化学氧化La-214体系的超导电性及相关物性的实验研究

【作者】 涂清云；

【导师】 马本堃； 陈小龙；

【作者基本信息】 北京师范大学 ， 凝聚态物理， 2002， 博士

【摘要】 本论文以La-214高温超导体系为研究对象，利用X射线衍射直流磁化率测量扫描电镜观察电子衍射及碘量法氧含量测定等分析测试手段，对化学氧化的La₂CuO_4+δLa₂Cu_1-xM_xO_4+δ（M=Zn N）和La_2-xNd_xCuO_4+δ三个子体系的超导电性及相关物性进行了较为系统的实验研究对ka₂CuO_4+δ体系主要研究了化学氧化对体系超导电性的影响，对La₂Cu_1-xM_xO_4+δ（M=Zn N）和La_2-xNd_xCuO_4+δ二体系则分别着重研究了CuO₂面内Zn Ni部分替代Cu以及CuO₂面外等价元素Nd部分替代La对化学氧化La₂CuO_4+δ体系超导电性的影响。 首次使用NaClO作为氧化剂分别在室温和50℃对La₂CuO_4+δ进行了2-120小时的化学氧化，研究了氧掺杂对化学氧化La₂CuO_4+δ体系超导电性及相关物性的影响研究结果表明，所有样品在氧化前后均为单相正交结构，晶粒的大小和形貌在氧化前后无明显变化大量的额外氧可通过化学氧化插入具有正交结构的La₂CuO_4+δ的晶格中，氧化样品的额外氧含量与氧化时间间遵从双指数饱和规律，推测额外氧原子在晶体中的扩散可能同时存在空位和间隙原子两种扩散机制，且在不同的氧化温度下，起主导作用的扩散机制不同额外氧的插入可使La₂CuO_4+δ由绝缘体转变为超导体，所有室温氧化样品的起始超导转变温度T_c均为45K；50℃氧化样品的起始超导转变温度T_c则随氧化时间的增加从45K逐渐降至43K表明高的氧化温度于超导电性是不利的实验证实NaClO是一种对La₂CuO_4+δ体系非常有效的氧化剂，并首次在化学氧化的La₂CuO_4+δ中获得45K的起始超导转变温度KMnO₄氧化La₂CuO_4+δ样品的最显著特征是样品早现两相超导转变，选区电子衍射（SAED）花样证实样品包含两个超导相。 通过Zn Ni部分替代CuO₂面内的Cu，再经化学氧化，研究了CuO₂面内元素替代对化学氧化La₂CuO_4+δ体系超导电性及相关物性的影响研究结果表明，La₂Cu_1-xM_xO_4+δ（M=Zn Ni）样品在氧化前后均为单相正交结构，晶粒的大小和形貌在氧化前后无明显变化氧化Zn Ni部分替代样品的额外氧含量明显高于纯La₂CuO_4+δ氧化样品，额外氧含量随替代元素Zn Ni含量的变化无显著变化，表明元素替代样品更有利于额外氧的插入氧化La₂Cu_1-xM_xO_4+δ（M=Zn Ni）样品的起始超导转变温度T_c和超导相含量都随替代元素含量的增加而急剧下降，T_c抑制率分别为dT_c/dx=-11.5K／％Zn和-6.8K／％Ni。表明无论磁性元素还是非磁性元素对CuO₂面内Cu的部分替代对La₂CuO_4+δ的超导电性均有强烈的抑制作用氧化Zn Ni部分替代样品中替代离子所诱导或呈现的平均有效磁矩分别在1μB和0.8μB左右，且前者的有效磁矩随Zn含量的增加而减小，后者的有效磁矩则随Ni含量的改变无显著变化。通过对化学氧化La₂Cu_1-xM_xO_4+δ（M=Zn Ni）与Sr最佳掺杂的La_2-xSr_xCu_1-yM_yO₄（M=Zn Ni）两体系的T_c抑制率Zn Ni诱导或呈现磁矩大小及其随替代元素含量的变化的比较，认为Zn、Ni对二体系超导电性的破坏作用是基本相同的。 用Nd部分替代CuO₂面外的La，并经化学氧化，研究了CuO₂面外等价元素替代对化学氧化La₂CuO_4+δ体系超导电性及相关物性的影响对于La_2-xNd_xCuO_4+δ（0≤x≤0.6）体系，当0≤x≤0.4时，样品为单相正交结构，x=0.6样品为单相四方结构，而x=0.5样品则是四方结构和正交结构两相的混合，即在x=0.5附近存在一个窄的两相区大量的额外氧可通过化学氧化插入正交结构的La_2-xNd_xCuO_4+δ（0≤x≤0.4）的晶格中，而额外氧很难插入四方结构样品（x=0.6）的晶格中氧化正交结构样品的T_c随Nd含量的增加而缓慢降低，但所有样品的屏蔽体积百分比非常接近，说明CuO₂面外等价元素的替代对La₂CuO_4+δ体系的超导电性影响很小。氧化四方结构样品（x=0.6）直到5K仍无超导转变。更多还原

