Cubic Sr2ScGaO5 Perovskite: Structural Stability, Oxygen Defect Structure, and Ion Conductivity Explored on Single Crystals.

Oxygen-deficient Sr2ScGaO5 single crystals with a cubic perovskite structure were grown by the floating-zone technique. The transparent crystals of this pure 3D oxygen electrolyte are metastable at ambient temperature, showing one-sixth of all oxygen positions vacant. While neutron single-crystal diffraction, followed by maximum entropy analysis, revealed a strong anharmonic displacements for the oxygen atoms, a predominant formation of ScO6 octahedra and GaO4 tetrahedra is indicated by Raman spectroscopic studies, resulting in a complex oxygen defect structure with short-range order. Temperature-dependent X-ray powder diffraction (XPD) and neutron powder diffraction (NPD) studies reveal the cubic Sr2ScGaO5 to be thermodynamically stable only above 1400 °C, while the stable modification below this temperature shows the brownmillerite framework with orthorhombic symmetry. Cubic Sr2ScGaO5 remains surprisingly kinetically stable upon heating from ambient temperature to 1300 °C, indicating a huge inertia for the retransformation toward the thermodynamically stable brownmillerite phase. Ionic conductivity investigated by impedance spectroscopy was found to be 10-4 S/cm at 600 °C, while oxygen 18O/16O isotope exchange indicates a free oxygen mobility to set in at around 500 °C.

Polarized Neutron Diffraction as a Tool for Mapping Molecular Magnetic Anisotropy: Local Susceptibility Tensors in Co(II) Complexes.

Polarized neutron diffraction (PND) experiments were carried out at low temperature to characterize with high precision the local magnetic anisotropy in two paramagnetic high-spin cobalt(II) complexes, namely [Co(II) (dmf)6 ](BPh4 )2 (1) and [Co(II) 2 (sym-hmp)2 ](BPh4 )2 (2), in which dmf=N,N-dimethylformamide; sym-hmp=2,6-bis[(2-hydroxyethyl)methylaminomethyl]-4-methylphenolate, and BPh4 (-) =tetraphenylborate. This allowed a unique and direct determination of the local magnetic susceptibility tensor on each individual Co(II) site. In compound 1, this approach reveals the correlation between the single-ion easy magnetization direction and a trigonal elongation axis of the Co(II) coordination octahedron. In exchange-coupled dimer 2, the determination of the individual Co(II) magnetic susceptibility tensors provides a clear outlook of how the local magnetic properties on both Co(II) sites deviate from the single-ion behavior because of antiferromagnetic exchange coupling.

Neutron scattering studies of K3H(SO4)2 and K3D(SO4)2: the particle-in-a-box model for the quantum phase transition.

In the crystal of K(3)H(SO(4))(2) or K(3)D(SO(4))(2), dimers SO(4)···H···SO(4) or SO(4)···D···SO(4) are linked by strong centrosymmetric hydrogen or deuterium bonds whose O···O length is ≈2.50 Å. We address two open questions. (i) Are H or D sites split or not? (ii) Is there any structural counterpart to the phase transition observed for K(3)D(SO(4))(2) at T(c) ≈ 85.5 K, which does not exist for K(3)H(SO(4))(2)? Neutron diffraction by single-crystals at cryogenic or room temperature reveals no structural transition and no resolvable splitting of H or D sites. However, the width of the probability densities suggest unresolved splitting of the wavefunctions suggesting rigid entities H(L1/2)-H(R1/2) or D(L1/2)-D(R1/2) whose separation lengths are l(H) ≈ 0.16 Å or l(D) ≈ 0.25 Å. The vibrational eigenstates for the center of mass of H(L1/2)-H(R1/2) revealed by inelastic neutron scattering are amenable to a square-well and we suppose the same potential holds for D(L1/2)-D(R1/2). In order to explain dielectric and calorimetric measurements of mixed crystals K(3)D((1-ρ))H(ρ)(SO(4))(2) (0 ≤ ρ ≤ 1), we replace the classical notion of order-disorder by the quantum notion of discernible (e.g., D(L1/2)-D(R1/2)) or indiscernible (e.g., H(L1/2)-H(R1/2)) components depending on the separation length of the split wavefunction. The discernible-indiscernible isostructural transition at finite temperatures is induced by a thermal pure quantum state or at 0 K by ρ.

