Ge Sites Research Articles

Kinetic origin of hysteresis and the strongly enhanced reversible barocaloric effect by regulating the atomic coordination environment

Hysteresis is an inherent property of first-order transition materials that poses challenges for solid-state refrigeration applications. Extensive research has been conducted, but the intrinsic origins of hysteresis remain poorly understood. Here, we report a study of the kinetic origin of hysteresis and the enhanced barocaloric effect (BCE) in MnCoGe-based alloys with ~2% nonmagnetic In atoms. First-principles calculations demonstrate that substituting In atoms at Ge sites rather than Co sites results in a lower energy barrier, indicating a narrower hysteresis for the former. Combining neutron powder diffraction (NPD) with magnetic and calorimetric measurements completely verified the theoretical prediction. Electron local function (ELF) calculations further reveal the atomic coordination origin of regulated hysteresis due to weaker Co–Ge bonds when In atoms replace Ge, which is opposite to Co sites. Moreover, we experimentally investigate the BCE and find that although MnCo(Ge0.98In0.02) has a lower barocaloric entropy change ΔSP than does Mn(Co0.98In0.02)Ge, the reversible ΔSrev of the former is advantageous owing to a smaller hysteresis. The maximum ΔSrev of MnCo(Ge0.98In0.02) is 1.7 times greater than that of Mn(Co0.98In0.02)Ge. These results reveal the atomic-scale mechanism regulating hysteresis and provide insights into tailoring the functional properties of novel caloric refrigeration materials.

An ab-initio investigation of Sc and Y doped chalcopyrite MgGeN[formula omitted]

The doping of Scandium and Yttrium within chalcopyrite MgGeN2 were computationally investigated using density function theory (DFT) calculations. By calculating the formation energy, it was shown that independent single Sc and Y dopants energetically favor the occupation of the Mg site, inducing a shallow level within the conduction band. However, in the presence of a substitutional defect, the second dopant will occupy the neighboring Ge site. There is strong attractive interaction between two dopants, so that they are easily combined into a composite defect. In this case a shallow defect level appears above the valence band edge. Similar behavior is observed for the composite defects containing one dopant and an intrinsic cation vacancy, while those with one dopant and a nitrogen vacancy induce a deep defect level and greatly reduce the band gap. The investigation deepens the knowledge of Sc and Y doped II-IV-V2 compounds and may hence lead to more implementations.

Understanding the structural relation and electrochemical evolution between ZnGeP2 and ZnSiP2 twin phosphides for advanced Li-ion batteries

Ternary ZnGeP2 with large capacity and suitable plateau is considered to be alternative anode for LIBs, while it suffers heavy-use of Ge raw-material inducing high cost-price thus failure of commercialization. Herein, a unique atomic substitution strategy is proposed by using Si-atom to totally replace Ge site in ZnGeP2. Interestingly, novel ZnSiP2 as twin-analogue is achieved, keeping the great advantages of high ICE (90 %), low plateau (∼0.35 V) and comparable rate performance (4000 mA/g, 629 mAh/g) for LIBs. More specially, such ZnSiP2 twin even exhibits higher thermo-stability (1046 vs 891 °C), larger capacity (1554 vs 1387 mAh/g) and lower voltage-hysteresis (0.28 vs 0.44 V) than ZnGeP2, which are unexpected and attractive for battery-safety and energy improvement. The above unusual phenomena are investigated in-detail, and the results show that owing to the similar radius and electronegativity between Si&Ge, the ZnSiP2 enables the same structural phase with ZnGeP2 matrix thus keeping its electrochemical advantages. Meanwhile, the ZnSiP2 exhibits larger formation energy (−1.24 eV) toward better structural-stability, lighter Si-component to higher capacity, lower Si-lithiated potential oriented smaller voltage-hysteresis, thus showing high performances in LiCoO2//ZnSiP2 full-cell. The above atomic substitution strategy can be easily extended to other good performance yet high cost electrode materials toward application.

