EPR probing of bonding and spin localization of the doublet-quartet states in a spin-frustrated equilateral triangular lattice: Cu3(O2C16H23)6·1.2 C6H12

Abstract

We present high-frequency (34 and 95 GHz) EPR spectroscopic measurements of the magnetic parameters of both the ground state (spin-doublet) and the excited state (spin-quartet) of the model frustrated-spin triangular lattice of Cu 3 (O 2 C 16 H 23 ) 6 ·1.2 C 6 H 12 , containing the Cu 3 6+ core. From 295 down to about 100 K, the EPR spectra from single crystals consist of a well-resolved triplet, but with the central component being overlapped by a single peak. At 4 K, the triplet is replaced by a singlet. The triplet is shown to arise from the quartet state, located at 324 K above the ground state. Its magnetic parameters are: D = –535 G, E = 0, g // = 2.209, g ⊥ = 2.057 with the parallel direction being the three-fold axis of the Cu 3 6+ core. The singlet is assigned to the S = 1/2 ground state, with g xx = 2.005, g yy = 2.050, and g zz = 2.282. Its hyperfine structure was that from a single Cu nucleus, with A zz = 157 G, and A xx = A yy < 60 G, demonstrating that in the doublet state the unpaired electron is localized on only one of the three Cu 2+ ions. We ascribe this localization to an antiferromagnetic exchange interaction between Cu 3 6+ cores, with zJ ′ = –0.15 K. These results serve as a basis for detailed theoretical calculations of spin dynamics and electronic bonding in a frustrated triangular magnetic lattice.