The impact of A-site cations on the crystal structure and magnetism of the new double perovskites ALaCoTeO 6 (A = Na and K).

In this work, we report the structural and magnetic characterization of two new B-site rock-salt ordered double perovskites ALaCoTeO 6 (A = K + and Na + ) with mixed A-site cations. KLaCoTeO 6 crystallizes in the space group P 4/ nmm with a long-range ordering degree of 84.8% for the A-site K + /La 3+ cations, whereas NaLaCoTeO 6 adopts an unexpected triclinically distorted I 1̄-structure with Na/La 3+ 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. KLaCoTeO 6 exhibits the highest AFM transition temperature of 20 K amongst all the Co/Te-ordered 3C-type A 2 CoTeO 6 (A = Pb 2+ , Sr 2+ , and Ca 2+ ) and ALaCoTeO 6 double perovskites due to its larger Co 2+ -O-Te 6+ bond angle and A-site cationic ordering-induced larger distortion of the Co 2+ -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.