The first purely organic BEDT-TTF spin--Peierls system, beta'-(ET)(2)SF5CF2
SO3, has been confirmed using a high-frequency electron paramagnetic resona
nce (EPR) cavity perturbation technique. The material exhibits the characte
ristics of a quasi-one-dimensionai (1D) Heisenberg antiferromagnetic spin s
ystem above 30 K, but undergoes a second-order transition, at Tsp = 33 K, t
o a Singlet ground state, due to a progressive spin-lattice dimerization. T
he spin--Peierls state is evidenced by a sharp drop in the spin susceptibil
ity below 24 K for the magnetic field (of order 2.5 T) parallel to each of
the three principal axes (i.e., H \ \a, H \ \b, and H \ \c). The spin-Peier
ls distortion based on g value shift analysis appears to be predominately a
long the crystallographic b axis. The singlet-triplet gap, Delta (sigma)(0)
= 114 (+/-21) K, was determined using a modified BCS theory. Also, we desc
ribe in some detail the millimeter-wave vector network analyzer (MVNA) for
researchers who have interest in precision EPR measurements at magnetic fie
lds and corresponding resonance frequencies higher than conventional X-band
measurements.