Some observations on the Gd-rich side of the Gd-C system

The phase relationships at the Gd-rich end of the Gd-C system were established. The addition of C expands the lattice parameters of pure Gd, indicating that C dissolves in Gd, but the solid solubility of C is less than 2 at.% below 600°C. Below 600°C the ‘Gd₂C’ phase appears to exist as a line compound, but exhibits a narrow solid solution region above this temperature. This phase is slightly C deficient, i.e. Gd₂C_1−x, where x=0.015 [Gd₂C_0.985]. ‘Gd₂C’ has the rhombohedral C19, CdCl₂-type structure with a=6.29 Å and =33.70°. The Gd-C alloys are quite reactive in moist air, forming gaseous hydrocarbons and a white powder which flakes off of the metallic ingots, and must be handled in glove boxes. Magnetic measurements indicated that ‘Gd₂C’ orders ferromagnetically at 350°C. This is the highest known magnetic ordering temperature for any binary lanthanide compound which does not contain Fe or Co.     

Neutron diffraction studies of the magnetic structure of Ho3Pd4Ge4

Ho₃Pd₄Ge₄ crystallizes in the orthorhombic Gd₆Cu₈Ce₈-type of structure (space group Immm) in which the Ho atoms occupy two nonequivalent crystallographic positions: 2a and 4j. Neutron diffraction measurements indicate that the Ho moments in the 4j site below 6.7 K form a collinear antiferromagnetic structure with the magnetic moments parallel to the a axis, whereas the Ho moments in the 2a site below 5 K form a sine-wave modulated structure with the magnetic moments parallel to the c axis.     

Synthesis and properties of Ba3NaRu2O9−δ and Ba3(Na, R)Ru2O9−δ (R=rare earth) crystals

Crystals of Ba₃NaRu₂O_9−δ (δ≈0.5) and Ba₃(Na, R)Ru₂O_9−δ (R=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) were grown by an electrochemical method, and their crystallographic, magnetic, and electric properties were studied. All crystals have a hexagonal structure of space group P6₃mmc. Ba₃NaRu₂O_9−δ and Ba₃(Na, R)Ru₂O_9−δ (except Ce) have a negative asymptotic Curie temperature suggesting the existence of an antiferromagnetic order; however, they are paramagnetic at temperatures above 1.7 K. Ba₃NaRu₂O_9−δ has an effective magnetic moment P_eff of 0.91 μ_B, while P_eff of Ba₃(Na, R)Ru₂O_9−δ (except Ce) reflects the large free-ion moment of the rare earth ions. Ba₃(Na, Ce)Ru₂O_9−δ shows peculiar magnetic behavior that differs from the magnetism of other Ba₃(Na, R)Ru₂O_9−δ crystals. The resistivity of all crystals exhibits an activation-type temperature dependence with an activation energy in the range of 0.10.2 eV.     

Electron energy loss spectroscopy of Gd compounds

Electron energy loss spectroscopy (EELS) has been used to study excitations in Gd metal and Gd compounds: Gd₅Ge₄, Gd₅Si₂Ge_2, GdTiSi and GdTiGe. The energy of the primary electrons was changed in the range from 140 to 1346 eV, and the losses up to 60 eV were analyzed. The spectra show several features that can be attributed mainly to atomic excitations in Gd, like 4f → 4f and 5p → 5d. These transitions appear to be independent of the specific compound. The pronounced losses in the region from 13 to 17 eV are attributed to plasmon-like excitations and exhibit a dependence on the primary energy and on the specific compound.     

