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Crystal structure and optical properties of Ln2(Ca,Mn)Ge4O12

Nadezda V. Tarakina 1,  Vladimir G. Zubkov 1,  Alexander P. Tyutyunnik 1,  Ivan I. Leonidov 1,  Ludmila L. Surat 1,  Inna V. Baklanova 1,  Lina A. Perelyaeva 1,  Olga V. Koriakova 2,  Joke Hadermann 3,  Gustaaf Van Tendeloo 3

1. Russian Academy of Sciences, Ural Division, Institute of Solid State Chemistry (ISSC), Pervomaiyskay, 91, Ekaterinburg 620219, Russian Federation
2. Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, 22 S. Kovalevskaya str., Ekaterinburg 620219, Russian Federation
3. University of Antwerp, EMAT, Groenenborgerlaan 171, Antwerp B-2020, Belgium

Abstract
The new group of compounds with cyclic anions Ln2M2+Ge4O12 (Ln = Eu - Lu; M = Ca, Mn) which shows record Stokes shifts (3500 - 4200 cm-1) upon laser pumping at λ = 976 nm has been synthesized and studied for the first time. All compounds convert the monochromatic line of the excitation laser to a shift band consisting of selected lines with a width of 5 - 8 cm-1. The compounds Ln2M2+Ge4O12 (Ln = Eu - Lu; M = Ca, Mn) are isostructural, crystallizing in the space group P4/nbm. A peculiarity of their crystal structure is the presence of cyclic anions [Ge4O12]8-, consisting of four GeO4 tetrahedra. The crystal structure of these compounds can be described as two alternating layers: one formed by Ln and (Ca,Mn) atoms and another by [Ge4O12]8- anions in boat-type conformation. Between these layers octahedral and square antiprismatic cavities are formed. The Ln and (Ca,Mn) atoms are positioned inside elongated oxygen octahedrons with ratio 0.5/0.5. The square antiprisms are occupied only by rare earth cations. The decreasing atomic radius of the rare earth elements leads to a change of the island structure motive to the 2D layered type. Analysis of the crystal structure and optical properties allows to conclude that the high values of the Stokes shifts are caused by inelastic interactions of excitation quanta and tetracyclic groups [Ge4O12], which are harmonic oscillators. This type of vibration is almost absent in compounds with the 2D layout of tetracycles in the structure (Yb2CaGe4O12, Lu2CaGe4O12).

This work was supported by the RFBR (grant 07-03-00143), by Belgium Science Policy, by the Councils for Grants of the President of Russia for Support of Young Scientists (grant MK-84.2007.3) and for Support of Leading Scientific Schools (grant no. NSh - 1170.2008.3).

 

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Presentation: Poster at 11th European Powder Diffraction Conference, Poster session, by Nadezda V. Tarakina
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-04-25 13:53
Revised:   2009-06-07 00:48