Double peak behavior of resistivity curves in Cd doped LaMnO3 perovskite systems

The effect of Cd doping on transport, magnetotransport, and magnetic properties has been investigated in the perovskite La_1−xCd_xMnO₃ (0x0.5) systems. The ρ(T) curves exhibit a sharp metal insulator transition (T_p1), which is close to paramagnetic to ferromagnetic transition (T_c) obtained from M−T curves for all samples. In addition, ρ(T) curves for Cd doped samples exhibit another broad transition (T_P2) below T_p1. This transition becomes more prominent and the transition temperature (T_p2) shifts to lower temperature with increasing Cd content. Such double peak behavior in the ρ(T) curve is attributed to the phase separation between the ferromagnetic metallic phases and the ferromagnetic insulating phases induced by the electronic inhomogeneity in the samples.     

Oxygen overdoping in superconducting and non-superconducting Y1−xPrxBa2Cu3Oy

We present the evolution of magnetic and structural properties of Y_1−xPr_xBa₂Cu₃O_y (x0.5 and x=1) single crystals and polycrystalline materials when the oxygen concentration y is varied from under- to overdoping. We have found a monotonous evolution of the Pr Néel temperature for x=1 samples and a maximum of the superconducting critical temperature for the x0.5 samples. The structural properties as detected by X-ray diffraction and Raman spectroscopy show no instabilities when crossing the optimal doping region as was found in the x=0 material.     

Disruption of antiferromagnetic order in Zn-doped La2CuO4

The magnetic phase diagram of La₂(Cu_1−xZn_x)O₄ has been determined from zero-field muon-spin-rotation (ZF-μSR) data taken at LAMPF for 0 ≤ x ≤ 0.10. Antiferromagnetic onset temperatures follow T_N(x) from susceptibility measurements on the same samples. However, the order becomes long range, as evidenced by a well-defined internal magnetic field, only at temperatures well below T_N. Extrapolation of our results yields T_N → 0 K at x = 0.11 for a single (Cu_1−xZn_x)O₂ plane, and comparison with YBa₂(Cu_1−xZn_x)₃O₆ implies identical disruption of magnetism by Zn doping in the single- and double-plane systems.     

Normal-state transport properties of Ba1−xKxBiO3 crystals

The normal-state transport properties of Ba_1−xK_xBiO₃ crystals with a wide range of potassium compositions (0≤x≤0.62) were studied. Although the host material BaBiO₃ has a monoclinic structure, the system changes from a monoclinic to an orthorhombic structure with a small doping of potassium (0≤x<0.35) and behaves similar to a doped semiconductor, without exhibiting superconductivity. In the composition range, holes are majority carriers in the transport phenomena. When x exceeds a critical value (0.35), the system goes into a cubic superconducting phase with a single metallic band. The vicinity of the critical composition transport phenomena is easy to understand assuming the existence of two conducting channels that are made up of metallic and semiconducting phases. Maximum T_c exceeding 30 K was observed at x0.4, where carrier density was at its maximum. Overdoping with potassium suppresses superconductivity. In the metallic composition of x>0.45, transport seems to correlate with the phonon mode with an energy distribution of 15–43 meV.     

Effect of Sn doping on the magnetic and transport properties of LaMnO3+δ

We report the influence of the Sn doping on the magnetotransport properties of the LaMnO_3+δ perovskite. Two series of samples with nominal LaSn_xMn_1−xO_3+δ (I series) and La_{(1−x)/(1+x)}Sn_xMn_1−xO_3+δ (II series) compositions (x=0, 0.025, 0.05 and 0.10) were prepared at T_s=750°C. The M(T) data under 0.01 and 0.5 T for the I series reveal a depressed magnetization as the Sn content increases suggesting the presence of magnetic clusters with a superparamagnetic behavior. Resistivity measurements indicate an insulator material for all Sn content independently of the applied magnetic field. On the contrary, for the II series the M(T) and M(H) data reveal FM behavior and an improvement of the magnetization as Sn increases. These samples show magnetoresistance. The magnetotransport properties are discussed in terms of the presence of A-site cation vacancies.     