ESR bottleneck effect in dilute Gd1−xLaxT2Si2 (T=Cu, Ni) systems

The bottleneck effect observed in electron spin resonance (ESR) was removed by replacing the Gd^3. spins by non-magnetic lanthanum ions in two compounds: the most bottlenecked GdCu₂Si₂ and GdNi₂Si₂ where the bottleneck is weaker. The unbottlenecked limit was reached for very small Gd concentrations to (1 −x) = 0.0005. From these results the ν(E_F)J_Sc values have been estimated, where ν(E_F) is the electron density of states at the Fermi level and J_Sc is the exchange coupling constant between conduction electrons and Gd spins. It was found that these values are twice as large for Ni compounds (15 × 10³) as for Cu compounds (7.5 × 10³). On the basis of X-ray photoemission spectra of the valence band and theoretical calculations of ν(E_F) for LaCu₂Si₂, the density of states is estimated as 0.15 states per eV spin atom for GdCu₂Si₂ and 0.3 states per eV spin atom for GdNi₂Si₂ (for J_Sc = 0.05 eV). The relaxation processes in the investigated compounds are also discussed.     

Changes in the phase structure and magnetic characteristics of Gd5Si2Ge2 when alloyed with Mn

Gd₅Si₂Ge₂ parent compounds were alloyed with Mn in order to understand the underlying relation between the structural phases and the magnetic behavior of the pseudo ternary compounds formed. The alloying mechanism in Gd₅Si₂Ge₂ causes simultaneous substitution of the nonmagnetic Si and Ge atoms from the (Si + Ge) sublattice in equal amounts. No subsequent heat treatment was made on alloyed compounds. X-ray powder diffraction, magnetization versus temperature and isothermal magnetization measurements were carried out. X- ray diffraction patterns were used to qualitatively determine the existence of different structural phases in the alloys. It was observed that the starting, as-melted alloy with z = 0 has Gd₅Si₄-type orthorhombic structure at room temperature with traces of 1:1 stoichiometry phase which transforms totally to a Gd₅Si₂Ge₂-type monoclinic phase when heat treated. Similarly, increase in the Mn content leads to an increase in the monoclinic phase content of the originally orthorhombic compounds. Curie temperatures were determined from M(T) measurements and the magnetocaloric characterization was made using M(H) measurements by plotting the magnetic entropy change values against temperature. No giant magnetocaloric effect was observed for non heat treated samples.     

Phase relationships and crystallography in the pseudobinary system Gd5Si4---Gd5Ge4

A study of phase relationships and crystallography in the pseudobinary system Gd₅(Si_xGe_1−x)₄ revealed: (1) that both terminal binary compounds Gd₅Si₄ and Gd₅Ge₄ crystallize in the Sm₅Ge₄-type orthorhombic structure, and (2) the appearance of an intermediate (ternary) phase with a monoclinic crystal structure which is similar to both Gd₅Si₄ and Gd₅Ge₄. The formation of the monoclinic phase at 0.24≤x≤0.5 [between Gd₅(Si_0.96Ge_3.03)Gd₅(Si₁Ge₃) and Gd₅(Si₂Ge₂)] is probably due to the large difference in bonding characteristics of Si and Ge in the Gd₅Si₄-Gd₅Ge₄ pseudobinary system which limits the ability of the mutual substitution of Si for Ge and vice versa without a change of the crystal structure. For the composition Gd₅(Si₂Ge₂) the lattice parameters of the monoclinic structure (space group P112₁/a) are a=7.580865), b=14.802(1), c=7.7799(5)Å, γ=93.190(4)°. A distinct difference in the magnetic behaviors of the alloys from three different phase regions in this system follows the distinct difference in the crystal structures observed for the alloys from the three phase regions.     

Dependency of c-lattice parameter, activation energy and its magnetic power constant on x in Y1−xYbx/2Gdx/2Ba2Cu3O7−y superconductors

