Structure and luminescence of K2TaF7 and K2NbF7

The crystal structure of K2TaF7 has been refined using single-crystal X-ray diffraction data. It is monoclinic with lattice parameters a = 5.8559(6), b = 12.708(1), and c = 85125(9) Å and β = 90.17° in the space group P21c. The structure is composed of TaF7 units that are interconnected by potassium ions. TaF7 polyhedra may be described as monocapped trigonal prisms with the capping atom located on one of the rectangular faces. Potassium atoms are 9-coordinated and may be viewed as distorted monocapped square prisms. The previously unreported luminescence properties of K2TaF7 and K2NbF7 are reported and discussed. In addition to the intrinsic emission, impurity luminescence due to OH− and Eu2+ were observed.