Superconductivity and crystal structural origins of the metal-insulator transition in Ba_(6-x)Sr_xNb_(10)O_(30) tetragonal tungsten bronzes

摘要

Ba_(6-x)Sr_xNb_(10)O_(30) solid solution with 0 ≤ x ≤ 6 forms the filled tetragonal tungsten bronze (TTB) structure. The Ba-end member crystallizes in the highest symmetry PA/mbm space group (a = b = 12.5842(18)A and c = 3.9995(8)A) 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)A, b~* = 34.932(7)A, and c~* = 7.7777(2)A). The latter space group is related to the parent PA/mbm TTB structure as a~* ≈ 2a~(1/2), b~* ≈s 2(2a)~(1/2), c~* = 2c. Low-temperature specific heat measurements indicate that the Ba-rich compositions with x ＜ 2 are conventional BCS superconductors with T_C≤1.6 K and superconducting energy gaps of ≤ 0.38 meV. The values of the T_C in the cation-filled Nb-based TTBs reported here are comparable with those of the unfilled K_xWO_3 and Na_xWO_3 TTBs having large alkali ion deficiency. As the unit cell volume decreases with increasing x, an unexpected metal-insulator transition (MIT) in Ba_(6-x)Sr_xNb_(10)O_(30) occurs at x ≥ 3. We discuss the possible origins of the MIT in terms of the carrier concentration, symmetry break, and Anderson localization.