Solid State Phenomena Vol. 289

Abstract: Crystals of the solid solution Tm(Al_1-xMo_x)B₄ (x = 0.002 - 0.050) were grown from Tm₂O₃, metal Mo and crystalline boron powders as starting materials under an Ar atmosphere at 1773 K for 5 h. Tm(Al_1-xMo_x)B₄ crystals were obtained as a single phase in the case of Al_1-xMo_x flux (x = 0.005 and 0.010). Tm(Al_1-xMo_x)B₄ crystals at a maximum size of about 2.2 mm were obtained in the shape of a flake, and together with needle-like crystals of MoAlB and an unknown phase using Al_0.970Mo_0.030 flux. The lattice constants of Tm(Al_1-xMo_x)B₄ (x = 0.005 - 0.010) crystals were determined to be larger compared to those obtained for TmAlB₄. This is related to the fact that the ionic radius of Mo is larger than the ionic radius of Al. When the Al position in the TmAlB₄ structure was substituted with Mo having a large ionic radius, the hardness hardly changed. In contrast to TmAlB₄, no antiferromagnetic transition could be observed for Tm(Al_0.995Mo_0.005)B₄, indicating that the disorder introduced by Mo-doping disrupted the magnetic transition.

Abstract: The crystal structures of the binary compounds ZrAl₃ and HfAl₃ at 600°C belong to the structure type ZrAl₃ (Pearson symbol tI16, space group I4/mmm, a = 4.00930(11), c = 17.2718(7) Å for ZrAl₃ and a = 3.9849(3), c = 17.1443(15) Å for HfAl₃). Substitution of Ge atoms for Al atoms in ZrAl₃ and HfAl₃ led to the formation of the ternary compounds ZrAl_2.52(1)Ge_0.48(1) and HfAl_2.40(1)Ge_0.60(1), respectively, where the latter is probably part of a solid solution extending from the high-temperature modification of HfAl₃. The crystal structures belong to the tetragonal structure type ht-TiAl₃ (tI8, I4/mmm, a = 3.92395(11), c = 9.0476(4) Å for ZrAl_2.52Ge_0.48 and a = 3.9021(2), c = 8.9549(8) Å for HfAl_2.40Ge_0.60). The structure types ZrAl₃ and ht-TiAl₃ are both members of the family of close-packed structures.

Abstract: The new ternary silicide Dy₃Ni_11.83(1)Si_3.98(1) was synthesized from the elements by arc-melting and its crystal structure was determined by X-ray single-crystal diffraction. The compound crystallizes in a Sc₃Ni₁₁Ge₄-type structure: Pearson symbol hP38, space group P6₃/mmc (No. 194), a = 8.1990(7), c = 8.6840(7) Å, Z = 2; R = 0.0222, wR = 0.0284 for 365 reflections. The structure belongs to a large family of structures related to the EuMg_5.2 type, with representatives among ternary aluminides, silicides, germanides, etc.

Abstract: We have succeeded in synthesizing new arsenic-based filled skutterudite compounds CaFe₄As₁₂ and BaOs₄As₁₂ under high pressure. These compounds have lattice constants of 8.3288 and 8.5809 Å, respectively. The magnetic properties of CaFe₄As₁₂ and BaOs₄As₁₂ have been studied by means of electrical resistivity, magnetic susceptibility and magnetization measurements. The electrical resistivity and magnetic measurements indicate that CaFe₄As₁₂ is a nearly ferromagnetic metal with spin fluctuations of Fe 3d electrons and BaOs₄As₁₂ is a new superconductor with a transition temperature around 3 K.

Abstract: A study on the formation and stability of new quaternary compounds with the general chemical formula Gd₃TAl₃Ge₂ (T = Mn, Cu) has been undertaken by experimental investigations (SEM-EDX, DTA and XRD) and density functional theory (DFT) calculations. These compounds crystallize in the hexagonal Y₃NiAl₃Ge₂-type structure (hP9, P–62m, Z = 1) (an ordered, quaternary derivative of the ternary ZrNiAl or of the binary Fe₂P prototypes), with lattice parameters values a = 7.0239(2) Å and c = 4.2580(1) Å for Gd₃MnAl₃Ge₂ and a = 7.0434(1) Å and c = 4.2089(1) Å for Gd₃CuAl₃Ge₂. DTA suggests a peritectic reaction for the formation of these compounds (at 1245°C for Gd₃CuAl₃Ge₂). The existence and stability of these phases has been explained on the basis of DFT calculations, and a comparison of ground state properties of the studied compounds with the earlier known Gd₃CoAl₃Ge₂ phase is outlined. The negative formation energies in all three cases govern the stability of compounds from theory as well, predicting Gd₃MnAl₃Ge₂ as the most stable phase with highest formation energy (–13.01 eV/f.u.). The total DOS are generic in nature and suggest the robust magnetism, with the Gd-f moments of ≈7 μ_B. An antiparallel coupling among Gd-f and T-d states is observed for all compounds, as usually seen in rare earth (R) - transition metal (T) compounds. Preliminary magnetization measurements on Gd₃MnAl₃Ge₂ show two ferromagnetic/ferrimagnetic (FM/FIM) like transitions at T_C1 = 142 K and T_C2 = 97 K, with another anomaly seen at ≈15 K. Isothermal magnetization data show no hysteresis even at 5 K, and the magnetization does not saturate up to 50 kOe, further suggesting a possible FIM behavior.

