Polar compounds constructed with the [Cr₂O₇]^2- anion

Crystalline KTiOPO₄ (KTP), an inorganic nonlinear optical material with a waveguide figure-of-merit that is twice that of other mixed-metal oxides, contains helical chains of TIO_4/2O_2/2 octahedra in which a long, short Ti-O bond motif results in a net c-directed polarization. The alternating long and short Ti-O bonds that occur along these chains are the major contributors to the large nonlinear optic and electrooptic coefficients. Analogous chains have been constructed using dichromate [Cr₂O₇]^2-anions and [M(py)₄]²⁺ (M = Cu, Zn) cations; these new transition metal oxides crystallize in the same space group as KTP. Crystal data for Cu(py)₄Cr₂O₇: Orthorhombic, space group Pna2₁ (No. 33), with a = 15.941(7) Å, b = 16.324(3) Å, c = 8.857(2) Å, and Z = 4; for Zn(py)₄Cr₂O₇, orthorhombic, space group Pna2₁ (No. 33), with a = 16.503(1) Å, b = 16.005(1) Å, c = 8.8130(5) Å, and Z = 4; for Cd(py)₄Cr₂O₇, monoclinic, space group C2/c (No. 15), with a = 14.8034(9) Å, b = 11.1847(7) Å, c = 15.788(1) Å, β = 110.023(1)°, and Z = 4.