Synthesis, Crystal Structure, and Characterizations of Two Tantalum Phosphates A 3 TaP 2 O 9 (A=K, Na).

Two new compounds K 3 TaP 2 O 9 and Na 3 TaP 2 O 9 were obtained by spontaneous crystallization with KH 2 PO 4 and K 2 HPO 4 or NaH 2 PO 4 and Na 2 HPO 4 as fluxes, respectively. K 3 TaP 2 O 9 belongs to the P 2 1 / m space group of the monoclinic system, with cell parameters of a = 9.8912(7) Å, b = 7.1861(5) Å, c = 13.6457(9) Å, β = 94.047(2)°, and Z = 4. In contrast, Na 3 TaP 2 O 9 pertains to the P 2 1 2 1 2 1 space group of the orthorhombic system, with cell parameters of a = 7.1241(7) Å, b = 9.3071(10) Å, c = 12.3752(13) Å, and Z = 4. Both of the structures feature the existence of the zigzag (TaP 2 O 9 ) ∞ chains with the -(O 4 Ta)-O-(TaO 4 )- connection. Also, two compounds have high transparency of about 90% in the 400-2500 nm range. Theoretical calculations and powder second-harmonic generation test show that the compound Na 3 TaP 2 O 9 has a large band gap of 5.5 eV, a moderate powder second-harmonic generation response of 0.6 × KDP, and a large birefringence of 0.11.