  The existing controversy about the room temperature structure of multiferroic
Pb(Fe_1/2Nb_1/2)O_3 is settled using synchrotron powder x-ray diffraction data.
Results of Rietveld refinements in the temperature range 300 to 12K reveal that
the structure remains monoclinic in the Cm space group down to 12K, but the
lattice parameters show anomalies at the magnetic transition temperature (T_N)
due to spin lattice coupling. The lattice volume exhibits negative thermal
expansion behaviour, with Alpha = - 4.64 x 10^-6 K^-1, below T_N.
