A new sodium cobaltophosphate with a tunnel structure, ionic conductor

Abstract

The exploration of the Na–Co–P–O system has allowed a new cobaltophosphate Na₂Co₈(PO₄)₆ with an original tunnel structure to be synthesized. The framework [Co₈P₆O₂₄]_∞ consists of a complex arrangement of corner and edge sharing CoO₅ pyramids and PO₄ tetrahedra forming large tunnels running along and . In this structure two partially occupied Na⁺ sites distributed along the axis of the “hexagonal” tunnels ( axis) are evidenced. Complex impedance measurements show that this phosphate is an ionic conductor above 600 K. The mobility of the Na⁺ ions in the structure is explained by their particular location, which allows successive jumps between the two different sodium sites in the “hexagonal” tunnels.