In situ XRPD and DSC study of order-disorder phase transition in nanosize Li-ferrite

An order-disorder phase transition of nanosize Li-ferrite (Li0.50Fe2.50O4) was studied by DSC measurements and in situ XRPD technique. The phase transition temperature is found to be 762 °C (DSC) and (745±5) °C (XRPD).Physical chemistry 2004 : 7th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 21-23 September 200

Biquadratic coupling between two ordering parameters in 1 : 3 B-site ordered spinels

The Letter presents an analysis of the order-disorder phase transition mechanism, Fd (3) over bar LT ---- GT P4(3)32, in 1:3 B-site ordered spinels, Li1.33xCo2-2xTi1+0.67xO4 and Li1-0.5xFe2.5xMn2-2xO4, by using the Landaus theory of phase transitions. Two ordering parameters were used to describe cation distribution: Q(1) for cation distribution between tetrahedral and octahedral sites. and Q(2) for cation distribution between two octahedral sites. Analysis of the topology of the order parameter vector space indicates biquadratic coupling between Q(1) and Q(2). For the Li1.33xCo2-2xTi1+0.67xO4 (x = 0.50) in the low-temperature ferroelastic phase both ordering parameters differ from zero and they both influence on the spontaneous strain scalar values. (C) 2004 Elsevier B.V. All rights reserved

Structure and microstructure of SmxY2-xO3

SmxY2-xO3 solid solutions are semimagnetic semiconducting materials with wide energy gaps. Magnetic properties of these materials are coupled with their structural and microstructural parameters. Evidently, it is of interest to investigate these parameters in order to explain magnetic properties of diluted magnets. All mixed sesquioxides SmxY2-xO3 (x = 0.2, 0.5, 0.7, 1.0, 1.4 and 1.7) crystallize in C-M2O3 crystal type in space group Ia3 with Y3+ and Sm3+ cations in 24(d) and 8(b) Wyckoff positions, respectively. The X-ray diffraction data were used to establish the distribution of magnetic ions over crystallographic positions 8(b) and 24(d). A random distribution was found. The same data were used for evaluation of the crystal size, microstrain and paracrystallinity. The concentration dependence of these values was concluded. Results show that unit cell parameters obey Vegards rule, That fact leads to indirect information about well mixing of Y2O3 and Sm2O3. Agreement of the unit cell parameters with Vegards rule is connected with the difference between cationic radii and electronegativity, The Sm3+-O2--Sm3+ bond geometry, important for the superexchange interaction, strongly depends on Sm3+ concentration. By comparing difference of unit cell parameters of the constitutive components Y2O3 and Sm2O3 and values of microstrains of their solid solutions it can be concluded that clusters are randomly formed.XIIIth International Symposium on the Reactivity of Solids, Sep 08-12, 1996, Hamburg, German

ZnFe2O4 antiferromagnetic structure redetermination

Magnetic structure of ZnFe2O4 normal spinel is re- examined. Antiferromagnetic structure non-collinear model is established within C(a)2 space group having four different crystallographic/magnetic sites for 32 Fe3+ spins within magnetic unit cell

Particle size effect on Neel temperature in Er2O3 nanopowder synthesized by thermolysis of 2, 4-pentadione complex

New synthesis routine based on thermal decomposition of an erbium complex with acetylacetonato ligand (2, 4-pentadione) was used to prepare nanosized Er2O3 with a particle size of 5 nm and a narrow size distribution. For the sake of comparison, the ultrafine-Er2O3 powder was prepared by precipitation and subsequent sintering. The magnetic susceptibility of nanosized Er2O3 indicates the antiferromagnetic LT - GT paramagnetic transition at 1.8 K or lower, while in ultrafine such as in a bulk counterpart the transition was observed at noticeably higher temperature of similar to 3.4 K. The decrease in Neel temperature, observed in the case of nanosized Er2O3 sample, was attributed to both particle surface and core effects. (c) 2007 Elsevier Ltd. All rights reserved

True magnetic structure of the ferrimagnetic garnet Y3Fe5O12 and magnetic moments of iron ions

To examine and emphasize the difference between approximative and true magnetic structure, we choose ferrimagnetic garnet Y3Fe5O12 whose magnetic properties are well known. In order to study the magnetic structure, neutron diffraction experiments were done on powder sample of Y3Fe5O12 at 10 and 295 K at zero field and at 295 K in the applied held B = 0.2 T. Using these data the crystal and magnetic structures were first refined in the space group Ia3d. Beside the use of a cubic space group, for a magnetically ordered crystal, this solution suffers from unrealistic magnetic moment per formula unit, which are all significantly above 5 mu(B) and temperature factors at 10 K which lead to unreliable Debye temperature. To satisfy compatibility of the symmetry of the magnetic moments (oriented along the body diagonal in cubic system) and crystal symmetry, the crystal and magnetic structures were also refined in the trigonal space group R (3) over bar. In this group the magnetic moments orientation, along the principal axis, is compatible with the symmetry of local positions. The magnetic R factors are significantly lower than in cubic system. The magnetic moment per formula unit: 3.1 mu(B), from the diffraction measurements in the field, is close to the result obtained by classical measurements of magnetization. (C) 1999 Elsevier Science B.V. All rights reserved

