Nax/2Bi1−x/2MoxV1−xO4 and Bi1−x/3MoxV1−xO4 New Scheelite-related phases

New Scheelite-related solid solutions of the compositions Na_x/2Bi_1−x/2Mo_xV_1−xO₄ (0≤x≤1) and Bi_1−x/3 Mo_xV_1−xO₄(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO₄ type ferroelastic BiVO₄. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na_x/2Bi_1−x/2Mo_xV_1−xO₄, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi_1−x/3 Mo_xV_1−xO₄ where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.     

Itinerant metamagnetic transition in Y(Co1−xFex)2

High field magnetization measurements have been performed to examine the existence of itinerant metamagnetism in exchange-enhanced systems related to YCo₂ together with Fe_1−x Co_xSi. In the Y(Co_−xFe_x)₂ system, the meta magnetism inherent in YCo₂ has been observed in 0.04 x0.07. The transition is not as sharp as in the Y(Co_1−xAl_x)₂ system. Other exchange-enhanced paramagnets Y(Co_−xCu_x)₂ and Y_1−xLa_xCo_x2 and weakly itinerant ferromagnet Fe_1−xCo_xSi exhibit no metamagnetic transition up to 430 kOe.     

High field magnetization of the Laves phase compounds M(Co1−xAlx)2 (M = Y, Lu)

Magnetization measurements have been carried out on the Laves phase compounds M(Co_1−xAl_x)₂(M = Y, Lu) up to 42 T. Sharp metamagnetic transitions with small hysteresis are found in Y(Co_1−xAl_x)₂, while broad transitions with large hysteresis are found in Lu(Co_1−xAl_x)₂. The results suggest that the former compounds are magnetically homogeneous but the latter inhomogenous.     

A Raman study of the structural properties of YBa2(Cu1 − xFex)3Oy

The effects of Fe-substitution of YBa₂Cu₃O_y have been investigated by means of Raman scattering, X-ray diffraction, resistivity and susceptibility measurements. A series of samples of YBa₂(Cu_{1 − x}Fe_x)₃O_y with different dopant concentration (0 x 0.15) has been prepared in two batches, the second set having undergone twice the heat and mechanical treatment used to produce the first batch. Considerable improvement in the superconducting transition temperature, T_c, is obtained upon reprocessing. A phase transformation from orthorhombic to tetragonal symmetry is observed for x=0.05 from the X-ray measurements in agreement with previous work. Using a micro-Raman technique, all five A_g vibrational modes have been measured and their dependence on Fe-concentration is analyzed. There are indications that iron substitutes for copper at both sites and that the structure is a mixture of orthorhombic and tetragonal microdomains for all x.     

The anomalous behaviour of Gd atoms (Fe1−xMnx)2 compounds

The educed Gd atoms in the X-structure (Th₆Mn₂₃-type) of Gd(Fe_1−xMn_x)₂ were magnetically investigated by comparing with Gd₆(Fe_1−yMn_y)₂₃ whose structure is Th₆Mn₂₃-type. The magnetic properties of Gd(Fe_1−xMn_x)₂ (0.4≤x≤0.7) were observed to be quite similar to those of Gd₆(Fe_1−yMn_y)₂₃ (0.4≤y≤0.7).     

Magnetic moments and Fe hyperfine field interactions in Y(Fe1−xRux)2 ternaries

Measurements of magnetization and ⁵⁷Fe Mössbauer spectra have been made for Y(Fe_1−xRu_x)₂. The C15 type cubic structure is stabilized for xx 0.7. The C15 compounds is ferromagnetic with T_c200 K and its saturation moment decreases monotonically with increasing x, while the ⁵⁷Fe hyperfine field decreases only slightly with x. From these results, it is deduced that the Ru atoms have an induced moment of ≈1μ_B in the range x 0.2. In the C14 type phase, no magnetic ordering develops even at 4.2 K.     

