Two Different Mechanisms of Metal Cluster Formation in the Rhombohedral Spinel Structures: AlV₂O₄ and CuZr_1.86(1)S₄

An unusual case of the structural mechanisms of inorganic crystals formation is considered theoretically on the example of the AlV2O4 and CuZr1.86(1)S4 isosymmetric rhombohedral spinels with the same character of the atom distribution on the Wyckoff positions. It is shown that the difference in structures is due to the critical influence of the order parameter value that describes the real (in the case of AlV2O4) and virtual (in the case of CuZr1.86(1)S4) structural phase transitions. Owing to the low value of the order parameter, the phase transition to the rhombohedral AlV2O4 structure is associated with relatively small atomic displacements and the formation of heptamer structural blocks in the V sublattices. In contrast, the virtual phase transition to the rhombohedral CuZr1.86(1)S4 structure is associated with high value of the order parameter and giant atomic displacements. As a result, the Cu and Zr atoms form metal nanoclusters—a “bunch” of dimers. The CuZr1.86(1)S4 is a second substance (the firs...