Alignment of acentric MoO₃F₃^3- anions in a polar material: (Ag₃MoO₃F₃)(Ag ₃MoO₄)Cl

(Ag₃MoO₃F₃)(Ag₃MoO ₄)Cl was synthesized by hydro(solvato)thermal methods and characterized by single-crystal X-ray diffraction (P3m1, No. 156, Z=1, a=7.4488(6)Å, c=5.9190(7)Å). The transparent colorless crystals are comprised of chains of distorted fac-MoO₃F₃^3- octahedra and MoO₄^2- tetrahedra anions, as suggested by the formulas Ag₃MoO₃F₃ and Ag ₃MoO₄⁺, and are connected through Ag ⁺ cations in a polar alignment along the c-axis. One Cl^- anion per formula unit serves as a charge balance and connects the two types of chains in a staggered fashion, offset by ∼1/2×c. In MoO ₄^2-, the Mo atom displaces towards a single oxide vertex, and in MoO₃F₃^3-, the Mo displaces towards the three oxide ligands. The ordered oxide-fluoride ligands on the MoO ₃F₃^3- anion is important to prevent local inversion centers, while the polar organization is directed by the Cl ^- anion and interchain dipole-dipole interactions. The dipole moments of MoO₃F₃^3- and MoO₄ ^2- align in the negative c-axis direction, to give a polar structure with no cancellation of the individual moments. The direction and magnitude of the dipole moments for MoO₃F₃^3- and MoO ₄^2- were calculated from bond valence analyses and are 6. 1 and 1.9debye (10^-18esucm) respectively, compared to 4.4debye for polar NbO₆ octahedra in LiNbO₃, and 4.5debye for polar TiO₆ octahedra in KTiOPO₄ (KTP).