The color of the pellet changes due to the change in the microstructure of the sample. As the grain boundaries are formed and the porosity is reduced after the sintering, the absorption and scattering of the light by the sample also change.

The color form after a sintering depends on the type of furnace used in the sintering process. Have you been using the vacuum furnace, inert gas or normal pressure and the presence of air furnace?. In the case of the use of the vacuum furnace, the samples prepared used the raw materials such as carbonate for example and polymeric binders will burn in the formation of carbon remains in these samples and interacts with other materials to be undesirable materials. While in the case of the furnace with air will burn these materials disintegrate and turn carbon into CO2 gas.

It needs a little of background for pure perovskites ABO3[SrTiO3] and doped perovskites.

First considering the SrTiO3 alone.

It has been repored in the literature that in the perovskite SrTiO3( being a nonstoichiometric compound), on heating , there occurs a change in morphology( the grain size, crystal structure etc) . So the number of oxygen vacancies change,i.e. there occurs a change electronic transitions to exhibit wide range of colors, from yellow or light brown to blue-violet and finally black. This effect is called the thermochromism of SrTiO3.[It can be completely reversed by reheating in an oxygen atmosphere or cooling slowly to room temperature from any temperature above 850 C].

Now considering a Ce dopped ABO3[SrTiO3] . It is reported in literature,that

there occurs a decrease in the lattice parameter of doped ABO3 to suggest that a majority of the cerium is Ce(III) substituting on the A-site. B-site substitution would result in an increase in the lattice parameter and A-site substitution by Ce(IV) would not be expected based on the relative ionic radii of Sr(II) and Ce(IV).

In the oxidized samples, there is a significant amount of CeO2 present in the cation stoichiometric sample. Both Ce(III) and Ce(IV) being yellow/ orange yellow( with differing intensities) to make look yellow ( the intensity may vay due to temperature) The A-site deficient sample was single phase.

When the cation stoichiometric sample was sintered for 24 hr., the CeO2 amount decreases significantly and in the A-site deficient sample there is a Ti-rich phase present which being analogous to SrTiO3 and is expected to finally look black.

For Rampyar Singh Kushwaha

Since, you happen to be a scholar working on doping of perovskites( I hope) with RE ions and must the needing the references of the paper/s which corroborate with my above said answer,I submit below two references- first one for the pure ABO3[SrTiO3] and second one for the Ce doped ABO3[SrTiO3] for your perusal and authentication..

[A][Materials Research

Print version ISSN 1516-1439

Mat. Res. vol.1 n.1 São Carlos Oct. 1998

http://dx.doi.org/10.1590/S1516-14391998000100004

Single-Crystal SrTiO3 Fiber Grown by Laser Heated Pedestal Growth Method: Influence of Ceramic Feed Rod Preparation in Fiber Quality

D. Reyes Ardila, M.R.B. Andreeta, S.L. Cuffini, A.C. Hernandes,

J.P. Andreeta, Y.P. Mascarenhas]

[B][J. Am. Ceram. Soc., ]] []]] 1–8 (2011)

Electrical Properties and Dimensional Stability of Ce-Doped SrTiO3-delta

for Solid Oxide Fuel Cell Applications

Denis J. Cumming,w,z Vladislav V. Kharton,y Aleksey A. Yaremchenko,y Andrei V. Kovalevsky,y and John A. Kilnerz z

Department of Materials, Imperial College London, London SW7 2AZ, U.K.

Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal]

Thank you sir @ Manohar sir

Following  a simple rule(unless other factors  come to play their part), this is consistent with a LATTICE  COMPACTION  and STRENGTHENING OF LOCAL CATION-ANION BONDS.[Ce(III)=115ppm; Sr(II)=128ppm; from Wikipedia, the free encyclopedia].

 This means Ce(III)- O(-2) would be stronger/ shorter than Sr(II)- O(-2) bond.  The best evidence  for this reasoning comes   from the fact that if we scan the Raman spectra of STO with  increasing   %   Cerium  doping, we  will notice  that the blue shift[ increase in wave number] becomes more and more  pronounced  up to   certain % Ce limit).