Metallic Transport Behavior Research Articles

Half metallic ferromagnetic and transport behavior of rare earth based CdGd2(S/Se)4 spinels

Spinel chalcogen compounds have garnered significant interest in recent ages because of their promising potential in optoelectronic and thermoelectric device applications. Here, we undertake an inclusive analysis of the structural, mechanical, electronic, magnetic, optical, and transport response of CdGd2(S/Se)4 utilizing WIEN2k code based on DFT. The determination of the crystal structure's ground state and the material's thermodynamic stability is investigated through energy vs. volume plots and ∆Hf calculation. Our analysis, including Poisson (v) and Pugh (B/G) ratios, reveals the ductile nature of the studied composition. Band gap assessments are conducted using the TB-mBJ approach, revealing the semiconducting nature of investigated compounds with direct band characteristics. Specifically, calculated band gap values are 2.2 for CdGd2S4 and 1.8 eV for CdGd2Se4. Additionally, analysis of optical properties and transport response underscores the perspective of these compositions for diverse optoelectronic and thermoelectric devices.

Realizing metallicity in Sr2IrO4 thin films by high-pressure oxygen annealing

Perovskite iridates are a promising material platform for hosting unconventional superconductivity. Transport measurements of Sr2IrO4 thin-film field-effect transistors are expected to provide irrefutable evidence for the existence of superconductivity. However, these experiments have revealed a remarkably robust insulating state over wide electron and hole doping ranges; this finding is in contrast to the case of the bulk material, in which metallicity appears upon moderate electron doping by substituting cations in place of Sr. The nature of this robust insulating state and whether any metallic state can be realized in the Sr2IrO4 thin film are two remaining challenges that preclude further progress in the search for superconductivity in this system. Here, we show that this insulating state is enhanced in Sr2IrO4 thin films by thermal annealing under vacuum conditions, while it can be destroyed upon annealing in an oxygen atmosphere within restricted ranges of oxygen pressure, annealing temperature and ion substitution levels. The resulting films exhibit metallic transport behavior near room temperature and a metal–insulator crossover at ~200 K. Our results point to the potentially important roles of the oxygen vacancies at different atomic sites in the formation of the robust insulating state and the new metallic state and to their interplay in the Sr2IrO4 thin film. This finding opens new possibilities in the search for unconventional superconductivity by further tailoring the as-found metallic state in properly oxygen-annealed Sr2IrO4 thin films.

Air-stable van der Waals PtTe2 conductors with high current-carrying capacity and strong spin-orbit interaction

High-performance van der Waals (vdW) integrated electronics and spintronics require reliable current-carrying capacity. However, it is challenging to achieve high current density and air-stable performance using vdW metals owing to the fast electrical breakdown triggered by defects or oxidation. Here, we report that spin-orbit interacted synthetic PtTe2 layers exhibit significant electrical reliability and robustness in ambient air. The 4-nm-thick PtTe2 synthesized at a low temperature (∼400°C) shows intrinsic metallic transport behavior and a weak antilocalization effect attributed to the strong spin-orbit scattering. Remarkably, PtTe2 sustains a high current density approaching≈31.5 MA cm-2, which is the highest value among electrical interconnect candidates under oxygen exposure. Electrical failure is caused by the Joule heating of PtTe2 rather than defect-induced electromigration, which was achievable by the native TeOx passivation. The high-quality growth of PtTe2 and the investigation of its transport behaviors lay out essential foundations for the development of emerging vdW spin-orbitronics.

(Invited) Metallic Transport Behaviors in Monolayer and Multi-Layer MoS2 By Surface-Charge Transfer Interaction with Redox-Active Molecules

