Electron Glass Phase with Resilient Zhang-Rice Singlets in LiCu_{3}O_{3}.

LiCu_{3}O_{3} is an antiferromagnetic mixed valence cuprate where trilayers of edge-sharing Cu(II)O (3d^{9}) are sandwiched in between planes of Cu(I) (3d^{10}) ions, with Li stochastically substituting Cu(II). Angle-resolved photoemission spectroscopy (ARPES) and density functional theory reveal two insulating electronic subsystems that are segregated in spite of sharing common oxygen atoms: a Cu d_{z^{2}}/O p_{z} derived valence band (VB) dispersing on the Cu(I) plane, and a Cu 3d_{x^{2}-y^{2}}/O 2p_{x,y} derived Zhang-Rice singlet (ZRS) band dispersing on the Cu(II)O planes. First-principle analysis shows the Li substitution to stabilize the insulating ground state, but only if antiferromagnetic correlations are present. Li further induces substitutional disorder and a 2D electron glass behavior in charge transport, reflected in a large 530 meV Coulomb gap and a linear suppression of VB spectral weight at E_{F} that is observed by ARPES. Surprisingly, the disorder leaves the Cu(II)-derived ZRS largely unaffected. This indicates a local segregation of Li and Cu atoms onto the two separate corner-sharing Cu(II)O_{2} sub-lattices of the edge-sharing Cu(II)O planes, and highlights the ubiquitous resilience of the entangled two hole ZRS entity against impurity scattering.

Dynamically disordered hydrogen bonds in the hureaulite-type phosphatic oxyhydroxide Mn5[(PO4)2(PO3(OH))2](HOH)4.

We report the temperature evolution of hydrogen bond (HB) chains and rings in Mn5[(PO4)2(PO3(OH))2](HOH)4 to reveal conduction pathways based on difference Fourier maps with neutron- and synchrotron x-ray diffraction data. Localized proton dynamics for the five distinct hydrogen sites were observed and identified in this study. Their temperature evaluation over ten orders of magnitude in time was followed by means of quasielastic neutron scattering, dielectric spectroscopy, and ab initio molecular dynamics. Two out of the five hydrogen sites are geometrically isolated and are not suitable for long-range proton conduction. Nevertheless, the detected dc conductivity points to long-range charge transport at elevated temperatures, which occurs most likely (1) over H4-H4 sites between semihelical HB chains (interchain-exchanges) and (2) by rotations of O1-H1 and site-exchanging H4-O10-O5 groups along each semihelical HB chain (intrachain-exchanges). The latter dynamics freeze into a proton-glass state at low temperatures. Rotational and site-exchanging motions of HOH and OH ligands seem to be facilitated by collective motions of framework polyhedra, which we detected by inelastic neutron scattering.

Ligand influence in Li-ion battery hybrid active materials: Ni methylenediphosphonate vs. Ni dimethylamino methylenediphosphonate.

The influence of a ligand on the structural, morphological and electrochemical properties of organic-inorganic hybrid nickel diphosphonates was assessed using Ni methylenediphosphonate (NiMeDP) and Ni dimethylamino methylenediphosphonate (NiDMAMDP) as model electrode materials.

Origin of the Spin-Orbital Liquid State in a Nearly J=0 Iridate Ba_{3}ZnIr_{2}O_{9}.

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of Ba_{3}ZnIr_{2}O_{9} is a realization of a novel spin-orbital liquid state. Our results reveal that Ba_{3}ZnIr_{2}O_{9} with Ir^{5+} (5d^{4}) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J=0 state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural Ir_{2}O_{9} dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK.

Characterization of selenium in UO2 spent nuclear fuel by micro X-ray absorption spectroscopy and its thermodynamic stability.

