The nature of the unconventional ordered phase occurring in CeRu2Al10 below T0 = 27 K was investigated by neutron scattering. Powder diffraction patterns show clear superstructure peaks corresponding to forbidden (h + k)-odd reflections of the Cmcm space group. Inelastic neutron scattering experiments further reveal a pronounced magnetic excitation developing in the ordered phase at an energy of 8 meV. The low-temperature behavior of intermetallic cerium compounds can be broadly typified in terms of the competition between several interaction channels (intra-atomic couplings, on-site Coulomb repulsion, hybridiza-tion between local f-electron states and itinerant conduction-band states), forming the basis of the well-known and highly successful Anderson model. 1 However, there has also been continued interest in Ce-based materials which do not seem to fit into this general framework. Among those are, for instance, the "Kondo insula-tors", as well as various compounds exhibiting multipole ordering 2 or other types of elusive "hidden order" transitions. One example of such unconventional ordering properties has been discovered very recently by Strydom 3 in the ternary compound CeRu 2 Al 10. CeRu 2 Al 10 is an YbFe 2 Al 10-type orthorhombic compound belonging to the Cmcm space group, with room-temperature lattice constants a = 9.1272Å1272Å, b = 10.282Å 282Å, and c = 9.1902Å1902Å. It has been described as a "cage" crystal structure, in which Ce atoms are separated from each other by an exceptionally large distance of 5.2 ˚ A. From the lattice constants, the Ce valence state was estimated to be close to 3+. The transport properties below room temperature are indicative of a gap in the electronic structure, 3 although the Hall effect still suggests a dominant metallic character. In this regime, the material exhibits considerable magnetic anisotropy (a: easy axis, b: hard axis). 4-6 Upon application of pressure, the system rapidly changes, first to a Kondo insulator, then to a metal above 5 GPa. 4 The striking feature of this compound is the phase transition taking place at T 0 = 27 K, which causes pronounced anomalies in various physical properties. Whereas the origin of this transition remains highly controversial, there is growing evidence that it cannot reduce to a conventional ordering of local Ce magnetic moments. The transition temperature is far too high in view of the large Ce-Ce distance and, more specifically, of the magnetic ordering temperature of 16.5 K found in GdRu 2 Al 10. 4 The drop in the magnetic susceptibility below T 0 occurring for all three magnetic field orientations H a, b, c, with an exponential behavior χ = χ 0 + A exp(−∆/T) and ∆ ∼ 100 K, is also difficult to reconcile with the behavior expected for an antiferromagnet. 4,7 Finally, 27 Al NQR/NMR experiments did not find the splitting of peaks below T 0 expected for a static order of Ce magnetic moments. 8 Alternative mechanisms such as charge-or spin-density-wave formation also have serious shortcomings. 4,8 Recently, Tanida et al. 5,7 sugge...
