Pressure-induced Structural Phase Transition Research Articles

Structural, vibrational, and electronic topological transitions of Bi1.5Sb0.5Te1.8Se1.2 under pressure

Topological insulators have been the subject of intense research interest due to their unique surface states that are topologically protected against scattering or defects. However, the relationship between the crystal structure and topological insulator state remains to be clarified. Here, we show the effects of hydrostatic pressure on the structural, vibrational, and topological properties of the topological insulator Bi1.5Sb0.5Te1.8Se1.2 up to 45 GPa using X-ray diffraction and Raman spectroscopy in a diamond anvil cell, together with first-principles theoretical calculations. Two pressure-induced structural phase transitions were observed: from ambient rhombohedral R3¯m phase to a monoclinic C2/m phase at ∼13 GPa, and to a disordered I4/mmm phase at ∼22 GPa. In addition, the alloy undergoes several electronic transitions within the R3¯m phase: indirect to direct bulk band gap transition at ∼5.8 GPa, bulk gap closing with an appearance of Dirac semimetal (DSM) state at ∼8.2 GPa, and to a trivial semimetal state at ∼12.1 GPa. Anomalies in c/a ratio and Raman full width at half maximum that coincide with the DSM phase suggest the contribution of electron-phonon coupling to the transition. Compared to binary end members Bi2Te3, Bi2Se3, and Sb2Te3, the structural phase transition and anomaly were observed at higher pressures in Bi1.5Sb0.5Te1.8Se1.2. These results suggest that the topological transitions are precursors to the structural phase transitions.

Half metallic ferromagnetism in Ni based half Heusler alloys

The search for emerging materials with ferromagnetic and spin flip properties has attracted widespread interest in material science. Ni based half Heusler alloys have been one of the benchmark half metallic ferromagnetic material owing to their magnetic interaction and promising figure of merit in magnetic memory element. A systematic pathway to design novel Heusler materials is by analyzing structural phase stability, electronic structure, mechanical and magnetic properties. Specifically, we carried out first principles calculations to identify the ferromagnetic and half-metallic properties of Ni based half Heusler alloys XYZ (X = Ni; Y = Cr; Z = Si, Ge, Ga, Al, In, As). The predicted phase stability shows that α-phase is found to be the lowest energy phase compared with β and γ phases. A pressure-induced structural phase transitions from α-phase to γ-phase in NiCrSi, NiCrGe, NiCrGa, NiCrAl, NiCrIn and α-phase to β-phase in NiCrAs are found. Due to the presence of d8 and d5 electronic configuration within Ni2+ and Cr2+ ions respectively, all the Heusler materials show half metallic behavior. Furthermore, the magnetic moments for these half Heusler alloys in all the three different phases (α, β and γ) have been reported. Our work paves the way for designing novel Heusler materials at normal and high pressure.

Competition between spin-orbit coupling, magnetism, and dimerization in the honeycomb iridates: α−Li2IrO3 under pressure

Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate $\alpha$-Li$_{2}$IrO$_{3}$ reveal a pressure-induced structural phase transition with symmetry lowering from monoclinic to triclinic at a critical pressure of $P_{c}$ = 3.8 GPa. According to the evolution of the lattice parameters with pressure, the transition mainly affects the $ab$ plane and thereby the Ir hexagon network, leading to the formation of Ir--Ir dimers. These observations are independently predicted and corroborated by our \textit{ab initio} density functional theory calculations where we find that the appearance of Ir--Ir dimers at finite pressure is a consequence of a subtle interplay between magnetism, correlation, spin-orbit coupling, and covalent bonding. Our results further suggest that at $P_{c}$ the system undergoes a magnetic collapse. Finally we provide a general picture of competing interactions for the honeycomb lattices $A_{2}$$M$O$_{3}$ with $A$= Li, Na and $M$ = Ir, Ru.

Pressure-induced structural phase transition in transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt): a DFT study

First-principles calculations based on density functional theory was performed to analyse the structural stability of transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt). It is observed that zinc-blende phase is the most stable one for these carbides. Pressure-induced structural phase transition from zinc blende to NiAs phase is predicted at the pressures of 248.5 GPa, 127 GPa and 142 GPa for OsC, IrC and PtC, respectively. The electronic structure reveals that RuC exhibits a semiconducting behaviour with an energy gap of 0.7056 eV. The high bulk modulus values of these carbides indicate that these metal carbides are super hard materials. The high B/G value predicts that the carbides are ductile in their most stable phase.

