Chandra Prakash

Dielectric behaviour of tetravalent titanium-substituted Ni-Zn ferrites

Abstract The dielectric constants and dielectric loss tangents of tetravalent titanium-substituted Ni-Zn ferrites Zn 0.25 Ni 0.75 + t Ti t Fe 2−2 t O 4 with various titanium concentrations were measured at room temperature in the frequency range 100 Hz – 500 kHz. The variation of these parameters with composition and frequency is explained qualitatively. Peaks in the dielectric loss tangent versus frequency curves are observed in two of the ferrites. The possible mechanisms are discussed.

Mössbauer studies on hyperfine field measurement in titanium doped nickel-zinc ferrites

Abstract 57Fe Mossbauer effect study in the polycrystalline solid solutions of Zn0.25Ni0.75+tTitFe2−2tO4 (t=0.0-0.5) has been carried out at room temperature. All the samples exhibited two pure Zeeman sextets corresponding to tetrahedral and octahedral sites. The present work has been aimed at investigating the influence of tetravalent titanium substitution on the isomer shift, quadrupole splitting, nuclear magnetic fields at 57Fe nuclei for tetrahedral and octahedral sites and the cation distribution. The isomer shift was found to be independent of substitution level t. Nuclear magnetic field HA decreases faster than HB with t. From the variation of HA and HB with t it is concluded that for higher titanium substituted samples, titanium occupies both the B as well as A sites.

Cation distribution and quadrupole interaction in tin-substituted zinc ferrites

Mossbauer and X-ray diffraction studies of the tin-substituted zinc ferrites Zn1+xSnxFe2-2xO4 have been carried out and the cation distribution Zn(ZnxSnxFe2-2x)O4 has been established using both the methods. 57Fe Mossbauer spectra in these spinel solid solutions with x=0.0 to 0.9 are reported at 77K and 295K. The 119Sn Mossbauer spectra have been used to identify the site occupancy of tin and show its presence at octahedral sites only. 57Fe Mossbauer spectra show well resolved quadrupole doublets for all the compositions studied. A monotonic increase in the value of quadrupole splitting with increasing tin content is observed. This is attributed to the change in the oxygen parameter of the spinel structure with addition of tin. No detectable change in the value of quadrupole splitting is observed with the change in temperature, which suggests that the quadrupole splitting is mainly of lattice origin. The value of the isomer shift does not change with tin concentration, which indicates that the s-electron charge distribution of Fe3+ ions is not influenced by tin substitution. An increase in the value of the isomer shift with decreasing temperature is attributed to the second-order Doppler shift.

A convenient synthesis of linear pyranocoumarins. Xanthyletin and 3-Phenylxanthyletin

A convenient synthesis of linear pyranocoumarins, viz., 8,8-dimethyl-2H,8H-benzo[1,2-b; 5,4-b′]dipyran-2-one (xanthyletin,1) and 8,8-dimethyl-3-phenyl-2H,8H-benzo[1,2-b; 5,4-b′]dipyran-2-one (3-phenylxanthyletin,2) is described. The key steps are blocking the 8-position of appropriate 7-hydroxy-2H-1-benzopyran-2-one derivatives with iodine and 1,1-dimethyl-2-propynylation followed by cyclisation.ZusammenfassungEs wird ein vorteilhafter Weg zur Synthese von linearen Pyranocumarinen am Beispiel von Xanthyletin und 3-Phenylxanthyletin gezeigt. Das Syntheseprinzip besteht in einer Blockierung der 8-Position des entsprechenden 7-Hydroxy-2H-1-benzopyran-2-ons mit Jod und einer 1,1-Dimethyl-2-propinylierung mit nachfolgender Cyclisierung.

