Smaller rare-earth cation and mixed valent Mn incorporation as a dual strategy to enhance ferrimagnetic ordering temperatures in A-site ordered quadruple perovskites, LnCu3Mn1+xTi3−xO12 (Ln = La, Nd; x = 0, 0.3)

Abstract

High-T_C ferro-/ferrimagnetic quadruple perovskites constitute an important class of oxides that has garnered a lot of research attention in recent times, but their synthesis is commonly achieved under high-pressure conditions. Thus, the development of high-T_C quadruple perovskites that can be synthesized under ambient pressure can be a key to the above problem. Herein, we report ambient pressure synthesis of a series of new A-site ordered quadruple perovskites, LnCu₃Mn_1+xTi_3−xO₁₂ (Ln = La, Nd; x = 0, 0.3), by coupled aliovalent-cation manipulation in CaCu₃Ti₄O₁₂. The effect of smaller lanthanide Nd incorporation in place of La has been investigated. Furthermore, 30% mixed valency of Mn per Mn³⁺ has been introduced in place of Ti⁴⁺ following similar strategies adopted to achieve giant magnetoresistive manganites La_0.7A_0.3MnO₃ (A = Ca, Sr, Ba) from LaMnO₃. Mn is present in the 3+ state for x = 0 and in a mixed valent state (3+ and 4+) for x = 0.3, whereas Cu exists in the 2+ state in all the compounds. While LaCu₃MnTi₃O₁₂ and LaCu₃Mn_1.3Ti_2.7O₁₂ show the onset of ferrimagnetic order at ∼60 and 110 K, respectively, the corresponding Nd analogs, NdCu₃MnTi₃O₁₂ and NdCu₃Mn_1.3Ti_2.7O₁₂, exhibit enhanced T_C's of ∼80 and 140 K, respectively. This work reveals an effective strategy of mixed-valent Mn incorporation in the B-sublattice and smaller rare-earth cation incorporation to achieve higher ferrimagnetic ordering temperatures in quadruple perovskites.