Direct disposal of spent nuclear fuel (SNF) in deep geological formations is the preferred option for the final storage of nuclear waste in many countries. In order to assess to which extent radionuclides could be released to the environment, it is of great importance to understand how they are chemically bound in the waste matrix. This is particularly important for long-lived radionuclides such as (79)Se, (129)I, (14)C or (36)Cl, which form poorly sorbing anionic species in water and therefore migrate without significant retardation through argillaceous repository materials and host rocks. We present here X-ray absorption spectroscopic data providing evidence that in the investigated SNF samples selenium is directly bound to U atoms as Se(-II) (selenide) ion, probably replacing oxygen in the cubic UO2 lattice. This result is corroborated by a simple thermodynamic analysis, showing that selenide is the stable form of Se under reactor operation conditions. Because selenide is almost insoluble in water, our data indirectly explain the unexpectedly low release of Se in short-term aqueous leaching experiments, compared to iodine or cesium. These results have a direct impact on safety analyses for potential nuclear waste repository sites, as they justify assuming a small fractional release of selenium in performance assessment calculations.

Layer-preferential substitutions and magnetic properties of pyrrhotite-type Fe7-yMyX8 chalcogenides (X = S, Se; M = Ti, Co).

A comparative study of four series of pyrrhotite-type chalcogenide compounds Fe(7-y)M(y)X(8) (X = S, Se) with substitution of Ti or Co for iron has been performed by means of x-ray and neutron powder diffraction, and by magnetization measurements. In Fe(7-y)M(y)X(8) compounds having a ferrimagnetic order at y = 0, the substitution of either Ti or Co for iron is observed to result in a monotonous decrease of the magnetic ordering temperature, while the resultant magnetization shows a non-monotonous behavior with a minimum around y = 1.0-1.5 in all the Fe(7-y)M(y)X(8) families except Fe(7-y)Co(y)Se(8). Suppression of a magnetically ordered state with substitutions in Fe(7-y)M(y)X(8) is ascribed to nearly zero values of Ti and Co magnetic moments, while the non-monotonous changes of the resultant magnetization are explained by the compensation of the sublattice magnetizations due to the non-random substitutions in alternating metallic layers. The difference in the cation partitioning observed in Fe(7-y)Ti(y)X(8) and Fe(7-y)Co(y)X(8) is attributed to the difference in the spatial extension of Ti and Co 3d orbitals. High coercive field values (20-24 kOe) observed at low temperatures in the Ti-containing compounds Fe(7-y)Ti(y)X(8) with y ⩾ 3 are suggested to result from the enhancement of Fe orbital moment due to the Ti for Fe substitution.

{110}-Layered B-cation ordering in the anion-deficient perovskite Pb2.4Ba2.6Fe2Sc2TiO13 with the crystallographic shear structure.

A novel anion-deficient perovskite-based compound, Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13), was synthesized via the citrate-based route. This compound is an n = 5 member of the AnBnO(3n-2) homologous series with unit-cell parameters related to the perovskite subcell a(p)≈ 4.0 Å as a(p)√2 ×a(p)× 5a(p)√2. The crystal structure of Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13) consists of quasi-2D perovskite blocks with a thickness of three octahedral layers separated by the 1/2[110](1[combining macron]01)(p) crystallographic shear (CS) planes, which are parallel to the {110} plane of the perovskite subcell. The CS planes transform the corner-sharing octahedra into chains of edge-sharing distorted tetragonal pyramids. Using a combination of neutron powder diffraction, (57)Fe Mössbauer spectroscopy and atomic resolution electron energy-loss spectroscopy we demonstrate that the B-cations in Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13) are ordered along the {110} perovskite layers with Fe(3+) in distorted tetragonal pyramids along the CS planes, Ti(4+) preferentially in the central octahedra of the perovskite blocks and Sc(3+) in the outer octahedra of the perovskite blocks. Magnetic susceptibility and Mössbauer spectroscopy indicate a broadened magnetic transition around T(N)∼ 45 K and the onset of local magnetic fields at low temperatures. The magnetic order is probably reminiscent of that in other AnBnO(3n-2) homologues, where G-type AFM order within the perovskite blocks has been observed.

k=0 magnetic structure and absence of ferroelectricity in SmFeO3.

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675 K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

Rich crystal chemistry and magnetism of "114" stoichiometric LnBaFe4O7.0 ferrites.

