Fig. 2. Three-dimensional CDW order in undoped BaSbO_3−δ.

a,b, Schematic diagrams of expanded and contracted octahedra in BaSbO_3−δ (a) and BBO (b). d denotes bond length between metal and oxygen ions. From the neutron diffraction investigations (Supplementary Fig. 2), two distinct Sb–O bond lengths are estimated to be 2.24(1) and 2.01(1) Å, respectively (Supplementary Table 3 for detailed structural parameters). The difference between the two bond lengths is found to be larger than that of BBO (ref. ⁴). c, Optical absorbance of BaSbO_3−δ at 300 K shows a wide bandgap E_CDW of 2.54 eV caused by the formation of the CDW order. By comparison, the optical conductivity (σ₁) of BBO (ref. ⁵⁰) is plotted as a reference, indicating the CDW gap of BaSbO_3−δ is larger than that of BBO.