**5. The influence of hole doping on the antiferromagnetic order of CuO2 planes**

Doping is the operation that modifies the concentration of charge carriers in the CuO2 planes. There are two ways of doping the compound either by substituting a cation with another of different valences, for example, in the La2–xSrxCuO4 system, the La3+ is substituted by Sr2+ [49], or simply by adding oxygen. In the undoped compounds, the planes adjacent to the CuO2 planes consist of trivalent

**Figure 6.** *Structures of (a) T-La2CuO4, (b) T*′*-Nd2CuO4, and (c) S-Sr2CuO3.*

cations X3+ (La3+ in the phase La2CuO4 or Bi3+ for Bi2Sr2CaCu2O8). Only two of the three electrons provided by X3+ are needed for bonding X3+-O2 − . The electron remaining is transferred to blueprints Cu2+(O2 – )2. The unit cell CuO2 takes an electron from the two neighboring layers XO, thus, ensuring electronic neutrality. However, when a divalent Z2+ ion is partially substituted for the trivalent X3+ ion, an electron deficit is created in the CuO2 planes [50]. This process can also be described as introducing holes in the copper-oxygen planes. These holes transform the 3d9 states of Cu2+ into Cu3+ (S = 0), and these ions represent a Cu2+ (S = ½) bonding state with a hole residing mainly in the four neighboring 2p orbitals of the oxygens that is called a "Zhang-Rice" singlet [51]. The introduction of hole-like charge carriers significantly alters the long-range antiferromagnetic ordering of the system as shown in **Figure 7**.

#### **Figure 7.**

*Schematic representation of the influence of hole doping on the antiferromagnetic order of CuO2 planes [52].*