Realizing high average zT in GeTe through band modulation and suppressing Ge vacancies

GeTe is regarded as an excellent thermoelectric material, while its intrinsically high hole concentration impedes its further enhancement in thermoelectric performance. In this work, we introduce a two-step strategy by Bi and Y co-doping and Pb alloying to optimize the thermoelectric properties of the GeTe compound. Aliovalent Bi doping on the Ge site is found to effectively reduce the hole concentration, and a small substitution of Bi by Y atoms can further increase the effective mass, resulting in a high thermoelectric figure of merit (zT) of ∼ 1.92 at 723 K. Alloying Pb with high concentration elaborately decreases the carrier concentration to around 1.09 × 1020 cm−3, leading to an enhancement of Seebeck coefficient. In addition, Pb alloying maintains stable carrier mobility, contributing to a high power factor. PbTe alloying induces a strong point defect scattering and strain field fluctuation, leading to a low lattice thermal conductivity. Benefiting from the synergistically optimized effects of Bi and Y co-doping and Pb alloying, a peak zT of ∼ 2.26 at 723 K and an average zT of 1.64 over 300–773 K are realized in Ge0.8Pb0.15Bi0.04Y0.01Te.

Efficient rhombohedral GeTe thermoelectrics for low-grade heat recovery

The existence of Ge vacancy induces an inherently hole concentration up to 1021 cm−3 in pristine thermoelectric GeTe, which results in a need of ∼10 % aliovalent dopants for optimization. However, the extra carrier scattering by the dopants would notably deteriorate the carrier mobility. In this work, CuSbTe2-alloying is revealed to increase the formation energy of Ge-vacancy, leading to a rapid reduction in hole concentration down to 1.3 × 1020 cm−3 but with a mobility above 100 cm2/V-s. Along with a further Pb-substitution at Ge sites for lattice thermal conductivity reduction and band alignment, a peak figure of merit zT of ∼2.4 at 625 K is eventually realized in the rhombohedral structure, leading to a specific power density of as high as 70 W/m and a conversion efficiency of ∼10 % under a temperature difference of 302 K. This illustrates r-GeTe as truly efficient thermoelectrics particularly for low-grade heat recovery applications.

The impact of Si substitution on Ge sites in full heusler compound Ru2VGe: stability analysis

The impact of Si substitution on Ge sites in full heusler compound Ru2VGe: stability analysis

Fabrication of Cu2Sn1-xGexS3 Thin-Film Solar Cells via Sulfurization of Cu2GeS3/Cu2SnS3 Stacked Precursors.

Cu2Sn1-xGexS3 (CTGS) is a compound composed of relatively abundant elements in the crust of the earth. The band gap of CTGS can be tuned by substituting elements at the Sn and Ge sites, making it an attractive material for low-environmental-impact solar cells. In this study, CTGS thin films were fabricated with a controlled [Ge]/([Ge] + [Sn]) composition ratio (x) by combining the co-evaporation method and sulfurization in an infrared furnace. Furthermore, the effect of Na on the CTGS and changes in the solar cell properties were investigated by stacking and sulfurizing NaF on the precursor fabricated using the co-evaporation method. As a result, CTGS with varying x was successfully fabricated by varying the deposition time of the Cu2GeS3 layer using co-evaporation. Additionally, CTGS prepared by doping with Na showed enlarged CTGS crystals compared to Na-free CTGS. The fabricated CTGS solar cells achieved a power conversion efficiency of more than 4.5% after doping with Na.

Structural and electronic transformations of GeSe2 glass under high pressures studied by X-ray absorption spectroscopy

Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above [Formula: see text] 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure.

Investigation on the valence state stability and optical properties of Mg2GeO4:Cr

In this work, the valence states and optical properties of chromium-doped Mg2GeO4 are investigated via first-principles methods. The formation energies and thermal charge transition levels show that the site occupation and valence states of chromium are strongly related to the growth condition: Cr3+ dominates in the Mg sites under Mg-poor conditions, and Cr4+ dominates in the Ge sites under Ge-poor conditions. With the lithium co-doping, the Fermi-level is pinned below the midgap, and the formation energies of chromium-doped Mg sites become the lowest either Mg-poor or Ge-poor conditions, suggesting that the co-doping of lithium ions promotes the chromium to preferentially occupy Mg sites, in good agreement with experiments. Furthermore, the excitation and emission processes of chromium ions are investigated, and our results recommend that the near-infrared emission is related to the 4T2 to 4A2 transition of Cr3+ at the Mg sites and the infrared emission is related to the 3T2-3A2 transition of Cr4+ at the Ge site. Our results help to understand the stability of the valence states of chromium ions doped Mg2GeO4 and the effects of ions co-doping in regulating the chromium valence states, as well as the corresponding optical properties of different types of chromium ions.

