Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Crystal-structure and magnetic phase transformations in solid solutions of BiFeO<Subscript>3</Subscript>-AFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (A = Ca,Sr, Ba,Pb)

Solid solutions of Bi_{1 ? x} A _x (Fe_{1 ? x/2}Nb _x/2)O₃, where A = Ca, Ba, and Pb, are obtained and their crystal structure and magnetic properties are investigated. It is shown that for A = Ca and x ≈ 0.15, the symmetry of the unit cell changes from rhombohedral (space group R3c) to orthorhombic (Pbnm). The transformation leads to the emergence of spontaneous magnetization due to the Dzyaloshinskii-Moriya interaction. Solid solutions with A = Pb remain rhombohedral up to a concentration of x = 0.3. Spontaneous magnetization sharply increases in the compound with x ≈ 1 at low temperatures and is due to the formation of the spin-glass component.     

Ferroelectric phase transition in orthorhombic CdTiO<Subscript>3</Subscript>: First-principles studies

Parameters of the crystal structure and phonon spectra for orthorhombic cadmium titanate with space group Pbnm and its two possible ferroelectrically distorted phases (with space groups Pbn2₁ and Pb2₁ m) were calculated from first principles within the density functional theory. The obtained structural parameters and frequencies of Raman- and infrared-active modes are in good agreement with available experimental data for the Pbnm phase. Expansion of the total energy in a Taylor series of two order parameters showed that the ground state of the system corresponds to the Pbn2₁ structure into which the Pbnm phase transforms through a second-order phase transition without intermediate phases. A substantial discrepancy between calculated and experimentally observed lattice distortions and spontaneous polarization in the polar phase was explained by quantum fluctuations, as well as by existence of twins and competing long-period structures.     

Lattice dynamics of BiFeO<Subscript>3</Subscript> under hydrostatic pressure

The vibrational frequencies of the BiFeO₃ crystal lattice in the cubic phase (Pm3m) and the rhombohedral paraelectric phase (R3c) are calculated in terms of the ab initio model of an ionic crystal with the inclusion of the dipole and quadrupole polarizabilities. In the ferroelectric phase with the symmetry R3c, the calculated spontaneous polarization of 136 μC cm^?2 agrees well with the experimental data. The dependences of the unit cell volume, the elastic modulus, and the vibrational frequencies on the pressure are calculated. It is found that the frequency of an unstable ferroelectric mode in both the cubic (Pm3m) and rhombohedral (R3c) phases are almost independent of the applied pressure, in contrast to classical ferroelectrics with a perovskite structure, where the ferroelectric instability is very sensitive to a variation in the pressure.     

Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

Optical investigations of structural phase transformations in PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-<Emphasis Type="Italic">x</Emphasis>PbTiO<Subscript>3</Subscript> single crystals located at the morphotropic phase boundary

A study has been made of the effect of a dc electric field (0 < E < 4 kV/cm) on the optical transmittance of single-crystal compounds PbMg_1/3Nb_2/3O₃-xPbTiO₃ (PMN-xPT) located at the boundaries of the morphotropic region (x = 32.0 and 36.5%) and directly at the center of the morphotropic region (x = 35%). It is shown that, at temperatures close to the morphotropic phase transition point, the electric field induces two phase transitions in PMN-32PT and PMN-35PT crystals and only one phase transition in PMN-36.5PT. The tetragonal (T) phase induced in all three compounds remains stable after the electric field is removed only in crystals with x = 35.0 and 36.5%, whereas the T phase is metastable and transforms into the monoclinic M _c phase after the field is switched off in the PMN-32PT crystals lying at the boundary of the morphotropic region on the rhombohedral side. It is found that the electric-field-induced intermediate phase M _c in PMN-35PT is inhomogeneous and that M _c transforms into the tetragonal phase in a continuous transition. It is suggested that only the presence of a third orthorhombic phase can account for the continuous character of the transition between the M _c and T phases in PMN-35PT crystals. The results obtained are interpreted in terms of the Devonshire theory for strongly anharmonic crystals. The E-T phase diagrams are constructed for all the crystals.     

Neutron-diffraction study of the crystal structure of BaTiO<Subscript>3</Subscript> ferroelectric doped with iron

The crystal structure of iron-doped barium titanate BaTi_1–x Fe _x O₃ is studied by neutron diffraction in the range of 0 ≤ x ≤ 0.12. At low concentrations of iron, x < 0.01, and at room temperature, these compounds have a polar structure with tetragonal symmetry with space group P4mm. The temperature of the transition of the tetragonal ferroelectric phase into the cubic paraelectric phase with space group Pm \(\bar 3\) m for an iron concentration of x = 0.01 is 390 K (for pure BaTiO₃, it is 410 K). At an iron concentration of x = 0.07, the crystal structure of the studied compounds varies, and it is described by the centrosymmetric hexagonal space group P6₃/mmc. The structural parameters of various phases of compound BaTi_1–x Fe _x O₃ are determined from the experimental data.     

Superposition of M<Subscript>5</Subscript>X<Subscript>5</Subscript> superstructures and X-ray diffraction in TiO<Subscript>1.0</Subscript> titanium monoxide

The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO_1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3?m) M₅X₅ superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M₅X₅ cubic superstructure. The transition state model can be considered as an M_(5–i)X_(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1).     

