Valenti and colleagues made mixtures of diazepam and nordazepam and mapped their melting behavior across compositions. The eutectic, or lowest-melting composition, was approximately 18 mole percent nordazepam, with a melting temperature near 395 K. The authors also melted and cooled mixtures to study the resulting noncrystalline material.
The mixture near the eutectic composition resisted recrystallization better than the pure amorphous substances under the study conditions. Dielectric measurements characterized molecular motion in the material. Physical stability here refers to persistence of the amorphous phase. Chemical potency, absorption and stability in a biological sample are separate properties.
The experiment involved deliberately prepared bulk solids. It cannot tell an analyst how diazepam and nordazepam will behave in stored blood, urine or a commercial tablet. Those questions require matrix-specific evidence and documented storage conditions.