Singlet ground state in the spin-1/2 dimer compound Sr3Cr2O8

Magnetic susceptibility chi and specific heat C versus temperature T measurements on polycrystalline samples of Sr3Cr2O8 and the isostructural, nonmagnetic compound Sr3V2O8 are reported. A Curie-Wiess fit to the high-T chi(T) data for Sr3Cr2O8 indicates that the Cr atoms are in the rare Cr5+ (spin S= 1/2) valence state as expected from the composition. The ground state was found to be a spin singlet with an excitation gap Delta/k(B)=61.9(1) K to the magnetic triplet states, and a weak interdimer coupling of 6(2) K was inferred. The C and chi measurements on Sr3V2O8 reveal a phase transition at 110 K, which is evidently a structural transition.