Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

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 (La_1−2x 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.     

Crystal and molecular structure of benzophenone azine: evidence for an Fe(II) carbene intermediate

Benzophenone azine crystallizes in the monoclinic space groupA2/a (No. 15) witha=16.303(3) Å,b=5.4864(2) Å,c=21.973(5) Å, =85.52(2)°,V=1959.4(8) ų, andD _calc=1.22 g cm^–3 forZ=4. The structure was solved by direct methods and refined againstF to a finalR value of 0.047. The unit cell contains four molecules of the title compound; the asymmetric unit consists of half a molecule. Despite the high crystallographic symmetry, the two halves of the molecule arenot related by a center of symmetry; instead, the molecule adopts C₂ symmetry; the molecular C₂ axis is coincident with the crystallographic twofold axis. The formation of benzophenone azine from the reaction of the iron-containing Lewis acid complex [(⁵-C₅H₅)Fe(CO)₂(THF)]⁺ [BF₄]^– and diphenyldiazomethane provides strong evidence for an Fe(II) carbene intermediate.     

A structural study of sodium dithionite and its ephemeral dihydrate: A new conformation for the dithionite ion

Raman spectroscopy on solid anhydrous sodium dithionite reveals that it exists in at least two forms in the solid state. MAS²³Na NMR spectra and X-ray powder diffraction patterns of the solids indicate that sodium ions are in different environments in each form of the material. The results suggest that the dithionite anion is conformationally different in each lattice. A single crystal X-ray diffraction study of the recrystallized form of anhydrous sodium dithionite reveals an anion with C₂ geometry and a 16° O-S-S-O torsional angle; nearly eclipsed. (Crystal Data for recrystallized Na₂S₂O₄ area=6.539(1) Å,b=6.552(1) Å,c=6.578(1) Å,V=240.0(1) ų,=121.61(1)°, space group=P2/c,Z=2). Raman spectra of sodium dithionite dihydrate reveal that the dithionite ion is in a different conformation than in either of the anhydrous materials. A single crystal X-ray diffraction study of Na₂S₂O₄·2H₂O reveals a dithionite anion with a substantially shorter S-S bond length than in the anhydrous structure and an O-S-S-O torsional angle of 56°; approximately gauche. (Crystal Data for Na₂S₂O₄·2H₂O area=8.134(1) Å,b=5.756(2) Å,c=14.528(5) Å,V=653.3(3) ų,=106.20(2)°, space group=P2₁/n,Z=4). The structure of the dithionite anion is found to depend critically upon the nature of its external environment.     

Crystal and molecular structure of trans-(tricarbonyl)bis(trans-2,2,3,4,4-pentamethyl-1-phenylphosphetane)iron

The title compound crystallizes in the monoclinic space groupP2₁/a witha=13.092(2) Å,b=15.853(2) Å,c=15.406(3) Å,=103.00(1)°,V=3116(1) ų, andD _calc=1.237 g cm^–3 forZ=4. The structure was solved by direct methods, and refined to a finalR value of 0.057. The phosphetane ligands adopt a nonlinear trans relationship. The four-membered rings are non-planar and have different flap angles of 6.1 and 20.3°. Virtual coupling is evident in the¹³C NMR spectrum of the title compound. Comparison of the¹³C NMR spectral data of this molecule with the parent phosphine ligand, the ligand oxide, and the mono-phosphine iron complex is provided.     

Cr(VI) Oxidation of Cholesterol—A Kinetic Study Using N-Cetylpicolinium Dichromates,A Class of Novel Phase Transfer Oxidants

Kinetics and Catalysis - Kinetic study of cholesterol oxidation has been studied using a series of N-cetylpicolinium dichromates (CPDC), a class of phase transfer oxidants, in acetic acid medium...     

Elastic properties of the Ta–V system: bcc Ta0.33V0.67 and C15 TaV2

Resonant ultrasound spectroscopy was used to measure the elastic constants of bcc Ta_0.33V_0.67 over the temperature range 3.5–300?K; the results were compared to earlier measurements on C15 TaV₂. The temperature dependence of the polycrystalline shear modulus is completely different in the two phases; that of the bcc phase decreases with temperature whereas that of the C15 phases increases in an anomalous fashion. This difference is consistent with a model involving doubly-degenerate levels at the X point of the Brillouin zone in the C15 phase with the Fermi level lying near the doubly degenerate level. This model accounted for the unusual behaviour of the C15 phase. Debye temperatures were determined from the ultrasonic measurements: 295?K for the C15 phase and 315?K for the bcc phase.     

Irradiation of a cone by a magnetic dipole

Th electromagnetic field produced by a magnetic dipole in thepresence of a perfectly conducting cone of arbitrary cross-sectionis determined. The solution is used to find out how a currenton the cone travelling towards the apex is reflected. Some valuesof the reflection coefficient are calculated. In particular,it is shown that there is a sort of resonance with the reflectionincreasing significantly as the cone approaches a plane.