Direct Bi3+–Bi3+ contacts mediated by lone pairs in the HP-BiNiO(PO4) polymorph

Abstract

A high-pressure polymorph of BiNiPO₅ was prepared at 5 GPa. In the HP phase, the parent 1D-[BiNiO]³⁺trans chains of edge-sharing O(Bi₂Ni₂) oxo-centered tetrahedra are conserved but reorganized into a more compact crystal-structure. It returns short Bi–Bi contacts (3.42 Å) with interacting lone pair (LP) electrons via a weakly bonding σ bond. The main density of states topology is preserved bringing a calculated bandgap about 3.45 eV for the HP-form against 3.52 eV for the ambient pressure (AP) form. The magnetic properties are mainly held by the 1D subunits, which order antiferromagnetically at T_N = 16.3 K compared to 17.5 K in AP-BiNiPO₅. Our work attests the oxo-centered polyhedra’s role and principles, where the polyhedra form the robust framework of the 3D-lattice which is able to survive structural transformation in extreme conditions.