Revealing the Complex Nature of Bonding in the Binary High-Pressure Compound FeO_{2}.

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe_{2}O_{3} and the appearance of FeO_{2}. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory+dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO_{2} and isostructural FeO_{2}H_{0.5} is ferric (Fe^{3+}), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites.

Pressure-induced symmetry lowering in Nb3Sn1-xsuperconductor.

CubicPm-3nNb3Sn0.92superconductor (Tc∼ 16 K) was found to exhibit tetragonal instabilities at the superconducting state (T= 10 K). These instabilities are manifested through the appearance of reflections which are forbidden in thePm-3nsymmetry but are compatible with theP42/mmcstructure which is observed in the Nb3Sn1-xsystem for higher Sn content at temperatures lower than ∼43 K. Nevertheless, the low-temperature structure of Nb3Sn0.92remains metrically fully cubic, as concluded from single crystal synchrotron radiation diffraction experiments. Subsequent application of external pressure amplifies the observed instabilities with a resulting pseudo-cubic-tetragonal transformation atP= 3 GPa at 10 K and this transition is energy driven, as concluded fromab initiocalculations. The electronic structures of the corresponding phases are virtually identical and, therefore, the pseudo-cubic-tetragonal transformation does not influence significantly the underlying electronic interactions. Consequently, no anomalies in the behavior of the critical temperature,Tc, are expected at this pressure. However, anomalies in the upper critical field are anticipated during this transition, in analogy to the corresponding behavior observed during the cubic-tetragonal transformation in Nb3Sn1-xinduced by increase in Sn content. Therefore targeted changes in composition could be used to enhance upper critical field of Nb3Sn1-xfor specific extreme conditions of temperature and pressure.

High-pressure polymorphism of BaFe2Se3.

BaFe2Se3 is a potential superconductor material exhibiting transition at 11 K and ambient pressure. Here we extended the structural and performed electrical resistivity measurements on this compound up to 51 GPa and 20 GPa, respectively, in order to distinguish if the superconductivity in this sample is intrinsic to the BaFe2Se3 phase or if it is originating from minor FeSe impurities that show a similar superconductive transition temperature. The electrical resistance measurements as a function of pressure show that at 5 GPa the superconducting transition is observed at around 10 K, similar to the one previously observed for this sample at ambient pressure. This indicates that the superconductivity in this sample is most likely intrinsic to the BaFe2Se3 phase and not to FeSe with T c > 20 K at these pressures. Further increase in pressure suppressed the superconductive signal and the sample remained in an insulating state up to the maximum achieved pressure of 20 GPa. Single-crystal and powder x-ray diffraction measurements revealed two structural transformations in BaFe2Se3: a second order transition above 3.5 GPa from Pnma (CsAg2I3-type structure) to Cmcm (CsCu2Cl3-type structure) and a first order transformation at 16.6 GPa. Here, γ-BaFe2Se3 transforms into δ-BaFe2Se3 (Cmcm, CsCu2Cl3-type average structure) via a first order phase transition mechanism. This transition is characterized by a significant shortening of the b lattice parameter of γ-BaFe2Se3 (17%) and accompanied by an anisotropic expansion in the orthogonal ac plane at the transition point.

Pressure-induced chemical reactions in the N2(H2)2 compound: from the N2 and H2 species to ammonia and back down into hydrazine.

Theory predicts a very rich high pressure chemistry of hydronitrogens with the existence of many NxHy compounds. The stability of these phases under pressure is being investigated by the compression of N2-H2 mixtures of various compositions. A previous study had disclosed a eutectic-type N2-H2 phase diagram with two stoichiometric van der Waals compounds: (N2)6(H2)7 and N2(H2)2. The structure and pressure induced chemistry of the (N2)6(H2)7 compound have already been investigated. Here, we determine the structure of the N2(H2)2 compound and characterize using Raman spectroscopy measurements the chemical changes under a pressure cycle up to 60 GPa and back to ambient conditions. A N2(H2)2 single crystal was grown from a 1 : 2 N2-H2 mixture and its crystalline structure was solved using synchrotron X-ray diffraction. Similar to the (N2)6(H2)7 solid, N2(H2)2 has a remarkable host-guest structure containing N2 molecules orientationally disordered with spherical, ellipsoidal and planar shapes. Above 50 GPa, N2(H2)2 was found to undergo a chemical reaction. The reaction products were determined to be of the azane family, with NH3 as the main constituent, along with molecular nitrogen. Upon pressure decrease, the reaction products are found to react in such a way that below 10 GPa, hydrazine is the sole azane detected. Observed down to the opening of the diamond anvil cell, the formation of metastable hydrazine instead of the energetically favorable ammonia is puzzling and remains to be elucidated. That could change the current view of Jovian planets' atmospheres in which ammonia is assumed the only stable hydronitrogen molecule.

Superconductivity in alkali metal intercalated iron selenides.

Alkali metal intercalated iron selenide superconductors A x Fe2-y Se2 (where A = K, Rb, Cs, Tl/K, and Tl/Rb) are characterized by several unique properties, which were not revealed in other superconducting materials. The compounds crystallize in overall simple layered structure with FeSe layers intercalated with alkali metal. The structure turned out to be pretty complex as the existing Fe-vacancies order below ~550 K, which further leads to an antiferromagnetic ordering with Néel temperature fairly above room temperature. At even lower temperatures a phase separation is observed. While one of these phases stays magnetic down to the lowest temperatures the second is becoming superconducting below ~30 K. All these effects give rise to complex relationships between the structure, magnetism and superconductivity. In particular the iron vacancy ordering, linked with a long-range magnetic order and a mesoscopic phase separation, is assumed to be an intrinsic property of the system. Since the discovery of superconductivity in those compounds in 2010 they were investigated very extensively. Results of the studies conducted using a variety of experimental techniques and performed during the last five years were published in hundreds of reports. The present paper reviews scientific work concerning methods of synthesis and crystal growth, structural and superconducting properties as well as pressure investigations.