Magnetic and electromagnetic properties of RuZn and RuCo substituted BaFe12O19

Polycrystalline samples of M-type hexaferrites BaFe_12−2xRu_xZn_xO₁₉ and BaFe_12−2xRu_xCo_xO₁₉ with 0x0.45 have been prepared by a classical sintering method. The evolutions with x of the cell parameters, the saturation magnetization and the magnetic transition temperature have been measured; in this range of small doping ratios, saturation magnetization and Curie temperature of substituted hexaferrites remain close to those of the undoped BaFe₁₂O₁₉. X-ray diffraction measurements on oriented powders show that a change of magnetocrystalline anisotropy from axial to planar occurs in both cases for a small substituting ratio x_c=0.375. Microwave electromagnetic characteristics have been studied on the ceramic samples from 0.1 to 10 GHz. The behaviour of the magnetic losses (μ″) corroborates the anisotropy change when doping; a convolution of the dissipation mechanisms (domain wall motions and gyromagnetism) is obtained for x_c. The level of the magnetic losses is discussed in relation with others substituted Ba-hexaferrites.     

Effect of Pr and Ca substitution on superconductivity in Y(1−x−yPrxCay)Ba2Cu3O7−δ

Pr substituted at constant Ca concentration for Y in (Y_1−x−yPr_xCa_y)Ba₂Cu₃O_7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of T_c decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of T_c cannot be compensated by appropriate hole doping with Ca.     

The relationship between crystal structure and electrical conductivity in the LaY1−xInxO3 (x=0.0–0.7) system

The electrical conductivity of the LaY_1−xIn_xO₃ (x=0.0–0.7) system has been studied from the viewpoint of crystal chemistry. The high temperature form of LaYO₃ (x=0.0) was ascertained to be the Sm₂O₃-type (B-type rare earth) structure, not perovskite-type one. The X-ray diffraction (XRD) experiments revealed that the samples with x=0.05 and 0.10 were the mixed phase of Sm₂O₃-type and perovskite-type structure, and changed to perovskite phase in the range of x0.20. From oxygen partial pressure dependence of the electrical conductivity, it was found that both the Sm₂O₃-type and the perovskite-type single phases showed hole conduction, but the mixed phase did oxide-ion one. The electrical conductivity of the LaY_1−xIn_xO₃ (x=0.0–0.7) system increased with increasing x, and showed the maximum value in the range of x=0.05–0.10, and then decreased with increasing x. The occurrence of oxide-ion conduction was discussed from the viewpoint of lattice distortion in the mixed phase.     

Pr含量对Bi_5Fe_(0.5)Co_(0.5)Ti_3O_(15)室温多铁性的影响

采用改良的固相烧结工艺制备了Bi5-xPrxFe0.5Co0.5Ti3O15(BPFCT-x,x=0.25,0.50,0.75,0.80)陶瓷样品.X射线衍射结构分析表明:镨(Pr)含量对样品微观结构产生了影响,但所有样品均为层状钙钛矿结构;BPFCT-x样品的剩余极化强度(2Pr)随着掺杂量的增加呈现出先增大后减小的变化趋势,当Pr含量为0.75时,样品的2Pr达到最大值,为6.43μC/cm2.样品的磁性与铁电性能具有相同的变化规律,室温下样品的剩余磁化强度(2Mr)也呈现出先增大后减小的趋势,并且也在x=0.75时达到最大为0.097 emu/g.随着Pr掺杂量增大,样品的室温下铁电和铁磁性能得到明显改善,并且当掺杂量为0.75时,样品室温多铁性最好.Pr掺杂降低了样品中的缺陷浓度,从而提高了样品铁电畴动性,这有助于提高样品铁电性能.而样品铁磁性能的改善可能与Pr对样品晶格畸变产生的影响有关.     

Non-Fermi liquid regimes in U1−xMxPd2Al3(M=Y,Th)

The temperature-composition (T–x) phase diagram and NFL characteristics in the electrical resistivity ρ(T), specific heat C(T), and magnetic susceptibility χ(T) at low temperatures for the systems U_1−xM_xPd₂Al₃ (M=Y,Th) are described. The T–x phase diagram, the NFL characteristics, and the underlying mechanism for the NFL behavior are distinctly different for M=Y³⁺ and Th⁴⁺, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.     

