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 I-structure with Na/La³⁺ disordering, validated by combined Rietveld refinements against high-resolution neutron diffraction data and Cu K_α1 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.