Linear and nonlinear AC susceptibility studies in La(Mn1−xCux)O3

The linear and nonlinear AC susceptibility as a function of temperature were measured on LaMn_1−xCu_xO₃ compounds for x=0.05–0.30$x=0.05–0.30$. Samples with x?0.10$x?0.10$ exhibit paramagnetic to ferromagnetic transitions followed by low temperature spin glass like transition. The linear susceptibility exhibits strong frequency dependence and is analyzed in terms of standard theoretical model for spin glass. The magnitude and peak temperature of nonlinear susceptibility vary with AC field amplitudes. They are analyzed in terms of critical behaviour in the vicinity of spin glass transition temperature and the critical exponent is found to be 3.2.     

Structure, magnetic and transport properties of La1−xBixMnO3

We have investigated the structural, magnetic and transport properties of La_1−xBi_xMnO₃ samples. As the Bi content increases, a structural transition from rhombohedral to pseudocubic and a magnetic phase transition from ferromagnetic ordering to cluster glass are identified. Metal–insulator (MI) transitions and large magnetoresistance (MR) effects are observed at low Bi doping levels, while insulating behavior of resistivity is found in the whole measured temperature range at high-doping levels. Two distinct ferromagnetic insulating (FI) states are found at low temperatures in this system. One can be suppressed and the other can be enhanced by applying magnetic fields. Possible reasons for the observed structural, magnetic phase transitions and changes of resistivity behavior with Bi doping are discussed.     

Structural and lithium intercalation studies of Mn(0.5−x)CaxTi2(PO4)3 phases (0≤x≤0.50)

Materials from the Mn_(0.5−x)Ca_xTi₂(PO₄)₃ (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, ³¹P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R space group. Mn²⁺/Ca²⁺ ions occupy only the M1 sites in the Ti₂(PO₄)₃ framework. The divalent cations are ordered in one of two M1 sites, except for the Mn_0.50Ti₂(PO₄)₃ phase, where a small departure from the ideal order is observed by XRD and ³¹P MAS NMR. The electrochemical behaviour of Mn_0.50Ti₂(PO₄)₃ and Mn_(0.5−x)Ca_xTi₂(PO₄)₃ phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti₂(PO₄)₃ framework. In the 0≤y≤2 range, the OCV curves of Li//Li_yMn_0.50Ti₂(PO₄)₃ cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li_(1+y)Ti₂(PO₄)₃ and Li//Li_yMn_0.50Ti₂(PO₄)₃ shows that the intercalation occurs first in the unoccupied M1 site of Mn_0.50Ti₂(PO₄)₃ at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn_0.50Ti₂(PO₄)₃ on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn_0.50Ti₂(PO₄)₃, the stability of Mn²⁺ either in an octahedral or tetrahedral environment facilitates cationic migration.     

Magnetic properties of transition metal substituted La0.85Ag0.15Mn1−yMyO3 compounds (M=Co, Cr and Al)

In this paper we report a systematic study of Mn-site substitution by M=Co, Cr and Al in La_0.85Ag_0.15MnO₃ series to understand the magnetic interactions between Mn and other transition metals. The long-range ferromagnetic (FM) ordering of the parent compound was significantly affected by Mn-site substitution. The measured magnetic properties of Co-doped samples have been explained on the basis of FM interactions in Mn³⁺-O-Mn⁴⁺, Co²⁺-O-Mn⁴⁺, Co³⁺-O-Mn⁴⁺ networks and simultaneous antiferromagnetic (AFM) interactions in Mn⁴⁺-O-Mn⁴⁺, Co²⁺-O-Mn³⁺ networks. The magnetic properties of Cr-doped compounds could be understood on the basis of double exchange FM interactions in Mn³⁺-O²⁻-Mn⁴⁺ networks and competing AFM in Cr³⁺-O-Mn⁴⁺, Mn⁴⁺-O-Mn⁴⁺, Cr³⁺-O-Mn³⁺ networks. However, it is found that the doping of Al ions play a role of magnetic dilution, without contributing any other competing magnetic interaction. The field variations of magnetization of all the above three series could be analysed by fitting to Brillouin function model and the effective spin contribution for FM has been determined. The measured saturation magnetization has been explained quantitatively.     

