  The isotopic mass of constituent elements of materials has a well-known
effect on the energy of vibrational modes. By means of monochromated scanning
transmission electron microscopy we have experimentally studied the phonon
bandstructure of hexagonal BN, where a phonon band gap appears between in-plane
optical phonon modes and the lower energy part of the phonon spectrum. The size
of the phonon band gap can be manipulated by the isotopic mass of the boron.
While in $^{11}$BN the phonon band gap is about 7 meV wide, in $^{10}$BN the
gap nearly closes, being an order of magnitude smaller (below 0.5 meV). This
opens exciting options for manipulating terahertz wave propagation through
isotopically structured devices having otherwise no interfaces between
chemically distinct components.
