Superconductivity and crystal structural origins of the metal-insulator transition inBa6−xSrxNb10O30tetragonal tungsten bronzes

Abstract

${\mathrm{Ba}}_{6\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nb}}_{10}{\mathrm{O}}_{30}$ solid solution with $0\ensuremath{\le}$ $x$ $\ensuremath{\le}6$ forms the filled tetragonal tungsten bronze (TTB) structure. The Ba-end member crystallizes in the highest symmetry $P4/mbm$ space group $(a=b=12.5842(18)\AA{}$ and $c=3.9995(8)\AA{})$ and so do all the compositions with $0\ensuremath{\le}$ $x$ $\ensuremath{\le}5$. The Sr-end member of the solid solution crystallizes in the tentatively assigned $Amam$ space group $(a{}^{*}=17.506(4)\AA{}$, $b{}^{*}=34.932(7)\AA{}$, and $c{}^{*}=7.7777(2)\AA{})$. The latter space group is related to the parent $P4/mbm$ TTB structure as $a{}^{*}$ $\ensuremath{\approx}$ $\sqrt{2}a,b{}^{*}$ $\ensuremath{\approx}2\sqrt{2}a,c{}^{*}=2c$. Low-temperature specific heat measurements indicate that the Ba-rich compositions with $x$ $\ensuremath{\le}2$ are conventional BCS superconductors with ${T}_{C}$ $\ensuremath{\le}1.6$ K and superconducting energy gaps of $\ensuremath{\le}0.38$ meV. The values of the ${T}_{C}$ in the cation-filled Nb-based TTBs reported here are comparable with those of the unfilled ${\mathrm{K}}_{x}{\mathrm{WO}}_{3}$ and ${\mathrm{Na}}_{x}{\mathrm{WO}}_{3}$ TTBs having large alkali ion deficiency. As the unit cell volume decreases with increasing $x$, an unexpected metal-insulator transition (MIT) in ${\mathrm{Ba}}_{6\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nb}}_{10}{\mathrm{O}}_{30}$ occurs at $x$ $\ensuremath{\ge}3$. We discuss the possible origins of the MIT in terms of the carrier concentration, symmetry break, and Anderson localization.