  One-dimensional Heisenberg spin 1/2 chains with random ferro- and
antiferromagnetic bonds are realized in systems such as $Sr_3 CuPt_{1-x} Ir_x
O_6$. We have investigated numerically the thermodynamic properties of a
generic random bond model and of a realistic model of $Sr_3 CuPt_{1-x} Ir_x
O_6$ by the quantum Monte Carlo loop algorithm. For the first time we
demonstrate the separation into three different temperature regimes for the
original Hamiltonian based on an exact treatment, especially we show that the
intermediate temperature regime is well-defined and observable in both the
specific heat and the magnetic susceptibility. The crossover between the
regimes is indicated by peaks in the specific heat. The uniform magnetic
susceptibility shows Curie-like behavior in the high-, intermediate- and
low-temperature regime, with different values of the Curie constant in each
regime. We show that these regimes are overlapping in the realistic model and
give numerical data for the analysis of experimental tests.
