xMnxTe Alloys Research Articles

Theoretical Study of the Stability and Electronic Properties of HgMnTe Alloys

Herein, preliminary theoretical results for the structural and electronic properties of Hg1−xMnxTe alloys in the zincblende structure using density functional theory calculations are presented. The results suggest that there is a strong dependence of the electronic structure of the alloys on the exchange correlation functional used. When using the generalized gradient approximation, the evaluated data show that this material behaves as a semimetal in almost all Mn concentration range, in disagreement with the experimental data. However, with the hybrid Heyd–Scuseria–Ernzerhof functional calculations, results show that the band gap of these alloys opens for the diluted Mn concentration range 0.05 < x < 0.2, consistent with the experimental observations. Moreover, when considering the magnetic ordering of the Mn spins in the alloy, the results show that, in this material, the lower total spin configurations is energetically more favorable.

Interplay between magnetic, metal/insulator and topological phases in Hg1−xMnxTe alloys: prediction of a ferromagnetic Weyl semimetal at x = 0.25

We report a theoretical investigation of magnetic, electronic, and topological properties of Hg1−xMnxTe alloys. We consider periodic structures with Mn concentrations as x = 0, 0.25, 0.5, 0.75, and 1. Our hybrid DFT/Hartree–Fock calculations for the bandgaps of antiferromagnetic (ground-state) phases are in good agreement with experiments. The calculations also show that the modification of the magnetic ordering from anti- to ferromagnetic leads to a significant bandgap reduction, resulting in a metal/insulator transition at higher Mn concentrations. We show that a ferromagnetic Weyl semimetal phase is achieved for x = 0.25, where a single pair of Weyl nodes mirrored by the point in the momentum space is observed. The non-trivial topological property of the ferromagnetic Hg0.75Mn0.25Te is confirmed by the calculation of the chirality of each Weyl node, which are connected by a surface Fermi arc of a semi infinite Hg0.75Mn0.25Te.

Microstrucutre and thermoelectric properties of rapidly prepared Sn1−xMnxTe alloys

Sn1−xMnxTe (x = 0, 0.09, 0.15, 0.20) bulk materials were prepared by melt spinning combined with spark plasma sintering process. Nanoscale grains were obtained, and the solid solubility of Mn was much enhanced by the ultrafast-cooling synthesis technique. The maximum of Seebeck coefficient and power factor are 242 µVK−1 and 19.97 µW cm−1K−2 at 873 K with the doping concentration of 15 at% Mn. A large amount of grain boundaries and doped atoms improve the scattering of heat-carrying phonons in a wide range of frequencies, and the scattering mechanisms are also explained by theoretical calculation. As a result, the minimum of lattice thermal conductivity is 0.66 µVK−1 at 873 K, the corresponding figure of merit is 1.26 for Sn0.85Mn0.15Te sample. This value is improved by 35% comparing with previously reported result. Our work indicates that melt spinning process is effective to develop SnTe related thermoelectric materials with excellent thermoelectric properties, which has the widespread commercial value and the prospects for development.

Phases and thermoelectric properties in stoichiometric Sn1 − xMnxTe and non-stoichiometric Sn1 − yMn1.1yTe alloys

Both series of stoichiometric Sn1−xMnxTe and non-stoichiometric Sn1−yMn1.1yTe alloys were prepared through melting, quenching and spark plasma sintering to investigate their phases and thermoelectric properties. The experimental results show that the SnTe-based solid solution single phase was formed in both series of samples with lower Mn content. The minor secondary MnTe2 phase was formed in the stoichiometric Sn1−xMnxTe alloys with x≥0.06 due to the Mn needed for filling in the intrinsic Sn lattice vacancies of SnTe and volatilization lost during the preparation. The excess Mn in Sn1−yMn1.1yTe alloys compensates this need to avoid the formation of block MnTe2 phase, which changed the phase relationship and guaranteed more Mn effectively substituting Sn in SnTe. All the samples show p-type conduction. The substitution of Mn in SnTe enhances its Seebeck coefficient and reduces its thermal conductivity. Much greater improvement on thermoelectric properties was found in the non-stoichiometric Sn1−yMn1.1yTe alloys, resulting in the maximum figure of merit ZT of 1.22 in Sn0.9Mn0.11Te alloy at 873K, due to more Mn substituting for Sn in SnTe lattice. The excess Mn in the non-stoichiometric Sn1−yMn1.1yTe alloys is important for preparing the Mn-doped SnTe materials with high thermoelectric property.

