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CM Thaker S Rayaprol Krushna Mavani DS Rana MS Sahasrabudhe SI Patil DG Kuberkar 《Pramana》2002,58(5-6):1035-1039
The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO3 perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good
example of these properties is (La1−2x
Pr
x
Ca
x
)MnO3 (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T
c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using
X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for
understanding the structural, transport and magnetic properties are discussed in detail. 相似文献
3.
Janzen DE Vanderveer DG Mehne LF da Silva Filho DA Brédas JL Grant GJ 《Dalton transactions (Cambridge, England : 2003)》2008,(14):1872-1882
We report the synthesis and full characterization for a series of cyclometallated complexes of Pt(II) and Pd(II) incorporating the fluxional trithiacrown ligand 1,4,7-trithiacyclononane ([9]aneS3). Reaction of [M(C insertion mark N)(micro-Cl)]2 (M = Pt(II), Pd(II); C insertion mark N = 2-phenylpyridinate (ppy) or 7,8-benzoquinolinate (bzq)) with [9]aneS3 followed by metathesis with NH4PF6 yields [M(C insertion mark N)([9]aneS3)](PF6). The complexes [M(C insertion mark P)([9]aneS3)](PF6) (M = Pt(II), Pd(II); Cinsertion markP = [CH2C6H4P(o-tolyl)2-C,P]-) were synthesized from their respective [Pt(C insertion mark P)(micro-Cl)]2 or [Pd(C insertion mark P)(micro-O2CCH3)]2 (C insertion mark P) starting materials. All five new complexes have been fully characterized by multinuclear NMR, IR and UV-Vis spectroscopies in addition to elemental analysis, cyclic voltammetry, and single-crystal structural determinations. As expected, the coordinated [9]aneS3 ligand shows fluxional behavior in its NMR spectra, resulting in a single 13C NMR resonance despite the asymmetric coordination environment of the cyclometallating ligand. Electrochemical studies reveal irreversible one-electron metal-centered oxidations for all Pt(II) complexes, but unusual two-electron reversible oxidations for the Pd(II) complexes of ppy and bzq. The X-ray crystal structures of each complex indicate an axial M-S interaction formed by the endodentate conformation of the [9]aneS3 ligand. The structure of [Pd(bzq)([9]aneS3)](PF6) exhibits disorder in the [9]aneS3 conformation indicating a rare exodentate conformation as the major contributor in the solid-state structure. DFT calculations on [Pt([9]aneS3)(ppy)](PF6) and [Pd([9]aneS3)(ppy)](PF6) indicate the HOMO for both complexes is primarily dz2 in character with a significant contribution from the phenyl ring of the ppy ligand and p orbital of the axial sulfur donor. In contrast, the calculated LUMO is primarily ppy pi* in character for [Pt([9]aneS3)(ppy)](PF6), but dx2-y2 in character for [Pd([9]aneS3)(ppy)](PF6). 相似文献
4.
Novel chromophores formed in the solvent reactions of α-amino acids and small peptides were identified by crystal structure analysis and characterized by UV absorption. The formation of these chromophores in basic solutions was attributed to two strong driving forces—conjugation and cyclization. The discussion of possible reaction pathways could benefit the future design of α-amino acid-based chromophores. 相似文献
5.
The metal ion-complexing properties of 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) are reported. The protonation constants (pK1 = 4.75, pK2 = 2.53) and formation constants (log K(1)) for PDA with Mg(II) (3.53), Ca(II) (7.3), Sr(II) (5.61), Ba(II) (5.43), La(III) (13.5), Gd(III) (16.1), Zn(II) (11.0), Cd(II) (12.8), Pb(II) (11.4), and Cu(II) (12.8) were determined by UV-vis spectroscopy in 0.1 M NaClO4 at 25 degrees C. The log K(1) values for most of these metal ions were high enough that they were not displaced from their PDA complexes even at pH 2. The log K(1) values were determined using the UV spectra to monitor the competition with EDTA (or DTPA; EDTA = ethylendiamine tetraacetic acid, DTPA = diethylenetriamine pentaacetic acid) as a function of pH according to the equilibrium: M(EDTA) + PDA + nH+ = M(PDA) + EDTAHn. The log K1 values indicate that the rigid extended aromatic backbone of PDA leads to high levels of ligand preorganization and selectivity toward large metal ions (e.g., Ca(II), Cd(II), Gd(III)) with an ionic radius of about 1.0 A and greatly enhanced thermodynamic stability as compared to similar ligands without the reinforcing aromatic backbone. The structure of [Ca(PDA)(H2O)2].2H2O (1) is reported: orthorhombic, Fdd2, a = 44.007(9) A, b = 18.945(4) A, c = 7.2446(14) A, V = 6040(2) A(3), Z = 16, R = 0.0882. The Ca(II) ion has a coordination number of eight, lying in the plane of the tetradentate PDA, with Ca-N bonds averaging 2.55 A and Ca-O bonds to the two acetate groups of PDA averaging 2.45 A. These are very close to the normal Ca-L bonds of this type, supporting the idea that a metal ion the size of Ca(II) (ionic radius approximately 1.0 A) will fit into PDA in a low-strain manner. The remaining four coordination sites on Ca(II) in 1 come from two coordinated water molecules and a chelating carboxylate bridging from an adjacent [Ca(PDA)(H2O)2].2H2O complex. Potential applications of PDA as a ligand in biomedical applications such as Gd(III) contrast agents in MRI are discussed. 相似文献
6.
