Planar Bond Length Research Articles

Investigations of the local lattice distortions and the EPR parameters for the orthorhombic Pt3+ center in YAG

The local lattice distortions and the EPR parameters (anisotropic g factors and the hyperfine structure constants) for the orthorhombic Pt(3+) center in YAG are theoretically investigated from the perturbation formulas of these parameters for a 5d(7) ion in an orthorhombically elongated octahedron. The [ PtO(6)] (9-) cluster on Al(3+) site is found to experience the axial elongation of about 0.01 along Z axis, and the planar impurity-ligand bond lengths suffer the relative variation of about 0.11 along X and Y axes due to the Jahn-Teller effect. As a result, the above local lattice distortions produce significant orthorhombic deformation, whereas the original slight trigonal distortion of the host [ AlO(6)] (9-) octahedron is entirely depressed by the Jahn-Teller effect. The calculated EPR parameters based on the above lattice distortions show good agreement with the experimental data, and the local structure of the impurity center is also discussed.

Investigations of the EPR parameters and local structures for the substitutional Cu2+centers in the tungstates

The high order perturbation formulas of the electron paramagnetic resonance (EPR) parameters (the g factors and the hyperfine structure constants) are established for a 3d(9) ion in orthorhombically compressed octahedra. These formulas are applied to the investigations of the EPR spectra and local structures for the substitutional Cu(2+) centers in the wolframite-type tungstates AWO(4) (where A = Cd, Zn, Mg). Based on the studies, the [CuO(6)](10-) clusters are found to experience the local planar bond length variations delta R' (approximate to 0.096, 0.021 and 0.028 angstrom for CdWO(4), ZnWO(4) and MgWO(4), respectively) along X and Y-axes. These values are quite different from the host planar bond length variations delta R (approximate to 0.013, 0.069 and 0.005 angstrom) for the A(2+) sites in the pure tungstates. The above local bond length variations in the impurity centers can be attributed to the Jahn-Teller effect and size mismatching substitution. The theoretical EPR parameters based on the above local structures agree well with the experimental data. By adopting the uniform theoretical formulas and fewer adjustable parameters in this work, the improvements are achieved for the EPR parameters as compared with the previous calculation results.

THE STRAIN QUANTUM CRITICAL POINT FOR SUPERSTRIPES IN THE PHASE DIAGRAM OF ALL CUPRATE PEROVSKITES

The metallic phase in doped cuprate perovskites is determined by both the hope doping δ and the micro-strain ε of the planar Cu-O bond length. The micro-strain ε in the CuO 2 plane has been measured by Cu K-edge EXAFS and x-ray diffraction using synchrotron radiation. The critical micro-strain ε c for the onset of local lattice distortions (LLD) and stripe formation has been determined. The strain quantum critical point (QCP) is found at (ε c ,δ c ). The superconducting critical temperature is measured as a function of two variables T c (ε,δ) and it reaches its maximum at the strain QCP. The superconducting phase occurs in the region of critical fluctuations around this QCP. The critical fluctuations near the strain QCP drives the self-organization of the metallic plane forming a particular superlattice of quantum stripes called "superstripes" that favors the amplification of the superconducting critical temperature.

Evidence for the strain critical point in high Tc superconductors

We report experimental evidence for the phase diagram of doped cuprate superconductors as a function of the micro-strain \(\) of the planar Cu-O bond length, measured by Cu K-edge EXAFS, and hole doping \(\). The local lattice distortions are measured by EXAFS and the charge ordering is measured by synchrotron radiation diffuse X-ray diffraction. This phase diagram shows a QCP at P(\(\)) where for \(\) charge-orbital-spin stripes and free carriers co-exist. The superconducting phase occurs in the region of critical fluctuations around this QCP. The function \(\) of two variables shows its maximum at the strain QCP. The critical fluctuations near this strain QCP give the self-organization of a metallic superlattice of quantum wires “superstripes" that favors the amplification of the critical temperature.

