Apical Bond Research Articles

Pressure-induced phase segregation in single-crystalLa2−2xSr1+2xMn2O7(x=0.32)

The temperature dependence of resistivity $\ensuremath{\rho}(T)$ and of the thermoelectric power $\ensuremath{\alpha}(T)$ of a single crystal of the ferromagnetic Ruddlesden-Popper compound ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}(x=0.32)$ have been measured under different hydrostatic pressures. Pressure induces segregation into antiferromagnetic electron-rich and ferromagnetic electron-poor ${\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ layers. With increasing pressure, the ferromagnetic majority phase exhibits contraction of the apical (Mn-O) bond and spin reorientation from the c axis to the basal plane at a critical pressure ${P}_{c}(6<~{P}_{c}<~7\mathrm{kbar}).$ The phase segregation appears to reflect a first-order inversion of the exchange coupling between ferromagnetic ${\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ layers.

Evidence of bilayer structure in V2O5 xerogel.

Polarized X-ray absorption spectroscopy has been used to study the short-range structure of deposited films of V2O5 xerogel. The material is characterized by a layer of VO5 units with the V-O apical bond perpendicular to the basal (xy) plane. We have focused our attention along the z axis. Experiments were carried out by extended X-ray absorption fine structure (EXAFS) spectroscopy in a grazing incidence geometry and showed evidence for close interactions between neighboring layers of V2O5. The structure is described by two sheets of V2O5 facing each other. Fitting of the EXAFS data has confirmed the existence of a vanadium-vanadium interaction between two different V2O5 layers and an oxygen bridge between them.

Transport phenomena in La(2-x)Ba(x)CuO(y) epitaxial thin films

The (001) oriented thin films of La(2-x)Ba(x)CuO(4+d) on SrAlO4 substrates with barium composition x = 0 to 2 were grown by DC sputtering and characterized by X-ray diffraction and resistivity measurement. The films with oxygen composition d = 0 showed superconductivity for x between 0.055 and 0.30. For x = 0.15, the superconducting transition was maximized to 4.3 K. When the films had d > 0, Tc reached 47 K. These values are higher than those for bulk samples. The depression of T(c) around x = 0.125 is smaller than that for the bulk samples. The compressive strain expands the c-axis and suppresses formation of the low-temperature tetragonal phase. Both effects result in an increase in the bond length between Cu and the apical oxygen, which is responsible for the enhancement of T(c). The samples with lower residual resistivity show a higher T(c), so we suppose that the T(c) enhancement is caused by reduced antiferromagnetic spin fluctuation in the CuO2 planes due to the change in the Cu-O apical bond length.

Conductivity and Structure of Poly(ethylene glycol) Complexes Using Energy Dispersive X-ray Diffraction

Triflate salts of Mg(II), Zn(II), and Cu(II) in liquid, low weight poly(ethylene glycol) dimethyl ether (PEGM(400)) to determine their conductivity and to ascertain their possible use as electrolyte media, have been studied. Their liquid structures have been studied by energy-dispersive X-ray diffraction. By this technique, the cation geometrical coordination has been determined. Furthermore, the local structure of the bivalent cations has been singled out by application of the difference method to isomorphic solutions. We found that the investigated cations have an octahedral coordination, with the exception of Cu, which displays two apical bonds longer than the planar ones. We also found that the polymer chain is coordinated to the cations Mg and Zn through the oxygen atoms of the polymer units 1, 5, and 9.

The chemical control of colossal magnetoresistance (CMR) in the new two-dimensional La1.2(Sr1.8−xCax)Mn2O7 system

The magnetic and electrical properties of the colossal magnetoresistance (CMR) La1.2(Sr1.8−xCax)Mn2O7 materials have been investigated. Based on the chemical titration method, the valence of Mn was determined to be 3.39±0.04 for all measured La1.2(Sr1.8−xCax)Mn2O7. The magnetoresistance ratio [ρ(0)/ρ(H)] was efficiently increased from ∼192% (135 K, 1.5 T) for x=0 to 208% (102 K, 1.5 T) for x=0.4 in La1.2(Sr1.8−xCax)Mn2O7. Moreover, the Curie temperature (Tc) decreased with increasing x and was strongly dependent on the apical bond lengths of Mn–O(2).

