A competition between 2D and 3D magnetic orderings in novel mixed valent copper frameworks

Abstract

Low-dimensional hybrid inorganic-organic frameworks exhibit high structural flexibility and allow for the inclusion of various magnetic and optically-active species into their host structures. The emergence of copper-based hybrid structures for various optical applications provides a promising foundation for exploring the integration of magnetic sublattices, paving the way for advancements in magneto-optical coupling and multifunctional materials. Herein, we introduce a novel class of hybrid copper frameworks with covalently-connected alternating magnetic 2D copper (II) formate and non-magnetic copper (I) bromide layers. The anionic framework is stabilized by A+ cations to form ACu5Br4(COOH)4 (A+ = Na+, K+, Rb+, NH4+) semiconductors (bandgaps 2.1-2.2 eV) with optical transitions suitable for optoelectronic applications. Comprehensive magnetometry studies show that ACu5Br4(COOH)4 compounds exhibit low-dimensional 2D short-range antiferromagnetic order within the formate layers, characterized by strong exchange coupling (J/kB ~ –100 K). Upon further temperature reduction, interactions between Cu(II) layers give rise to 3D long-range magnetic order at ~ 40 K, despite the large (8.6-8.8 Å) spatial separation of the magnetic Cu(II) formate layers by nonmagnetic Cu(I)-Br bridging layers. This transition is further supported by electron paramagnetic resonance (EPR) spectroscopy. This study expands our understanding of low-dimensional hybrid frameworks and opens new avenues for the design of 2D multifunctional materials.