The
soft, dynamic lattice of inorganic lead halide perovskite CsPbX3 (X = Cl–, Br–, I–) 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 A2BX6. Here we study Cs2TeX6 (X = Cl–, Br–, I–) 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 [TeX6]2– 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 Cs2TeCl6 and Cs2TeBr6as expected from the effective 0D nature of these
single crystalsto a dispersive electronic structure in Cs2TeI6, typical of three-dimensional (3D) bulk single
crystals.