Publications

The First Open-Framework Cadmium Phosphate, K4[Cd3(HPO4)4(H2PO4)2], with a Layered Structure

A layered cadmium phosphate of the formula K4[(Cd3(HPO4)4(H2PO4)2], I, containing six-membered apertures, constituting the first example of an open-framework cadmium phosphate, has been synthesized under hydrothermal conditions. Crystal data: monoclinic, P21/n (No. 14), a=9.0698(3), b=6.7341(3), and c=18.2663(7) Å, β=93.092(1)°, V=1114.03 Å3, Z=4. The structure consists of a network of CdO6 octahedra and PO3(OH) and PO2(OH)2 tetrahedra forming a spiro unit, which are connected through Cd2O2 two-membered rings forming one-dimensional chains. The chains are further connected forming two-dimensional macroanionic layers with six-membered apertures possessing hanging phosphate units on either side of the layer. The layers are stabilized by strong hydrogen bond interactions between the terminal P–OH groups forming pseudo channels in which the charge-compensating extraframework K+ ions are located. The presence of the spiro unit and the infinite one-dimensional Cd–O–Cd chains, along with the hanging phosphates are noteworthy structural features.

Dielectric properties, thermal decomposition and related aspects of BiAlO3

BiAlO3, prepared under high-pressure and high-temperature conditions has a rhombohedral perovskite structure ( R 3 c with a = 5.3747 ( 4 )  Å and c = 13.391 ( 1 )  Å) and decomposes around 550 ∘C to form the intermediate Bi2Al4O9 phase along with the Bi 26 − x Al x O 40 − y and Al2O3. Above 580 ∘C, the intermediate phase changes into the Bi 26 − x Al x O 40 − y . Dielectric measurements show an anomaly at the decomposition temperature and hysteresis at room temperature with a maximum polarization of ∼16 μC/cm2. Raman scattering measurements suggest softening of optical phonon modes, and are consistent with the occurrence of thermal decomposition of the rhombohedral phase to form the thermodynamically stable phase.

Probing the hydrogen bond through experimental charge densities

Experimental charge density analysis has been carried out by using low-temperature X-ray diffraction data on molecular crystals exhibiting a variety of hydrogen bond interactions. The various types of hydrogen-bonded interactions examined include O–H⋯O, N–H⋯O, C–H⋯O, O–H⋯N, N–H⋯N, N–H⋯S and C–H⋯S bonds. Static deformation density maps reveal depletion of electron density in the hydrogen-acceptor (H⋯A) regions. While the total density and the Laplacian at the H⋯A critical points exhibit an exponential dependence on the H⋯A distance, the positive curvature of the density, λ 3, closely follows the exponential relation with a considerably better correlation factor. The relation derived based on the data for 218 hydrogen bonds of various types is given by 0.47×103 exp(−2.5×d H⋯A ).

Anderson transitions in Ln_1-<i>x</i>Sr_<i>x</i>CoO₃(Ln═Pr or Nd)

Electrical conductivity measurements show that Ln1-x Sr x CoO3, (Ln = Pr or Nd) undergoes a non-metal-metal transition when x≈0 3. The d.c. conductivity of compositions with 0<x<0.l obeys the T−1/4law up to 400 K and the a.c. conductivity data follow the ωδ law with δ in the range 0.6–0.8. Surprisingly, however, δ seems to increase with increasing temperature. Thermopower measurements show clear evidence for a change in the mechanism of conduction (around 350 K) from variable range hopping to transport at E c. All these transport properties suggest that Co4+ ions in Ln1-x Sr x CoO3 (0<x<0.1), considered as moving in a lattice of Co3+, form a degenerate gas and that the states at the Fermi energy are Anderson localized.

Certain Novel Aspects of Thallium and Bismuth Cuprate Superconductors

Mo1−W S2 nanotubes and related structures

(NH4)2MoS4 and (NH4)2WS4 form solid solutions over the entire composition range. Nanotubes of Mo1−x W x S2 solid solutions have been prepared by employing these solid solutions precursors. Thus, the thermal decomposition of (NH4)2Mo1−x W x S4 in an inert or a hydrogen atmosphere yields Mo1−x W x S2 nanotubes, those with x<0.1 being more readily obtained. The nanotubes exhibit novel tip architectures and defects. Single-walled nanotubes have also been observed in some samples.

Structure and properties of oxyanion derivatives of 123 cuprate superconductors: Effect of substitution of carbonate and nitrate ions in the Cu(1) position of YBaSrCu3O7−δ

When the Cu(1) site in orthorhombic YBaSrCu3O7−δ (Tc = 80 K), is partly substituted by the carbonate ion (upto 50%), the crystal structure becomes tetragonal and the electron diffraction pattern shows evidence for 2a × 2c superstructure; the material however is not superconducting. The same is true when Y is partly replaced by Ca as in YCaBa2Sr2Cu5(CO3)Oy. When the CO3 group is partly replaced by NO3 group as in the YCaBa2Sr2Cu5(CO3)1−x(NO3)xOy, the structure remains the same but superconductivity is retained. IR spectroscopic studies show that both CO3 and NO3 coordinate strongly and are not present as free ions in these oxyanion cuprate derivatives. Cu K-EXAFS studies on the carbonate and carbonato-nitrate derivatives confirm the presence of the oxyanions in the place of CuO4 units in the CuO chains.

