Interruptions of (CG)n sequences by GG, TG and CA need not prevent B to Z transition in solution

Oligonucleotides containing alternating purines-pyrimidines with AT base pairs have been shown to exist in the Z-form preferably in solid state. We report that oligodeoxyribonucleotides with GG, TG and CA interruptions in their alternating CG sequences can undergo B to Z transition in solution in the absence of any chemical modification or topological constraint. The sequences, d(CGCGCGGCGCGC) and d(CGTGCGCACG) have been synthesised and shown to adopt Z- conformation in presence of millimolar concentrations of Ni2+ under low water activity conditions. Significance of GG, TG and CA interruptions in the B to Z transition is discussed.

Structure of nonactin-calcium perchlorate, C40H64O12.Ca(ClO4)2, and a comparative study of metal-nonactin complexes

M r= 975.9, orthorhombic, Pnna, a = 20.262 (3), b= 15.717 (2), c= 15.038 (1)A, V= 4788.97 A 3, z = 4, D x = 1.35 Mg m -3, Cu Kct radiation, 2 = 1.5418 A, /t = 2.79 mm -1, F(000) -= 2072, T = 293 K, R = 0.08, 3335 observed reflections. The molecular structure and the crystal packing are similar to those observed in the nonactin complexes of sodium thiocyanate and potassium thiocyanate. The eight metal-O distances are nearly the same in the potassium complex whereas the four distances involving carbonyl O atoms are shorter than the remaining four involving the tetrahydrofuran-ring O atoms in the Na and the Ca complexes. This observation can be explained in terms of the small ionic radii of Na + and Ca 2+, and leads to a plausible structural rationale for the stronger affinity of nonactin for K + than for the other two metal ions.

Dependence of magnetism on GdFeO3 distortion in the t(2g) system ARuO(3) (A = Sr, Ca)

We have examined the stability of the ferromagnetic (FM) state in CaRuO3 and SrRuO3 as a function of the GdFeO3 distortion. Model calculations predict the dependence of the FM transition temperature (T-c) on the rotation angle theta to vary as cos(2)(2 theta) for e(g)-electron systems. However, here, we find an initial increase and then the expected decrease. Furthermore, a much faster decrease is found than predicted for e(g)-electron systems. Considering the specific case of CaRuO3, a larger deviation of the Ru-O-Ru angle from 180 degrees in CaRuO3 as compared to SrRuO3 should result in a more reduced bandwidth, thereby making the former more correlated. The absence of long-range magnetic order in the more correlated CaRuO3 is traced to the strong collapse of various exchange interaction strengths that arises primarily from the volume reduction and increased distortion of the RuO6 octahedra network that accompanies the presence of a smaller ion at the A site.

Substitution of chromium for univalent copper in superconducting Pb2Sr2(Ca,Y)Cu3O8+delta

Following considerations of geometry and the similarity between chromate and carbonate groups in terms of size and charge, we have investigated the possibility of replacing the two-coordinate Cu-I in superconducting lead cuprates of the general formula Pb2Sr2(Ca, Y)CU3O8 by Cr. A high-resolution electron microscopy study coupled with energy dispersive X-ray analysis on small crystals of the title phases suggests that between 10 and 15% of the Cu-I can be replaced by Cr. While from the present structural study using HRTEM and Rietveld refinement of X-ray powder data we are unable to precisely obtain the oxidation state and oxygen coordination of Cr, we suggest in analogy with Cr substitution in other similar cuprates that in the title phases (CuO2)-O-I rods are partially replaced by tetrahedral CrO42- groups. Infrared spectroscopy supports the presence of CrO42- groups. The phases Pb1.75Sr2Ca0.2Y0.8O8+delta and Pb1.75Sr2Ca0.2Y0.8CCu2.85Cr0.15O8+delta are superconducting as-prepared, but the substitution of Cr for Cu-I results in a decrease of the Te as well as the superconducting volume fraction. (C) 1996 Academic Press, lnc.

