Synthesis, Crystal Structure, and Characterizations of Two Tantalum Phosphates A₃TaP₂O₉ (A=K, Na)

Two new compounds K₃TaP₂O₉ and Na₃TaP₂O₉ were obtained by spontaneous crystallization with KH₂PO₄ and K₂HPO₄ or NaH₂PO₄ and Na₂HPO₄ as fluxes, respectively. K₃TaP₂O₉ belongs to the P2₁/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, Na₃TaP₂O₉ pertains to the P2₁2₁2₁ 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 (TaP₂O₉)_∞ chains with the -(O₄Ta)-O-(TaO₄)- 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 Na₃TaP₂O₉ 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.