Observation of the cubic-field splitting of an excited S=2 manifold in a cubic copper tetramer

EPR measurements on single crystals of Cu4OCl6(triphenylphosphine oxide)4 at liquid helium temperatures in the frequency ranges 14–17 and 34–35 GHz were fitted to a simple cubic S=2 spin Hamiltonian with g=2.10±0.01 and a zero-field splitting of (0.53±0.01) cm−1. From the decrease in intensity of the S=2 spectrum on cooling below 4.2 K and the absence of an S=1 spectrum, the S=2 manifold was deduced to lie (14±1) cm−1 above a nonmagnetic ground state. The EPR results are used as a test of the various theories developed to explain the magnetic susceptibility of copper tetramers.