Interplay between crystal and magnetic structures in YFe2(HαD1−α)4.2 compounds studied by neutron diffraction

Hydrogen order in hydrides of Laves phases

Many Laves phases AM 2 takes up hydrogen to form interstitial hydrides in which hydrogen atoms partially occupy A 2 M 2, AM 3, and/or M 4 tetrahedral interstices. They often exhibit temperature-driven order-disorder phase transitions, which are triggered by repulsion of hydrogen atoms occupying neighboring tetrahedral interstices. Because of the phase widths with respect to hydrogen a complete ordering, i.e., full occupation of all hydrogen positions is usually not achieved. Order-disorder transitions in Laves phase hydrides are thus phase transitions between crystal structures with different degrees of hydrogen order. Comparing the crystal structures of ordered and disordered phases reveals close symmetry relationships in all known cases. This allows new insights into the crystal chemical description of such phases and into the nature of the phase transitions. Structural relationships for over 40 hydrides of cubic and hexagonal Laves phases ZrV2, HfV2, ZrCr2, ZrCo2, LaMg2, CeMg2, PrMg2, NdMg2, SmMg2, YMn2, ErMn2, TmMn2, LuMn2, Lu0.4Y0.6Mn2 YFe2, and ErFe2 are concisely described in terms of crystallographic group-subgroup schemes (Bärnighausen trees) covering 32 different crystal structure types, 26 of which represent hydrogen-ordered crystal structures.

Relation between the weak itinerant magnetism in A 2Ni7 compounds (A = Y, La) and their stacked crystal structures

The weak itinerant magnetic properties of A ₂Ni₇ compounds with A  =  {Y, La} have been investigated using electronic band structure calculations in the relation with their polymorphic crystal structures. These compounds crystallizes in two structures resulting from the stacking of two and three blocks of [A ₂Ni₄  +  2 ANi₅] units for hexagonal 2H-La₂Ni₇ (Ce₂Ni₇ type) and rhombohedral 3R-Y₂Ni₇ (Gd₂Co₇ type) respectively. Experimentally, 2H-La₂Ni₇ is a weak itinerant antiferromagnet whereas 3R-Y₂Ni₇ is a weak itinerant ferromagnet. From the present first principles calculation within non-spin polarized state, both compounds present an electronic density of state with a sharp and narrow peak centered at the Fermi level corresponding to flat bands from 3d-Ni. This induces a magnetic instability and both compounds are more stable in a ferromagnetic (FM) order compared to a paramagnetic state (ΔE  ≈  -35 meV/f.u.). The magnetic moment of each of the five Ni sites varies with their positions relative to the [A ₂Ni₄] and [ANi₅] units: they are minimum in the [A ₂Ni₄] unit and maximum at the interface between two [ANi₅] units. For 2H-La₂Ni₇, an antiferromagnetic (AFM) structure has been proposed and found with an energy comparable to that of the FM state. This AFM structure is described by two FM unit blocks of opposite Ni spin sign separated by a non-magnetic layer at z  =  0 and ½. The Ni (2a) atoms belonging to this intermediate layer are located in the [La₂Ni₄] unit and are at a center of symmetry of the hexagonal cell (P6₃/mmc) where the resultant molecular field is cancelled. Further non-collinear spin calculations have been performed to determine the Ni moment orientations which are found preferentially parallel to the c axis for both FM and AFM structures.