In general, any magnetic ordering breaks the symmetry of the system, and structural distortions compatible with the new symmetry can happen due to magneto-structural coupling. Often this coupling is too weak to be detected, but in other cases like NiO is strong enough.
And this is indeed what happens for bcc Cr (where the initial question was about). Quoting from pages 173-174 of "Phase diagrams of the elements" (David A. Young):
"At room pressure and temperature, Cr is bcc. This structure is modified very slightly by two first order magnetic phase transitions. From 0 to 123 K, Cr is antiferromagnetic with a small tetragonal distortion of the bcc lattice. From 123 K to 311 K, Cr is antiferromagnetic with as small orthorhombic distortion of the bcc lattice."
The original experiments can be found in http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.23.979
Stefaan -- Stefaan Cottenier Center for Molecular Modeling (CMM) & Department of Materials Science and Engineering (DMSE) Ghent University Tech Lane Ghent Science Park – Campus A building 903 BE-9052 Zwijnaarde Belgium http://molmod.ugent.be http://www.ugent.be/ea/dmse/en email: stefaan.cotten...@ugent.be my conference talks on Youtube: http://goo.gl/P2b1Hs for China: http://i.youku.com/cottenierlectures _______________________________________________ Wien mailing list Wien@zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://firstname.lastname@example.org/index.html