Pressure-induced electronic and structural phase transitions in Dirac semimetal Cd3As2: Raman study

We report on the high-pressure Raman study of , a three-dimensional Dirac semimetal, up to 19 GPa at room temperature. Our study shows that light scattering by intervalley and intravalley density fluctuations give rise to electronic Raman scattering (ERS), with Lorentzian-like lineshape at low frequency. The strength and linewidth of the ERS are pressure dependent and exhibit a significant drop at , signifying a breakdown of the Dirac semimetal to a semiconducting phase. The first phase transition at Pc1 is also clearly identified by the significant changes in the pressure derivatives of phonon frequencies and linewidths. Pressure dependence of phonon parameters also reveal a second phase transition at , being reported for the first time. This semiconductor-to-semiconductor transition coincides with the abrupt changes seen in the activation energy (band gap) of the semiconductor phase.

Giant Pressure-Induced Enhancement of Seebeck Coefficient and Thermoelectric Efficiency in SnTe.

The thermoelectric properties of polycrystalline SnTe have been measured up to 4.5 GPa at 330 K. SnTe shows an enormous enhancement in Seebeck coefficient, greater than 200 % after 3 GPa, which correlates to a known pressure-induced structural phase transition that is observed through simultaneous in situ X-ray diffraction measurement. Electrical resistance and relative changes to the thermal conductivity were also measured, enabling the determination of relative changes in the dimensionless figure of merit (ZT), which increases dramatically after 3 GPa, reaching 350 % of the lowest pressure ZT value. The results demonstrate a fundamental relationship between structure and thermoelectric behaviours and suggest that pressure is an effective tool to control them.

Quasiparticle energies and dielectric functions of diamond polytypes

We perform ab initio many-body Green's function calculations to investigate the quasiparticle energies and optical properties of diamond polytypes that have been predicted to be producible via a pressure-induced structural phase transition from carbon nanotube solids. We find, through quasiparticle band-structure calculations within the $GW$ approximation, that the band gaps of two hexagonal $(2H$- and $4H$-type) polytypes of diamond differ significantly from that of cubic diamond as well as from that of the crystalline $s{p}^{3}$ carbon phase with a body-centered-tetragonal structure, called bct ${\mathrm{C}}_{4}$. We also examine the dielectric functions of three polytypes of diamond (cubic, $2H$, and $4H)$ by employing the $GW$ plus Bethe-Salpeter equation approach. The calculated optical absorption spectra are found to be distinct from each other. The lattice mismatches of carbon layers of these diamond polytypes are very small and the total-energy differences are also small. Our work opens up the possibility of fabricating diamond superlattices with various electronic and optoelectronic properties by utilizing and controlling the different stacking sequences of carbon layers.

High-pressure phase transition and thermodynamic properties from first-principles calculations: Application to cubic copper iodide

The pressure-induced B3-B1 structural phase transition and some interesting thermodynamic properties for high-pressure B1 structure of cubic copper iodide (CuI) have been studied systematically using first-principles calculations based on density functional theory. It is found that CuI has a B3 ground-state phase at zero pressure and the transition pressure from B3 to B1 structure determined by the relative enthalpy versus pressure curves is about 6.9 GPa with a volume contraction of 14.42%. As a high-pressure phase, the B1 structure will be mechanically stable up to 57 GPa according to the generalized elastic stability criteria. Through the quasi-harmonic Debye model, the dependences of the primitive cell volume, isothermal bulk modulus, volumetric thermal expansion coefficient, heat capacity, entropy, and Debye temperature of CuI with high-pressure B1 phase on pressure and temperature are successfully predicted. The thermodynamic properties are summarized in the pressure range of 0–50 GPa and the temperature up to 800 K.

Anomalous compression behavior in a C15 Laves compound CeAl2 under high pressure

Although there have been quite a number of discrepancies in the compression curves of CeAl2, such results come from the lattice compression at non-hydrostatic condition by solidification of pressure transmitting medium. We have carried out the powder X-ray diffraction measurement of CeAl2 under hydrostatic pressure with helium gas as pressure-transmitting medium which achieves the best hydrostatic conditions. Although no splitting of the peak is observed in the present diffraction pattern, it is found that a full width at half maximum is enhanced above 20 GPa for several Bragg reflections. Such behaviors suggest the existence of a new pressure-induced structural phase transition which is not observed in the previous reports under non-hydrostatic pressure.

