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 lambda = 1.249 Angstroms. A series of samples with La2-xDyxCa2xBa2Cu4+2xOz stoichiometric composition, for x = 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, exhibiting no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties are discussed in detail

Neutron scattering study of combustion-synthesized Ce1−XCuxO2−xCe_{1-X} Cu_{x}O_{2-x}

Powder neutron diffraction and Hi-Q neutron diffraction data have been recorded and analysed in order to obtain the local and long range order of Cu in Cu-doped CeO2 with three doping levels of Cu. Rietveld method and MCGR techniques of data analysis for the two types of data reveal that the Cu ion is in the 2+ oxidation state and has a vacancy in its first coordination shell. These deductions from the data analysis fit well with the mechanism of catalysis we propose

Neutron scattering study of combustion-synthesized Ce1−xCuxO2−x

Powder neutron di®raction and Hi-Q neutron di®raction data have been recorded and analysed in order to obtain the local and long range order of Cu in Cu-doped CeO2 with three doping levels of Cu. Rietveld method and MCGR techniques of data analysis for the two types of data reveal that the Cu ion is in the 2+ oxidation state and has a vacancy in its ¯rst coordination shell. These deductions from the data analysis ¯t well with the mechanism of catalysis we propose

Magnetic ordering in La_0.75Sr_0.25CrO₃: a neutron diffraction study

Low-temperature neutron diffraction measurements were carried out on a powder sample of the compound La0.75Sr0.25CrO3 in order to elucidate its magnetic structure. Rietveld analysis of the neutron diffraction data, as a function of temperature, showed that it possesses a G-type antiferromagnetic alignment of Cr spins at all temperatures below 300 K. Down to the lowest achievable temperature, viz. 17 K, the Cr site moments were found to be the weighted average of the 75% Cr3+ and 25% Cr4+ spin-only ionic moments. At 17 K, the Cr site moment was 2.71(5) μB/Cr ion. There is no observable change in the Cr–O bond lengths as a function of temperature. The tilt angles of the CrO6 octahedra marginally increase with decreasing temperature

Low-temperature neutron diffraction study of La_0.95Nd_0.05CrO₃

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

Low-temperature neutron diffraction study of La_1-xNd_xCrO₃ (x = 0.05, 0.1, 0.2 and 0.25)

Low-temperature neutron diffraction measurements have been carried out on the La_1-xNd_xCrO₃ (x = 0.05, 0.1, 0.2 and 0.25) compounds, which are all orthorhombic perovskites (space group Pbnm), in order to elucidate their magnetic structures. Thermal evolution of magnetic structure using Rietveld analysis showed that they all had a G-type antiferromagnetic alignment of spins at all temperatures below their respective Néel temperatures, between Cr³⁺ ions and with moments close to the free Cr³⁺ ionic moment at the lowest measured temperature. There was no indication of Nd³⁺ moment ordering down to 12 K

Structural stability of orthorhombic and rhombohedral La_0.75Nd_0.25CrO₃: a high-temperature neutron diffraction study

A high-temperature neutron diffraction study of La_0.75Nd_0.25CrO₃ 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

Structural study of La_0.75Sr_0.25CrO₃ at high temperatures

A high-temperature neutron diffraction study has been carried out on La_0.75Sr_0.25CrO₃ compound in the temperature range 300–1400 K. On doping the parent compound LaCrO₃ with Sr at the La site, the orthorhombic (Pbnm) to rhombohedral (R3c) structural transition shifts to lower temperatures. From quantitative Rietveld analysis it is found unequivocally that there is a two-phase coexistence (orthorhombic and rhombohedral phases with ~89 and 11 weight%, respectively) in the temperature range 300–470 K and a three-phase coexistence (with a new cubic phase with space group Pm3m) in the temperature range 480–1400 K. The weight percentages of the orthorhombic, rhombohedral and cubic phases were found to be ∼49%, 37% and 14%, respectively, in the temperature range 480–1300 K, while over 1350–1400 K, the average weight percentages of orthorhombic, rhombohedral and cubic phases were found to be ∼41%, 41% and 18%, respectively. The coefficients of volume thermal expansion and linear thermal expansion have been determined for all three phases. The importance of the present study has been discussed for practical applications of the studied compound in solid oxide fuel cells