 °C is in line with the extrema in the curves of the electrical resistivity and Hall constant (Figure 6). In Figure 7 we compared the temperature dependencies of the electrical resistivity measured at zero magnetic field and at a field of 13− 13.6 T across the annealing temperature of 325 °C. This figure explicitly demonstrates the dramatic changes in the electrical conduction type after the annealing. For instance, the curves measured after the annealing at 325 °C looked different from all of the others, but those measured after the anneals at 300 and 350 °C, on the contrary, looked similar (Figure 7). These data also supported the above observations that a landmark point in the annealing of Mg2Si:Al lies between 300 and 350 °C, probably near 325 °C.

1 1 1 = + ρ ρ1 ρ2

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(1)

where ρ1 = ρ01 + anTn and ρ2 = ρ02 − aL ln T + a2T2 as discussed above. This simple two-band model could fit well all of the temperature dependencies of the electrical resistivity of Mg2Si:Al (Figure 4a). We found that the power, n, in the function of anTn for the curves, lying below the values of ρ < 102 mΩ cm, was well-defined and varied in the range of n = (0.35 ÷ 0.6) = 0.5 ± 0.15. The resistivity curves lying above 102 mΩ cm were also fitted by this two-band model, but the fitting parameters, such as an and n, were already not so well refined.

DATA ANALYSIS Analysis of the evolution of the temperature dependencies of the electronic transport properties (Figure 4) after the sequent anneals can give information about the role and contribution of the radiation defects. Concerning the electrical resistivity, we figured out that for the low-resistive states with ρ ∼ 1 ÷ 5 mΩ cm, the low-temperature part of these dependencies, that is, in the range of 50−150 K, looks like ρ ∼ −ln T. Likewise, the D


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Figure 7. Comparison of temperature dependencies of the electrical resistivity of Mg2Si:Al, measured at zero magnetic field and in the field of 13 or 13.6 T (shown near the curves) on the initial sample #1 (a), and after its fast-neutron irradiation and different anneals at 300 °C (b), 325 °C (c), and 350 °C (d).

Figure 5. Magnetic-field dependencies of the magnetoresistance effect (Δρ/ρ) of Mg2Si:Al, measured at a temperature of 4.2 K on the initial as-grown sample #1, and after its fast-neutron irradiation and consequent annealing at different temperatures, from 100 to 500 °C. The annealing temperatures are given near the curves. The points are experimental data, and the lines are fitting curves based on eq 4.

Figure 8. Examples of temperature dependencies of the measured full electrical resistivity (a) and the calculated by eq 1 partial contributions to the resistivity of the first, ρ1 (b), and the second, ρ2 (c), conduction bands of Mg2Si:Al sample #1, found for the initial sample and after its irradiation with fast neutrons and different subsequent anneals at 100, 450, and 500 °C. Figure 6. Dependencies of the values of the electrical resistivity, ρ (a), and Hall constant, RH (b), at 4.2 and 300 K for two samples of Mg2Si:Al, #1 and #2, irradiated with the fast neutrons and consequently annealed at different temperatures up to 500 °C, as functions of the annealing temperature, Tann.

8b,c). For the carriers of the first conduction band, the magnetic scattering is absent at low temperatures, and at higher temperatures the electrical resistivity of this band behaves as a function of ρ ∼ T1/2 (Figure 8b). For the carriers of the second band, the magnetic scattering is dominant at low temperatures, and the electrical resistivity of this band behaves as a function of ρ ∼ −ln T. Yet at higher temperatures, this resistivity dependence of the second band transforms to a function of ρ ∼ T2, corresponding to conventional electron−electron scattering (Figure 8c).