Thermoelectric power of Y(Mn1−xAlx)2

Electrical resistivity and thermoelectric power of Y(Mn_1−xAl_x2 and (Y_1−xSc_x)Mn₂ were measured from 2 to room temperature. Anomalous behavior of the thermoelectric power of YMn₂ disappears rapidly with the increase of Al content in Y(Mn_1−xAl_x)₂. Based upon spin fluctuations of the itinerant antiferromagnetism of the samples, the anomaly of the thermoelectric power is discussed.     

六角相ErAx (A=H,He)体系弹性性质的第一性原理研究

本文采用第一性原理研究了六角相ErA_x (A=H, He)的弹性性质, 其中x=0, 0.0313, 0.125, 0.25, 分别讨论了体系中不同浓度的氢和氦对体系弹性性质的影响. 计算结果表明, 六角相铒-氢体系晶体的弹性常数、体弹模量、剪切模量、杨氏模量基本上随着晶体中氢的浓度增加而增加, 然而, 铒-氦体系的弹性常数、体弹模量、剪切模量和杨氏模量几乎随着氦浓度的增加而降低. 从电荷转移方面分析了氢和氦与Er的相互作用, 发现六角相ErH_x的弹性性质随H浓度的变化机理与ErHe_x随He浓度的变化机理不同. 关键词： 第一性原理 弹性性质 x')" href="#">六角相ErH_x x')" href="#">六角相ErHe_x     

Spin fluctuations in weakly itinerant ferromagnets Y(Co1−xAlx)2

The nuclear spin-lattice relaxation rate, 1/T₁, has been measured in weak itinerant ferromagnets Y(Co_1−xAl_x)₂. The temperature and magnetic field dependence of 1/T₁T has been found to be well described by the self-consistent renormalization (SCR) theory of spin fluctuations. The parameters characterizing spin fluctuations in this system were estimated from NMR and magnetic measurements. The temperature dependence of susceptibility calculated from these parameters well reproduces the experimental results.     

Magnetic behavior of Pr1 − xMxCo5 compounds (M = Ti, Zr and Hf)

Magnetic characteristics of Ti-, Zr- and Hf-substituted PrCo₅ alloys have been studied over the temperature range from 77 to 300 K and for applied fields up to 20 kOe. It is established that Ti, Zr and Hf substitute for Pr. Single-phase materials are formed for all values of x up to 0.2 in the system Pr_1−xZr_xCo₅ but for x only up to 0.1 for Pr_1−xTi_x Co₅ and Pr_1−xHfxCo₅ alloys. Larger amounts of Zr can be substituted if the material is made hyperstoichiometric in Co, e.g., Pr_0.7Zr_0.3Co_5.5. All the alloys show a decrease in magnetic moment and an increase in Curie temperature as x increases. Anisotropy fields decrease as x increases at 295 K. Anomalous behavior is observed at 77 K, suggested that these ternary alloys may have a cone structure at this temperature.     

Specific heat measurements of (Ho, Y)Co2 and (Dy, Y)Co2

We report on concentration- and temperature-dependent specific heat measurements of (Dy_xY_1−x)Co₂ in the temperature range from 1.5 to 60 K in comparison with recent results of (Ho_xY_1−x)Co₂. The magnetic contribution to the specific heat, C_m, is analysed with regard to the appearance of spin-fluctuations and in terms of spin-glass phenomena. The loss of magnetic entropy in the dilute RE-concentration range indicates presumably an instability of the RE-moments.     

On the appearance of ferromagnetism in Y(Co1−xAlx)2 alloys

The appearance of ferromagnetism in Y(Co_1−xAl_x)₂ is discussed in terms of a d band model. The approximate d bands for YCo₂ and Y(Co_1−x)₂ are calculated and the decrease in the electronic energy due to magnetization of the spin of estimated. The energy decrease is the largest in YCo₂, and it gradually decreases as the Al content increases, if the lattice constant is fixed, while this energy decrease increases if the lattice constant increases with increasing Al content. These results of calculations give a good account of the appearance of ferromagnetism in Y(Co_1−xAl_x)₂ around x = 0.15. The ferromagnetism in Sc(CO_1−xAl_x)₂ is also discussed, leading to the appearance of ferromagnetism between x = 0.15 and 0.30.     