Dopant-site occupancies and superconducting properties of the Fe-substituted La2.5Nd0.5CaBa3(Cu1−xFex)7Oz perovskite

The structural and superconducting properties of single-phase Fe-substituted La_2.5Nd_0.5CaBa₃(Cu_1−xFe_x)₇O_z (LNCBCuFe) with 0.0x0.06 compounds having triple-perovskite structure are investigated using X-ray diffraction, a.c. susceptibility, d.c. magnetization, oxygen content and Mössbauer effect measurements. Mössbauer spectral analysis of x=0.03 sample displays unusual Fe-dopant site occupancies and the Cu(2) plane to Cu(1) chain site ratio in the LNCBCuFe are quite different from those of the usual Fe-doped YBa₂Cu₃O_7−δ. Specifically, we observe substantial occupation of a new chain-associated quasi-octahedral site, E, at 300 K which transforms into the well-known distorted tetrahedron chain site, A, on lowering the temperature to 78 K. The observed reduction of T_c with increasing x in LNCBCuFe supports the view that the hole filling mechanism contributes predominantly to the suppression of superconductivity by Fe.     

Structure, ferroelectric and piezoelectric properties of (Bi1−x−yNa0.925−x−yLi0.075)0.5BaxSryTiO3 lead-free piezoelectric ceramics

Lead-free multi-component ceramics (Bi_1−x−yNa_{0.925−x−y}Li_0.075)_0.5Ba_xSr_yTiO₃ have been prepared by an ordinary sintering technique and their structure and electrical properties have been studied. All the ceramics can be well-sintered at 1100 °C. X-ray diffraction patterns shows that Li⁺, Ba²⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) is formed at 0.04 < x < 0.08. As compared to pure Bi_0.5Na_0.5TiO₃ ceramic, the coercive field E_C of the ceramics decreases greatly and the remanent polarization P_r of the ceramics increases significantly after the formation of the multi-component solid solution. Due to the MPB, lower E_C and higher P_r, the piezoelectricity of the ceramics is greatly improved. For the ceramics with the compositions near the MPB (x = 0.04–0.08 and y = 0.02–0.04), piezoelectric coefficient d₃₃ = 133–193 pC/N and planar electromechanical coupling factor k_P = 16.2–32.1%. The depolarization temperature T_d reaches a minimum value near the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions at temperatures near/above T_d.     

Superparamagnetic properties of La1−xBaxCoO3 (0.0<x<0.18)

The static and dynamic magnetic properties of La_1−xBa_xCoO₃ (0<x<0.18) (LBCO) have been investigated by the dc and ac susceptibility measurements systematically. The spin-glass-like characteristics of magnetic behavior, which are always considered as an evidence of spin glasses for La_1−xSr_xCoO₃ (0<x<0.18) (LSCO), have been observed. However, just like that of LSCO, some magnetic behaviors, which are different from the canonical spin-glass characteristics, for example, the difference far above peak temperature T_p of zero-field-cooled curve between field-cooled magnetization (FCM) and zero-FCM (ZFCM) and the very broad peaks in the samples for x>0.10, are also observed simultaneously. Based on the comparisons with canonical spin-glass Au₉₆Fe₄ alloy and conventional superparamagnetic (SPM) Cu₉₇Co₃ alloy, the nature of the magnetic behaviors in LBCO is ascribed to the superparamagnetic cluster rather than spin glass. Though the magnetic properties behave as SPM, LBCO has one critical point different from that of the conventional SPM Cu₉₇Co₃ alloy. The segregated Co nanoparticles in the Cu₉₇Co₃ alloy are isolated by nonmagnetic background while the clusters in LBCO are embedded in an antiferromagnetic matrix. The SPM behavior of LBCO can only exist at low temperature because the antiferromagnetic matrix transfers to paramagnetic phase at Neél temperature T_N and the SPM cluster turns into ferromagnetic (FM) above T_N.     

