Y 2 Ti 2 O 5 S 2 was first reported in a study of the structural and optical properties of the Ln 2 Ti 2 O 5 S 2 series. The material crystallises in the tetragonal space group I4/mmm (no. 139) in a unique quasi-layered structure consisting of two connected building blocks, viz. layers of two-dimensional corner-sharing octahedra and rocksalt layers (Figure ). Unlike some layered systems, which are held together by weak dispersion forces, the titanium and yttrium motifs in Y 2 Ti 2 O 5 S 2 are held together by Ti-S and Y-O bonds. Y 2 Ti 2 O 5 S 2 has been studied as a potential battery anode but has also proven to be an excellent candidate for hydrogen production by photocatalytic water splitting. The performance of the material in these applications stems from its uniquely complex structure, which renders it stable to Li intercalation and gives rise to relatively disperse electronic bands. The complex structure is also expected to facilitate strong phonon scattering and low lattice thermal conductivity, making Y 2 Ti 2 O 5 S 2 a promising thermoelectric candidate.