Substitution of Sn in the superconducting Bi---Pb---Sr---Ca---Cu---O system

X-ray powder diffraction, magnetic susceptibility and electrical resistivity measurements have been used to investigate the effect of Sn substitution in the Bi_1.7Pb_0.3Sr₂Ca₂(Cu_3−xSn_x1.2O_y system for different values of Sn concentrationx(0×1.33). The substitution of Sn is found to decrease the superconducting volume fraction of the 2223 phase. The 2212 phase is enhanced until it reaches a maximum at x=0.67. The system has a complete 2212 phase at x=0.88. An increase in the normal-state resistivity with a corresponding suppression of the temperature at which the resistivity goes to zero is observed with increasing x for the samples which are superconducting. The substitution of Sn probably causes a spatial disorder in the CuO₂ planes, which in turn enhances the Coulomb interaction. Samples with x1.0 are found to be insulating.     

Crystal structure, superconductivity and magnetic properties of the superconducting ferromagnets Gd1.4−xDyxCe0.6Sr2RuCu2O10 (x=0–0.6)

The structural, electrical and magnetic properties of the superconducting ferromagnets, Gd_1.4−xDy_xCe_0.6Sr₂RuCu₂O₁₀ (x=0–0.6) are systematically investigated as a function of Dy doping and temperature. These compounds are characterized by high temperature superconductivity (T_c ranging from 20 to 40 K depending upon the Dy content) co-existing with weak ferromagnetism with two magnetic transitions (T_M2 ranging from 95 to 106 K and T_M1 around 120 K). Doping with Dy gives no significant structural changes except for a minor change in the c/a ratio. However the superconducting transition temperature is significantly suppressed and magnetic ordering temperature enhanced on Dy doping. These effects are described and discussed.     

钙钛矿锰氧化物(La1-xGdx)4/3Sr5/3Mn2O7 (x=0, 0.025) 磁性和输运性质研究

采用传统的高温固相烧结法制备了双层钙钛矿锰氧化物(La_1-xGd_x)_4/3Sr_5/3Mn₂ O₇ (x=0, 0.025)多晶样品. 通过X射线衍射仪研究发现样品为Sr₃Ti₂O₇型四方结构, 空间群为I4/mmm; 磁性测量表明, Gd³⁺掺杂后的样品(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂O₇的三维磁有序转变温度(T_C1^3D)、磁化强度(M)均降低, 这是由于Gd³⁺的掺杂引起晶格的畸变, 从而使得晶格常数发生改变, 减弱了铁磁耦合而导致的; 通过电子自旋共振谱测量发现, 在T_C^3D<T<300 K温度范围内, 两样品在顺磁的基体上均有短程的铁磁团簇存在, 出现了相分离现象. 电性测量表明: 两样品分别在T_C1^3D (La_4/3Sr_5/3Mn₂O₇ 样品的三维磁有序转变温度, T_C0^3D)<T<300 K温度范围内均以三维变程跳跃的方式导电, 分析得出Gd³⁺的掺杂使得载流子局域长度的减小. 这表明载流子需要吸收更多的能量才能克服晶格的束缚进行跳跃, 因此(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂ O₇ 样品的电阻较高.     

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

The maximum solid solubility of gallium in the perovskite-type La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.40–0.80; y=0–0.60) was found to vary in the approximate range y=0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O_3−δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10⁻⁶ K⁻¹ at 300–800 K and (19.3–26.7)×10⁻⁶ K⁻¹ at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.7–0.8; y=0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La_1−xSr_xFe_1−yGa_yO_3−δ increases with strontium content up to x=0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.     

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.     

Resistivity and thermoelectric power of Bi2Sr2Ca−xPrxCu2Oy system

Samples of Bi₂Sr₂Ca_1−xPr_xCu₂O_y have been characterized by resistivity and thermoelectric power measurements. All metallic samples show superconductivity with a maximum T_c = 90 K at X = 0.2. The sample of x = 0.6 shows a crossover from hopping conduction at low temperature above T_c to metallic conduction at high temperature. For the metallic samples below x = 0.6, the results of thermoelectric power are well fitted by both of a phenomenological band spectrum model and the Nagaosa and Lee model.     

