THE CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF CUGEO3

The crystal structure of CuGeO3, which has been reported to undergo a spin-Peierls transition at 14 K, is determined by neutron powder diffr action at 300, 20 and 4.2 K and the structure refined in the space gro up Pmma at all three temperatures, ie without displacement of the Cu a toms from a uniform one-dimensional chain; anisotropic broadening of ( h0l) reflections, observed in all three data sets, is successfully mod elled by an orthorhombic micro-strain along a and c ba perpendicular t o Cu-Cu chains. Below 10 K the magnetic susceptibility fits very well to the Bulaevskii model, indicating a ratio of 0.69 between the Cu-Cu exchange constants in the dimerised chain and a mean exchange constant of 88 K.