In the last decade, transition metal chalcogenides (TMDCs), e.g., MoS2 and WSe2, have gathered attention as semiconducting 2D materials. Modulation of carrier concentrations in TMDCs is a crucial matter for applying to semiconductor devices. So far, we have developed a method to gain the carrier concentrations of MoS2 by a junction with organic molecules and achieved obtaining a degenerately doped state in the MoS2. The mechanism of the doping is a surface-charge transfer interaction at the interface. Although the transfer characteristic behaviors show the degenerately doped state (small ON/OFF ratio), the details of the transport behavior have not been well understood. In this presentation, I will discuss the details of the device characteristics of the monolayer and multilayer MoS2 metal-oxide-semiconductor field-effect-transistors (MOSFETs) with a junction of redox-active molecules in a manner of the surface-charge transfer interaction. The devices were fabricated via standard lithography (e-beam and photolithography) techniques to prepare each monolayer and multilayer MoS2 MOSFETs. Transport properties in the MoS2 MOSFETs were measured in the as-prepared device and the device after the molecular doping. The dopant molecule is benzyl viologen (BV) molecule which is known as a redox-active molecule with high-reduction potential. After the doping with BV molecules, the MoS2 MOSFET showed a small ON/OFF ratio in the transfer characteristic curves, indicating a degenerately doped state. The identical device further showed the metallic behavior in the temperature dependence (the conductivity increases with decreasing the temperature). Both monolayer and multilayer MoS2 MOSFETs showed similar metallic behavior. This metallic transport behavior changes to an insulative regime when applying gate-voltage to reduce the carrier concentrations in the channel MoS2. Therefore, the gate-electrostatic induced metal-insulator-transition was suggested. At the conference, I will discuss the details of the transport behaviors, gate-potential and temperature dependence of the contact resistance. In addition, I will also discuss potential candidates of other molecules for obtaining doping to TMDCs for going forward the 2D materials-based electronics.REF: Matsuyama, et al., "Metallic transport in monolayer and multilayer molybdenum disulfides by molecular surface-charge transfer doping", ACS Appl. Mater. Interfaces, 2022, 14, 8163-8170.

A promising layered thermoelectric metallic ceramic with ultra-high temperature stability: Mo2Ti2AlC3

As a structural ceramic, MAX phases have attracted the most attentions because of light weight, excellent machinability, good thermal shock resistance and thermal stability at ultra-high temperature. Herein, for the first time we reported the thermoelectric property of Mo2Ti2AlC3, one of MAX phases. It has similar resistivity (~2.3 μΩ·m) and higher Seebeck coefficient (−15μV/K) compared with Pt-Rh alloy. Mo2Ti2AlC3 has higher power factor (~46% improvement) and much lower thermal conductivity (6.2 W/(m·K), one-third) compared with SiC thermoelectric ceramics at 700 °C. The ZT value is about 0.0138 at 700 °C, which is three times as high as that of SiC-7%Si3N4 thermoelectric composite. These suggest that Mo2Ti2AlC3 and other MAX phases have an important application prospect in the field of thermoelectric generator and temperature sensor at ultra-high temperature. Meanwhile, although Mo2Ti2AlC3 ceramic shows metallic transport behavior, the electric thermal conductivity far deviates from traditional Wiedemann-Franz law, indicating that there are strong correlations or couplings between electrons and 2-D layered lattices, and Mo2Ti2AlC3 is a new strong related system with spin-orbital coupling.

Metallic Transport in Monolayer and Multilayer Molybdenum Disulfides by Molecular Surface Charge Transfer Doping.

Carrier modulation in transition-metal dichalcogenides (TMDCs) is of importance for applying electronic devices to tune their transport properties and controlling phases, including metallic to superconductivity. Although the surface charge transfer doping method has shown a strong modulation ability of the electronic structures in TMDCs and a degenerately doped state has been proposed, the details of the electronic states have not been elucidated, and this transport behavior should show a considerable thickness dependence in TMDCs. In this study, we characterize the metallic transport behavior in the monolayer and multilayer MoS2 under surface charge transfer doping with a strong electron dopant, benzyl viologen (BV) molecules. The metallic behavior transforms to an insulative state under a negative gate voltage. Consequently, metal-insulator transition (MIT) was observed in both monolayer and multilayer MoS2 correlating with the critical conductivity of order e2/h. In the multilayer case, the BV molecules strongly modulated the topmost surface layer in the bulk MoS2; the transfer characteristics suggested a crossover from a heterogeneously doped state with a doped topmost layer to doping in the deep layers caused by the variation in the gate voltage. The findings of this work will be useful for understanding the device characteristics of thin-layered materials and for applying them to the controlling phases via carrier modulation.

