We report the structural, vibrational, and electrical transport properties up to $\ensuremath{\sim}16\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ of $1T\text{\ensuremath{-}}\mathrm{TiT}{\mathrm{e}}_{2}$, a prominent layered 2D system. We clearly show signatures of two isostructural transitions at $\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and $\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ obtained from the minima in $c/a$ ratio concomitant with the phonon linewidth anomalies of ${E}_{g}$ and ${A}_{1g}$ modes around the same pressures, providing a strong indication of unusual electron-phonon coupling associated with these transitions. Resistance measurements present nonlinear behavior over similar pressure ranges shedding light on the electronic origin of these pressure-driven isostructural transitions. These multiple indirect signatures of an electronic transition at $\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and $\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ are discussed in connection with the recent theoretical proposal for $1T\text{\ensuremath{-}}\mathrm{TiT}{\mathrm{e}}_{2}$ and also the possibility of an electronic topological transition from our electronic Fermi surface calculations. Between 4 GPa and $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, the $c/a$ ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi-3D crystal network. First-principles calculations suggest that the 2D to quasi-3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition, we observed a first-order structural phase transition from the trigonal ($P\overline{3}m1$) to monoclinic ($C2/m$) phase at higher pressure regions. We estimate the start of this structural phase transition to be $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and also the coexistence of two phases [trigonal ($P\overline{3}m1$) and monoclinic ($C2/m$)] was observed from $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ to $\ensuremath{\sim}16\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$.

/pdf/structural-vibrational-and-electrical-properties-of-1-t-tit-3ik33wkzz5.pdf

Structural, vibrational, and electrical properties of 1 T − TiT e 2 under hydrostatic pressure: Experiments and theory

Cr2Ge2Te6 is a rare layered ferromagnetic semiconductor that represents a promising material for novel thermoelectric and spintronic devices. High pressure is an effective way to change the electronic structure, and profoundly affects the physical and chemical properties of two-dimensional (2D) layered materials. Here we combined experimental results and theoretical calculations to investigate the structural evolution and properties of Cr2Ge2Te6under high pressure. It is found that Cr2Ge2Te6 undergoes an isostructural phase transition from layered to non-layered structure at ~14 GPa, accompanied with a semiconductor to metal transition, which is distinct from the previous reports. The pressure-induced metallization is verified by the overlap of the valence and conduction bands, and the layered-to-non-layered isostructural transition is attributed to the enhanced interlayer Te–Te interaction by First-principles calculations. Our results provide a potential pathway to control the structure and property of layered materials and possibility for conceptually new devices via pressure engineering.

Effects of pressure on the structure and properties of layered ferromagnetic Cr2Ge2Te6

We show that polycrystalline $\mathrm{GeS}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ in the fcc phase (f-GST), which is an insulator at low temperature at ambient pressure, becomes a superconductor at elevated pressures. Our study of the superconductor-insulator transition versus pressure at low temperatures reveals a second-order quantum phase transition with linear dependence of the transition temperature on the pressure above the critical zero-temperature pressure. In addition, we demonstrate that at higher pressures the f-GST goes through a structural phase transition via amorphization to bcc GST (b-GST), which also becomes superconducting. We also find that in the pressure regime where an inhomogeneous mixture of amorphous and b-GST exists, there is an anomalous peak in magnetoresistance, and we suggest an explanation for this anomaly.

