Characterization and Solid-State UV–Vis Investigations of Photoelectrocatalytically Active La₅Cl₇[TeO₃]₄, a Mixed Anion Compound with Alternating 2D Layers of Oxygen and Chlorine

An oxide chloride, La₅Cl₇[TeO₃]₄, was synthesized using the conventional high-temperature solid-state synthesis technique in an inert atmosphere. This compound possesses a novel crystal structure that can be described with the triclinic space group <i>P</i>1̅ (No. 2) and unit cell parameters: <i>a</i> = 7.2634(3) Å, <i>b</i> = 8.1241(3) Å, <i>c</i> = 9.1993(3) Å, α = 79.373(1)°, β = 83.599(1)°, and γ = 82.511(1)°. The preference of Te(IV) to coordinate to oxygen and direct its lone pair toward the lower charged chlorine results in 2D layers of both oxygen and chlorine, alternating along the crystallographic <i>b</i>-direction. Homoleptic coordination, solely to oxygen, and heteroleptic coordination to oxygen and chlorine are observed for lanthanum, forming layers connected through edge-sharing polyhedra. In the crystal structure, two distinct tellurium positions are observed, with three close Te-O distances, emphasizing an active lone pair. The compound has been investigated by solid-state UV-vis measurements, and a band gap of 3.44 eV has been determined by DFT calculations. Detailed photoelectrochemical measurements clearly indicate that the title compound is photoelectrocatalytically active, showing an n-type behavior. Raman spectroscopy confirms that complex tellurite ions are present in the crystal structure; several observed bands can be assigned to Te-O stretching, reflecting the relatively low crystallographic symmetry of the title compound.