Keka R. Chakraborty

Effect of Dy substitution for La in La0.7Ca0.3MnO3 perovskite

AC susceptibility, resistivity and dc magnetization measurements on the (La 0.757 Dy 0.243 ) 0.7 Ca 0.3 MnO 3 perovskite reveal a clear departure from the well-behaved (collinear) ferromagnetic nature observed in the parent material La 0.7 Ca 0.3 MnO 3 at low temperatures. Neutron diffraction structural study shows that the buckling of the MnO 6 octahedra increases with the Dy substitution. This in turn gives a weaker double exchange (ferromagnetic) interaction, as the transfer integral, t o, describing the hopping of eg electrons between Mn 3+ and Mn 4+ decreases resulting in the observed deterioration of ferromagnetism and metallic conduction.

Low-temperature neutron diffraction study of La0.95Nd0.05CrO3

We have synthesized polycrystalline La0.95Nd0.05CrO3 sample by doping the La-site of LaCrO3 with Nd and its magnetic properties have been studied using DC magnetization and neutron diffraction techniques. DC magnetization study shows a paramagnetic to a weak ferromagnetic-like transition at ∼295 K followed by signatures of a spin reorientation phenomenon at 233 and 166 K and, finally a transition to an antiferromagnetic-like phase at ∼21 K. Low-temperature neutron diffraction measurements confirm a weak ferrimagnetic ordering of Cr3+ moments at all temperatures below 295 K.

A neutron diffraction study on ceria - neodia solid solutions

Neutron diffraction studies were carried out to ascertain the structural details of the composition Ce 0.5 Nd 0.5 O 1.750 . The structure was unequivocally found to be that of C-type cubic. The refinement on an F-type cubic lattice was found to be unacceptable because of high R values. Selected bond distances are also being reported. In addition, the neutron diffraction studies on a typical defective F-type composition Ce 0.75 Nd 0.25 O 1.875 were also carried out.

Spin-phonon coupling in scandium doped gallium ferrite

We embarked on a study of Scandium (Sc) doped (onto Ga site) gallium ferrite (GaFeO3) and found remarkable magnetic properties. In both doped as well as parent compounds, there were three types of Fe3+ ions (depending on the symmetry) with the structure conforming to space group Pna21 (Sp. Grp. No. 33) below room temperature down to 5 K. We also found that all Fe3+ ions occupy octahedral sites, and carry high spin moment. For the higher Sc substituted sample (Ga1−xScxFeO3: x = 0.3), a canted magnetic ordered state is found. Spin-phonon coupling below Neel temperature was observed in doped compounds. Our results indicated that Sc doping in octahedral site modifies spin-phonon interactions of the parent compound. The spin-phonon coupling strength was estimated for the first time in these Sc substituted compounds.

Structural elucidation of stabilized tetragonal ThSiO4 : A neutron diffraction study

A new compound Th 0.9 Ce 0.1 SiO 4 , iso-typic to zircon, was prepared by heating predried ThO 2 , CeO 2 , and SiO 2 (in the mole ratio 0.9:0.10:1.0) by a two-step heating protocol. The polycrystalline sample obtained was characterized by Rietveld refinement of the observed neutron diffraction data with the starting model of tetragonal ThSiO 4 . It has a body centered tetragonal structure with space group I 4 1 ∕ a m d and four formula units per unit cell. The unit-cell parameters are a =7.1238(4) A and c =6.3186(95) A. The R P , R w p , and R e factors are 7.77%, 10.9%, and 4.85%, respectively. The incorporation of about 10 mol % cerium was used to stabilize the tetragonal modification of ThSiO 4 .