梯子型超伝導体Ｓｒ１４−ｘＣａｘＣｕ２４Ｏ４１の結晶構造と高圧物性

The spin ladder system Sr14-xCaxCu24O41 is reviewed from the viewpoint of relations be-tween the crystal structure and superconductivity under high pressure. The compound is a composite crystal, which consists of CuO2 1-D chain and Cu2O3 tow-leg ladder subsystems. As the interplay of the subsystems is increased with Ca substitution or pressurization, holes are removed from the 1-D chain to the ladder via locally formed apical oxygen bonds. Then, the superconductivity occurs owing to the hole doping into the ladder. The hole distribution, magnetic interaction, and electrical property of the normal state are also discussed in terms of the structural modulation.

Bis(1,10-phenanthroline-N,N')copper(II) Tetrathionate and Tris(1,10-phenanthroline-N,N')copper(II) Tetrathionate Pentahydrate

The structures of [Cu(C12H8N2)2(S4O6)], (I) and [Cu(C12H8N2)3](S4O6).5H2O, (II), are reported. Compound (I) consists of infinite polymeric chains where copper displays a typical (4 + 2) coordination and S4O6 connects Cu(phen)2 (phen = phenanthroline) units through rather long Cu—O contacts. Compound (II), instead, has a purely ionic character displaying [Cu(phen)3]2+ and S4O62− monomeric units. Equatorial and apical bonds in the copper coordination spheres have mean values of 2.003 (5)/2.467 (12) Å and 2.056 (3)/2.311 (13) Å for (I) and (II), respectively. Some short O⋯OW contacts in (II) suggest the existence of hydrogen bonding involving some of the water molecules of hydration.

Stereochemistry of complexes with n‐alkylated amino acids. xii. crystal structure and molecular mechanics calculations for aqu abis(l‐n, n‐dimethylalaninato)copper(ii) hexahydrate

The structure of blue, monoclinic (C2) crystals of trans-aquabis(L-N, N-dimethylalaninato-)copper(II) hexahydrate was determined by X-ray diffraction methods and refined to R = 0.026. The structure consists of discrete molecules in the same (equatorial-equatorial) conformation linked together in a comprehensive array of water molecules with the structure resembling that of ice. The copper coordination polyhedron is highly distorted [N-Cu-N′ = 166.2(1)° and O-Cu-O′ 161.5(1)°] and the apical bond length [Cu-OW = 2.222(3) Å] is extremely short, which is a direct consequence of the distortion of the coordination polyhedron. Conformational analysis shows that the conformation of the molecule in the crystal state is 1.4 kJ mol−1 higher in energy than the most stable conformer (with axial-axial conformation).

Synthesis and X-ray structure of the thioethereal mercury(II) complex [N(CH2CH2SCHMe2)3HgCl]2(Hg2Cl6)

The thioethereal mercury complexes [N(CH2CH2SCHMe2)3HgX]2(Hg2X6) (X = Cl, Br) have been prepared by reaction of HgX2 with the tripodal ligand N(CH2CH2SCHMe2)3. The chloro derivative has been fully characterized by a crystal structure determination: space group Pl¯, a = 10.114(2), b = 11.230(5), c = 12.196(7)Å, α = 86.11(4), β = 73.47(4), γ = 74.45(4)°, Z = 2. The complex shows a geometry intermediate between the trigonal bipyramidal and the tetrahedron. These distortions have been compared with those found in a series of mercury complexes [LHgX]+ containing the ligand tris(2-diphenylphosphinoethyl)amine, np3, or tris(2-dimethylaminoethyl)amine, Me6tren, the differential weakening of the apical NHg bond being related to the electronegativity and to the σ-bonding capability of the ligand trans to the apical nitrogen.

