We have calculated the phase diagrams of one--component fluids made of five
types of biaxial particles differing in their cross sections. The orientation
of the principal particle axis is fixed in space, while the second axis is
allowed to freely rotate. We have constructed a free-energy density functional
based on fundamental--measure theory to study the relative stability of nematic
and smectic phases with uniaxial, biaxial and tetratic symmetries. Minimization
of the density functional allows us to study the phase behavior of the biaxial
particles as a function of the cross-section geometry. For low values of the
aspect ratio of the particle cross section, we obtain smectic phases with
tetratic symmetry, although metastable with respect to the crystal, as our MC
simulation study indicates. For large particle aspect ratios and in analogy
with previous work [Phys. Chem. Chem. Phys. 5, 3700 (2003)], we have found a
four--phase point where four spinodals, corresponding to phase transitions
between phases with different symmetries, meet together. The location of this
point is quite sensitive to particle cross section, which suggests that
optimizing the particle geometry could be a useful criterion in the design of
colloidal particles that can exhibit an increased stability of the biaxial
nematic phase with respect to other competing phases with spatial order.Comment: 13 pages, 12 figures, submitted to PR