Defects and Defect Association Determine the Actual Entropy of Perovskites Derived from Lanthanum−Calcium Ferrite

Abstract

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 o r t h o r h o m b i c a l l y d i s t o r t e d p e r o v s k i t e , w h e r e a s 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 Lapoor positions and high-valence 3d-metals. In addition, oxygen vacancies are displaced toward the latter layers.