Structure and thermal conductivity of Na1-xGe3+z

Structural analyses as well as low temperature thermal conductivity is reported for the binary phase Na1-xGe3+z. Specimens were characterized by thermal analysis, conventional and synchrotron powder X-ray diffraction, neutron powder diffraction, 23Na nuclear magnetic resonance spectroscopy, and electrical and thermal transport measurements. With structural characteristics qualitatively analogous to some aluminum-silicate zeolites, the crystal structure of this phase exhibits an unconventional covalently bonded tunnel-like Ge framework, accommodating Na in channels of two different sizes. Observed to be non-stochiometric, Na1-xGe3+z concurrently exhibits substantial structural disorder in the large channels and a low lattice thermal conductivity, of interest in the context of identifying novel low thermal conductivity intermetallics for thermoelectric applications. - Graphical abstract: A zeolite-like intermetallic, Na1-xGe3+z, is prepared by thermal decomposition of Na4Ge4. This binary phase exhibits an unusual crystal structure in which germanium forms large tunnels with significant disorder, and is found to possess a very low lattice thermal conductivity.