Check this article

http://www.sciencedirect.com/science/article/pii/S0926860X06008519

Given the interest in lithium vanadium phosphate batteries, I am surprised you cannot find the spectrum?

Then measure it yourself. 1 hour job including sample preparation. You might want to dissolve and measure the UV-vis spectra of vanadium phosphate in a certain concentration of phosphoric acid. Different V oxidation states display different colours so it should be informative and pretty :-)

I agree with Samar Layek and the article given by her

Thank you so much for the valuable information.

how do you know the material you synthesized is Vanadium phosphate not vanadyl phosphate?

I agree with Amir Reza Judy Azar. Vanadium in solution always forms the Vanadyl iom, i.e.= VO2+^ - RC Ropp

Dear Amir Reza Judy Azar and Rc Ropp,

Thank you so much for noticing the mistake I made, and that should be vanadyl instead of vanadium phosphate.

Vanadyl phosphate, besides being used as catalyst and electrode materials in electrochemical capacitor, can it be used in optoelectronics (such as vanadium pentoxide in IR detector)? Could anyone provide information (books, journal, patents, etc.) on that?

Thanks in advance.

To-Boon-Hong Wee: There is a book which gives an abbreviated summary of vanadium chemistry- Reference Book of Inorganic Chemistry by Lundell & Hildebrarnd (1940) which I have found to be fairly accurate. And, of course, Remy- Treatise on Inorganic Chemistry - which gives some of the old work (1800's to 1920's) some of which is accurate but most has been superseded; also Sidgwick- Chemical elements and Their Chemical Compounds (1950) is the least accurate since it relies on 1800's work but gives the references- RC Ropp