Charge density wave (CDW) systems are proposed to exhibit application potential for electronic and optoelectronic devices. However, CDWs often develop at cryogenic temperatures, which hinders their applications. Therefore, identifying new materials that exhibit a CDW state at room temperature is crucial for the development of CDW-based devices. Here, we present a nonlayered tetragonal polymorph of Gd 2 Os 3 Si 5 , which exhibits a CDW state at room temperature. Gd 2 Os 3 Si 5 assumes the tetragonal Sc 2 Fe 3 Si 5 structure type with the space group P4/mnc. Singlecrystal X-ray diffraction (SXRD) analysis shows that Gd 2 Os 3 Si 5 possesses an incommensurately modulated structure with modulation wave vector q = (0.53, 0, 0), while the modulation reduces the symmetry to orthorhombic Cccm(σ00)0s0. This differs from isostructural Sm 2 Ru 3 Ge 5 , where the modulated phase has been reported to possess monoclinic symmetry Pm(α0γ)0. Reinvestigation of Sm 2 Ru 3 Ge 5 suggests that its modulated crystal structure can alternatively be described by Cccm(σ00)0s0, with modulations similar to Gd 2 Os 3 Si 5 . The temperature-dependent magnetic susceptibility indicates an antiferromagnetic transition at T N ≈ 5.5 K. Furthermore, it shows an anomaly at around 345 K, suggesting a CDW transition at T CDW = 345 K, in agreement with high-temperature SXRD measurements. The temperature-dependent electrical resistivity has a maximum at a lower temperature, which we nevertheless identify with the CDW transition and can be described as an insulator-to-metal transition. The calculated electronic band structure indicates q-dependent electron−phonon coupling as the dominant mechanism of CDW formation in tetragonal Gd 2 Os 3 Si 5 . The modulated structure then indicates a major involvement of the Si2a atom in the CDW modulations. Compounds RE 2 T 3 X 5 (RE = rare earth, T = transition metal, X = Si, Ge) have been reported with either the tetragonal Sc 2 Fe 3 Si 5 structure type or the orthorhombic U 2 Co 3 Si 5 structure type. Not all of these compounds undergo CDW phase transitions. We find that RE 2 T 3 X 5 compounds will exhibit a CDW transition if the condition < < c ab 0.526 / 0.543 is satisfied.
