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.     

Impedance spectroscopy study of Pb<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript> compound

The lead pyrophosphate, Pb₂P₂O₇, compound was prepared by conventional solid-state reaction and identified by X-ray powder diffractometer. Pb₂P₂O₇ has a triclinic structure whose electrical properties were studied using impedance spectroscopy technique. Both impedance and modulus analysis exhibit the grain and grain boundary contribution to the electrical response of the sample. The temperature dependence of the bulk and grain boundary conductivity were found to obey the Arrhenius law with activation energies E _g = 0.66 eV and E _gb = 0.67 eV, respectively. The scaling behavior of the imaginary part of the complex impedance suggests that the relaxation describes the same mechanism at various temperatures.     

Phase transitions in FeBO<Subscript>3</Subscript> under pressure: DFT + DMFT study

We present a theoretical study of spectral, magnetic, and structural properties of the iron borate FeBO₃. Within the DFT + DMFT method combining density functional theory with dynamical mean-field theory FeBO₃ was investigated under pressures up to 70 GPa at 300 K. We found that FeBO₃ is an insulator with a gap of 2.0 eV with antiferromagnetic ordering at ambient pressure in agreement with experiments. In our calculations, we showed that Fe ions in FeBO₃ undergo a high-spin to low-spin transition under pressure with change from antiferromagnetic to paramagnetic state, and demonstrate that the spin and magnetic transitions occur simultaneously with an isostructural transition at 50.4 GPa with the volume collapse of 13%.     

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.     

Study of crystal and magnetic structures of manganite Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of manganite Pr_0.7Ba_0.3MnO₃ have been studied at high pressures of up to 5.1 GPa and temperatures from 10 to 300 K by means of the neutron diffraction. At normal pressure and a temperature T _C = 200 K, a ferromagnetic state forms in Pr_0.7Ba_0.3MnO₃. At high pressures P ≥ 1.9 GPa and T < T _N ≈ 153 K, a new antiferromagnetic state of A-type have been observed. Under high pressure, the Curie temperature T _C increases with the characteristic quantity dT _C/dP ≈ 2.4 K/GPa. A possible reason for the appearance of an A-type antiferromagnetic phase in Pr_0.7Ba_0.3MnO₃ at high pressures may be anisotropic uniaxial compression of oxygen octahedra along the b axis of the orthorhombic structure.     

Surface impedance of <Emphasis Type="Italic">k</Emphasis>-(BEDT-TTF)<Subscript>2</Subscript>Cu[N(CN)<Subscript>2</Subscript>]Br crystals

Precision measurements of the real and imaginary parts of the microwave surface impedance Z _ac(T) = R _ac(T) + iX _ac(T) of the conducting ac layers of the k-(BEDT-TTF)₂Cu[N(CN)₂]Br crystals in the temperature interval of 0.5 < T < 100 K have demonstrated a series of features: (i) the temperature course of the field penetration depth is close to linear Δλ_ac(T)∞ΔX _ac(T) in the superconducting state at T T _c ∼ 11.5 K; (ii) the curves R _ac(T) = X _ac(T) coincide at T _c < T < 40 K; (iii) the X _ac(T) value at T > 40 K increases in comparison with R _ac(T); (iv) the dependence R _ac(T) at T > 40 K is nonmonotonic in thin crystals. These features of the impedance Z _ac(T) with increasing T are interpreted in terms of (i) the d-type symmetry of the superconducting order parameter, (ii) normal skin effect, (iii) manifestations of the antiferromagnetic fluctuations, and (iv) the size effect. The electrodynamic parameters of k-(BEDT-TTF)₂Cu[N(CN)₂]Br have been determined.     

