**10. Synthesis of new Ln2CuO3.5 compounds by reduction with CaH2**

New compounds were synthesized using hydrogen reduction as a synthetic method to obtain compounds of the Ln2CuO3.5 (Ln: Pr, Nd, Sm, Eu, Gd) [23, 79, 85] and La2CuO3.67 type [78]. These interesting phases are obtained by heating the compounds of the Ln2CuO4 mother phases under a reducing atmosphere of the order of 5% moles of H2/He at a moderate temperature of 300°C. After inspection with X-ray diffraction, the structures obtained for La, Pr, and Nd are all similar to those of the Sr2CuO3 [86], whereas those of the compounds containing Sm, Eu, and Gd are different. The oxidation of the compound La2CuO3.67 in the temperature range of 300–500°C leads to a new La2CuO4+δ system of the T′ type similar to the structure

of Nd2CuO4 [82]. The oxygen excess is exceptionally high, ranging from 4.10 to 4.42. Annealing beyond 620°C in the presence of atmospheric oxygen transforms the structure back to the original structure of the K2NiF4 (T phase) type, although the excess oxygen varies up to 4.06 and with a small orthorhombicity compared with the starting product La2CuO4. The oxidation of the other S-Ln2CuO3.5 phases (Ln: Pr, Nd, Sm, Eu, and Gd) above 300°C leads to the original T′ structure, but the excess oxygen varies between 3.98 for the compounds (Eu, Gd) and 4.03 for the compound Nd [39, 87].

There is a notable change in the lattice parameters between the S, T′, and T phases. For La2CuOx, the transition from the T structure to the S structure leads to the elongation of the a- and b-axes and to compression of the c-axis, with respect to the structure T. Consequently, a large increase in the volume of the unit cell (~6%) is observed [88, 89]. The decrease in the c-axis was associated with the loss of oxygen in the CuO2 plane, as in the example of La2–xSrxCuO4–y. The transformation of the T′-La2CuO4+δ phase to the T phase produces a big decrease of about 4.3% in the unit cell volume.
