Nonstoichiometry and Defects in Hydrothermally Synthesized ϵ-LiVOPO₄

ϵ-LiVOPO₄ has been synthesized through the hydrothermal method by adjusting the pH of the hydrothermal solution and the reaction temperature. This phase is formed between 180 and 220 °C, as diamond-like crystals around 10-15 μm in size. X-ray diffraction (XRD) analysis shows that hydrothermal ϵ-LiVOPO₄ lattice parameters a and b linearly decrease, while c linearly increases when the synthesis temperature increases. Thermogravimetric analysis with mass spectroscopy reveals 1.5 to 0.5% water loss at about 350 °C for ϵ-LiVOPO₄ synthesized at 180 and 220 °C, suggesting water or protons incorporation into the structure. Magnetic studies reveal ferrimagnetism in hydrothermal ϵ-LiVOPO₄ below 10 K, as opposed to antiferromagnetic ordering below 14 K found in samples synthesized at high temperature. In-situ XRD upon heating of the hydrothermal ϵ-LiVOPO₄ synthesized at 180, 200, and 220 °C reveals that the temperature dependences of their lattice parameters merge at about 500 °C; furthermore, at the same temperature the structure reversibly changes from triclinic to monoclinic. The lattice parameters and the magnetic properties of the hydrothermal samples heated to 750 °C are similar to those of solid-state synthesized ϵ-LiVOPO₄. Based on structure and composition analysis, we suggest that hydrothermal samples can be described as an ϵ-Li_1+xH_yV_1-zOPO₄ (x, y, z < 0.1) solid solution. The electrochemical characterization of hydrothermal ϵ-LiVOPO₄ reveals the first cycle capacity of about 300 mAh/g, which holds for about five cycles, gradually decreasing thereafter. The low-voltage region does not reveal voltage plateaus corresponding to Li_1.5VOPO₄ and Li_1.75VOPO₄ phases found in the solid-state material, further suggesting structural disorder in the low-temperature samples evidenced from the lattice parameters and the magnetic properties.