The Cuprate Ln2CuO4 (Ln: Rare Earth): Synthesis, Crystallography, and Applications

*Basma Marzougui, Amira Marzouki, Youssef Ben Smida and Riadh Marzouki*

### **Abstract**

This chapter is concerned with a study of undoped and doped cuprates of the general formula Ln2CuO4 (Ln = rare-earth metal) and Ln2–xMxCuO4±δ (Ln = rare earth and M = Sr, Ba, Ca, Ln', Bi, and 3d metal). The crystal structures of the undoped and doped cuprates having the notations (T, T′, T\*, S, and O), significantly depend, however, on the synthetic route. The topotactic synthesis is a specific method, which allows the transformation of the cuprate from the T to T′ structure. The importance of these materials originates from the discovery of the unconventional superconductors of the Ce-doped Ln2CuO4. The cuprate materials could function as insulators or semiconductors which are valuable tools in optoelectronic applications. The doped cuprate materials are good ionic conductors and are found useful as electrodes in fuel cell applications. The undoped cuprates reveal high dielectric properties.

**Keywords:** cuprate, synthesis, crystal structure, superconductivity, ionic conductivity, optical properties

#### **1. Introduction**

Perovskite with the general formula ABO3 is an important structure in solid-state chemistry which has been applied in many fields. From the ABO3 structure, several approximate structures can be derived, which are equally important and reveal excellent physical and chemical properties. Historically, Perovskite was first depicted by geologist Gustav Rose in 1830 as a particular mineral CaTiO3 calcium titanate [1]. Today, 'Perovskite' refers to a group of compounds with the same crystal structure and similar unit cell parameters. The partial and total substitution of the cationic atoms of the stochiometric ABO3 allows to obtain several structures with attractive physical and chemical properties [2]. **Figure 1** shows the phases obtained after the modification of the central compound ABO3.

Horizontally, the diagram shows that phases of layered structure can be formed by the interlacing of motifs (AO) or (BO2) and motifs (ABO3). Vertically, it shows the intermediate phases that can be obtained by varying the oxygen content through the oxidation/reduction process [3, 4].

#### **Figure 1.** *Derivatives reached from the central structure perovskite ABO3.*

The Ruddlesden–Popper phases (RP), of the general formula An+1BnO3n+1, have a structure derived from perovskite, which can be described as the stacking of n successive perovskite layers (ABO3) alternating with one sheet (AO) of NaCl structure along axis c [5].

Like the perovskites, the RP phases show high structural flexibility and more particularly the cuprates of the general formula ACumOn. They are copper-based oxides alloyed with other elements, with different coordination numbers for Cu and consequently different geometry of CuO2 polyhedra. They may be a chemical compound in which copper forms an anion or complex with an overall negative charge [4].

## **2. The discovery of the superconductivity of cuprate**

Although the undoped cuprates are considered electrical insulators, the "doped" cuprates are regarded as unconventional superconductors [6]. Site A may be rare earth or shared by other rare earth (Sc, Y, and the lanthanides elements) or alkaline earth of different valences (Be, Mg, Sr, Ca, Ba) [7–12]. This gives these materials' different physical properties in relation to the substitution coefficient. These compounds have all different structures but have in common the "active" CuO2 plans in which the superconductivity is formed [13].

The first cuprate superconductor was discovered in 1986 in lanthanum barium copper oxide by the scientists Georg Bednorz and Karl Alexander Müller [14]. The critical temperature for this material was 35K, much higher than the previous record of 23K [15]. This discovery resulted in a significant increase in research on cuprates, which resulted in hundreds of publications between 1986 and 2001. Bednorz and Müller received the Nobel Prize in Physics in 1987 [16], just one year after their discovery.

Superconductivity in cuprates is considered non-conventional and is not explained by BCS theory (Bardeen-Cooper-Schrieffer) [17]. The potential pairing mechanisms for cuprates superconductivity continue to be the subject of extensive discussion and research. In 1987, Philip Anderson proposed that super exchange could be used as a mechanism for coupling high-temperature superconductors [18]. In 2016, Chinese physicists observed a correlation between a cuprates' critical temperature and the size of the charge transfer gap in that cuprate, offering an explication for the super

#### *The Cuprate Ln2CuO4 (Ln: Rare Earth): Synthesis, Crystallography, and Applications DOI: http://dx.doi.org/10.5772/intechopen.109193*

exchange hypothesis (the strong antiferromagnetic coupling between two neighbor cations through a non-magnetic anion) [19].