Molecular Structure of Tin Tetrachloride

No abstract is provided for this article.

Evaluation of solute-solvent interactions from solvent blue-shifts of n → π∗ transitions of CO, CS, NO2 and NN groups: hydrogen bond energies of various donor-acceptor systems

Effects of non-polar, polar and proton-donating solvents on the n → π* transitions of CO, CS, NO2 and NN groups have been investigated. The shifts of the absorption maxima in non-polar and polar solvents have been related to the electrostatic interactions between solute and solvent molecules, by employing the theory of McRAE. In solvents which can donate protons the solvent shifts are mainly determined by solute-solvent hydrogen bonding. Isobestic points have been found in the n → π∗ bonds of ethylenetrithio-carbonate in heptane-alcohol and heptane-chloroform solvent systems, indicating the existence of equilibria between the hydrogen bonded and the free species of the solute. Among the different proton-donating solvents studied water produces the largest blue-shifts. The blue-shifts in alcohols decrease in the order 2,2,2-trifluoroethanol, methanol, ethanol, isopropanol and t-butanol, the blue-shift in trifluoroethanol being nearly equal to that in water. This trend is exactly opposite to that for the self-association of alcohols. It is suggested that electron-withdrawing groups not merely decrease the extent of self-association of alcohols, but also increase the ability to donate hydrogen bonds. The approximate hydrogen-bond energies for several donor-acceptor systems have been estimated. In a series of aliphatio ketones and nitro compounds studied, the blue-shifts and consequently the hydrogen bond energies decrease with the decrease in the electron-withdrawing power of the alkyl groups. It is felt that electron-withdrawing groups render the chromophores better proton acceptors, and the alcohols better donors. A linear relationship between n → π∗ transition frequency and the infrared frequency of ethylenetrithiocarbonate has been found. It is concluded that stabilization of the electronic ground states of solute molecules by electrostatic and/or hydrogen-bond interactions determines the solvent shifts.

Virtues of marginally metallic oxides

Transition-metal oxides at the metal–insulator boundary, especially those belonging to the perovskite family, exhibit fascinating phenomena such as insulator–metal transitions controlled by composition, high-temperature superconductivity and giant magnetoresistance (GMR). Interestingly, many of these marginally metallic oxides obey the established criteria for metallicity and have a finite density of states at the Fermi level. The perovskite manganates exhibiting GMR, on the other hand, are unusual in that they possess very high resistivities in the 'metallic' state and show no significant density of states at the Fermi level. Marginal metallicity in oxide systems is a problem of great complexity and contemporary interest and its understanding is of crucial significance to the diverse phenomena exhibited by these materials.

Structure and superconducting properties of a new family of thallium cuprates, TlSr3−xLnxCu2O7

Cuprates of the formula TlSr3−x Ln x Cu2O7 (Ln=Pr, Nd or Y) derived from the hypothetical TlSr3Cu2O7 show superconductivity with T cs up to 95 K when 0.5≲x≤0.75, the x=1.0 compositions being insulators. Rietveld analysis of X-ray diffraction profiles has been carried out for two superconducting members of this family. The unit cell a-parameter, and hence the in-plane Cu-O distance, increases with increase in x. The T c value decreases with increase in x or the in-plane Cu-O distance in all the series of cuprates. Superconductivity in the Tl1−y Pb y Sr3−x Nd x Cu2O7 systems is found with the highest T c of 95 K when y=0.2 and x=0.5. The in-plane Cu-O distances in all the cuprates studied fall in the range found in the Sr-class of cuprate superconductors.

Role of the spin magnitude of the magnetic ion in determining the frustration and low-temperature properties of kagome lattices

In view of the variety of low-temperature magnetic properties reported recently for kagome lattices with transition-metal ions in different oxidation states, we have investigated the low-energy spectrum and low-temperature thermodynamic properties of antiferromagnetic kagome lattices with varying magnitudes of site spins, employing quantum many-body Heisenberg models. The ground state and the low-lying excitation spectrum are found to depend strongly on the nature of the spin magnitude of the magnetic ions. The system remains highly frustrated if spins are half-odd-integer in magnitude, while the frustration is very weak or almost absent for integer spins or mixed-spin systems. In fact, for a mixed-spin kagome system with a certain magnitude, the whole system behaves as a classical magnet with a ferrimagnetic ground state without any frustration. These theoretical findings are consistent with a few experimental observations recently reported in the literature and would be of value in designing new kagome systems with unusual and interesting low-temperature magnetic properties.

