Structural Evolution in Hollandite Solid Solutions Across the A-Site Compositional Range from Ba1.33Ga2.66Ti5.34O16 to Cs1.33Ga1.33Ti6.67O16

Hollandite solid solutions along the A‐site compositional range from the pure barium end‐member Ba1.33Ga2.66Ti5.34O16 to the pure cesium end‐member Cs1.33Ga1.33Ti6.67O16 have been synthesized using a solid‐state reaction technique. The crystal structure of the hollandite across the entire compositional range remained in the I4/m space group. Structural evolution was resolved by neutron diffraction, total scattering data, and density functional theory calculations. A trend of decreasing thermodynamic stability with smaller tunnel cations was attributed to increased structural distortion observed in the system. In addition, the tunnel cations' local environment was studied in the eightfold coordinated oxygen cavities. Local binding features of the tunnel cations reveals that the hollandite structure can strongly stabilize tunnel cations, even at elevated temperatures up to 500 K.