Stoichiometric LnBaFe4O7.0 oxides with Ln = Dy to Lu have been synthesized and protected in order to prevent oxidation at room temperature. The structural study of these compounds, using laboratory and synchrotron X-ray as well as neutron powder diffraction, shows the extraordinary flexibility of the tetrahedral [Fe4] sublattice of these compounds, which exhibit various distortions. At room temperature they all are tetragonal (I4), and at higher temperature (T > 580 K) they exhibit a cubic symmetry (F43m). Moreover, the low-temperature structures of these oxides are dependent on the nature of the Ln(3+) cation. At 110 K, compounds with Ln = Dy and Ho adopt the same monoclinic (P12(1)1) structure as YBaFe4O7.0, whereas YbBaFe4O7.0 possesses a new centered monoclinic cell (I121), and members with Ln = Er and Lu keep the tetragonal (I4) symmetry. Neutron diffraction patterns evidence long-range magnetic ordering only for the most distorted structures (Ln = Dy and Ho), showing that the geometric frustration generated by the tetrahedral [Fe4]∞ sublattice can be lifted only with the most severe distortions. The other oxides (Ln = Er, Yb, and Lu) with weakly distorted [Fe4]∞ sublattices do not exhibit magnetic ordering down to 4 K, demonstrating the importance of magnetic frustration. The behavior of these "114" iron oxides is compared to the cobalt family, showing in both cases a striking underbonding of barium.

1D to 2D Na+ ion diffusion inherently linked to structural transitions in Na0.7CoO2.

We report the observation of a stepwise "melting" of the low-temperature Na-vacancy order in the layered transition-metal oxide Na0.7CoO2. High-resolution neutron powder diffraction analysis indicates the existence of two first-order structural transitions, one at T1≈290 K followed by a second at T2≈400 K. Detailed analysis strongly suggests that both transitions are linked to changes in the Na mobility. Our data are consistent with a two-step disappearance of Na-vacancy order through the successive opening of first quasi-1D (T1>T>T2) and then 2D (T>T2) Na diffusion paths. These results shed new light on previous, seemingly incompatible, experimental interpretations regarding the relationship between Na-vacancy order and Na dynamics in this material. They also represent an important step towards the tuning of physical properties and the design of tailored functional materials through an improved control and understanding of ionic diffusion.

Structural and magnetic phase transitions in the A(n)B(n)O(3n-2) anion-deficient perovskites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16.

Novel anion-deficient perovskite-based ferrites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 were synthesized by solid-state reaction in air. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 belong to the perovskite-based A(n)B(n)O(3n-2) homologous series with n = 5 and 6, respectively, with a unit cell related to the perovskite subcell a(p) as a(p)√2 × a(p) × na(p)√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]p(101)p crystallographic shear (CS) planes. The CS operation results in (101)p-shaped perovskite blocks with a thickness of (n - 2) FeO6 octahedra connected to each other through double chains of edge-sharing FeO5 distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above T ≈ 750 K for Pb2Ba2BiFe5O13 and T ≈ 400 K for Pb(1.5)Ba(2.5)Bi2Fe6O16 the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned Fe-Fe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all n = 4, 5, 6 A(n)Fe(n)O(3n-2) (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623-632 K.

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.

Evidence for the strong effect of quenched correlated disorder on phase separation and magnetism in (La(1-y)Pr(y))0.7Ca0.3MnO3.

High resolution neutron diffraction shows that the mesoscopic separation into ferromagnetic (FM) and antiferromagnetic (AFM) phases and the FM transition temperature T(C) in the perovskite manganite (La(1-y)Pr(y))(0.7)Ca(0.3)MnO(3) strongly depend on the quenched correlated disorder. The different disorder strengths are achieved by different procedures of the sample synthesis and are quantitatively characterized by the microstrain-type diffraction peak broadening. The system shifts to predominantly a one-phase state with smaller T(C) as the correlated disorder strength is decreased, supporting the viewpoint that the origin of phase separation in the indicated manganite system is the correlated quenched disorder. The ground state of an ultimately chemically homogeneous sample is FM-like containing about 20% of the AFM minority phase. This FM-like state can be readily transformed to the AFM-like one having < 20% of the FM phase by the decrease of the effective charge carrier bandwidth via oxygen isotope substitution.

A neutron diffraction study of structural and magnetic transformations in AFeO(2) (A = K, Rb and Cs).

