Origin of the first order magnetostructural transition in YFe2D4.2

The deuterium order-disorder and the first order magnetic transitions in YFe2D4.2 have been studied by X-ray and neutron diffraction, magnetization measurements and Fe57 Mössbauer spectroscopy. From 363 down to 290 K, a progressive lowering of the crystal symmetry from cubic to rhombohedral and then monoclinic structure is observed. At 290 K YFe2D4.2 crystallizes in a primitive monoclinic space group P1c with a=9.429(1)  Å, b=11.474(1)  Å, c=5.508(1)  Å, β=122.37°. This lowering of crystal symmetry leads to very different quantity of D neighbours around each Fe sites. The magnetic study shows that the moment of the Fe3 atoms, surrounded by nearly 5 D atoms, collapses at 84 K, leading to a transition from a ferromagnetic to an antiferromagnetic structure. The first order transition at 84 K is associated with a 0.55% volume decrease and the magnetization curves above 84 K display the same behaviour than the collective electron metamagnetism (CEM) observed in RCo2 compounds. This is related to the fact that for critical deuterium content between 4 and 5 D atoms in YFe2 the Fe 3d ferromagnetism becomes instable due to a strong modification of the electronic band structure.