Interplay between Two Structure Types in the Incommensurately Modulated A14+2/3Ta8Se46+2/3 Compounds (A = Rb, Cs)

Incommensurately modulated crystals are a rare class of materials that are notoriously difficult to characterize properly. We have synthesized two new incommensurately modulated compounds, Rb14+2/3Ta8Se46+2/3 and Cs14+2/3Ta8Se46+2/3, based on the M2Q11 (M = Nb, Ta; Q = S, Se) unit using high-temperature solid-state synthesis. Using superspace crystallography in combination with second harmonic generation measurements, we confirmed both materials to be noncentrosymmetric, falling into the superspace group P1(αβγ)0, while the basic cell suggests C2/c. These materials can be structurally understood as ordered combinations of two known structure types, A6Ta4Se22 and A12Ta6Se35 (A = K, Rb, Cs). While both modulated compounds share structural similarities with the aforementioned known phases, they represent novel structures rather than a literal combination of the two phases, such as a composite. Additionally, both Rb14+2/3Ta8Se46+2/3 and Cs14+2/3Ta8Se46+2/3 were optically and thermally characterized, revealing identical band gaps of 1.63 eV and congruent melting points at 434 and 417 °C, respectively.