To investigate the effect of x on c-lattice parameter, the activation energy, and the magnetic power constant of activation energy, the nominal composition of Y_1−xYb_x/2Gd_x/2Ba₂Cu₃O_7−y superconducting samples for x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by solid-state reaction technique. c-Lattice parameters of the samples were calculated using XRD data. From the resistivity curves which were obtained under 11 different magnetic fields from 0 up to 5 T in the temperature range from 70 to 100 K steps by 0.25 K in zero field cooling regime, the activation energies of the samples were determined using Arrhenius activation energy law. The magnetic power constants of activation energy of the samples were calculated from the slopes of the plots ln U versus ln H. The results showed that the value of c-lattice parameters of samples x = 0 (Y123), and x = 1.0 (YbGd123) are in good agreement with the reported works. It was found that c-lattice parameter decreased with the increasing the content x except samples of x = 0.4 and 1.0. It was thought that the origin of the separation from the linearity comes from the lack of Y in nominal composition for x = 1.0. As for the sample x = 0.4, it was thought to be due to the fact that Y, Yb and Gd atoms affect the superconductivity properties dominantly. On the other hand, there is a similarity among U(0 T), U(1 T), its magnetic power constant and the c-lattice parameter with increasing content x. The desired activation energy and its magnetic field dependency constant can be controlled with x and then this can be useful for the superconducting applications.     

Phase transformations in iron-rich Tb–Fe–Si alloys

RRhO₃ (R=rare earth except Ce and Pm) was prepared by a solid-state reaction, and its crystallographic, magnetic, and electric properties were investigated. RRhO₃ has an orthorhombic perovskite-type structure of the space group Pbnm. RRhO₃ shows Curie–Weiss paramagnetism above 5 K. On the other hand, EuRhO₃ shows antiferromagnetic behavior. The Rh³⁺ ion, which seems to be in the low-spin state, has a very small effective magnetic moment (P_eff=0.295 μ_B/ion). The P_eff value of the RRhO₃ compounds shows the same dependence on the number of 4f electrons as the gJ value of the rare-earth ions. The rare-earth ions make a major contribution to the magnetic moment of RRhO₃. The resistivity of all RRhO₃ shows an activation-type (or semiconductor-like) temperature dependence.     

Magnetocrystalline anisotropy of R2Fe17Hx (x=0, 3) single crystals

The magnetocrystalline anisotropy of R₂Fe₁₇ (R=Y, Gd, Tb, Ho and Er) and their hydrides are studied by analyzing the magnetization curves of single crystal samples in the temperature range 4.2–300 K in magnetic fields up to 140 kOe. There is no noticeable influence of hydrogenation on the magnetic anisotropy in the Y₂Fe₁₇ and Gd₂Fe₁₇ compounds. An easy-plane to easy-cone transition is detected for a Tb₂Fe₁₇H₃ single crystal after hydrogenation. A significant change of the magnetization process has been observed in the hydrides Ho₂Fe₁₇H₃ and Er₂Fe₁₇H₃. Hydrogenation induces a FOMP-type transition in the Ho₂Fe₁₇H₃ compound and, on the contrary, leads to the disappearance of the FOMP type transition in the Er₂Fe₁₇H₃ compound. The obtained results are discussed on the basis of a model based on the interaction of the quadrupolar moment and the magnetic moment of the 4f electron shell of the rare earth ion with the interstitial hydrogen. It is established that the orientation of the quadrupolar moment of the asymmetric 4f shell with respect to the direction of the resulting magnetic moment of 4f electrons plays an important role.     

Magnetic properties of Pr1−xGdxMn2Ge2 compounds

The structure and magnetic properties of the Pr_1−xGd_xMn₂Ge₂ (0.0≤x≤1.0) compounds have been investigated by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. The lattice constants and the unit cell volume obey Vegard’s law. Samples in this alloy system exhibit a crossover from ferromagnetic ordering for PrMn₂Ge₂ to antiferromagnetic ordering for GdMn₂Ge₂ as a function of Gd concentration x. At low temperatures, the rare earth sublattice also orders and reconfigures the ordering in the Mn sublattice. The results are summarized in the x–T magnetic phase diagram.     

New RRh5Ge3 compounds of the new SmRh5Ge3 structure type and their magnetic properties (R=Sm, Gd, Tb)

Powder X-ray diffraction results and macroscopic magnetic properties of new ternary RRh₅Ge₃ compounds (R=Sm, Gd, Tb) are reported. The compounds SmRh₅Ge₃ (a=2.2744(4) nm, c=0.3888(1) nm), GdRh₅Ge₃ (a=2.2711(5) nm, c=0.3872(1) nm) and TbRh₅Ge₃ (a=2.2628(7) nm, c=0.3851(1) nm) crystallize in the hexagonal SmRh₅Ge₃-type structure (space group P6₃/m; No. 176). The GdRh₅Ge₃ and TbRh₅Ge₃ compounds are Curie–Weiss paramagnets down to 5 K.     