Evidence of macroscopically entangled protons in a mixed isotope crystal of KH(p)D(1-p)CO3.

We examine whether protons and deuterons in a crystal of KH(0.76)D(0.24)CO(3) at 300 K are particles or matter waves. The neutron scattering function measured over a broad range of reciprocal space reveals the enhanced diffraction pattern anticipated for antisymmetrized macroscopic states for protons (fermions). These features exclude a statistical distribution of protons and deuterons. Raman spectra are consistent with a mixture of KHCO(3) and KDCO(3) sublattices whose isomorphous structures are independent of the isotope content. We propose a theoretical framework for decoherence-free proton and deuteron states.

The neutron diffraction study, calorimetry and spontaneous polarization of pyridinium perrhenate at 350, 300 and 100 K.

The crystal and molecular structure of pyridinium perrhenate [H(5)C(5)NH](+) [ReO(4)](-) (hereafter referred to as PyReO(4)) was determined by single-crystal neutron diffraction at 350, 300 and 100 K. The neutron study confirmed the x-ray diffraction results in all three phases. The three temperature-dependent polymorphs are orthorhombic, with the following sequence of phases: Cmcm − → Cmc2(1) − → Pbca, with the a lattice parameter doubled. In the two high temperature phases the pyridinium cations display a rotational disorder while the perrhenate anions are well ordered. The low temperature phase is fully ordered. The neutron results allow for a very precise description of the distribution of the nitrogen atoms in the disordered pyridinium cation, which enables us to analyse the calorimetric and spontaneous polarization measurements. The results from the DSC and pyroelectric measurements point to a paraelectric (350 K), ferroelectric (300 K) with the Curie point at 336 K and antiferroelectric (100 K) crystal phases. The phase transition at 336 K can be classified as an order-disorder ferroelectric with a small displacive component.

Structure, magnetic properties, polarized neutron diffraction, and theoretical study of a copper(II) cubane.

The paper reports the synthesis, X-ray and neutron diffraction crystal structures, magnetic properties, high field-high frequency EPR (HF-EPR), spin density and theoretical description of the tetranuclear CuII complex [Cu4L4] with cubane-like structure (LH2=1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one). The simulation of the magnetic behavior gives a predominant ferromagnetic interaction J1 (+30.5 cm(-1)) and a weak antiferromagnetic interaction J2 (-5.5 cm(-1)), which correspond to short and long Cu-Cu distances, respectively, as evidence from the crystal structure [see formulate in text]. It is in agreement with DFT calculations and with the saturation magnetization value of an S=2 ground spin state. HF-EPR measurements at low temperatures (5 to 30 K) provide evidence for a negative axial zero-field splitting parameter D (-0.25+/-0.01 cm(-1)) plus a small rhombic term E (0.025+/-0.001 cm(-1), E/D = 0.1). The experimental spin distribution from polarized neutron diffraction is mainly located in the basal plane of the CuII ion with a distortion of yz-type for one CuII ion. Delocalization on the ligand (L) is observed but to a smaller extent than expected from DFT calculations.

A neutron scattering study of strong-symmetric hydrogen bonds in potassium and cesium hydrogen bistrifluoroacetates: Determination of the crystal structures and of the single-well potentials for protons.

