NiAs-type Phase Research Articles

High-pressure structure prediction and high-temperature structural stability of periclase

Periclase is the terminal component of the ferropericlase, and its chemical composition is MgO. It is well known that there exists a huge difference between the melting curves of MgO determined experimentally and theoretically. A feasible way to clarify the nature of the melting temperature is to investigate the possible new phase of MgO. Meanwhile, it is very important to study the new phase and the influence of temperature on structural stability of MgO in high-pressure condensed matter physics and geophysics. In the present work, we study in detail the phase stability and the possible existing structures of MgO, which include the structure predicted by particle swarm optimization algorithm through using the first-principles pseudopotential density functional method. We find that MgO crystallizes into a rocksalt structure in a pressure range from 0 to 580 GPa and that the CsCl-type structure is of a high-pressure phase at up to 800 GPa. Although an NiAs-type hexagonal phase perhaps explains the volume discontinuity at (170 ± 10) GPa along the MgO Hugoniot in a shock-compression experiment (Zhang L, Fei Y W 2008 <i>Geophys. Res. Lett</i>. <b>35</b> L13302) and a wurtzite phase perhaps explains the huge difference between the melting curves of MgO determined experimentally and theoretically (Aguado A, Madden P A 2005 <i>Phys. Rev. Lett</i>. <b>94</b> 068501), neither of them is existent in the entire range of pressures studied, according to the thermodynamic stability calculations. The calculations of phonon spectra indicate that the B3, B4, B8<sub>1</sub>, B8<sub>2</sub>, and <i>P</i>3<i>m</i>1 phases of MgO are dynamically stable at zero pressure. That is to say, all of the predicted structures are the metastable structures of MgO. In addition, the high-temperature structural stability of MgO is investigated by using very similar Lewis-Catlow and Stoneham-Sangster shell model potential based on the classical molecular dynamics (MD) simulations. In order to take into account the non-central force in crystal, the breathing shell model is also introduced in simulation. The thermodynamic melting curves are estimated on the basis of the thermal instability MD simulations and compared with the available experimental data and other theoretical results in the pressure range of 0－150 GPa.

Composition and annealing effects on superconductivity in sintered and arc-melted Fe1+εTe0.5Se0.5

We present the results of x-ray diffraction, electrical resistivity, and ac magnetic susceptibility measurements on specimens of the “11”-structure superconductor Fe1+εTe0.50Se0.50 (0 ≤ ε ≤ 0.15). Samples were initially either sintered in sealed quartz tubes or melted in a zirconium-gettered arc furnace. Sintered samples were fired two to three times at temperatures of 425°C, 600°C, or 675°C, while arc-melted samples were studied both asmelted and after annealing at 650°C. X-ray diffraction data show a predominant PbO-type tetragonal phase, with a secondary hexagonal NiAs-type phase; for sintered specimens annealed at 600°C, the secondary phase decreases as ε increases over the range 0 ≤ ε ≤ 0.10, with the composition Fe1.10Te0.5Se0.5 exhibiting x-ray phase purity. A higher annealing temperature of 675°C provided such tetragonal phase purity at the composition Fe1.05Te0.5Se0.5. The resistive superconducting transition temperature Tc was nearly independent of the iron concentration 1+ε, suggesting a single superconducting phase, while the magnetic screening fraction varied greatly with concentration and conditions, peaking at ϵ = 0.07, indicating that the amount of superconducting phase is strongly dependent on conditions. We propose that the behaviour can also be viewed in terms of an electron-doped, chalcogen-deficient stoichiometry.