X-ray investigations of ternary R3Pd4Ge4 samples

Ternary R₃Pd₄Ge₄ samples (R=Nd, Eu, Er) were investigated by means of X-ray single crystal (four circle diffractometer Philips PW1100, MoK radiation) and powder diffraction (MX Labo diffractometer, CuK radiation). The Er₃Pd_3.68(1)Ge₄ compound belongs to the Gd₃Cu₄Ge₄ structure type, space group Immm, a=4.220(2) Å, b=6.843(2) Å, c=14.078(3) Å, R₁=0.0484 for 598 reflections with F_o>4σ(F_o) from X-ray single crystal diffraction data. No ternary R₃Pd₄Ge₄ compound when R is Nd or Eu was observed. The Nd and Eu containing samples appeared to be multiphase. Ternary phases observed in the Nd₃Pd₄Ge₄ and Eu₃Pd₄Ge₄ alloys and their crystallographic characteristics are the following: NdPd₂Ge₂, CeGa₂Al₂ structure type, space group I4/mmm, a=4.3010(2) Å, c=10.0633(2) Å (X-ray powder diffraction data); NdPd_0.6Ge_1.4, AlB₂ structure type, space group P6/mmm, a=4.2305(2) Å, c=4.1723(2) Å (X-ray powder diffraction data); Nd(Pd_0.464(1)Ge_0.536(1))₂, KHg₂ structure type, space group Imma, a=4.469(2) Å, b=7.214(2) Å, c=7.651(3) Å, R₁=0.0402 for 189 reflections with F_o>4σ(F_o) (X-ray single crystal diffraction data); Eu(Pd,Ge)₂, AlB₂ structure type, space group P6/mmm, a=4.311(2) Å, c=4.235(2) Å; EuPdGe, EuNiGe structure type, space group P2₁/c, and ternary compound with unknown structure (X-ray powder diffraction data).     

Spectroscopic and dielectric studies on MnO doped PbO–Nb2O5–P2O5 glass system

PbO–Nb₂O₅–P₂O₅ glasses containing different concentrations of MnO ranging from 0 to 2.5 mol% were prepared. A number of studies viz., differential thermal analysis, infrared, optical absorption, luminescence, Raman and ESR spectra, magnetic susceptibility and dielectric properties (constant ′, loss tan δ, ac conductivity σ_ac over a range of frequency and temperature) of these glasses have been carried out. The results have been analyzed in the light of different oxidation states of manganese ions. The analysis indicates that when the concentration of MnO is around 1.0 mol%, manganese ions mostly exist in Mn²⁺ state, occupy network forming positions with MnO₄ structural units and increase the rigidity of the glass network. When MnO is present in higher concentrations, these ions seem to exist mostly in Mn³⁺ state and occupy modifying positions.     

Magnetic properties, crystal and electronic structure of GdNi5−xCux series

The effect of Ni/Cu substitution on the magnetic properties, crystal and electronic structure of the polycrystalline GdNi_5−xCu_x series has been studied. All compounds crystallize in the hexagonal CaCu5 type of crystal structure (space group P6/mmm). The temperature dependence of magnetic phase transition (T_mag) estimated from χ_AC(T) susceptibility as well as magnetization M(T) below room temperature indicates the maximum for x = 1.0 copper concentration. In the paramagnetic range (above 300 K) the magnetic susceptibilities follow a Curie-Weiss-type dependence. The effective magnetic moments are higher than theoretical value for free Gd³⁺.From X-ray photoelectron spectroscopy (XPS) data the valence band as well as the core level spectra have been analyzed. The filling of Ni3d band in the GdNi_5−xCu_x system by charge transfer of Gd conduction electrons is revealed by a reduction of the satellite intensities in the Ni2p core level spectrum. The obtained results exhibit that the valence bands at the Fermi level are dominated by hybridized Ni3d and Gd5d states, when Cu3d states are rather localized about 3 eV below the Fermi level. Quite good relation between the magnetic properties and electronic structure has been found.     