Abstract: The crystal structure and overall magnetic properties of RE(Fe_0.25Co_0.75)₂H_y with RE = Ho and Er, were analyzed versus y, the hydrogen content. The single phase C15 type compounds, synthesized using cold crucible HF melting, were hydrogenated and controlled using a PCI apparatus. The impact of hydrogen insertion on the cell parameter, the Curie temperature T_C and the magnetization saturation were determined. All compounds and hydrides were found to be ferrimagnets, the magnetic moment of RE and 3d elements being opposite since Ho and Er belong to the 2^nd row of rare earth elements. Depending on the formula, a typical compensation point was in evidence. The 2^nd order character of the ferri ↔ paramagnetic transition was established using the Arrott plot method. Magnetization isotherms vs. magnetic field gives quantified results for the magnetic entropy variation by application of the Maxwell relation. Control of y, the hydrogen content, shows it is interesting for improvement of the MCE of the starting compounds. However increasing y leads to decreased T_C at the lowest temperatures, suggesting potential cryogenic uses.

Abstract: The crystal structure of the (Cr,Ni)₄Si phase with and without Co was refined from X-ray powder diffraction data. The compound crystallises with an Au₄Al-type structure (Pearson symbol cP20, space group P2₁3): unit-cell parameter a = 0.611959(6) nm for the composition (Cr_0.312Ni_0.688)₄Si, a = 0.612094(6) nm for (Cr_0.375Ni_0.625)₄Si, and a = 0.612316(6) nm for (Cr_0.337Co_0.063Ni_0.600)₄Si. The magnetic susceptibility was measured in external fields up to 7 T at temperatures between 1.8 and 400 K. The three investigated samples exhibited paramagnetic behaviour described by the modified Curie-Weiss law: χ₀ = 146∙10^-6 emu g-at.^-1, μ_eff = 0.21 μ_B/atom, θ_P = -13 K for (Cr_0.312Ni_0.688)₄Si; χ₀ = 158∙10^-6 emu g-at.^-1, μ_eff = 0.20 μ_B/atom, θ_P = -15 K for (Cr_0.375Ni_0.625)₄Si; χ₀ = 169∙10^-6 emu g-at.^-1, μ_eff = 0.18 μ_B/atom, θ_P = -52 K for (Cr_0.337Co_0.063Ni_0.600)₄Si.

Abstract: The crystal structure of phases in the pseudo-binary system CeCo_1–хNi_хC₂ (x = 0, 0.33, 0.5, 0.67, 0.79, 0.80, 0.83, 1) was investigated by means of X-ray powder diffraction. Co richer solid solutions CeCo_1–хNi_хC₂ (0≤ x ≤0.5) crystallize in the monoclinic CeCoC₂-type structure; a = 5.3968(2) Å, b = 5.4013(3) Å, c = 7.4762(3) Å, β = 102.136(3)°, V = 213.06(3) ų for x = 0.5. Ni-rich CeNi_1–yCo_yC₂ (0≤ y ≤0.2) are isotypic with the orthorhombic CeNiC₂-type structure, a = 3.8486(2) Å, b = 4.5479(2) Å, c = 6.1531(3) Å, V = 107.70(1) ų for y = 0.2. In the intermediate region (0.5< x <0.79) both phases, CeCo_0.21Ni_0.79C₂ and CeCo_0.5Ni_0.5C₂, coexist. The non-isoelectronic substitution of Ni by Co in solid solutions CeNi_1–yCo_yC₂ causes a continuous reduction of the Néel temperature and finally, for CeCoC₂, results in a paramagnetic Kondo-lattice ground state.

Abstract: Crystals of the quaternary compounds α-Lu(Al_1-xT_x)B₄ (T = Fe, Cr) (YCrB₄-type, orthorhombic, Pbam), obtained from the nominal composition of Lu(Al_1-xT_x)B₃, were grown by using Al flux mixed with T metal at 1773 K for 5 h under an Ar atmosphere. The maximum dimensions and morphology of the crystals obtained were about 0.7 mm × 0.5 mm for flake-type crystals of Lu(Al_1-xCr_x)B₄ and about 5.2 mm × 0.2 mm for prism crystals of Lu(Al_1-xFe_x)B₄. The lattice constants determination and chemical analyses of Lu(Al_1-xT_x)B₄ (T = Fe, Cr) compounds were carried out for Fe 0.5 - 10.0 at% and Cr 0.5 - 1.0 at%. The lattice constants and the unit lattice volume in Lu(Al_1-xT_x)B₄ crystals decreased with increase of the concentration of Fe or Cr. The values of micro-Vickers hardness of Lu(Al_1-xFe_x)B₄ (x = 0.005 - 0.030) and Lu(Al_1-xCr_x)B₄ (x = 0.005 - 0.010) samples are in the ranges of 16(2) - 20(3) GPa and 13(2) - 16(3) GPa, respectively. The hardness values showed a little increase as the solid solution of Fe or Cr was realized in Lu(Al_1-xT_x)B₄, possibly because of distortion to the crystal structure. The magnetic susceptibility of as-grown Lu(Al_0.995Fe_0.005)B₄ crystals for example, exhibited diamagnetic behavior with a small ferromagnetic component.

Abstract: Disordered and frustrated magnetization of different surface coated (Cr₂O₃, Co₃O₄, ZrO₂, and SiO₂) MnFe₂O₄ nanoparticles have been studied using SQUID-magnetometry. Magnetic measurements, such as ZFC/FC and ac-susceptibility evidence surface spin-glass behavior. ZFC/FC curves were also compared with numerical simulation to get information about effective anisotropy constants. Frequency dependent ac susceptibility results were analyzed by using Arrhenius, Vogel Fulcher and dynamic scaling laws to further confirm the spin-glass behavior. It is observed that the strength of surface spins disorder and frustration strongly depends upon the type of the coating material. All these analyses signify that disordered and frustrated surface magnetization in MnFe₂O₄ nanoparticles greatly depend on the type of the surface coating materials and are useful for controlling the nanoparticle’s magnetism for different practical applications.