Anomalous crystal-structure distortion at the antiferromagnetic transition in the layered cobalt oxide ${\mathrm{Sr}}_2{\mathrm{Y}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{Co}}_2{\mathrm{O}}_6$

The temperature-dependent crystal structure and magnetic structure of the layered Ruddlesden-Popper phase ${\mathrm{Sr}}_2{\mathrm{Y}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{Co}}_2{\mathrm{O}}_6$ are reported. The crystal structure is orthorhombic below 270 K [Immm; $a=3.840\mathrm{}29\left(7\right)\mathrm{}\AA ,$ $b=3.806\mathrm{}91\left(6\right)\mathrm{}\AA ,$ and $c=19.4980\mathrm{}\left(3\right)\mathrm{}\AA$ at 20 K], transformed from tetragonal [$I4/mmm;$ $a=3.827\mathrm{}65\left(6\right)\mathrm{}\AA ,$ and $c=19.5795\mathrm{}\left(3\right)\mathrm{}\AA$ at 295 K]. The crystal structure distortion is accompanied by an anomalous temperature dependence of the degree of buckling of the ${\mathrm{CoO}}_2$ plane and is correlated with the development of anisotropic antiferromagnetic ordering of magnetic moments [$2.93\left(2\right)\mathrm{}{\mu }_B$ per Co at 20 K].