The phase diagram of a model describing doped CuGeO3 is derived. The model
emphasizes the role of local moments released by the impurities and randoml
y distributed inside the gaped singlet background. The phase diagram is inv
estigated by two methods: (i) in a mean field treatment of the interchain c
oupling and (ii) in a real space decimation procedure in a two-dimensional
model of randomly distributed moments. Both methods lead to similar results
, in a qualitative agreement with experiments. In particular, a transition
to an inhomogeneous Neel phase is obtained for arbitrary small doping. From
the decimation procedure, we interpret this phase at very low doping as a
Griffith antiferromagnet. Namely, it does not have a true long range order
down to zero temperature. Nonetheless, large magnetically ordered clusters
appear already at relatively high temperatures. This demonstrates the role
of disorder in the theoretical description of doping in CuGeO3. A detailed
comparison with other approaches is also given.