Zn-O tetrahedral bond length variations in normal spinel oxides

Six synthetic single crystals of spinel phases with different compositions along the ZnAl2O4-ZnCr2O4 solid solution were structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. As predicted, unit-cell parameters and octahedral bond lengths (M-O) increase with increasing replacement of Al3+ by Cr3+. Despite the constant occupancy of the T site by Zn, also the tetrahedral bond length Zn-T-O shows significant variations along this binary. These variations are positively correlated with variations in M-O bond lengths. The present data in conjunction with data from literature provide a basis for quantitative analyses of the variation in Zn-T-O in normal spinel structures. A negative correlation between Zn-T-O and the ionic potential at M ((IP)-I-M) suggests that increasing (IP)-I-M is related to a stronger electrostatic cation-cation repulsion across the shared octahedral edge M(O-O)(shared) of the structure. An observed negative correlation between (IP)-I-M and M(O-O)(shared) suggests that a decrease of M(O-O)(shared) provides a more efficient shielding effect to reduce the octahedral cation interactions. In normal ZnB2O4 spinels (where B = Al3+, Cr3+, Ga3+, V3+, Fe3+, and Mn3+) cations with a smaller size provides a higher charge density. Increasing charge density at the M site causes shortening of M(O-O)shared, which in turn results in shorter Zn-T-O bond length. In general, variations in TZn-O are required by the structure to better provide an oxygen shielding effect to the octahedral cationcation repulsion.