Tris(triphenylphosphaniminato)boran, B(NPPh3)3

Tris(triphenylphosphoraneiminato)borane, B(NPPh3)3 B(NPPh3)3 has been prepared by the reaction of BF3 · OEt2 with LiNPPh3 in toluene/tetrahydrofuran. Moisture sensitive single crystals of B(NPPh3)3 · 0.5 C7H8 were obtained and characterized by NMR and IR spectroscopy as well as by a crystal structure determination. Space group P21/n, Z = 4, lattice dimensions at –70 °C; a = 2147.8(3), b = 978.5(2), c = 2423.8(2) pm, β = 114.11(1)°, R1 = 0.070. The compound forms monomeric molecules with a planar BN3 skeleton and BN bond lengths of 144.7 pm on average.

Structural Properties and Oxygen Stoichiometry of (Pr1.5Ce0.5)Sr2Cu2TaO10−δand (R1.5−xPrxCe0.5)Sr2Cu2NbO10−δ,R=Nd, Sm, Eu: Correlations with Electronic and Magnetic Properties

The structural parameters, phase purity, and oxygen stoichiometry of the (Pr1.5Ce0.5)Sr2Cu2TaO10−δand (R1.5−xPrxCe0.5) Sr2Cu2NbO10−δ,R=Nd, Sm, Eu, compounds are investigated with powder X-ray diffraction (XRD), Rietveld refinements, and thermogravimetric analysis (TGA). The XRD data indicate that all the (R1.5−xPrxCe0.5)Sr2Cu2NbO10−δcompounds are isostructural and typically 98% pure. Rietveld refinements reveal a contraction of theclattice parameter and features in the rare earth–planar oxygen bond length (as functions of Pr doping) that are correlated with the unusual electronic and magnetic transitions induced by the Pr ion in these materials. Results for deoxygenated samples show that the lattice expands upon deoxygenation and suggest that the oxygen is removed primarily from the SrO layers. TGA results reveal reliable techniques for determining oxygen stoichiometries and deoxygenating these compounds and that these compounds are oxygen deficient withδ's of typically 0.045. The structural features of these compounds are compared to similar highTCcuprates, and related to the electronic and magnetic transitions and the suppression of superconductivity induced by the Pr ion in these materials. We discuss our results in terms of the current understanding of highTCcuprates and models for the suppression of superconductivity by the Pr ion in the related (R1−xPrx)Ba2Cu3O7system.

Temperature-dependent modulation amplitude of theCuO2superconducting lattice inLa2CuO4.1

The temperature-dependent amplitude of the Cu-O(planar) bond modulation in the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4.1}$ superconducting crystal has been measured by in-plane-polarized Cu K-edge extended x-ray-absorption fine structure. Minority domains characterized by a long Cu-O(planar) bond length are found at T150 K. The separation between the two Cu-O(planar) distances provides a measure of the amplitude of the ${\mathrm{CuO}}_{2}$ lattice modulation assigned to a charge-density wave that coexists with the superconducting phase, providing evidence for a broken symmetry.

Synthesis and Structure of Tris(trialkylstannyl)‐ and Tris(dialkylhalostannyl)amines; Stabilization of the Sn3N Skeleton by Intramolecular Sn–X–Sn Bridges

AbstractTris(triorganylstannyl)amines (R2R'Sn)3N (1, 2) with substituents R = R' = Me, Bu or R = Me and R' = iPr, tBu are obtained by metathesis from R2R'SnX and NaNH2 in liquid ammonia or by transamination of R3SnNMe2 with NH3. Tris(diorga‐nylhalostannyl)amines (R2XSn)3N (3) are synthesized by stannazane cleavage of (Me3Sn)3N (1) with R2SnX2. Information from multinuclear magnetic resonance spectra ascertain the planarity of the Sn3N skeleton of type 2 and 3, as well as the relationship between the coupling constants 1J(119Sn15N) and 2J(119Sn117Sn) and the Sn–N bond length as determined by the X‐ray structure analysis of 1, 3b and 3r. Compound 3b shows an almost undistorted D3h symmetry with a planar Br3Sn3N skeleton and SnN bond lengths of 1.99 Å, which beside those of 3a are the shortest found so far. According to MNDO approximate and ab initio calculations π interactions between the lone electron pair at the N atom and empty orbitals at the Sn atoms can be excluded. Therefore, the tristan‐nylamines discussed here have a trigonal planar nitrogen center with its lone electron pair in a p‐type orbital. Further characteristic features of the molecular structures of typ 3 compounds are the intramolecular Sn–X–Sn bridges (X = Cl, Br, I) found in the solid state as well as in solution. The molecular geometries of the tristannylamines are supported by MS fragmentation patterns as well as by infrared and Raman spectra.

