Crystal structures and electronic properties in the crossover region between the spin-gap system CaV2O5and the linear-chain system NaV2O5

The structural and electronic properties of the Ca 1- x Na x V 2 O 5 system, where the composition CaV 2 O 5 exhibits an isolated dimer-like spin-gap state, and NaV 2 O 5 indicates a linear-chain magnetic behaviour and undergoes a spin-singlet transition at T c = 34 K, have been explored by means of x-ray four-circle diffraction and through magnetization and electron paramagnetic resonance measurements. The crystal structures with 0< x 0.1 and 0.8 x 1 are isomorphous with that of CaV 2 O 5 , where the space group is Pmmn . The effective V valence is correlated with the V-O bond length in the VO 5 pyramid, and the transfer integral for the next-nearest-neighbour V-V path is suggested to be larger than those for other paths, which may be consistent with the formation mechanisms of the dimer in the Ca-rich compounds and the V-O-V molecular orbital in the Na-rich compounds. The magnetic properties for 0< x 0.1 are interpreted as being governed by contributions from the dimer which is formed between the V zigzag chains and from the isolated spins that are present due to the breaking of the dimer. On the other hand, the properties for 0.9 x <1 still exhibit linear-chain behaviours, and for x 0.97 the singlet transition with a large reduction of the exchange coupling constant disappears, and no magnetic ordering takes place. The characteristic spin dynamics for the Na-rich compounds is also discussed