High-pressure Monoclinic Phase Research Articles

First-principles study of moderate phonon-mediated pairing in high-pressure monoclinic phase of BiS2-based superconductors

Isotope effect on superconductive transition temperature (T c) is an essential indicator to examine whether the mechanism of superconductors is conventional. Unconventional isotope effect of BiS2-based superconductors has been previously reported in ambient-pressure tetragonal phase. However, to comprehensively ascertain the nature of superconductivity, the investigation of BiS2-based system in high-pressure structure is highly desirable. In this work, we carried out the first-principles calculations of phonon spectra and superconductivity in high-pressure monoclinic phase of LaO0.5F0.5BiS2 with 32S and 34S, and observed that the corresponding isotope coefficient is 0.13 ≤ α ≤ 0.20. This value is much greater than that of BiS2-based superconductors in ambient-pressure phase, but slightly smaller than that of conventional MgB2. Taking into account the calculated T c lower than experimental results, we finally conclude that the moderate phonon-mediated pairing plays a significant role in forming superconductivity of BiS2-based system in high-pressure phase, moreover, the cooperative multiple paring interactions should also be considered.

Pressure-Induced Phase Transition in Multilayered Vanadium Diselenide Nanosheets

Layered transition-metal dichalcogenides have recently attracted considerable attention due to their unique mechanical and opto-electronic properties. Here, we report the investigation of structural, vibrational, and electronic properties of vanadium diselenide (VSe2) nanosheets up to a pressure of 33 GPa by diamond anvil cell-based pressure-induced studies. The experimental results indicate a structural transition from the metallic trigonal (P3̅m1) to a metallic monoclinic (C2/m) phase at ∼7 GPa, consistent with our ab initio calculations. A decrease in the metallic nature of the trigonal phase is evident from the reduction in the width of the Fermi level band crossing in the high-pressure monoclinic phase. Transmission electron microscopy analyses reveal that VSe2 nanosheets recover the original ambient structure upon decompression. Raman spectroscopy studies at high pressures identify an A1g soft phonon (∼236 cm–1) and an Eg phonon (208 cm–1) that show normal hardening and consequently phase instability at ∼7 GPa. Using our experimental Raman mode Grüneisen parameters γi, the thermal expansion coefficient αv of the Vse2 nanosheets at ambient temperature is obtained as −0.96 × 10–6 K–1. This pressure-tuned behavior of these layered nanomaterials can be beneficial in the calibration and development of novel nanodevices using Vse2 nanosheets under an extreme stress environment.

X-ray diffraction reveals two structural transitions in szomolnokite

AbstractHydrated sulfates have been identified and studied in a wide variety of environments on Earth, Mars, and the icy satellites of the solar system. The subsurface presence of hydrous sulfur-bearing phases to any extent necessitates a better understanding of their thermodynamic and elastic properties at pressure. End-member experimental and computational data are lacking and are needed to accurately model hydrous, sulfur-bearing planetary interiors. In this work, high-pressure X-ray diffraction (XRD) and synchrotron Fourier-transform infrared (FTIR) measurements were conducted on szomolnokite (FeSO4·H2O) up to ~83 and 24 GPa, respectively. This study finds a monoclinic-triclinic (C2/c to P1) structural phase transition occurring in szomolnokite between 5.0(1) and 6.6(1) GPa and a previously unknown triclinic-monoclinic (P1 to P21) structural transition occurring between 12.7(3) and 16.8(3) GPa. The high-pressure transition was identified by the appearance of distinct reflections in the XRD patterns that cannot be attributed to a second phase related to the dissociation of the P1 phase, and it is further characterized by increased H2O bonding within the structure. We fit third-order Birch-Murnaghan equations of state for each of the three phases identified in our data and refit published data to compare the elastic parameters of szomolnokite, kieserite (MgSO4·H2O), and blödite (Na2Mg(SO4)2·4H2O). At ambient pressure, szomolnokite is less compressible than blödite and more than kieserite, but by 7 GPa both szomolnokite and kieserite have approximately the same bulk modulus, while blödite’s remains lower than both phases up to 20 GPa. These results indicate the stability of szomolnokite’s high-pressure monoclinic phase and the retention of water within the structure up to pressures found in planetary deep interiors.

