Constitution, structural chemistry and magnetism in the ternary system Ce–Ag–Si

Phase relations were established for the Ce–Ag–Si system at 850°C by means of X-ray diffraction, light optical microscopy and quantitative electron probe microanalysis. Phase equilibria are characterised by the existence of extended solid solutions starting from the binaries: Ce(Ag_xSi_1−x)_2−y (ThSi₂-type), Ce(Ag_1−xSi_x)_1−y (unknown structure type) and Ce(Ag_1−xSi_x)_2−y (unknown structure type). Three ternary phases were found to exist, CeAg₂Si₂ (ThCr₂Si₂-type), Ce(Ag_xSi_1−x)_2−y (AlB₂-type) and the new ternary compound CeAgSi₂ with unknown structure type. Magnetic behaviour was studied from magnetic susceptibility and magnetisation measurements down to 1.7 K and employing magnetic fields up to 5 T. Soft ferromagnetism is observed for CeAg_xSi_2−x (AlB₂-type) below 5 K. Alloys Ce(Ag_xSi_1−x)_2−y with 0.08<x_Ag<0.30 (ThSi₂-type) encounter ferromagnetic order below 7 K. For x_Ag=0.31 the ferromagnetic interaction changes to antiferromagnetism with T_N=5.7 K. For CeAgSi₂ ferrimagnetic or canted antiferromagnetic order is indicated below 7 K.     

Oxygen release behaviour of Ce(1−x)ZrxO2 powders and appearance of Ce(8−4y)Zr4yO(14−δ) solid solution in the ZrO2–CeO2–CeO1.5 system

To clarify the existence of metastable phases in the ZrO₂–CeO₂–CeO_1.5 system, evolved-oxygen gas analyses, (EGA), by heating a single phase of t′ and t″ (Ce_(1−x)Zr_xO₂) with various compositions, x, in a reducing gas and successive oxidation were carried out repeatedly. The oxygen release behaviour of the t′ and t″ phases was very complicated. The single κ phases, (Ce_(1−x)Zr_xO₂) with the composition, x=0.5 and 0.6, which were obtained by oxidizing the resulting pyrochlore as a precursor in O₂ gas at 873 K, exhibited a sharp oxygen release at the lowest temperature; the composition range of κ phase may be x=0.450.65. A new tetragonal phase t*, (Ce_(1−x)Zr_xO₂), which was attained by cyclic redox process together with annealing in O₂ gas at 1323 or 1423 K, exhibited a sharp oxygen release at the highest temperature; the composition range of t* phase may be as wide as x=0.200.65. A metastable solid solution expressed by a chemical formula of Ce_(8−4y)Zr_4yO_(14−δ) (y=01) possessing a CaF₂-related structure appeared on deoxidation of the t* phase. A ternary phase diagram containing the t* and Ce_(8−4y)Zr_4yO_(14−δ) solid solution was proposed.     

Recent improvement of hard ferrite permanent magnets based on La–Co substitution

Recent improvements performed on hard ferrite magnets with the Sr_1−xLa_xFe_12−yCo_yO₁₉ composition are presented. The influence of the composition on both structural and magnetic properties of samples with y=x and y/x=0.75 is emphasized. Our investigations have allowed to optimize the permanent magnet properties of La–Co substituted hard ferrite magnets. The optimum composition contains more lanthanum than cobalt at the benefit of both lower irreversible losses and lower raw material costs.     

Boron content dependence of crystallization, glass forming ability and magnetic properties in amorphous Fe-Zr-B-Nb alloys

The glass forming ability (GFA) was investigated in Fe_91−xZr₅B_xNb₄ alloys with B contents of 0–36 at.%. The GFA changes with B content, and fully amorphous alloys were prepared by melt spinning for B contents between 5 and 30 at.%. The amorphous alloys crystallize with a primary crystallization mode in the low B content range of 5≤x≤20 at.%, but in the eutectic mode in the high B content range of 20<x<30 at.%. A single new metastable Fe-Zr-B-Nb cubic phase with a lattice constant of 1.0704 nm, a saturation magnetization of 137 emu/g and a coercivity of 7.3 Oe at room temperature is formed when crystallizing in a polymorphous mode at x=30 at.%. The glass transition temperature (T_g), crystallization temperature (T_x), Curie temperature (T_c) and saturation magnetizations (M_s) of the amorphous alloys increase with increasing B content, but the coercivity (H_c) decreases. As the B content exceeds 20 at.%, not only increase the T_g, T_x and GFA sharply, due to the change of crystallization mode, but also the concentration dependence of the T_c and M_s changes. It is concluded that the amorphous alloys have better GFA, thermal stability and soft magnetic properties for the high B contents of 25–30 at.% than for the low B contents of 5–20 at.%.     