Metamagnetic effects in epitaxial BaFe<Subscript>1.8</Subscript>Cr<Subscript>0.2</Subscript>As<Subscript>2</Subscript> thin films

Epitaxial BaFe_1.8Cr_0.2As₂ thin films with the tetragonal c-axis perpendicular to the thin film surface were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) single crystalline substrates using pulsed laser deposition (PLD). Resistive measurements indicate the existence of two transitions at temperatures of about 80 K and 40 K. The transition at 80 K is attributed to the structural transition from the high temperature tetragonal phase to the low temperature orthorhombic phase accompanied with the magnetic transition from a paramagnetic to an antiferromagnetic state as known for doped bulk systems. Below T ≈ 40 K the magnetization curves measured perpendicularly to the orthorhombic c-axis in fields up to 9 Tesla show two inflexion points indicating metamagnetic transitions.     

Heat capacity,structure, and <Emphasis Type="Italic">p-T</Emphasis> phase diagram of elpasolite (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>KMoO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Thermophysical and structural studies of an (NH₄)₂KMoO₃F₃ crystal show that this crystal belongs to the family of elpasolites (space group \(Fm\bar 3m\)) and undergoes an order-disorder phase transition at T ₀ = 241.5 K. Under hydrostatic pressure, this phase transition splits into two consecutive transitions at the tricritical point with parameters T _tr = 232.5 K and p _tr=0.21 GPa. It was found that anomalous hysteresis and relaxation phenomena accompany the transitions from the cubic to both distorted phases. The results are analyzed taking into account the data on the phase transition in the related elpasolite (NH₄)₂KWO₃F₃.     

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.     

Comparative study of the spectral and structural properties of EuAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals with different morphologies

Europium alumoborate EuAl₃(BO₃)₄ microcrystals have been synthesized by the flux method at a temperature of 1050°C. The obtained crystals have different morphologies: both plane-faced and skeletal microcrystals have been observed. Infrared spectroscopy, cathodeluminescence, and transmission electron microscopy investigations of individual microcrystals showed that the spectral and structural characteristics of these morphological forms coincide. The obtained crystals are characterized by the rhombohedral symmetry (sp. gr. R32) with the inclusions of C2/c monoclinic phase domains.     

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.     

The two-phase crystal structure and magnetic properties of the LaCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>x</Subscript>O<Subscript>3−<Emphasis Type="Italic">d</Emphasis></Subscript> system

The preparation conditions and magnetic properties of the La(Co_1?xFe_x)O3?d system of solid solutions were studied. Irrespective of synthesis conditions, the samples with x ≤ 0.15 were rhombohedral (space group \(R\bar 3c\)), and those with x ≥ 0.7, orthorhombic (space group Pnma). In the intermediate region of 0.15 < x < 0.7, the samples could be mixtures of rhombohedral and orthorhombic phases. Structural heterogeneity considerably decreased as the temperature of the synthesis increased or after annealing in a reducing medium. The samples containing more than 40% iron exhibited weak spontaneous magnetization, as is characteristic of weak ferromagnets. The magnetic properties and structure depend on the content of oxygen. A decrease in the concentration of oxygen sharply increases magnetic anisotropy; the coercive force is then close to 10 kOe, which is more than that known for other oxide systems. The magnetic properties are explained in terms of the model according to which Co³⁺ ions are in the low-spin state.     

Thermophysical study of structural phase transitions in Na<Subscript>0.95</Subscript>Li<Subscript>0.05</Subscript>NbO<Subscript>3</Subscript> solid solution

Temperature dependences of specific heat C_p(T) and coefficient of thermal expansion ;(T) for Na_0.95Li_0.05NbO₃ sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The C_p(T) and α(T) anomalies at T₃ = 310 ± 3 K, T₂ = 630 ± 8 K, and T₁ = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ? Q ? S(R) ? T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition Q → R/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the C_p(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed.     

Band structure of (Sr<Subscript>3</Subscript>Sc<Subscript>2</Subscript>O<Subscript>5</Subscript>)Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> as a possible basis phase of new FeAs superconductors

The results of the ab initio FLAPW-GGA computations of the band structure of the recently synthesized layered tetragonal (space group I4/mmm) arsenide (Sr₃Sc₂O₅)Fe₂As₂ as a possible basis phase of a new group of FeAs superconductors are presented. For (Sr₃Sc₂O₅)Fe₂As₂, the energy bands, electron state density distributions, Fermi surface topology, low-temperature electron specific heat, molar Pauli paramagnetic susceptibility, and effective atomic charges have been determined. These results are discussed compared to similar data for the layered tetragonal crystals LaFeAsO, SrFeAsF, SrFe₂As₂, and LiFeAs that are the basis phases of the recently discovered high-temperature (T _C ～ 26–56 K) 《1111》, 《122》, and 《111》 FeAs superconductors.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

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.     

Peculiarities of magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu_0.55Sr_0.45MnO₃ ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=10⁶ Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field H_C1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and H_C1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρ _H ?ρ_H=0)/ρ _H is positive for H<H_C1 and passes through a maximum at 41 K, where Δρ/ρ = 6%. For H>H_C1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×10⁵ % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10^?4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu_0.55Sr_0.45MnO₃, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ～3% of the sample volume is occupied by the FM phase; ～67%, by the CE-type AFM phase; and ～30%, by the A-type AFM phase.     

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)].