Photoluminescence study of proton-implanted InP1−xAsx:Yb

Photoluminescence (PL) studies of low-energy (60 keV, H⁺₂), proton-implanted InP_1-xAs_x (0 x 17) crystals doped with Yb are reported. In the implanted samples we observed remarkable intensity reduction of all the PL lines. After annealing, the PL spectra did not recover to their preirradiation values indicating high thermal stability of the generated damage. We have not seen any influence of hydrogen or lattice defects on the shape of the Yb intra-4f-shell luminescence. We show that hydogen-implantation-induced defects stabilize surfaces of InP and InPAs compounds, preventing their decomposition during capless thermal annealing up to 650°C. It is suggested that this enhanced surface stabilization is due to the presence of defects saturated with hydrogen atoms which are bound tightly to the phosphorus atoms.     

A solid-state V NMR characterization of vanadium sites in LiCoxNi1−xVO4

LiCo_xNi_1−xVO₄ compounds (x=0, 0.2, 0.5, 0.8, 1.0) have been prepared for lithium ion batteries by different preparation methods, namely, wet-chemistry (WC) and solid-state (SS) routes. ⁵¹V nuclear magnetic resonance (NMR) has been employed to accurately assess the vanadium environments in these materials and results show that vanadium atoms are distributed in octahedral and tetrahedral sites. The fractions of vanadium atoms in tetrahedral/octahedral sites depends on composition and sample preparation conditions. The tetrahedral site, which is particularly dominant in samples produced via the WC route, is more affected by paramagnetic interactions and its ⁵¹V NMR spectrum is essentially Gaussian in shape. The octahedral site, on the other hand, exhibits first-order quadrupolar broadened satellites. Even though tetrahedral sites for a given x are comparable for WC and SS materials, there are some differences amongst octahedral environments, namely in the distributions of quadrupolar broadened satellites. The larger satellite distribution observed for WC materials is indicative of a larger distribution of structural defects for these as compared to SS materials.     

Ferromagnetic Zr1−xMnxCo2 laves phase compounds

Structure and magnetic properties of the Zr_1−xMn_xCo_2+δ alloys were studied for 0 x <0.7, δ=0, 0.45. The cubic C15 Laves phase structure shows Mn solubility up to x≈0.4. The other Laves phase with the hexagonal C36 structure found for x0.5 apparently has a small region of Mn solubility in the vicinity of Zr_0.4Mn_0.6Co₂. Though the parent Mn-free compounds are known to be paramagnetic, the Mn-substituted alloys show ferromagnetic behavior with the Curie temperatures up to 625 K and the room-temperature saturation magnetization of about 100 emu/g. The onset of ferromagnetism with the Mn substitution for Zr may be caused by polarization of itinerant 3d electrons, like it was earlier supposed for the off-stoichiometric ZrCo_2+δ. The universal composition dependencies of the intrinsic magnetic properties for different δ can be obtained, if plotted against the amount of zirconium atoms missing in its sublattice. The room-temperature anisotropy with the noticeable anisotropy field of 24 kOe and the 1 1 0 easy magnetization direction laying in a basal plane was found in the hexagonal Zr_0.5Mn_0.5Co₂.     

