【作者】 陈志军；

【导师】 刘卫；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2009， 硕士

【摘要】 自从以La₂CuO4+δ为母体的超导体系发现以来,额外氧在K₂NiF4结构中的行为就受到了广大科学家的关注。La₂CuO4+δ体系在相分离温度之下会发生相分离现象,分离为富氧相和贫氧相,其中富氧相具有金属导电性,低温下具有超导电性,贫氧相在奈尔温度以下为反铁磁有序。体系的结构以及性质受到额外氧的有序方式和性质的影响,比如额外氧的有序无序分布、相分离或者团簇结构以及价态等等。La2NiO4+δ体系跟La2CuO4+δ结构相同,但是可以容纳更多的额外氧,为研究额外氧的性质提供了很好的一个参照体系。随着额外氧的增多,La₂NiO4+δ体系表现出了非常丰富的物理图象,比如相分离现象、额外氧一维有序到三维有序的转变、极化子有序现象、磁有序结构公度到非公度转变现象等等,这些现象同样引起了各国科学家的广泛的关注和研究。而与La₂NiO4+δ同构的Pr2NiO4+δ可以容纳比La2NiO4+δ还要多的额外氧,正交-四方的相转变出现在700K附近温区,也与额外氧大小有关,该转变温度依赖于δ值,并且处在高温四方相的额外氧是无序的,而低温正交相中的额外氧处于有序状态。对K2NiF4结构额外氧的认识还存在着很多不清楚的地方,因而研究La2MO4+δ（M=Cu,Ni）掺杂体系额外氧在低温下是否有序以及多大程度上有序以及它们对体系性质的影响是一个很重要的问题。内耗作为一种检测机械能损耗的实验手段对固体材料中存在的缺陷及其组态变化异常敏感,能够给出体系中缺陷运动的动态信息。本论文主要内容就是利用内耗手段来研究La2MO4+δ（M=Cu,Ni）掺杂体系中额外氧的状态和性质。第一章简要介绍了K2NiF4结构氧化物的晶体结构和额外氧的存在状态,并提出了本论文的研究内容。第二章介绍了滞弹性理论;内耗的测量原理;内耗的唯象理论,包括标准滞弹性固体的概念和性质;内耗的微观理论,内耗仪工作原理和本论文用到的实验方法和原理,包括碘滴定以及内耗等。第三章研究了La2Cu1-xMxO4+δ（M=Zn,Li,Ni）体系低频内耗,分析认为在250K温区观察到的弛豫型内耗峰是由氧对形式的额外氧跳跃弛豫引起的。对La2Cu1-xMxO4+δ（M=Zn,Ni）体系,当掺杂量到一定值时,会由于额外氧的增加而出现额外氧从一维有序到三维有序的转变。对La2Cu1-xLixO4+δ体系,并没有发现证据显示额外氧三维有序,但在低温发现了另外一个内耗峰。第四章研究了La2-xMxNixO4+δ（Pr, Nd, Sr）体系低频内耗峰现象。分析认为La2-xPrxNixO4+δ体系额外氧跳跃存在一个玻璃化冻结过程。对内耗测量过程中的内耗峰的变化情况进行了分析,认为随着Pr含量的增加,额外氧出现三维有序化,对气氛处理的Pr2NiO4+δ的内耗研究也证实了这一点。在四方结构La2-xPrxNixO4+δ体系中,只存在一维有序额外氧对弛豫引起的一个内耗峰,而正交结构中,除了这个峰以外,还存在一个低温侧内耗峰。在Nd2NiO4+δ体系中,我们同样发现了这两个弛豫内耗峰,还发现了一个一级相变内耗峰。第五章研究了La2-xMxCuO4+δ（M=Nd, Pr, Sr）体系内耗峰现象。实验发现了La1.9Nd0.1CuO4+δ体系的结构相变内耗峰,而在四方相的该体系中都没有发现弛豫内耗峰。更多还原

