Structure of the luminescent Ca 4K 2(CrO 4) 5 · 2H 2O crystal

• Published in: Journal of luminescence Vol. 69; no. 5; pp. 319 - 323
• Main Authors: Spek, A.L., Duisenberg, A.J.M., Coremans, C.J.M., van der Waals, J.H.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 1996
• Subjects: Ca 4K 2(CrO 4) 5 · 2H 2O; Chromate luminescence; Crystal structure; Chromate luminescence; Ca 4K 2(CrO 4) 5 · 2H 2O; Crystal structure
• ISSN: 0022-2313; 1872-7883
• DOI: 10.1016/S0022-2313(96)00111-1

Chromate crystals with the composition Ca 4K 2(CrO 4) 5 · 2H 2O have been identified which under UV excitation show luminescence with a lifetime of the order of 1 ms, i.e. a thousandfold longer than expected for CrO 2− 4 ions of tetrahedral symmetry. In order to find out whether the long lifetime is related to a distortion of the chromate ions in the mixed salt, the structure has been determined by X-ray analysis. The crystal is orthorhombic, space group Fdd2, with a = 12.005(1), b = 44.137(4), c = 7.361(1) A ̊ at T = 296 K. The two independent Ca ions are 7 and 8 coordinated with oxygen atoms. Three inequivalent CrO 2− 4 tetrahedra occur, with an average Cr-O bond length of 1.647 Å. The modest deviations from tetrahedral symmetry of these ions are not held responsible for the long luminescence lifetime. EPR signals of the triplet species from which the emission originates show excessive inhomogeneous broadening, which points to luminescence associated with imperfections in the crystal.