Rigid unit modes (RUMs) represent a class of lattice vibrations or phonons that exist in network materials such as quartz, cristobalite or zirconium tungstate.
RUMs in crystalline materials are the counterparts of floppy modes in glasses, as introduced by Jim Phillips and Mike Thorpe.
The simplest way to understand the origin of RUMs is to consider the balance between the numbers of constraints and degrees of freedom of the network, an engineering analysis that dates back to James Clerk Maxwell and which was introduced to amorphous materials by Jim Phillips and Mike Thorpe.
What appears to happen is that symmetry reduces the number of constraints so that structures such as quartz and cristobalite are slightly floppy and thus support some RUMs.
A simple counting analysis would in fact suggest that such structures are rigid, but in the ideal cubic phase symmetry allows some degree of flexibility.