The effect of cation disorder on magnetic properties of new double perovskites La2NixCo1-xMnO6 (x = 0.2 - 0.8)

Solid solutions of new double perovskite oxides La2NixCo1-xMnO6 (x = 0.2, 0.25, 0.5, 0.75, 0.8) were synthesized by solid-state reaction method. The X-ray powder diffraction data show that all the compounds crystallize in the monoclinic structure with space group P21/n at room temperature. The elementary composition of the powders was determined by the electron Probe Microanalysis. Raman and IR spectra show strong bands at (520, 650 cm?1) and (426, 600 cm?1) respectively, attributed to the stretching vibration of Ni/Co-O and Mn-O bonds in the structure. The magnetic studies for all the compounds have been performed in both DC and AC magnetic fields in the temperature range from 2 to 300 K. All samples exhibit a main paramagnetic to ferromagnetic (PM-FM) transition between 232 K and 260 K, and their Curie temperature increases rapidly with increasing x values. Three samples with x = 0.2, 0.25 and 0.5 respectively display also a secondary PM-FM transition between 200 K and 208 K. The thermal variation of out of phase component of AC susceptibility presents also frequency-dependent transitions between 65 K and 110 K unfolding the existence of super-paramagnetic mono-domains in all samples. The band gap energy has been calculated and revealing semiconductor behavior for all samples

Phase boundary between Na–Si clathrates of structures I and II at high pressures and high temperatures

Understanding of the covalent clathrate formation is a crucial point for the design of new superhard materials with intrinsic coupling of superhardness and metallic conductivity. It has been found that silicon clathrates have the archetype structures, which can serve an existent model compounds for superhard clathrate frameworks Si–B, Si–C, B–C and C with intercalated atoms (e.g., alkali metals or even halogens) that can assure the metallic properties. Here we report our in situ and ex situ studies of high-pressure formation and stability of clathrates Na₈Si₄₆ (structure I) and Na₂₄₊xSi₁₃₆ (structure II). Experiments have been performed using standard Paris–Edinburgh cells (opposite anvils) up to 6 GPa and 1500 K. We have established that chemical interactions in the Na–Si system and transition between two structures of clathrates occur at temperatures below silicon melting. The strong sensitivity of crystallization products to the sodium concentration has been observed. A tentative diagram of clathrate transformations has been proposed. At least up to ~ 6 GPa, Na₂₄₊xSi₁₃₆ (structure II) is stable at lower temperatures as compared to Na₈Si₄₆ (structure I).Изучен in situ и ex situ процесс образования при высоких давлениях и стабильности клатратов Na₈Si₄₆ (структура I) и Na₂₄₊xSi₁₃₆ (структура II). Эксперименты были проведены в стандартных Париж-Эдинбургских ячейках (opposite anvils) при давлениях и температурах до 6 ГПа и 1500 K соответственно. Установлено, что химическое взаимодействие в системе Na–Si и переходы между двумя структурами клатратов происходят при температурах ниже температуры плавления кремния. Предложено первое приближение диаграммы превращений клатратов. Отмечена большая чувствительность продуктов кристаллизации к концентрации натрия. Na₂₄₊xSi₁₃₆ (структура II) является стабильной при более низких температурах по сравнению с Na₈Si₄₆ (структура I), по крайней мере до ~ 6 ГПа.Вивчено in situ і ex situ процес утворення при високих тисках і стабільності клатратів Na₈Si₄₆ (структура I) і Na₂₄₊xSi₁₃₆ (структура II). Експерименти було проведено в стандартних Париж-Единбурзький комірках (opposite anvils) при тисках і температурах до 6 ГПа і 1500 K відповідно. Встановлено, що хімічна взаємодія в системі Na–Si і переходи між двома структурами клатратов відбуваються при температурах нижче температури плавлення кремнію. Запропоновано перше наближення діаграми перетворень клатратів. Відзначено велику чутливість продуктів кристалізації до концентрації натрію. Na₂₄₊xSi₁₃₆ (структура II) є стабільною при більш низьких температурах у порівнянні з Na₈Si₄₆ (структура I), принаймні до ~ 6 ГПа

Tailoring the Griffiths-like cluster formation in the insulator ferromagnet spin-glass Gd 2 Ni x Co 1 − x Mn O 6 double perovskite

International audienceIn this paper we present the effects of the compositionon structural, vibrational, magnetic, and electronic properties of double perovskite (DP) 3d based manganese oxides Gd 2 Ni x Co 1-x MnO 6 (GNCMO) powder (for 0 < x < 1) prepared by high temperature solid state chemistry. These compounds have great research interest due to their specific physical properties. We have shown that the synthesized compounds exhibit ferromagnetic interaction between the spins of (Mn 4+ , Co 2+), and (Mn 4+ , Ni 2+) cations via oxygen ions and antiferromagnetic interactions between (Gd 3+ , Co 2+), (Gd 3+ , Ni 2+) and (Gd 3+ , Gd 3+) and present a paramagnetic to ferromagnetic phase transition. The paramagnetic Curie-Weiss temperature of GNCMO (T c) increases from 121K to 128K as the Nickel content increases from 0.2 to 0.8. Further, temperature dependent inverse magnetic susceptibility χ(T)-1 shows deviation from Curie Weiss behaviour around T G which depicts that Griffiths phase already observed in several perovskite compounds. The value of the GP percentage and the exponent of (λ < 1) are found to increase as the Nickel content increases. The hysteresis curve at T = 20K for x = 0.2 displays a meta-magnetic transition explained by the reorientation of the Gd spin moments towards the Mn, Co and Ni spin moments. The imaginary part of susceptibility χ'' showing a small shift of the peak indicating the present of spin or cluster glass behavior