Phase relations and superconductivity in the Fe7(Se1−yTey)8 system: Effect of phase coexistence

The effect of the Te for Se substitution in the Fe7Se8 compound having a layered NiAs-type structure has been studied by means of X-ray diffraction, scanning electron microscopy, thermal expansion, electrical resistivity and magnetization measurements. The growth of Te tellurium content in Fe7(Se1−yTey)8 above y ∼ 0.15 is found to result in the phase segregation; besides the NiAs-type phase the superconducting tetragonal PbO-type phase appears in these samples. It has been observed that the crystal structure and composition of the hexagonal Fe7(Se,Te)8 phase (P63/mmc) transforms with increasing Te content to the CdI2-type phase (P3¯m1) and then to the monoclinic Fe3(Se,Te)4-type phase (I12/m1). The concentration intervals in which these NiAs-type phases coexist with the tetragonal phase have been determined. At y ≥ 0.4, the Fe7(Se1−yTey)8 samples show bulk superconductivity with higher superconducting transition temperatures (Tc up to 17.8 K) compared with single-phase samples of Fe(Se,Te). Such an enhancement is suggested to result from the interactions between the superconducting and non-superconducting phases coexisting in Fe7(Se1−yTey)8.

Phase equilibria and structural investigations of the general NiAs-type in the ternary system Ni–Pt–Sn

The ternary alloy system Ni–Pt–Sn was investigated in detail, with special focus on the general NiAs-type phases. Phase diagram and crystallographic parameters were analysed by means of light optical microscopy (LOM), powder X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDX). Two isothermal sections at 550 and 700°C, respectively, were studied experimentally. In both of them a continuous phase field of the general NiAs type structure was detected between PtSn and Ni3Sn2 (high temperature modification), covering a continuous transition from the NiAs- to the Ni2In-type. Lattice parameter variation and occupation of the atomic positions throughout this phase field were analysed and partial ordering of the transition metals was discovered.Furthermore, seven vertical sections were determined at 15at.% Sn, 20at.% Sn, 50at.% Sn and 65at.% Sn, and diagonal series from Pt60Sn40–Ni70Sn30, Pt42Sn58–Ni60Sn40 and Pt41.7Sn58.3–Ni48.1Sn51.9. According to these sections, eight ternary invariant phase reactions were determined: five transition reactions, one eutectic reaction, one peritectoid reaction and a maximum. Moreover, a liquidus surface projection and a complete reaction scheme of the system were constructed.

Bromine-doping dependence of crystal structure and superconductivity in FeSe1−xBrx

Our experimental data indicate that the crystal structure formation in FeSe1−xBrx is dependent on the amount of Br. For the samples with x between 0.18 and 0.3, the single tetragonal β-FeSe phase with space group P4/nmm could be obtained by carrying out the low-temperature (400°C) annealing after reaction at 680°C. As to the sample with x=0.1, a heat firing at 680°C is in favor of forming single hexagonal δ-FeSe phase with space group P63/mmc. The compound FeSe0.9Br0.1 with single δ hexagonal NiAs-type phase shows neither magnetic order nor superconductivity above 1.8K according to dc magnetization measurements while the bulk character of the superconducting state with a Tc around 5K for the single β tetragonal PbO-type phase in FeSe1−xBrx is confirmed. Larger superconducting volume fraction appears in the single-phase sample with x near 0.2.

Reduced thermal conductivity in nanolamellar composite comprising half-Heusler and NiAs-type phases

The major drawback of half-Heusler-based thermoelectric materials lies in their relatively high thermal conductivity . In this work, nanolaminated structures composed of the half-Heusler phase M CoSb or M (Fe 1− x Ni x )Sb and NiAs-type phase M Sb ( M = Ti, Zr) are fabricated by the arc-melting method. A significant reduction in the thermal conductivity is achieved due to effective phonon scattering in the nanolaminated structure, while the electrical resistivity is low enough for thermoelectric applications because of the formation of coherent interfaces.

Structure of the oxygen-annealed chalcogenide superconductor Fe1.08Te0.55Se0.45Ox

The as-grown single crystal Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$ with the tetragonal PbO-type structure is nonsuperconducting owing to the excess Fe. Superconductivity is induced after oxygen annealing with an onset and zero resistance transition temperature around 14.5 K and 11.5 K, respectively. The oxygen doping is evidenced by electron energy loss spectroscopy and accompanied by improved homogeneity in the remaining PbO-type phase, as well as an increase in the L${}_{3}$/L${}_{2}$ intensity ratio of the Fe-L${}_{2,3}$ edge, indicating an increase in Fe valence. Local phase transformation from the tetragonal PbO-type phase to the hexagonal NiAs-type phase is also observed after oxygen annealing.

