  Based on density functional calculations, we propose that ZrMn_2 is a
polymorphic material. We predict that at low temperatures the cubic C15, and
the hexagonal C14 and C36 structures of the Laves phase compound ZrMn_2 are
nearly equally stable within 0.3 kJmol^{-1} or 30 K. This degeneracy occurs
when the Mn atoms magnetize spontaneously in a ferromagnetic arrangement
forming the states of lowest energy. From the temperature dependent free
energies at T approx 160K we predict a transition from the most stable C15 to
the C14 structure, which is the experimentally observed structure at elevated
temperatures.