Pressure-induced superconductivity and quantum phase transitions in the Rashba material BiTeCl

Recent theoretical predictions have indicated that BiTeX (X = Cl, Br, I) type strong spin orbital coupling semiconductor compounds with strong Rashba effects are suitable for use as topological insulators under compressed conditions and novel phenomena have been identified in experiments. The compound BiTeCl is expected to exhibit a topological nontrivial state under pressure. In this study, we obtained measurements based on the resistivity, Hall coefficient, synchrotron X-ray diffraction, and Raman spectroscopy using the Rashba semiconductor BiTeCl crystal at high pressures up to 54.8 GPa. Our experiments indicated that the Rashba semiconductor BiTeCl exhibited a pressure-induced metal to insulator-like transition at 2.8 GPa. A pressure-induced structural phase transition was observed above 5.5 GPa, which was followed by a superconducting transition. The superconducting transition temperature (TC) increased to a maximum 4.2 K with pressures up to 32 GPa. Compared with BiTeI, the structural phase transition of BiTeCl occurs at a relatively lower pressure due to the weaker Rashba effects, where the inner chemical pressure is affected by the replacement of I with Cl.

Interplay between structure and superconductivity: Metastable phases of phosphorus under pressure

Pressure-induced superconductivity and structural phase transitions in phosphorous (P) are studied by resistivity measurements under pressures up to 170 GPa and fully $ab-initio$ crystal structure and superconductivity calculations up to 350 GPa. Two distinct superconducting transition temperature (T$_{c}$) vs. pressure ($P$) trends at low pressure have been reported more than 30 years ago, and for the first time we are able to reproduce them and devise a consistent explanation founded on thermodynamically metastable phases of black-phosphorous. Our experimental and theoretical results form a single, consistent picture which not only provides a clear understanding of elemental P under pressure but also sheds light on the long-standing and unsolved $anomalous$ superconductivity trend. Moreover, at higher pressures we predict a similar scenario of multiple metastable structures which coexist beyond their thermodynamical stability range. Metastable phases of P experimentally accessible at pressures above 240 GPa should exhibit T$_{c}$'s as high as 15 K, i.e. three times larger than the predicted value for the ground-state crystal structure. We observe that all the metastable structures systematically exhibit larger transition temperatures than the ground-state ones, indicating that the exploration of metastable phases represents a promising route to design materials with improved superconducting properties.

Structural evolution behavior of manganese monophosphide under high pressure: experimental and theoretical study

The influence of external pressure on the structural properties of manganese monophosphides (MnP) at room temperature has been studied using in situ angle dispersive synchrotron x-ray powder diffraction (AD-XRD) with a diamond anvil cell. The crystal structure of MnP is stable between 0 to 15 GPa. However, the compressibility of b-axis is much larger than those of a- and c-axes. From this result we suggested that the occurrence of superconductivity in MnP was induced by suppression of the long-range antiferromagnetically ordered state rather than a structural phase transition. Furthermore, the present experimental results show that the Pnma phase of MnP undergoes a pressure-induced structural phase transition at ~15.0 GPa. This finding lighted up-to-date understanding of the common prototype B31 structure (Strukturbericht Designation: B31) in transition metal monophosphides. No additional structural phase transition was observed up to 35.1 GPa (Run 1) and 40.2 GPa (Run 2) from the present AD-XRD results. With an extensive crystal structure searching and ab initio calculations, we predict that MnP underwent two pressure-induced structural phase transitions of Pnma → P213 and P213 → Pm-3m (CsCl-type) at 55.0 and 92.0 GPa, respectively. The structural stability and the electronic structures of manganese monophosphides under high pressure are also briefly discussed.

