Crystal Structures and Long-Range Spin Orders for the Delithiated Phase of Li9V3(P2O7)3(PO4)2 Insertion Electrode System

The crystal structures and electronic properties for the LixV3(P2O7)(3)(PO4)(2) insertion electrode system, where 7.82 <= x <= 8.76 for single crystals and 6.56 <= x <= 8.63 for polycrystals, are explored through measurements of x-ray four-circle diffraction and magnetization. The system may be delithiated uniformly for x >= 7.5, where the V atom is in a mixed-valence state of V3+ and V4+, consistent with the chemical composition, and for x >= 8, the octahedral Li ion is mainly extracted through the characteristic tunnel. The crystal field of V3+ in the delithiated phase is intermediate, as in the case of x = 9, which suggests the significant ionicity of V atoms and a high voltage in rechargeable lithium batteries. This phase may be regarded as being the distorted kagome lattice with a sizable ferromagnetic superexchange coupling through V-O-P-O-V pathways. For x <= 8, a long-range spin order, specifically a ferromagnetic state with uniaxial magnetocrystalline anisotropy based on the single-ion anisotropy of V3+, appears above 2 K.