Structural,magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure

At ambient pressure CaFe₂As₂ has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T ～ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ～0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ～1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe₂As₂ transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.     

Magnetic ordering in Fe<Subscript>1.087</Subscript>Te under pressure

The crystal and magnetic structures of Fe_1.087Te have been studied by neutron powder diffraction in the temperature range from 1.7 to 80 K at pressures of  ≈0.4 and ≈1.2 GPa. No symmetry change of the tetragonal paramagnetic ambient pressure phase (space group P4/nmm) was observed for temperatures above 60 K and pressures up to  ≈1.2 GPa. A novel pressure-induced phase of Fe_1.087Te having orthorhombic symmetry (space group Pmmn) and incommensurate antiferromagneticbicollinear order was observed in the temperature range from 50 to 60 K at  ≈1.2 GPa. The known monoclinic ambient pressure phase of Fe_1.087Te (space group P2 ₁/n) with commensurate antiferromagnetic order was found to be stable up to at least  ≈1.2 GPa at low temperature.     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

A synchrotron study of high-pressure transformations in TiH0·74

Abstract

The crystal structure of the TiH_0·74 alloy was studied by the energy dispersive X-ray diffraction technique in the pressure range to 30·5 GPa at temperatures to 630 K. A phase transformation to the (η + ω) two-phase state was found to occur above 7 GPa at room temperature, then (η+ω)-TiH_0·74 remained stable up to P=30·5 GPa. Another phase transformation resulting in a single-phase state, ζ-TiH_0·74, was found to occur upon heating (η+ω)-TiH_0·74 above T ? 560 K. Both high-pressure phases, η and ζ, were indexed on the basis of the tetragonal sublattices of the Ti atoms with nearly the same specific volumes. It is assumed from the relation of the specific volumes that the hydrogen atoms occupy the tetrahedral interstices in the ζ-phase and the octahedral interstices in the η-phase.     

Distribution of light alkanes in the reaction of graphite hydrogenation at pressure of 0.1–7.8 GPa and temperatures of 1000–1350°C

ABSTRACT

We studied the effect of pressure and temperature on the hydrocarbon (HC) chain length distribution and total amount of HCs in the reaction of direct graphite hydrogenation at pressures of 0.1–7.8?GPa and temperatures of 1000–1350°C. An increase in pressure was found to lead both to an increase in the absolute yield of HCs due to direct graphite hydrogenation and to chain elongation of HC products. Light alkanes predominate among HCs in the entire studied range of P–T parameters. However, their concentration in quenched fluids increases as pressure is elevated, from less than 10?rel.% at 0.1?GPa to more than 40–50?rel.% at P?≥?3.8?GPa. Methane is actually the only light alkane among reaction products at 0.1?GPa and 1000°C, while it is a minor component at 7.8?GPa and 1350°C. The most stable alkane at pressures above 3.8?GPa is ethane (C₂H₆).     

Pressure-induced superconductivity and structural transitions in Ba(Fe<Subscript>0.9</Subscript>Ru<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

Electrical transport and structural characterizations of isoelectronically substituted Ba(Fe_0.9Ru_0.1)₂As₂ have been performed as a function of pressure up to ～ 30 GPa and temperature down to ～ 10 K using designer diamond anvil cell. Similar to undoped members of the AFe₂As₂ (A = Ca, Sr, Ba) family, Ba(Fe_0.9Ru_0.1)₂As₂ shows anomalous a-lattice parameter expansion with increasing pressure and a concurrent ThCr₂Si₂ type isostructural (I4/mmm) phase transition from tetragonal (T) phase to a collapsed tetragonal (cT) phase occurring between 12 and 17 GPa where the a is maximum. Above 17 GPa, the material remains in the cT phase up to 30 GPa at 200 K. The resistance measurements show evidence of pressure-induced zero resistance that may be indicative of high-temperature superconductivity for pressures above 3.9 GPa. The onset of the resistive transition temperature decreases gradually with increasing pressure before completely disappearing for pressures above ～ 10.6 GPa near the T-cT transition. We have determined the crystal structure of the high-T _c phase of Ru-doped BaFe₂As₂ to remain as tetragonal (I4/mmm) by analyzing the X-ray diffraction pattern obtained at 10 K and 9.7 ± 0.7 GPa, as opposed to inferring the structural transition from electrical resistance measurement, as in a previous report [S.K. Kim, M.S. Torikachvili, E. Colombier, A. Thaler, S.L. Bud’ko, P.C. Canfield, Phys. Rev. B 84, 134525 (2011)].     

