Lacunary lead apatite Pb6Ca2K2(PO4)6: multi-methodological structural investigation, optical and dielectric properties

Apatite-type materials have attracted considerable attention due to their potential applications in optical, electronic, and energy devices. In this study, the synthesis and physicochemical characterization of the novel lacunary lead apatite Pb₆Ca₂K₂(PO₄)₆ were investigated to evaluate its structural, optical, and dielectric properties. The compound was synthesized via solid-state reaction. Structural characterization, performed by powder and single-crystal X-ray diffraction, revealed that the compound crystallizes in the hexagonal P6₃/m space group. The single-crystal analysis further indicated that the apatite framework contains total anionic vacancies, which are stabilized by the stereochemically active lone electron pair of Pb2+ ions. Infrared spectroscopy confirmed the absence of (OH)/(CO) groups in the structure. Optical properties were examined through UV–visible absorption spectroscopy. The material exhibited a strong absorption in the UV region with characteristic absorption bands at 230 and 315 nm. The optical band gap was determined to be a direct band gap of 3.50 eV. Under excitation at 375 nm, fluorescence measurements revealed emission in the green region of the visible spectrum. Dielectric properties were studied using complex impedance spectroscopy over a range of temperatures and frequencies. Results indicated a phase transition around 740 K, likely associated with an order–disorder process. The electrical conduction mechanism is attributed to the thermally activated migration of Ca2+ ions through the structural tunnels along the c-axis. Overall, the structural stability, the semiconductor-like behavior, and the luminescent properties suggest that Pb₆Ca₂K₂(PO₄)₆ is a promising candidate for technological applications in photonic devices, photovoltaic systems, batteries, and light-emitting diodes (LEDs).