Crystal Structure and Spin-Singlet State of the LixVOPO4 Insertion Electrode System with Alternating-Bond Chain

The crystal structure and electronic properties of the triclinic LixVOPO4 insertion electrode system with 0.9 <= x <= 1 are explored through measurements of x-ray four-circle diffraction, magnetization, and electron paramagnetic resonance. For x = 0.95 at 295 K, the one-dimensional corner-sharing chain of distorted VO6 octahedra has the alternating V-V distances of 3.5986 and 3.6219 angstrom with mixed-valence states of V4+ and V5+. All of the compositions exhibit a one-dimensional paramagnetism at temperatures above T-c1 similar or equal to 15K due to the compensation from V-O-V bond angles in spite of the bond alternation. They have a long-range order to spin-singlet states at T-c2 similar or equal to 10 K due to the alternating-exchange couplings, accompanied by spin-dimer fluctuations between T-c1 and T-c2.