Ca掺杂对YBa2Cu3O7-δ超导块材性能影响的研究

用固相反应法制备了掺杂 Ca和不掺杂 Ca的 Y1 - x Cax Ba2 Cu3O7-δ超导块材。主要研究了 Ca掺杂对 Y1 - xCax Ba2 Cu3O7-δ(x≤ 0 .3)超导体性能的影响 ,采用标准四引线法测量了样品的 R - T和 I - V曲线。实验结果表明 :在YBCO超导体中掺杂适量的 Ca后临界温度 TC有所下降 ,样品的临界电流 IC有所增加 ,弱磁场下的 IC- H曲线下降幅度很平稳 ,这表明适量的 Ca掺杂有利于改善 YBCO超导体的临界电流性能     

Ca掺杂氧化物超导体正常态电阻反常及其磁输运行为

本文研究了对Y1-xCaxBa2Cu3O7-δ进行Ca掺杂后样品的输运特性.实验中发现:随Ca含量增加,样品正常态电阻随之下降.出现反常电阻现象原因可能是Ca替代Y位,导致体系载流子浓度发生变化,从而使得晶界附近的电子能带结构发生改变.另外;低温下的样品伏安特性在外磁场下出现反常,动态电阻dV/dI随磁场增加而下降.有关该现象的物理机制有待于进一步研究.     

Ca掺杂氧化物超导体正常态电阻反常及其磁输运行为

本文研究了对Y1-xCaxBa2Cu3O7-δ进行Ca掺杂后样品的输运特性．实验中发现：随Ca含量增加，样品正常态电阻随之下降．出现反常电阻现象原因可能是Ca替代Y位，导致体系载流子浓度发生变化，从而使得晶界附近的电子能带结构发生改变．另外；低温下的样品伏安特性在外磁场下出现反常，动态电阻dV／dI随磁场增加而下降．有关该现象的物理机制有待于进一步研究．     

Y替代La2/3Ca1/3MnO3体系的结构与输运行为

系统研究了(La1-xYx)2/3Ca1/3MnO3(0.0≤x≤0.3)体系的结构和输运行为.结果表明，实验样品具有很好的单相结构，随Y掺杂浓度的增加，金属—绝缘体（M—I）转变温度T-MI向低温区移动，对应的峰值电阻率ρp升高，对x=0.3样品，较未替代样品（x=0.0）增幅达8个数量级.在外加磁场下，材料表现出很强的磁电阻效应.同时，从实验结果出发，直接给出了输运特性与晶体结构之间的关联，并从双交换模型和可变程跃迁理论出发，对实验结果进行了初步讨论. 关键词： La2/3Ca1/3MnO3锰氧化物 Y替代 晶体结构 输运行为     

YBCO中钙掺杂对CuO_2面空穴浓度的影响

本文通过固相反应法制备了(Y1-xCax)Ba2Cu3O7-δ(x=0,0.06,0.12)多晶样品并在流动氧气下退火,研究了Ca掺杂对超导体CuO2面空穴浓度的影响.热重分析结果显示,相同退火条件下Ca2+/Y3+替代所引入的空穴不会被氧含量的降低所完全补偿.热电势测量数据显示,低温退火就可使未掺杂样品处于"临界掺杂浓度",而低剂量Ca掺杂则有可能降低CuO2面的空穴浓度.观察发现无论是否掺Ca样品的p(δ)函数都呈非线性关系,可能来源于CuO1-δ链的有序性.计算显示Ca2+/Y3+替代可能降低材料的超导性能.     

Y1-xRxBa2Cu3O7—δ体系（R=Eu、Gd）的正电子寿命和超导性研究

本文报道了对Y1-xRxBa2Cu3O7-δ（R=Eu、Gd）两个系列样品的超导电性研究结果．首先，发现随着Eu、Gd掺杂量x的增加，Y1-x]EuxBa2Cu3O7-δ和Y1-xGdxBa2Cu3O7-δ的晶胞体积均随之增大．但所有样品均保持与Y-123样品相同的单相正交结构；正电子平均寿命实验结果表明Cu—O链处局域电子密度ne随Eu、Gd掺杂浓度x的增加而减小．说明Eu、Gd对Y位的替代引起晶胞结构参量的变化不汉改变了Y位的电子密度．同时也影响了Cu-O链周围的电子密度分布；超导转变温度Tc随Eu、Gd掺杂浓度x增加而略有增加，反映了Eu、Gd掺杂虽然改变了体系的局域电子结构．但没有破坏超导所需的Cu-O链与CuO2面之间的耦合：Y位掺杂体系的超导电性与掺杂离子的磁性无关，而与掺杂离子的半径有关．     

