Magnetic and transport properties of low-dimensional oxi-borates

Transition metal oxi-borates present a variety of interesting physical behavior associated with the presence of low-dimensional sub-units (ribbons or walls) in their structure. When transition metal ions with incomplete shells and spin S = 1/2 occupy randomly the octahedral sites in the ribbons we observe the formation of a random singlet phase in these one-dimensional structures. The magnetic behavior of ribbons randomly occupied by transition ions of spin S = 1 is also investigated. When the sub-units are walls, we observe the formation of independent magnetic sub-systems which order separately, Homo-metallic systems with mixed valence ions present different types of charge ordering depending on the relative strength of the electron-electron or electron-phonon interaction. (C) 2001 Elsevier Science B.V. All rights reserved.