**9. Topotactic reduction of cuprates**

Ln2CuO4 compounds (Ln = lanthanide) can also be reduced to lead to Ln2CuO4–δ oxygen-deficient phases. Topochemical deintercalation of O2− ions from transition metal oxides can be used to prepare a large number of metastable phases that contain coordination geometries and transition metal centers in unusual oxidation states [80]. The Ln2CuO4–δ oxygen-deficient phases obtained after reduction constitute an obligatory intermediate step for the transformation of the T to T' phase [81].

The reduction reaction of a transition metal oxide is normally conducted by artificially reducing the partial pressure of oxygen in the system, either by pumping oxygen or purging with an inert atmosphere (stream of H2 gas) or by the use of highly electropositive binary metal hydrides (LiH, NaH, MgH2, CaH2, SrH2, and BaH2) in a vacuum tube.

The importance of the T' phase is due to the superconductivity properties of the cuprates. Therefore, numerous attempts have investigated the possibility of transforming the T phase into the T′ phase [10, 80–82] use of a topotactic transformation. For cuprates, e.g., topotactic transformation can be performed in two steps: reduction using hydrides of electropositive metals such as NaH, LiH, and CaH2 or using H2. The reduction step creates it possible to find an intermediate phase that is transformed by oxidation to the required structure. For example, using CaH2, La1.8Nd0.2CuO3.5 is synthesized from La1.8Nd0.2CuO4 [10]. T′-(La-Ln)2CuO4 phase was first synthesized by Tsukada et al. [83] at a low temperature of 600°C. In this context, researchers are chasing stability conditions to realize whether T′-La2CuO4 phase exists only as a thin layer. Thus, using the co-precipitation method, Takayama Muromachi et al. [84] stabilize T′-La1.8Y0.2CuO4 phase at low temperature (600°C).
