Anomalous dielectric response in the triple perovskite ruthenate Ba 3 BiRu 2 O 9 .

We have investigated the magnetic, dielectric and thermal properties of the S Ru= 1 magnetic dimer Ba 3 BiRu 2 O 9 , which is known to exhibit a spin-gap opening in conjunction with a first-order magneto-elastic phase transition at ∼175 K. Above the spin-gap temperature, the temperature dependence dielectric constant shows a peak like feature with pronounced frequency dependence. The critical slowing down behavior of this frequency dispersion suggests that a ferroelectric relaxor like electrical glassy state exists above the spin-gap opening temperature regime. The extermination of frequency dispersion-right at the magneto-elastic phase transition, is suggestive of a strong coupling between the lattice and charge-spin degrees of freedom in this triple perovskite system.