Study on the thermochromic behavior of V⁵⁺ doped BiPO₄

To investigate the reasons for the thermochromism of V⁵⁺ doped BiPO₄, a series of V⁵⁺ doped BiPO₄ powders (BiP_1−xV_xO₄) were prepared by hydrothermal method. The crystal structure, morphology and chemical composition of BiP_1−xV_xO₄ were characterized using XRD, SEM, TEM and XPS. The crystalline phase of V⁵⁺ doped BiPO₄ changes from monoclinic to hexagonal. The morphology of BiP_1−xV_xO₄ tends from polyhedra to ellipses. TEM displays that BiVO₄ and BiPO₄ crystals are stacked together. With the increase of V⁵⁺ doping, the color of sample gradually changed from grayish-green to green olive at room temperature. When the temperature rose to 500℃, the color of the samples gradually turned to gray-brown and red-brown. Compared with BiPO₄, V⁵⁺ doped BiPO₄ exhibited reversible thermochromic behavior. Among BiP_1−xV_xO₄, BiP_0.65V_0.35O₄ showed the most obvious reversible change of "yellow-green↔red-brown" between 20℃ and 500℃, color difference value ∆E=28.35. The research results of in-situ UV-Vis spectrum showed that the absorptions of BiP_0.65V_0.35O₄ were enhanced in visible light with increasing temperature. And the spectra of sample showed a red shift from 529 nm to 560 nm. During the heating and cooling process, BiVO₄ in BiP_0.65V_0.35O₄ exhibited a reversible phase transition between monoclinic and tetragonal phases using in-situ XRD. This transition is the main reason for reversible thermochromic of BiP_0.65V_0.35O₄.