Thermal expansion and compressibility ofSr2RuO4

The thermal expansion and compressibility of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ have been investigated using neutron-powder diffraction. Our results confirm that the temperature dependence of the Ru-O2 apical bond length is linear with no structural anomaly (reported previously) corresponding to the metal-to-insulator transition. The response of the crystal lattice of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ to pressure is similar to that of the superconducting cuprate ${(\mathrm{L}\mathrm{a},\mathrm{S}\mathrm{r})}_{2}{\mathrm{CuO}}_{4}.$ However, the Ru-O2 apical bond is much harder to compress than the corresponding Cu-O2 bond.

Charge-Transfer-Induced Insulator−Superconductor Transition in TlBa2-xSrxCa0.4Pr0.6Cu2O7-δ: A Joint Neutron/X-ray Rietveld Refinement, Infrared Absorption, and Raman Scattering Studies

Insulator-superconductor transition in TlBa2-xSrxCa0.4Pr0.6Cu2 O7-δ has been investigated by a joint Rietveld refinement of neutron and X-ray diffraction data, Raman scattering, and infrared absorption studies. For x = 0, the system is an insulator. Substitution of the Ba2+ ions by smaller Sr2+ ions leads to a transition from semiconductor to metal then superconductor for x = 1.0. Further, the superconducting transition temperature (Tc) increases with x. Results of the joint neutron/X-ray Rietveld refinement revealed that the Tl−O(1) apical bond length does not change appreciably while the Cu−O(1) bond length decreases largely when x increases, consistent with an electron transfer from Cu to Tl. This idea is supported by the Raman scattering measurements showing a large frequency shift of the phonon mode involving the motion of O(1) toward Tl, when Ba is replaced by Sr.

Effect of lattice distortions on the competition between the double and superexchange mechanisms inLaMnO3

Double exchange (DE), superexchange (SE), and higher-order contributions to the exchange interaction energy between Mn layers in ${\mathrm{LaMnO}}_{3}$ are studied using the linear-muffin-tin-orbital method as generalized to treat noncollinear magnetic configurations. The degree of the internal lattice distortion $(f)$ and the angle $(\ensuremath{\theta})$ between ferromagnetically ordered Mn layers are considered as simulation parameters. We find that both global and internal lattice distortions dramatically influence the character of the exchange interactions: global distortions associated with variations of the apical Mn-O bond length promotes the DE contribution; the bending of the Mn-O bonds in the $(a\ensuremath{-}b)$ plane suppresses the DE and promotes the antiferromagnetic SE contribution to the interlayer exchange energy. Overall, the character of exchange interactions is determined by SE contributions: in hypothetical ferromagnetic phases DE interactions mediated by itinerant electrons is present but still rather small compared with SE; in antiferromagnetic phases this DE is negligible and non-Heisenberg terms are small compared with SE. We find further that a metal-to-semiconductor transition (in the sense of a band-gap opening) occurs in the range of parameters of magnetic and lattice distortion variations given by $\ensuremath{\theta}g~120\ifmmode^\circ\else\textdegree\fi{}$ and $fg~$ 0.7.

High-T c superconductors with apical halogens

Various HTSC systems-oxyhaloid, two dimensional (2D) and three dimensional (3D) oxide -can be described from unified position. All of them have apical bonds with localized (heavy) holes, ordered in a crystal. Such ordering and interaction of the heavy holes with light carriers give high T c superconductivity and anomalous physical properties of HTSC systems.