New high-pressure monoclinic phase of Sn

High-pressure structure phase transitions of Sn were studied by using the unbiased CALYPSO structure prediction method combined with first-principles calculations. The experimental structures of Fd-3m, I41/amd, I4/mmm, Immm, Im-3m and P63/mmc were successfully reproduced in our crystal structure searches. Furthermore, we firstly uncovered a novel high-pressure structure of Sn with space group P21/m symmetry stabilized at the pressure range of 6.1–21.5 GPa. The results of elastic constants and phonon dispersion curves show that the P21/m structure is dynamically and mechanically stable at the considered pressures. Atomic structure of the P21/m phase was very similar to that of I4/mmm (body-centered tetragonal) under the same pressure. Our findings enrich the high pressure phases of Sn and lay the foundation for further theoretical and experimental research.

High-pressure monoclinic phase of MoAlB

The pressure-induced phase transitions of MoAlB have been fully studied to provide insights into the typical MAB phases under high pressure. A novel high-pressure monoclinic structure (C2/m, Z = 4) was firstly identified for the MoAlB above 234 GPa, using the first principles crystal structure prediction approach. First-order structure transition character of MoAlB from the ambient-pressure orthorhombic Cmcm phase to this high-pressure monoclinic C2/m phase was demonstrated by a volume drop of 1.28%. Inspections of crystal structures showed that the occurrence of C2/m phase follows the slip in the metallic Al-Al layers and the fracture of linear zigzag B-B chains in the Cmcm phase under compression, confirming the earlier reports. Our argument has been also supported by the softening of elastic constant under high pressure. Phonon curves calculations suggest the high-pressure C2/m phase can be quenchable to ambient pressure. The electronic and mechanical properties of this novel C2/m phase have thus been systematically investigated.

High Pressure Behaviors and a Novel High-Pressure Phase of Cuprous Oxide Cu2O

In the present study, we extensively explored the phase stabilities and elastic behaviors of Cu2O with elevated pressures up to 29.3 GPa based on single-crystal X-ray diffraction measurements. The structural sequence of Cu2O is different than previously determined. Specifically, we have established that Cu2O under pressure, displays a cubic-tetragonal-monoclinic phase transition sequence, and a novel monoclinic high-pressure phase assigned to the P1a1 or P12/a1 space group was firstly observed. The monoclinic phase Cu2O exhibits anisotropic compression with axial compressibility βb > βc > βa in a ratio of 1.00:1.64:1.45. The obtained isothermal bulk modulus of cubic and monoclinic phase Cu2O are 125(2) and 41(6) GPa, respectively, and the KT0’ is fixed at 4. Our results provide new insights into the phase stability and elastic properties of copper oxides and chalcogenides at extreme conditions.

Retainable Superconductivity and Structural Transition in 1T-TaSe2 Under High Pressure.

As a prominent platform possessing the properties of superconductivity (SC) and charge density wave (CDW), transition-metal dichalcogenides (TMDCs) have attracted considerable attention for a long time. Moreover, extensive efforts have been devoted for exploring the SC and/or the interplay between SC and CDW in TMDCs in the past few decades. Here, we systematically investigate the electronic properties and structural evolution of 1T-TaSe2 under pressure. With increasing pressure, pressure-induced superconductivity is observed at ∼2.6 GPa. The superconductive transition temperature (Tc) increases with the suppression of the CDW state to the maximum value of ∼5.1 K at 21.8 GPa and then decreases monotonously up to the highest pressure of 57.8 GPa. 1T-TaSe2 transforms into a monoclinic C2/m structure above 19 GPa. The monoclinic phase coexists with the original phase as the pressure is released under ambient conditions and the retainable superconductivity with Tc = 2.9 K is observed in the released sample. We suggest that the retained superconductivity can be ascribed to the retention of the superconductive high-pressure monoclinic phase in the released sample. Our findings demonstrate that both the structure and CDW order are related to the superconductivity of TaSe2.

Pressure-induced octahedral tilting distortion and structural phase transition in columbite structured NiNb2O6

