Heat capacity of the highly frustrated triangulated Kagome lattice Cu9Cl2(cpa)6

Magnetic field-dependent heat capacity measurements on the tunable molecular magnet Cu9Cl2(cpa)6, a metal organic framework (MOF) based on copper(II) ions and carboxypentonic acid, suggest that this molecular magnetic material could serve as a candidate for quantum spin liquid (QSL) studies. The 2-D framework has been described as a ‘triangles-in-triangles’ (TIT) Kagomé or triangulated-Kagomé-lattice (TKL), and previous magnetic experiments and theoretical studies have suggested this Kagomé Heisenberg antiferromagnet (KHAF) is highly frustrated. Here we report heat capacity data down to 50mK that confirms this frustration. Above 5K the material exhibits field-independent heat capacity that largely fits C = αT3+βT behavior with parameters that vary with the temperature window of analysis. Below 5K, the zero-field heat capacity exhibits no obvious phase transition down to 0.050K and levels off at a nonzero value. Upon the application of magnetic field, a peak in the heat capacity (Cmax) emerges from below 0.050K. The temperature at which (Cmag/T)max occurs, Tmax, scales linearly with applied field. As Tmax increases, the magnitude of (Cmag/T)max decreases and the peak broadens, eventually subsumed into the αT3 behavior above 3T. Below Cmax, the heat capacity decreases to zero at finite temperatures in large fields. These data corroborate magnetic data and confirm that this frustrated TKL material does not exhibit a phase transition in low-to-zero field down to 0.050 K. Further, the temperature Tmax nominally extrapolates to 0K in zero-field, yielding a T-H phase diagram similar to Herbertsmithite; one that suggests CPA could be a possible candidate to develop additional chemical variants for QSL studies.