Synthesis, Structure, and Characterization of Two New Layered Mixed-Metal Phosphates, BaTeMO₄(PO₄) (M = Nb⁵⁺ or Ta⁵⁺)

Two new isostructural mixed-metal phosphates, BaTeMO₄(PO₄) (M = Nb⁵⁺ or Ta⁵⁺), have been synthesized as bulk phase powders and single crystals by standard solid-state techniques using BaCO₃, TeO₂, Nb₂O₅ (or Ta₂O₅), and NH₄H₂PO₄ as reagents. The materials have novel layered crystal structures consisting of [M⁵⁺O_6/2]^- corner-sharing octahedral chains that are connected to [Te⁴⁺O_4/2]⁰ polyhedra and [P⁵⁺O_2/1O_2/2]^- tetrahedra. The Ba²⁺ cations reside between the layers and maintain charge balance. The Te⁴⁺ cations are in asymmetric coordination environments attributable to their lone pairs. The Nb⁵⁺ distorts along the local C₄ direction of its octahedron resulting in a “short−long−short−long” Nb−O−Nb bond motif. The Nb⁵⁺ cation displaces away from the oxide ligands that are bonded to Te⁴⁺ or P⁵⁺ cations, attributable to the structural rigidity of the TeO₄ and PO₄ polyhedra. Thus, the TeO₄ and PO₄ polyhedra support and reinforce the intraoctahedral distortion observed within the NbO₆ octahedra. Infrared and Raman spectroscopy, thermogravimetric analysis, and ion-exchange experiments are also presented. Crystal data:  BaTeNbO₄(PO₄), orthorhombic, space group Pbca (No. 61), with a = 6.7351(9) Å, b = 7.5540(10) Å, c = 27.455(4) Å, V = 1396.8(3) ų, and Z = 8; BaTeTaO₄(PO₄), orthorhombic, space group Pbca (No. 61), with a = 6.734(2) Å, b = 7.565(3) Å, c = 27.435(9) Å, V = 1372.6(8) ų, and Z = 8.