Co–Ln Mixed-Metal Phosphonate Grids and Cages
as Molecular Magnetic Refrigerants
ZhengYan-Zhen
EvangelistiMarco
TunaFloriana
E. P. WinpennyRichard
2016
The synthesis, structures, and magnetic properties of
six families
of cobalt–lanthanide mixed-metal phosphonate complexes are
reported in this Article. These six families can be divided into two
structural types: grids, where the metal centers lie in a single plane,
and cages. The grids include [4 × 3] {Co<sub>8</sub>Ln<sub>4</sub>}, [3 × 3] {Co<sub>4</sub>Ln<sub>6</sub>}, and [2 × 2]
{Co<sub>4</sub>Ln<sub>2</sub>} families and a [4 × 4] {Co<sub>8</sub>Ln<sub>8</sub>} family where the central 2 × 2 square
is rotated with respect to the external square. The cages include
{Co<sub>6</sub>Ln<sub>8</sub>} and {Co<sub>8</sub>Ln<sub>2</sub>}
families. Magnetic studies have been performed for these compounds,
and for each family, the maximum magnetocaloric effect (MCE) has been
observed for the Ln = Gd derivative, with a smaller MCE for the compounds
containing magnetically anisotropic 4f-ions. The resulting entropy
changes of the gadolinium derivatives are (for 3 K and 7 T) 11.8 J
kg<sup>–1</sup> K<sup>–1</sup> for {Co<sub>8</sub>Gd<sub>2</sub>}; 20.0 J kg<sup>–1</sup> K<sup>–1</sup> for
{Co<sub>4</sub>Gd<sub>2</sub>}; 21.1 J kg<sup>–1</sup> K<sup>–1</sup> for {Co<sub>8</sub>Gd<sub>4</sub>}; 21.4 J kg<sup>–1</sup> K<sup>–1</sup> for {Co<sub>8</sub>Gd<sub>8</sub>}; 23.6 J kg<sup>–1</sup> K<sup>–1</sup> for {Co<sub>4</sub>Gd<sub>6</sub>}; and 28.6 J kg<sup>–1</sup> K<sup>–1</sup> for {Co<sub>6</sub>Gd<sub>8</sub>}, from which we can see these
values are proportional to the percentage of the gadolinium in the
core.