a new n-type thermoelectric material, was synthesized via a high-temperature solid-state routine. The quasi-layered structure features of were established by a comprehensive study including variable-temperature single-crystal X-ray diffraction, synchrotron powder X-ray diffraction, DFT calculations, and resonant ultrasound spectroscopy. The structural relationship between and two previously reported compounds, and , is addressed. The quasi-layered structure of coupled with point defects accounts for its ultralow lattice thermal conductivity. First-principles simulations predict that the electrical properties of are sensitive to Cu content, which is confirmed by the thermoelectric property measurements of (x = 0, 0.1, and 0.2) samples. Through tuning the Cu content, shows the best performance due to the highest Seebeck coefficient combined with a moderate electrical conductivity, achieving zT = 0.42 at 775 K. This work proves that crystal structure engineering can achieve extremely low lattice thermal conductivity in crystalline solids.