We studied Mn 12 -acetate by inelastic neutron scattering and diffraction. We separated the energy levels corresponding to the splitting of the lowest S multiplet (S 10 ground state). The irregular spacing of the transition energies unambiguously shows the presence of high-order terms in the spin Hamiltonian [D 20.457͑2͒ cm 21 , B 0 4 22.33͑4͒ 3 10 25 cm 21 ]. The relative intensity of the lowest energy peaks is very sensitive to the small transverse term that is responsible for quantum tunneling, providing the first determination of this term in zero magnetic field ͓B 4 4 63.0͑5͒ 3 10 25 cm 21 ͔. PACS numbers: 75.25. + z, 75.45. + j, 78.70.Nx 0031-9007͞99͞83(3)͞628(4)$15.00

We studied the Nb-H system over extended pressure and temperature ranges to establish the highest level of hydrogen abundance we could achieve from the resulting alloy. We probed the Nb-H system with laser heating and x-ray diffraction complemented by numerical density functional theory-based simulations. New quenched double hexagonal close-packed (hcp) NbH2.5 appears under 46 GPa, and above 56 GPa cubic NbH3 is formed as theoretically predicted. Nb atoms are arranged in close-packed lattices which are martensitically transformed in the sequence: face-centered cubic (fcc) → hcp → double hcp (dhcp) → distorted body-centered cubic (bcc) as pressure increases. The appearance of fcc NbH2.5−3 and dhcp NbH2.5 cannot be understood in terms of enthalpic stability, but can be rationalized when finite temperatures are taken into account. The structural and compressional behavior of NbHx>2 is similar to that of NbH. Nevertheless, a direct H-H interaction emerges with hydrogen concentration increases, which manifests itself via a reduction in the lattice expansion induced by hydrogen dissolution

We have studied the magnetic and crystal structures of different Laves hydrides RMn 2 H 4.5 (RϭY, Gd, Tb, Dy, Ho͒, having the cubic C15 structure at high temperature. We observe a strong coupling between the hydrogen and magnetic order in the frustrated Mn sublattice. The Néel temperature coincides with the ordering temperature in the hydrogen sublattice, resulting in a single magnetostructural transition. In contrast to the RMn 2 compounds, in the hydrides the Mn-Mn magnetic interaction dominates and it imposes the magnetic order in the rare-earth sublattice. On the other hand, the anisotropy of the rare-earth ion strongly influences the orientation of the magnetic moments at low temperature. The Laves hydrides show a very unusual case where the structural and magnetic orders strongly interact with each other. They also offer many examples of the interplay between the localized Mn moments and the rare-earth moments. ͓S0163-1829͑99͒13213-2͔

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