The crystal structures of potassium and cesium bistrifluoroacetates, KH(CF(3)COO)(2) and CsH(CF(3)COO)(2), respectively, were determined at room and cryogenic temperatures with the single crystal neutron diffraction technique. The crystals belong to the monoclinic space groups, I2a and A2a, respectively, and there is no evidence of any structural phase transition. In both crystals, trifluoroacetate entities in centrosymmetric dimers are linked by very short hydrogen bonds lying across a center of inversion. The thermal parameters provide no evidence of any double minimum potential for hydrogen bond protons. Single-minimum potentials were determined via best fitting to the inelastic neutron scattering spectral profiles of the stretching vibrations. They comprise a narrow well for the ground state and a very broad quasiharmonic well for excited states. The spread out of the wave functions of these states shows that protons are no longer confined between the oxygens. Presumably, they are attracted by the lone pairs of oxygen atoms. These potentials emphasize the covalent nature of the OO bond and the ionic character of the hydrogen bond proton.

The solid solution Na(0.39)(NH(4))(1.61)SO(4)·Te(OH)(6).

The title compound, sodium ammonium sulfate-telluric acid (1/1), Na(0.39)(NH(4))(1.61)SO(4)·Te(OH)(6), is isostructural with other solid solutions in the series M(1-x)(NH(4))(x)SO(4)·Te(OH)(6), where ammonium is partially replaced with an alkali metal (M = K, Rb or Cs). The structure is composed of planes of Te(OH)(6) octa-hedra alternating with planes of SO(4) tetra-hedra. The Na(+)/NH(4) (+) cations are statistically distributed over the same position and are located between the planes. The structure is stabilized by O-H⋯O and N-H⋯O hydrogen bonds between the telluric acid adducts and the O atoms of sulfate groups, and between the ammonium cations and O atoms, respectively. Both Te atoms lie on centres of symmetry.

Structure elucidation of a dihydropyranone from Tapinanthus dodoneifolius.

A new dihydropyranone, ( R)-6-[( S)-2-hydroxy-4-(4-hydroxyphenyl)butyl]-5,6-dihydropyran-2-one ( 1), was isolated from Tapinanthus dodoneifolius. The structure was determined from spectroscopic and X-ray crystallographic analysis. Compound 1 (named dodoneine) showed a relaxing effect on preconstricted rat aortic rings (IC 50 of 81.4 +/- 0.9 microM).

Ferromagnetic interaction in an asymmetric end-to-end azido double-bridged copper(II) dinuclear complex: a combined structure, magnetic, polarized neutron diffraction and theoretical study.

A new end-to-end azido double-bridged copper(II) complex [Cu(2)L(2)(N(3))2] (1) was synthesized and characterized (L=1,1,1-trifluoro-7-(dimethylamino)-4-methyl-5-aza-3-hepten-2-onato). Despite the rather long Cu-Cu distance (5.105(1) A), the magnetic interaction is ferromagnetic with J= +16 cm(-1) (H=-JS(1)S(2)), a value that has been confirmed by DFT and high-level correlated ab initio calculations. The spin distribution was studied by using the results from polarized neutron diffraction. This is the first such study on an end-to-end system. The experimental spin density was found to be localized mainly on the copper(II) ions, with a small degree of delocalization on the ligand (L) and terminal azido nitrogens. There was zero delocalization on the central nitrogen, in agreement with DFT calculations. Such a picture corresponds to an important contribution of the d(x2-y2) orbital and a small population of the d(z2) orbital, in agreement with our calculations. Based on a correlated wavefunction analysis, the ferromagnetic behavior results from a dominant double spin polarization contribution and vanishingly small ionic forms.

Time-resolved in situ studies of oxygen intercalation into SrCoO2.5, performed by neutron diffraction and X-ray absorption spectroscopy.

