Electronic and optical properties of Fe<Subscript>2</Subscript>SiO<Subscript>4</Subscript> under pressure effect: ab initio study

We report first-principles studies the structural, electronic, and optical properties of the Fe₂SiO₄ fayalite in orthorhombic structure, including pressure dependence of structural parameters, band structures, density of states, and optical constants up to 30 GPa. The calculated results indicate that the linear compressibility along b axis is significantly higher than a and c axes, which is in agreement with earlier work. Meanwhile, the pressure dependence of the electronic band structure, density of states and partial density of states of Fe₂SiO₄ fayalite up to 30 GPa were presented. Moreover, the evolution of the dielectric function, absorption coefficient (α(ω)), reflectivity (R(ω)), and the real part of the refractive index (n(ω)) at high pressure are also presented.     

First-Principles Study of Pressure-Induced Phase Transition in CuGaO<Subscript>2</Subscript>

We have studied the structural, elastic, electronic properties, and pressure-induced phase transition of CuGaO₂ by using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The obtained ground state properties of three phases were in agreement with previous works. The calculated enthalpy variations with pressure showed that the structural phase transition (β → 3R/2H) appeared at 65.5 ± 1 GPa. The changes in volume and band gap of β phase showed that there was a break between 30 and 40 GPa. The independent elastic constants of three phases were calculated. The 3R, 2H, and β phases were all mechanical stability and behaved in ductile manner under zero pressure.     

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)].     

Phase diagrams of bosonic AB<Subscript>n</Subscript> chains

The AB_{N ?1} chain is a system that consists of repeating a unit cell withN siteswhere between the A and B sites there is an energy difference ofλ. Weconsidered bosons in these special lattices and took into account the kinetic energy, thelocal two-body interaction, and the inhomogenous local energy in the Hamiltonian. We foundthe charge density wave (CDW) and superfluid and Mott insulator phases, and constructedthe phase diagram for N =2 and 3 atthe thermodynamic limit. The system exhibited insulator phases for densitiesρ =α/N, with α being an integer. Weobtained that superfluid regions separate the insulator phases for densities larger thanone. For any N value, we found that for integer densitiesρ, thesystem exhibits ρ +1 insulator phases, a Mott insulator phase, and ρ CDW phases. Fornon-integer densities larger than one, several CDW phases appear.     

Dissimilarity between metallic-like transport in the dielectric spin density wave and field-induced spin density wave states in (TMTSF)<Subscript>2</Subscript>PF<Subscript>6</Subscript>

We report similarities and differences of the transport features in the spin density wave (SDW) and in the field-induced SDW (FISDW) phases of the quasi-one-dimensional compound (TMTSF)₂PF₆. As temperature decreases below ≈2 K, the resistance in both phases exhibits a maximum and a subsequent strong drop. However, the characteristic temperature of the R(T) maximum and its scaling behavior in different magnetic fields B are evidence that the nonmonotonic R(T) dependences have different origin in SDW and FISDW regions of the phase diagram. We also found that the borderline T₀(B, P) which divides the FISDW region of the P-B-T phase diagram into the hysteresis and nonhysteresis domains terminates in the N=1 subphase; the borderline thus has no extension to the SDW N=0 phase.     

Study of the compressibility of FeSi,MnSi, and CoS<Subscript>2</Subscript> transition-metal compounds at high pressures

Silicides and sulfides of transition metals attract great attention of researchers because of a wide spectrum of interesting magnetic, electronic, and optical properties. The crystal structure of FeSi, MnSi, and CoSi silicides is P2₁3(B20), whereas FeS₂, CoS₂, and MnS₂ sulfides have a structure of pyrite Pa3. Despite the great interest in these systems and the cubic symmetry of crystals, the structure and compressibility of these compounds at high pressures are still insufficiently studied. There is a significant spread (more than a factor of two!) in the bulk modulus and its pressure derivative for a single compound. Most studies were performed under nonhydrostatic conditions. In this work, the compressibility of FeSi and MnSi silicides (at pressures up to 35 GPa) and CoS₂ sulfide (up to 22 GPa) has been studied by the X-ray diffraction method in a diamond anvil cell with the use of helium as the softest pressure-transmitting medium. The values obtained for the bulk modulus and its derivative—B = 178 ±3 GPa and B_p = 5.6 ± 0.5 for FeSi, B = 167 ± 3 GPa and B_p^' = 4.6 ± 0.5 for MnSi, and B = 94 ± 2 GPa and B_p^' = 6.9 ± 0.5 for CoS₂—can be considered as the most reliable and can be used to test numerous theoretical models. The results for the compressibility of FeSi are important for the verification of models of the Earth’s core.     

