The impact of A-site cations on the crystal structure and magnetism of the new double perovskites ALaCoTeO6 (A = Na and K).

Abstract

In this work, we report the structural and magnetic characterization of two new B-site rock-salt ordered double perovskites ALaCoTeO6 (A = K+ and Na+) with mixed A-site cations. KLaCoTeO6 crystallizes in the space group P4/nmm with a long-range ordering degree of 84.8% for the A-site K+/La3+ cations, whereas NaLaCoTeO6 adopts an unexpected triclinically distorted I1̄-structure with Na/La3+ disordering, validated by combined Rietveld refinements against high-resolution neutron diffraction data and Cu Kα1 X-ray powder diffraction data. Magnetic susceptibility at low temperatures shows clear antiferromagnetic (AFM) transitions for both compounds. KLaCoTeO6 exhibits the highest AFM transition temperature of 20 K amongst all the Co/Te-ordered 3C-type A2CoTeO6 (A = Pb2+, Sr2+, and Ca2+) and ALaCoTeO6 double perovskites due to its larger Co2+-O-Te6+ bond angle and A-site cationic ordering-induced larger distortion of the Co2+-based face-centered cubic sublattice. Moreover, we found that the average radius of the A-site cations plays a decisive role in the AFM transition temperatures of all these ordered double perovskites, that is, a larger A-site cation always results in a higher AFM transition temperature. This provides a strategy to subtly manipulate the magnetic properties of ordered double perovskites.