First principles study on Fe based ferromagnetic quaternary Heusler alloys

The study of stable half-metallic ferromagnetic materials is important from various fundamental and application points of view in condensed matter Physics. Structural phase stability, electronic structure, mechanical and magnetic properties of Fe-based quaternary Heusler alloys XX′YZ (X=Co, Ni; X′=Fe; Y=Ti; Z=Si, Ge, As) for three different phases namely α, β and γ phases of LiMgPdSn crystal structure have been studied by density functional theory with generalized gradient approximation formulated by Perdew, Burke and Ernzerhof (GGA-PBE) and the Hubbard formalism (GGA-PBE+U). This work aims to identify the ferromagnetic and half-metallic properties of XX′YZ (X=Co, Ni, X′=Fe; Y=Ti; Z=Si, Ge, As) quaternary Heusler alloys. The predicted phase stability shows that α-phase is found to be the lowest energy phase at ambient pressure. A pressure-induced structural phase transition is observed in CoFeTiSi, CoFeTiGe, CoFeTiAs, NiFeTiSi, NiFeTiGe and NiFeTiAs at the pressures of 151.6GPa, 33.7GPa, 76.4GPa, 85.3GPa, 87.7GPa and 96.5GPa respectively. The electronic structure reveals that these materials are half metals at normal pressure whereas metals at high pressure. The investigation of electronic structure and magnetic properties are performed to reveal the underlying mechanism of half metallicity. The spin polarized calculations concede that these quaternary Heusler compounds may exhibit the potential candidate in spintronics application. The magnetic moments for these quaternary Heusler alloys in all the three different phases (α, β and γ) are estimated.

Transport and structural properties of Cu0.25Bi2(TexSe1−x)3 (x = 0.01) under high pressure

Cu0.25Bi2(TexSe1−x)3 is a material in which tellurium atoms (Te) partially occupy selenium (Se) sites in a crystal structure of the superconductor Cu0.25Bi2Se3. This study focuses on the composition of x = 0.01, Cu0.25Bi2(Te0.01Se0.99)3, and we performed electrical resistivity measurement and X-ray diffraction under high pressures. Cu0.25Bi2(Te0.01Se0.99)3 shows slight diamagnetism and a drop in resistivity at ambient pressure; this suggests possible superconductivity at the transition temperature of Tc = 3.2 K, which is suppressed at P = 1 GPa. On further compression, a slight drop in resistivity is observed at temperatures of 4.3–5.4 K above 10 GPa. This drop is suppressed under magnetic fields, suggesting the occurrence of superconducting transition. The pressure-induced structural phase transition from a trigonal phase to a sevenfold monoclinic C2/m phase occurs at 10.9 GPa. These results suggest that the superconducting transition above 10 GPa occurs in the high-pressure C2/m phase.

Structural, electronic, mechanical and magnetic properties of Mn based ferromagnetic half Heusler alloys: A first principles study

Abstract The search for stable half-metallic ferromagnetic materials remains a high priority in condensed matter Physics. Ab initio calculations are performed using density functional theory to analyze the structural phase stability, electronic structure, mechanical and magnetic properties of Mn based half Heusler alloys XYZ (X = Ir, Pt, Au; Y = Mn; Z = Sn, Sb) for three different phases namely α, β and γ phases of C1 b crystal structure. This work aims to identify the ferromagnetic and half-metallic behavior of XYZ (X = Ir, Pt, Au; Y = Mn; Z = Sn, Sb) half Heusler alloys. To accomplish this, density functional theory (DFT) with generalized gradient approximation formulated by Perdew, Burke and Ernzerhof (GGA-PBE) and the Hubbard formalism (GGA-PBE + U) are used to describe the strong correlations present in these alloys. Among the considered phases, α-phase is found to be the lowest energy phase for IrMnSn, IrMnSb, PtMnSn, PtMnSb, AuMnSn and AuMnSb at normal pressure. A pressure-induced structural phase transition is observed in IrMnSn, IrMnSb, PtMnSn, PtMnSb, AuMnSn and AuMnSb at the pressures of 62.1 GPa, 47.8 GPa, 26.1 GPa, 25.3 GPa, 23.7 GPa and 14.0 GPa respectively. The electronic structure reveals that these materials are metals at normal pressure whereas half metals at high pressure. The magnetic moments for these half Heusler alloys in all the three different phases (α, β and γ) are estimated.

Structural, Electronic, and Mechanical Properties of CoN and NiN: An Ab Initio Study

Abstract The structural stabilities of cobalt mononitride (CoN) and nickel mono-nitride (NiN) were investigated among the crystal structures, namely, NaCl (B1), CsCl (B2), and zinc blende (B3). It was found that the zinc blende (B3) phase was the most stable phase for both nitrides. A pressure-induced structural phase transition from B3 to B1 phase was predicted in these nitrides. The computed lattice parameter values were in agreement with the experimental values and other theoretical values. The electronic structures reveal that these nitrides are metallic at zero pressure. The computed elastic constants indicate that CoN and NiN are mechanically stable in the B1 and B3 phases. The variations of the elastic constants, bulk modulus, shear modulus, Poisson’s ratio, and elastic anisotropy factor with pressure were investigated. The Debye temperature θ D values are reported for both the nitrides in their B1 and B3 phases. The high-pressure NaCl phase of both CoN and NiN were found to be ferromagnetic.