Theoretical investigation on the structural and thermodynamic properties of FeSe at high pressure and high temperature

A theoretical investigation on structural and thermodynamic properties of 11-type iron-based superconductor FeSe at high pressure and high temperature was performed by employing the first-principles method based on the density functional theory. Some structural parameters of FeSe in both tetragonal and hexagonal phases are reported. According to the fourth-order Birch-Murnaghan equation of states, the transition pressure P(t) of FeSe from the PbO-type phase to the NiAs-type phase was determined. The calculated results are found to be in good agreement with the available experimental data. Based on the quasi-harmonic Debye model, the pressure and temperature dependence of the thermodynamic properties for hexagonal phase FeSe were investigated. Our theoretical calculations suggest that the pressure and temperature have significant effects on the heat capacity, vibrational internal energy, vibrational entropy, vibrational Helmholtz free energy, thermal expansion coefficient and Debye temperature. Even though few theoretical reports on the structural properties of FeSe are found in the current literature, to our knowledge, this is a novel theoretical investigation on the structural and thermodynamic properties of FeSe at high temperature. We hope that the theoretical results reported here can give more insight into the structural and thermodynamic properties of other iron-based superconductors at high temperature.

Structural transition of FeSe under high pressure

The density functional calculations of the energy band structure and density of state for the tetragonal PbO-type phase α-FeSe and hexagonal NiAs-type phase β-FeSe are reported in this paper. The structural phase transition from tetragonal to hexagonal FeSe under high pressure is investigated, it is found that the calculated transition pressure for the α → β phase transformation is 8.5 GPa. Some fluctuations in the transition pressure maybe occurred by different external factors such as temperature and stress condition. There is about 17% volume collapse accompanying the α → β phase transformation.

Low temperature amorphization and superconductivity in FeSe single crystals at high pressures

In this study, we report low temperature x-ray diffraction studies combined with electrical resistance measurements on single crystals of iron-based layered superconductor FeSe to a temperature of 10 K and a pressure of 44 GPa. The low temperature high pressure x-ray diffraction studies were performed using a synchrotron source and superconductivity at high pressure was studied using designer diamond anvils. At ambient temperature, the FeSe sample shows a phase transformation from a PbO-type tetragonal phase to a NiAs-type hexagonal phase at 10 ± 2 GPa. On cooling, a structural distortion from a PbO-type tetragonal phase to an orthorhombic Cmma phase is observed below 100 K. At a low temperature of 10 K, compression of the orthorhombic Cmma phase results in a gradual transformation to an amorphous phase above 15 GPa. The transformation to the amorphous phase is completed by 40 GPa at 10 K. A loss of superconductivity is observed in the amorphous phase and a dramatic change in the temperature behavior of electrical resistance indicates formation of a semiconducting state at high pressures and low temperatures. The formation of the amorphous phase is attributed to a kinetic hindrance to the growth of a hexagonal NiAs phase under high pressures and low temperatures.

MnTe and ZnTe grown on sapphire by molecular beam epitaxy

We report on growth of MnTe layers by molecular beam epitaxy on Al 2O 3 substrates and of ZnTe layers on hybrid MnTe/Al 2O 3 substrates. The aim of our work was to prepare hexagonal phases of epitaxial thin films of these two materials. In the case of MnTe, the hexagonal NiAs-type phase was prepared by depositing the film directly on Al 2O 3 substrates. On the other hand, the crystal structure of ZnTe layers grown on hybrid MnTe/Al 2O 3 substrates was found to depend on the layer thickness: layers thinner than 0.05 μm grew in a metastable hexagonal wurtzite structure, but with further increases of the thickness, the cubic zinc blende phase of ZnTe tended to appear. The structural properties of MnTe and ZnTe layers were characterized by high energy electron and X-ray diffraction methods. Electrical properties of MnTe films were assessed by the Hall effect measurements. The topography and microstructure were analyzed by atomic force microscope. The Néel temperature and magnetic domains structure of antiferromagnetic hexagonal MnTe layers were obtained from neutron experiments.