High-spin-low-spin transition in magnesiowüstite (Mg<Subscript>0.75</Subscript>,Fe<Subscript>0.25</Subscript>)O at high pressures under hydrostatic conditions

The spin states of Fe²⁺ ions in (Mg_0.75,Fe_0.25)O magnesiowüstite crystals at hydrostatic pressures up to 90 GPa created in a diamond-anvil cell with helium as a pressure-transmitting medium have been investi-gated by transmission and synchrotron Mössbauer spectroscopy at room temperature. An electron transition from the high-spin (HS) state to the low-spin (LS) state (HS-LS crossover) has been observed in the pressure range of 55–70 GPa. The true HS-LS transition occurs in a narrow pressure range and the extension of the electron transition to ～15 GPa is attributed to the effect of the nearest environment and to thermal fluctuations between the high-spin and low-spin states at finite temperatures. It has been found that the lowest pressure at which the electron HS-LS transition can occur in the Mg_{1 ? x} Fe _x system is 50–55 GPa.     

Exploration of large amplitude motions in the Ca+Ar2 complex

ABSTRACT

We present a theoretical study of the ground electronic state potential of the Ca⁺Ar₂ complex and of its photoabsorption spectra, simulated at temperatures ranging between 20 and 220?K. These calculations exploit a Monte-Carlo (MC) method, based on a one-electron pseudo-potential approach. A pairwise additive potential fitted to coupled cluster ab initio points, is used to model the Ca⁺Ar₂ complex. Our study shows that the most stable form of Ca⁺Ar₂ is a bent C_2v structure, whereas the linear isomer is located at around 90?±?10?cm^?1 above in energy. The analysis of the photoabsorption spectra establishes that a structural transition from bent Ca⁺Ar₂ to linear ArCa⁺Ar occurs at T～100?K. Trends in binding energies of both isomers, bond lengths and bond angles are also discussed. Molecular orbital overlaps provide an explanation for the order of stability between the bent and linear structures.     

Anomalies in the velocity of longitudinal ultrasonic waves in cesium under the pressure up to 5 GPa

Abstract

The velocity of longitudinal ultrasonic waves, ν₁,(P), in polycrystalline cesium was measured at 293 ±1K in the pressure interval 0–5 GPa. v ₁(P) alterations at BCC-FCC phase transition at 2.3 GPa and at the electronic-structure transformation at 4.3 GPa were obtained. Decrease of v ₁(P) to 4.3 GPa after a maximum at ～3.8 GPa were found, that gave evidence to the appearance of a corresponding soft mode in the FCC-Cs phonon spectrum. The peculiarities of dependence v ₁(P) correlate with s-conduction electrons promotion to the empty d-band in accordance with the theoretically predicted continuous electronic s-d transformation in Cs.     

In-situ X-ray diffraction study on β-CrOOH at high pressure and high-temperature

ABSTRACT

High pressure hydrous phases with distorted rutile-type structure have attracted much interest as potential water reservoirs in the Earth’s mantle. An in-situ X-ray diffraction study of β-CrOOH was performed at high pressures of up to 6.2?GPa and high-temperatures of up to 700?K in order to clarify the temperature effect on compression behaviors of β-CrOOH. The P-V-T data fitted to a Birch–Murnaghan equation of state yielded the following results: isothermal bulk modulus K_T0?=?191(4)?GPa, temperature derivative (?K_T/?T)_P?=??0.04(2)?GPa?K^?1, and volumetric thermal expansion coefficient α?=?3.3(2)?×?10^?5?K^?1. In this study, at 300?K, the a-axis became less compressible at pressures above 1–2?GPa. We found that the pressure where the slopes of a/b and a/c ratios turned positive increased with temperature. This is the first experimental study indicating the temperature dependence of the change in the axial compressibility in distorted rutile-type M³⁺OOH.     

Pressure induced anisotropic magnetovolume phenomena in Lu2Fe17 Intermetallics

Magnetisation and magnetic susceptibility of a Lu₂Fe₁₇ single crystal have been studied under hydrostatic pressure up to 1.2 GPa at temperatures down to 5 K using a SQUID magnetometer. The ferromagnetic phase of Lu₂Fe₁₇ is suppressed rapidly above a critical pressure P _C = 0.4 GPa in the whole temperature range below the critical temperature T _C . A magnetic phase diagram of Lu₂Fe₁₇ has been constructed using results of the magnetic susceptibility measurements under pressure. A pressure induced incommensurate antiferromagnetic phase exhibits metamagnetic transitions with the increasing critical magnetic field H _C under pressure. Taking into account recent neutron diffraction data, the pressure induced anisotropic changes of the lattice parameters of the Lu₂Fe₁₇ are discussed.     