Realizing High Thermoelectric Performance in GeTe‐Based Supersaturated Solid Solutions

AbstractAs a highly promising thermoelectric material in the mid‐temperature region, pristine GeTe shows deteriorated thermoelectric properties due to the low Seebeck coefficient and the intrinsic high thermal conductivity. Herein, it is discovered that the introduction of strongly correlated d, f electrons by alloying rare‐earth atoms at Ge sites will increase the band effective mass and promote band convergency, which can largely improve the electric transport property. Moreover, the heavy atoms (Pb, Bi) doping induces optical phonon softening and the avoided crossing between acoustic and optical phonon branches, decelerating both optical and acoustic phonon velocities. The modulated composition wave from the fluctuated multi‐component distribution introduces a complicated hierarchical structure including point defects and nanoprecipitates, which provides all‐scale scattering sources for heat‐carrying phonons and results in low lattice thermal conductivity. Consequently, a high zT of 2.4 at 800 K can be obtained in the Ge0.86Pb0.09Bi0.03Ce0.005Te sample due to the combination of strong correlation for d, f electrons and the full‐spectrum scattering for phonons. This work provides an effective strategy to increase the thermoelectric performance of IV–VI compounds by combining the modulated band structure and the softening phonon dispersion.

Giant magnetocaloric effect in the Co-doped Tb5Si2Ge2 by establishing magnetostructural coupling

The magnetostructural coupling materials exhibit excellent magnetocaloric properties, making them ideal candidates for magnetic refrigerants. Tb5Si2Ge2 reveals diverse phase transition behaviors, but its structural and magnetic phase transitions are decoupled, which restricts its potential applications in magnetic refrigeration technology. Here, we demonstrate that upon the addition of Co into the Ge site of Tb5Si2Ge2, the compound can obtain a strong magnetostructural coupling between the structural and the magnetic transition, which can significantly improve the magnetocaloric properties. It is observed that the magnetic entropy change under a magnetic field change of 5 T is increased from − 18.30 Jkg−1K−1 to − 31.34 Jkg−1K−1 after adding the magnetic element Co into the compound. The temperature-averaged entropy change (TEC) at the working range of 3 K under 1 T is also improved 2.1 times, exhibiting an enhanced low-field magnetocaloric performance. In addition, theoretical calculations show that a large lattice entropy change of 21.5 Jkg−1K−1 can be achieved after the magnetostructural transformation in the Co-doped compound. The results demonstrate that Co doping can be used to construct a stable magnetostructural transition, transforming Tb5Si2Ge2 into an excellent magnetic refrigerant.

Insight into physical properties of carbon-doped BeSiP2 and BeGeP2 chalcopyrite: An ab initio study

Our present article reports the carbon substitution-driven changes in the behavior of BeSi1-xCxP2 and BeGe1-xCxP2 (x = 0.25 and 0.5) chalcopyrite alloys using the full-potential augmented plane wave plus local orbital (FP-APW + lo) based first-principles calculations. The lattice constants (a and c) of BeSi1-xCxP2 and BeGe1-xCxP2 are shortened, and the bulk moduli are improved as carbon atoms replace Si and Ge atoms. The computed negative formation energy (Ef) indicate adequate thermodynamic stability of the BeSi1-xCxP2 and BeGe1-xCxP2 alloys. The bandgaps pristine BeSiP2 and BeGeP2 computed as 1.84 eV and 1.54 eV, respectively, using the modified Tran-Blaha (TB) Becke Johnson (mBJ) transition potential, matching well to the experimental results. However, the C substitution is found to narrow the bandgap of BeSiP2 and BeGeP2 considerably. Similarly, C substitution is found to reduce electrons' and holes’ effective mass and enhance the absorption coefficient of BeSi1-xCxP2 and BeGe1-xCxP2 alloys up to (α∼105cm−1). Our predictions indicate that the physical properties of BeSiP2 and BeGeP2 chalcopyrite can be modified effectively by replacing C in the Si and Ge sites, which makes them promising for optoelectronic applications.

EXAFS investigations on the pressure induced local structural changes of GeSe2 glass under different hydrostatic conditions

Pressure-induced transformations in glassy GeSe2 have been studied using the x-ray absorption spectroscopy. Experiments have been carried out at the scanning-energy beamline BM23 (European Synchrotron Radiation Facility) providing a micrometric x-ray focal spot up to pressures of about 45 GPa in a diamond anvil cell. Both Se and Ge K-edge experiments were performed under different hydrostatic conditions identifying the metallization onsets by accurate determinations of the edge shifts. The semiconductor-metal transition was observed to be completed around 20 GPa when neon was used as a pressure transmitting medium (PTM), while this transition was slightly shifted to lower pressures when no PTM was used. Accurate double-edge extended x-ray absorption fine structure (EXAFS) refinements were carried out using advanced data-analysis methods. EXAFS data-analysis confirmed the trend shown by the edge shifts for this disordered material, showing that the transition from tetrahedral to octahedral coordination for Ge sites is not fully achieved at 45 GPa. Results of present high pressure EXAFS experiments have shown the absence of significant neon incorporation into the glass within the pressure range up to 45 GPa.

