Compositional Design Guides Property Control in A₂M_2+xTi_1-(x/4)Q₄ Semiconductors

We introduce 15 new members to the A₂M_2+xTi_1-(x/4)Q₄ family (A = K, Rb, Cs; M = Cu, Ag; Q = S, Se, Te; x = 0 - 1$\frac{1}{3}$) of semiconductors, for which all compositions afford the ThCr₂Si₂ structure-type with two-dimensional (2D) [M_2+xTi_1-(x/4)Q₄]^2- layers. We unveil how A, M, Q, and the vacancies inherent to the [M_2+xTi_1-(x/4)Q₄]^2- layers serve as four tunable compositional variables that influence both the local atomic and electronic structures with systematic impacts on the optoelectronic and thermal properties in a predictable manner. Namely, increased distortion within the [M_2+xTi_1-(x/4)Q₄]^2- layers occurs when M = Ag, lowering the melting temperatures of all Ag compounds relative to their Cu counterparts for a given Q. The electronic structures are primarily influenced by Q, with a substitution of S → Se → Te narrowing the average band gaps from 2.30 → 1.49 → 0.74 eV. For a given Q, however, Ag lowers the energy of the valence band maximum (VBM) compared to Cu, and relative to vacuum. Increasing the Ag/Ti ratio inserts additional Ag into the inherent vacancies within the [Ag_2+xTi_1-(x/4)S₄]^2- layers of Cs₂Ag_3.3Ti_0.675S₄ compared to Cs₂Ag₂TiS₄, raising the valence band maximum from -5.26 → -4.83 eV, which in turn narrows the band gap from 2.48 → 1.85 eV. Such broad ranges in thermal and optoelectronic properties within this family provide interest for diverse applications. Moreover, because this work establishes the independent influences of A, M, Q, and the inherent vacancies on properties for a constant structure-type, all composition-property relationships should apply to other structure-types containing any of these elements or inherent vacancies.