JP West D Sulejmanovic SJ Hwu J He D Vanderveer BK Johnson 《Inorganic chemistry》2012,51(18):9723-9729
Single crystals of three new noncentrosymmetric (NCS) phosphates, α (1) and β (2) forms of Cs(3)KBi(2)Mn(4)(PO(4))(6)Cl and α-Cs(3)KBi(2)Fe(4)(PO(4))(6)Cl (3), were grown in a reactive CsCl/KCl molten-salt media. Their structures were determined by single-crystal X-ray diffraction methods showing that the α form crystallizes in the space group Cc (No. 9), which is in one of the 10 NCS polar crystal classes, m (2/m) while the β form crystallizes in P4(3) (No. 78) of another polar class, 4 (4/m). The unit cell parameters of the α form can be approximately correlated with that of the β form via the 3 × 3 orientation matrix [0.5, 0.5, 0; -0.5, 0.5, 0; 0, 0, 2 sin β]. The structures of these otherwise complicated phosphates exhibit two types of channels with circular and elliptical windows where the Cl-centered Cl(Bi(2)Cs) acentric unit is located. The neighboring acentric units are arranged in a parallel fashion in the α form, resulting in the monoclinic (Cc) lattice, but "antiparallel" in the β form, thus giving the tetragonal (P4(3)) unit cell. 1-3 feature the compatible M-O-P unit that contains four crystallographically independent MO(x) (x = 4, 5) polyhedra, which are connected to the Cl(Bi(2)Cs) acentric unit through one short and one long M(II)···Cl bond. The compositions of 1 and 2 consist of three Mn(2+) (d(5)) and one Mn(3+) (d(4)) per formula unit and that of 3 has three Fe(2+) (d(6)) and one Fe(3+) (d(5)). Bond valence sums reveal that, in the α phase, the trivalent site adopts distorted tetrahedral M(1)(3+)O(4) coordination and, in the β phase, distorted trigonal-bipyramidal M(4)(3+)O(5). Thus far, the iron phase has only been isolated in the α form presumably because of little extra stabilization energy gain if the Fe(2+) d(6) ion were to occupy the M(1)O(4) site. The possible origins pertaining to the structural differences in the α and β forms are discussed. 相似文献
7.
Michelle A. Meierhoefer Matthew J. Walters S. Patrick Dunn Jarrett H. Vella Bonnie J. Grant Carolyn L. Sober Nidhi S. Patel Laela M. Hajiaghamohseni Sara B. Lioi Clyde R. Metz Charles F. Beam William T. Pennington Donald G. Vanderveer N. Dwight Camper 《Journal of heterocyclic chemistry》2006,43(2):307-313
8.
§ 1.Introduction WearegivenkindependentWishartdensitiesofthe (p +q)× (p +q)randomsymmetricpositivedefinitematricesG1,… ,Gktobeg(Gi) =Kexp -12 trR- 1i Gi Gi12 (ni- q-p- 1) ,(1 )wherei=1 ,… ,k,andRidenotesthepopulationcorrelationmatrixofthei thpopulationandKasagenericletterdenote… 相似文献
9.
Xiao Hu Guillaume Guery Joshua Boerstler J. David Musgraves Don Vanderveer Peter Wachtel Kathleen Richardson 《Journal of Non》2012,358(5):952-958
Glass ceramic materials with composition 75TeO2–xBi2O3–(25-x)ZnO (x = 13, 12, 11) possessing transparency in the near- and mid-infrared (MIR) regions were studied in this paper. It was found that as the Bi2O3 content increased in the glass composition, the observed crystallization tendency is enhanced, and high crystal concentrations were obtained for the glasses with high Bi2O3 content while maintaining transparency in the MIR region. Crystal size in the glass ceramic was reduced by adjusting the heat treatment conditions; the smallest average size obtained in this study is 700 nm. Bi0.864Te0.136O1.568 was identified using X-ray Diffraction (XRD) and found to be the only crystal phase developed in the glass ceramics when the treatment temperature was fixed at 335 °C. The morphology of the crystals was studied using Scanning Electron Microscopy (SEM), and crystals were found to be polyhedral structures with uniform sizes and a narrow size distribution for a fixed heat treatment regime. Infrared absorption spectra of the resulting glass ceramics were studied. The glass ceramic retained transparency in the infrared region when the crystals inside were smaller than 1 μm, with an absorption coefficient less than 0.5/cm in the infrared region from 1.25 to 2.5 μm. The mechanical properties were also improved after crystallization; the Vickers Hardness value of the glass ceramic increased by 10% relative to the base glass. 相似文献
10.
The structure of the compound Fe4(CO)12S(CSNMe2)(CNMe2) has been determined by X-ray crystallography. The compound crystallizes in space group P21/c with four molecules in a unit cell of dimensions a 8.840(2), b 20.174(9), c 16.856(5)Å, β 114.82(3)°. Full-matrix least-squares refinement of 2881 counter data yielded R = 0.046. The molecule consists of two Fe2(CO)6 units bridged by thiocarboxamido and immoniocarbene ligands and by a common bridging sulfur atom. The structure of this compound is compared with those of related molecules and a detailed comparison is made of the bonding properties of the thiocarboxamido ligand in bridging and chelating configurations. 相似文献