  We report a comprehensive temperature-dependent investigation of the 1:2
ordered triple perovskite system Sr$_3$CaRu$_2$O$_9$. It crystallizes in the
monoclinic structure with space group $P21/c$, consisting of corner-sharing
CaO$_6$ and RuO$_6$ octahedra. Using DC magnetization and neutron diffraction
measurements, we show that this system undergoes successive magnetic
transitions ~190 K and ~160 K. From the analysis of the temperature-dependent
neutron diffraction and dielectric data, we demonstrate two distinguishing
features of the Sr$_3$CaRu$_2$O$_9$ system: (i) magnetoelastic coupling
associated with the two magnetic transitions, as revealed by a change in the
unit cell volume, direct Ru-Ru distance, and bond angles and (ii)
magnetodielectric coupling, as revealed by the anomalies across the two
magnetic transitions. The present results would add significantly to the
current understanding of the triple perovskites with incipient spin-orbit
coupling.
