Three-dimensional complete photonic bandgap materials or photonic crystals block light propagation in all directions. The rod-connected diamond structure exhibits the largest photonic bandgap known to date and supports a complete bandgap for the lowest refractive index contrast ratio down to nhigh/nlow ∼ 1.9. We confirm this threshold by measuring a complete photonic bandgap in the infrared region in Sn–S–O (n ∼ 1.9) and Ge–Sb–S–O (n ∼ 2) inverse rod-connected diamond structures. The structures were fabricated using a low-temperature chemical vapor deposition process via a single-inversion technique. This provides a reliable fabrication technique of complete photonic bandgap materials and expands the library of backfilling materials, leading to a wide range of future photonic applications.