Growth facets of SrIrO 3 thin films and single crystals

International audienceThe crystallographic orientation of SrIrO3 surfaces is decisive for the occurrence of topological surface states. We show from density functional theory computations that (001) and (110) free surfaces have comparable energies, and, correspondingly, we experimentally observe that single micro-crystals exhibit both facet orientations. These surfaces are found to relax over typically the length of one oxygen octahedron, defining a structural critical thickness for thin films. A reconstruction of the electronic density associated to tilts of the oxygen octahedra is observed. At the opposite, thin films have invariably been reported to grow along the (110) direction. We show that the interfacial energy associated to the oxygen octahedra distortion for epitaxy is likely at the origin of this specific feature, and propose leads to induce (001) SrIrO3 growth

Fermi- to non-Fermi-liquid crossover and Kondo behavior in two-dimensional (Cu 2/3 V 1/3 )V 2 S 4

International audienceBy means of a specific heat (C) and electrical resistivity () study, we give evidence of a pronounced Fermi liquid (FL) behavior with sizable mass renormalization, m * /m = 30, up to unusually high temperatures ∼70 K in the layered system (Cu 2/3 V 1/3)V 2 S 4. At low temperature, a marked upturn of both C and is suppressed by magnetic field, which suggests a picture of Kondo coupling between conduction electrons in the VS 2 layers and impurity spins of the V 3+ ions located between layers. This picture opens the possibility of controlling electronic correlations and the FL to non-FL crossover in simple layered materials. For instance, we envisage that the coupling between layers provided by the impurity spins may realize a two-channel Kondo state

Fermi- to non-Fermi-liquid crossover and Kondo behavior in two-dimensional (Cu 2/3 V 1/3 )V 2 S 4

International audienceBy means of a specific heat (C) and electrical resistivity () study, we give evidence of a pronounced Fermi liquid (FL) behavior with sizable mass renormalization, m * /m = 30, up to unusually high temperatures ∼70 K in the layered system (Cu 2/3 V 1/3)V 2 S 4. At low temperature, a marked upturn of both C and is suppressed by magnetic field, which suggests a picture of Kondo coupling between conduction electrons in the VS 2 layers and impurity spins of the V 3+ ions located between layers. This picture opens the possibility of controlling electronic correlations and the FL to non-FL crossover in simple layered materials. For instance, we envisage that the coupling between layers provided by the impurity spins may realize a two-channel Kondo state

Understanding the magnetic, electronic and optical propertiesof Iron titanium Oxyphosphate Fe 0.5 TiOPO 4 using DFT, GGA and GGA+U approaches

An investigation into the structural, electronic, magnetic and optical properties of oxyphosphate Fe0.5TiOPO4 was conducted using first‐principles calculations based on density functional theory (DFT) with the Plane‐Wave Self‐Consistent Field (PWSCF). The results are used to discuss the quality of the generalized‐gradient expansion GGA and GGA+U approximation for describing these atoms. We showed that the obtained symmetry lattice parameter agree well with the experimental results. The influence of iron element on the magnetic and electronic properties of oxyphosphate Fe0.5TiOPO4 is analyzed. The electronic structure calculations for monoclinic oxyphosphate Fe0.5TiOPO4 predict an energy‐gap in both spin‐up and spin‐down. The partial density of states (PDOS) of Fe0.5TiOPO4 show that the oxygen 2p‐derived states hybridize strongly with iron‐3d at the conduction band when it’s hybridize with titanium and iron at the valance band and the theoretical band gap energy is found to be Eg=0.62 eV (GGA) and 2.2 eV (GGA+U)

Understanding the magnetic, electronic and optical properties of Iron titanium Oxyphosphate Fe0.5TiOPO4 using DFT, GGA and GGA+U approaches.

An investigation into the structural, electronic, magnetic and optical properties of oxyphosphate Fe0.5TiOPO4 was conducted using first-principles calculations based on density functional theory (DFT) with the Plane-Wave Self-Consistent Field (PWSCF). The results are used to discuss the quality of the generalized-gradient expansion GGA and GGA+U approximation for describing these atoms. We showed that the obtained symmetry lattice parameter agree well with the experimental results. The influence of iron element on the magnetic and electronic properties of oxyphosphate Fe0.5TiOPO4 is analyzed. The electronic structure calculations for monoclinic oxyphosphate Fe0.5TiOPO4 predict an energy-gap in both spin-up and spin-down. The partial density of states (PDOS) of Fe0.5TiOPO4 show that the oxygen 2p-derived states hybridize strongly with iron-3d at the conduction band when it’s hybridize with titanium and iron at the valance band and the theoretical band gap energy is found to be Eg=0.62eV (GGA) and 2.2 eV (GGA+U)

Electronic structure studies of oxyphosphate Cu0.5TiOPO4

The electronic structure and magnetic properties of Cu0.5TiOPO4 have been calculated using density functional theory (DFT) employing generalized gradient approximation (GGA) the result of calculation show that the antiferromagnetic state is more stable than the ferromagnetic and  the partial densities of states (PDOS) results show that the valence band is formed by O-2p and Cu-3d states and the conduction band formed by Ti-3d state with gap energy Eg=2.24 eV and spin polarization around the Fermi level appears due to the magnetic impurity