Magnetic phase diagram and correlation between metamagnetism and superconductivity in Ru<Subscript>0.9</Subscript>Sr<Subscript>2</Subscript>YCu<Subscript>2.1</Subscript>O<Subscript>7.9</Subscript>

The magnetic superconductorRu_0.9Sr₂YCu_2.1O_7.9 (Ru-1212Y) has beeninvestigated using neutron diffraction under variable temperature and magnetic field. Withthe complementary information from magnetization measurements, we propose a magnetic phasediagram T-H for the Ru-1212 system. Uniaxialantiferromagnetic (AFM) order of 1.2μ _B /Ruatoms with moments parallel to the c-axis is found below the magnetictransition temperature at  ~140 K in the absence of magnetic field. In addition,ferromagnetism (FM) in the ab-plane develops below  ~120 K, butis suppressed at lower temperature by superconducting correlations. Externally appliedmagnetic fields cause Ru-moments to realign from the c-axis to theab-plane, i.e. along the ?1,1,0? direction, and induce ferromagnetismin the plane with  ~1μ _B at 60 kOe.These observations of the weak ferromagnetism suppressed by superconductivity and thefield-induced metamagnetic transition between AFM and FM demonstrate not only competingorders of superconductivity and magnetism, but also suggest a certain vortex dynamicscontributing to these magnetic transitions.     

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.     

Stabilization of Ferromagnetism in BiFeO<Subscript>3</Subscript>:Ho at Hydrostatic Pressure

The isothermal magnetization of the Bi_{1 – x}Ho _x FeO₃ (x = 0?0.2) multiferroic has been studied at a hydrostatic pressure up to 9 GPa in the range of room temperatures. A new anomaly at P_C ≈ 3.81 GPa related to intermediate phases between the structural transition R3c → Pnma has been found against the background of the pressure-induced antiferromagnetic ordering in BiFeO₃ (BFO) at P ≈ 2.59 GPa. It is established that the ferromagnetic behavior under pressure depends on the Ho impurity concentration: P_C decreases at 0.05 ≤ x ≤ 0.1 because of the decrease in R3c bond lengths in the structure, and the stabilization of ferromagnetism is implemented at 0.1 ≤ x ≤ 0.2 probably because of the coexistence of the R3c and Pnma phases. The results of studies indicate that, in Bi_{1 – x}Ho _x FeO₃ with x = 0.2, the transition pressure P_C = 3.7 GPa exceeds the values for BFO doped with other 4f elements (Eu, Y, Sm) in the region R3c → Pnma of the transition.     

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.     

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 structural transition at high hydrostatic pressures in the BiFeO<Subscript>3</Subscript> crystal

Change in the crystal structure of the BiFeO₃ multiferroic at high pressures up to 70 GPa in a diamond anvil cell has been studied by the method of synchrotron x-ray diffraction at room temperature. The experiment has been carried out under hydrostatic conditions with helium as a pressure-transferring medium. An anomaly has been observed in the behavior of the structural parameters at pressures P _c ≈ 40?50 GPa. This anomaly correlates with the effect of the magnetic collapse of iron moments revealed in this pressure range. It has been found that the bulk compression modulus is equal to B ₀ = (75.5 ± 15.5) GPa in the interval 0 < P < P _c and is almost quadrupled to a value of B = (292 ± 9) GPa in the interval P > P _c. When the pressure decreases, the behavior of the structural parameters is completely reversible in correlation with the reversibility of the magnetic transition. The “diffuseness” of the structural transition in pressure is explained by thermal fluctuations between the high-and low-spin states of Fe³⁺ ions in the transition region.     

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.     

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.     

Studies on structural and electrical properties of Mg<Subscript>0. 5+y</Subscript> (Zr<Subscript>2-y</Subscript>Fe<Subscript>y</Subscript>) <Subscript>2</Subscript> (PO<Subscript>4</Subscript>) <Subscript>3</Subscript> ceramic electrolytes

