Magnetic properties and electronic structure of R3T4X4 compounds (R = Pr,Nd; T = Cu,Ag; X = Ge,Sn)

Polycrystalline samples of R₃T₄X₄ (R = Pr, Nd; T = Cu, Ag; X = Ge, Sn) intermetallics were investigated by means of magnetometric and XPS methods. All these compounds crystallize in the orthorhombic Gd₃Cu₄Ge₄-type structure. They were found to be antiferromagnets, except Pr₃Ag₄Ge₄ which is a collinear ferromagnet. Analysis of the XPS valence band data indicates strong overlaps of the Pr 4f states with the Cu 3d states and the Nd 4f states with the Ag 4d states. Analysis of the R 3d states on the basis of the Gunnarsson–Schönhammer model gave the information on the hybridization of the 4f orbital with the conduction band. The results showed that the Pr-based compounds are characterized by larger values of the hybridization energy than the Nd-based compounds.     

Half-metallic ferromagnetism in the RbX (X = Sb,Te) compounds with the rock salt and zinc-blende structures: A first-principles calculation

The first-principles density functional theory (DFT) calculations have been employed to investigate the electronic structures, magnetic properties and half-metallicity of the RbX (X = Sb, Te) compounds with the rocksalt and zinc-blende structures. The RbSb and RbTe compounds with both structures are half-metallic ferromagnet although these compounds do not include transition metal atoms. The compounds with the rock salt structure are found to be more stable energetically than the compounds with the zinc-blende structure. Magnetic moments, independent of crystal structure, are evaluated to be 2 μ_B/f.u. for RbSb and 1 μ_B/f.u. for RbTe. The half-metallic band gaps are 2.94 and 3.61 eV for the RbSb and RbTe compounds with the rock salt structure, respectively, while the RbSb and RbTe compounds with the zinc-blende structure have the half-metallic gaps of 3.00 and 3.25 eV, respectively. The lattice distortion does not affect the half-metallic properties of the RbX (X = Sb, Te) compounds with both structures.     

Theoretical and experimental study of the phase formation for Ti2YAl and Ti2Y′Ga (Y = Co,Fe; Y′ = Cr,Fe)

The site preference and phase formation of Ti₂YAl and Ti₂Y′Ga (Y = Co, Fe; Y′ = Cr, Fe) have been investigated by theoretical calculation and experimental study. It has been found that Ti₂CoAl and Ti₂FeAl are stable in Hg₂CuTi-type structure, while Ti₂CrGa and Ti₂FeGa compounds are most likely to form Cu₂MnAl-type structure. These results indicate that not all full-Heusler alloys follow the site preference rule, especially when the transition metals are low-valent metals. The differences of band structures and magnetic properties between Hg₂CuTi-type and Cu₂MnAl-type structures for the above compounds are also studied by first-principles calculation. It has been predicted that Ti₂FeAl, Ti₂FeGa and Ti₂CoAl are half-metallic compounds in Hg₂CuTi-type structure, while their half-metallic properties are completely destroyed as they formed Cu₂MnAl-type structure.     

Structural,electronic, elastic,mechanical and thermal behavior of RESn3(RE = Y,La and Ce) compounds: A first principles study

The structural, electronic, elastic, mechanical and thermal properties of the isostructural and isoelectronic nonmagnetic RESn₃ (RE = Y, La and Ce) compounds, which crystallize in AuCu₃-type structure, are studied using first principles density functional theory based on full potential linearized augmented plane wave (FP-LAPW) method. The calculations are carried out within PBE-GGA, WC-GGA and PBE-sol GGA for the exchange correlation potential. Our calculated ground state properties such as lattice constant (a₀), bulk modulus (B) and its pressure derivative (B′) are in good agreement with the experimental and other available theoretical results. We first time predict the elastic constants for these compounds using different approximations of GGA. All these RESn₃ compounds are found to be ductile in nature in accordance with Pugh's criteria. The computed electronic band structures and density of states show metallic character of these compounds. The elastic properties including Poisson's ratio (σ), Young's modulus (E), shear modulus (G_H) and anisotropy factor (A) are also determined using the Voigt–Reuss–Hill (VRH) averaging scheme. The average sound velocities (v_m), density (ρ) and Debye temperature (θ_D) of these RESn₃ compounds are also estimated from the elastic constants. We first time report the variation of elastic constants, elastic moduli, Cauchy's pressure, sound velocities and Debye temperatures of these compounds as a function of pressure.     