基于择优掺杂取向研究Y掺杂量对ZnTe电子结构和吸收光谱的影响

Zn Te由于其特有的禁带宽度,光学性质以及可重掺杂等特性,使得众多学者对其进行了系列的相关研究,但关于Y掺杂浓度和掺杂方式对Zn Te性质的影响却鲜有报道.作者采用密度泛函理论框架下的广义梯度近似方法,分别计算了Y在掺杂浓度为1.56at%、3.12at%、4.69at%下Zn Te的几何结构、能带结构、态密度分布、吸收光谱等性质,以及不同掺杂方式对体系的影响.结果表明:在掺杂浓度为3.12at%,掺杂方式不相同时,掺杂原子沿[111]晶向排布的形成能最低,即[111]晶向为择优晶向.当掺杂浓度为4.69at%时,择优晶面为(111)面.若要实现更高浓度的Y掺杂,沿(111)晶面掺杂更容易实现.对于实验而言,更高浓度的Y掺杂,掺杂原子在Zn Te体系中更容易沿(111)晶面进行集中排列. Y掺杂Zn Te后,体系的禁带宽度变大,吸收光谱发生蓝移,对可见光的吸收强度减小.在浓度为3.12at%时禁带宽度最大,蓝移现象最明显,吸收强度最小. Y掺杂后体系变为n型半导体,可以使用这种掺杂方式制作P-N结二极管．     

Co掺杂对La5／8Ca3／8MnO3体系铁磁有序的影响与电子的局域化效应

系统研究了强关联锰氧化物La5／8Ca3／8Mn1-xCoxO3（x=0，0．05）体系的结构和输运特性．结果表明，样品均为很好的正交O′单相结构，Mn位5％Co掺杂明显影响了样品铁磁—顺磁（FM-PM）转变和金属-绝缘体（M-I）转变，M-I转变温度TM-I从未掺杂时的271K降至227K，对应的峰值电阻率ρp增大；随着TM-I的降低，Tc同时降低，磁电阻MR％亦相应增加；La5／8Ca3／8MnO3样品在Tc以下表现出长程铁磁有序态，Mn位5％Co掺杂样品则表现为团簇玻璃型短程铁磁有序行为．证明Co掺杂引起电子的局域化效应乃是导致体系输运行为发生变化的主要原因．同时，从Mn^3＋-O^2--Mn^4＋双交换（DE）作用和超交换（SE）相互作用机制等出发，对样品的输运行为、CMR效应以及与Co掺杂之间的关联进行了讨论．     

Y,Zr,Nb掺杂SnO_2电子结构的第一性原理研究

本文基于第一性原理方法研究了Y,Zr,Nb在Sn位掺杂SnO_2的键长变化、稳定性、能带结构以及态密度.结果表明:Y,Zr,Nb在Sn位掺杂SnO_2使附近的键长发生改变,改变量最大是Y掺杂SnO_2体系;掺杂体系的杂质替换能都为负值,表明体系为稳定结构;掺杂使SnO_2能级增多,能较好的调节带隙值;而Y掺杂SnO_2体系价带顶端有一条能级越过了费米线表明该体系呈现出半导体的特征;同时,Y,Zr,Nb掺杂SnO_2使导带底端的能级出现分离;在低能区的态密度仍主要由Sn、O的s轨道贡献;在高能区态密度的掺杂体系出现sp杂化的现象; Zr掺杂SnO_2的态密度能量向低能区移动.     

二价金属元素掺杂对LiCoO2体系电子输运性质的影响

为了解释Ca掺杂与Mg掺杂在影响锂离子二次电池正极材料LiCoO2体系电子输运性质方面的不同效应,采用基于密度泛函理论的第一性原理方法研究了该体系的电子结构.计算结果表明,虽然在LiCoO2体系中用Ca或Mg替代Co都会在费米能级附近产生部分占据的受主带,但两者对应的电子态都具有明显的局域化特征;此外,与Mg掺杂体系明显不同的是,Ca掺杂体系的受主带与价带之间存在清晰的带隙.这一带隙的存在正是Ca掺杂不能明显提高LiCoO2体系电导率的主要原因.此外,Ca2+与Mg2+离子半径的较大差别也是造成这两个掺杂体系的电导率存在明显差异的一个重要因素.     

