Thermopower of lead chalcogenides at high pressures

Thermopower measurements on PbX crystals (X=Te, Se, S) at high hydrostatic pressures of up to 35 GPa are reported. New data were obtained on the magnitude and on the pressure dependence of the thermopower of high-pressure semiconducting and metallic phases. The phase transitions occurring in PbX are treated in terms of a model in which the transition to an insulator electronic spectrum is caused by the Peierls lattice distortion.     

Transport and optical properties of iron borate FeBO<Subscript>3</Subscript> under high pressures

The optical absorption spectra of iron borate FeBO₃ were measured in diamond anvil cells at high pressures up to P=82 GPa. The electronic transition with an abrupt jump in the absorption edge from ～3 to 0.8 eV was observed at P≈46 GPa. The resistance and its temperature dependence were directly measured for FeBO₃ at high pressures up to 140 GPa. It was established that the electronic transition at P≈46 GPa was accompanied by the insulator-semiconductor transition. In the high-pressure phase, the thermoactivation gap decreases smoothly at 46<P<140 GPa approximately from 0.55 to 0.2 eV following the linear law. The extrapolated value of the pressure at which the sample becomes fully metallic is equal to about 210 GPa.     

Electronic transitions in the VBO<Subscript>3</Subscript> single crystal at high pressures

Optical absorption spectra of single crystals of the ferromagnetic semiconductor VBO₃ are studied at high pressures up to 70 GPa achieved in a diamond-anvil cell. An electronic transition accompanied by sharp changes in the optical parameters and a decrease in the optical gap from E ₀ = 3.02 eV to 2.25 eV is found at the pressure P _C ～ 30 GPa. The gap does not disappear in the high-pressure phase and its value becomes typical of semiconductors. This is indicative of a semiconductor-semiconductor transition. The transition to the metallic state may occur at the critical pressure P _met ≈ 290 GPa.     

Thermoelectric study of the phase transitions in cerium at ultrahigh pressures from 0 to 20 GPa

The pressure dependences of the thermoelectric power S of Ce samples were measured at pressures P from 0 to 20 GPa in a synthetic diamond cell. The dependence of S on P was found to be nonmonotonic both in the region of transitions from the fcc (γ) phase to the modified fcc (α) phase followed by the transition to the body-centered monoclinic (α″) and the tetragonal (ε) phases at pressures of about 1, 5–6, and 12–15 GPa, respectively, and in the stability region of these phases. The thermoelectric power sign was found to be positive for all high-pressure Ce phases. The found S(P) dependence was compared with the published computational data on the electronic structure of the cerium phases. Cerium was taken as an example to demonstrate the advantage of the thermoelectric studies over other methods of investigation of phase transitions.     

Damage of Dusty Optical Elements in the Field of Continuous-Wave Laser Radiation

The structure and properties of high-pressure phases of iron nitrides Fe₇N₃ in the pressure range of 50–150 GPa have been studied with ab initio calculations within the electron density functional theory. A new phase Amm2-Fe₇N₃, which is the most energetically favorable in the pressure range of 43–128 GPa, has been found using the USPEX (Universal Structure Predictor: Evolutionary Xtallography) algorithms. It has been thermodynamically shown that another high-pressure phase β-Fe₇N₃ is isostructural to a similar phase of iron carbide. The elastic properties have been calculated for all modifications ε-, β-, and Amm2-Fe₇N₃ stable at high pressures.     

Thermophysical studies of the phase transitions in (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>NbOF<Subscript>6</Subscript> crystals

The thermophysical properties of oxyfluoride (NH₄)₃NbOF₆ were studied in detail over wide ranges of temperatures and pressures. At atmospheric pressure, a sequence of four structural phase transitions was established with the following changes in entropy: ΔS ₁ = Rln 2.7, δS ₂ = Rln38.3, ΔS ₃ = 0.08R, and ΔS ₄ = 0.17R. An external hydrostatic pressure was found to narrow the region of existence of the initial cubic phase. A triple point was detected in the p-T diagram; at a pressure above 0.07 GPa, the transition between the tetragonal and monoclinic phases occurs through a distorted high-pressure phase.     

Equation of state and electrical resistivity of the heavy fermion superconductor CeCoIn<Subscript>5</Subscript> to 51 GPa

We have used X-ray diffraction to study the structural phase of CeCoIn₅ in external pressure. Using high-pressure X-ray diffraction, we find that the crystalline phase is stable in the P4/mmm phase for pressures ≤51.2 GPa. From our measured equation of state, we find a bulk modulus given by B ₀ = 72.8 ± 2.9 GPa and a first pressure derivative of B ^′ = 5.1 ± 0.3. Measurement of the electrical resistivity of CeCoIn₅ to pressures as high as 34.4 GPa shows the existence of a peak in resistivity at p ^? = 8.2 ± 0.2 GPa.     

