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

Abstract

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 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.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 T_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 −ΔS_M = 11.7 J kg⁻¹ K⁻¹ at 5 T, much more promising than those reported for other Tb-comprising compounds.