First-principles study on half-metallic properties of the CoMnZ (Z = S,Se, Te) half-Heusler compounds

The first-principles calculations based on the density functional theory have been employed to explore the electronic structure and magnetic properties of the CoMnZ (Z = S, Se, Te) half-Heusler compound. CoMnTe is predicted to be half-metallic ferromagnet with an energy gap of 1.04 eV in the minority spin and a completely spin polarization at the Fermi level. CoMnS and CoMnSe compounds are nearly half-metallic with spin polarization of 98.9 and 97.9%, respectively. All compounds have a total magnetic moment of 4 μ_B/f.u., which agrees with the Slater–Pauling rule. CoMnTe compound keeps half-metallicity within a wide range of lattice constants between 5.65 and 6.05 Å. Under tetragonal distortions, high spin polarization at the Fermi level is maintained for the CoMnTe compound.     

Structural,electronic and magnetic properties of the Mn54−xAl46Tix (x = 2; 4) alloys

The structural, electronic and magnetic behavior of the as-cast and annealed Mn₅₂Al₄₆Ti₂ and Mn₅₀Al₄₆Ti₄ alloys have been studied through electronic band structure calculations, X-ray diffraction and magnetic measurements in the temperature range 4–850 K and magnetic field up to 7 T. Band structure calculations show a preference for Ti atoms to occupy the Mn sites in the plane of Al atoms with their magnetic moments (−0.68 μ_B/Ti) coupled antiparallel relative to the Mn magnetic moments in the plane of Mn atoms (2.33 μ_B/Mn). The as-cast and annealed samples were phase mixtures with different values of the hard ferromagnetic τ phase content. Except the as-cast and annealed at 1050 °C Mn₅₂Al₄₆Ti₂ alloys, all the analyzed samples include, along with the τ and γ₂ phases, a soft κ phase (CsCl - structure type) with T_C around 530 K. The best magnetic characteristics were obtained for Mn₅₂Al₄₆Ti₂ alloy annealed at 470 °C for 6 h: M_S = 116 A m²/Kg at 4 K and T_C = 668 K, in good agreement with the values reported in the literature for the τ phase of the MnAl system. The effects of the composition and of the preparation route on the electronic and magnetic properties are discussed in comparison with the properties of Mn₅₄Al₄₆ parent alloy.     

Electronic properties and crystal structures of RE3Rh2Ga2 and RE3Rh3Si2 (RE = La,Ce)

Polycrystalline samples of Ce₃Rh₂Ga₂, Ce₃Rh₃Si₂ and their La-based isostructural analogues were studied by means of magnetic susceptibility, magnetization and electrical resistivity measurements. The crystal structure of La₃Rh₃Si₂ (Ce₃Rh₃Si₂ type) was investigated by single-crystal X-ray diffraction. The cerium gallide was found to order ferromagnetically at T_C = 3.5 K, whereas for the silicide more complex magnetic behaviour was established with antiferromagnetic order setting at T_N = 6.5 K and subsequent change in the magnetic structure occurring at T_t = 5.5 K. Both cerium compounds exhibit weak Kondo effect. The L_III-edge XAS data indicated rather stable 4f¹ character of cerium in both compounds. The electronic band structures of Ce₃Rh₂Ga₂ and Ce₃Rh₃Si₂ were calculated by the LMTO method and compared with those of Ce₃Rh₂Ge₂ and La₃Rh₂Ge₂. For each Ce-based compound the total electronic DOS is dominated by a peak of Ce 4f states near the Fermi level and a Rh 4d band located about 2 eV below the Fermi energy.     