Ionic conductivity in single-crystal LiAlSi2O6: influence of structure on lithium mobility

With the increased interest in application of Li aluminosilicate materials as solid electrolytes, this study focuses on investigating the influence of structure on conductivity properties in single-crystal LiAlSi2O6 which is characterized by three crystal polymorphs where only structural arrangements differ while the amount of mobile carriers is identical. Two single-crystal polymorphic phases produced at ambient pressure are with tetragonal P4(1)2(1)2 symmetry and hexagonal P6(2)22 symmetry, also referred to as beta- and gamma-spodumene in the older literature. For this study, flux-grown hexagonal spodumene single-crystals were measured for conductivity parallel to the crystallographic c-axis and the results were compared with previously published results for tetragonal spodumene, both as single-crystal and polycrystalline aggregate, monoclinic spodumene (alpha-spodumene) and LiAlSi2O6 glass. The activation energy E (a) of 79.69 +/- A 0.38 kJ/mol for hexagonal phase is very similar to the other crystalline polymorphs with the same orientation. However, the sigma (DC) in hexagonal spodumene was determined to be higher than other crystalline phases and about 1.5 orders of magnitude lower than the conductivity in glass LiAlSi2O6 phase. Additionally, the densities of atomic packing were compared between phases, and the differences in the openness of the structures indicate that the more dominant effect on the Li mobility lies with the actual spatial arrangement of Li sites and the Si/Al sublattice

The microstructural changes of cobalt powder in consequence of cold pressing process

The application of pressure as a technique for obtaining dense and mechanically strengthen materials has been investigated from the structural aspect by the use of X-Ray Powder Diffraction (XRPD) and Scanning Tunneling Microscopy (STM)

Influence of the cation distribution on the physical properties of the diluted magnetics Li1.33Ti1.67O4/Co2TiO4

Spinel samples of Li1.33xCo2-2xTi1+0.67xO4 crystallize in two space groups: Fd 3 m for 0less than or equal tox:less than or equal to0.40 and x=1, and P4(3)32 for 0.50less than or equal toxless than or equal to0.875. In S.G. P4(3)32 magnetic Co2+ ions occupy 8c sites (for x=0.75 and 0.875), or 8c, 4b and 12d sites (for x=0.50). In x=0.25 and 0.40 samples Co2+ occupy both 8a and 16d cation sites of the S.G. Fd 3 m [1]. Observed differences in magnetic 2+ behaviour of the samples are induced by crystallographic distribution of Co2+. Possible paths for exchange interactions in S.G. P(4)332 were analysed. It was found that the superexchange is the dominant exchange mechanism where Co2+(8c)-O(24e)-CO2+(12d) path was the most important one. On the basis of the results on crystallographic distribution we suggest x=0.50 sample as potential electrode material in Li ion rechargeable batteries.8th European Powder Diffraction Conference, May 23-26, 2002, Uppsala, Swede

Exchange-Bias and Grain-Surface Relaxations in Nanostructured NiO/Ni Induced by a Particle Size Reduction

Transition-metal-oxide/transition-metal nanocomposites, such as NiO/Ni, FeO/Fe, and CoO/Co, have been the subject of much recent investigation (i) because of their potential applications and (ii) because they are good model systems for studies of some effects on the nanoscale. They are used, for example, as catalysts, fuel-cell electrodes, magnetic memories, etc. When a nanocomposite is composed of both ferromagnetic (FM) and antiferromagnetic (AFM) nanoparticles, interesting physical properties can occur, such as the phenomenon of exchange bias (EB). A Ni/NiO nanocomposite obtained by the thermal decomposition of nickel(II) acetate tetrahydrate, Ni(CH3COO)(2)center dot 4H(2)O, at 300 degrees C, is composed of NiO (62%) and Ni (38%) with crystallite sizes of 11 and 278 nm, respectively. We observed an increase in the crystallite size for NiO and decrease of crystallite size for Ni, a decrease in the microstrain for both and an increase in the NiO phase content with thermal annealing in air, while high-energy ball milling leads to a decrease of the crystallite size, an increase in the size of the agglomerates, and microstrain as well as reduction, NiO - GT Ni. The lattice parameters of the nanosized NiO and Ni show a deviation from the value for the bulk counterparts as a consequence of crystallite size reduction and the grain-surface relaxation effect. The exchange bias found in a milled sample with particles of 10 nm (NiO) and 11 nm (Ni) disappears for larger particles as a consequence of a coupling-area decrease between the antiferromagnetic and ferromagnetic particles. Due to reduction/oxidation (NiO LT - GT Ni) and size as well as surface-relaxation effects the saturation magnetization value increases/decreases with milling/annealing, respectively. Having in mind the effect of size on the exchange bias, coercivity, and magnetization values, it is possible, by annealing/milling, to tailor the composition and particle size and then control the exchange bias and improve the other magnetic properties of the Ni/NiO