The 6H-Perovskite Dimer Lattice with Antiferromagnetic Interactions: Ba$_3$ARu$_2$O$_9$

Abstract: We investigate the magnetic behavior of the 6H-perovskite dimer lattice Ba$3$Zn${1-x}$Ca$x$Ru$_2$O$_9$ using analytical theory, density functional theory, inelastic neutron scattering, and modeling of historical magnetization and neutron-scattering data. A dimer mean-field theory built upon classical Luttinger-Tisza analysis generates a phase diagram revealing a transition from a nonmagnetic singlet to a finite-moment ground state as interdimer couplings increase. A (generalized) linear spin-wave theory captures multiplet mixing, excitation gap closing, and fluctuation-induced moment suppression. Density functional theory on select compounds and neutron spectroscopy on dilute Ba$_3$Zn(Ru${1-x}$Sb$_x$)$_2$O$_9$ confirm the exchange hierarchy, enabling quantification of previously published experiments within this framework. Our results identify three mechanisms for magnetic moment suppression: quantum fluctuations, ligand hybridization, and nonmagnetic-singlet/magnetic-multiplet mixing.