Mo¨ssbauer study of LnBaCuFeO 5+δ (Ln [dbnd]La, Pr, Y) phases: Disappearance of the spectrum at low temperature

Among the title materials, the La and Pr-doped ones show an abrupt drop of the resonant area of the Mo¨ssbauer spectra below ∼180K probably associated with the softening of a vibrational mode at the Cu/Fe-O apical bond at a pseudo-octahedral (Cu/Fe)O 6 position. This effect is similar to that observed in some superconducting materials. However, it must be noticed, that the fact that the title materials are not superconducting down to 1.7 K clearly shows that a phonon softening is not necessarily related to superconductivity.

High nuclearity ruthenium carbonyl cluster chemistry VI. Cyclic voltammetric and spectroelectrochemical studies of [Ru 10( μ- H) ( μ6-C) (CO) 24] - and [Ru 10( μ6-C)(CO) 24] 2−

The reductive electrochemistry of the decaruthenium clusters [Ru 10( μ-H( μ 6-C)(CO 24] − (1) and [Ru 10( μ 6-C) (CO) 24] 2− (2) has been examined by cyclic voltammetry and UV-Vis spectmelectrochemistry, the latter using an optically transparent thin-layer electrode (OTTLE) cell. Two-electron reduction of both 1 and 2 proceeds in an electrochemically non-reversible fashion to give [HRu 10( μ 6-C(CO) 24] 3− and [Ru 10( μ 6-C)(CO) 24] 4−, respectively, with an associated structural change (indicated #) which is proposed to involve apical Ru-Ru bond cleavage. Two-electron oxidation of these electrochemically-generated species at low temperature proceeds in a reversible fashion to afford [HRu 10( μ 6-C)(CO) 24] − and [ Ru 10( μ 6-C)(CO) 24] 2−, respectively, with no further gross structural change, whereas two-elecron oxidation at room temperature proceeds by way of apical RuRu bond reformation to regenerate 1 and 2. The effect of the hydride ligand in the decaruthenium clusters is to favor reduction and disfavor oxidation by ∼0.3 V. Results with these clusters contrast with the reported behavior of stepwise reductions observed for isostructurai [Os 10( μ 6-C)(CO) 24] 2−, where no structural adjust was observed to accompany the first one-electron reduction, and only a minor change (cluster vertex expansion) accompanied the second one-electron reduction, differences between these systems being assigned to varying M-M bond strengths.

The Deformation of Di-μμ-halide Dinuclear Five-Coordinate Copper(II) Complexes in the Crystalline State

The coordination geometries of 154 dinuclear di-\mu-chloro, di-\mu-bromo and di-\mu-fluoro five-coordinate (4 + 1) copper(II) complexes have been analyzed by the structural correlation method. Two reference CuII polyhedra have been established: (i) a trigonal bipyramid (TBP) in which the equatorial bond lengths are equal, but with the symmetry at the metal often deformed from D 3h to C 2, and (ii) an elongated square pyramid (SQP), with either a pyramidally or tetrahedrally distorted base, in which the trans valence angles are equal. Six different paths for the SQP⇌TBP deformation of the CuII coordination have been established from three criteria: (a) the location of the bridging ligands (axial and equatorial or equatorial and equatorial in TBP); (b) the nature of the deformation of the SQP base plane (pyramidal or tetrahedral); (c) whether a bridging or nonbridging bond is elongated in a SQP. The causes of the angular distortions from C 2 symmetry along the apical bond of a SQP, typical for the TBP⇌SQP Berry path, are analyzed. A progressive reduction of the tetragonal elongation of the Cu—D apical (SQP) bond length along the SQP⇌TBP transformation path and trigonal equalization of the three equatorial Cu—D (TBP) bonds are observed. The dependence of the average Cu—Cu′ distance and Cu—X—Cu′ angles on the deformation path are also established.

