UV–vis-NIR spectroscopy was performed by diffuse reflectance with integrating sphere (Perkin-Elmer l35, USA)
in the 300–1100 nm range, step 0.3 nm, using BaSO4 as a reference. Reflectance measurements (R) were converted
into absorbance data (A) by the Kubelka–Munk equation: A = (1 R)22R1 [13]. Optical spectra were deconvolved
by Gaussian bands for crystal field transitions and a Lorentzian band for metal–oxygen charge transfer, using the
software PFM (OriginLab).