Modeling of Phase Transitions of Graphites to Diamond-Like Phases

The method of the density functional theory is used to study structural transformations between graphites and diamond-like phases. The calculations have been carried out in two approximations: a local density approximation and a generalized gradient approximation. It is found that the phase transitions of hexagonal graphene layers to a cubic diamond and diamond-like phases must occur at uniaxial compressions of ~57–71 GPa, whereas some diamond-like phases can be obtained from tetragonal graphene layers at significantly lower pressures of 32–52 GPa. The X-ray diffraction patterns have been calculated for the phase transition of graphite I4₁/amd to tetragonal LA10 phase that takes place at the minimum pressure that can be used for experimental identification of these compounds.     

Superconducting transition temperature in hafnium under pressures up to 64 GPa

The superconducting transition temperature T _c of hafnium is measured as a function of pressure up to 64 GPa. The character of the pressure dependence of T _c observed at α–ω–β transitions in Hf is found to be similar to that observed for Zr. In the regions of α and β phases, T _c increases with pressure with the slopes dT _c /dP=0.05 and 0.16 K/GPa, respectively. At the α–ω transition, T _c (P) exhibits a tendency to a decrease, while at the ω–β transition, T _c increases stepwise from 5.8 to 8.0 K. The α–ω transition occurs at pressures between 31.2 and 35.9 GPa, and the ω–β transition, at a pressure of 62±2 GPa.     

Structural phase transitions and the equation of state of SnTe at high pressures up to 2 mbar

Synchrotron X-ray diffraction studies of the structure of SnTe have been performed at room temperature and high pressures under the conditions of quasihydrostatic compression up to 193.5 GPa created in diamond anvil cells. Two structural phase transitions have been detected at P ≈ 3 and 23 GPa. The first phase transition is accompanied by a stepwise decrease in the volume of the unit cell by 4% because of the orthorhombic distortion of the initial SnTe-B1 cubic structure of the NaCl type. It has been found that two intermediate rhombic phases of SnTe with the space groups Cmcm and Pnma coexist in the pressure range of 3–23 GPa. The second phase transition at 23 GPa occurs from the intermediate rhombic modification to the SnTe-B2 cubic phase with the CsCl structure type. This phase transition is accompanied by an abrupt decrease in the volume of the unit cell by 8%. The pressure dependence of the volumes per formula unit at room temperature has been determined.     

<Emphasis Type="Italic">In situ</Emphasis> study of the mechanism of formation of pressure-densified Sio<Subscript>2</Subscript> glasses

The volume of glassy a-SiO₂ upon compression to 9 GPa was measured in situ at high temperatures up to 730 K and at both pressure buildup and release. It was established that the residual densification of a-SiO₂ glass after high-pressure treatment was due to the irreversible transformation accompanied by a small change in volume directly under pressure. The bulk modulus of the new amorphous modification was appreciably higher (80% more than its original value), giving rise to residual densification as high as 18% under normal conditions. It was shown that the transformation pressure shifted to a lower pressure of about 4 GPa with a rise in temperature. A conclusion was drawn about the existence of at least two pressure-induced phase transitions accompanied by structure rearrangement in a-SiO₂. A nonequilibrium phase diagram is suggested for glassy SiO₂. It accounts for all the presently available experimental data and is confirmed by the existing modeling data.     

Pressure-induced antiferromagnetism in La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> manganite

The crystal and magnetic structures of La_0.75Ca_0.25MnO₃ manganite are studied under high pressures up to 4.5 GPa in the temperature range 12–300 K by the neutron diffraction method. At normal pressure and temperature T _C = 240 K, a ferromagnetic state is formed in La_0.75Ca_0.25MnO₃. At high pressures P ≥ 1.5 GPa and at temperatures T < T _N ≈ 150 K, a new A-type antiferromagnetic state appears. A further increase in pressure leads to an increase in the volume fraction of the antiferromagnetic phase, which coexists with the initial ferromagnetic phase. The effect of high pressure causes a considerable increase in T _C with the slope dT _C /dP ≈ 12 K/GPa. Calculations performed in the framework of the double exchange model with allowance for the electron-phonon interaction make it possible to explain this pressure dependence of T _C on the basis of experimental data.     

Structural Phase Transitions and the Equation of State in SnSe at High Pressures up to 2 Mbar

The crystal structure of tin selenide SnSe has been studied under quasihydrostatic compression at pressures up to 205 GPa created in diamond anvil cells at room temperature. Two structural phase transitions have been detected at and P ≈ 2.5 32 GPa. The former phase transition is continuous from the GeS-type structure (space group Pbnm) to the TlI-type structure (space group Cmcm). The phase transition to the CsCl-type cubic structure (space group Pm\(\bar 3\)m) occurs at 32 GPa and is accompanied by a stepwise decrease in the volume of the unit cell by 7%. The pressure dependence of the specific volume of the unit cell at room temperature has been constructed up to 205 GPa.     

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.     