In Figure 8 we give several examples of the resistivity curves in the low-resistance states (below 2 mΩ cm) together with the partial contributions of both conduction bands (Figure 8b,c), calculated by using eq 1. We noted that at low temperatures the electrical resistivity of the first band was lower than that of the second band, and vise verse at the higher temperatures (Figure E


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Figure 9. Concentration and mobility values of charge carriers at 4.2 K of two electron bands of two Mg2Si:Al samples, #1 and #2, after their fastneutron irradiation and postirradiation annealing at different temperatures, from 100 to 500 °C. The carrier concentrations, n1 and n2 (a), and mobility values, μ1 and μ2 (b), for the first and the second conduction bands as functions of the annealing temperature, Tann. (c) Parametric dependencies of the mobility values on the carrier concentration values for these two bands for these two samples. The symbols in (c) are the same as those in (a, b). The arrows in (c) show the directions of increase of the annealing temperature. We attributed the second conduction band, which has the electron concentration similar to that in the initial sample, to the original Al-doping. Hence, the first conduction band may be attributed to the radiation defects.

where ⟨σ⟩ = ∑σi/[1 + (RHiσiH)2] and ⟨Rσ2⟩ = ∑RHi(σi)2/[1 + (RHiσiH)2], where σi = 1/ρi and RHi are the electrical conductivity and Hall coefficient for different conduction bands, i = 1, 2. Similar expressions were also used in other works.78 The above model (eqs 1 and 2) permitted one to separate the partial Hall coefficients of both bands and the parameters of their charge carriers in Mg2Si:Al. Both bands were characterized by the n-type electrical conduction in the whole temperature range for all of the annealing steps. In Figure 9a,b, we summarized the carrier concentration and mobility values for both bands at 4.2 K, found from the data for samples #1 and #2. Thus, the growth observed in the electrical resistivity values after the annealing at 275−325 °C (Figure 6) may be attributed to a dramatic decrease in the carrier concentrations of both bands (Figure 9a). The carrier mobility value of the second band also dramatically diminished after the annealing at these temperatures (Figure 9b). In Figure 9c, we replotted the carrier mobility values as functions of the carrier concentrations for both bands. From this plot, one can grasp the difference in characters of the electrical conduction of these two bands (Figure 9c). The second conduction band in the irradiated samples is characterized by an electron concentration (Figure 9a) similar to that in the initial samples, and hence this band may be assigned to the original Al-doping. Hence, the first conduction band should be assigned to a superposition of conduction bands linked to different radiation defects.

For analysis of the Hall constant dependencies, we applied the same two-band model, in which the Hall constant comes as follows:76,77 RH =

RH1/(ρ1)2 + RH2/(ρ2 )2 1/(ρ1)2 + 1/(ρ2 )2

(2)

where RH1 and RH2 are temperature-independent Hall constants for the first and the second conduction bands, respectively, and ρ1 and ρ2 are temperature-dependent electrical resistivities of these bands, determined from eq 1. This model fit well the experimental Hall data (Figure 4b). Similarly, the magnetoresistance effect dependencies (Figure 5) may be described by a superposition of two contributions, including (i) a nearly quadratic positive term in the high magnetic fields (Δρ/ρ ∼ (μH)2, where μ is the mobility and H is the magnetic field), and (ii) a logarithmic term related to suppression of the Kondo effect by magnetic field (Δρ/ρ0 = −b1 ln(1 + BHH), where b1 and bH are two fitting parameters). In application to the initial sample, this model gave the mobility values as μMR ≈ 140 cm2/(V·s), in harmony with the value of μH ≈ 96 cm2/(V·s), found above from the Hall effect data. In a more general case, the expressions for RH and ρ(H) come as follows:76,77 Rσ 2

RH =

2

σ

2

+( Rσ 2 H )

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(3)

DISCUSSION The main outcome of a fast-neutron bombardment of a substance consists in the appearance of strong displacements of

σ

ρ (H ) =

2

σ

2

+( Rσ 2 H )