X-ray diffraction and spectroscopy study of nano-Eu2O3 structural transformation under high pressure

Nanoscale materials exhibit properties that are quite distinct from those of bulk materials because of their size restricted nature. Here, we investigated the high-pressure structural stability of cubic (C−type) nano-Eu2O3 using in situ synchrotron X−ray diffraction (XRD), Raman and luminescence spectroscopy, and impedance spectra techniques. Our high-pressure XRD experimental results revealed a pressure-induced structural phase transition in nano−Eu2O3 from the C−type phase (space group: Ia-3) to a hexagonal phase (A−type, space group: P-3m1). Our reported transition pressure (9.3 GPa) in nano-Eu2O3 is higher than that of the corresponding bulk-Eu2O3 (5.0 GPa), which is contrary to the preceding reported experimental result. After pressure release, the A−type phase of Eu2O3 transforms into a new monoclinic phase (B−type, space group: C2/m). Compared with bulk−Eu2O3, C-type and A-type nano−Eu2O3 exhibits a larger bulk modulus. Our Raman and luminescence findings and XRD data provide consistent evidence of a pressure-induced structural phase transition in nano-Eu2O3. To our knowledge, we have performed the first high-pressure impedance spectra investigation on nano-Eu2O3 to examine the effect of the structural phase transition on its transport properties. We propose that the resistance inflection exhibited at ∼12 GPa results from the phase boundary between the C−type and A−type phases. Besides, we summarized and discussed the structural evolution process by the phase diagram of lanthanide sesquioxides (Ln2O3) under high pressure.

Theoretical Analysis of the Structural Phase Transformation and Elastic Properties in the ZrN under High Pressure

We report a phenomenological model based calculation of pressure-induced structural phase transition and elastic properties of ZrN compound. Gibb’s free energy is obtained as a function of pressure by applying an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. From the present study, we predict a structural phase transition from NaCl structure (B1) to the CsCl structure (B2). The variations of elastic constants with pressure follow a systematic trend identical to that observed in others compounds of NaCl type structure family.

Exploring the coordination change of vanadium and structure transformation of metavanadate MgV2O6 under high pressure.

Raman spectroscopy, synchrotron angle-dispersive X-ray diffraction (ADXRD), first-principles calculations, and electrical resistivity measurements were carried out under high pressure to investigate the structural stability and electrical transport properties of metavanadate MgV2O6. The results have revealed the coordination change of vanadium ions (from 5+1 to 6) at around 4 GPa. In addition, a pressure-induced structure transformation from the C2/m phase to the C2 phase in MgV2O6 was detected above 20 GPa, and both phases coexisted up to the highest pressure. This structural phase transition was induced by the enhanced distortions of MgO6 octahedra and VO6 octahedra under high pressure. Furthermore, the electrical resistivity decreased with pressure but exhibited different slope for these two phases, indicating that the pressure-induced structural phase transitions of MgV2O6 was also accompanied by the obvious changes in its electrical transport behavior.

Infrared study of the magnetostructural phase transition in correlated CrN

We report on the pressure and temperature dependence of the electronic and vibrational properties of polycrystalline CrN studied by optical transmission and reflection measurements over the frequency range 0.012--2.48 eV. The optical conductivity spectrum of CrN at ambient conditions shows a phonon mode at $\ensuremath{\approx}55\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ with a shoulder at $\ensuremath{\approx}69\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, a pronounced midinfrared absorption band centered at $123\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, and a high-energy absorption band at $\ensuremath{\approx}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The absorption bands are discussed in terms of the charge-transfer insulator picture. Following the reflectance spectrum with increasing pressure, the activation of an additional phonon mode above 0.6 GPa indicates the occurrence of a pressure-induced structural phase transition. Furthermore, the absorption spectrum exhibits significant changes in the far-infrared range with decreasing temperature: The phonon mode shows a sudden broadening followed by a splitting below 270 K. These changes observed under pressure or while cooling down can be associated with the magnetostructural phase transition reported previously.