Chains, Layers, and Twists: Structural Evolution in the VSn₂-VSb₂ Pseudo-Binary System

Single crystals of different V(Sn,Sb)₂ phases were grown via self-flux method by employing excess of Sn and Sb. The obtained crystals were studied using X-ray diffraction and EDX spectroscopy, which revealed the existence of two new ternary phases and two solid solutions based on the binary VSn₂ to VSb₂ phases showing peculiar multi-step structural evolution upon going from the VSn₂ side to the VSb₂ side of the system. The first novel phase τ₁ crystallizes in the monoclinic unit cell, while the second phase – τ₂ – adopts orthorhombic crystal structure. The discovered phases derive their structure from the parent phases VSn₂ and VSb₂ and are built from linear chains of fused V(Sn,Sb)₈ antiprisms. These chains are then coupled into layers, which are stacked either right on top of each other or with a with 60° twist. The twists are supported by Sn atoms located at a boundary between the layers. Thus, upon going from VSn₂ to VSb₂, the number of twists per layer decreases from 1 in VSn₂ to 2/3 in τ₁, and to 1/2 in τ₂, ultimately ending at 0 in VSb₂. While the Sn atoms have larger radius compared to that of Sb, the volume per one atom decreases in the VSn_x’Sb_2-x’ solid solution with the increasing Sn content before returning to the expected increase in the τ₁-τ₂-VSn_2-xSb_x sequence. The decrease in volume in the former is caused by the decrease in V-V distances inside the chains, which is caused by the change in character of the V-E interactions, where Sb atoms form two-center V-Sb bonds, while Sn atoms act as bridges forming three-center V-Sn-V bonds as revealed by our DFT-calculations.