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La5Ti2Cu1−xAgxS5O7, a solid solution of La5Ti2CuS5O7 and La5Ti2AgS5O7, is a semiconductor studied for sunlight-driven water splitting. In this study, the band structure and effective mass of La5Ti2Cu1−xAgxS5O7 solid solutions are analyzed on the basis of density functional theory (DFT) calculations to reveal the effect of the composition on the band structure and the anisotropy in the conductivity. The band gap of La5Ti2Cu1−xAgxS5O7 materials exhibits a bowing with respect to the value of x. The effective masses of holes and electrons in La5Ti2Cu1−xAgxS5O7 are estimated from the band structure. Both holes and electrons have smaller effective masses in the b-axis direction than in the other directions, testifying the highly anisotropic electronic properties. The wave functions at the valence band maximum and the conduction band minimum are both delocalized along the b-axis only, forming parallel channels for electron and hole conduction. These results support the experimental observation that photoexcited carriers tend to migrate one dimensionally. The outcome of the theoretical calculation is qualitatively consistent with the experimental observations and helps to understand the electronic properties of La5Ti2Cu1−xAgxS5O7 with varying compositions. [ABSTRACT FROM AUTHOR]