On the properties of the quasi-one-dimensional compound NbSe3 doped with tantalum

We suggest here that the dependence on the impurity concentration y of the two resistive anomalies R₁, R₂ and the superconducting critical temperature T_c of the quasi-one-dimensional compound Ta_yNb_1−ySe₃ may be interpreted within the recently proposed model based on a CDW-induced regularity in the impurity distribution.     

Study of the antiferromagnetic phase in Sm(Mn1−xCrx)2Ge2

Magnetic and thermal expansion measurements have been carried out on the polycrystalline Sm(Mn_1−xCr_x)₂Ge₂ samples to see how the antiferromagnetie (AFMII) region in SmMn₂Ge₂ is affected by Cr substitution. It is found that the antiferromagnetic region disappears for samples with less than 2 at.% of Cr. Sharp changes in the thermal expansivity (Δl/l) at FMI–AFMII and AFMII–FMII transitions are observed, indicating first order transitions. The decrease in relative thermal expansivity at the two transitions with the increase of Cr concentration is related to the decrease in the stability and the temperature-range of the AFMII phase observed in magnetization measurements. A spin reorientation transition (T_SR) has been observed for x=0, at 148 K. It is found that the T_SR increases with the increase of Cr concentration. A magnetic phase diagram as a function of Cr concentration in Sm(Mn_1−xCr_x)₂Ge₂ has been constructed.     

Power factor of La1−xSrxFeO3 and LaFe1−yNiyO3

The electrical conductivity (σ), Seebeck coefficient (S), and power factor (σS²) of perovskite-type LaFeO₃, La_1−xSr_xFeO₃ [0.1 ≤ x ≤ 0.4] and LaFe_1−yNi_yO₃ [0.1 ≤ y ≤ 0.6] were investigated in the temperature range of 300–1100 K to explore their possibility as thermoelectric materials. The electrical conductivity of LaFeO₃ showed semiconducting behavior, and its Seebeck coefficient changed from positive to negative around 650 K with increasing temperature. The electrical conductivity of LaFeO₃ increased with the substitutions of Sr and Ni atoms, while its Seebeck coefficient decreased. The Seebeck coefficient of La_1−xSr_xFeO₃ was positive, whereas that of LaFe_1−yNi_yO₃ changed from positive to negative with increasing Ni content. The substitutions of Sr and Ni were effective in increasing the power factor of LaFeO₃; 0.0053 × 10⁻⁴ Wm⁻¹ K⁻² for LaFeO₃ (1050 K), 1.1 × 10⁻⁴ Wm⁻¹ K⁻² for La_1−xSr_xFeO₃ (x = 0.1 at 1100 K) and 0.63 × 10⁻⁴ Wm⁻¹ K⁻² for LaFe_1−yNi_yO₃ (y = 0.1 at 1100 K).     

On the crystal structure of new series of compounds, RPt2+xSb2−y (x = 0.125, y = 0.25; R = La, Ce, Pr)

A new compound CePt_2+xSb_2−y (x = 0.125, y = 0.25) was synthesized by arc-melting of the elements. The chemical and structural characterizations were carried out at room temperature on as-cast samples using X-ray diffractometry, metallographic analysis and EDS-microanalysis. According to the results of X-ray single crystal diffraction this antimonide crystallizes in I4cm space group (no. 108), Z = 32, ρ = 12.19 Mg/m³, μ = 89.05 mm⁻¹ (a = 12.5386(3) Å, c = 21.4692(6) Å (crystal I) and a = 12.5455(2) Å, c = 21.4791(5) Å (crystal II)). The structure and composition were confirmed by powder X-ray diffraction (a = 12.4901(2) Å, c = 21.3620(4) Å) and EDS-microanalysis respectively. Isotypic compounds were observed with La and Pr from X-ray powder diffraction of as-cast alloys at room temperature (a = 12.6266(4) Å, c = 21.4589(6) Å for LaPt_2+xSb_2−y and a = 12.5184(5) Å, c = 21.4178(7) Å for PrPt_2+xSb_2−y). The CePt_2+xSb_2−y structure is derived from CaBe₂Ge₂ (a = 2a₀ − 2b₀, b = 2a₀ + 2b₀, c = 2c₀) and comprises a new atomic arrangement with both vacancy on 4(b) pyramidal site and substitution of antimony atoms (X) by platinum (B) in the B–XX–B layers (referring to the subcell structure) forming two B––1/2B1/2XX–3/4B and two X–BB–X layers per cell. The structure of CePt_2+xSb_2−y is compared with those reported before for URh_1.6As_1.9 and CeNi_1.91As_1.94.     