Quadrupolar susceptibility and magnetic phase diagram of PrNi2Cd20 with non-Kramers doublet ground state

ABSTRACT

In this study, ultrasonic measurements were performed on a single crystal of cubic PrNi₂Cd₂₀, down to a temperature of 0.02?K, to investigate the crystalline electric field ground state and search for possible phase transitions at low temperatures. The elastic constant (C₁₁?C₁₂)/2, which is related to the Γ₃-symmetry quadrupolar response, exhibits the Curie-type softening at temperatures below ～30?K, which indicates that the present system has a Γ₃ non-Kramers doublet ground state. A leveling-off of the elastic response appears below ～0.1?K toward the lowest temperatures, which implies the presence of level splitting owing to a long-range order in a finite-volume fraction associated with Γ₃-symmetry multipoles. A magnetic field–temperature phase diagram of the present compound is constructed up to 28?T for H || [110]. A clear acoustic de Haas–van Alphen signal and a possible magnetic-field-induced phase transition at H ～26?T are also detected by high-magnetic-field measurements.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Direct measurement of magnetization isotherms of Fe64Ni36 Invar alloy in a diamond anvil cell

Magnetization isotherms of the Fe₆₄Ni₃₆ Invar alloy have been measured under pressure up to 5.3?GPa in magnetic field up to 5?T using a diamond anvil cell and SQUID magnetometer. The unambiguous change of the pressure parameter dlnM_S/dP (from ?9 to ?13×10^?2?GPa^?1) has been observed in a narrow pressure interval from 2.5 to 3.5?GPa at all temperatures in the range from 5 to 300?K. The pressure interval, where the sharp decrease in magnetization was observed, coincides with the critical pressures of the pressure-induced decrease in Fe-moment that were determined by the X-ray Magnetic Circular Dichroism and the X-ray Emission Spectra studies, recently. The pronounced decrease in the Curie temperature of the Fe₆₄Ni₃₆ alloy under pressure, dT_C/dP = ?44 ±2?K/GPa, has been confirmed.     

In situ X-Ray study of thermal expansion and phase transition of iron at multimegabar pressure

The density of varepsilon-iron has been calculated at pressures and temperatures up to 300 GPa and 1300 K, respectively. We observe varepsilon to beta phase transition at pressures between 135 and 300 GPa and temperature above 1350 K; the pattern can be interpreted in terms of double hexagonal close-packed structure. The density calculated at high pressure and temperature (330-360 GPa and 5000-7000 K) closely matches with preliminary reference Earth model density, thereby imposing constraint on the composition of the Earth's inner core.     

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

Abstract

The phase transition of orthorhombic sulphur α-S₈ to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S₈ molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S₈. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm² at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S₆ can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.     

Phase Transition and vibration properties of MnCO3 at high pressure and high-temperature by Raman spectroscopy

The high pressure and high-temperature behavior of MnCO₃ was investigated up to 55?GPa at ambient temperature and up to 573?K at ambient pressure by Raman spectroscopy, respectively. Some new modes were detected at ～16 and ～32?GPa, which were assigned to MnCO₃-I below 16?GPa and to MnCO₃-II above 32?GPa, and to a coexisting phase of them in between. The high pressure vibration properties of all Raman modes, especially high frequency modes, were systematically reported. The coexisting phase of MnCO₃-I and MnCO₃-II had much easier compressibility than the MnCO₃-II phase. The thermal stability of MnCO₃ was at least to 573?K and its thermal expansion along the c axis was easier than a and b axes.     

The Fe–Fe2P phase diagram at 6 GPa

Here, we report experimental results on melting and subsolidus phase relations in the Fe–Fe₂P system at 6?GPa and 900–1600°C. The system has two P-bearing compounds: Fe₃P and Fe₂P. X-ray diffraction patterns of these compounds correspond to schreibersite and barringerite, respectively. The Fe–Fe₃P eutectic appears at 1075°C and 16?mol% P. Schreibersite (Fe₃P) melts incongruently at 1250°C to produce barringerite (Fe₂P) and liquid containing 23?mol% P. Barringerite (Fe₂P) melts congruently at 1575°C. Maximum solid solution of P in metallic iron at 6?GPa is 5?mol%. As temperature increases to 1600°C, the P solubility in the metallic iron decreases to 0.5?mol%, whereas the P content in coexisting liquid decreases to 3?mol%. The composition of quenched phases from Fe–P melt coincides with the compositions of equilibrium phases at corresponding temperature. Consequently, the composition of quenched products of Fe-P melts in meteorites can be used for reconstruction of P–T conditions of their crystallization under ambient or low pressures or during shock melting by impact collisions.     

High–pressure electrical conductivity and Raman spectroscopy of chalcanthite

Abstract

The phase transitions and dehydration of chalcanthite were investigated by electrical conductivity and Raman spectroscopy at 1.0–24.0?GPa and 293–673?K in a diamond anvil cell. At ambient temperature, two secondary phase transitions were observed according to discontinuous changes in the slope of Raman shifts, full width at half maximum and electrical conductivities at ～7.3 and ～10.3?GPa. The dehydration temperatures were determined by the splitting of Raman peaks and changes in electrical conductivity as ～350 and ～500?K at respective ～3.0 and ～6.0?GPa. A positive relationship for chalcanthite between dehydration temperature and pressure is established.