Interplay between Two Structure Types in the Incommensurately Modulated A_14+2/3Ta₈Se_46+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, Rb_14+2/3Ta₈Se_46+2/3 and Cs_14+2/3Ta₈Se_46+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 Rb_14+2/3Ta₈Se_46+2/3 and Cs_14+2/3Ta₈Se_46+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.