Phase relationships at 500 °C in the Y---Pr---Mg system

Phase equilibria in the ternary system Y---Pr---Mg have been studied by X-ray powder diffraction analysis, optical and scanning electron microscopy, and electron probe microanalysis. The isothermal section at 500 °C has been determined in the range 50–100 at.% Mg. The phase equilibria are characterized by the formation of the following rather extended solid solutions: (Y_xPr_1−x)Mg (continuous solid solution of cP2-CsCl type) and Y_xPr_1−xMg₃ (0 x 0.5) of cF16-BiF₃ type. A cubic phase of (Y_xPr_1−x)Mg₂, (cF24-CuMg₂ type) derived from the binary PrMg₂ phase by a substitution of Pr with Y has been observed in a narrow range close to x ≈ 0.6. A solid solution based on the binary YMg₂ phase, i.e. (Y_xPr_1−x)Mg_{2 + y} of hP12-MgZn₂ type (0.79 x 1, for y = 0), extends into the ternary field. The ternary compound Y_xPr_{1 − x}Mg₅ (0.3 x 0.9) of the cF440-GdMg₅ type has been established. Finally, the Y---Pr---Mg section has been compared with the similar Y---La---Mg section studied previously and with an isothermal section predicted on the basis of the systematic trends of the rare earth alloy properties.     

Structural, magnetic and magnetostrictive studies of Tb0.27Dy0.73(Fe1 − xAlx)2

Measurements of magnetic properties, X-ray diffraction and magnetostriction were made on Tb_0.27Dy_0.73(Fe_{1 − x}Al_x)₂ (x = 0.1, 0.2, …, 0.7) compounds. It was found that the system has the cubic MgCu₂ structure over almost the whole (Fe,Al) concentration range investigated, except for a narrow intermediate range (x = 0.4–0.6) where the hexagonal MgZn₂ structure appears. With increasing Al content x, the lattice constant a increases linearly with x. The first replacement of Fe results in a marked decrease in the Curie temperature, which is followed by a slight decrease in T_C with x. A linear decrease in magnetostriction of |λ_| − λ| at room temperature with x was also observed from 1530 × 10⁻⁶ for x=0 to 36×10⁻⁶ for x=0.3. The saturation magnetization σ_s exhibits a complex concentration dependence in the Tb_0.27Dy_0.73(Fe)_{1 − x}Al_x)₂ system: in the range x < 0.5, σ_s increases linearly with x and, for x = 0.5–0.6, σ_s decreases and then increases again. An enhancement of the magnetic ‘hardness’ in this system was also observed at low temperature.     

High-pressure ultrasonic properties of spin-density-wave Cr–Re alloy single crystals

The pressure dependence of the elastic constants (c_ij) of Cr–Re alloy single crystals, containing 0.3 and 0.5 at.% Re, are reported. For 0.3 at.% Re the concentration (c) is below the triple point concentration (c_t) on the (c–T) magnetic phase diagram. This crystal therefore remains in the incommensurate (I) spin-density-wave (SDW) antiferromagnetic phase at all temperatures below the ISDW–paramagnetic (P) Néel transition temperature (T_N). The pressure derivatives of the elastic constants, dc_ij/dp, were measured for the Cr+0.3 at.% Re crystal as a function of temperature through T_N. The Cr+0.5 at.% Re crystal has c>c_t and exhibits an incommensurate–commensurate (I–C) SDW phase transition at a temperature T_IC<T_N on heating. In the case of this crystal dc_ij/dp was studied at temperatures close to and above T_IC. The acoustic mode Grüneisen parameters (γ_n), which quantify the lattice vibrational anharmonicity, were calculated as a function of temperature from the dc_ij/dp data for each crystal. These calculations indicate much larger coupling of the SDW to the long-wavelength longitudinal phonons than to the shear modes for both Cr–Re crystals. Longitudinal mode softening under applied pressure, due to strong magnetoelastic interactions between the SDW and the longitudinal acoustic phonons, in Cr+0.3 at.% Re at T<T_N, gives nearly discontinuously way to extremely large mode stiffening as the crystal is heated through T_N. γ_n in the CSDW and pressure-induced ISDW phases of the Cr+0.5 at.% Re crystal shows unusual behaviour with temperature. This is particularly so for the shear mode Grüneisen parameters which are relatively large and negative just above T_IC, implying significant shear mode softening under applied pressure. γ_n (shear) then increases on further increasing the temperature of this crystal. In comparison, γ_n (shear) for both the CSDW and pressure-induced ISDW phases of other Cr alloys, with c>c_t, for instance Cr–Ru and Cr–Ir alloys, is near zero, slightly positive, and remains constant with temperature. The measurements on the Cr+0.5 at.% Re crystal suggest the existence of a new phase line, separating the ISDW phase present at atmospheric pressure from that induced from the CSDW phase by applying high pressure, on the pressure–temperature magnetic phase diagrams of dilute Cr alloys with c>c_t. More experimentation, particularly high pressure neutron diffraction studies, are needed to verify this point.     

