NaGe3P3: a new ternary germanium phosphide featuring an unusual [Ge3P7] ring

Abstract

A new ternary germanium phosphide, NaGe₃P₃, was obtained for the first time with the use of NaP as the reactive flux. It crystallizes in the orthorhombic space groupPmc2₁. The basic structural unit is an unprecedented [Ge₃P₇] ring built from one Ge(P)₄ tetrahedron, one Ge(Ge)(P)₃ tetrahedron and one Ge(Ge)(P)₂ trigonal pyramid with Ge in mixed valences of 4+, 3+ and 1+. The bonding between a tetrahedrally coordinated Ge atom and a trigonal pyramidally coordinated Ge atom (with 4s² lone pair of electrons) is observed for the first time in inorganic compounds. These [Ge₃P₇] rings are connected with each other to form two-dimensional ²_∞[Ge₃P₃]⁻ layers separated by Na⁺ cations. An optical band gap of 2.06(2) eV was deduced from the diffuse reflectance spectrum. Based on the electronic structure calculation, NaGe₃P₃ is an indirect gap semiconductor with the Ge 4s, Ge 4p and P 3p orbitals strongly hybridizing around the Fermi level.