(4) F


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length of a Si−Si bond (e.g., in the diamond-type structure of elemental silicon) is about 1.86 Å and the lattice parameter of Mg2Si is more than 3 times longer, a ≈ 6.35 Å (Figure 1b). Thus, taking into account all of the above circumstances, we can propose that the most probable radiation damage in the lattice of Mg2Si:Al consists in the ejection of the compact Mg2+ ions toward the interstitial 4b site with a concurrent formation of a vacancy in the Mg sublattice, as shown in Figure 1b. Notice that the interstitial Mg ions were reported to act as donors, and the Mg vacancies, on the contrary, were found to act as acceptors.66 Therefore, the simultaneous formation of these two types of defects should give comparable contributions of nand p-types to the electrical conduction, and hence these contributions could partly compensate each other. The very moderate growth in the electrical resistivity we observed in the irradiated samples (Figure 3) suggested that a number of new charge carriers arising because of the introduction of the radiation defects should be less than the number of the original charge carriers linked to the Al-doping. Withal, the introduction of the radiation defects is capable of enhancing the scattering of the charge carriers, which could also lead to such a growth in the electrical resistivity (Figure 3). As the data analysis revealed, the carrier concentration related to the first conduction band, assigned to the radiation defects, is roughly by 1.5−2 orders less than that of the second conduction band, attributed to the Aldoping (Figure 9a). Meanwhile, we should note that the tiny “swelling” of the crystal lattice that we detected in the irradiated samples of Mg2Si:Al (right inset in Figure 2) may hint that some other more specific interstitial defects, related to radiation “damages” in the lattice, might be introduced as well, but in much less concentrations. The central interstitial 4b site in the crystal lattice of Mg2Si is potentially six-coordinated for Mg ions (Figure 1c). We can propose that upon shifting the Mg2+ ions to this central site, they most likely retain three of the four of their original chemical bonds with the Si4− ions, but, in addition, should form at least one more chemical bond with one of the three opposite Si4− ions, to form a stable point defect. For example, this could happen in a way as illustrated in Figure 1b. Such nonfully bonded interstitial Mg ions in the lattice could induce local distortions and deviations from the cubic symmetry, and their bonding in the lattice should be sensitive to external factors, such as annealing temperature. Thus, we can further propose that the surprising “semimetal−semiconductor”-type electronic transition we observed after the annealing at 275−325 °C (Figure 6) could be driven by completing the chemical bonding of all of these interstitial Mg ions, as shown in Figure 1c. This process should be concurrent with a local “symmetrization” of the crystal lattice. The sudden disappearance of the conduction electrons linked to both the first and the second conduction bands after the annealing at 275−325 °C (Figure 9a) suggests that these electrons have been involved in the formation of the new chemical bonds of the interstitial Mg ions (Figure 1c). Annealing at somewhat higher temperatures should eventually assist in turning the crystal lattice back to its initial state, which is more thermodynamically stable at ambient conditions. This means, in particular, that the interstitial Mg ions should move back to their regular sites in the lattice, as shown in Figure 1a. We observed that after the annealing at temperatures above 350 °C, the electronic properties of both irradiated samples of Mg2Si:Al gradually returned to their initial states, and nearly reached them after the final annealing at 500 °C (Figures 4 and 5). Thus, the asymmetry of the peaks in both