Glass-forming ability and magnetic properties of mechanically solid-state reacted Co100−xTix alloy powders

A high-energy ball milling technique using the mechanical alloying method has been employed for fabrication of glassy Co_100−xTi_x (25≤x≤67) alloy powders at room temperature. The fabricated glassy alloys in the Co-rich (33≥x) side exhibit good soft magnetic properties. The binary glassy alloys for which the glass transition temperatures (T_g) have rather high temperatures (above 800 K), show large supercooled liquid regions before crystallization (ΔT_x larger than 50 K). The reduced glass transition temperature (ratio between T_g and liquidus temperatures, T_l (T_g/T_l)) was found to be larger than 0.56. We have also performed post-annealing experiments on the mechanically deformed Co/Ti multilayered composite powders. The results show that annealing of the powders at 710 K leads to the formation of a glassy phase (thermally enhanced glass formation reaction), of which the heat of formation was measured directly. The similarity in the crystallization and magnetization behaviors between the two classes of as-annealed and as-mechanically alloyed glassy powders implies the formation of the same glass state.     

Low-temperature synthesis and characterization of (Zn,Ni)TiO3 ceramics by a modified sol–gel route

Hexagonal ilmenite-type (Zn_1−xNi_x)TiO₃ (x = 0, 0.85–1.0) ceramic powders were successfully synthesized by a sol–gel route with low temperature (800 °C) sintering, which was modified by using the two-step heat treatment so as to obtain pure products. The thermal stability of the hexagonal (Zn,Ni)TiO₃ was enhanced with the increasing amount of nickel addition. FE-SEM observations demonstrated that the average crystallite sizes of (Zn_1−xNi_x)TiO₃ remarkably decreased from more than 200 nm to less than 100 nm with the increasing solubility x. The dielectric properties of (Zn_1−xNi_x)TiO₃ were measured at different frequencies and the results showed that there existed maximum values both for the dielectric constants and the loss tangents at x = 0.85.     

The effects of the combined substitution of Y and Ga on the crystallographic structure of Nd2−xYxFe17−yGay intermetallic compounds

The effects of the combined substitution of Y and Ga on the crystallographic structure of Nd_2−xY_xFe_17−yGa_y compounds with x = 0, 0.5, 1.0, 1.5 and y = 0, 1, 2, 3 have been investigated using X-ray and neutron powder diffractions. Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th₂Zn₁₇-type structure with only small amounts of alpha iron. It is found that the addition of Ga atoms lessens the decreasing rates of the a-axis and unit cell volume V on the Y content but almost does not affect the decreasing rates of the c-axis. However, the substitution of Y has a positive effect on the increasing rates of the a-axis and unit cell volume V on the Ga content but has a very slight effect on the increasing rate of the c-axis. The c/a ratio of Nd_2−xY_xFe_17−yGa_y as a function of Ga content exhibits a different increase for different Y content owe to the combined effects of Y and Ga on the crystallographic structure. The substitution of Y is found to have little effect on the site occupancy of Ga in Nd_2−xY_xFe_17−yGa_y. The combined effects of Y and Ga on the bond lengths and ASBL of Nd_2−xY_xFe_17−yGa_y indicate that more bonds detrimental to ferromagnetic exchange can be modulated into the desirable ferromagnetic exchange distance range through suitable combined substitution, which provides a valuable way to improve the magnetic properties of rare earth-transition intermetallic compounds.     

Microstructure and tribological properties of LaxCeyO1 − x − y composite nanofilms

A series of La_xCe_yO_{1 − x − y} films (x = 0–0.54, y = 0–0.58) with thickness of 35–45 nm was deposited by unbalanced magnetron sputtering. High-resolution transmission electron microscope observation shows that La_0.24Ce_0.34O_0.42 film has polycrystalline structure. La₂O₃ and CeO₂ are formed within the La_xCe_yO_{1 − x − y} films confirmed by the X-ray diffraction and X-ray photoelectron microscopy. The friction coefficient and residual compressive stress of five kinds of three-element compound films exhibit symmetric distribution with the relative equilibrium of La and Ce atomic concentration within the films. The critical load of all deposited films is between 28 and 33 mN. The friction coefficient of two kinds of rare earth complex oxide films is in the range of 0.08–0.09, which is lower than that of only one kind of rare earth oxide films, and the friction mechanism is discussed.     