Band and scattering tuning for high performance thermoelectric Sn1−xMnxTe alloys

The thermoelectric figure of merit zT of SnTe, an analogue to PbTe, has long been known to be about 0.6, mainly due to its single band transport behavior. Similar to what has been found in PbTe, alloying with group II monotellurides such as CdTe, HgTe, MgTe enables a reduced energy separation between the two valence bands of SnTe, leading to converged bands for a significantly increased zT to ∼1.1 with demonstrated further improvements by other independent strategies such as nanostructuring. Here we show alloying with highly soluble MnTe not only tunes the band structure but also reduces the lattice thermal conductivity, leading to a record zT of ∼1.3 at 900 K in the alloy form that does not rely on additional mechanisms for lattice thermal conductivity reduction. This work demonstrates Sn1−xMnxTe as a potential alternative for PbTe with toxic Pb.

Effects of Mn substitution on the phases and thermoelectric properties of Ge0.8Pb0.2Te alloy

The series of (Ge0.8Pb0.2)1−xMnxTe alloys with x=0, 0.03, 0.05, 0.08, 0.10, 0.13 and 0.18 were prepared by melting, quenching and spark plasma sintering (SPS) techniques. The effects of Mn on the phases and thermoelectric properties of the alloys were investigated. Experimental results show that all alloys are composites containing minor NaCl-type structure PbTe-based and major GeTe-based phases with the rhombohedral and the cubic structures without any phase arising from Mn. All Mn atoms were dissolved in these phases and reduce their unit cell. Mn atoms in GeTe-based phase can stabilize its high temperature cubic structure. The layer-like microstructures are formed by the Spinodal decomposition. All the samples show p-type conduction. Although the electrical resistivity for the alloy (Ge0.8Pb0.2)1−xMnxTe increases, its Seebeck coefficient increases while its thermal conductivity reduces significantly as Mn content x increases due to the enhanced phonon scattering from the solute atoms Mn in these three phases and the microstructures. As a result, the figure of merit ZT of the (Ge0.8Pb0.2)1−xMnxTe composites can be enhanced with proper Mn content. The maximum ZT of 1.3 was obtained in the sample (Ge0.8Pb0.2)0.9Mn0.1Te at 723K, which is much higher than that of Ge0.8Pb0.2Te alloy.

Half-metallicity, magnetism and electrical resistivity of Sn1−xMnxTe alloys in rock salt and zinc blende structures

This work presents the results for the magnetic properties of the compound SnTe doped with 3 d transition metal Mn from the viewpoint of potential application in spintronics . We report a systematic density-functional study of the electronic structure and magnetic properties of these ternary compounds in both rock salt and zinc blende structures. In both cases, it is the Sn-sublattice that is doped with Mn. Among the various compounds studied, we identify one as a possible half-metallic candidate: 25% Mn-doping of the Sn-sublattice in the rock salt structure. Others are either magnetic semiconductors or metals. Exchange interaction between the Mn atoms is discussed and results for resistivities for various compositions in the two structures are compared. For low Mn concentration, the zero temperature ground state of the compound is found to be semiconducting and the Mn–Mn exchange interactions are dominantly antiferromagnetic. The weakly ferromagnetic interaction and ordering found in experiments is shown to be ‘carrier induced’. It is shown that ferromagnetic interactions between the Mn atoms are caused by the presence of ‘holes’. Another likely cause is the presence of extra Te atoms in interstitial locations, which seems to suppress Mn–Mn antiferromagnetic interactions. This is supported by the earlier experimental observation that a weak ferromagnetism occurs in these alloys for low Mn concentration only if the samples are subjected to additional Te-flux. • We study electronic structure and magnetic properties of Mn-doped semiconductor SnTe. • We find one antiferromagnetic half-metallic case: Sn 3 Mn 1 Te 4 in rock salt structure. • We show that the presence of ‘holes’ may turn antiferromagnetic Mn–Mn interactions to ferromagnetic. • Results are compared with available experimental data.

Thermopower enhancement in Pb1−xMnxTe alloys and its effect on thermoelectric efficiency