Impedance analysis of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric ceramic

AC impedance spectroscopy technique has been used to study electrical properties of Bi_3.25La_0.75Ti₃O₁₂ (BLT) ceramic. Complex impedance plots were fitted with three depressed semicircles, which are attributed to crystalline layer, plate boundary and grain boundary and all three were found to comprise of universal capacitance nature [C = C0w ⁿ⁻¹]. Grain boundary resistance and capacitance evaluated from complex impedance plots have larger values than that of plate boundary and crystalline layer. The activation energies (E _a) for DC-conductance in grain boundary, plate boundary and crystalline layer are 0.68 eV, 0.89 eV and 0.89 eV, respectively. Relaxation activation energies calculated from impedance plots showed similar values, 0.81 eV and 0.80 eV for crystalline layer and plate boundary, respectively. These activation energy values are found to be consistent with the E _a value of oxygen vacancies in perovskite materials. A mechanism is offered to explain the generation of oxygen vacancies in BLT ceramic and its role in temperature dependence of DC-conductance study.      

Effect of high pressure on the crystal and magnetic structure and on the raman spectra in Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite

The crystal and magnetic structure and the Raman spectra in Pr_0.7Ba_0.3MnO₃ manganite have been studied by the neutron diffraction technique at pressures up to 5 GPa as well as by the X-ray diffraction and Raman spectroscopy at pressures up to 30 GPa. The pressure dependence is determined for the lattice parameters, unit cell volume, Mn-O bond lengths in the orthorhombic structure of the Imma symmetry, and bending and stretching vibration modes for oxygen octahedra. In the low-temperature range at pressure P = 1.9 GPa, the magnetic transition from the initial ferromagnetic (FM) ground state (T _C = 197 K) to the A-type antiferromagnetic (AFM) state (T _N = 153 K) has been revealed. The FM and AFM phases coexist at pressures up to 5.1 GPa and exhibit negative and positive values of the pressure coefficient for the Curie and Néel temperature, respectively (dT _C/dP = −2.3 K/GPa and dT _N/dP = 8 K/GPa). The pressure dependence of the Curie temperature in Pr_0.7Ba_0.3MnO₃ differs drastically from that observed in other manganites of nearly the same composition with the orthorhombic Pnma and rhombohedral R[`3]cR\bar 3c structures, where the FM phase is characterized by the positive values of dT _C/dP. The structural mechanisms of these phenomena are discussed.     

Effect of high pressure on the crystal and magnetic structures of La<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> cobaltite

The atomic and magnetic structures of La_0.5Ca_0.5CoO₃ cobaltite have been studied by the neutron diffraction technique at high pressures of up to 4 GPa in the 10- to 300-K temperature range. The pressure dependences of the structural parameters have been obtained. The Curie temperature increases with the pressure with the coefficient dT _C/dP = 1 K/GPa, demonstrating the stability of the ground ferromagnetic (FM) state. The pressure dependence of the ground FM state in La_0.5Ca_0.5CoO₃ is in drastic contrast with that for La_{1 − x} Ca _x CoO₃ at a lower calcium content (x < 0.3). For the latter compound, the pressure suppressed the ground FM state and a large negative pressure coefficient of the Curie temperature (dT _C/dP ∼ −10 K/GPa) was observed. The nature of such a phenomenon is analyzed in the framework of the double exchange model also taking into account the changes in the electron configuration of Co³⁺ ions.     

High-precision measurements of the compressibility and the electrical resistivity of bulk <Emphasis Type="Italic">g</Emphasis>-As<Subscript>2</Subscript>Te<Subscript>3</Subscript> glasses at a hydrostatic pressure up to 8.5 GPa