Investigation of valence states in the perovskite oxides, LaAFeNbO(6) (A = Ca, Sr)

Compounds of the type, LaAFeNbO(6) (A = Ca Sr) have been synthesized to study the electrical and magnetic properties and to examine valence degeneracy. The results show that valence degeneracy is not operative and the compounds are insulating. Magnetic susceptibility data show that part of the Fe is in Fs(2+) state, thus oxidizing part of Nb4+ to Nb5+ by an internal redox mechanism. The presence of mixed valent Fe is confirmed by Mossbauer spectra. (C) 1999 Elsevier Science B.V. All rights reserved.

LiSr1.65 square 0.35B1.3B ' O-1.7(9) (B = Ti, Zr; B ' = Nb, Ta): New lithium ion conductors based on the perovskite structure

We describe the design and synthesis of new lithium ion conductors with the formula, LiSr(1.65)rectangle(0.35)B(1.3)B'O-1.7(9) (rectangle = vacancy; B = Ti, Zr; B' = Nb, Ta), on the basis of a systematic consideration of the composition-structure-property correlations in the well-known lithium-ion conductor, La-(2/3-x)Li(3x)rectangle((1/3)-2x)TiO3 (I), as well as the perovskite oxides in Li-A-B,B'-O (A = Ca, Sr, Ba; B = Ti, Zr; B' = Nb, Ta) systems. A high lithium-ion conductivity of ca. 0.12 S/cm at 360 degrees C is exhibited by LiSr(1.65)rectangle(0.35)Ti(1.3)Ta(1.7)O(9) (III) and LiSr(1.65)rectangle(0.35)Zr(1.3)Ta(1.7)O(9) (IV), of which the latter containing stable Zr(IV) and Ta(V) oxidation states is likely to be a candidate electrolyte material for all-solid-state lithium battery application. More importantly, we believe the approach described here could be extended to synthesize newer, possibly better, lithium ion conductors.

Raman evidence for the superconducting gap and spin-phonon coupling in the superconductor Ca( Fe0.95Co0.05)(2)As-2

Inelastic light scattering studies on a single crystal of electron-doped Ca(Fe0.95Co0.05)(2)As-2 superconductor, covering the tetragonal-to-orthorhombic structural transition as well as the magnetic transition at T-SM similar to 140 K and the superconducting transition temperature T-c similar to 23 K, reveal evidence for superconductivity-induced phonon renormalization. In particular, the phonon mode near 260 cm(-1) shows hardening below T-c, signaling its coupling with the superconducting gap. All three Raman active phonon modes show anomalous temperature dependence between room temperature and T-c, i.e. the phonon frequency decreases with lowering temperature. Further, the frequency of one of the modes shows a sudden change in temperature dependence at TSM. Using first-principles density functional theory based calculations, we show that the low temperature phase (T-c < T < T-SM) exhibits short-ranged stripe antiferromagnetic ordering, and estimate the spin-phonon couplings that are responsible for these phonon anomalies.

Electronic structure of ${\rm Ca}_{1-x}{\rm Sr}_x{\rm VO}_3$: A tale of two energy scales

We investigate the electronic structure of Ca1-xSrxVO3 using photoemission spectroscopy. Core level spectra establish an electronic phase separation at the surface, leading to a distinctly different surface electronic structure compared to the bulk. Analysis of the photoemission spectra of this system allowed us to separate the surface and bulk contributions. These results help us to understand properties related to two vastly differing energy scales, namely the low-energy scale of thermal excitations ( $\sim\!k_{\rm B}T$) and the high-energy scale related to Coulomb and other electronic interactions.

