Nasrin Azad

Perhaps during sintering an aluminum salt of chromic acid H2CrO4 was formed. Salts of chromic acid are yellow.

Nasrin Azad

Perhaps during sintering an aluminum salt of chromic acid H2CrO4 was formed. Salts of chromic acid are yellow.

Dear Nasrin,

In an alkaline environment and elevated temperature under oxygene or air, Cr3+ is easily oxidised to Cr6+ (CrO42- or Cr2O72-), which are yellow to orange due to a LMCT absorption band in the blue spectral range. I assume that Cr2O3 reacts with your crucible to α-Al2(CrO4)2Cr2O7, a structure which was solved a long time ago. Therefore, the choice of crucible material must be done with care. For instance, Cr can be leached out from stainless steel as CaCr2O4 and CaCrO4 due to the contact to lime (which is alkaline too!) and high temperature.

I would recommen BN or W crucibles under inert gas.

All the best for your research,

Thomas

Dear Nasrin Azad, I fully agree with Yuri Mirgorod, Artur Braun and Thomas Jüstel in that oxidation to Cr+6 is most likely the origin of the yellow coloration. Especially chromates (CrO42–) are often yellow, and none of the other metals in your system is typically known to form yellow compunds. The original paper on the structure of α-Al2(CrO4)2Cr2O7·4H2O is available on RG. What puzzles me is that this compound has been described as red crystals, but perhaps the anhydrous form is yellow.

Article Crystal structure of α-Al2(CrO4)2Cr2O7·4H2O

related to the calcination process

Dear Nasrin, Cr(VI) gives a significant yellow coloration, Cr(III) - red in stronger crystal field and green in weaker cfrystal field. Check the oxidation states in your materials.

I fully agree with Yuri Mirgorod, Artur Braunand Thomas Jüstelin that oxidation to Cr+6is most likely the origin of the yellow coloration.