Phosphorus diffusion and deactivation during SiGe oxidation

Dopant profiles near the semiconductor–oxide interface are critical for microelectronic device performance. As the incorporation of Si1−xGex into transistors continues to increase, it is necessary to understand the behavior of dopants in Si1−xGex. In this paper, the diffusion and electrical activation of phosphorus within a strained, single-crystal Si0.7Ge0.3 layer on Si during oxidation are reported. Both layers were uniformly doped, in situ, with an average phosphorus concentration of 4 × 1019 atoms/cm3. After high-temperature oxidation, secondary ion mass spectrometry measurements revealed that the bulk of the phosphorus diffuses out of only the SiGe layer and segregates at the oxidizing SiGe–SiO2 interface. Hall effect measurements corroborate the observed phosphorus loss and show that the phosphorus diffusing to the oxidizing interface is electrically inactive. Through density functional theory (DFT) calculations, it is shown that phosphorus interstitials prefer sites near the SiGe–SiO2 interface. Finally, based on a combination of experimental data and DFT calculations, we propose that the phosphorus atoms are displaced from their lattice sites by Ge interstitials that are generated during SiGe oxidation. The phosphorus atoms then migrate toward the SiGe–SiO2 interface through a novel mechanism of hopping between Ge sites as P–Ge split interstitials. Once they reach the interface, they are electrically inactive, potentially in the form of interstitial clusters or as part of the reconstructed interface or oxide.

Structural characteristics of mixed nido‐[Si9−xGex]4− (x=1, 2) Zintl clusters in solution and within solvent crystals

AbstractMixed Zintl‐type anionic clusters of silicon and germanium comprise an important class of potential materials in electronic applications. Herein, we present quantum chemical investigations to describe the mixing behavior of Si and Ge within [Si9−xGex]4− (x=0, 1, 2) Zintl anions that can be considered as precursors for novel Si−Ge materials. To understand the mixing behavior of such clusters in solution, we analyze the molecular clusters at using Density Functional Theory (DFT) and Coupled Cluster methods. Systematic assessment of relative energies of various structural isomers indicates that there is a preference to substitute Si with Ge in the open square of the monocapped square antiprismatic [Si8Ge]4− and [Si7Ge2]4− clusters. Population analysis reveals that the highest negative partial charge is also located at these positions. Investigation of Rb4Si9−xGex(NH3)5 solvent crystals using DFT methods and periodic boundary conditions further elucidates the behavior during crystallization. It is shown that in addition to the favored [Si8Ge]4− open‐square isomers arising in solution, the weak intermolecular interactions in the crystalline environment affect which structural isomers are observed in the crystal structure. The experimentally observed Ge site occupations could be explained by analysis of their energetics. This analysis contributes to the understanding of the mixing behavior of these important building blocks for material design.

Computational Investigation of a NASICON-Type Solid Electrolyte Material LiGe2(PO4)3

Phosphate-based electrolyte materials are of great interest in the field of Li-ion batteries due to their rigid structural integrity. LiGe2(PO4)3 is a NASICON-type phosphate material with high thermal and electrochemical stability. Computational simulation techniques were employed to study the defects, diffusion, and dopant properties of LiGe2(PO4)3. Furthermore, the reaction energies for the formation of LiGe2(PO4)3 and the incorporation energies for the insertion of additional Li into this material were calculated. The calculations revealed that the Li Frenkel is the lowest-energy defect. The second most favorable defect is the Ge-P anti-site defect cluster. A low Li migration energy of 0.44 eV implies high Li ionic conductivity. The most favorable isovalent dopants on the Li and Ge sites are Na and Si, respectively. The formation of Li interstitials and oxygen vacancies can be facilitated through the doping of Ga on the Ge site. The doping of Ga slightly enhances the Li ionic conductivity. Li incorporation (up to four Li) is thermodynamically feasible.