Sr掺杂对CaMnO_3基氧化物电子性质及热电输运性能的影响

采用密度泛函理论计算分析的方法研究了Ca位Sr掺杂的CaMnO_3基氧化物的电子性质和电性能;采用柠檬酸溶胶-凝胶法结合陶瓷烧结制备工艺制备了Ca位Sr掺杂的CaMnO_3基氧化物块体试样,分析研究了所得试样的热电传输性能.结果表明,Sr掺杂CaMnO_3氧化物仍然呈间接带隙型能带结构,带隙宽度由0.756 eV减小到0.711 eV.Sr掺杂CaMnO_3氧化物费米能级附近的载流子有效质量均得到调控,载流子浓度也有所增大.Sr比Ca具有更强的释放电子能力,其掺杂在CaMnO_3氧化物中表现为n型.Sr掺杂的CaMnO_3基氧化物材料电阻率大幅度降低,Seebeck系数绝对值较本征CaMnO_3基氧化物材料有一定程度的增大,Sr掺杂量为0.06和0.12的Ca_(1-x)Sr_xMnO_3(x=0.06,0.12)试样,其373 K的电阻率分别降低至本征CaMnO_3基氧化物材料的25%和21%,其373 K的Seebeck系数绝对值分别是本征CaMnO_3基氧化物材料的112.9%和111.1%,Sr掺杂有效提高了CaMnO_3基氧化物材料的热电性能.     

Lithium insertion into Li4V3O8

The lithium insertion characteristics of lithium vanadate, Li₄V₃O₈, were investigated using LiV₃O₈ prepared by the precipitation technique as the starting material. The Li₄V₃O₈ phase was formed by lithiation over x=1.5 in Li_1+xV₃O₈, and the diffusion of lithium in this phase determined the reaction rate of insertion more than x=1.5. Improvement of insertion kinetics in the Li₄V₃O₈ phase extended the lithium insertion limit from x=3.2 to x=4.0, compared with the case of LiV₃O₈ by conventional high temperature synthesis. Lithium insertion proceeds as the single-phase reaction in the range of 3.2<x<4.0.     

Synthesis and structural characterisation of YSr2Cu3−xMxO7±y (M = Fe, Ti, Al, Co, Ga, Pb; 0.5 < x < 1), and the non-existence of the parent phase YSr2Cu3O7

The possibility of synthesising phases of general composition YSr₂Cu_3−xM_xO_7±y has been examined for a wide variety of metals M. Single phase tetragonal products (typically a = 3.83 Å, c = 11.5 Å) can be synthesised provided that the concentration of M is sufficiently large (x > 0.5), and M has a preference for the Cu(1) sites (the four-coordinate sites) of the YBa₂Cu₃O₇, structure. Neutron diffraction has demonstrated that a high degree of disorder exists in the (001) planes containing the M cations and, in particular, significant oxygen displacements are found. The non-existence of the unsubstituted variant YSr₂Cu₃O₇ is explained in terms of the high degree of compressive stress on the Cu(1) sites which could exist in such a material.     

Simultaneous doping with La and Y for Ba- and Ce-sites in BaCeO3 and the ionic conduction

Powder X-ray diffraction (XRD) analysis showed that the single phase perovskite-type structure of Ba_1−xLa_xCe_0.90−xY_0.10+xO₃₋ (0 x 0.40, =0.05) could be maintained in a wide region of doping level by simultaneous partial substitution of La³⁺ for Ba²⁺-site and Y³⁺ for Ce⁴⁺-site in BaCeO₃. The conduction properties of these oxides were investigated using various electrochemical methods in the same concentration of oxygen vacancy (=0.05). At high oxygen partial pressure, these oxides exhibited a mixed oxide ionic and p-type electronic conduction while at low oxygen partial pressure their conduction was almost protonic. Among these oxides, BaCe_0.90Y_0.10O₃₋ exhibited the highest conductivities with a value of 1.24×10⁻¹ S/cm in dry oxygen, and 5.65×10⁻² S/cm in wet hydrogen at 1000°C. Both of the proton and oxide ion conductivities under oxygen and under hydrogen atmospheres decreased monotonically with the increasing substitution for Ba²⁺- and Ce⁴⁺-sites. The decreases in ion conductivities appear to relate to the decreased free volume (V_f) of crystal lattice as well as the increased distortion of lattice from ideal cubic perovskite structure.