Hyperfine interaction studies in the doped lithium ferrite system Li0.5+0.5xCr0.3TixFe2.2-1.5xO4

Mossbauer spectra of Li0.5+0.5xCr0.3TixFe2.2-1.5xO4 with x varying from 0.0 to 1.0 in steps of 0.2 have been obtained at 300 K and 78 K. The effect of Ti4+ substitution on the various hyperfine interactions has been discussed. For lower Ti4+ concentrations (0.0 and 0.2) the spectra showed a clear Zeeman pattern whereas, for x>or=0.4, a central quadrupole doublet was present together with the magnetic sextet at both temperatures. The intensity of the doublet was observed to increase with increase in Ti4+ content. This anomalous behaviour of the Mossbauer spectra in the present system has been discussed.

Effect of doping on the spin state of a tris(dithiocarbamato)iron(III) complex

Abstract This Communication reports magnetic, Mossbauer and EPR studies on Fe 1− x Co x (dhe-dtc) 3 · 3H 2 O with x = 0.0, 0.2, 0.5 and 0.7. The complex with x = 0.0 is predominantly high-spin at room temperature and shows a gradual spin transition to low-spin state on decreasing the temperature. At and below 20 K, it is exclusively in low-spin state. Doping increases the population of the low-spin state and shifts the spin cross-over towards higher temperature.

Hyperfine field in Li-Zn-Ti ferrites

The paper reports the57Fe Mössbauer effect investigations on the solid solutions of Li0.5ZnxTixFe2.5−2xO4 (x=0 to 0. 6) at 300K. The samples with x⩽0.4 exhibit well defined Zeeman pattern consisting of two sextets corresponding to Fe3+ ions at tetrahedral and octahedral sites. The composition with x=0.5 shows a strong ferrimagnetic relaxation whereas the composition with x=0.6 exhibits paramagnetic behaviour. The internal magnetic field at57Fe nuclei decreases with increasing x at both the sublattices. This variation is explained in terms of cation distribution and super-exchange interactions. The s-electron density has been observed to be independent of substitution level.

Electrical conductivity and dielectric behaviour of Fe2−2xSnxZn1 + xO4

Abstract D.c. resistivity measurements on solid solutions of Fe 2−2 x Sn x Zn 1 + x O 4 are reported as a function of tin concentration and temperature in the present paper. The resistivity is observed to increase with increasing x . This variation is explained in terms of the cation distribution and a hopping mechanism. The activation energy of each sample is determined from the temperature dependence of the resistivity. The variation in the dielectric constant and the dielectric loss as a function of composition and frequency of applied field are also reported. The behaviour is explained in terms of the Koops model.

An elegant synthesis of some naturally occurring linear acetylchromenes: eupatoriochromene, methyleupatoriochromene (encecalin); evodionol and methylevodionol

Abstract An elegant synthesis of linear acetylchromenes, viz eupatoriochromene, methyleupatoriochromene (encecalin), evodionol and methlevodionol, has been achieved by blocking the reactive position position C-3 of the appropriate ketones with an iodo group, prenylation with 3-chloro-3-methylbut-1-yne and subsequent cyclisation. Regiospecific introduction of C-prenyl group in the less reactive C-5 has been achieved by the reaction of the appropriate 3-iodo ketones with 2-methylbut-3-en-2-ol. The 5-prenyl ketones are also essential intermediates for the synthesis of linear acetylchromenes.

Synthesis of linear 3-phenylpyranocoumarins: synthesis of robustin, robustic acid, and their methyl ethers

The linear 3-phenylpyranocoumarins, viz., robustin (5b; R4= H), its methyl ether (5b; R4= Me), robustic acid (5c; R4= H), its methyl ether (5c; R4= Me), 4-methoxy-3-phenylxanthyletin (5a; R4= H), and its methyl ether (5a; R4= Me), have been synthesised starting from the corresponding 7-(1,1-dimethylprop-2-ynyloxy)-coumarin derivatives having the 8-position blocked with an easily introduceable and removable group like iodine. The subsequent step consists of heating the prop-2-ynyl ethers in NN-dimethylaniline, which resulted in cyclization to linear pyranocoumarins, accompanied by the removal of iodine and also partial demethylation of the methoxy-group at the 4-position. The compounds obtained were identical with the natural samples and were different from authentic samples of the corresponding angular isomers.