Pressure-Induced Collapse Transition in BaTi₂Pn₂O (Pn = As, Sb) with an Unusual Pn-Pn Bond Elongation

Making and breaking bonds in a solid-state compound greatly influences physical properties. A well-known playground for such bonding manipulation is the ThCr₂Si₂-type structure AT₂X₂, allowing a collapse transition where a X-X dimer forms by a chemical substitution or external stimuli. Here, we report a pressure-induced collapse transition in the structurally related BaTi₂Pn₂O (Pn = As, Sb) at a transition pressure P_c of ∼15 GPa. The Pn-Pn bond formation is related with Pn-p band filling, which is controlled by charge transfer from the Ti-3d band. At P_c, the Sb-Sb distance in BaTi₂Sb₂O shrinks due to bond formation, but interestingly, the Sb-Sb expands with increasing pressure above P_c. This expansion, which was not reported in ThCr₂Si₂-type compounds, may arise from heteroleptic coordination geometry around titanium, where a compression of the Ti-O bond plays a role. Electrical resistivity measurements of BaTi₂Sb₂O up to 55 GPa revealed an increasing trend of the superconducting transition temperature with pressure. This study presents structure motifs that allow flexible bonding manipulation and property control with heteroleptic coordination geometry.