Crystal and magnetic structures of electrochemically delithiated Li1-xCoPO4 phases

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 LiCoPO4, LizCoPO4 with a specific intermediate Li-content z = 0.60(10) and CoPO4, which are obtained by electrochemical Li-extraction from LiCoPO4. 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 LiCoPO4 and LizCoPO4, but with a significantly higher Neel temperature of 76 K for the latter compound in comparison with 23 K for LiCoPO4. Olivine-type CoPO4 can only be prepared from LiCoPO4 by delithiation and its physical properties were investigated for the first time. An antiferromagnetic arrangement along the [100] direction is observed for CoPO4 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) mu(B) per Co-ion indicates a mainly high-spin state for Co3+ in the octahedral coordination of CoPO4, 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 CO2+ <-> Co3+ 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. (c) 2008 Elsevier Masson SAS. All rights reserved.