Cr-substituted polycrystalline Ba₂(In_2-xCr_x)O₅·(H₂O)_δ powders (0.04 ≤ x ≤ 0.60) were synthesized by solid state reaction to investigate the relation of crystal structure, thermochemical, magnetic, and optical properties. The Cr-substitution results in an unit cell expansion and formation of the higher-symmetric tetragonal phase together with increased oxygen and hydrogen contents. Magnetic property measurements reveal that the diamagnetic pristine Ba₂In₂O₅·(H₂O)_δ becomes magnetically ordered upon Cr-substitution. By UV–vis spectroscopy a gradual shift of the absorption-edge energy to lower values was observed. Numerical calculations showed that the observed bandgap narrowing was ascribed to the Cr induced states near the Fermi level. The correlation between the changes of crystal chemistry, magnetic, and optical properties of Cr-substituted Ba₂(In_2-xCr_x)O₅·(H₂O)_δ can be explained by the replacement of In by Cr. Consequently, an enhanced photocatalytic CO₂ reduction activity was observed with increasing Cr substitution, compatible with the state-of-the-art high surface area TiO₂ photocatalyst (P-25).

Cr-substituted and pristine Ba₂In₂O₅·(H₂O)_x powders were synthesized by solid state reaction. The influence of Cr-substitution on the crystal structure, chemical composition, magnetic and optical properties were investigated. Powder X-ray diffraction (XRD), elemental analysis and TGA-MS reveal that with substitution of In for Cr, the unit cell volume and the unit cell parameter b increase together with the oxygen and hydrogen content. Magnetic property measurements indicate that Ba₂In₂O₅·(H₂O)_x is diamagnetic in the temperature range of 2 K < T < 300 K becoming ferromagnetic upon Cr-substitution. In the UV–vis spectra of the Cr-substituted sample a distinctive shift of the absorption-edge energy from 430 to 690 nm was observed corresponding to a bandgap narrowing from 2.88 to 1.80 eV. The replacement of tetrahedral InO₄ units by octahedral CrO₆ units was found to be the main factor for the drastic change of the magnetic and optical properties.