A sodium tantalum phosphate with a noncentrosymmetric structure: Na5Ta8P5O35

A sodium tantalum phosphate Na5Ta8P5O35 has been successfully obtained by spontaneous crystallization in high temperature solution. It belongs to the noncentrosymmetric R32 space group with a ​= ​8.895 (2) Å, c ​= ​29.992 (12) Å and Z ​= ​3. The structure resembles those of tungsten bronzes with mai...
Ausführliche Beschreibung

A sodium tantalum phosphate Na5Ta8P5O35 has been successfully obtained by spontaneous crystallization in high temperature solution. It belongs to the noncentrosymmetric R32 space group with a ​= ​8.895 (2) Å, c ​= ​29.992 (12) Å and Z ​= ​3. The structure resembles those of tungsten bronzes with main building blocks of the [Ta6O27] unit composed of 2 ​× ​3 corner-sharing Ta1O6 octahedra. [Ta6O27] units are joined by two P1O4 tetrahedra and two Ta2O6 octahedra forming a [Ta8P2O29]∞ layer in the bc plane. Those layers are further connected via P2O4 groups along the c direction leaving triangular and pentagonal tunnels in the 3-dimensional framework. Both sodium atoms reside in the pentagonal tunnel with Na1 (9d) site partially occupied (2/3). The Ta5+ ions show second-order Jahn-Teller effects with TaO6 distortions that are favorable to obtain large hyperpolarizabilities. Powder Kurtz-Perry test and reflectance spectroscopy studies reveal that it exhibits moderate second harmonic effect of about 0.5 ​× ​KH2PO4 and an optical band gap of 3.93 ​eV. Ausführliche Beschreibung