X-Ray Scattering Study of Spin-Lattice Dimerization in a Quasi One-Dimensional Heisenberg Antiferromagnet

X-ray measurements on the quasi one-dimensional Heisenberg antiferromagnet TTF-(tetrathiafulvalenium-)CuS₄{C}₄(CF₃)₄ reveal a new crystallographic phase below T₀=11 K in which spin-{} TTF⁺ units are paired. The temperature dependence of the order parameter below 11 K is accurately given by the BCS gap equation. Unexpectedly, the pairing is found along a diagonal direction in the →bF-→cF plane rather than along the molecular stacking axis →cF. Furthermore, strong diffuse scattering dominates the x-ray cross section at the dimerization superlattice positions for temperatures as high as 225 K.