Interplay between Two Structure Types in the Incommensurately Modulated ATaSe 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, RbTaSe and CsTaSe, based on the MQ (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 1(αβγ)0, while the basic cell suggests 2/. These materials can be structurally understood as ordered combinations of two known structure types, ATaSe and ATaSe (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 RbTaSe and CsTaSe were optically and thermally characterized, revealing identical band gaps of 1.63 eV and congruent melting points at 434 and 417 °C, respectively.