  We use combine high resolution neutron diffraction (HRPD) with density
functional theory (DFT) to investigate the exchange striction at the Curie
temperature (TC) of Fe2P and to examine the effect of boron and carbon doping
on the P site. We find a significant contraction of the basal plane on heating
through TC with a simultaneous increase of the c-axis that results in a small
overall volume change of ~0.01%. At the magnetic transition the FeI-FeI
distance drops significantly and becomes shorter than FeI-FeII . The shortest
metal-metalloid (FeI-PI) distance also decreases sharply. Our DFT model reveals
the importance of the latter as this structural change causes a redistribution
of the FeI moment along the c-axis (Fe-P chain). We are able to understand the
site preference of the dopants, the effect of which can be linked to the
increased moment on the FeI-site, brought about by strong magneto-elasticity
and changes in the electronic band structure.
