Thermometer or Freezer: Dual Functionality in a 2D Mixed-Anion Terbium (III) Oxide Carbodiimide

Advances in the chemistry of compounds containing the NCN2– 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 NCN2– anion, offering a conjunction of luminescent thermometry and magnetocaloric effect. 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.e., 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 TN = 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 –∆SM = 11.7 J kg–1 K–1 at 5 T, much more promising than those reported for other Tb-comprising compounds.