Nonmagnetic spin-singlet dimer formation and coupling to the lattice in the 6H perovskite Ba3CaRu2O9.

Non-conservation of magnetic neutron scattering from the six-layer hexagonal perovskite Ba3CaRu2O9 on cooling reveals that (Ru(5+))2 dimers form nonmagnetic spin-singlet ground states. This is in contrast to Ba3BRu2O9 (B=Co or Ni) analogues which display long range spin ordered ground states containing antiferromagnetic (Ru(5+))2 dimers (Lightfoot and Battle 1990 J. Solid State Chem. 89 174). Dimer formation is weakly coupled to the lattice, resulting in an excess [101] microstrain broadening of diffraction peaks at low temperatures. Non-conservation of magnetic neutron scattering is also observed around the charge ordering transition in Ba3NaRu2O9 showing that the ground state structure is coupled to spin-singlet formation in charge ordered Ru2O9 dimers.