Ionic Liquid Gating Control of Oxygen Vacancies in the La0.8Ba0.2MnO3 Ultrathin Films

In this work, we show ionic liquid field-effect of La0.8Ba0.2MnO3 (LBMO) untrathin films prepared by the sol-gel method. When applying a positive gate voltage (V g) in vacuum, it is found that the film resistivity continuously increases tenfold within hours or minutes depending on the magnitude of V g, accompanied with disappearance of the low-temperature metallic transport behavior. In contrast, the film resistivity changes little in air or under a negative applied V g. Such a difference reveals that the increased resistivity is related to the oxygen depletion in the films under the positive V g, especially at the grain boundaries. After removing the positive V g in vacuum, the room-temperature resistivity begins to fall off and the low-temperature metallic state is partially restored in several tens hours, suggesting oxygen diffusion from the relatively oxygen-rich grains to the seriously oxygen-deficient grain boundaries. Furthermore, the oxygen content in the oxygen-deficient films can be almost fully restored in ten minutes by external annealing (200 °C) or Joule self-heating (21 mA) the films in air. These experimental findings provide an additional method in tuning oxygen vacancies in transition-metal oxide films.

Reconciling the bulk metallic and surface insulating state in 1T−TaSe2

The origin of different electronic states of $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ and $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ remains controversial due to the complicated correlated electronic properties. We apply scanning tunneling microscopy to study the electronic state of bulk $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$. Both insulating and metallic states are identified in different areas of the same sample. The insulating state is similar to that in $1T\ensuremath{-}{\mathrm{TaS}}_{2}$, concerning both the $dI/dV$ spectrum and the orbital texture. With detailed investigations in single-step areas, the electronic state measured on the upper-layer surface is found to be associated with different stacking orders and the lower layer's electronic state. The insulating state is most possibly a single-layer property, perturbed to a metallic state by particular stacking orders. Both the metallic and large-gap insulating spectra, together with their corresponding stacking orders, are stable states in $1T\text{\ensuremath{-}}{\mathrm{TaSe}}_{2}$. The connected metallic areas lead to the metallic transport behavior. We then reconcile the bulk metallic and surface insulating state in $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$. The rich phenomena in $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ deepen our understanding of the correlated electronic state in bulk $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ and $1T\ensuremath{-}{\mathrm{TaS}}_{2}$.

Metallic transport properties and electrostatic resistance modulations in LaNiO3 ultrathin channels electrochemically etched in electric-double-layer transistors

We fabricated electric double-layer transistors with perovskite nickelate LaNiO3 epitaxial films as a channel and an ionic liquid as a gate insulator and investigated the channel's transport behavior at negative gate bias voltages. We found that when the negative VG is low enough, the channel resistance undergoes reversible changes due to electrostatic carrier accumulation. On the other hand, with increasing VG, electrochemical decomposition of the channels due to the VG-induced instability of the unusually high Ni valence state [Ni(3+δ)+] occurs and the channels are etched, which is seen as an irreversible increase in the channel resistance. We also show that channel layers electrochemically etched to a few nanometers thick exhibit metallic transport behavior and the electrostatic carrier accumulation in the etched metallic channels results in 30% reductions in the channel resistance at 220 K, which are about twice as large as those seen for the ultra-thin insulating film channels. These observations indicate that surface defects and dangling bonds in the electrochemically etched channel layers differ from those in ultrathin films. Our results highlight that the electrochemical decomposition and etching in electric double-layer transistors is a unique approach for fabricating ultrathin oxide films and exploring electric field-induced effects on their functional properties.

Electronic and magnetic properties of α−FeGe2 films embedded in vertical spin valve devices

We studied metastable $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Ge}}_{2}$, a layered tetragonal material, embedded as a spacer layer in spin valve structures with ferromagnetic ${\mathrm{Fe}}_{3}\mathrm{Si}$ and ${\mathrm{Co}}_{2}\mathrm{Fe}\mathrm{Si}$ electrodes. For both types of electrodes, spin valve operation is demonstrated with A metallic transport behavior of the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Ge}}_{2}$ spacer layer. The spin valve signals are found to increase with both temperature and spacer thickness, which is discussed in terms of a decreasing magnetic coupling strength between the ferromagnetic bottom and top electrodes. The temperature-dependent resistances of the spin valve structures exhibit characteristic features, which are explained by ferromagnetic phase transitions between 55 and 110 K. The metallic transport characteristics as well as the low-temperature ferromagnetism are found to be consistent with the results of first-principles calculations.

