A sodium tantalum phosphate with a noncentrosymmetric structure: Na5Ta8P5O35

A sodium tantalum phosphate Na 5 Ta 8 P 5 O 35 has been successfully obtained by spontaneous crystallization in high temperature solution. It belongs to the noncentrosymmetric R 32 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 [Ta 6 O 27 ] unit composed of 2 ​× ​3 corner-sharing Ta1O 6 octahedra . [Ta 6 O 27 ] units are joined by two P1O 4 tetrahedra and two Ta2O 6 octahedra forming a [Ta 8 P 2 O 29 ] ∞ layer in the bc plane. Those layers are further connected via P2O 4 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 (9 d ) site partially occupied (2/3). The Ta 5+ ions show second-order Jahn-Teller effects with TaO 6 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 ​× ​KH 2 PO 4 and an optical band gap of 3.93 ​eV.