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

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 group Pmc2(1). 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 4s(2) 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)(infinity)[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.