Crystal chemistry in the Ag 2O-Nb 2O 5 system . AgNb 3O 8 structure determination

The investigation of the AgNbO ₃-Nb ₂O ₅ system is carried out using solid-state routes. This investigation allows to confirm the existence of four compounds with structure related to the Na-based homologous. A new form of AgNb ₃O ₈ is evidenced and its structure is determined on the basis of single-crystal X-ray diffraction investigations. This compound crystallizes in the orthorhombic system (SG Pbam) with cell parameters a=12.453(4) Å; b=12.416(1) Å; c=3.9700(4) Å. It presents a TTB type host network in which triangular tunnels remain empty, square ones are fully filled with Ag ⁺ and pentagonal ones show mixed occupancy with Ag ⁺ and [NbO] ³⁺ entities. Crystal-chemistry investigations show that despite a complex and more or less disordered structure, no evidence for solid solution domain is observed.