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 <i>P</i>1(αβγ)0, while the basic cell suggests <i>C</i>2/<i>c</i>. 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.