【Abstract】 In this dissertation, a systematically experimental study has been made on the structure, oxygen content, superconductivity and the etc. of the La₂CuO_4+δ, La₂Cu_1-xM_xO_4+δ （M = Zn, Ni） and La_2-xNd_xCuO_4+δ system by means of powder X-ray diffraction, iodometric titration, DC susceptibility and so on. The main results are as follows,La₂CuO_4+δ was chemically oxidized by using NaClO as oxidizing reagent with oxidation time from 2 to 120h at room temperature （RT） and 50℃, respectively, for the first time. The effects of the oxygen doping have been studied on superconductivity and related physical properties of the La₂CuO_4+δ system. The results show that all the samples are single phase with orthorhombic structure, and the grain size and its morphology have no obvious change before and after oxidation. A large amount of extra oxygen can be intercalated into the lattice of the La₂CuO_4+δ with orthorhombic structure, the relation between the extra oxygen content and the oxidation time obeys the double exponential saturation law, thus we speculate that there may exist simultaneously two different kinds of diffusion mechanism, the vacancy mechanism and the interstitial mechanism, during the oxidizing process. All the samples oxidized at RT are bulk superconductors with onset superconducting transition at 45K. While all the samples oxidized at 50℃ are bulk superconductors, and the T_c’s of which decrease gradually from 45K to 43K with increasing oxidation time. This result indicates that the higher oxidation temperature is unfavorable for the superconductivity of the La₂CuO_4+δ system. The experiments have been confirmed that NaCIO is a very efficient oxidizing reagent for La₂CuO_4+δ system, and the T_c at 45K has been obtained in chemically oxidized La₂CuO_4+δ for the first time. The most evident feature for the samples oxidized by KMnO₄ is the appearance of the two-step superconducting transition. The SAED patterns confirm that there are two superconducting phases existing in the sample.The effects of the element substitution in the copper site in CuO₂ plane have been studied on superconductivity and related physical properties of the La₂CuO_4+δ system by substituting Zn or Ni for Cu under chemical oxidation. The results show that all the samples La₂Cu_1-xM_xO_4+δ （M=Zn, Ni） are single phase with orthorhombic structure, and the grain size and its morphology have no obvious change before and after oxidation. The excess oxygen content of the element-substituted sample after oxidation is evidently larger than that of the oxidized sample of pure La₂CuO_4+δ. This indicates that the element-substituted sample is more favorable for the insertion of the extra oxygen. The onset superconducting transition temperature T_c and the superconducting phase content of the oxidized La₂Cu_1-xM_xO_4+δ （M = Zn, Ni） samples markedly decrease with increasing the Zn and Ni content. The average T_c suppression rates are -11.5K/%Zn and -6.8K/%Ni, respectively. The effective moment induced by Zn is 1μ_B order, which decreases with increasing the Zn content. The effective moment of per Ni is about 0.8μ_B, which doesn’t obviously change with increasing the Ni content. By comparing the related properties of the chemically oxidized La₂Cu_1-xM_xO_4+δ system and the La_2-xSr_xCu_1-yM_yO₄ system with the Sr optimal doping, we can conclude that the effects of Zn and Ni on superconductivity are basically the same in both of the two systems.更多还原