YBa2Fe3O8+w的湿化学法制备以及电磁性质研究

本文成功采用湿化学法制备了高品质的YBa2Fe3O8+w(YBFO)材料,并利用XRD、M～T、R～T和SEM 等测试与分析手段研究了其结构、电导性质和磁性质.结果表明,YBFO在结构上具有与高温超导体YBa2Cu3O7(YBCO)高度的相似性,在电磁学行为上呈现出反铁磁(AFM)的半导体特征.YBFO与YBCO的相似...     

退火温度对YBa2Fe3O8的结构与磁性质影响分析

YBa2Fe3O8(YBFO)是具有三方钙钛矿结构的反铁磁(AFM)体,研究它的结构和磁性质对于我们认识三方钙钛矿结构具有重要作用.目前,关于固相法制备YBFO中退火温度方面的研究并不多,而控制退火温度是这种工艺中的重要环节.为制备高含量的YBa2Fe3O8,本文分别在760℃、950℃和1100℃退火温度下,采用固相...     

Tl2Ba2Ca2Cu3O10的化学键性质和同质异能位移研究

在Tl2Ba2Can-1CunO2n 4(n=1,2,3,4)系列超导性氧化物中,Tl2Ba2Ca2Cu3O10具有最高Tc值(125 K)．利用电介质的平均能带模型计算了Tl2Ba2Ca2Cu3O10的局域化学键参数,得到Cu(1)-O键的共价性为0．561,Cu(2)-O键的平均共价性为0．296．应用化学环境因子计算了57Fe在Tl2Ba2Ca2Cu3O10中的穆斯堡尔同质异能位移,证实了57Fe在低掺杂时以Fe3 和Fe4 离子形式占据Cu(1)位置,而在高掺杂时以Fe3 和Fe4 离子形式分别占据Cu(1)和Cu(2)位置．结果表明,化学键理论计算有助于复杂晶体的穆斯堡尔谱的正确分析．     

Mössbauer effect study and crystallographic structure of chlorinated green rust one compounds

Mössbauer spectra of green rust one run at 78 and 4.4 K exhibit two ferrous and one ferric sites in the 2/1/1 intensity ratio. Chemical formula is set at 3Fe(OH)₂, Fe(OH)₂Cl, nH₂O. By replacing Fe²⁺ with Ni²⁺ ions, the compound is stabilized and the structure is determined on a ABiBCiCai stacking sequence. Space group is P3₁21 with an hexagonal unit cell of parameters a=6.17 and c=23.5 A⁰.     

LiAl_5O_8∶Fe~(3+)发光粉的发光性能(英文)

Fe3+激活的铝酸锂是深红色发射的红色荧光粉,其发射的峰值波长为675nm,呈现出少有的纯正的深红色发光。本文对LiAl5O8:Fe3+荧光粉的基质组成和激活剂浓度进行了研究。结果表明:Fe3+掺杂LiAl5O8的激发光谱,在λem=673nm的波长监测下,有序相的激发光谱在284nm处有强吸收带,为Fe3+-O2-电荷迁移带;激发波长λex=254nm的有序相样品的发射波长峰值为673nm,并伴随一个在长波方向轻微不对称得,发射是属于4T1(4G)-6A1(6S)的跃迁。当原料Li2CO3与Al2O3的量的比为0.21时,样品的发光强度最好;样品的发光强度随激活剂Fe3+的浓度的增加而提高,当浓度达到0.5%时,发光强度达到最大值,超过0.5%时呈现出浓度猝灭效应。     

High-pressure phase transition of Bi2Fe4O9

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.     