Pressure dependence of the carrier concentration in electron - and hole-doped Ln 2CuO 4 superconductors

Results of electronic structure calculations for the T-, T ★-, and T′-phase superconductors show that the planar copperapical oxygen bond length does not play a crucial role in determining the charge redistribution under pressure. The rare earth element, the contribution of which has been ignored upto now, plays a more important role. In all three structures we find that there is a small electron transfer from the CuO 2 planes to the rare earth sites which act as electron reservoirs. This results in an increase in the carrier concentration in the CuO 2 planes in the hole-doped superconductors and a decrease in the electron-doped superconductors. Assuming a direct relationship between T c and the carrier concentration we obtain an increase in T c in the hole-doped superconductors and a modest decrease in the electron-doped superconductors.

The geometry of 4-(1-ethoxyethylidene)-5-oxazolones and thiazolones: 1H and 13C studies and the crystal structure of 4-[(Z)-1-ethoxyethylidene]-2-phenyl-5-oxazolone

A series of E and Z isomers of substituted 4-(1-ethoxyethylidene)-5-oxazolones and thiazolones have been prepared and their 1H and 13C spectra recorded. The vinylic methyl 1H chemical shifts showed minimal differences between E and Z isomers whereas the vinylic OCH21H signals differed by 0.15–0.43 ppm, with the Z isomer being consistently the more deshielded. Both vinylic methyl and OCH2 groups showed different 13C resonances for each isomer, with the Z isomers being the more deshielded. The Z geometry was conclusively defined for one isomer of 4-(1-ethoxyethylidene)-2-phenyl-5-oxazolone, 5, by X-ray crystallography and this was sufficient to assign the geometry of the remaining pairs of E and Z isomers. Oxazolone 5 has the space group P21/n and cell dimensions a = 9.219(3), b = 19.899(5), c = 7.459(1) Å, β = 118.01(2)°, and has four formula units in the unit cell. Intensities were measured with use of MoKα radiation and a Nicolet P3 diffractometer. The crystal structure was determined by standard methods and refined to R1 = 0.0709, R2 = 0.0696 based on 1419 independent reflections. The molecule is essentially planar and most bond lengths and angles are normal. Exceptions are the very short C(olefin)—O(ether) bond (1.339(4) Å) and the large ether C—O—C angle (122.1(3)°) caused by extreme delocalization in the O(ether)CCCO(carbonyl) system. The planarity causes a number of strong intramolecular repulsive interactions, causing an exceptionally small external olefin angle, O(ether)CC(methyl), of 108.1(4)°. The ethoxyl side chain of 5 adopts a conformation in the solid state which places the methylene of the OCH2 group adjacent to the oxazole ring nitrogen. This conformation is proposed to persist in solution phases and is consistent with the observed 13C chemical shifts and known γ and δ substituent effects.

The Molecular Structure and Arrangement in Stretched Natural Rubber

Abstract Although there are still some minor discrepancies to be resolved, the appearance of the diffuse-zone electron-diffraction pattern arising from thin films of stretched natural rubber is adequately explained on the basis of the thermal oscillations of the long-chain rubber molecules in the crystalline regions, this pattern being analogous to that which would be given by a stream of oriented molecules in a pseudo-gaseous condition. As Charlesby, Finch and Wilman pointed out, the diffuse-zone pattern has distinct advantages in the examination of complex molecules, since it gives a direct indication of the configuration of the molecule, independent of its mode of fitting into any particular crystalline lattice. In spite of the diffuseness of the patterns with which we are dealing a study of the diffuse zone or molecular pattern has shown that it is possible to discriminate decisively between two postulated atomic configurations which do not differ greatly. In the case of natural rubber, neither of the two configurations investigated agrees in every respect with experiment, but the evidence brought forward above supports the view that the atomic arrangement in the stretched rubber molecule approximates to a simple form having planar units and standard bond lengths and angles, rather than to the more complex form hitherto considered.