The phase diagram of a polydisperse mixture of uniaxial rodlike and platelike hard parallelepipeds is determined for aspect ratios $\kappa =5$ and 15. All particles have equal volume, and polydispersity is introduced in a highly symmetric way. The corresponding binary mixture is known to have a biaxial phase for $\kappa =15$, but to be unstable against demixing into two uniaxial nematics for $\kappa =5$. The phase diagram for $\kappa =15$ is qualitatively similar to that of the binary mixture, regardless of the amount of polydispersity, while for $\kappa =5$ a sufficient amount of polydispersity stabilizes the biaxial phase. This provides clues for designing an experiment to observe this long searched biaxial phase.

• Received 24 June 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.185701