Temperature- and Pressure-Induced Spin Crossover in Co1+xCr2-xSe4 (x = 0.24): A Diffraction Study.

The Co(2+) ions of the Co1+xCr2-xSe4 phase (Co1.24Cr1.76Se4 composition with x = 0.24, C2/m space group, Cr3S4-type structure) undergo a high- to low-spin-state transition around 230 K, as concluded from the temperature-dependent single-crystal synchrotron radiation diffraction experiments and the previously reported physical property studies. The change of the spin state is not instantaneous and goes through a wide spin-crossover (SCO) region of 75 K. A similar Co(2+) high- to low-spin-state transition is suggested at a pressure of 14.5 GPa, as is evident from the pressure-dependent single-crystal synchrotron radiation diffraction experiments. The corresponding SCO region is equal to 5 GPa, and the structural behavior is different from the one observed during the temperature-dependent transition. Coupling between the spin-conversion process in Co1+xCr2-xSe4 and the concomitant changes in the physical properties opens a way for a controlled tuning of the observed physical response through compositional and structural modifications.

Crystal structure of BaFe2Se3 as a function of temperature and pressure: phase transition phenomena and high-order expansion of Landau potential.

BaFe2Se3 (Pnma, CsAg2I3-type structure), recently assumed to show superconductivity at ~11 K, exhibits a pressure-dependent structural transition to the CsCu2Cl3-type structure (Cmcm space group) around 60 kbar, as evidenced from pressure-dependent synchrotron powder diffraction data. Temperature-dependent synchrotron powder diffraction data indicate an evolution of the room-temperature BaFe2Se3 structure towards a high-symmetry CsCu2Cl3 form upon heating. Around 425 K BaFe2Se3 undergoes a reversible, first-order isostructural transition, which is supported by the differential scanning calorimetry data. The temperature-dependent structural changes occur in two stages, as determined by the alignment of the FeSe4 tetrahedra and corresponding adjustments of the positions of Ba atoms. On further heating, a second-order phase transformation into the Cmcm structure is observed at 660 K. A rather unusual combination of isostructural and second-order phase transformations is parameterized within phenomenological theory assuming high-order expansion of the Landau potential. A generic phase diagram mapping observed structures is proposed on the basis of the parameterization.

Intrinsic crystal phase separation in the antiferromagnetic superconductor Rb(y)Fe(2-x)Se2: a diffraction study.

The crystal and magnetic structures of the superconducting iron-based chalcogenides Rb(y)Fe(2-x)Se(2) have been studied by means of single-crystal synchrotron x-ray and high-resolution neutron powder diffraction in the temperature range 2-570 K. The ground state of the crystal is an intrinsically phase-separated state with two distinct-by-symmetry phases. The main phase has the iron vacancy ordered √5 × âˆš5 superstructure (I4/m space group) with AFM ordered Fe spins. The minority phase does not have √5 × âˆš5-type of ordering and has a smaller in-plane lattice constant a and larger tetragonal c-axis and can be well described by assuming the parent average vacancy disordered structure (I4/mmm space group) with the refined stoichiometry Rb(0.60(5))(Fe(1.10(5))Se)(2). The minority phase amounts to 8-10% mass fraction. The unit cell volume of the minority phase is 3.2% smaller than the one of the main phase at T = 2 K and has quite different temperature dependence. The minority phase merges with the main vacancy ordered phase on heating above the phase separation temperature T(P) = 475 K. The spatial dimensions of the phase domains strongly increase above T(P) from 1000 to >2500 Å due to the integration of the regions of the main phase that were separated by the second phase at low temperatures. Additional annealing of the crystals at a temperature T = 488 K, close to T(P), for a long time drastically reduces the amount of the minority phase.

Temperature and pressure evolution of the crystal structure of A(x)(Fe(1-y)Se)2 (A = Cs, Rb, K) studied by synchrotron powder diffraction.

Temperature-dependent synchrotron powder diffraction on Cs(0.83)(Fe(0.86)Se)(2) revealed first-order I4/m to I4/mmm structural transformation around 216 °C associated with a disorder of the Fe vacancies. Irreversibility observed during the transition is likely associated with a mobility of the intercalated alkali atoms. Pressure-dependent synchrotron powder diffraction on Cs(0.83)(Fe(1-y)Se)(2), Rb(0.85)(Fe(1-y)Se)(2), and K(0.8)(Fe(1-y)Se)(2) (y ~ 0.14) indicated that the I4/m superstructure reflections are present up to pressures of 120 kbar. This may indicate that the ordering of the Fe vacancies is present in both superconducting and nonsuperconductive states.

The synthesis, and crystal and magnetic structure of the iron selenide BaFe2Se3 with possible superconductivity at Tc = 11 K.

We report on the synthesis of single crystals of BaFe(2)Se(3) and study their crystal and magnetic structures by means of synchrotron single-crystal x-ray and neutron powder diffraction. The crystal structure has orthorhombic symmetry and consists of double chains of FeSe(4) edge connected tetrahedra intercalated with barium. Below 240 K, long range spin-block checkerboard antiferromagnetic order is developed. The magnetic structure is similar to one observed in A(0.8)Fe(1.6)Se(2) (A = K, Rb or Cs) superconductors. The crystals exhibit a transition to the diamagnetic state with an onset transition temperature of T(c) ∼ 11 K. Though we observe FeSe as an impurity phase (<0.8% mass fraction) it is not likely that the diamagnetism is attributable to the FeSe superconductor, which has T(c) ≈ 8.5 K.