Entropy and phase diagram of valence transition

The electronic and lattice entropies associated with the valence transition are estimated. The electronic entropy in metallic phase is evaluated based on the model which includes the mixing between ?-level and d-band states, and the d-band superimposes the hybridized ?-level. The quasiharmonic approximation together with the Debye approximation are used to calculate the lattice entropy. For the first order transition occurring at low temperature the entropy of semiconducting phase is found to be lower than that of metallic phase. The reverse situation is obtained for high transition temperature. This explains the experimental fact that the slope of the phase boundary of valence transition in SmS changes its sign with temperature. The specific heat calculated in this model shows a broad maximum at low temperature.     

On the Einstein Relation in Ternary Semiconductors in the Presence of Crossed Electric and Magnetic Fields

We investigate theoretically the Einstein relation for the diffusivity-mobility ratio of the electrons in ternary semiconductors at low temperatures in the presence of crossed electric and quantizing magnetic fields on the basis of threeband Kane model. It is found, taking n-Hg_1-xCd_xTe as an example, that DMR shows an oscillatory magnetic field dependence. Besides, the DMR increase both with increasing electron concentration and decreasing alloy composition respectively.     

Superparamagnetic-like behaviour of magnetic alloys

The effect of antiferromagnetic interaction on a binary Ising ferromagnetic alloy A_pB_1?p is studied based on cluster-variational method and assuming either J_AB or J_BB or both exchange interactions to be of antiferromagnetic type. In a ferromagnetic state the moment saturates to a value much lower than its full value. Within the temperature interval where moment saturates, the system exhibits superparamagnetic-like behaviour. The induced moment becomes a function of $\text{h}\text{T}$ only, and the zero-field susceptibility follows Curie law. The incomplete saturation of moment persists up to a large value of $\text{h}\text{T}$. The short-range order parameter at T = 0 is much less than that of homogeneous ferromagnet.     

Effect of carrier degeneracy on the diffusivity-mobility ratio in n-Cd3As2

An attempt is made to study the dependence of the diffusivity-mobility ratio on carrier concentration in degenerate n-Cd₃As₂ according to the Bodnar model which has recently been shown in the literature from studies on magnetic quantization to be the most valid model for Cd₃As₂. The results obtained are then compared with those derived on the basis of the Kane model to indicate the amount of error that would be involved with the use of the same model since many authors have continued to use it for Cd₃As₂.     

Synthesis,characterization and cyclic voltammetric studies of monopicolinate complexes of ruthenium(II)

Summary Two stable monopicolinate complexes of ruthenium(II), [Ru(bipy)₂(pic)]ClO₄ and [Ru(pap)₂(pic)]ClO₄ [bipy = 2,2-bipyridine, pic = picolinate anion, pap = 2-(phenylazo)-pyridine], were prepared and characterized. The complexes are diamagnetic and behave as 1:1 electrolytes in MeCN solution. In the i.r. spectra, they show characteristic vibrations of bipy or pap, pic and ClO _inf4 ^p– . In MeCN solution, both complexes display three intense absorption bands in the visible region, which have been assigned to metal-to-ligand charge-transfer transitions. Each complex shows a reversible ruthenium(II)-ruthenium(III) oxidation in MeCN, the formal potential (E _inf298 ^p0 ) being 0.75 V versus a saturated calomel reference electrode (SCE) for [Ru-(bipy)₂(pic)]⁺ and 1.44 V versus SCE for [Ru(pap)₂(pic)]⁺. Multiple reductions of the coordinated bipy and pap ligands have also been observed.Author to whom all correspondence should be directed.     

Synthesis, characterization and cyclic voltammetric studies of ruthenium oximates

Summary Using 1,2-naphthoquinone-1-oxime (HL) as the principal ligand, four mixed-ligand ruthenium oximate complexes - namely [Ru(bipy)₂(L)]ClO₄, [Ru(pap)₂(L)]-ClO₄, [Ru(bipy)(L)₂] and [Ru(PPh₃)₂(L)₂], where bipy = 2,2′-bipyridine and pap = 2-(phenylazo)pyridine- have been synthesized and characterized. In all these complexes, Ru exists in the +2 state. They are diamagnetic and, in solution, show several intense metal-to-ligand charge transfer (MLCT) transitions in the vis. region. In MeCN solution, all four complexes show a reversible Ru^II-Ru^III oxidation on the positive side of a standard calomel electrode (s.c.e.), the potential of which varies with the compositions of the complexes. Reductions of the coordinated co-ligands (bipy or pap) are also observed.     

Synthetic studies towards complex diterpenoids—VII : Stereoselective synthesis and conformational analysis of all four possible racemates of 1,2,3,4,4a,9a-hexadydro-1-methylfluorene-1-carboxylic acids

Synthesis of the unsaturated acid 3 through H₂SO_4? catalysed cyclodehydration of the keto-ester 1 and its conversion to lactone 4 are described. The PPA induced cyclization of the keto-acid 9, on the other hand resulted in a stereoisomeric mixture of the octahydroanthracene derivatives 10a and 10b. The four possible racemates of the hydrofluorene derivatives 5, 6, 7, and 8 have been synthesized by chemical and catalytic reduction of 3 and 4. Li-liquid ammonia induced reductive cleavage of the lactone 4 proceeds with 65% retention and 35% inversion of configuration at the benzylic C-4a asymmetric centre to afford the trans and cis acids 5 and 8 respectively, while catalytic hydrogenolysis of 4 proceeds with inversion to give the cis acid 8. Li-liquid ammonia reduction of 3 gives trans acid 6 and cis7 in 23% and 53% yields respectively, whereas catalytic hydrogenation of 3 affords 81% of cis acid 8 and 13% of cis acid 7. Some conformational properties of the methyl esters 17, 18, 19, and 20 have been deduced from chemical and NMR spectral data.     

On the gate capacitance of MOS structures of kane-type semiconductors under magnetic quantization

An attempt is made to formulate the gate capacitance of MOS structures of Kane-type semiconductors under magnetic quantization, without any approximations of weak or strong electric field limits, on the basis of the fourth-order effective mass theory and taking into account the interactions of the conduction, light-hole, heavy-hole, and split-off bands. It is found, taking n-channel Hg_1–x Cd _x Te as an example, that the gate capacitance exhibits spiky oscillations with changing magnetic field, which is in qualitative agreement with experimental observations, reported elsewhere, in MOS structures of the same semiconductor. The corresponding results for n-channel inversion layers on parabolic semiconductors are also obtained from the expressions derived.     

Reduction of Sulfur Dioxide to Sulfur Monoxide by Ferrous Porphyrin**

The reduction of SO₂ to fixed forms of sulfur can address the growing concerns regarding its detrimental effect on health and the environment as well as enable its valorization into valuable chemicals. The naturally occurring heme enzyme sulfite reductase (SiR) is known to reduce SO₂ to H₂S and is an integral part of the global sulfur cycle. However, its action has not yet been mimicked in artificial systems outside of the protein matrix even after several decades of structural elucidation of the enzyme. While the coordination of SO₂ to transition metals is documented, its reduction using molecular catalysts has remained elusive. Herein reduction of SO₂ by iron(II) tetraphenylporphyrin is demonstrated. A combination of spectroscopic data backed up by theoretical calculations indicate that Fe^IITPP reduces SO₂ by 2e⁻/2H⁺ to form an intermediate [Fe^III−SO]⁺ species, also proposed for SiR, which releases SO. The SO obtained from the chemical reduction of SO₂ could be evidenced in the form of a cheletropic adduct of butadiene resulting in an organic sulfoxide.