We report the observation of a new type of charge-density wave (CDW) in the large magnetic-moment rare-earth intermetallic compound, Er 5 Ir 4 Si 10 , which then orders magnetically at low temperatures. Single crystal x-ray diffraction shows the development of a 1D incommensurate CDW at 155 K, which then locks into a purely commensurate state below 55 K. The well-localized Er 31 moments are antiferromagnetically ordered below 2.8 K. We observe very sharp anomalies in the specific heat at 145 and 2.8 K, signifying the bulk nature of these transitions. Our data suggest the coexistence of strongly coupled CDW with local-moment antiferromagnetism in Er 5 Ir 4 Si 10 . PACS numbers: 71.45.Lr, 75.20.Hr, 71.20.Lp Charge-density-wave (CDW) transitions occur in lowdimensional solids where it is possible to achieve nesting of Fermi surfaces that leads to the appearance of a periodic lattice distortion with an accompanying energy gap D [1-4]. These materials show striking nonlinear and anisotropic properties as well as unusual elastic and dynamic behaviors: This makes them an appealing field for experimental and theoretical studies on Fermi surface gapping and electron-phonon coupling. A number of such systems have been previously studied, including quasi-1D organic salts (e.g., TTF-TCNQ) [5] and inorganic chain compounds (e.g., NbSe 3 ) [2]. Here a Peierls-Fröhlich-type [4] phase transformation is observed at a finite temperature T CDW : Above this temperature the coupled electronphonon system is unstable with respect to a deformation of wave vector q 2k F . Below T CDW , the ground state is characterized by a gap in the single-particle excitation spectrum.Although many of the properties of these quasi-1D compounds could be understood within the framework of this continuous phase transition with weak electronphonon and interchain coupling, the thermodynamic behavior of the CDW transition in compounds such as TaSe 2 [6] and the blue bronzes [3] requires a description beyond the weak-coupling model. McMillan [7], using strong electron-phonon coupling and small interchain correlation lengths, was able to provide a microscopic theory, which has given a semiquantitative explanation for the peculiar behavior of these latter systems, like their specific heat jumps and D͞T CDW ratios. In recent years interest in this area has focused on detailed investigation of these conventional CDW systems. However, in order to probe the theory further and search for novel CDW behaviors, especially those involving the interplay with magnetism, new classes of materials are needed. To the best of our knowledge there does not exist a local-moment magnet (mediated by the RKKY interaction between the f and conduction electrons) exhibiting a CDW.We have begun a quest for new CDW systems in intermetallic rare-earth (RE) compounds and found indications in the literature [8][9][10] for such behavior in a series of RE 5 Ir 4 Si 10 materials. Polycrystalline samples showed anomalies in the resistivity above 20 K, which were tentatively attributed to CD...

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