Unraveling the Complex Structural Transformations in Cu 3– x Te 2

High Resolution Image Download MS PowerPoint Slide The Cu–Te system contains particularly complex copper chalcogenides due to their intricate crystal structures and multiple phase transitions. This study investigates the structural transitions of Cu 3– x Te 2 through high-resolution synchrotron measurements conducted from room temperature (RT) up to 693 K. At room temperature, Cu 3– x Te 2 displays incommensurately modulated orthorhombic structure with lattice parameters a = 4.00912(1) Å, b = 12.22806(3) Å, and c = 3.98424(1) Å, along with a modulation wavevector q = 0.3999(1) c *. A transformation to a tetragonal structure (space group P 4/ nmm; a = 4.01877(2) Å, c = 6.11105(3) Å) occurs at ∼423 K, followed by a transition to a hexagonal structure (space group P 6 m 2; a = 7.28869(1) Å and c = 7.85504(1) Å) at ∼623 K. Both transitions are reversible upon cooling. High-resolution transmission electron microscopy (HRTEM) and high-resolution annular dark field scanning TEM (HR-ADF STEM) reveal nanoscale origin of the modulation. Most notably a robust doubling along c and occasional longer-period supercells, linked to ordering of the copper sublattice with fractional occupancy. Density functional theory (DFT), quantum theory of atoms in molecules (QTAIM), and electron localizability indicator (ELI-D) reveal very small charge transfer and the emergence of lone-pair-like basins on Te upon Cu vacancy formation, suggesting that ordering/reordering of Te lone pairs coupled to Cu vacancy ordering drives the orthorhombic ↔ tetragonal ↔ hexagonal transformations. The high-temperature hexagonal modification exhibits a well-ordered Te framework but a highly disordered Cu sublattice, reminiscent of fast-ion conductors such as Cu 2 Se, indicating potential ionically dynamic behavior and motivating transport studies toward superionic functionality.