for another M 2+ , while if it were assigned to M 2+ -O-M 3+ stretching, it would be sensitive to both M 2+ and M 3+ composition. Given that we do observe shifts of this band with both M 2+ and M 3+ substitution, we propose that this band can be assigned to M 3+ -O-M 2+ stretching. However, it should be noted that the shift of the 550 cm -1 band is small when M 2+ cation has comparable radius to that of Mg 2+ (e.g. Cu, Zn, Ni), as the change in bond lengths is relatively small. The band at ~470 cm -1 is assigned to M 3+ -O-M 3+ , as this band shifts only with substitution of M 3+ . The even weaker band at 300 cm -1 can be assigned to an M-O vibration, which seems to be more profound for the samples with iron and nickel. Bands around 1040 cm -1 and 1060 cm -1 are both assigned to carbonates anions in different environments . HT, e) Cu HT, f) Zn HT.