Crystal structure and thermoelectric properties of the incommensurate chimney-ladder compound RhGe_γ (γ ∼ 1.293)

The (3 + 1)-dimensional crystal structure of the Nowotny chimney-ladder compound Rh17Ge22 was revealed using powder x-ray diffraction technique. As in the case of the higher manganese silicide MnSiγ (known as Mn11Si19, etc.), this germanide consists of two tetragonal subsystems of [Rh] and [Ge] with an irrational c-axis ratio γ = c Rh/c Ge, and hence the structural formula can be represented as RhGeγ. As expected from first-principles calculations of the approximate structure and the valence electron count, n-type (negative Seebeck coefficient) conduction was experimentally confirmed over the whole temperature range of 333-984 K. Although the absolute value of the Seebeck coefficient was limited to |S| ≤ 53 μV/K, a high electrical conductivity (σ = 4.8 × 10³ S/cm) yielded a reasonable power factor of S ²σ ∼ 1 mW/K² m above 600 K. A maximum dimensionless figure-of-merit of ∼0.1 at 900 K was expected using thermal conductivity data of a sintered pellet.