Properties of the Ferrimagnetic Double-Perovskites A2FeReO6 (A=Ba and Ca)

We have synthesized ceramics of A2FeReO6 double-perovskites A2FeReO6 (A=Ba, Ca). Structural characterizations indicate a cubic structure with a=8.0854(1) Å for Ba2FeReO6 and a distorted monoclinic symmetry with a=5.396(1) Å, b=5.522(1) Å, c=7.688(2) Å and β=90.4° for Ca2FeReO6. The barium compound is metallic from 5K to 385K, i.e. no metal-insulator transition has been seen up to 385K, and the calcium compound is semiconducting from 5K to 385K. Magnetization measurements show a ferrimagnetic behavior for both materials, with Tc =315 K for Ba2FeReO6 and above 385K for Ca2FeReO6. At 5K we observed, only for Ba2FeReO6, a negative magnetoresistance of 10% in a magnetic field of 5T. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr2Fe(Mo,Re)O6 recently studied.

Gibbs energies of formation of intermetallic phases in the systems Pt Mg, Pt Ca and Pt Ba and some applications

The standard Gibbs energies of formation of platinum-rich intermetallic compounds in the systems Pt-Mg, Pt-Ca, and Pt-Ba have been measured in the temperature range of 950 to 1200 K using solid-state galvanic cells based on MgF2, CaF2, and BaF2 as solid electrolytes. The results are summarized by the following equations: ΔG° (MgPt7) = −256,100 + 16.5T (±2000) J/mol ΔG° (MgPt3) = −217,400 + 10.7T (±2000) J/mol ΔG° (CaPt5) = −297,500 + 13.0T (±5000) J/mol ΔG° (Ca2Pt7) = −551,800 + 22.3T (±5000) J/mol ΔG° (CaPt2) = −245,400 + 9.3T (±5000) J/mol ΔG° (BaPt5) = −238,700 + 8.1T (±4000) J/mol ΔG° (BaPt2) = −197,300 + 4.0T (±4000) J/mol where solid platinum and liquid alkaline earth metals are selected as the standard states. The relatively large error estimates reflect the uncertainties in the auxiliary thermodynamic data used in the calculation. Because of the strong interaction between platinum and alkaline earth metals, it is possible to reduce oxides of Group ILA metals by hydrogen at high temperature in the presence of platinum. The alkaline earth metals can be recovered from the resulting intermetallic compounds by distillation, regenerating platinum for recycling. The platinum-slag-gas equilibration technique for the study of the activities of FeO, MnO, or Cr2O3 in slags containing MgO, CaO, or BaO is feasible provided oxygen partial pressure in the gas is maintained above that corresponding to the coexistence of Fe and “FeO.”

Structural and dielectric behavior of pulsed laser ablated Sr(0.6)Ca(0.4)TiO(3) thin film and asymmetric multilayer of SrTiO(3) and CaTiO(3)

Homogeneous thin films of Sr(0.6)Ca(0.4)TiO(3) (SCT40) and asymmetric multilayer of SrTiO(3) (STO) and CaTiO(3) (CTO) were fabricated on Pt/Ti/SiO(2)/Si substrates by using pulsed laser deposition technique. The electrical behavior of films was observed within a temperature range of 153 K-373 K. A feeble dielectric peak of SCT40 thin film at 273 K is justified as paraelectric to antiferroelectric phase transition. Moreover, the Curie-Weiss temperature, determined from the epsilon'(T) data above the transition temperature is found to be negative. Using Landau theory, the negative Curie-Weiss temperature is interpreted in terms of an antiferroelectric transition. The asymmetric multilayer exhibits a broad dielectric peak at 273 K. and is attributed to interdiffusion at several interfaces of multilayer. The average dielectric constants for homogeneous Sr(0.6)Ca(0.4)TiO(3) films (similar to 650) and asymmetric multilayered films (similar to 350) at room temperature are recognized as a consequence of grain size effect. Small frequency dispersion in the real part of the dielectric constants and relatively low dielectric losses for both cases ensure high quality of the films applicable for next generation integrated devices. (C) 2011 Elsevier B.V. All rights reserved.

