Pressure effects on the dimerization in the spin-Peierls state of CuGeO3

The dimerization in the spin-Peierls state of CuGeO3 has been studied by neutron diffraction under high pressures up to 1.65 GPa. Effects of pressure on the chain direction along the c axis are fairly small; however, the dimerization of Cu along this direction tends to decrease. In comparison with this small change, the effects on the oxygen displacements in the ab plane are rather large. The shift of oxygen atoms decreases also along the a axis; on the other hand, the shift along the b axis increases. These characteristic changes in the dimerization together with those in the bond lengths and bond angles are briefly discussed in connection with the large enhancement of the transition temperature Tsp and also with the decrease in the exchange energy J which were recently observed under high pressure.