Alkali and alkaline-earth ion vibrations in solvent cages, coordination compounds, glasses and related systems

Alkali metal ion—oxygen donor solvent cages as well as coordination compounds, alkali metal oxide glasses and oxyanion salts of alkali metals exhibit low-frequency bands in the infrared spectrum characteristic of the cation—oxygen polyhedra. Similar bands are seen with nitrogen donors. Alkaline earth ions also show such absorption bands due to quantised vibration of cations. These bands not only provide a means of establishing the nature of cation coordination in diverse systems, but also serve as probes to examine ion-solvent interactions.

Offene Metallcarboxylat‐Architekturen

Alumosilicate und Metallphosphate sind die wichtigsten Beispiele für anorganische Materialien mit offenen Gerüsten. In den letzten Jahren stieß zu dieser Gruppe die Familie der Metallcarboxylate, zu der nicht nur Mono‐ und Dicarboxylate von Übergangs‐, Seltenerd‐ und Hauptgruppenmetallen, sondern auch eine Vielzahl von Hybridstrukturen zählen. Einige Carboxylate weisen neuartige magnetische Eigenschaften und Adsorptionseigenschaften auf. Mithilfe von Dicarboxylaten und verwandten Verbindungen kann man effektiv neue poröse Hybridstrukturen erhalten. In einigen dieser Strukturen verbrücken die Dicarboxylateinheiten zwei anorganische Einheiten. Bemerkenswert sind auch die neuen Hybridnanokomposite, in denen Gitter aus Cadmiumoxalat als Wirte für aufgeweitete Alkalimetallhalogenidstrukturen dienen. In diesem Aufsatz beschreiben wir die Synthesen, Strukturen und Eigenschaften von Metallcarboxylaten mit verschiedenartigen offenen Gerüsten.

Electron Paramagnetic Resonance Study of Intrinsic Phase Segregation in CMR Manganite Nd$_{0.5}$Sr$_{0.5}$MnO$_3$

We present results of an electron paramagnetic resonance (EPR) study of Nd$_{1-x}$Sr$_x$MnO$_3$ with x = 0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T$_c$) and the antiferromagnetic, charge ordering transition (T$_N$ = T$_{co}$) at 150 K. The results are compared with those on Nd$_{0.45}$Sr$_{0.55}$MnO$_3$ which undergoes a transition to a homogeneous A-type antiferromagnetic phase at T$_N$ = 230 K and on La$_{0.77}$Ca$_{0.23}$MnO$_3$ which undergoes a transition to coexisting ferromagnetic metallic and ferromagnetic insulating phases. For x = 0.5, the EPR signals below T$_c$ consist of two Lorentzian components attributable to the coexistence of two phases. From the analysis of the temperature dependence of the resonant fields and intensities, we conclude that in the mixed phase ferromagnetic and A-type antiferromagnetic (AFM) phases coexist. The x = 0.55 compound shows a single Lorentzian throughout the temperature range. The signal persists for a few degrees below T_N. The behaviour of the A-type AFM phase is contrasted with that of the two ferromagnetic phases present in La$_{0.77}$Ca$_{0.23}$MnO$_3$. The comparison of behaviour of A-type AFM signal observed in both Nd$_{0.5}$Sr$_{0.5}$MnO$_3$and Nd$_{0.45}$Sr$_{0.55}$MnO$_3with the two FM phases of La$_{0.77}$Ca$_{0.23}$MnO$_3$, vis-a-vis the shift of resonances with respect to the paramagnetic phases and the behaviour of EPR intensity as a function of temperature conclusively prove that the Nd$_{0.5}$Sr$_{0.5}$MnO$_3$undergoes phase separation into A-type AFM and FM phases.

Giant magnetoresistance and related properties of rare earth manganates and related systems

Rare earth manganates of the formula Ln1−x A x MnO3 become ferromagnetic and undergo an insulator-metal transition around the Curie temperature, T c , when the Mn4+ content is around 30%. These materials also show GMR especially around T c . Various features affecting GMR in these materials and certain novel features such as charge-ordering exhibited by manganates are discussed.

CCDC 644079: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Mechanism of the addition of alcohols to substituted phenylisothiocyanates

The addition reaction of alcohols to substituted phenylisothiocyanates is found to be a second-order reaction. The reaction is catalysed by triethylamine. First-order rate constants of the addition reaction have been determined in excess of ethanol, for a number of substituted phenylisothiocyanates and the rate data give a satisfactory linear correlation with Hammett σ constants of groups. While the energies of activation vary randomly with substitution, the entropies of activation bear a linear relationship to the energies of activation. Infra-red spectra indicate that the thiourethanes which are the products of the addition reaction exist in the thioamide form. The most prominent resonance form which can satisfactorily explain both the kinetic and infrared data, has been suggested.