Mg掺杂对Li(Co,Al)O2电子结构影响的第一原理研究

基于密度泛函理论的第一原理赝势法，研究了Mg在Li(Co，Al)O2中掺杂前后的电子结构的变化.通过能带和态密度的分析,发现Mg掺杂后在价带中引入了电子空穴，同时价带展宽，这两个电子结构的显著变化是引起Li(Co，Al)O2导电率提高的主要机理.通过对Co3d电子态密度的分析发现，在二价Mg掺杂后，Li(Co，Al)O2中的Co价态升高，介于Co3+和Co4+之间.从能带计算出发，进一步定量给出了Co和O的平均价态的变化. 关键词： Li(Co Al)O2 电子结构 第一原理 电导     

Crystal orientation dependence of the in-plane dielectric properties for Ba(Sn0.15Ti0.85)O3 thin films

Barium tin titanate Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films with (1 0 0), (1 1 0) and (1 1 1) orientation were grown on (1 0 0), (1 1 0) and (1 1 1) LaAlO₃ (LAO) single-crystal substrates through sol–gel process, respectively. The in-plane dielectric properties of the films were measured on interdigital capacitor (IDC). Films with the (1 1 1) orientation had larger relative dielectric constant and larger tunability against the dc bias electric field than (1 0 0)- and (1 1 0)-oriented films. This difference in dielectric properties in these three kinds of oriented BTS films may be attributed due to change in the direction and magnitude of electric polarization in orientation engineered BTS films. This work clearly reveals the dielectric properties of BTS films exhibited a strong sensitivity to crystal orientation.     

Piezoelectric properties of [Li0.03(K0.48Na0.52)0.97](Nb0.97Sb0.03)O3-(Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 lead-free piezoelectric ceramics

[Li_0.03(K_0.48Na_0.52)_0.97](Nb_0.97Sb_0.03)O₃-(Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ [(1−x)LKNNS-xBCTZ] lead-free piezoelectric ceramics were prepared by the conventional solid state method, and effects of BCTZ content on the piezoelectric properties of LKNNS ceramics were mainly investigated. A stable solid solution has been formed between LKNNS and BCTZ, and a morphotropic phase boundary of (1−x)LKNNS-xBCTZ ceramics is identified in the range of 0 < x ≤ 0.02. The Curie temperature of (1−x)LKNNS-xBCTZ ceramics decreases with increasing BCTZ content. A higher ?_r value and a lower tan δ value are demonstrated for the (1−x)LKNNS-xBCTZ ceramic with x = 0.02. The (1−x)LKNNS-xBCTZ ceramic with x = 0.02 has an enhanced electrical behavior of d₃₃ ∼ 237 pC/N, k_p ∼ 48.6%, ?_r ∼ 1451, tan δ ∼ 0.037, and T_c ∼ 335 °C. As a result, (1−x)LKNNS-xBCTZ ceramics are promising candidate materials for the field of lead-free piezoelectric materials.     

Magnetic properties of Gd2(Fe1−x−yCoyTix)17

Serial single-phase Gd₂(Fe_1−x−yCo_yTi_x)₁₇ compounds have been synthesized. These compounds have a crystal structure belonging to rhombohedral lattice with space group. The lattice parameters of compounds decrease with cobalt content and increase with titanium content, respectively. The saturation magnetization decreases with increasing cobalt and titanium contents. The anisotropy fields increase to maximum then decrease with cobalt concentration. The magnetocrystalline anisotropy constants increase with cobalt content from negative to positive maximum and then decrease with Co concentration. The saturation moment of the compounds decreases linearly with cobalt concentration and decreases nonlinearly with titanium concentration.     

