  Several spin systems with low dimensionality develop a spin-dimer phase
within a molecular orbital below TS, competing with long-range
antiferromagnetic order. Very often, preferential orbital occupancy and
ordering are the actual driving force for dimerization, as in the so-called
orbitally-driven spin-Peierls compounds (MgTi2O4, CuIr2S4, La4Ru2O10,
NaTiSi2O6, etc.). Through a microscopic analysis of the thermal conductivity k
(T) in La4Ru2O10, we show that the orbital occupancy fluctuates rapidly above
TS, resulting in an orbital-liquid state. The strong orbital-lattice coupling
introduces dynamic bond-length fluctuations that scatter the phonons to produce
a k (T) proportional to T (i.e. glass-like) above TS. This phonon-glass to
phonon-crystal transition is shown to occur in other spin-dimer systems, like
NaTiSi2O6, pointing to a general phenomenon.
