DOUBLE EXCHANGE IN TETRAMERIC MIXED-VALENCE CLUSTERS

The energy levels of mixed-valence d1-d1-d1-d2 tetrameric clusters are studied. As an initial the model was taken in which every transition metal atom in its ground state contains two non-degenerate orbitals phi(i) and psi(i), the two-electron states being the high-spin ones. The energy levels of a series of distorted pseudo-tetrahedral configurations of rhombic symmetry are calculated as well as the limiting cases of this series - the square and tetrahedral configurations. It is shown that the correlation diagram of the square-planar clusters is symmetrical relative to the sign reverse of the double exchange,parameter. A strong double exchange always leads to the ferromagnetic ground state of the square-planar cluster. While passing from the square-planar to the tetrahedral systems the following types of deformation variations of d1-d1-d1-d2 tetramer spectrum were revealed: a) in the case of weak positive double exchange the deformation does not lead to the change of spin of the ground anti-ferromagnetic cluster state; b) in the case of strong positive double exchange the deformation changes the ground ferromagnetic state of the square system or the state with spin S = 3/2 for the antiferromagnetic state of the tetrahedral system; c) in the case of negative double exchange irrespective of its absolute value within the series of structures D4h --> D2d --> Td the ground state spin is not changed. In the case of weak double exchange this ground state remains antiferromagnetic while when strong double exchange takes place the ground state is ferromagnetic (S = 5/2) or the intermediate one (S = 3/2).