Magnetic properties of NiAs-type Fe–Sb–Te alloys

The influence of the addition of Te on the physical and magnetic properties of the binary alloy Fe 0.540Sb 0.460 (i.e. Fe l.173Sb) was studied using X-ray diffraction, Mössbauer spectroscopy and magnetic susceptibility measurements. The specimens are characterized as single NiAs-type phases and their lattice parameters are calculated. The temperature dependence of the hyperfine interaction parameters has shown that iron occupies two distinct crystallographic sites: FeI in octahedrally coordinated regular sites is ordered antiferromagnetically, while FeII occupies pyramidal interstitial positions and remains paramagnetic. The phase transition temperature of FeI is influenced by Fe and/or Te/Sb substitutions. The strength of the covalent bonding of the resonant atom to the lattice is described in terms of the Mössbauer temperature θ M deduced from the temperature dependence of the area under the resonance curve. The findings are clearly correlated with the ionicity variations of the chemical bonds as indicated by the changes in the axial ratio c/ a and the magnetic moments.

Phase relationships and equations of state for FeS at high pressures and temperatures and implications for the internal structure of Mars

In situ X-ray diffraction experiments on FeS up to 22 GPa and 1600 K were carried out using large volume multianvil apparatus, combined with synchrotron radiation at SPring-8. We investigated phase stability relationships of FeS and determined the straight phase boundaries between FeS III (monoclinic phase) and FeS IV (hexagonal phase) to be T ( K)=20P (GPa)+170 and between FeS IV and FeS V (NiAs-type phase) to be T ( K)=39.6P ( GPa)+450 . We also found anomalous behavior in the c/ a ratio, thermal expansion, and isothermal compression of FeS V as well as FeS IV, in the pressure range 4–12 GPa. These anomalies in FeS can be attributed to the spin-pairing transition of Fe, and divides FeS IV and FeS V into the high-spin low-pressure phase (LPP) and the possibly low-spin high-pressure phase (HPP). In order to investigate the internal structure of Mars, we evaluated the equations of state for FeS IV (HPP) and FeS V (HPP). A least square fit to the experimental data yielded K 0 T =62.5±0.9 GPa at T=600 K and (d K 0/d T) P =−0.0208±0.0028 GPa/K for FeS IV (HPP), and K 0 T =54.3±1.0 GPa at T=1000 K and (d K 0/d T) P =−0.0117±0.0015 GPa/K for FeS V (HPP) with fixed K′=4. Thermal expansion coefficients were α=7.16×10 −5+6.08×10 −8 T for FeS IV (HPP) and α=10.42×10 −5 for FeS V (HPP), respectively. Using these equations of state, we examined the internal structure of Mars that has a model mantle composition [Meteoritics 20 (1985) 367] and Fe–FeS core. Our models show that an Mg-silicate perovskite-rich lower mantle is stable only with the Fe-rich core having less than 20 wt.% sulfur. The polar moment of inertia factor C derived from Mars Pathfinder data [Science 278 (1997) 1749] is consistent with any compositions between Fe and FeS for the Martian core, but it excludes the presence of a crust thicker than 100 km.

High-Pressure Phase Relationships for FeS

Phase relationships for FeS polymorph have been studied at the pressure of 16-20 GPa and temperature of 300-1350 K by in situ X-ray observation using a large volume high-pressure apparatus and a synchrotron radiation. Contrary to Fei's prediction [1], we found the stability field of NiAs-type phase of FeS extending at least to 18 GPa. Results of in situ X-ray observation correspond with the NiAs-hexagonal phase boundary determined by Kusaba et al . [2, 3]. Assuming the straight NiAs-hexagonal phase boundary, we estimate that the NiAs-hexagonal-liquid triple junction is located at 39.5 GPa and 2300 K.

