  The anomalous thermal expansion in a layered 3$d$-5$d$ based triple
perovskite iridate Ba$_{3}$CoIr$_{2}$O$_{9}$ is investigated using high
resolution synchrotron diffraction. Below the magneto-structural transition at
107\,K, the onset of antiferromagnetic order is associated with a monoclinic
distortion of the hexagonal structure. Deeper within the magnetically ordered
state, a part of the monoclinic phase distorts even further, and both these
structural phases co-exist down to the lowest measured temperatures. We observe
negative thermal expansion in this phase co-existence regime, which appears to
be intimately connected to the temperature driven relative fractions of these
monoclinic phases. The significant NTE observed in this system could be driven
by magnetic exchange striction, and is of relevance to a number of systems with
pronounced spin orbit interactions.