Neutron-powder-diffraction study of structural and magnetic structure of La0.7Sr0.3Mn1−xTixO3 (x=0, 0.10, 0.20, and 0.30)

Neutron-diffraction and magnetic measurements have been used in order to study the structure and magnetic changes of La_0.7Sr_0.3Mn_1−xTi_xO₃ (x=0, 0.10, 0.20, and 0.30) perovskites. Magnetic measurements show that our samples exhibit a ferromagnetic–paramagnetic transition with decreasing temperature. Effects of Ti doping on the magnetic and nuclear structures are studied here by neutron-powder-diffraction for La_0.7Sr_0.3Mn_1−xTi_xO₃ (x=0.10, 0.20, and 0.30). Our analysis indicates that the three representative samples, with x=0.10, 0.20, and 0.30, have a hexagonal system of R3¯c space group, and that the magnetic moments order ferromagnetically into the (1 0 1) crystallographic planes, i.e. the magnetic moments of the Mn ion are in the ac plane, with no component along the b-axis. There is no crystalline structure changes when the Ti doping level increases.     

Magnetocaloric and magnetoresistance properties of La2/3Sr1/3Mn1−xCoxO3 compounds

Structural, magnetic, magnetoresistance and magnetocaloric studies on La_2/3Sr_1/3Mn_1−xCo_xO₃ compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |ΔS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples.     

Oxygen permeability of La2Cu(Co)O4+δ solid solutions

Formation of the La₂Cu_1−xCo_xO_4+δ solid solutions with orthorhombic K₂NiF₄-type structure was found to be in the range of 0≤x≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La₂Cu_1−xCo_xO_4+δ (x=0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10⁻⁶ K⁻¹. Studying the dependence of oxygen permeation fluxes through La₂Cu(Co)O_4+δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La₂Cu_1−xCo_xO_4+δ solid solutions was ascertained to increase with cobalt concentration at x=0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport.     

New red Y0.85Bi0.1Eu0.05V1−yMyO4 (M=Nb, P) phosphors for light-emitting diodes

The Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) as new near-ultraviolet excited phosphors were synthesized and their luminescence properties under 365 nm excitation were investigated in detail. It indicated that by doping small amount of P⁵⁺ into V⁵⁺ sites, the excitation intensity of charge transfer (CT) band of Bi–O (330–400 nm) was greatly improved. By substituting Nb⁵⁺ for V⁵⁺, both the CT bands of Bi–O and Eu–O (240–320 nm) were significantly enhanced. As a result, the emission intensity of Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) could be improved about 90% by doping 5 mol% P⁵⁺ and 110% by doping 5 mol% Nb⁵⁺. Comparing with the commercial Y₂O₂S:Eu³⁺ phosphors, the Y_0.85Bi_0.1Eu_0.05V_0.95M_0.05O₄ (M=Nb, P) phosphors exhibited excellent color purity and much higher brightness. The results showed that these Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) phosphors could be considered as promising red phosphors for application in LED.     

Pyroelectric properties in three phases coexistence Pb[(Mn0.33Nb0.67)0.5(Mn0.33Sb0.67)0.5]0.08(ZrxTi1−x)0.92O3 lead ceramics

The bulk dense Pb[(Mn_0.33Nb_0.67)_0.5(Mn_0.33Sb_0.67)_0.5]_0.08(Zr_xTi_1−x)_0.92O₃ pyroelectric ceramics have been successfully prepared by the conventional solid method. The effect of three phases coexistence in the ceramics is studied. When x = 0.95 and 0.85 in the ceramics, the maximum pyroelectric coefficient peaks appear at 23 °C and 45 °C, and the maximum values are 26.5 × 10⁻⁴ C/m² °C and 25.5 × 10⁻⁴ C/m² °C, respectively. The maximum pyroelectric coefficient appears large while the peaks widths are small. When the two kinds of ceramic powders mixed with the mol ratio of 2:1, the pyroelectric coefficient of the ceramics is above 10.0 × 10⁻⁴ C/m² °C in a broad temperature range from 20 °C to 55 °C. The possible physical mechanism of the temperature broadened phenomenon is briefly discussed.     