【Abstract】 The behavior of the excess oxygen in the K2NiF4-type structure has attracted much attention of researchers since superconductivity was found in La2CuO4+δ-based compounds. In La2CuO4+δthe excess oxygen remains mobile in low temperature and has a tendency to phase segregation into an oxygen-rich superconductive phase and oxygen-poor antiferromagnetic phase below the temperature of the phase separation. The structure and the properties were affected by the manner and characters of the excess oxygen, such as random or ordered distribution of over the interstitial sites, segregation or clustering, valence states, etc.La2NiO4+δwith the same structure of La2CuO4+δ, has served as a prototype in researching the behavior of the excess oxygen because it has greater capacity for absorbing excess oxygen compared to La2CuO4+δ. With increasing of excess oxygen, La2NiO4+δshows fruitful phenomena, such as phase separation, one-dimensional ordering and three-dimensional ordering, charge ordering and the magnetic order transition from the one-dimension to three-dimension and etc, which also have attracted much attentions. Pr2NiO4+δ, has the same structure as La2NiO4+δ, can absorb more excess oxygen than La2NiO4+δ, the phase transition of it changes from the orthorhombic phase to the tetragonal phase around 700K, and the phase-transition temperature is related with the content of excess oxygen. The state of the excess oxygen remains disordering in high temperature tetragonal phase and ordering in low temperature orthorhombic phase. The state of the excess oxygen still remains unclear in K2NiF4-type structure oxides and there are some discrepancies among the reported results; thus, whether the ordering of the excess oxygen really exists and to what extent the disorder in the arrangement of the excess oxygen atoms remains in the low-temperature phase in doped La2MO4+δ(M=Cu, Ni) system is an important issue. The internal friction technique, which in general measures the dissipation of elastic energy (Q-1), has proved to give valuable information about defects in ceramic samples. In the thesis we present the results of the internal friction study of a series of doped La2MO4+δ(M=Cu, Ni) samples and discuss the relation of the state of the excess oxygen and the internal friction spectra.The chapter 1 deals with the structure and the state of the excess oxygen in La2MO4+δ(M=Cu, Ni) system, and put forward what studied in the thesis. The chapter 2 deals with the theory of anelasticity, the methods used in the measurement, the standard anelastic solid and its dynamic properties as function of temperature, the theory of point-defect relaxations and the theory of the first-order phase transition, working principle of internal friction instrument, including the theory of iodometric titration and internal friction measurement.In chapter 3, we studied the low frequency internal friction and shear modulus of La2Cu1-xMxO4+δ(M=Zn, Li, Ni) system, the internal friction peak at 250K is due to the hopping of excess oxygen pairs. The excess oxygen transits from one-dimensional ordering to three-dimensional ordering with the excess oxygen increasing in Zn and Ni doped La2Cu1-xMxO4+δsystem. There is no evidence to approve that having three-dimensional ordering excess oxygen formation in La2Cu1-xLixO4+δsystem.In chapter 4, the low frequency internal friction and shear modulus of La2-xMxNixO4+δ(Pr, Nd, Sr) system. The glass freezing-in phenomenon of excess oxygen in La2-xPrxNiO4+δ(0≤x≤2.0) system is observed and discussed, the excess oxygen transits from one-dimensional ordering to three-dimensional ordering with the excess oxygen increasing, which also approved by the measurements of Pr2NiO4+δannealed in deferent atmospheres, there is only one relaxation internal friction peak in tetragonal phase La2-xPrxNiO4+δsystem while another peak exist in orthorhombic phase. The two internal friction peak also exist in Nd2NiO4+δsystem, and the first ordering phase transition internal friction peak appears in the system.The chapter 5 deals with the low frequency internal friction peaks and shear modulus of La2-xMxCuO4+δ(M=Nd, Pr, Sr) system. The structure phase transition internal friction peak was observed in La1.9Nd0.1CuO4+δ, and no relaxation internal friction peak appears in tetragonal phase of this system.更多还原