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

Abstract

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 Cs2TeBr6as expected from the effective 0D nature of these single crystalsto a dispersive electronic structure in Cs2TeI6, typical of three-dimensional (3D) bulk single crystals.

Supramolecular Assembly of Halide Perovskite Building Blocks

“…The (18C6@Cs) 2 TeCl 6 dumbbell structural unit belonged to the S 6 point group, where two Cs + cations and the Te 4+ cation sit on the S 6 axis, and the six-fold symmetry of the 18C6 and the S 6 axis of the O h -symmetric [TeCl 6 ] 2– octahedron were perfectly aligned. In the (18C6@Cs) 2 TeCl 6 dumbbell unit, the Te–Cl bond length was determined as 2.546 Å, which was comparable to that in the Cs 2 TeCl 6 vacancy-ordered double perovskite (2.570 Å) . The high symmetry of this dumbbell building block offered a unique rhombohedral packing of the octahedra at the macroscopic level.…”

“…The halide perovskite has been the spotlight of semiconductor research in the past decade owing to its superior optoelectronic properties: a high optical absorption coefficient, tunable band gap, long free carrier diffusion length, high defect tolerance, and efficient photo-/electro-luminescence. , There are increasing studies revealing the fact that the [MX 6 ] n– (M = Pb 2+ , − Sb 3+ , − Te 4+ , − Sn 4+ , , Pt 4+ , etc. ; X = Cl – , Br – , I – ) metal halide ionic octahedral units are the fundamental building blocks and functional units in metal halide perovskites.…”

Mapping the Real-Time Vibrational Infrastructure of Cs2SnI6 Nanocrystals through Coherent Phonon Dynamics

“…Contrasting these defect-ordered Cs 2 SnI 6 architectures with their counterparts exhibiting an ideal corner-shared octahedral network reveals that the isolated [SnI 6 ] 4– polyhedra in the former allow greater degrees of freedom for free motion. , This dynamic motion originating primarily from the coupling of Cs + displacements and octahedral tilting results in divergence from pure harmonic approximation, setting in intense anharmonic vibrations . Moreover, both the optical and electronic traits of the charge carriers eventually rely on how the mobile charge carriers couple to the low-energy phonon modes and are fundamental for device performance germinating from such dynamics. , Thus for further significant technological advancements deploying these Sn 4+ based all-inorganic perovskites, it is of utmost importance to have an extensively fundamental inspection concerning the phonon vibrational modes and the subsequent carrier phonon scattering pathways. , However, to date, not only do the vibrational phonon features in the ground state configuration of this tin-based all-inorganic DP system lack comprehensive exploration but also the correlation of the underlying phonon modes and electronic transitions is scarcely investigated.…”

“…20−22 Contrasting these defect-ordered Cs 2 SnI 6 architectures with their counterparts exhibiting an ideal corner-shared octahedral network reveals that the isolated [SnI 6 ] 4− polyhedra in the former allow greater degrees of freedom for free motion. 23,24 This dynamic motion originating primarily from the coupling of Cs + displacements and octahedral tilting results in divergence from pure harmonic approximation, setting in intense anharmonic vibrations. 25 Moreover, both the optical and electronic traits of the charge carriers eventually rely on how the mobile charge carriers couple to the low-energy phonon modes and are fundamental for device performance germinating from such dynamics.…”

Ab-initio Study of Structural and Electronic Properties of Perovskite Nanocrystals of the CsSn[Br1−xIx]3 Family

“…The PDOS plots can be explained by two aspects: orbitals, which contribute near the band edges, and the general contribution of states. In the CsSnBr3 system, electrons of the I (d) -state and Sn (p) -state contribute near the valence band (CB) and conduction bands (CB), respectively (Figure 9 Thus, our results can contribute to understanding some of the features of their optical properties, which are important for the practical application of the studied systems, and may turn out to be of interest for researchers searching for materials with predetermined and programmed optoelectronic properties [39][40][41][42].…”