Pressure effect on the magnetism and structure of a spin-Peierls substance:MEM-[TCNQ]2

An anomalous pressure effect on the magnetic susceptibility of a spin-Peierls substance, MEM-[TCNQ]2, is observed. As the pressure increases, the susceptibility at low temperatures increases as if free magnetic spins are produced and it overwhelms the spin-Peierls transition. In order to clarify the pressure effect, the crystal structure of the substance under pressure has been studied by using synchrotron radiation X-rays. The reflection resulting from the superlattice of the spin-Peierls state below 18 K disappears under pressure and the crystal structure is significantly affected even at room temperature