Defect Properties of Li2NiGe3O8

There is a growing interest in finding a suitable electrolyte material for the construction of rechargeable Li-ion batteries. Li2NiGe3O8 is a material of interest with modest Li-ionic conductivity. The atomistic simulation technique was applied to understand the defect processes and Li-ion diffusion pathways, together with the activation energies and promising dopants on the Li, Ni, and Ge sites. The Li-Ni anti-site defect cluster was found to be the dominant defect in this material, showing the presence of cation mixing, which can influence the properties of this material. Li-ion diffusion pathways were constructed, and it was found that the activation energy for a three-dimensional Li-ion migration pathway is 0.57 eV, which is in good agreement with the values reported in the experiment. The low activation energy indicated that Li-ion conductivity in Li2NiGe3O8 is fast. The isovalent doping of Na, Fe and Si on the Li, Ni and Ge sites is energetically favorable. Both Al and Ga are candidate dopants for the formation of Li-interstitials and oxygen vacancies on the Ge site. While Li-interstitials can improve the capacity of batteries, oxygen vacancies can promote Li-ion diffusion.

Distribution Characteristics and Influencing Factors of Germanium in Soil in the Eastern Mountainous Area of the Nanyang Basin

Based on the land quality geochemical survey results in the southwest cultivated area of Nanyang Basin, the content, spatial distribution, and enrichment characteristics of Ge in surface soil (0-20 cm) and deep soil (150-200 cm) in the eastern mountainous area of Nanyang Basin were studied, and the influencing factors of Ge in the surface soil were analyzed. The results showed that the average ω(Ge) in the surface soil and deep soil were 1.39 mg·kg-1 and 1.45 mg·kg-1, respectively. In the study area, 32.22% of surface soil and 12.77% of surface soil was rich in Ge, and the rich areas of the surface soil Ge were mainly distributed in the metamorphic rock and granite-dominant development areas. The optimal theoretical model of surface soil Ge variogram in the study area was a spherical model, and the nugget effect value was 0.434, indicating that surface soil Ge had moderate spatial correlation due to the joint influence of random factors and structural factors. The enrichment factor showed that 93.61% of Ge sites in the topsoil were mainly affected by natural factors, whereas 6.39% of Ge sites were significantly affected by human factors. The source of Ge in soil in the study area was mainly affected by the parent materials of soil formation, but the enrichment of Ge in surface soil was mainly affected by the Fe, Mn oxides, quartz, and pH in the soil.

Band Modulation and Strain Fluctuation for Realizing High Average zT in GeTe

AbstractRecently, GeTe has emerged as a very promising thermoelectric material. However, the pristine GeTe exhibits intrinsically low Seebeck coefficient and high thermal conductivity. Therefore, resolving these issues is critical for further promoting its thermoelectric performance. Herein, PbTe and AgSbTe2 are chosen to form a solid solution with GeTe. Benefitting from the converged electronic bands and reduced thermal conductivity after AgSbTe2 alloying, the thermoelectric performance of GeTe‐based materials is effectively improved. In addition, PbTe alloying can optimize the carrier concentration and further improve the effective mass, and thus a remarkable enhancement in the Seebeck coefficient is realized. Moreover, intense phonon scattering occurs due to the introduced point defects at the Ge site, and the lattice thermal conductivity is effectively reduced to be as low as ≈0.47 W m−1 K−1. As a result, a high zT of ≈0.7 at 323 K and ≈2.4 at 723 K can be achieved in (Pb0.15Ge0.85Te)0.8(AgSbTe2)0.2, leading to a record‐high average zT of ≈1.8 and a high conversion efficiency of ≈14.8% under the temperature difference of 500 K.

The effect of rare earth element doping on thermoelectric properties of GeTe

GeTe has attracted extensive attention due to high thermoelectric performance. Herein, we comprehensively investigate the effect of rare earth element (Eu, Gd, Er and Tm) substitutions at Ge site of GeTe on its thermoelectric performance. It is found that the substitution limit of Eu, Gd, Er and Tm at Ge site of GeTe increases from 0.8 to 1.8 mol.% with increasing atomic number. The local magnetic moment arising from the substituted rare earth element enhances the Seebeck coefficient and in turn results in enhanced power factor at the entire measurement temperature range (from 303 to 723 K). Additionally, the increasing substitution limit with increasing atomic number of rare earth element strengthens the mass and strain-field fluctuations, which contributes to strengthened phonon scattering, and leads to reduced lattice thermal conductivity. The simultaneously enhanced power factor and reduced lattice thermal conductivity enhances the dimensionless figure of merit at the entire measurement temperature range, and correspondingly enhances the average dimensionless figure of merit from 0.38 of GeTe to 0.63 of Ge0.98Er0.02Te. This work indicates rare earth element can be used an effective dopant in GeTe to improve the thermoelectric properties.