Semimetallic features in thermoelectric transport properties of 2H–3R phase niobium diselenide

Abstract In two-dimensional van der Waals crystals, the interlayer stacking sequence often leads to a change in crystal symmetry and, thus, new polymorphs, leading to an abundant array of physical properties. In this paper, we report the polymorphic form of 2H–3R–NbSe2 that exhibits a substantial difference in terms of the gate dependence of semimetallic behavior and Seebeck coefficient, compared to the well-known 2H–NbSe2 with metallic transport behavior. The semimetallic features of 2H–3R–NbSe2 indicate the presence of minor carriers, confirmed through theoretical calculations, which is in good agreement with the transport behavior. Our results reveal perspectives for understanding the metastable 2H–3R phase NbSe2, which is not far from equilibrium, and for engineering the materials necessary for efficient energy harvesting.

Influence of Coulomb interaction effect on the electrical transport for few-layered PtSe2 films

The Coulomb effect is a particularly significant topic in quasi-two-dimensional systems. Here, we carried out comprehensive measurements of electrical transport properties of fewer-layer PtSe2 films with and without antidot arrays. For as-grown PtSe2 films, the sheet conductance σ□ varies linearly with the logarithm of temperature at low temperature, which can be well described by the Altshuler and Aronov electron-electron interaction (EEI) effect, and then the result was further quantitatively confirmed by the ratios of a relative change of the Hall coefficient ΔRH/RH to that of sheet resistance ΔR□/R□. While after milling of antidot arrays the PtSe2 film possesses inhomogeneous structure, it still presents metallic transport behavior. The overall σ□ and RH characteristics can be explained by the current EEI theories in granular metals. Our results not only reveal the fundamental physics of PtSe2 films, but also provide powerful experimental evidence for the presence of the Coulomb EEI effect in the family of transition metal dichalcogenides. This work presents the subtle change of charge transport properties by introducing antidot arrays, which may be of benefit in improving sensitivity and stability of nanoscale electronic devices.

Effects of Rb Intercalation on NbSe2: Phase Formation, Structure, and Physical Properties.

Here, we report the crystal structures and properties of Rb xNbSe2, with 0 ≤ x ≤ 0.5. With Rb intercalation, Rb xNbSe2 evolves from 2H (Phase I) for 0 ≤ x ≤ 0.025, to 6R (Phase II) for x ∼ 0.2 with space group R3 m (no. 166), and finally to 2H (Phase IV) for 0.375 ≤ x ≤ 0.5 with space group P63/ mmc (no. 194). In addition, Phase II is found to transform to a rare 6H structure (Phase III) with space group P63/ mmc (no. 194) by annealing at a relatively low temperature. We show the first 6H phase in the intercalated transition-metal dichalcogenides (TMDs) family obtained through a solid-state reaction. Moreover, both 6R and 6H phases are new polymorphs in the NbSe2 system. For the range 0.2 ≤ x ≤ 0.5 in Rb xNbSe2, we show metallic electronic transport behavior and a paramagnetic feature. The lack of superconductivity (SC) down to 2 K is most probably due to the decrease of hole carrier density with increasing Rb content. Through careful analysis of the structural data, we were able to assemble a phase diagram covering the range of 0 ≤ x ≤ 0.5 in Rb xNbSe2.

