Polymorphism within the Quasi-One-Dimensional Au₂MP₂ (M = Tl, Pb, Pb/Bi, and Bi) Series

Quasi-1-dimensional (q1D) materials have attracted significant interest for experimentally realizing fundamental physical models. A plethora of q1D systems have been discovered in previous decades, although many have not been extensively examined with modern computational or experimental techniques. Herein, we reexamine the Au₂MP₂ q1D system (formerly, M = Hg, Tl, and Pb) and extend the range of M substituents to Pb_0.53Bi_0.43 and Bi. Remarkably, this q1D system resists changing its highly anisotropic structure type over a range of three valence electrons per formula unit. However, at the highest valence electron count, we find that Au₂BiP₂ exhibits polymorphism: in addition to the previously reported orthorhombic phase, a slight distortion generates a closely related monoclinic structure type, which is found more frequently. Analysis of local packing tensions in the orthorhombic phase reveals strains within the [Au₂P₂] framework hold open channel spaces for the chains of M atoms, which are relatively free to move as a group within the channels. Consistent with this picture, the calculated phonon dispersions show that the monoclinic distortion in Au₂BiP₂ resolves imaginary phonon frequencies seen in the orthorhombic structure type. Using density functional theory computations, we determine that this structural change is driven through rectifying both electronic and atomic packing frustrations. Furthermore, electronic transport measurements substantiate calculations of the band structures and density of states of these materials, which suggest that the compositions within this series can be tuned to band structure and property design.