Crystal and electronic structure of heteroanionic praseodymium chlorosilicate Pr₃Cl₅[SiO₄]

Novel heteroanionic tripraseodymium pentachloride orthosilicate, Pr₃Cl₅[SiO₄], was synthesized from a stoichiometric mixture of Pr₂O₃, PrCl₃, and SiO₂ using a self-flux-assisted high-temperature reaction under vacuum-sealed conditions. Single-crystal X-ray diffraction revealed that the compound crystallizes in the orthorhombic space group Pnma (No. 62) with unit-cell parameters a = 16.0012 (10), b = 4.1766 (2), and c = 14.0348 (9) Å (Z = 4). The structure consists of discrete [SiO₄]^4- tetrahedra within the anionic sublattice, embedded within a three-dimensional network of distorted [PrCl₇O₃] polyhedra, in which each Pr³⁺ cation is ten-coordinated in a distorted tetracapped trigonal prismatic or bicapped square-antiprismatic fashion. Refinement revealed coupled positional splitting of the Si atom and the apical O atom aligned approximately along the crystallographic b axis, corresponding to two mirror-related `up-down' orientations of the discrete [SiO₄]^4- tetrahedra in the average Pnma structure. Electronic structure calculations based on a symmetry-lowered disorder-free model generated using ISODISTORT predict an indirect bandgap of ca 4.53 eV, classifying Pr₃Cl₅[SiO₄] as a wide-bandgap insulator. The density of states (DOS) analysis indicates that the valence band is dominated by O 2p and Cl 3p states, while the conduction band is primarily derived from Pr 5d orbitals. Crystal orbital Hamilton population (COHP) and crystal orbital bond index (COBI) analyses establish a clear hierarchy of bonding strengths: Si-O (-iCOHP = 7.35 eV, iCOBI = 0.72) ≫ Pr-O (3.18 eV, 0.48) > Pr-Cl (1.62 eV, 0.27), highlighting the strongly covalent nature of Si-O bonds and weaker, but still appreciable, polar Pr-Cl and Pr-O interactions. These findings provide new insights into the crystal and electronic structure characteristics of rare-earth (RE) chlorosilicates, extending the structural chemistry of the RE₃Cl₅(SiO₄) family.