Thermoelectric properties of chemically substituted full-HeuslerFe2TiSn1−xSbx(x=0,0.1,and0.2)compounds

According to theoretical predictions [Phys. Rev. Lett. 114, 136601 (2015)], Sb substitution in $\mathrm{F}{\mathrm{e}}_{2}\mathrm{TiSn}$ full-Heusler compounds is thought to enhance the $n$-type thermoelectric power factors. We prepare $\mathrm{F}{\mathrm{e}}_{2}\mathrm{TiSn}$ samples with 0%, 10%, and 20% Sb substitution on the Sn site and study the effects of the heat treatment protocol and of Sb substitution by structural characterization, electrical, thermoelectrical, and thermal transport measurements from room temperature down to 10 K, and by comparison with ab initio calculations. For the undoped $\mathrm{F}{\mathrm{e}}_{2}\mathrm{TiSn}$ phase, we find a bad metallic gapless transport behavior in the as-cast sample and a small gap semiconducting behavior for the annealed samples. These observations, together with the $p$-type character emerging from the Hall effect and Seebeck effect, are explained by ab initio calculations, provided that antisite disorder and vacancy defects are included. In these undoped $\mathrm{F}{\mathrm{e}}_{2}\mathrm{TiSn}$ samples, we find that increasing thermal annealing temperature up to 800 \ifmmode^\circ\else\textdegree\fi{}C slightly decreases the carrier concentration, enhances the Seebeck coefficient in the low-temperature regime (below 100--200 K), induces magnetic ordering at low temperature $(l150\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, and improves the thermoelectric figure of merit and power factor in the low-temperature regime (below \ensuremath{\sim}200 K). As for Sb substitution, we find that 10% substitution increases hole carrier concentration and induces a weakly metallic behavior as compared to the semiconducting behavior of the corresponding undoped annealed samples. On the other hand, 20% substitution lowers the carrier density and increases the resistivity by a factor \ensuremath{\sim}50, with semiconducting behavior measured in the whole temperature range. We interpret the observed effect of Sb doping as a complement to disorder. Indeed, using first-principles techniques we unearth the experimental signature related to density of states features induced by native impurities in the realized samples. The Seebeck coefficient exhibits a maximum at around 200--250 K for the undoped and 20%-doped samples, as typical of semiconducting compounds, while it increases monotonically with temperature for the metallic 10%-doped sample. The latter behavior, joined with the good electrical properties of the 10% sample, determines the room-temperature power factor of $\ensuremath{\sim}1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ in the 10% Sb-doped sample, more than twice as much as to the value of the undoped sample.

Possible half-metallicity and variable range hopping transport in Sb-substituted Fe2TiSn Heusler alloys

The investigation of the magnetotransport properties on Sbx with 0 0.6 are presented in this paper. The substitution of Sb in place of Sn decreases the anti-site disorder as evident from x-ray diffraction patterns as well as from transport properties measurement. The much-disputed upturn in low temperature electrical resistivity of TiSn has been demonstrated to be a result of weak localization induced by anti-site disorder. With increased Sb substitution (⩾25%) the metallic transport behavior of TiSn changes to semiconductor-like. At low temperature, carrier transport in such compositions occurs via the variable range hopping mechanism. Moreover, a systematic increase in the anomalous Hall voltage is observed with increasing Sb-content, attributable to a side jump or Berry phase curvature effect. Electrical resistivity in the entire temperature regime hints towards half metallicity of the system. Our ab initio electronic structure calculations using generalised gradient approximation formalism further supports the results of our magnetotransport study.

Engineering the Charge Transport of Ag Nanocrystals for Highly Accurate, Wearable Temperature Sensors through All-Solution Processes.

All-nanocrystal (NC)-based and all-solution-processed wearable resistance temperature detectors (RTDs) are introduced. The charge transport mechanisms of Ag NC thin films are engineered through various ligand treatments to design high performance RTDs. Highly conductive Ag NC thin films exhibiting metallic transport behavior with high positive temperature coefficients of resistance (TCRs) are achieved through tetrabutylammonium bromide treatment. Ag NC thin films showing hopping transport with high negative TCRs are created through organic ligand treatment. All-solution-based, one-step photolithography techniques that integrate two distinct opposite-sign TCR Ag NC thin films into an ultrathin single device are developed to decouple the mechanical effects such as human motion. The unconventional materials design and strategy enables highly accurate, sensitive, wearable and motion-free RTDs, demonstrated by experiments on moving or curved objects such as human skin, and simulation results based on charge transport analysis. This strategy provides a low cost and simple method to design wearable multifunctional sensors with high sensitivity which could be utilized in various fields such as biointegrated sensors or electronic skin.