atoms from their equilibrium positions, toward relatively stable interstitial sites.79−81 This type of irradiation does not change a charge balance inside crystal lattice, in contrary to other common treatments, like electron irradiation.77,82,83 Another potential implication of a fast-neutron bombardment, such as transmutations in a lattice because of thermal capture of neutrons by nuclei, was reported to give negligible contributions for the overwhelming majority of materials.79−81,84 Irradiation with fast neutrons typically creates a limited number of stable radiation defects; some of those may be electrically active, and hence capable to contribute to electronic transport, either as charge carriers or as scattering factors for the original charge carriers in pristine material. In pure semiconducting materials, a contribution of radiation defects may be significant,79,80,85−87 whereas in metals or semimetals with high carrier concentrations, this contribution may be minor or very moderate.88,89 Some materials subjected to a fast-neutron irradiation also demonstrated signatures of moderate structural disordering.90 Our examination of the irradiated samples of Mg2Si:Al by Xray diffraction did not reveal any detectable traces of structural disordering (Figure 2). We observed only a tiny “swelling” of the lattice (increase in the lattice parameter by 0.05%), which is typical for materials with radiation defects. Considering the cubic crystal structure of Mg2Si consisting of a face-centered lattice of Si with an embedded simple cubic lattice of Mg (Figure 1a), we can note that the central part of the unit cell is unfilled, thereby providing enough space for stable interstitial defects. Theoretical investigations of structural defects in this lattice of Mg2Si also proposed that the central position (4b site in Wyckoff notation) is the only stable position for interstitial atoms.65−67 These theoretical works considered the equilibrium crystal growth mechanisms under minor excesses of either Mg or Si atoms, and established that the most energetically favorable point defects are interstitial Mg ion at the central 4b site and Mg vacancy (Figure 1b).65,66 In addition to that, these works proposed that the formation of vacancies of two and more neighbor cations, like MgSi and Mg2Si, have the minimal energy barriers as well, and may be realized during the crystal growth process too.65,66 On the contrary, the energies of the formation of both interstitial Si and Si vacancy were found to be incomparably higher.65,66 From analysis of the chemical bonds in this cubic antifluorite structure of Mg2Si with the lattice parameter of a ≈ 6.35 Å, one can figure out the ionic radii of the Mg2+ and Si4− ions. Thus, from the Mg−Si bond distance as ∼2.75 Å, it has been determined that the radii of the Mg2+ and Si4− ions are strongly different, as ∼0.6 and ∼2.1 Å, respectively.65 Thus, it seems to be feasible that the small Mg2+ ions could be relatively easily shifted toward the central interstitial site under action of the fast-neutron bombardment (Figure 1b). Likewise, the large Si4− ions hardly can be moved to this site without a prior rearrangement of their electronic configuration. Furthermore, introduction of the Si4− ions to the central 4b site does not look that promising for modification of the physical properties of this material. The Si4− ions in this crystal structure form eight chemical bonds with the Mg2+ ions, and a similar bonding with eight Mg ions, should be conserved even though the Si4− ions are moved toward the central 4b site; in this case, some Mg ions may become “underbonded”, and the others “overbonded”. Whereas the formation of qualitatively new covalent Si−Si bonds between the interstitial Si atom occupying the central 4b site and its neighbors to form a stable point defect seems to be hardly possible, because a typical G


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The Journal of Physical Chemistry C the resistivity and the Hall constant values at near 325 °C (Figure 6) may be explained by the fact that recovering the lattice from the radiation damages and defects is a more sluggish process than the formation of the chemical bonds. Thus, on the example of Mg2Si:Al thermoelectrics, we have revealed a remarkable electronic transition, which interconnects a tuning of the chemical bonding, local distortion of the crystal lattice symmetry, and “arresting” and “releasing” the free charge carriers. In other words, the case of Mg2Si:Al has brightly demonstrated how the crystal chemistry and the electronic properties may be tightly interconnected.

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CONCLUSIONS In this work, we have revealed an unusual temperature-assisted electronic transition of a “semimetal−semiconductor”-type in fast-neutron-irradiated Al-doped Mg2Si thermoelectrics. We have inferred that the fast-neutron bombardment led to the formation of stable radiation defects; the majority of those are the energetically favorable interstitial Mg ions and associated Mg vacancies. We have proposed a simple model interrelating the temperature-controlled tuning of the chemical bonding of this defect structure, on the one hand, and the electronic transport properties, on the other hand. Fast-neutron irradiation permits one to create the high concentrations of stable defects in the bulk of material, and together with a series of postirradiation anneals they provide a tunable doping of a single sample. Our findings suggest that this sort of investigation can be effective in the detection of features of the electronic transport properties. We propose that a temperature-assisted chemical bonding of defect structure can be an efficient strategy for modification of the physical properties of different electronic materials, for example, thermoelectrics. For practical realization of this strategy in different thin-film or nanometric materials, other methods of defects generation may be applied as well.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 (0) 921 55 3839. E-mail: sergey.ovsyannikov@ uni-bayreuth.de, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (research project no. 14-02-00622a), and in part by IMP Neutron Material Science Complex within the state assignment of theme “Flux” (no. 01201463334).

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REFERENCES

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Unconventional Electronic Properties of Mg2Si Thermoelectrics

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