WEAK INTERLAYER AND INTRALAYER EXCHANGE COUPLING BETWEEN COPPER(II) DIMERS AND A TRIPLET DENSITY EFFECT IN EPR OF TRIS(ETHYLENEDIAMINE) COBALT(III) BIS(MU-CHLORO)BIS[TRICHLOROCUPRATE(II)] DICHLORIDE DIHYDRATE

Low-temperature EPR spectra of Co(en)(3)CuCl5 . H2O single crystals we re studied at X-band. The crystals contain layers of antiferromagnetic ally coupled Cu2Cl84- dimers, with singlet-triplet splitting 2J(0) = - 14.8 cm(-1), separated by diamagnetic Co(en)(3) molecules. Superexchan ge coupling between dimers located in neighboring layers, determined f rom the merging effect of EPR lines from inequivalent sites, is J(1) = 0.00712 cm(-1) at room temperature, decreases to 0.00325 cm(-1) below 100 K, and then is temperature independent. Below 15 K the fine struc ture lines from S = 1 are resolved with the following zero-field split ting parameters: D-z = -0.0200 cm(-1), D-y = 0.0147 cm(-1), and D-x = 0.0053 cm(-1). The appearance of the structure is a result of a decrea se in the number of magnetic dimers due to a rapid thermal depopulatio n of the triplet state when kT < 2J(0). A detailed analysis of this pr ocess allowed us to determine intralayer exchange coupling between dim ers which amounts to J(2) = 0.041 cm(-1) at 100 K and decreases on coo ling following triplet density rho(T) = [1 + 1/3 exp(-2J(0)/kt)](-1)) behavior. Hyperfine splitting is not resolved down to 4.2 K, and the r elatively narrow lines indicate a strong mobility of the still remaini ng triplet excitons. The line width decreases on cooling down to about 100 K, and then a strong broadening appears resulting in the fine str ucture splitting. Contributions from the narrowing effect, triplet sta te density, and temperature variations of the zero-field splitting hav e been separated in Delta B-rho rho(T). A decrease in the zero-field s plitting occurs on heating and, in the temperature range 4.2-100 K, is phenomenologically described by the equation D(T) = D-0(1 - aT - bT(- 2)).