Oxide-ion conductivity of (Ba1−xLax)2In2O5+x system based on brownmillerite structure

(Ba_1−xLa_x)₂In₂O_5+x, whose end member is Ba₂In₂O₅, is an oxygen-deficient perovskite oxide showing high oxide-ion conductivity. In order to clarify the reason why the high oxide ion conductivity appeared in this system, the electrical conductivity was measured as a function of temperature and La content. With an increasing La content, the discontinuous jump of ion conductivity in the Arrhenius plot, which is related to the disordering of the oxygen vacancies, disappeared for the sample with x0.2. Above x=0.12, the ion conductivity linearly increased with La content, while the activation energy remained constant with respect to the La content. Moreover, the conductivity for x=0.6 was 0.042 (S/cm) at 1073 K, which exceeded that of 8 mol% yttria-stabilized zirconia. The higher oxide-ion conductivity of this system could be dominated by the amount of mobile oxygen ions.     

NMR study of coexistence of ferro- and antiferromagnetism in (Zr1−xNbx)Fe2

The magnetic phase diagram has been investigated in the C14 type (Zr_1−xNb_x)Fe₂ with x0.7 from ⁹³Nb NMR and magnetization measurements. In the compound with x = 0.825 a first order-like transition has been found to occur around 25 K from a canted state with the ferromagnetic moment in the basal plane to a ferromagnetic state with decreasing temperature.     

The crystal structure of (Pb0.5Cd0.5)Sr2 (Y1−xCax)Cu2O7

The crystal structures of (Pb_0.5Cd_0.5)Sr₂YCu₂O₇, (Pb_0.5Cd_0.5)Sr₂(Y_0.6Ca_0.4)Cu₂O₇ and (Pb_0.5Cd _0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O₇ have been refined by the Rietveld method for the X-ray diffraction data. The refinement results indicate that both Pb and Cd atoms in the (Pb, Cd)O layers and O(3) atoms are displaced from their ideal sites, and that there is a complicated occupation in the (Pb, Cd)O layers, i.e., other cations, such as Sr²⁺, Cu²⁺ and Ca²⁺, can occupy the (Pb, Cd) site for these samples. This may be the reason why the lattice constants do not vary monotanically with the calcium content, and why there exist broad superconducting transitions in the system (Pb_0.5Cd_0.5)Sr₂(Y_1−xCa_x)Cu₂O₇. The refinement results also indicate that, as the calcium content increases, the O(2) atoms move close to the CuO₂ planes and far from the (Pb, Cd)O layers. Thus, the increase of calcium content results in charge transfer from the charge reservoir layer (Pb, Cd)O to the CuO₂ planes. This result is consistent with the bond valence sums of the Cu ions in the CuO₂ planes.     

Superconductivity and crystallographic properties of La2 − xMxCuO4 − δ (M = Na, K)

Superconductivity and crystallographic properties of La_{2 − x}M_xCuO_{4 − δ} (M = Na, K) are studied. In the La_{2 − x}M_xCuO_{4 − δ} system, superconductivity is detected for x 0.2. Oxygen content analysis shows that the system has more oxygen vacancies than the La_{2 − x}Sr_xCuO_{4 − δ} system. These oxygen vacancies may reduce the hole concentration, and high Na-doping is needed to produce superconductivity. In the La_{2 − x}K_xCuO_{4 − δ} system, superconductivity is observed for the first time. Resistivity and magnetic susceptibility measurements show that T_c(onset) is 40 K and the Meissner volume fraction is about 4% for x = 0.7. The system changes from orthorhombic to a tetragonal K₂NiF₄ structure at x ≈ 0.3 and only tetragonal samples show superconductivity.     

NMR study of hyperfine interactions in R1−xLuxFe2 (R = Gd, Tb)

Spin-echo NMR measurements on ¹⁵⁵Gd, ¹⁵⁹Tb and ¹⁷⁵Lu have been done for the cubic Laves phase compounds Gd_1−xLu_xFe₂ and Tb_1−xLu_xFe_x. The observed hyperfine fields of Gd and Lu in those compounds increase with increasing Lu concentration. The concentration dependence of the Lu hyperfine field is qualitatively discussed.