Lattice,magnetic and electronic transport behaviors of Ge-doped Mn3XC (X = Al,Zn, Ga)

The lattice, magnetic, and electronic transport properties of Mn₃X_1?yGe_yC (X = Al, Zn, and Ga; y = 0, 0.5) were investigated. Ge-doping does not change the antiperovskite structure, and only decreases the lattice constant in different degree. For all Mn₃X_1?yGe_yC, Ge-doping makes the magnetic transition temperature decrease. The total behavior of Mn₃Al_xGe_1?xC in temperature dependence of resistivity is metallic. For Mn₃Ga_xGe_1?xC, since the carrier densities in the ferromagnetic phase and antiferromagnetic one are different, with decreasing the temperature, the resistivity appears an abrupt increase.     

Structural variations in the ternary system HfAl2−xCux (x = 0.2–1.0)

Synthesized ternary intermetallic phases HfAl_2?xCu_x in a series with (x = 0.49, 0.88, 1.04) are characterized as Laves phase structures. X-ray diffraction revealed homogeneity within ranges 0.2 ≤ x ≤ 0.5 and 0.7 ≤ x ≤ 0.9 for structure types MgCu₂ and MgNi₂, respectively. When Cu atoms gradually replaced the Al atoms, the structure type altered in the sequence MgCu₂ → MgNi₂ → MgZn₂, and the Kagomé nets were distorted with varied bond lengths. Measurements of physical properties revealed these phases to be metallic, with resistances 4.35 (x = 0.5), 5.85 (x = 0.7), and 6.50 (x = 0.9) mΩ cm, respectively, at temperature 300 K. The coloring schemes reveal that, upon increasing the proportion of Cu atoms, the stability of these phases correlated with the arrangements of the Al and Cu atoms. Calculated electronic structures indicate that the bonding character is consistent with the experimentally observed phase width.     

New ternary Yb5Sb3-type R5T1−x{Sb,Bi}2+x phases (R = Y,Dy, Ho,T = Co,Ru, Rh,Pd) and their magnetic properties

Several novel R₅T_1?x{Sb,Bi}_2+x phases having the Yb₅Sb₃-type structure (space group Pnma) have been synthesized. The cell parameters are: a = 1.20668(9), b = 0.88396(7), c = 0.78745(7) nm for Y₅CoSb₂; a = 1.19939(7), b = 0.88364(5), c = 0.78283(5) nm for Dy₅CoSb₂; a = 1.19633(8), b = 0.89231(6), c = 0.78450(6) nm for Ho₅RhSb₂; a = 1.18650(6), b = 0.90455(4), c = 0.79260(4) nm for Ho₅PdSb₂; a = 1.2215(3), b = 0.8948(2), c = 0.7977(2) nm for Y₅CoBi₂; a = 1.1781(8), b = 0.9071(7), c = 0.7936(6) nm for Tm₅Co_0.5Bi_2.5; a = 1.1972(1), b = 0.92096(9), c = 0.80048(9) nm for Ho₅RuBi₂; a = 1.2082(1), b = 0.90346(9), c = 0.79413(8) nm for Ho₅RhBi₂ and a = 1.20374(5), b = 0.91076(4), c = 0.80135(4) nm for Ho₅PdBi₂, respectively. Magnetization measurements indicate ferromagnetic transitions for Dy₅CoBi₂, Ho₅RhSb₂, Ho₅RhBi₂, Ho₅PdSb₂ and Ho₅PdBi₂ at T_C = 34, 38, 28, 42 and 38 K, respectively. The Ho₅RhSb₂ and Ho₅PdBi₂ compounds show additional magnetic transitions at about 18 K, probably associated with a spin reorientation. The magnetocaloric effect of Dy₅CoBi₂ in terms of the isothermal entropy change, ΔS_m, is ?6.2 J/(kg/K) at 38 K and in terms of the adiabatic temperature change, ΔT_ad, is 2.2 K for a 5 T field change.     

