Lattice Dynamics and Optoelectronic Properties of Vacancy-Ordered Double Perovskite Cs₂TeX₆ (X = Cl^–, Br^–, I^–) Single Crystals

The soft, dynamic lattice of inorganic lead halide perovskite CsPbX<sub>3</sub> (X = Cl<sup>⁻</sup>, Br<sup>⁻</sup>, I<sup>⁻</sup>) leads to the emergence of many interesting photophysical and optoelectronic phenomena. However, probing their lattice dynamics with vibrational spectroscopy remains challenging. The influence of the fundamental octahedral building block in the perovskite lattice can be better resolved in zero-dimensional (0D) vacancy-ordered double perovskites of form A<sub>2</sub>BX<sub>6</sub>. Here we study Cs<sub>2</sub>TeX<sub>6</sub> (X = Cl<sup>⁻</sup>, Br<sup>⁻</sup>, I<sup>⁻</sup>) single crystals to yield detailed insight into the fundamental octahedral building block and to explore the effect that its isolation in the crystal structure has on structural and electronic properties. The isolated [TeX<sub>6</sub>]<sup>2-</sup> octahedral units serve as the vibrational, absorbing, and emitting centers within the crystal. Serving as the vibrational centers, the isolated octahedra inform the likelihood of a random distribution of 10 octahedral symmetries within the mixed-halide spaces, as well as the presence of strong exciton-phonon coupling and anharmonic lattice dynamics. Serving as the absorbing and emitting centers, the isolated octahedra exhibit compositionally tunable absorption (1.50-3.15 eV) and emission (1.31-2.11 eV) energies. Due to greater molecular orbital overlap between neighboring octahedra with increasing halide anion size, there is a transition from a more molecule-like electronic structure in Cs<sub>2</sub>TeCl<sub>6</sub> and Cs<sub>2</sub>TeBr<sub>6</sub>-as expected from the effective 0D nature of these single crystals-to a dispersive electronic structure in Cs<sub>2</sub>TeI<sub>6</sub>, typical of three-dimensional (3D) bulk single crystals.