Crystal and magnetic structures of electrochemically delithiated Li 1− xCoPO 4 phases

Abstract

Precise structural data have been determined from a combined Rietveld refinement, based on neutron and X-ray powder diffraction data simultaneously, for the three phases LiCoPO 4, Li z CoPO 4 with a specific intermediate Li-content z = 0.60(10) and CoPO 4, which are obtained by electrochemical Li-extraction from LiCoPO 4. All three phases are isopointal. Therefore, the transitions between these phases are necessarily of first order, in agreement with their observed coexistence. The same collinear antiferromagnetic structures with magnetic moments nearly parallel to the [010] direction are observed for LiCoPO 4 and Li z CoPO 4, but with a significantly higher Néel temperature of 76 K for the latter compound in comparison with 23 K for LiCoPO 4. Olivine-type CoPO 4 can only be prepared from LiCoPO 4 by delithiation and its physical properties were investigated for the first time. An antiferromagnetic arrangement along the [100] direction is observed for CoPO 4 with an additional weak ferromagnetic component along the [001] direction (magnetic space group Pn′ m′ a and T C = 45 K). The magnetic moment of 3.1(2) μ B per Co-ion indicates a mainly high-spin state for Co 3+ in the octahedral coordination of CoPO 4, which is exceptional and probably the first example in a phosphate. The easy axes and the magnetic exchange interactions between Co-ions change dramatically with the Co 2+ ↔ Co 3+ transition. A continuous change of the formal oxidation state of a transition element by electrochemical Li-extraction and a quasi-continuous in situ observation of the resulting magnetic structure by neutron diffraction appear feasible.