High-precision studies of the volume and the electrical resistivity of g-As₂Te₃ glasses at a high hydrostatic pressure up to 8.5 GPa at room temperature are performed. The glasses exhibit elastic behavior in compression only at a pressure up to 1 GPa, and a diffuse structural transformation and inelastic density relaxation (logarithmic in time) begin at higher pressures. When the pressure increases further, the relaxation rate passes through a sharp maximum at 2.5 GPa, which is accompanied by softening the relaxing bulk modulus, and then decreases, being noticeable up to the maximum pressure. When pressure is relieved, an unusual inflection point is observed in the baric dependence of the bulk modulus near 4 GPa. The polyamorphic transformation is only partly reversible and the residual densification after pressure release is 2%. In compression, the electrical resistivity of the g-As₂Te₃ glasses decreases exponentially with increasing pressure (at a pressure up to 2 GPa); then, it decreases faster by almost three orders of magnitude in the pressure range 2–3.5 GPa. At a pressure of 5 GPa, the electrical resistivity reaches 10^–3 Ω cm, which is characteristic of a metallic state; this resistivity continues to decrease with increasing pressure and reaches 1.7 × 10^–4 Ω cm at 8.1 GPa. The reverse metal–semiconductor transition occurs at a pressure of 3 GPa when pressure is relieved. When the pressure is decreased to atmospheric pressure, the electrical resistivity of the glasses is below the initial pressure by two–three orders of magnitude. Under normal conditions, both the volume and the electrical resistivity relax to quasi-equilibrium values in several months. Comparative structural and Raman spectroscopy investigations demonstrate that the glasses subjected to high pressure have the maximum chemical order. The glasses with a higher order have a lower electrical resistivity. The polyamorphism in the As₂Te₃ glasses is caused by both structural changes and chemical ordering. The g-As₂Te₃ compound is the first example of glasses, where the reversible metallization under pressure has been studied under hydrostatic conditions.     

Phase transformations in calcite from electrical impedance measurements

The phase transformation in calcite I-IV-V and calcite ? aragonite have been characterized by electrical impedance measurements at temperatures 600–1200°C and pressures 0.5–2.5?GPa in a piston cylinder apparatus. The bulk conductivity σ has been measured from Argand plots in the frequency range 10⁵–10^?2?Hz in an electric cell representing a coaxial cylindrical capacitor. The synthetic polycrystalline powder of CaCO₃ and natural crystals of calcite were used as starting materials. The transformation temperature T_c was identified from resistivity-temperature curves as a kink point of the activation energy. At pressure above 2?GPa in ordered phase calcite I, the activation energy E _σ is c. 1.05?eV, and in disordered phase calcite V E _σ is c. 0.75?eV. The pressure dependence of T_c for the rotational order–disorder transformation in calcite is positive for pressures <1?GPa and negative for pressures >1?GPa. The transformation boundary of calcite 1–IV is observed only during first heating in samples after a long annealing at low temperatures. The activation energy of calcite I???IV decreases gradually from 1.8 to 1.05?eV with the pressure increase from 0.5 to 2?GPa. The kinetics of calcite ? aragonite transformation has been monitored by measuring a time-variation of the electrical resistance of a calcite sample at 10³?Hz in the stability P-T field of aragonite. The variation of the impedance correlates with the degree of phase transformation, estimated from X-ray powder diffraction studies on quenched products of experiments. The kinetics of calcite ? aragonite transformation may be fitted to the Avrami kinetics with the exponent m???1–1.5.     

Transformation of the magnetic structure of FeBO<Subscript>3</Subscript> under high pressures

The transformation of magnetic structure under hydrostatic and quasi-hydrostatic pressures up to 4 GPa was studied for iron borate FeBO₃ by the neutron diffraction method. Under quasi-hydrostatic conditions, the orientation of iron magnetic moments changes at pressures P≥1.4 GPa. Under hydrostatic conditions, no changes in the magnetic structure of iron borate were observed up to 2.1 GPa. This behavior is caused by the influence of the inhomogeneity (in magnitude and direction) of elastic stresses on the configuration of magnetic sublattices.     

Thermoelectric properties of La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> manganite at ultrahigh pressures up to 20 GPa

Pressure dependences of the thermopower and electrical resistivity of the La_0.75Ca_0.25MnO₃ manganite are measured in the pressure range 0–20 GPa at room temperature. The absolute value of the thermopower increases in the pressure range 0–3 GPa and decreases at higher pressures. At the same time, the electrical resistivity decreases over the entire pressure range. It is found that the competing effect of the closing of the bandgap, which is determined by the activation energy for the thermopower, and the pressure broadening of the d bands is the cause of the observed behavior of the thermoelectric properties of La_0.75Ca_0.25MnO₃, which is untypical for the majority of dielectrics and semiconductors with single-band unipolar conductivity in the absence of phase transitions and is accompanied by a change in the sign of the pressure coefficient of the thermopower. The interrelation between the magnetic and thermoelectric properties of manganites under pressure is analyzed in the framework of the double exchange model. The causes of the considerable decrease in the pressure coefficients of the insulator-metal transition and Curie temperatures under pressure experimentally observed in manganites are discussed.     