Sur de nouveaux matériaux à base du bismuth trivalent de structures fluorine et pyrochlore

The study of the pseudo-binary systems Bi₂O₃-A₂B₂O₇, with A Cd²⁺, Pb²⁺, Mg²⁺ and B & Nb⁵⁺, Ta⁵⁺, reveals two types of solid solutions. The first one with Bi_1−2xCd_xB_xO_2−y (y = (1 − x)/2) formulation shows a fluorite-type structure whereas the second type with A_2−zB_2−zBi_2xO_7−z/2 formulation is obtained for A₂B₂O₇-rich compositions and possesses structures deriving from the pyrochlore type. The composition dependence of the unit cell parameters has been correlated to possible substitution mechanisms.

Résumé

D'une manière générale, l'étude des pseudo-binaires Bi₂O₃---A₂B₂O₇ (avec A Cd²⁺, Pb²⁺, Mg²⁺ et B Nb⁵⁺, Ta⁵⁺) a permis de mettre en évidence deux types de solutions solides: le premier de structure fluorine déficitaire en anions a pour formule Bi_1−2xCd_xB_xO_2−y (y = (1−x)/2); la seconde solution solide de composition A_2−z B_2−z Bi_2zO_7−z/2 est obtenue pour des compositions riches en A₂B₂O₇ et possède une structure qui dérive de celle du pyrochlore. Les paramètres cristallins des diverses phases isolées ont été déterminés à 298 K à partir des diffractogrammes X. Leur évolution a été corrélée avec le taux de substitution.     

Feromagnetism in NiAs-type Co---Fe---Te

The magnetic behaviour of NiAs-type (Co_1−yFe_y)_1−xTe (x = 0.36) has been characterized by magnetization measurements at low temperatures (4.2–300 K). All samples investigated (y = 0.3, 0.5, 0.7) were weakly ferromagnetic with a small magnetic moment of about 0.1 μ_B per transition metal atom; within this series of alloys magnetic ordering occurred at 30 K, 40 K and 15 K respectively. A competitive character of the magnetic interactions seemed to be indicated.     

Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys

Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (T_x) and melting temperature (T_m) of the Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of T_m and T_x, ΔT_m (=T_m − T_x), is as small as 135 K and the reduced crystallization temperature (T_x/T_m) is as high as 0.86. The small ΔT_m and high T_x/T_m values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloys are investigated.     

Stress-induced martensitic transformation in Fe–Ni bicrystals

The effect of applied stress on martensitic transformation behaviour near the grain boundary was examined using Fe–32 at.% Ni bicrystals containing a 90°2 1 1 tilt or a 90°{2 1 1} twist grain boundary focusing on the martensite-start stress (σ_M), the morphology of martensites and the variant selection. The σ_M for single crystals and tilt boundary bicrystals increases almost linearly with increasing temperature (T), and the σ_M−T relation for tilt boundary bicrystals is situated in the higher temperature side than that for single crystals. In contrast, the σ_M of the twist boundary bicrystals exists between the correlated σ_M−T relations for single crystals and tilt boundary bicrystals. The variant selection near the tilt boundary is not sensitive to applied stress; some variants with the habit plane almost parallel to the boundary were symmetrically formed similar to those in the thermal transformation. The effect of loading axis to grain boundary on the martensitic transformation behaviour was also investigated.     

Structural and thermodynamic properties of the pseudo-binary TiCr2−xVx compounds with 0.0≤x≤1.2

The TiCr_2−xV_x compounds with 0.0≤x≤1.2 series have been synthesised and characterised by X-ray powder diffraction. X-Ray qualitative and quantitative phase analysis has been carried out on the as-cast alloys using the Rietveld method. The refinements of the structure shows that the materials crystallise either in the hexagonal or in the cubic Laves phase type for low V contents. For x>0.6, the system is found of b.c.c.-type structure only. The pressure–composition–temperature (P–C–T) isotherms measured at 298 K show that the as-cast alloys absorb large amounts of hydrogen, from 4 to 5.2 H/f.u. The P–C–T diagrams reveal also the presence of a relatively flat plateau, and a large hysterisis effect, and correspondingly the hydride cannot be completely dehydrogenated.