Structural study and electrical properties of Zr-doped Nd2Sn2O7 pyrochlore compounds

Nd2Sn2O7 pyrochlores with the substitution of Zr4+ were prepared by conventional ceramic double sintering technique. The single-phase formation was confirmed by X-ray diffraction and neutron diffraction techniques. Relative intensity calculations for X-ray diffraction analysis were performed for oxygen positional parametersx = 0.331 and 0.375, while Rietveld refinements were employed for neutron diffraction data. The neutron diffraction study revealed that there are only two anion sites with 48f and 8b positions. This indicates that the 8a site, i.e. O(3) sublattice, is completely vacant and the structure is a perfect cubic pyrochlore with space group Fd3m (Oh7). From the conductivity measurements, it is observed that the electronic conductivity dominates from room temperature up to about 525 K and forT > 525 K, the oxygen ion conduction dominates the charge transport in these compositions. Complex impedance spectroscopy indicates the existence of grain and grain boundary as two separate elements.

Magnetic behaviour of TbMnFe

Neutron diffraction and Mössbauer measurements have been carried out on the cubic Laves phase intermetallic TbMnFe. The magnetic moment on the transition metal atom is found to be low, 0.2µB, at room temperature. This moment is temperature independent down to 10 K. Magnetic moment on the rare earth atom varies from 2.5µB at 296 K to 7.27µB at 10 K. Mössbauer spectra recorded at 298 K and 78 K have magnetic character but there is a large distribution of hyperfine field values. Both these features arise due to magnetic frustration created in the sample due to the competing ferro and antiferromagnetic interactions between the transition metal atoms.

Magnetic ordering: Neutron diffraction and depolarization studies at Dhruva Reactor

The neutron powder diffractometers and the polarized neutron spectrometer at the Dhruva reactor, Trombay have been used extensively for magnetic studies of several class of important materials, such as ferrites, perovskites, magnetic shape memory alloys, low dimensional magnetic oxides, molecular magnetic compounds, intermetallic alloys and compounds [1–19]. In present article, we report a few examples of such recent investigations [2, 3, 11, 19].

Structural stability of orthorhombic and rhombohedral La0.75Nd0.25CrO3: a high-temperature neutron diffraction study

A high-temperature neutron diffraction study of La0.75Nd0.25CrO3 compound has been carried out in the temperature range 300?1400?K. The sample has been studied for its structural stability of the two observed crystallographic phases at elevated temperatures. It was found to contain orthorhombic and rhombohedral phases with weight percentages 85% and 15%, respectively, at ambient temperature. It was observed that the two phases coexist even up to 1400?K; however, the fraction of the rhombohedral phase increases with increasing temperature at the expense of the orthorhombic phase. At 1400?K, the weight fractions of the orthorhombic and rhombohedral phases are about 11% and 89%, respectively. This study reveals a negative thermal expansion along the b-axis for the orthorhombic phase and along the c-axis for the rhombohedral phase from 950?K up to 1200?K, where a rapid transformation of one phase to the other occurs. Below 950?K as well as above 1200?K, both the phases show a normal positive thermal expansion. The La?La and Cr?Cr bond distances show an abrupt drop from 950?K up to 1200?K and a sharp rise after that, which corroborates the negative thermal expansion of the cell volume and b-axis of the orthorhombic phase. The Cr?Cr distance in the rhombohedral phase shows a reduction in magnitude in the temperature range 950?1200?K, explaining the negative thermal expansion of the c-axis of this phase. The coefficients of volume thermal expansion and linear thermal expansion have been determined for both the phases. The significance of the present study has been discussed for practical applications of the studied compound in solid oxide fuel cells.

NEUTRON DIFFRACTION STUDIES ON La2-xDyxCa2xBa2Cu4+2xOz SUPERCONDUCTORS

Structural studies on Dy-substituted La-2125 type superconductors have been carried out by neutron diffraction experiments at room temperature using a monochromatic neutron beam of wavelength (λ) = 1.249 Å. A series of samples with La2-xDyxCa2xBa2Cu4+2xOz stoichiometric composition, forx = 0.1–0.5, have been studied for their structural properties. A tetragonal Y-123 unit cell was taken as the starting model for the Rietveld analysis. All the samples fit into the starting model, with no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties will be discussed in detail.