https://link.aps.org/accepted/10.1103/PhysRevB.97.024513

Superconductor-insulator transition in fcc GeS b 2 T e 4 at elevated pressures

A practical and straightforward one-step co-electrodeposition process path for preparing electrode (Mo2CTx/PDTDA/rGO) based on poly (2, 2′- dithiodianiline) and 2D Mo2CTx-MXene was developed. The three-dimensional Mo2CTx/PDTDA composite on the rGO electrode facilitated electron transfer and ionic diffusion. Hence, the composite has been recognized as a multifunctional electrode with significant supercapattery and electrocatalytic activity. The laboratory-scale assembled symmetric supercapacitor devices (SSC) have a large energy density 82.5 mWh cm−2 (181.5 Wh kg−1) at a power density of 1000 mW cm−2 (2200 kW kg−1). In addition, the SSC device displays high durability with more than 90.5 % capacitance retention at the current density of 5 A cm−2 after 5000 cycles. The HER electrocatalytic activity of Mo2CTx/PDTDA/rGO electrode exhibited good catalytic activity, and superb stability for HER, demanded low overpotentials of 59 mV to achieve a current density of 10 mA cm−2 and also has a low corresponding Tafel slope of 44 mV dec−1 in 1 M KOH. The results based on the Mo2CTx/PDTDA show the practical application of the proposed composite as a highly efficient electrochemical catalyst in renewable green energy resources. 

Introduction of a three-dimensional flower-like Mo2CTx/poly (2, 2′-dithiodianiline) on reduced graphene oxide as an efficient electrode for supercapacitor and hydrogen evolution reaction

Here we firstly report the pressure-induced superconductivity in phase change materials SnSb2Te4. Single crystals of SnSb2Te4 were grown using a conventional melting-down method. The resistance under pressure was measured using an originally designed diamond anvil cell with boron-doped diamond electrodes. The temperature dependence of the resistance under different pressures has been measured up to 32.6 GPa. The superconducting transition of SnSb2Te4 appeared at 2.1 K ([Formula: see text]) under 8.1 GPa, which was further increased with applied pressure to a maximum onset transition temperature 7.4 K under 32.6 GPa.

Pressure-induced superconductivity in SnSb2Te4.

Here we report the discovery of superconductivity in multiple phases of the compressed $\mathrm{GeS}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ (GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous $(a$-GST) and for the crystalline $(c$-GST) phases. The superconducting critical temperature ${T}_{c}$ continuously increases with applied pressure, reaching a maximum ${T}_{c}=6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at $P=20\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ for $a$-GST, whereas the critical temperature of the cubic phase reaches a maximum ${T}_{c}=8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 30 GPa. This material system, exhibiting a superconductor-insulator quantum phase transition, has an advantage over disordered metals since it has a continuous control of the crystal structure and the electronic properties using pressure as an external stimulus.

Superconductivity in multiple phases of compressed GeS b 2 T e 4

Here we report the discovery of superconductivity in multiple phases of the compressed GeSb2Te4 (GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous (a-GST) and for the crystalline (c-GST) phases. The superconducting critical temperature, TC, continuously increases with the applied pressure reaching a maximum Tc =6K at P=20 GPa for a-GST, whereas the critical temperature of the cubic phase reaches a maximum Tc =8 K at 30 GPa. This new material system, exhibiting a superconductor-insulator quantum phase transition (SIT) has an advantage over disordered metals since it has a continuous control of the crystal structure and the electronic properties using pressure as an external stimulus, which was lacking in SIT studies until today.

Superconductivity in multiple phases of compressed GeSb2Te 4

In this work, we present high-pressure diffraction results of the Mo-based Mn + 1AXn phase, Mo2GaC. A diamond anvil cell was used to compress the material up to 30 GPa, and x-ray diffraction was used to determine the structure and unit cell parameters as a function of pressure. Somewhat surprisingly, we find that, at 295 ± 25 GPa, the bulk modulus of Mo2GaC is the highest reported of all the MAX phases measured to date. The c/a ratio increases with increasing pressure. At above 15 GPa, a splitting in the (1 0 0) reflection occurs. This result, coupled with new density functional theory calculations, suggests that a second order phase transition to possibly a mixture of hexagonal and monoclinic structures may explain this splitting. Such experimentally and theoretically supported phase transitions were not predicted in previously published calculations.

/pdf/possible-monoclinic-distortion-of-mo2gac-under-high-pressure-4hqle1y6cm.pdf

Possible monoclinic distortion of Mo2GaC under high pressure

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Superconductivity in multiple phases of compressed GeS b 2 T e 4

Superconductivity in multiple phases of compressed GeSb2Te 4

Possible monoclinic distortion of Mo2GaC under high pressure