Local lattice instability and stripes in theCuO2plane of theLa1.85Sr0.15CuO4system by polarized XANES and EXAFS

Temperature-dependent polarized Cu K-edge x-ray absorption has been used to investigate local structural distortions in the ${\mathrm{CuO}}_{2}$ plane of the ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ system. The Cu-O pair distribution shows the presence of a minority phase containing ${\mathrm{CuO}}_{6}$ octahedra characterized by a shorter Cu-O(apical) bond (\ensuremath{\Delta}R\ensuremath{\sim}-0.1 \AA{}) and two longer Cu-O(planar) bonds (\ensuremath{\Delta}R\ensuremath{\sim}+0.08 \AA{}) and a tilting angle of 16\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}. The temperature-dependent distortions show a maximum around T (\ensuremath{\sim}1.${6\mathrm{T}}_{\mathrm{c}}$) and a minimum at ${\mathrm{T}}_{\mathrm{c}}$ (\ensuremath{\sim}35 K). The data show the coexistence of two types of doped charges in different stripes in the superconducting phase.

Sign Reversal of the Mn-O Bond Compressibility in La1.2Sr1.8Mn2O7belowTC: Exchange Striction in the Ferromagnetic State

The crystal structure of the layered perovskite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ has been studied under hydrostatic pressure up to $\ensuremath{\sim}6\mathrm{kbar}$, in the paramagnetic and ferromagnetic states, with neutron powder diffraction. The compressibility of the Mn-O apical bonds in the double layer of ${\mathrm{MnO}}_{6}$ octahedra changes sign from the paramagnetic insulator (PI) to the ferromagnetic metal (FM) state; in the FM state the Mn-O-Mn linkage between ${\mathrm{MnO}}_{2}$ planes expands under applied pressure, whereas they contract in the PI state. This counterintuitive behavior is interpreted in terms of exchange striction, which reflects the competition between super and double exchange. An increase of the Mn moment with applied pressure in the FM state is consistent with a positive ${\mathrm{dT}}_{C}/dP$, as well as a cant angle ${\ensuremath{\theta}}_{0}$ between the magnetizations of neighboring ${\mathrm{MnO}}_{2}$ sheets that decreases with pressure.

Metal–α,ω-Dicarboxylate Complexes. I. Aqua(2,2'-bipyridyl-N,N')(malonato-O,O')copper(II) Monohydrate

The title compound, [Cu(C3H2O4)(C10H8N2)(H2O)].H2O, exists as discrete monomers. The CuII ion is square pyramidal, with two N atoms of the bipyridyl ligand and two O atoms, one from each of the carboxylates of the folded dibasic acid, providing the base and a water molecule forming a long apical bond [Cu—O5 2.523 (3) Å]. The `scorpion-like' complex possesses a plane of symmetry bisecting the molecule lengthwise, the non-planarity arising from severe puckering of the six-membered chelate ring of the acid. Hydrogen-bonding interactions between the water molecules and the carboxylate O atoms stabilize the crystal structure.

Resonance Raman Spectra of [M(6)X(8)Y(6)](2)(-) Cluster Complexes (M = Mo, W; X, Y = Cl, Br, I).

Resonance Raman spectra of the cubic metal-halide complexes having the general formula [M(6)X(8)Y(6)](2)(-) (M = Mo or W; X, Y = Cl, Br, or I) are reported. The three totally symmetric fundamental vibrations of these complexes are identified. The extensive mixing of the symmetry coordinates that compose the symmetric normal modes expected in these systems is not observed. Instead the "group-frequency" approximation is valid. Furthermore, the force constants of both the apical and face-bridging metal-halide bonds are insensitive to the identity of either the metal or the halide. Raman spectra of related complexes with methoxy and benzenethiol groups as ligands are reported along with the structural data for [Mo(6)Cl(8)(SPh)(6)][NBu(4)](2). Crystal data for [Mo(6)Cl(8)(SPh)(6)][NBu(4)](2) at -156 degrees C: monoclinic space group P2(1)/c; a = 12.588(3), b = 17.471(5), c = 20.646(2) Å; beta = 118.53(1) degrees, V = 3223.4 Å(3); d(calcd) = 1.664 g cm(-)(3); Z = 2.