Superconductivity and crystal structural origins of the metal-insulator transition in Ba6−xSrxNb10O30 tetragonal tungsten bronzes

Ba6−xSrxNb10O30 solid solution with 0≤ x ≤6 forms the filled tetragonal tungsten bronze (TTB) structure. The Ba-end member crystallizes in the highest symmetry P4/mbm space group (a=b=12.5842(18)Å and c=3.9995(8)Å) and so do all the compositions with 0≤ x ≤5. The Sr-end member of the solid solution crystallizes in the tentatively assigned Amam space group (a*=17.506(4)Å, b*=34.932(7)Å, and c*=7.7777(2)Å). The latter space group is related to the parent P4/mbm TTB structure as a* ≈ 2a,b* ≈22a,c*=2c. Low-temperature specific heat measurements indicate that the Ba-rich compositions with x ≤2 are conventional BCS superconductors with TC ≤1.6 K and superconducting energy gaps of ≤0.38 meV. The values of the TC in the cation-filled Nb-based TTBs reported here are comparable with those of the unfilled KxWO3 and NaxWO3 TTBs having large alkali ion deficiency. As the unit cell volume decreases with increasing x, an unexpected metal-insulator transition (MIT) in Ba6−xSrxNb10O30 occurs at x ≥3. We discuss the possible origins of the MIT in terms of the carrier concentration, symmetry break, and Anderson localization.