Na xCr xTi 8- xO 16, Priderite with Sodium Ions in the Tunnel—Structural Study for Stability and Na Ion Transport

Abstract

Single crystals of Na priderite Na x Cr x Ti 8- x O 16 ( x ∼ 1.7) with the hollandite-type tunnel structure have been grown by a flux method; tetragonal, I4/m with a = 10.058(1), c = 2.957(1) Å and Z = 1. Formation of Na priderite has been unexpected due to the size fitness between the framework structure and the tunnel ion because the cavity in the hollandite-type tunnel appears too large to stabilize Na ions. Single-crystals X ray diffraction analysis has revealed that the Na ion distribution in the tunnel is essentially different from that of large ions such as K, Rb, and Cs. In contrast with the usual case of K priderite, a part of the Na ions are located in particular around the center of the square plane of oxygens connecting adjacent cavities. This fact suggests this oxygen square would no longer act as a bottleneck for transport of the tunnel ion as in K priderite. Other Na ions are distributed at four equivalent positions around the tunnel axis in the cavity. Such a unique situation of the Na ions never seen in K priderite will give rise to characteristic behavior in Na ion conduction.