Magnetic structures of R2RhSi3 (R=Ho, Er) compounds

Powder X-ray and neutron diffraction and magnetic measurements have been performed on R₂RhSi₃ (R=Ho and Er) compounds at low temperatures. The compounds crystallize in a derivative of the hexagonal AlB₂-type structure. The crystal structure parameters have been refined on the basis of the X-ray and neutron diffraction patterns collected in the paramagnetic region. These compounds are antiferromagnets with Néel temperatures of 5.2 K for Ho₂RhSi₃ and 5 K for Er₂RhSi₃. Both compounds exhibit collinear magnetic structures, described by the propagation vector k=(1/2,0,0) for Ho₂RhSi₃ and k=(0,0,0) for Er₂RhSi₃. This magnetic order is stable in the temperature range between 1.5 K and the Néel temperature.     

Photoelectrochemical study of lanthanide titanium oxides, Ln2Ti2O7 (Ln = La, Sm, and Gd)

Photocatalytic activities for hydrogen evolution of lanthanide titanium oxides, Ln₂Ti₂O₇ (Ln = La, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb) prepared by a solid-state reaction were studied. Hydrogen gas was clearly evolved in distilled water suspension of La₂Ti₂O₇ and Sm₂Ti₂O₇. From the photoelectrochemical measurements, the values of the flat band potential were estimated to be −0.04, −0.02, +0.27 eV for La₂Ti₂O₇, Sm₂Ti₂O₇, Gd₂Ti₂O₇, respectively, versus the normal hydrogen electrode (NHE). The values of the band gap energy were calculated to be about 3.29, 2.79, 2.82 eV for La₂Ti₂O₇, Sm₂Ti₂O₇, Gd₂Ti₂O₇, respectively. The photocatalytic activities of Ln₂Ti₂O₇ (La, Sm, and Gd) were discussed along with detailed band structures estimated in this study. From the band structures, Sm₂Ti₂O₇ is a possible candidate of photocatalyst responding to visible light.     

Crystal field splitting and anomalous thermal expansion in YbVO4

We have investigated the Yb³⁺ crystal-field-level structure in YbVO₄ using inelastic magnetic neutron-scattering measurements and crystal-field model calculations. We determined the temperature dependence of the a and c lattice parameters of the tetragonal unit cell by neutron diffraction and observed a minimum at ca. 120 K in the c lattice parameter. This anomaly in the thermal expansion is interpreted as arising from a coupling of the anisotropic low-lying crystal-field states of the Yb³⁺ ions and the crystal lattice at low temperatures.     

Crystal structure of Mg3Pd from first-principles calculations

Crystal structure of Mg3Pd alloy was studied by first-principles calculations based on the density functional theory. The total energy, formation heat and cohesive energy of the two types of Mg3Pd were calculated to assess the stability and the preferentiality. The results show that Mg3Pd alloy with Cu3P structure is more stable than Na3As structure, and Mg3Pd alloy is preferential to Cu3P structure. The obtained densities of states and charge density distribution for the two types of crystal structure were analyzed and discussed in combination with experimental findings for further discussion of the Mg3Pd structure.     

XPS spectra of the AFe4Al8 compounds with A=Y, Sc, U and Th

The XPS electronic structures of the AFe₄Al₈ (A=Sc, Y, U, Th) single crystals were measured. The valence band of UFe₄Al₈ exhibits domination of the U 5f states at the Fermi level, while for the other AFe₄Al₈ compounds the valence states are shifted by 0.5 eV toward higher binding energy. The multiplet structure of the U 4f and Th 4f states was analyzed in comparison with the U 4f states of U₃Ge₅ and UNiSb₂.