High-pressure behavior of the technologically important compound NiNb2O6 adopting a columbite-type orthorhombic structure at ambient pressure and temperature conditions was investigated using synchrotron x-ray powder diffraction, Raman spectroscopic measurements, and first-principles calculations. The x-ray diffraction data indicate the occurrence of irreversible pressure-induced structural phase transition in the studied compound beyond 9 GPa. The high-pressure phase is found to be monoclinic with space group P2/m. The large volume collapse (∼4.4%) at the transition indicates the nature of the transition to be of the first order. There is a change in oxygen anion coordination number around Nb from 6 to 8; however, the coordination number around Ni remains 6. The experimental pressure–volume data when fitted to the Birch−Murnaghan equation of states yield the value of ambient pressure bulk modulus (B0) as 178.7 (17) GPa for the orthorhombic phase and 244 (6) for the high-pressure monoclinic phase. The changes in Raman spectra indicate the distortion of NbO6 octahedra resulting in structural phase transitions. The logarithmic variation of unit cell volume (V) with optical lattice mode frequency (ν) helped us to calculate their respective Grüneisen parameters (γ) and it also supports the instability of the orthorhombic columbite structure of NiNb2O6 beyond 9 GPa, originated due to strong octahedral deformation of NbO6 octahedra. The variation of structural, electronic, and optical properties with pressure has also been discussed through first-principles calculations based on density functional theory using the revised Perdew–Burke–Ernzerh of generalized gradient approximation. The theoretical bandgap collapses and the distortion of NbO6 octahedra through the O chains with pressure are emphasized from the density of states data.

Greigite (Fe3S4) is thermodynamically stable: Implications for its terrestrial and planetary occurrence

Iron sulfide minerals are widespread on Earth and likely in planetary bodies in and beyond our solar system. Using measured enthalpies of formation for three magnetic iron sulfide phases: bulk and nanophase Fe3S4 spinel (greigite), and its high-pressure monoclinic phase, we show that greigite is a stable phase in the Fe-S phase diagram at ambient temperature. The thermodynamic stability and low surface energy of greigite supports the common occurrence of fine-grained Fe3S4 in many anoxic terrestrial settings. The high-pressure monoclinic phase, thermodynamically metastable below about 3 GPa, shows a calculated negative P-T slope for its formation from the spinel. The stability of these three phases suggests their potential existence on Mercury and their magnetism may contribute to its present magnetic field.

High pressure behaviors and novel high-pressure phase of Si3N4 and TiN

Here we present synchrotron-based X-ray diffraction experiments combined with diamond anvil cell on the nitrogen-bearing compounds Si3N4 and TiN up to ~53 GPa and ~51 GPa, respectively. The pressure-induced phase transition of Si3N4 is evidenced at 42.3 GPa at room temperature and a new monoclinic high-pressure phase assigned to the P2, Pm, or P2/m space group was observed. The pressure-volume relationship is well described by the third-order Birch-Murnaghan equation of state, which yields KT0 = 271(4) GPa and KT0′ = 4.3(2), and the anisotropic compression with Mc > Ma is obtained. While TiN is stable up to 50.8 GPa without any evidence of phase transformation. Our results provide more information on the stability and elastic properties of nitrogen-bearing minerals in the Earth's mantle, and thus provide new insights for understanding the nitrogen stability and transport properties in the deep earth.

High-pressure characterization of multifunctional CrVO4

The structural stability and physical properties of CrVO4 under compression were studied by x-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, and ab initio calculations up to 10 GPa. High-pressure x-ray diffraction and Raman measurements show that CrVO4 undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is proposed to be isomorphic to the wolframite structure. Such a phase transition (CrVO4-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, are reported. Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

Pressure-induced structural and electronic transitions of thiospinel Fe3S4

We report the investigations on the structural and electronic properties of an inverse spinel Fe3S4 at high pressures using synchrotron x-ray diffraction (XRD) and electrical transport measurements. Our XRD measurements at high pressures reveal an irreversible structural phase transformation on compression above ∼3 GPa from a cubic spinel (Fd-3m space group) into a monoclinic Cr3S4-type structure (I2/m space group). Electrical transport measurements suggest that the high pressure monoclinic phase has a semiconducting behavior. This semiconducting behavior is found to persist up to the highest pressure of measurement of ∼23 GPa. These results show that while Fe3S4 possesses similar high pressure structural properties with other thiospinels, the electronic properties under pressure show a rather strong similarity to its oxide counterpart, Fe3O4, at high pressures.