The Seebeck coefficient of p-type PbTe can be enhanced at 300 K, either due to the addition of Tl-resonant states or by manipulation of the multiple valence bands by alloying with isovalent compounds, such as MgTe. PbTe alloyed with MnTe shows a similar thermopower enhancement that could be due to either mechanism. Here we investigate the characteristics that distinguish the resonant state mechanism from that due to multiple valence bands and their effect on the thermoelectric figure of merit, zT. Ultimately, we find that the transport properties of PbTe alloyed with MnTe can be explained by alloy scattering and multiple band model that result in a zT as high as 1.6 at 700 K, and additionally a ∼30% enhancement of the average zT. Thermoelectric materials, which can convert a heat flow into an electrical current, and vice versa, have potential for applications in cooling and in power generation. Yanzhong Pei, Jeffrey Snyder and co-workers from Tongji University in Shanghai and the California Institute of Technology have now enhanced the efficiency of a state-of-the-art thermoelectric material, lead telluride, by alloying it with manganese telluride (MnTe), and proposed a mechanism to account for this improvement. The properties of lead telluride can be improved by doping and alloying it with other species. This tuning in composition can reduce its thermal conduction or improve its electronic properties – the latter either by introducing resonant states or by utilizing multiple band conduction. Through comparisons with other doped PbTe materials, Pei, Snyder and co-workers suggest that the MnTe-induced performance enhancement arises from multiple band conduction. They also show that these band effects are more effective than the formation of resonant states. Increasing the Seebeck coefficient has long been pursued for increasing thermoelectric efficiency, but other changes in transport properties may compensate this effect and ultimately lead to no improvement in figure of merit. The Seebeck coefficient can be enhanced by either the addition of resonant states or the involvement of multiple band conduction. This work demonstrates the beneficial effect of multiple band conduction for highefficiency thermoelectrics.

Energy band gap and electrical conductivity of Cd1–xMnxTe alloys with different manganese content

Energy band gap and electrical conductivity of Cd1–xMnxTe alloys with different manganese content

First principles study of structural, electronic and magnetic properties of Mg 1− xMn xTe alloys

Density functional FP-LAPW + lo calculations have been performed to study the structural, electronic and magnetic properties of Mg 1− x Mn x Te for compositional parameter x = 0.25, 0.50, 0.75 and 1. Our calculations reveal the occurrence of ferromagnetism in these compounds in which the transition-metal atom is ordered in a periodical way thereby interacting directly with the host atoms. Results extracted from electronic band structure and density of states (DOS) of these alloys show the existence of direct energy band gap for both majority- and minority-spin cases, while the total energy calculations confirm the stability of ferromagnetic state as compared to anti-ferromagnetic state. The total magnetic moment for Mg 1− x Mn x Te for each composition is found to be approximately 5 μ B, which indicates that the addition of Mn content does not affect the hole carrier concentration of the perfect MgTe compound. Moreover, the s–d exchange constant ( N 0 α) and p–d exchange constant ( N 0 β) are also calculated which are in accordance with a typical magneto-optical experiment. The estimated spin-exchange splitting energies originated by Mn 3d states energies, i.e. Δ X (s–d) and Δ X (p–d), show that the effective potential for minority-spin is more attractive than that of the majority-spin. Also, the p–d hybridization is found to cause the reduction of local magnetic moment of Mn and produce small local magnetic moments on the nonmagnetic Mg and Te sites.

Thermal and Optical Properties of Zn1−x Mn x Te Diluted Magnetic Semiconductor Studied by Photoacoustic Spectroscopic Method

Using photoacoustic spectroscopy, the composition-dependent absorption coefficient (α), thermal diffusivity (σ), and optical bandgap (Eg) of Zn1−xMnxTe diluted magnetic semiconductor have been measured. For higher Mn compositions, the absorption spectrum of the Zn–Mn–Te system consists of three regions, viz., the high absorption region, the exponential region, and the weak absorption tail. The bandgap follows a nonlinear variation with composition, showing a downward bowing with a minimum around x = 0.31 as a consequence of the electro-negativity difference between the substituted atoms. The composition-dependent band-edge effective mass of the carriers does not show the bowing behavior indicating that the momentum matrix is not the same for all the Zn1−xMnxTe alloys due to different lattice constants. The absorption spectra show that the transition is allowed and direct.

Stretched-exponential photoionization of the DX-related centers in indium-and gallium-dopedCd1−xMnxTe alloys

The kinetics of photoconductivity build-up in indium- and gallium-dopedCd1−xMnxTe mixed crystals was studied at 80 K. The photoconductivity transients are non-exponential,due to the presence of metastable DX centers, formed by both elements in thesesemiconductors. It has been shown for the first time that the kinetics of theobserved photoconductivity transients can be properly described solely by theKohlrausch–Williams–Watts (KWW) function. This statement has been verified bythe careful analysis of the derivatives of the transients. It has been found thatthey obey short-time power-law behavior, regardless of the donor type and thecontent of manganese. This asymptotic property can be explained exclusivelyby the KWW approach. Such an approach results from a broad distribution ofrelaxation rates related to the DX centers. It is reasonable to assume that, inCd1−xMnxTe alloys, the broad distribution arises from the differences in local atomic configurations inthe vicinity of the DX centers.