Effects of substrate temperature and oxygen pressure on microstructures and conductivity of Ba0.95La0.05PbO3 thin films

By using pulsed laser deposition, Ba0.95La0.05PbO3 (BLPO) thin films with different substrate temperatures (550–750°C) and oxygen pressures (5–20Pa) were prepared on SrTiO3 (100) substrates. The charge valences of Ba and Pb cations in all the films are determined to be +2 and +4 by x-ray photoelectron spectroscopy. However, surface roughness of the films doesn’t keep unchanged, it increases with increasing temperature and oxygen pressure. The resistivity is significantly affected by substrate temperature and oxygen pressure, which increases with the former and decreases with the latter. Especially, the films deposited under 600°C and 10Pa oxygen pressure show metallic transport behavior with nearly temperature-insensitive values, as evidenced by the resistivity of 6.24×10−5Ωm and 5.41×10−5Ωm at 300K and 15K, respectively. Based on the structure and transport measurements, the optimized deposition conditions are determined to be close to 600°C and 10Pa. This work may provide some helpful clues for further work on tuning properties of BLPO films.

Room-Temperature Spin Filtering in Metallic Ferromagnet-Multilayer Graphene-Ferromagnet Junctions.

We report room-temperature negative magnetoresistance in ferromagnet-graphene-ferromagnet (FM|Gr|FM) junctions with minority spin polarization exceeding 80%, consistent with predictions of strong minority spin filtering. We fabricated arrays of such junctions via chemical vapor deposition of multilayer graphene on lattice-matched single-crystal NiFe(111) films and standard photolithographic patterning and etching techniques. The junctions exhibit metallic transport behavior, low resistance, and the negative magnetoresistance characteristic of a minority spin filter interface throughout the temperature range 10 to 300 K. We develop a device model to incorporate the predicted spin filtering by explicitly treating a metallic minority spin channel with spin current conversion and a tunnel barrier majority spin channel and extract spin polarization of at least 80% in the graphene layer in our structures. The junctions also show antiferromagnetic coupling, consistent with several recent predictions. The methods and findings are relevant to fast-readout low-power magnetic random access memory technology, spin logic devices, and low-power magnetic field sensors.

Giant Pressure‐Driven Lattice Collapse Coupled with Intermetallic Bonding and Spin‐State Transition in Manganese Chalcogenides

AbstractMaterials with an abrupt volume collapse of more than 20 % during a pressure‐induced phase transition are rarely reported. In such an intriguing phenomenon, the lattice may be coupled with dramatic changes of orbital and/or the spin‐state of the transition metal. A combined in situ crystallography and electron spin‐state study to probe the mechanism of the pressure‐driven lattice collapse in MnS and MnSe is presented. Both materials exhibit a rocksalt‐to‐MnP phase transition under compression with ca. 22 % unit‐cell volume changes, which was found to be coupled with the Mn2+(d5) spin‐state transition from S=5/2 to S=1/2 and the formation of Mn−Mn intermetallic bonds as supported by the metallic transport behavior of their high‐pressure phases. Our results reveal the mutual relationship between pressure‐driven lattice collapse and the orbital/spin‐state of Mn2+ in manganese chalcogenides and also provide deeper insights toward the exploration of new metastable phases with exceptional functionalities.

Giant Pressure-Driven Lattice Collapse Coupled with Intermetallic Bonding and Spin-State Transition in Manganese Chalcogenides.

Materials with an abrupt volume collapse of more than 20 % during a pressure-induced phase transition are rarely reported. In such an intriguing phenomenon, the lattice may be coupled with dramatic changes of orbital and/or the spin-state of the transition metal. A combined in situ crystallography and electron spin-state study to probe the mechanism of the pressure-driven lattice collapse in MnS and MnSe is presented. Both materials exhibit a rocksalt-to-MnP phase transition under compression with ca. 22 % unit-cell volume changes, which was found to be coupled with the Mn(2+) (d(5) ) spin-state transition from S=5/2 to S=1/2 and the formation of Mn-Mn intermetallic bonds as supported by the metallic transport behavior of their high-pressure phases. Our results reveal the mutual relationship between pressure-driven lattice collapse and the orbital/spin-state of Mn(2+) in manganese chalcogenides and also provide deeper insights toward the exploration of new metastable phases with exceptional functionalities.