Structural and magnetic characterizations of single-crystal (Na1−xCax)V2O5: charge-ordering and spin-ordering in vanadate systems

We report the X-ray diffraction and magnetic susceptibility measurements on single-crystal samples of the partially filled spin-ladder system (Na1−xCax)V2O5. X-ray powder diffraction shows a monotonic expansion of the b- and c-axis lattice parameters with increasing x, consistent with the larger atomic radius of Ca2+. Small Ca substitution (x<0.04) into NaV2O5 suppresses the charge-ordering transition at 34 K in a manner consistent with a “zig-zag” charge-ordered structure, but the ground state appears to remain a spin-singlet. Further Ca substitution results in a more traditional spin gap behavior with a persistent gapped spin state for x≥0.04. For x<0.04 the spin susceptibility is well described by the same linear chain model as NaV2O5, with coupling constant J∼560 K/kB. Analysis of the high-temperature spin susceptibility data suggests that the behavior of the x≥0.04 samples is better explained by a spin-ladder system with coupling Jladder≈Jrung∼500 K/kB.