Synthesis and properties of high-pressure phase [Er<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>](V<Subscript>4</Subscript>)O<Subscript>12</Subscript>

A new perovskite-like compound Er_0.73Cu₃V₄O₁₂ (space group Im \(\bar 3\), Z = 2, a = 7.266 Å) has been synthesized barothermally (P = 8.0 GPa, t = 1000°C). Its electrical and magnetic properties have been studied. It is found that the temperature dependence of the electrical conductivity (in the range 78–300 K) has of semiconductor type. The behavior of the impedance and admittance has been analyzed at 290 K and frequencies of 200 Hz to 200 kHz under atmospheric pressure and at high (15–42 GPa) pressures.     

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

High pressure study of the organic compound (TMTTF)<Subscript>2</Subscript>BF<Subscript>4</Subscript>

High pressure resistivity measurements of the organic compound (TMTTF)₂BF₄ have been performed in a newly developped Bridgman cell providing good pressure conditions on a wide pressure range. For the first time in this compound a zero resistance superconducting state is observed between 3 and 4 GPa. At temperatures above the superconducting transition, the resistivities of the two high quality samples show a different behavior. One sample, provides indications for a magnetic quantum critical point at the maximum of T_c, whereas in the other antiferromagnetic spin-fluctuations are present above T_c.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

Magnetic properties of nanoparticle La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> under applied hydrostatic pressure

Pressure effects on magnetic properties of two La_0.7Ca_0.3MnO₃ nanoparticle samples with different mean particle sizes were investigated. Both the samples were prepared by the glycine-nitrate method: sample S—as-prepared (10 nm), and sample S₉₀₀—subsequently annealed at 900 °C for 2 h (50 nm). Magnetization measurements revealed remarkable differences in magnetic properties with the applied pressure up to 0.75 GPa: (i) for S sample, both transition temperatures, para-to-ferromagnetic T _C = 120 K and spin-glass-like transition T _f = 102 K, decrease with the pressure with the respective pressure coefficients dT _C/dP = −2.9 K/GPa and dT _f/dP = −4.4 K/GPa; (ii) for S₉₀₀ sample, para-to-ferromagnetic transition temperature T _C = 261 K increases with pressure with the pressure coefficient dT _C/dP = 14.8 K/GPa. At the same time, saturation magnetization M _S recorded at 10 K decreases/increases with pressure for S/S₉₀₀ sample, respectively. Explanation of these unusual pressure effects on the magnetism of sample S is proposed within the scenario of the combined contributions of two types of disorders present in the system: surface disorder introduced by the particle shell, and structural disorder of the particle core caused by the prominent Jahn–Teller distortion. Both disorders tend to vanish with the annealing of the system (i.e., with the nanoparticle growth), and so the behavior of the sample S₉₀₀ is similar to that previously observed for the bulk counterpart.     

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-pressure neutron diffraction study of BaFe<Subscript>2</Subscript>As<Subscript>2</Subscript>

The crystal structure of BaFe₂As₂ was studied by high-pressure neutron powder diffraction in the pressure range from ambient to 6.5 GPa as well as in the temperature range from 12 K to 293 K at 4.4 GPa and no pressure or temperature induced phase changes were observed. The compression mechanism of BaFe₂As₂ was found to be anisotropic as the a- and c-axes are reduced by 2.49 and 3.66%, respectively at 6.5 GPa. Within the FeAs layers the Fe-As and Fe-Fe bonds decrease by 2.49 and 3.66%, respectively. The Ba-As distance decreases by 3.70% while the As-As inter-atomic distance along the c-axis exhibits a complex pressure dependence. The bulk modulus B ₀ and its pressure derivative B ₀' were determined to be B ₀ = 59(2) GPa and B ₀' = 6.1(7) at ambient temperature.