Pressure induced structural phase transition in rare earth sesquioxide Tm2O3: Experiment and ab initio calculations

Among the small cation sized rare earth sesquioxides, the reported transition pressure of cubic Tm2O3 is ambiguous. Pressure induced structural phase transition in cubic Tm2O3 has been reinvestigated using the synchrotron X-ray diffraction, Raman spectroscopy, and ab initio density functional theory (DFT) calculations up to a pressure of 25 GPa. Both the X-ray diffraction and Raman spectroscopic measurements revealed an irreversible polymorphic structural phase transition from type-C cubic to type-B monoclinic at around 12 GPa, whereas the same is predicted to be 8 GPa from the density functional theory. The phase transition observed at 12 GPa is in contrast to the literature and the reasoning has been established by other studies, viz., Raman spectroscopy and DFT. A third order Birch-Murnaghan equation of state fit to the experimental compressibility curve yielded a zero pressure bulk modulus of 149(2) GPa with the pressure derivatives 4.8(5) for the parent cubic phase and 169(2) GPa with the pressure derivative 4 for the high pressure monoclinic phase, respectively. These values are in good agreement with the calculated bulk modulus of 146 and 151 GPa for the cubic and monoclinic phases, respectively. Raman modes for the monoclinic phase of Tm2O3 are measured and reported for the first time. The mode Grüneisen parameter of different Raman modes for both cubic and monoclinic phases of Tm2O3 has also been determined. The experimental results are correlated with changes in the density of states near the Fermi level, which are indicative of structural instabilities in the parent cubic structure.

Pressure-induced anomalies and structural instability in compressed β-Sb2O3.

Here, we report a high-pressure study of orthorhombic structured β-Sb2O3 (valentinite) by the combination of synchrotron in situ X-ray diffraction and first-principles theoretical calculations at pressures up to 40.5 GPa. Our results reveal that the metastable β-Sb2O3 undergoes an isostructural phase transition at high pressure, yielding a distorted β phase at 7-15 GPa through symmetry breaking and structural distortion as inferred from our XRD analyses and DFT theoretical calculations where pressure-induced elasticity softening is observed at pressures of 7-15 GPa. At pressures higher than 15 GPa, a new high-pressure monoclinic phase is discovered from the current synchrotron X-ray diffraction data. Upon further compression up to ∼33 GPa, the monoclinic Sb2O3 starts to lose its long-range order and forms an amorphous component coexisting with the monoclinic one. To further explore the structural instability and understand the origin of pressure-induced phase transitions in β-Sb2O3 upon compression, we have performed first-principles calculations to track the evolution of its phonon velocities, density of states and phonon dispersion curves under high pressure. Our results may play an important role in determining the local structures as well as their structural relationship among sesquioxides.

High-pressure induced phase transition of FeS2: Electronic, mechanical and thermoelectric properties

Abstract Being as the most abundant natural mineral among the iron-based sulphides, iron pyrite (FeS2) has been extensively investigated. These investigations mainly focused on the known marcasite (Pnnm) and pyrite ( P a 3 ¯ ) phases, due to unknown of high pressure (HP) structural phases of FeS2. Here, we report a theoretical study up to 300 GPa to uncover a brand new monoclinic HP phase of C2/m structure at above 192 GPa. The stable of this structure is evaluated by the thermodynamical, dynamical and mechanical calculations. The C2/m structure is characterized by distorted FeS6 octahedron together with two connected 1D zigzag-like S chains. Analysis of the thermoelectric calculations indicates that the electronic thermal conductivity and electrical conductivity of C2/m structure linearly increasing with increasing temperature. Meanwhile, it is found that the C2/m phase behaves in a ductile manner through the elastic parameters calculations.

Pressure dependence of the structure and electronic properties ofSr3Ir2O7

We study the structural evolution of Sr$_3$Ir$_2$O$_7$ as a function of pressure using x-ray diffraction. At a pressure of 54 GPa at room temperature, we observe a first-order structural phase transition, associated with a change from tetragonal to monoclinic symmetry, and accompanied by a 4% volume collapse. Rietveld refinement of the high-pressure phase reveals a novel modification of the Ruddlesden-Popper structure, which adopts an altered stacking sequence of the perovskite bilayers. As the positions of the oxygen atoms could not be reliably refined from the data, we use density functional theory (local-density approximation+$U$+spin orbit) to optimize the crystal structure, and to elucidate the electronic and magnetic properties of Sr$_3$Ir$_2$O$_7$ at high pressure. In the low-pressure tetragonal phase, we find that the in-plane rotation of the IrO$_6$ octahedra increases with pressure. The calculations further indicate that a bandwidth-driven insulator-metal transition occurs at $\sim$20 GPa, along with a quenching of the magnetic moment. In the high-pressure monoclinic phase, structural optimization resulted in complex tilting and rotation of the oxygen octahedra, and strongly overlapping $t_{2g}$ and $e_g$ bands. The $t_{2g}$ bandwidth renders both the spin-orbit coupling and electronic correlations ineffectual in opening an electronic gap, resulting in a robust metallic state for the high-pressure phase of Sr$_3$Ir$_2$O$_7$.