Pressure-induced phase transitions in chromium thiospinels

The pressure and temperature dependence of the structure of ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$ and ${\mathrm{Co}}_{0.65}{\mathrm{Ni}}_{0.35}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ have been studied by in situ time-of-flight powder neutron diffraction. While the thiospinel ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$ transforms to a defect NiAs-type phase at 1.3 GPa and $800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ the nickel-doped thiospinel undergoes this phase change at 0.4 GPa and $750\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ Quenching of the hexagonal high-temperature and high-pressure phase resulted in a transition to a monoclinic ${\mathrm{Cr}}_{3}{\mathrm{S}}_{4}$ phase, due to the ordering of the metal vacancies in every second layer. Moreover, the spinel ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$ transforms at room temperature and 8.6 GPa to a phase which does not exhibit a NiAs-type structure.

硫化鉄（ＦｅＳ）の高温高圧下での２次相転移

High-pressure and high-temperature behavior of FeS was investigated up to 16 GPa and 750°C . A temperature-induced phase transition was observed at 4.6 GPa. The NiAs type phase was stable above 420°C, and it transformed to its super-structured phase with a hexagonal cell below 420°C in a temperature cooling process. The phase transition could be explained as a second-order transition, based on the analysis of X-ray diffraction intensities from the super-lattice.

Second-order phase transition of FeS under high pressure and temperature

A temperature-induced second-order phase transition of FeS was observed at 4.6 GPa. The NiAs type phase was stable at 500°C, and it transformed to its super-structured phase with a hexagonal cell near-by 420°C in a temperature cooling process. An abnormal temperature behavior of the c-axis causes an unusually low thermal expansion of FeS in this pressure range, and they can be explained by a relaxation of a repulsion along the c-axis in the hexagonal phase.

Giant magnetoelastic response in MnAs

MnAs has been prepared from elemental powders using a modified ball-milling technique, followed by hot pressing and subsequent annealing. Giant magnetic field-induced uniaxial strains of up to 0.7% were observed in a temperature range above the Curie temperature (TC=305 K). Dilatometric and magnetization measurements showed that the strains were associated with the volume change accompanying a field-induced magnetostructural paramagnetic to ferromagnetic transition above TC. The occurrence of a tricritical point (TCP) in temperature-field space, where the two low temperature phases and the high temperature NiAs-type phase are identical was shown to be responsible for the decrease of the magnitude of both the maximum strain and magnetization change with increasing temperature while approaching the TCP along the line of the first-order transitions. The coordinates of the TCP are approximately Ttr=405 K and Htr=165 kOe.

High pressure and temperature behavior of FeS

High pressure and temperature behavior of stoichiometric FeS (troilite) was studied up to 16 GPa and 800 °C using a combination of a large volume press apparatus and a synchrotron X-ray source. A pressuretemperature phase diagram of FeS was determined from more than 200 data points. The phase diagram shows that there are four stable phases; the troilite phase, the hexagonal phase, the NiAs type phase and the high pressure phase with a monoclinic cell. In the compression curve of FeS at 300 °C, an inflection was observed at 7.5 GPa. Minimum value of cubical expansion under high pressure was also observed around 6 GPa. These results suggested an electronic transition of the hexagonal phase between 6 and 8 GPa. The transition might explain a curved boundary between the hexagonal phase and the monoclinic high pressure phase.

Heat capacity and thermodynamic properties of millerite from 298.15 to 660 K and NiAs-type nickel(II) sulfide from 260 to 1000 K. thermodynamics of the NiAs-type to millerite transition

The heat capacity of synthetic millerite, NiS, has been determined by adiabatic-shield calorimetry from 300 to 660 K where it transforms endothermally to the NiAs polytype (niccolite), Δ trs H m = 6591 ± 50 J mol −1. The measurements were continued to 1000 K. The latter polytype is easily quenched and its heat capacity has been measured from 260 K, through the semimetal-to-metal transition at 270 K with Δ trs H m = 1394 ± 20 J mol −1, and up to 340 K where it transforms exothermally to millerite with Δ trs H m (340 K) = −5774 ± 50 J mol −1. Thermodynamic function values for the NiS phases have been derived through integration of the present results and combining them with low-temperature results from the literature. The enthalpy increment for millerite at 298.15 K to the NiAs-type phase at 1000 K is 47140 J mol −1, or about 7% higher than the drop-calorimetric results in the literature.