Defects and Defect Association Determine the Actual Entropy of Perovskites Derived from Lanthanum–Calcium Ferrite
来源：ACS Publications
Perovskites La1/2Ca1/2Mn1/3Fe1/3Co1/3O3−δ and iron-rich La3/4Ca1/4Mn1/4Fe1/2Co1/4O3−δ with five cations distributed through the A (La and Ca) and B (Mn, Fe, and Co) positions have been prepared by the combustion method. The corresponding nominal configurational entropies are 1.79R for the former and 1.61R for the latter. The actual configurational entropy may vary from these values due to metal ordering and the presence of oxygen vacancies; this is observed for the iron-rich phase whose configurational entropy is as high as 1.76R. The structures of both materials have been fully determined by combining complementary techniques. La1/2Ca1/2Mn1/3Fe1/3Co1/3O3−δ is an oxygen-stoichiometric, disordered, and orthorhombically distorted perovskite, whereas La3/4Ca1/4Mn1/4Fe1/2Co1/4O3−δ presents a previously undescribed (but theoretically predicted) columnar ordering of A and B cations. This order results in the alternation along the a-axis of layers composed of La-rich sites and low-valence 3d-metals with layers composed of La-poor positions and high-valence 3d-metals. In addition, oxygen vacancies are displaced toward the latter layers.
In this work, with the aim of selecting cations with geometric and electronic (charge) compatibility, we present two new perovskites with high nominal SConfig, having different stoichiometries with five cations distributed in the A (occupied by La and Ca) and B (occupied by Mn, Fe, and Co) positions of the perovskite. The first one follows the criterion of maintaining the same stoichiometry of the metals occupying the A and B positions to achieve electroneutrality while reaching the maximum configurational entropy, namely: La1/2Ca1/2Mn1/3Fe1/3Co1/3O3 (LCMFC), the second one is enriched in one of the 3d-metals (iron) which should result in a lower SConfig: La3/4Ca1/4Mn1/4Fe1/2Co1/4O3 (LCMFC-IR). These materials exhibit nominal configurational entropies as high as 1.79R for LCMFC and 1.61R for LCMFC-IR.