Structural phase relations in the La_2-x-yLn_xSr_yCuO_4-δ, the Ln_2-xCe_xCuO_4-δ and the Ln_2-ySr_yCuO_4-δ systems: A powder neutron diffraction study

High resolution powder neutron diffraction is used to study the structural phase diagram of the systems Ln_2-xCe_xCuO_{4-δ Ln2-ySrxCuO4-δ and La2-x-yLnxSryCuO4-δ, in particular the structural variants T′ (N CuO4 structure), T (K2NiF4 structure) and O (orthorhombically distorted T structure). The T′ phase which is characterised by flat (CuO2)∞ sheets is stable to both Ce⁴⁺ and Sr²⁺ doping and undergoes no structural phase transitions above 1.5 K, in contrast to the tilting mechanism operative in the T structure. This has important implications for the mechanism of antiferromagnetic ordering of Cu spins in the T′ phase because of the lack of static structural distortions. The value of the tolerance factor is within the stability range of the T′ phase and the basal plane Cu-O bond length is not compressed as in the T phase. Transport measurements indicate that the undoped and the hole doped T′ phases have much higher resistivities and activation energies than their O/T analogues. In contrast, electron doping of the T′ phase leads to reduced resistivities and activation energies, similar to hole doped O phases. The O phase of La2CuO4 transforms to the tetragonal T′ phase at a Pr³⁺ doping level of x = 0.6. Doping of the critical composition La1.4 Pr0.6Cu4 with Sr²⁺ induces a transition back to the T phase with abrupt changes in the lattice dimensions. The composition La1.2Pr0.6Sr0.2 CuO4 shows a T→O phase transition upon lowering the temperature [(a-b)/(a+b)] = 2.07(2) x 10^-3 at 1.5 K], compared with La1.8Sr0.2CuO4 and La1.15Pr0.6Sr0.25CuO4 which are tetragonal down to liquid helium temperatures. Furthermore, La1.2Pr0.6Sr0.2CuO4 is superconducting with a transition temperature of Tc = 29.5 K. The effect of Sr²⁺ doping on La2-xLnxCuO4-δ is t increase the critical xc value for the O→T′ phase transition. Furthermore, the critical Td value for the T→O phase transition also increases compared with the La2-ySryCuO4-δ} system, extending the stability range of the O phase into higher values of the formal Cu oxidation state (up to an effective value of +2.25).