The impact of A-site cations on the crystal structure and magnetism of the new double perovskites ALaCoTeO (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 (A = K and Na) with mixed A-site cations. KLaCoTeO crystallizes in the space group 4/ with a long-range ordering degree of 84.8% for the A-site K/La cations, whereas NaLaCoTeO adopts an unexpected triclinically distorted 1̄-structure with Na/La disordering, validated by combined Rietveld refinements against high-resolution neutron diffraction data and Cu K X-ray powder diffraction data. Magnetic susceptibility at low temperatures shows clear antiferromagnetic (AFM) transitions for both compounds. KLaCoTeO exhibits the highest AFM transition temperature of 20 K amongst all the Co/Te-ordered 3C-type ACoTeO (A = Pb, Sr, and Ca) and ALaCoTeO double perovskites due to its larger Co-O-Te bond angle and A-site cationic ordering-induced larger distortion of the Co-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.