  High-resolution synchrotron x-ray diffraction measurements were performed on
single crystalline and powder samples of BiMn2O5. A linear temperature
dependence of the unit cell volume was found between T_{N}=38$ K and 100 K,
suggesting that a low-energy lattice excitation may be responsible for the
lattice expansion in this temperature range. Between T* ~ 65 K and T_{N}, all
lattice parameters showed incipient magnetoelastic effects, due to short-range
spin correlations. An anisotropic strain along the a-direction was also
observed below T*. Below T_{N}, a relatively large contraction of the
a-parameter following the square of the average sublattice magnetization of Mn
was found, indicating that a second-order spin hamiltonian accounts for the
magnetic interactions along this direction. On the other hand, the more complex
behaviors found for $b$ and $c$ suggest additional magnetic transitions below
T_{N} and perhaps higher-order terms in the spin hamiltonian. Polycrystalline
samples grown by distinct routes and with nearly homogeneous crystal structure
above T_{N} presented structural phase coexistence below T_{N}, indicating a
close competition amongst distinct magnetostructural states in this compound.
