Thermometer or freezer: dual functionality in a 2D mixed-anion terbium(<scp>iii</scp>) oxide carbodiimide

Advances in the chemistry of compounds containing the NCN^2- complex anion are leading to the discovery of new materials with interesting and promising properties. Here we present a comprehensive investigation of the optical and magnetic properties of the 2D phosphor Tb₂O₂NCN whose structure presents a double layer of Tb-triangular lattices separated from each other by the NCN^2- anion, offering a conjunction of luminescent thermometry and magnetocaloric effects. Temperature-dependent luminescence studies reveal the typical behavior of thermal quenching, but when the temperature increases, a mechanism of direct relaxation to the ⁵D₄ levels of Tb³⁺ ions is activated, thereby giving rise to an anti-thermal quenching effect. This peculiar feature has prompted us to explore the characteristics of Tb₂O₂NCN in temperature sensing, using the classical approach of optical ratiometric thermometry as well as an approach based on linear transformations (<i>i.e.</i>, principal component analysis). The latter, by employing the entire emission spectrum, offers better sensitivity leading to more accurate temperature values. The magnetic properties of Tb₂O₂NCN reveal a long-range antiferromagnetic ordering below <i>T</i>_N = 6.5 K as well as first-order field-induced metamagnetic transitions. The absence of hysteresis and its high magnetic density make Tb₂O₂NCN a fine candidate for magnetic refrigeration, with a surprisingly large value of -Δ<i>S</i>_M = 11.7 J kg^-1 K^-1 at 5 T, much more promising than those reported for other Tb-comprising compounds.