Magnetic and transport properties of Laves phase Dy1−xMmxCo2 (x = 0–0.5) alloys

The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy_1?xMm_x)Co₂ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity (ρ(T)), ac susceptibility (χ(T)) and thermopower (S(T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature (T_C) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in T_C and sharpness of discontinuities in ρ(T) and S(T) with increasing Mm substitution has been discussed.     

Structural,half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb,As and Si) and IrMnAs from first-principles calculations

The structural, half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb, As and Si) and IrMnAs were investigated using first-principles calculations within the generalized gradient approximation (GGA) based on density function theory (DFT). The most stable lattice configurations about site occupancy are (Ni)_4a(Mn)_4c(Sb)_4d, (Ni)_4a(Mn)_4c(As)_4d, (Ni)_4a(Mn)_4c(Si)_4d and (Ir)_4a(Mn)_4c(As)_4d, respectively, and the exchange of elements in Wyckoff position 4c and 4d results in an identical (symmetry-related) phase. The half-Heusler compounds show half-metallic ferromagnetism with a half-metallic gap of 0.168 eV, 0.298 eV, 0.302 eV and 0.109 eV, respectively, and the total magnetic moments (M_tot) are 4.00 μ_B, 4.00 μ_B, 3.00 μ_B and 3.00 μ_B per formula unit, respectively, which agree well with the Slater–Pauling rule based on the relationship of valence electrons. The compound (Ir)_4a(Mn)_4c(As)_4d with half-metallic ferromagnetic character was reported for the first time. The individual elastic constants, shear modulus, Young's moduli, ratio B/G and Poisson's ratio were also calculated. The compounds are ductile based on the ratio B/G. The Debye temperatures derived from the average sound velocity (ν_m) are 327 K, 332 K, 434 K and 255 K, respectively. The predicted Debye temperature for NiMnSb agrees well with the available experimental value, and the Debye temperatures for the rest three compounds were reported for the first time.     

First-principles investigations on structural,elastic, thermodynamic and electronic properties of Ni3X (X = Al,Ga and Ge) under pressure

The structural, elastic, thermodynamic and electronic properties of L1₂-ordered intermetallic compounds Ni₃X (X = Al, Ga and Ge) under pressure range from 0 to 50 GPa with a step of 10 GPa have been investigated using first-principles method based on density functional theory (DFT). The calculated structural parameters of Ni₃X at zero pressure and zero temperature are consistent with the experimental data. The results of bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio v, anisotropy index A^U and Debye temperature Θ_D increase with the increase of external pressure. In addition, the Debye temperature of these compounds gradually reduce as the order of Ni₃Al > Ni₃Ga > Ni₃Ge. The ratio of shear modulus to bulk modulus G/B shows that the three binary compounds are ductile materials, and the ductility of Ni₃Al and Ni₃Ga can be improved with pressure going up, while Ni₃Ge is opposite. Finally, the pressure-dependent behavior of density of states, Mulliken charge and bond length are analyzed to explore the physical origin of the pressure effect on the structural, elastic and thermodynamic properties of Ni₃X.     

Magnetic and magnetocaloric properties of partially disordered RFeAl (R = Gd,Tb) intermetallic

Magnetic and magnetocaloric properties of TbFeAl and GdFeAl were studied in a wide temperature region in magnetic field up to 10 T. They order magnetically below T_c = 196 K and 259 K for TbFeAl and GdFeAl, respectively. The temperature change of ΔT = 1.4 K and 1.6 K was observed in GdFeAl and TbFeAl respectively for a field change of 4 T. Interestingly wide temperature region of significant magnetocaloric effect was observed in both compounds which give rise to relative cooling power of 350 J kg⁻¹ for TbFeAl and 348 J kg⁻¹ for GdFeAl for 4 T field span.     

