Unraveling the complex structural transformations in Cu3-x Te2

Abstract

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 Cu3-x Te2 through high-resolution synchrotron measurements conducted from room temperature (RT) up to 693 K. At room temperature, Cu3-x Te2 displays incommensurately modulated orthorhombic structure with lattice parameters a = 4.00912(1) & Aring;, b = 12.22806(3) & Aring;, and c = 3.98424(1) & Aring;, along with a modulation wavevector q = 0.3999(1)c*. A transformation to a tetragonal structure (space group P4/nmm; a = 4.01877(2) & Aring;, c = 6.11105(3) & Aring;) occurs at similar to 423 K, followed by a transition to a hexagonal structure (space group P 6 m2; a = 7.28869(1) & Aring; and c = 7.85504(1) & Aring;) at similar to 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 Cu2Se, indicating potential ionically dynamic behavior and motivating transport studies toward superionic functionality.