Ba(Mg1/3Nb2/3)O3电子结构第一性原理计算及光学性能研究

Ba(Mg_1/3Nb_2/3)O₃ (BMN)复合钙钛矿陶瓷具有高介电常数和高品质因子等介电性能, 预示了其在光学领域的应用前景. 本文采用第一性原理方法计算了BMN的电子结构, 对其本征光学性能进行分析和预测. 对固相合成六方相BMN的XRD 测试结果进行Rietveld精修(加权方差因子R_wp=6.73%, 方差因子R_p=5.05%), 在此基础上建立晶体结构模型并对其进行几何优化. 运用基于密度泛函理论(DFT)的平面波赝势方法, 对六方相BMN晶体模型的能带、态密度和光学性质进行理论计算. 结果表明BMN的能带结构为间接带隙, 禁带宽度E_g=2.728 eV. Mg-O和Ba-O以离子键结合为主, Nb-O以共价键结合为主, 费米面附近的能带主要由O-2p和Nb-4d 态电子占据, 形成了d-p轨道杂化. 修正带隙后, 计算了BMN沿[100]和[001]方向上的复介电函数、吸收系数和反射率等光学性质. 结果表明, BMN近乎光学各向同性, 在可见光区, 其本征透过率为77%< T <83%, 折射率为1.91< n <2.14, 并伴随一定的色散现象. 实验测试结果与理论计算结果相吻合.     

Unconventional magnetic and transport properties in Gd1−xLaxMn2Ge2 with two-dimensional arrangement of Mn atoms

In this paper, we present our recent experimental results of magnetic and transport properties of Gd_1−xLa_xMn₂Ge₂ intermetallic compounds with the ThCr₂Si₂-type layered structure. The results obtained indicate that, in GdMn₂Ge₂, a first-order transition from a collinear antiferromagnetic to a collinear ferrimagnetic state appears with decreasing temperature at T_t3, below the Néel temperature T_N. In Gd_1−xLa_xMn₂Ge₂ compounds with x=0.05 and 0.075, after ordering ferrimagnetically at T_t1, two kinds of first-order transitions from a canted ferrimagnetic to a non-collinear antiferromagnetic state and from a non-collinear antiferromagnetic to a reentrant canted ferrimagnetic state occur at T_t2 and T_t3. In Gd_0.925La_0.075Mn₂Ge₂, a field-induced metamagnetic transition from non-collinear antiferromagnetism to canted ferrimagnetism occurs at relatively low fields, accompanied by fractal like multi-step transitions, the so called “devil's stair-case”. Furthermore, a negative giant magnetoresistance (GMR) effect (Δρ/ρ15%) was observed at the field-induced metamagnetic transition. The mechanism of this negative GMR was clarified by comprehensive measurements of the resistivity on single crystals Gd_0.925La_0.075Mn₂Ge₂ and TbMn₂Ge₂. With further increasing x, only canted ferrimagnetism appears with a compensation temperature for 0.10<x<0.40, whereas no compensation behavior appears for x>0.50. The phase diagram obtained indicates that the overall magnetism is controlled by the Mn–Mn intralayer distance in the tetragonal c-plane, reflecting the two-dimensional arrangement of Mn atoms.     

Field-induced pinning effect of Nd(Ba1−xNdx)2Cu3O7−δ single crystals grown by the traveling-solvent floating-zone method

We have studied the superconducting properties of Nd(Ba_1−xNd_x)₂Cu₃O_7−δ (Nd123, x ≈ 0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O₂ in Ar atmosphere. The enhancement of the magnetization with increasing field is observed in the hysteresis (M-H) loop in fields both parallel and perpendicular to the c-axis of the Nd123 single crystals as well as in the bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The composition variation of Ba/Nd is observed in the matrix of Nd123 crystals by an analytical TEM equipped with a cold field-emission gun. It turns out that the enhancement is due to the field-induced pinning effect ascribed to the weak superconducting Nd---Ba substitution regions in the Nd123 matrix.