Magnetic ordering and magnetic properties of ErAuxNi1−xIn (0 ≤ x ≤ 1) solid solution

The ErAu_xNi_1−xIn (0 ≤ x ≤ 1) quasiternary compounds crystallize in the hexagonal layered crystal structure of ZrNiAl-type. ErAuIn was reported to be an antiferromagnet with T_N = 3 K and magnetic moments having triangular arrangement within the basal plane (the magnetic order is described by the propagation vector ). On the contrary ErNiIn is a ferromagnet with T_C = 9 K and magnetic moments pointing along the c-axis. The magnetic ordering in ErAu_xNi_1−xIn (0 < x < 1) solid solution, has been investigated by neutron diffractometry in the temperature range between 1.5 and 15 K. Moreover, bulk magnetic measurements have been carried out in the range 1.72–400 K. All alloys of intermediate composition were found to be antiferromagnets with T_N between 4.6 and 7 K. Below 2 K their magnetic order is described by the propagation vector and magnetic moments are aligned along the c-axis. However, for alloys with 0.2 ≤ x ≤ 0.7 the propagation vector was found to turn into with increasing temperature.     

High-temperature thermoelectric properties of Ln(Co,Ni)O3 (Ln = La,Pr, Nd,Sm, Gd and Dy) compounds

The high-temperature thermoelectric properties of polycrystalline LnCo_0.95Ni_0.05O_3±δ (Ln = La, Pr, Nd, Sm, Gd and Dy) compounds have been investigated. The substitution of the lanthanide element produces changes in the crystal structure and in the unit cell lattice parameters. A semiconductor to metal transition takes place for all the compounds at 500 K < T < 800 K. The transition temperature, also monitored by differential scanning calorimetry, shifts to higher temperature with the decrease in the size of the lanthanide element. The Seebeck coefficient has positive values up to T = 1240 K. The thermal conductivity is mainly dominated by the lattice contribution. The dimensionless figure of merit ZT can be enhanced at different temperatures depending on which lanthanide element occupies the A-site in the ABO₃ perovskite-type phases.     

Thermal expansion and melting temperature of the half-Heusler compounds: MNiSn (M = Ti,Zr, Hf)

Half-Heusler compounds: MNiSn (M = Ti, Zr, Hf) are considered as a candidate of environmentally friendly and low-cost thermoelectric (TE) materials. Although the thermomechanical properties are quite important when utilizing the half-Heusler compounds in TE devices, such properties have been scarcely reported. In the present study, we tried to collect the data of the thermal expansion coefficient and the melting temperature (T_m) of MNiSn. The thermal expansion coefficient was evaluated by means of two methods: the dilatometer measurement and the high temperature X-ray diffraction analysis. The T_m was evaluated from the differential thermal analysis. The relationship between the thermal expansion coefficient and the T_m of the half-Heusler compounds was studied.     

Magnetism and magnetocaloric effect in the ternary equiatomic REFeAl (RE = Er and Ho) compounds

Magnetic properties and magnetocaloric effect (MCE) in ErFeAl and HoFeAl intermetallic compounds have been studied systematically. Both compounds undergo a second order magnetic phase transition from paramagnetic to ferromagnetic state together with a probable spin reorientation transition at low temperature. A considerable reversible magnetocaloric effect was observed around its own Curie temperatures T_C ∼55 K and 80 K for ErFeAl and HoFeAl, respectively. For a magnetic field change of 0–7 T, the maximum values of magnetic entropy change (−ΔS_M^max) are found to be 8.3 and 9.9 J/kg K for RE = Er and Ho, respectively. The corresponding values of refrigerant capacity (RC) are evaluated to be 384 and 647 J/kg.