J. Ghose

Synthesis, characterisation and magnetic studies on nanocrystalline Co0.2Zn0.8Fe2O4

Abstract Nanocrystalline Co 0.2 Zn 0.8 Fe 2 O 4 have been synthesised by the co-precipitation method. XRD shows that the spinel ferrite is formed after sintering the sample at 393xa0K. Nanoparticles with average particle size of 5–65xa0nm were obtained by sintering the samples between 393 and 1173xa0K. FTIR show that spinel ferrites prepared by precipitation method retain some hydroxyl groups, which are not removed completely by sintering at temperatures ≤873xa0K. DTA/TG measurements show that between 573 and 823xa0K spinel is formed and a small weight loss above 873xa0K is due to removal of hydroxyl. Saturation magnetization of the samples increase with increasing average particle size upto 11xa0nm and for samples with particle size >11xa0nm, the saturation magnetization decreases with increasing particle size. This is due to a change in cation distribution with change in particle size.

Electrical properties of the CuCr2O4 spinel catalyst

Abstract Electrical resistivity of CuCr2O4 spinel obtained by a coprecipitation method was measured at various temperatures from 350 to 923K in air. Oxidation of its surface chromium ions at various temperatures was determined by an iodometric method. The temperature dependence of resistivity and the presence of higher-valent chromium ions at higher temperatures indicated CuCr2O4 to be an extrinsic semiconductor. Log ϱ vs 1 T plots yielded straight lines in the whole temperature range with two breaks, one around 550K and the other around 730K. The first break was attributed to the saturation of the surface with higher-valent chromium, while the second break was attributed to tetragonal-to-cubic phase transition in CuCr2O4. Log ϱ vs 1 T plots obtained while cooling exhibited hysteresis near the second break, confirming this break to be due to a first-order diffusionless transition.

Thermoelectric power measurements of CuCr2−xAlxO4 and Cu1−xMgxCr2O4 solid solution spinel oxides

Abstract Thermoelectric power of the solid solution system CuCr 2− x Al x O 4 for x = 0–2 was determined between 300 and 673 K. CuCr 2 O 4 shows p -type conduction and for x > 0.08 it changes to n -type. The results show that this is due to the presence of Cu 1+ tet ions produced during the redistribution of cations in the spinel lattice when aluminum is substituted for chromium ions. In the p -type samples charge hopping is between Cr 4+ oct and Cr 3+ oct and in the n -type, between Cu 2+ tet and Cu 1+ tet . The samples show n -type or p -type behavior depending on the dominant charge carrier present at a particular temperature.

Solid state studies on substituted CuCr2O4 spinel

Abstract Changes in crystallographic, electrical, and thermal properties of CuCr2O4 spinel were investigated by replacing Cu with Mg, i.e., Cu1−xMgxCr2O4, and Cr with Al, i.e., CuCr2−xAlxO4. The tetragonal distortion in CuCr2O4 disappeared with 60% replacement of Cu by Mg (x = 0.6) or 50% replacement of Cr by Al (x = 1.0). The temperature variation of electrical resistivity for all the tetragonal samples was similar to that of CuCr2O4. The first order, diffusionless phase transition was manifest in the hysteresis loops of log ϱ vs 1 T plots. The resistivity and activation energy for conduction changed sharply near the phase transition composition. With the replacement of Cr by Al, the conduction in CuCr2O4 was found to change from p type to n type. The low thermal stability of the spinel was found to be due to a high concentration of tetrahedral Cu2+ ions (>80%) and compressed tetragonal distortion which strains the spinel lattice. This strain is removed by replacing either Cu with Mg or Cr with Al, whereby the spinel becomes stable.

Investigation of nanocrystalline CoFe 2 O 4 by positron annihilation lifetime spectroscopy

Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in diffuse vacancy clusters or microvoids at the grain boundaries and at the grain-boundary triple points. The longest component corresponds to the pick-off annihilation of ortho-positronium formed at the larger voids. The variations in these lifetimes and their relative intensities with annealing temperature and crystallite size have been studied in detail.

Single crystal structure and Mössbauer studies of a new cation-deficient thiospinel : Cu5.47Fe2.9Sn13.1S32

Starting from pure metals and sulphur in evacuated silica tubes single crystals of n nCu 5.47(9) Fe 2.9(3) Sn 13.1(3) S 32 have been obtained by n nquenching from 680°C. It crystallizes in the Fd3 m space group with a = 10.3022 (6) A. The n natomic parameters, occupancies, and anisotropic thermal parameters were refined to give a final R-value of 0.0116. The n nSn 119 Mossbauer study shows the presence of Sn (IV) in an octahedral environment and 57 Fe Mossbauer indicates the presence of Fe (II) in an octahedral environment, statistically n ndistributed in both low- and high-spin configurations.

Effect of Fe3+ incorporation on the Fe2+ clustering in FeGa2O4

Abstract Magnetization and Mossbauer effect studies have been carried out on samples of Fe 1+ x Ga 2− x O 4 for x = 0.2, 0.5, 0.6, 0.8, and 1.0. The results show Fe 2+ ion clusters are present in the samples with x ≤ 0.2. On increasing x , clustering is removed and Fe 3+ ions are distributed between the two lattices giving rise to strong A - B interactions.

Mössbauer studies on Zn-substituted iron molybdate

Abstract Mossbauer spectra of Fe 2− y Zn y MoO 4 spinel ferrites were recorded in their paramagnetic state. Paramagnetic Mossbauer spectra of all the samples show broad absorption peaks due to the presence of Fe 2+ and Fe 3+ ions on both sites (A and B) of the spinel lattice. All the spectra have been fitted with four doublets, using a least squares fitting program. The isomer shift and quadrupole splitting values show that Fe A 2+ , Fe A 3+ , Fe B 3+ and Fe B 2.5+ ions are present. Fe B 2.5+ represents the presence of the Fe 2+ and Fe 3+ ions on the B-sites, which take part in charge hopping. The results of electrical resistivity and magnetic measurements support charge hopping between Fe 2+ and Fe 3+ ions on B-sites.

Solid state studies on rhodium-substituted CuCr2O4 spinel oxide

Abstract Rhodium-substituted CuCr 2 O 4 , i.e., CuCr 2− x Rh x O 4 , spinel oxides were prepared. X-ray analysis showed that single phase spinels were obtained for x = 0 to 0.8, 1.8, and 2.0. The other samples had an additional Cu 2 Cr 2 O 4 phase. The temperature variation of electrical resistivity for all the single phase samples except CuRh 2 O 4 was similar to that of CuCr 2 O 4 and with the substitution of Cr (3 d transition metal) by Rh (4 d transition metal) the conduction process did not change gradually from CuCr 2 O 4 type to CuRh 2 O 4 type.

Electrical studies on CuRh2O4

Electrical resistivity of the CuRh/sub 2/O/sub 4/ spinel has been determined in air while heating and cooling between 300-923K. The room temperature resistivity is of the order of 10 ohm cm and remains almost unchanged up to 563K, above which it decreases rapidly with temperature. A break in the log rho-1/T plot at 563 K is due to a change in the conduction process, and the phase change undergone by CuRh/sub 2/O/sub 4/ at 850 K is not manifest in the plot.