Far-infrared spectra of Hg 1− xMn xTe alloys

The infrared reflection spectra of Hg 1− x Mn x Te with x=0.02, 0.04, 0.08 and 0.10 have been measured at room temperature in the spectral region from 70 to 500 cm −1. The analysis of the far-infrared spectra was made by a fitting procedure based on the dielectric function which includes spacious distribution of free carrier influence on the plasmon–phonon interaction. The influence of energy gap opening (for x≈0.05) on far-infrared reflection spectra was analyzed. In spite of the strong plasmon–LO phonon interaction, we found that the long wavelength optical phonon modes of these mixed crystals showed a two-mode behavior. The model of phonon behavior based on Genzel’s model, was developed. This theoretical model shows very good agreement with experimental results.

Far-infrared spectra of Pb 1− xMn xTe alloys

Far-infrared reflectivity spectra of Pb 1− x Mn x Te (0.0001⩽ x⩽0.1) single crystals were measured in the 10–250 cm −1 range at room temperature. The analysis of the far-infrared spectra was made by a fitting procedure based on the model of coupled oscillators. In spite of the strong plasmon–LO phonon interaction, we found that the long wavelength optical phonon modes of these mixed crystals showed an intermediate one–two mode behavior.

Mn–Te bond in the rocksalt Sn 1− xMn xTe alloys and octahedral radius of Mn: X-Ray absorption- and diffraction study

The studies of local atomic structure in Sn 1− x Mn x Te ( x<0.07) alloys, with an attention to the nature of Mn–Te bond, were performed for the first time with use of extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) techniques, supplemented by photoelectron measurements. XRD unanimously indicated a single phase NaCl-type structure of these compounds. Analysis of Mn K-edge EXAFS data provided us with the bond length and Debye–Waller (DW) factor for Mn–Te bond in octahedral coordination of rocksalt Sn 1− x Mn x Te. The magnitude of DW factor suggested a moderate ionicity of Mn–Te bond (in the host SnTe matrix) indicative of its mixed covalent–ionic character. The determined value of Mn–Te bond length in Sn 1− x Mn x Te enabled us to estimate the octahedral covalent radius of Mn.

Differential thermal analysis of Hg 1− xMn xTe alloys in the x=0–0.3 range

The solidus and liquidus temperatures of seven Hg 1− x Mn x Te ( x=0–0.3) alloys were determined using differential thermal analysis. The phase diagram developed from these phase equilibrium measurements was used to clarify uncertainties in the existing Hg 1− x Mn x Te phase diagram and enabled the authors to calculate the variation of the segregation coefficient with solidus composition for the Hg 1− x Mn x Te system in the x=0–0.3 range.

Room temperature photoinduced Faraday rotation in Hg1−xMnxTe alloys at 1550 nm

Faraday rotation of the polarization for a 1550 nm probe beam induced by a pump beam at shorter wavelength is studied in Hg1−xMnxTe semimagnetic semiconductors. Photoinduced rotation decays in a nanosecond timescale and it is related to carrier-induced refractive index saturation. Observed decay is discussed in terms of photoexcited carrier dynamics.

Uncooled 10.6 μm mercury manganese telluride photoelectromagnetic infrared detectors

Photoelectromagnetic PEM detectors for 10.6 μm utilizing Hg1−xMnxTe alloys have been designed and fabricated. Numerical and experimental analysis of the PEM effect has been performed. The normalized responsivity Rv detectivity D* and time constant τ were calculated using Lile’s generalized theory of the PEM effect. The responsivity (Rv=0.1 V/W) and detectivity (D*=1.2×107 cm Hz1/2 W−1 ) were achieved with p-type of Hg1−xMnxTe having the composition x∼0.08 and acceptor concentration ND of about 2×1017 cm−3. The experimental values of Rr and D* are a factor of 3 below the calculated values.

Far-infrared absorption spectra of Cd1−xMnxTe from 10 to 300 K

Absorptivity spectra are presented in the far-infrared (15–700 cm−1) from 10 K to room temperature for Cd1−xMnxTe alloys (x=0–0.64); this covers almost the entire composition range in which the cubic structure is stable. The results are interpreted in terms of the known phonon spectra of these materials, as determined from far-infrared reflectivity, neutron scattering, and Raman scattering measurements, and in terms of anharmonic effects due primarily to an impurity-induced disorder.

Explanation of anomalous Curie constants of Cd 1−xMn xTe alloys

The anomalous Curie-constant data for Cd 1−xMn xTe alloys are explained in terms of spin-polarization of the valence band s and p states by Mn. The theory, which is based on an a priori spin-unrestricted pseudofunction local-density approximation, illustrates the consequences of Hund's first rule in solids.