N.T. Dang

Y-doped La0.7Ca0.3MnO3 manganites exhibiting a large magnetocaloric effect and the crossover of first-order and second-order phase transitions

We prepared orthorhombic La0.7−xYxCa0.3MnO3 samples (x = 0, 0.04, 0.06, and 0.08) by conventional solid-state reaction and then studied their magnetic properties and magnetocaloric (MC) effect based on magnetization versus temperature and magnetic-field measurements, M(T, H). The experimental results revealed that an x increase in La0.7−xYxCa0.3MnO3 reduced the ferromagnetic-paramagnetic transition temperature (TC) from 260 K (for x = 0) to ∼126 K (for x = 0.08). Around the TC, maximum magnetic-entropy changes for a magnetic-field variation interval H = 50 kOe are about 10.7, 8.5, 7.4, and 5.8 J·kg−1·K−1 for x = 0, 0.04, 0.06, and 0.08, respectively, corresponding to refrigerant capacities RC = 250–280 J·kg−1. These values are comparable to those of some conventional MC materials, revealing the applicability of La0.7−xYxCa0.3MnO3 in magnetic refrigeration. Using the Arrott method and scaling hypothesis as analyzing high-field M(H, T) data, and the universal-curve construction of the magnetic entropy chang...

Crystal structure, magnetic properties and conductivity mechanisms of La0.7Ca0.3Mn0.5Fe0.5O3

ABSTRACT La0.7Ca0.3Mn0.5Fe0.5O3 (LCMFO) ceramic samples were prepared by conventional solid-state reaction. X-Ray diffraction investigations show that samples have orthorhombic space group Pnma (a = 5.447(2) Ǻ, b = 7.709(3) Ǻ, c = 5.467(3) Ǻ). During the neutron powder diffraction measurements of LCMFO in the temperature range of 10 ÷ 300 K the antiferromagnetic long-range ordering of G-type was seen. It has been established that 50% substitution of Mn/Fe leads to a complete suppression of the ferromagnetism in LCMFO samples. Investigations of temperature dependence of the magnetization, M(T), suggests the frustration of magnetic ground state and the existence of a spin-glass or a cluster-glass phase below the magnetic transition temperature close to 350 K. The temperature dependences of conductivity for La0.7Ca0.3Mn0.5Fe0.5O3 sample investigated in magnetic fields B = 0 and 1 T. According to the research, the mechanisms of hopping conductivity were established. Micro parameters in density of localized states were calculated for LCMFO samples. The width of the Coulomb gap Δ = 0.67 eV, rigid gap δ = 0.324 eV and the localization radii of charge carriers were estimated.

Differential-Thermal and X-Ray Analysis of TlFeS2 and TlFeSe2 Chalcogenides

Triple compounds with the formulas TlFeS2 and TlFeSe2 and magnetic properties are investigated via X-ray diffraction, differential-thermal, and thermogravimetric analyses. Their crystal structures are found. It is ascertained that they are described by monoclinic space group C2/m under normal conditions. The atomic coordinates and interatomic distances are determined. From differential-thermal and thermogravimetric analyses performed at temperatures of 30–900°C, it is revealed that stronger covalent bonds are implemented in TlFeS2 crystals than in TlFeSe2 compounds. The effects associated with energy exchange in the compounds under study are observed. The physical nature of the observed effects is discussed.

Electronic structure and magnetism of SrTi1-xMnxO3 ceramics

Having reviewed the previous studies on Mn-doped SrTiO3 materials, we found that Mn2+ and/or Mn4+ ions are usually present in the SrTiO3 host lattice and lead to paramagnetic, spin-glass, or ferromagnetic (FM) behavior. Mn2+ can occupy at both octahedral (Ti) and dodecahedral (Sr) sites, while Mn4+ only occupies the Ti site. To get more insight into this material system, we have prepared polycrystalline SrTi1-xMnxO3 (x = 0–0.10) samples by solid-state reactions. The results obtained from analyzing the powder X-ray diffraction patterns, and Raman scattering, X-ray absorption, and electron-spin-resonance spectra indicate an incorporation of Mn2+ and Mn3+ ions into the SrTiO3 host lattice. Isolated Mn2+ ions are dominant in the samples with x ≤ 0.02 and can occupy both Ti and Sr sites, leading to two sets of the hyperfine sextet lines. An increase in the Mn content with x > 0.02 enhances the Mn3+ concentration and stimulates the formation of a minor tetragonal phase besides the main cubic phase of SrTiO3. This enhances the Lorentzian-shaped broad background line of dipolar and magnetic interactions between Mn ions. Besides the resonant signals of isolated Mn2+ ions and dipolar/magnetic interactions between Mn ions, there is also the resonant signal associated with intrinsic defects. We believe that intrinsic defects together with the Mn doping at suitable concentrations (x > 0.02) are an important pivot for tailoring room-temperature FM order in SrTi1-xMnxO3 ceramics.Having reviewed the previous studies on Mn-doped SrTiO3 materials, we found that Mn2+ and/or Mn4+ ions are usually present in the SrTiO3 host lattice and lead to paramagnetic, spin-glass, or ferromagnetic (FM) behavior. Mn2+ can occupy at both octahedral (Ti) and dodecahedral (Sr) sites, while Mn4+ only occupies the Ti site. To get more insight into this material system, we have prepared polycrystalline SrTi1-xMnxO3 (x = 0–0.10) samples by solid-state reactions. The results obtained from analyzing the powder X-ray diffraction patterns, and Raman scattering, X-ray absorption, and electron-spin-resonance spectra indicate an incorporation of Mn2+ and Mn3+ ions into the SrTiO3 host lattice. Isolated Mn2+ ions are dominant in the samples with x ≤ 0.02 and can occupy both Ti and Sr sites, leading to two sets of the hyperfine sextet lines. An increase in the Mn content with x > 0.02 enhances the Mn3+ concentration and stimulates the formation of a minor tetragonal phase besides the main cubic phase of SrTiO3. Th...

An intermediate antipolar phase in

ABSTRACT The crystal structure and vibration spectra of sodium niobate NaNbO3 have been studied by means of neutron diffraction, X-ray diffraction and Raman spectroscopy at high pressures. An isostructural phase transformation from the initial antiferroelectric phase to an intermediate antiferroelectric phase HP-I have been observed under 2 GPa. This transformation is caused by complex reorientations of NbO6 octahedra. Subsequent structural phase transition from the phase HP-I to an orthorhombic phase HP-III have been detected at pressures above 10 GPa. This transition leads to suppression of an antiferroelectric state of NaNbO3. The observed phase transitions are accompanied by anomalies in lattice parameters compression.

NaNbO_{3}

The crystal structure of iron-doped barium titanate BaTi1–xFexO3 is studied by neutron diffraction in the range of 0 ≤ x ≤ 0.12. At low concentrations of iron, x < 0.01, and at room temperature, these compounds have a polar structure with tetragonal symmetry with space group P4mm. The temperature of the transition of the tetragonal ferroelectric phase into the cubic paraelectric phase with space group Pm

Neutron-diffraction study of the crystal structure of BaTiO3 ferroelectric doped with iron

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Magnetic and magnetocaloric properties in La0.7Ca0.3−xNaxMnO3 exhibiting first-order and second-order magnetic phase transitions

m for an iron concentration of x = 0.01 is 390 K (for pure BaTiO3, it is 410 K). At an iron concentration of x = 0.07, the crystal structure of the studied compounds varies, and it is described by the centrosymmetric hexagonal space group P63/mmc. The structural parameters of various phases of compound BaTi1–xFexO3 are determined from the experimental data.