Zn-O tetrahedral bond length variations in normal spinel oxides

Abstract

Six synthetic single crystals of spinel phases with different compositions along the ZnAl 2 O 4 -ZnCr 2 O 4 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 Al 3+ by Cr 3+ . Despite the constant occupancy of the T site by Zn, also the tetrahedral bond length T Zn-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 T Zn-O in normal spinel structures. A negative correlation between T Zn-O and the ionic potential at M ( M IP ) suggests that increasing M IP 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 M IP 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 Zn B 2 O 4 spinels (where B = Al 3+ , Cr 3+ , Ga 3+ , V 3+ , Fe 3+ , and Mn 3+ ) 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 T Zn-O bond length. In general, variations in T Zn-O are required by the structure to better provide an oxygen shielding effect to the octahedral cation-cation repulsion.