In continuation of our recent x-ray study of the structural phase transitions in the AFeO(2) (A = K, Rb, Cs) family, we have systematically investigated the respective structural and magnetic phase transitions by neutron powder diffraction. While the temperatures of the first-order structural phase transitions are strongly different for the three compounds (~1003, ~737 and ~350 K for A = K, Rb, Cs) and systematically decrease with increasing ionic radius of the A-cation, the magnetic transition temperatures in all three compounds have been found to be almost the same-slightly above 1000 K. The magnetic ordering type is similar in all three compounds-antiferromagnetic ordering of magnetic Fe(3 + ) ions within the system of the three-dimensional Fe-O-Fe linkages such that the Fe-Fe exchange between the nearest neighboring ions is always antiferromagnetic. The directions of magnetic Fe moments were found to be parallel to the crystallographic axis c in RbFeO(2) and CsFeO(2) and parallel to the axis b in KFeO(2) in notations of their low-temperature orthorhombic modifications.

Structure of Ca(BD4)2 beta-phase from combined neutron and synchrotron X-ray powder diffraction data and density functional calculations.

We have investigated the crystal structure of Ca(BD4)2 by combined synchrotron radiation X-ray powder diffraction, neutron powder diffraction, and ab initio calculations. Ca(BD4)2 shows a variety of structures depending on the synthesis and temperature of the samples. An unknown tetragonal crystal of Ca(BD4)2, the beta phase has been solved from diffraction data measured at 480 K on a sample synthesized by solid-gas mechanochemical reaction by using MgB2 as starting material. Above 400 K, this sample has the particularity to be almost completely into the beta phase of Ca(BD4)2. Seven tetragonal structure candidates gave similar fit of the experimental data. However, combined experimental and ab initio calculations have shown that the best description of the structure is with the space group P4(2)/m based on appropriate size/geometry of the (BD4)tetrahedra, the lowest calculated formation energy, and real positive vibrational energy, indicating a stable structure. At room temperature, this sample consists mainly of the previously reported alpha phase with space group Fddd. In the diffraction data, we have identified weak peaks of a hitherto unsolved structure of an orthorombic gamma phase of Ca(BD4)2. To properly fit the diffraction data used to solve and refine the structure of the beta phase, a preliminary structural model of the gamma phase was used. A second set of diffraction data on a sample synthesized by wet chemical method, where the gamma phase is present in significant amount, allowed us to index this phase and determine the preliminary model with space group Pbca. Ab initio calculations provide formation energies of the alpha phase and beta phase of the same order of magnitude (delta H < or = 0.15 eV). This indicates the possibility of coexistence of these phases at the same thermodynamical conditions.

Low temperature crystal structures of apatite oxygen-conductors containing interstitial oxygen.

Oxygen-stoichiometric La(9.33) square(0.67)(Si(6)O(24))O2 and oxygen-excess La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) and La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) oxy-apatites have been structurally characterized at low temperatures by the Rietveld method. Oxygen-interstitial distribution has been studied at 15 K for La(9.33) square(0.67)(Si(6)O(24))O2 and La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) by time-of-flight neutron powder diffraction and at 4 K for La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) by constant-wavelength neutron powder diffraction. The low temperature structural study was undertaken in order to distinguish between the effects of static disorder, originated mainly from the presence of interstitial oxygens, and the anisotropic thermal vibrations. At such low temperatures, the influence of the anisotropic thermal vibrations is minimised. This structural study has firmly established the existence of interstitial oxygens in these materials, which may be useful as electrolytes for solid oxide fuel cells.

Pressure-induced quantum phase transition in the spin-liquid TlCuCl3.

The condensation of magnetic quasiparticles into the nonmagnetic ground state has been used to explain novel magnetic ordering phenomena observed in quantum spin systems. We present neutron scattering results across the pressure-induced quantum phase transition and for the novel ordered phase of the magnetic insulator TlCuCl3, which are consistent with the theoretically predicted two degenerate gapless Goldstone modes, similar to the low-energy spin excitations in the field-induced case. These novel experimental findings complete the field-induced Bose-Einstein condensate picture and support the recently proposed field-pressure phase diagram common for quantum spin systems with an energy gap of singlet-triplet nature.