What are the two separate compound's spectra? I mean, where do the standalone materials absorb?

Dear Andrea thank you.

CuBi2O4 strong and broad absorption in the range of 200–900 nm (band gap energy of 1.38eV ) and SnO2 absorbs in the UV region at 210-245 nm and at 270 nm (band gap energy of 3.59 eV).

The composite comosition is highly temperature dependent. Many a times , there is the formation of of Bi2O3 which absorbs at 280-300 and MASKS the actual spectra of the two constituents

Thank you so much for your answer. But, the same behaviour is observed at room temperature .

A very peculiar behavior of CuBi2O4 is reported at different temperature as:

At higher temperature(more than 500C), it can form UV-visible absorbing Bi2O3 showing VERY INTENSE band ( CT band) at 280-300 nm.

At low temperatures(less than 78C), it dissociates to form a mixture of CuBi2O4 and CuO; both absorbing in the UV-visible region.

J. Mater. Chem. A, 2014,2, 3661-3668.

Who knows that your case may fall in second category..

By the way can you send an attachment containing the spectrum?

At least, we will be able to rule out the formation of B2O3 completely as its spectra is available in the literature.

Thank you again, find the attached file of spectra

Alas the UV – Vis. is a replica of SnO2, So we can judge whether it is SnO2 or Bi2O3 or both.

Thank you sir Actually the the UV-vis spectra schould be belonging to SnO2 (matrix in our case. Bi2O3 should be present as traces.