Electrochemical oxidation of the antiferromagnetically ordered SrCoO(2.5), with brownmillerite-type structure, to the cubic ferromagnet SrCoO(3), with perovskite structure, has been investigated in situ by neutron diffraction as well as by X-ray absorption fine structure (XAFS) spectroscopy in specially designed electrochemical cells. The neutron diffraction experiments were performed twice, using two different wavelengths (lambda = 1.2921(2) and 4.74 A) in order to better discriminate structural and magnetic changes as functions of the charge transfer. From the neutron diffraction experiments, two intermediate phases, SrCoO(2.75) and SrCoO(2.82)(+/-)(0.07), were characterized. No superstructure reflections were observed for the corresponding SrCoO(2.75) phase. Instead we observed here, for the first time, 3D oxygen ordering during an oxygen intercalation reaction, as established for SrCoO(2.82)(+/-)(0.07), which can be described as a tetragonal unit cell, related to the perovskite cell by a approximately 2(a radical2) and c approximately 2a. The structure of this intermediate phase confirms the strongly topotactic character of the oxygen intercalation reaction. We were also able to prove, from in situ XAFS spectroscopy at the Co absorption edge, that the evolution of the Co valence state from formally +3 for SrCoO(2.5) to +4 for the final reaction product (SrCoO(3.0)) does not proceed continuously but gives evidence for the formation of O(-) species for stoichiometries corresponding to SrCoO(2.82)(+/-)(0.07). The use of neutrons (vs X-rays) in the diffraction experiments and the choice of the transmission (vs fluorescence) mode in the XAFS experiment guarantee that the obtained data well represent bulk and not just surface properties.

Contributions to the application of the transferability principle and the multipolar modeling of H atoms: electron-density study of L-histidinium dihydrogen orthophosphate orthophosphoric acid. I.

The electron density of L-histidinium dihydrogen orthophosphate orthophosphoric acid has been determined from X-ray and neutron diffraction data at low temperature (120 K). Topological analysis of the electron density has been used to analyse the effect of the multipolar refinement strategy on the electron-density model in the hydrogen-bonding regions. The electron density at low temperature has also been used to acquire high-quality experimental thermal parameters at room temperature using the transferability principle. Molecular vibrations, TLS and normal mode analysis are discussed and studied at both temperatures.

Synchrotron and neutron diffraction study of 4-methylpyridine-N-oxide at low temperature.

The structure of 4-methylpyridine-N-oxide has been determined at 250, 100 and 10 K by combined synchrotron (C(6)H(7)NO) and neutron (C(6)D(7)NO) powder diffraction experiments. At 250 K the space group is I4(1)/amd and the tetragonal unit cell [a = b = 7.941 (2), c = 19.600 (5) A] contains eight equivalent molecules. At 100 K the structure is orthorhombic, with space group Fddd, a = 12.138 (2), b = 10.237 (2) and c = 19.568 (3) A. The 16 equivalent molecules are rotated by about 8 degrees around the c axis with respect to positions at high temperature. At 10 K the best structural model corresponds to a tetragonal unit cell with the space group P4(1), a = b = 15.410 (2) A and c = 19.680 (3) A. The 32 molecules (eight molecules in the asymmetric unit) show complex reorientations around the three cell axes. Whereas at 250 and 100 K the deuterated methyl groups are largely disordered, at 10 K they are ordered in-phase along infinite chains parallel to a and b. Face-to-face methyl groups along c are in an eclipsed configuration. The structure at 10 K suggests that the manifold of rotational tunnelling transitions could be due to inequivalent lattice sites for crystallographically independent methyl groups.

The low-temperature phase of 1,3-dibromo-2,4,6-trimethylbenzene: a single-crystal neutron diffraction study at 120 and 14 K.