A new isomorph of ferrous chloride tetrahydrate: A Fe Mössbauer and X-ray crystallography study

This paper outlines the discovery of a newly characterised isomorph of ferrous chloride tetrahydrate, Fe(H₂O)₆·FeCl₄(H₂O)₂, which was initially identified by X-ray crystallography and confirmed by Mössbauer spectroscopy. The X-ray analysis identified the space group as P2₁/c with essentially the same unit cell dimensions as the well-known isomorph, FeCl₂·4H₂O, except that one edge is doubled due to two discrete [Fe(H₂O)₆]²⁺ and [FeCl₄(H₂O)₂]²⁻ species per unit cell. Time-series Mössbauer studies revealed this new isomorph to be unstable upon atmospheric exposure, decaying to the well-known structure over a period of days. Density functional theory calculations support an energetically favourable catalytic interconversion involving adsorbed water. A high-precision redetermination on the FeCl₂·4H₂O crystal structure, which is also in space group P2₁/c, is also reported, providing the unit cell parameters: a=5.8765(3) Å, b=7.1100(3) Å, c=8.4892(5) Å and β=111.096(1)°.     

Ca4Y6Si6O25:Eu2+绿色荧光粉的发光特性

采用高温固相法合成了Ca4 Y6Si6O25:Eu2+绿色荧光材料.通过X射线衍射分析得知,Ca4Y6Si6O25属于六方晶系,具有P63/m(176)空间点群结构.测定了Eu2+的激发光谱和发射光谱.Ca4Y6Si6O25:Eu2+的激发光谱为350～450 nm的宽带谱,这与近紫外光LED芯片相匹配.发射光谱是峰值...     

Y~(3+)掺杂对Sr_(1-x)Ca_xSi_2O_2N_2∶Eu~(2+)发光性能的影响

采用高温固相反应法制备了Sr_(1-x)Ca_xSi_2O_2N_2∶Eu~(2+)系列荧光粉,研究Y~(3+)离子掺入对荧光粉发光性能的影响。对于Sr Si_2O_2N_2∶Eu~(2+),Y~(3+)离子掺入主要起到稳定Eu~(2+)价态的作用,避免Eu~(2+)氧化为Eu~(3+),从而提高Sr Si_2O_2N_2∶Eu~(2+)的发光性能。对于Ca Sr Si_2O_2N_2∶Eu~(2+),Y~(3+)离子掺入除了稳定Eu~(2+)价态作用外,还能有效减小Eu~(2+)取代Ca~(2+)后晶格膨胀引起的应力,提高Eu~(2+)在晶格中的溶解度。Sr_(1-x)Ca_xSi_2O_2N_2∶Eu~(2+)(x=0,0.15,0.3,0.6,0.75,0.95)系列荧光粉中随着Ca含量的增加,共掺Y~(3+)离子对样品发光强度的提高程度也随之增加(20%~80%)。     

Electron energy-loss spectroscopy study of oxygen chemisorption and initial oxidation of Fe(110)

The initial stages of the interaction of oxygen with an Fe(110) surface have been studied at 300 K by electron energy-loss spectroscopy with in-situ combined low energy electron diffraction, Auger electron spectroscopy and work-function change measurement. From all the results, four different stages of the oxygen interaction are distinguished: (I) a first dissociative chemisorption up to 3 L, characterized by the c(2×2)-O structure, (II) a second dissociative chemisorption between 3 and 7 L, characterized by the c(3×1)-O structure, (III) incorporation of O adatoms into the selvage between 7 and 30 L, and (IV) oxidation above 30 L leading to the formation of FeO(111), characterized by the diffuse hexagonal diffraction pattern. The sticking probability was found to be initially near unity and fall off rapidly to a minimum value of ≈0.05 at ≈1 L. Particular emphasis was placed upon the investigation of the change in surface electronic properties from those characteristic of them metal to those of the oxide. In stage (I) an energy-loss peak, being attributed to the transition from the 2p orbital of the chemisorbed oxygen, was observed at 6.0 eV, while in stage (II) two additional peaks of the same origin appeared at 7.5 and 9.3 eV due to the formation of the O 2p band. The energy-loss spectrum in the oxide phase was characterized by the peaks at 4.8 and 7.5 eV due to the O²⁻ 2p → Fe²⁺3d charge-transfer transitions and by a peak at 2.4 eV due to the ligand-field d → d transitions of an Fe²⁺ ion in FeO. It is shown that the Fe 3d_yz,zx and 4sp electrons play a major role in the chemisorption bond (O adatoms located in the long-bridge site), and that for the incorporation process the Fe 3d_y² electrons are also involved in bonding by the symmetry breaking. The change in the Fe 3p-excitation spectrum during oxidation was also investigated. The differences between the experimental results on Fe(100) and (110) surfaces are summarized.