Pressure-induced reemergence of superconductivity in topological insulatorSr0.065Bi2Se3

The recent-discovered Sr$_x$Bi$_2$Se$_3$ superconductor provides an alternative and ideal material base for investigating possible topological superconductivity. Here, we report that in Sr$_{0.065}$Bi$_{2}$Se$_3$, the ambient superconducting phase is gradually depressed upon the application of external pressure. At high pressure, a second superconducting phase emerges at above 6 GPa, with a maximum $T_c$ value of $\sim$8.3 K. The joint investigations of the high-pressure synchrotron x-ray diffraction and electrical transport properties reveal that the re-emergence of superconductivity in Sr$_{0.065}$Bi$_{2}$Se$_3$ is closely related to the structural phase transition from ambient rhombohedral phase to high-pressure monoclinic phase around 6 GPa, and further to another high-pressure tetragonal phase above 25 GPa.

High pressure structural changes in aluminium triiodide: A first principles study.

First principles calculations identified a phase transition in aluminium triiodide (AlI3) and predicted its physical and spectroscopic properties under high pressure conditions. A high pressure monoclinic phase is predicted to exist above 1.3 GPa accompanied with a coordination change of aluminium resulting from a transformation from the ambient pressure 4-coordinated primitive monoclinic phase with space group P21/c to the monoclinic 6-coordinated structure with space group C2/m. Density functional phonon calculations predicted its dynamical and mechanical stability. Infrared effective charge intensities and Raman scattering tensors were obtained to characterize its spectroscopic properties. First-principles metadynamics simulations were employed to reconstruct this phase transition and provide the mechanism details for energetically favourable path from the ambient pressure P21/c structure to the predicted C2/m structure.

Structural distortions in the high-pressure polar phases of ammonium metal formates

The high-pressure behaviour of ammonium metal formates has been investigated using high-pressure single-crystal X-ray diffraction on ammonium iron and nickel formates, and neutron powder diffraction on ammonium zinc formate in the pressure range of 0–2.3 GPa. A structural phase transition in the pressure range of 0.4–1.4 GPa, depending on the metal cation, is observed for all three ammonium metal formates. The hexagonal-to-monoclinic high-pressure transition gives rise to characteristic sixfold twinning based on the single-crystal diffraction data. Structure solution of the single-crystal data and refinement of the neutron powder diffraction characterise the pressure-induced distortions of the metal formate frameworks. The pressure dependence of the principal axes shows significantly larger anisotropic compressibilities in the high-pressure monoclinic phase (K1 = 48 TPa−1, K3 = −7 TPa−1) compared to the ambient hexagonal phase (K1 = 16 TPa−1, K3 = −2 TPa−1), and can be related to the symmetry-breaking distortions that cause deformation of the honeycomb motifs in the metal formate framework. While high-pressure Raman spectroscopy suggests that the ammonium cations remain dynamically disordered upon the phase transition, the pressure-induced distortions in the metal formate framework cause polar displacements in the ammonium cations. The magnitude of polarisation in the high-pressure phase of ammonium zinc formate was calculated based upon the offset of the ammonium cation relative to the anionic zinc formate framework, showing an enhanced polarisation of Ps ∼ 4 μC cm−2 at the transition, which then decreases with increasing pressure.

Structural and vibrational properties of single crystals of Scandia, Sc2O3 under high pressure

We report the results of single-crystal X-ray diffraction and Raman spectroscopy studies of scandium oxide, Sc2O3, at ambient temperature under high pressure up to 55 and 28 GPa, respectively. Both X-ray diffraction and Raman studies indicated a phase transition from the cubic bixbyite phase (so-called C-Res phase) to a monoclinic C2/m phase (so-called B-Res phase) at pressures around 25–28 GPa. The transition was accompanied by a significant volumetric drop by ∼6.7%. In addition, the Raman spectroscopy detected a minor crossover around 10–12 GPa, which manifested in the appearance of new and disappearance of some Raman modes, as well as in softening of one Raman mode. We found the bulk modulus values of the both C-Res and B-Res phases as B0 = 198.2(3) and 171.2(1) GPa (for fixed B′ = 4), respectively. Thus, the denser high-pressure lattice of Sc2O3 is much softer than the original lattice. We discuss possible mechanisms that might be responsible for the pronounced elastic softening in the monoclinic high-pressure phase in this “simple” oxide with an ultra-wide band gap.