In the low-temperature phase of dibromomesitylene (1,3-dibromo-2,4,6-trimethylbenzene), C(9)H(10)Br(2), the molecule deviates significantly from the C(3h) molecular symmetry encountered in tribromomesitylene (1,3,5-tribromo-2,4,6-trimethylbenzene), even for the endocyclic bond angles. An apparent C(2v) molecular symmetry is observed. The angle between the normal to the molecular plane and the normal to the (100) plane is approximately 20 degrees. The overall displacement was analysed at 120 K with rigid-body-motion tensor analysis. The methyl group located intermediate between the two Br atoms is rotationally disordered at both temperatures. This disorder was treated using two different approaches at 14 K, viz. the conventional split-atom model and a model using the special annular shapes of the atomic displacement parameters that are available in CRYSTALS [Watkin, Prout, Carruthers & Betteridge (1999). Issue 11. Chemical Crystallography Laboratory, Oxford, England], but only through the latter approach at 120 K. The disorder locally breaks the C(2v) molecular symmetry at 14 K only. Intra- and intermolecular contacts are described and discussed in relation to this methyl-group disorder. The bidimensional pseudo-hexagonal structural topology of trihalogenomesitylenes is altered in dibromomesitylene insofar as the (100) molecular layers are undulated and are not coplanar as a result of an alternating tilt angle of approximately 34 degrees propagating along the [011] and [0-11] directions between successive antiferroelectric molecular columns oriented roughly along the a axis.

Two potential Ca(2+)-mobilizing processes depend on the abscisic acid concentration and growth temperature in the Arabidopsis stomatal guard cell.

The abscisic acid (ABA) stomatal closing signal might be transduced through different pathways, depending on the plant growth temperature (GT) and the applied ABA concentration. This was investigated in epidermal peels of Arabidopsis thaliana (L.) Columbia. Different Ca2+ buffers and guanosine-triphosphate-binding protein (G protein) modulators were tested on stomatal closing under light in response to 3 mumol/L ABA (ABA3 mu) and 30 mumol/L ABA (ABA30 mu) at the 15-17 degrees C and 23-25 degrees C GT ranges. The Ca2+ buffer, 1,2-bis(0-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, used as free acid (BAPTA) or acetoxymethyl ester (BAPTA-AM), similarly inhibited (up to approximately 70% inhibition) stomatal closing to ABA3 mu and ABA30 mu, whereas ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid specifically inhibited (up to approximately 70% inhibition) the ABA3 mu response at the 23-25 degrees C GT range. At the same GT range, the ABA3 mu response was specifically affected by the phospholipase C (PLC) inhibitor 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122). Moreover, the ABA30 mu response was specifically inhibited by the G protein antagonist pGlu-Gln-D-Trp-Phe-D-Trp-D-Trp-Met-NH2 (GP Ant-2) and by the inactive mastoparan analog, mas 17. The inhibitory effects of GP Ant-2 and mas 17 were additive. None of the tested pharmacological compounds were effective at the 15-17 degrees C GT range. Together, these results confirmed that, depending on GT and the exogenous ABA concentration, stomatal closing to ABA involves either one among two Ca2+ mobilizations or none of them.

Spin densities in a ferromagnetic bimetallic chain compound: polarized neutron diffraction and DFT calculations.

The spin population distribution in the ferromagnetically coupled hetero-bimetallic chain compound [MnNi(NO(2))(4)(en)(2)] (en = 1,2-ethanediamine) has been investigated by means of polarized neutron diffraction experiments, and the results compared with those from theoretical estimates obtained via calculations based on density functional theory on dinuclear molecular models of the chain. The spin distributions obtained from experiment and from theory are consistent and reflect a larger spin delocalization from the Ni atom due to the more covalent character of the Ni-N bonds compared to the Mn-O ones. Also a nearly isotropic spin distribution is observed for the more ionic d(5) Mn(2+) ion and a clearly anisotropic distribution for the d(8) Ni(2+) ion. The use of dinuclear molecular models for the calculation of the exchange coupling constant between Ni and Mn provide upper and lower limits (+17.6 and -4.2 cm(-)(1)) for the experimentally determined value (+1.3 cm(-)(1)), depending on how the missing part of the chain is simulated, but yield essentially the same spin distribution. The Mn(II)-Ni(II) weak ferromagnetic coupling in the chain is interpreted in a spin delocalization mechanism as resulting from the weakness of the overlap between the magnetic orbitals centered on nickel and those centered on manganese which are only weakly delocalized on the ligands.