The sample of Mg_{0. 5+y} (Zr_1-y Fe_y) ₂ (PO₄) ₃ (0.0 ≤y ≤0.5) was synthesized using the sol-gel method. The structures of the samples were investigated using X-ray diffraction and Fourier transform infrared spectroscopy measurement. XRD studies showed that samples had a monoclinic structure which was iso-structured with the parent compound, Mg_0.5Zr (PO₄) ₃. The complex impedance spectroscopy was carried out in the frequency range 1–6 MHz and temperature range 303 to 773 K to study the electrical properties of the electrolytes. The substitutions of Fe³⁺ with Zr⁴⁺ in the Mg_0.5Zr (PO₄) ₃ structure was introduced as an extrainterstitial Mg²⁺ ion in the modified structured. The compound of Mg_0.5+y (Zr_1-y Fe_y)₂(PO₄)₃ with y?=?0.4 gives a maximum conductivity value of 1.25?×?10^?5 S cm^?1 at room temperature and 7.18?×?10^?5 S cm^?1 at 773 K. Charge carrier concentration, mobile ion concentration, and ion hopping rate are calculated by fitting the conductance spectra to power law variation, σ _ac (ω)?=?σ _o ? +?Aω ^α . The charge carrier concentration and mobile ion concentration increases with increase of Fe³⁺ inclusion. This implies the increase in conductivity of the compounds was due to extra interstitial Mg²⁺ ions.     

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.     

Thermodynamics of a Magnetic Transition in MnS<Subscript>2</Subscript> at High Pressures

The behavior of the specific heat of MnS₂ at high pressures has been studied. A significant increase in the transition temperature T_N to an antiferromagnetic state with the pressure from 48.2 K at atmospheric pressure to 76 K at a pressure of 5.3 GPa has been revealed. The initial pressure derivative is dT _N /dP = 4.83 K/GPa. It has been found that the parameter α = d(logT _N )/d(logV ) = ?6.6 ± 0.1 is significantly different from the value α = ?10/3 ≈ ?3.3 (Bloch relation), which is typical of numerous antiferromagnetic insulators—transition- metal oxides and fluorides. The volume jump at the magnetic transition point has been estimated. The necessity of direct dilatometric measurements of the volume has been justified.     

Transport and magnetic properties of a Zn<Subscript>0.1</Subscript>Cd<Subscript>0.9</Subscript>GeAs<Subscript>2</Subscript> + 10 wt % MnAs composite with magnetic clusters at high pressure

The temperature (T = 77–420 K) dependences of the electrical resistivity and the magnetization, the magnetic-field (H ≤ 5 kOe) and pressure (P ≤ 7 GPa) dependences of the resistivity, the Hall coefficient, and the magnetization have been measured in the Zn_0.1Cd_0.9GeAs₂ + 10 wt % MnAs composite with the Curie temperature T _C = 310 K. The magnetoresistive effect has been observed at high hydrostatic pressure to 7 GPa. At nearly room temperature, the pressure dependence of the magnetization demonstrated a transition from the ferromagnetic to paramagnetic state at P ~ 3.2 GPa that was accompanied by the semiconductor–metal phase transition.     

Structural transformations in liquid,crystalline, and glassy B<Subscript>2</Subscript>O<Subscript>3</Subscript> under high pressure

We present in situ (x-ray diffraction) and ex situ (quenching) structural studies of crystalline, liquid, and glassy B₂O₃ up to 9 GPa and 1700 K, drawing equilibrium and nonequilibrium phase diagrams of B₂O₃. Particularly, we have determined the melting curve, the stability regions for crystalline B₂O₃ I and B₂O₃ II modifications, the regions of transformations, such as densification or crystallization, for both the liquid and glassy states, including the region of sharp first-order-like transition in liquid B₂O₃ to a high-density phase near 7 GPa. Quenching experiments also show that the transition to the high-density liquid can occur at much lower pressures in nonstoichiometric melts with an excess of boron. B₂O₃ is the first glassformer whose transformations in the disordered state have been comparatively studied for both liquid and glassy phases.     

Spontaneous ferroelectricity in strained low-temperature monoclinic Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>: A first-principles study

As a single-phase multiferroic material, Fe₃O₄ exhibits spontaneous ferroelectric polarization below 38 K. However, the nature of the ferroelectricity in Fe₃O₄ and effect of external disturbances such as strain on it remains ambiguous. Here, the spontaneous ferroelectric polarization of low-temperature monoclinic Fe₃O₄ was investigated by first-principles calculations. The pseudo-centrosymmetric FeB42–FeB43 pair has a different valence state. The noncentrosymmetric charge distribution results in ferroelectric polarization. The initial ferroelectric polarization direction is in the -x and -z directions. The ferroelectricity along the y axis is limited owing to the symmetry of the Cc space group. Both the ionic displacement and charge separation at the FeB42–FeB43 pair are affected by strain, which further influences the spontaneous ferroelectric polarization of monoclinic Fe₃O₄. The ferroelectric polarization along the z axis exhibits an increase of 45.3% as the strain changes from 6% to -6%.