Synthesis, structural characterization and ferroelectric properties of Pb(0.76)Ca(0.24)TiO(3) nanotubes

A capillary-enforced template-based method has been applied to fabricate Pb(0.76)Ca(0.24)TiO(3) (PCT24) nanotubes via filling PCT24 precursor solution, prepared by modified sol-gel method, into nanochannels of anodic aluminum oxide templates. The morphology and structure of as-prepared PCT24 were examined by scanning electron microscopy, transmission electron microscopy (TEM) and X-ray diffraction techniques. The obtained PCT24 nanotubes with diameter of similar to 200 nm and wall thickness of similar to 20 nm exhibited a tetragonal perovskite structure. High resolution TEM (HRTEM) analysis confirmed that as-obtained PCT24 nanotubes made up of nanoparticles (5-8 nm) which were randomly aligned in the nanotubes. Formation of some solid crystalline PCT24 nanorods, Y-junctions and multi-branches were observed. Interconnections in the pores of template are responsible for the growth of Y-junctions and multi-branches. The possible formation mechanism of PCT24 nanotubes/nanorods was discussed. Ferroelectric hysteresis loops of PCT24 nanotube arrays were measured, showing a room temperature ferroelectric characteristic of as-prepared PCT24 nanotubes. (C) 2011 Elsevier B.V. All rights reserved.

Enhancing the magnetic characteristics of BiFeO3 nanoparticles by Ca, Ba co-doping

Multiferroic nanoparticles (NPs) of pristine and Ca, Ba co-doped BiFeO3 were synthesized by a facile sal gel route. Co-doping was done by fixing the total dopant concentration at 5 mol% and then the relative concentrations of Ca and Ba was varied. Structural, optical and magnetic properties of the NPs were investigated using different techniques. UV-Vis absorption spectra of BiFeO3 NPs showed a substantial blue shift of similar to 100 nm (530 nm -> 430 nm) on Ca. Ba co-doping which corresponds to increase in band gap by 0.5 eV. Fe-57 Mossbauer spectroscopy confirmed that iron is present only in 3(+) valence state in all co-doped samples. The coercive field increased by 18 times for Bi0.95Ca0.01Ba0.04FeO3 samples, which is the maximum enhancement, observed amongst all the 5 mol% doped samples. At the equimolar (2.5 mol % each) concentration of co-dopants, the coercive field shows a significant enhancement of about 9 times (220 Oe -> 2014 Oe) with concomitant increase in saturation magnetization by 7 times. Thus, equimolar co-doping causes simultaneous enhancement of the twin aspects of magnetic properties thereby making them better suited for device applications. (C) 2012 Elsevier B.V. All rights reserved.

Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of the superconducting iron pnictide Ca(Fe0.944Co0.056)(2)As-2

We report the temperature evolution of coherently excited acoustic and optical phonon dynamics in the superconducting iron pnictide single crystal Ca(Fe0.944Co0.056)(2)As-2 across the spin density wave transition at T-SDW similar to 85 K and the superconducting transition at T-SC similar to 20 K. The strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range from 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below T-SC indicating a coupling of these low-energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T similar to 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before T-SDW. Copyright (c) EPLA, 2012

Ultrafast quasiparticle relaxation dynamics in superconducting iron pnictide Ca(Fe0.944Co0.056)(2)As-2

Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting Ca(Fe0.944Co0.056)(2)As-2 single crystals by measuring transient reflectivity changes using femtosecond time-resolved pump-probe spectroscopy. Large changes in the temperature-dependent differential reflectivity values in the vicinity of the spin density wave (T-SDW) and superconducting (T-SC) transition temperatures of the sample have been inferred to have charge gap opening at those temperatures. We have estimated the zero-temperature charge gap value in the superconducting state to be similar to 1.8k(B)T(SC) and an electron-phonon coupling constant lambda of similar to 0.1 in the normal state that signifies the weak coupling in iron pnictides. From the peculiar temperature-dependence of the quasiparticle dynamics in the intermediate temperature region between T-SC and T-SDW we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures.