Crystal and electronic structure of heteroanionic praseodymium chlorosilicate Pr ₃ Cl ₅ [SiO ₄ ]

Novel heteroanionic tripraseodymium pentachloride orthosilicate, Pr 3 Cl 5 [SiO 4 ], was synthesized from a stoichiometric mixture of Pr 2 O 3 , PrCl 3 , and SiO 2 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 [SiO 4 ] 4− tetrahedra within the anionic sublattice, embedded within a three-dimensional network of distorted [PrCl 7 O 3 ] polyhedra, in which each Pr 3+ 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 [SiO 4 ] 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 Pr 3 Cl 5 [SiO 4 ] as a wide-bandgap insulator. The density of states (DOS) analysis indicates that the valence band is dominated by O 2 p and Cl 3 p states, while the conduction band is primarily derived from Pr 5 d 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 RE 3 Cl 5 (SiO 4 ) family.