Metal-insulator transition in whiskers of the NbS<Subscript>3</Subscript> quasi-one-dimensional conductor under pressure

The effect of pressure on the conduction of the NbS₃ quasi-one-dimensional conductor is studied. A pressure-induced insulator-metal transition is observed. The transition is accompanied by an increase in conductivity by six orders of magnitude at room temperature. Under pressures of 3–4 GPa, an additional phase transition appears in the temperature dependences of resistance. This transition manifests itself in an increase in the local conduction activation energy. The quantity dln(R)/d(1/T) reaches its maximum under pressures of 4–5 GPa, and the temperature position of the maximum of dln(R)/d(1/T) depends on the pressure as T* ≈ 7.5P + 202 K.     

High-pressure effect on the crystal and magnetic structures of the frustrated antiferromagnet YMnO<Subscript>3</Subscript>

The high-pressure (to 5 GPa) effect on the crystal and magnetic structures of the hexagonal manganite YMnO₃ is studied by neutron diffraction in the temperature range 10–295 K. A spin-liquid state due to magnetic frustration on the triangular lattice formed by Mn ions is observed in this compound at normal pressure and T > T_N = 70 K, and an ordered triangular antiferromagnetic state with the symmetry of the irreducible representation Γ₁ arises at T < T_N. The high-pressure effect leads to a spin reorientation of Mn magnetic moments and a change in the symmetry of the antiferromagnetic structure, which can be described by a combination of the irreducible representations Γ₁ and Γ₂. In addition, it is observed that the ordered magnetic moment of Mn ions decreases from 3.27 μ_B (5 GPa) to 1.52 μ_B (5 GPa) at T = 10 K and diffuse scattering is enhanced at temperatures close to T_N. These effects can be explained within the model of the coexistence of the ordered antiferromagnetic phase and the spin-liquid state, whose volume fraction increases with pressure due to the enhancement of frustration effects.     

Optical detection of magnetoplasma resonances in indirect-gap AlAs/AlGaAs quantum wells

The high-pressure properties of a new multiferroic of the langasite family Ba₃TaFe₃Si₂O₁₄ were investigated in diamond-anvil cells (DAC) in the temperature range of 4.2–295 K by a new method of synchrotron Mössbauer spectroscopy. Strong enhancement of the Néel temperature T _N was observed at pressures above 20 GPa associated with the structural transformation. The highest value of T _N is about 130 K which is almost five times larger than the value at ambient pressure (about 27 K). It was suggested that the high value of T _N appears due to redistribution of Fe ions over 3f and 2d tetrahedral sites of the langasite structure. In this case, the short Fe-O distances and favorable Fe-O-Fe bond angles create conditions for strong superexchange interactions between iron ions, and effective two-dimensional (2D) magnetic ordering appears in the (ab) plane. The separation of the sample into two magnetic phases with different T _N values of about 50 and 130 K was revealed, which can be explained by the strong 2D magnetic ordering in the ab plane and 3D ordering involving inter-plane interaction.     

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)].     

Pressure-induced structural transformations,orbital order and antiferromagnetism in La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript>

High pressure evolution of structural, vibrational and magnetic properties of La_0.75Ca_0.25MnO₃ was studied by means of X-ray diffraction and Raman spectroscopy up to 39 GPa, and neutron diffraction up to 7.5 GPa. The stability of different magnetic ground states, orbital configurations and structural modifications were investigated by LDA + U electronic structure calculations. A change of octahedral tilts corresponding to the transformation of orthorhombic crystal structure from the Pnma symmetry to the Immaone occurs above P ~ 6 GPa. At the same time, the evolution of the orthorhombic lattice distortion evidences an appearance of the e _g d _{x² ? z²} orbital polarization at high pressures. The magnetic order in La_0.75Ca_0.25MnO₃ undergoes a continuous transition from the ferromagnetic 3D metallic (FM) ground state to the A-type antiferromagnetic (AFM) state of assumedly 2D pseudo-metallic character under pressure, that starts at about 1 GPa and extends possibly to 20–30 GPa.     

High-pressure magnetic properties and <Emphasis Type="Italic">P-T</Emphasis> phase diagram of iron borate

The high-pressure magnetic states of iron borate ⁵⁷FeBO₃ single-crystal and powder samples have been investigated in diamond anvil cells by nuclear forward scattering (NFS) of synchrotron radiation at different temperatures. In the low-pressure (0 < P < 46 GPa) antiferromagnetic phase, an increase of the Neél temperature from 350 to 595 K induced by pressure was found. At pressures 46–49 GPa, a transition from the antiferromagnetic to a new magnetic state with a weak magnetic moment (magnetic collapse) was discovered. It is attributed to the electronic transition in Fe³⁺ ions from the high-spin 3d⁵ (S = 5/2, ⁶A_1g) to the low-spin (S = 1/2, ²T_2g) state (spin crossover) due to the insulator-semiconductor-type transition with extensive suppression of strong d-d electron correlations. At low temperatures, NFS spectra of the high-pressure phase indicate magnetic correlations in the low-spin system with a magnetic ordering temperature of about 50 K. A tentative magnetic P-T phase diagram of FeBO₃ is proposed. An important feature of this diagram is the presence of two triple points where magnetic and paramagnetic phases of the high-spin and low-spin states coexist.