Crystal structure of ErAlGeO 5 and evidence of a peculiar double coordination sphere of Al(III) and Ge(IV) cations

Abstract

Single crystals of a new erbium alumino-germanate with gross formula ErAlGeO 5 have been isolated, using the flux method. The structure determined from X-ray single crystal diffraction data, was refined with the space group Pmcb (No 55), a = 5.580 ( 1 ) Å , b = 7.208 ( 1 ) Å , c = 8.309 ( 2 ) Å , Z = 4 , D calc = 3.326 g ⋅ cm −1 , R 1 = 4.29 % ; w R 2 = 11.76 % ). When regarded along [100], the structure can be seen as made of rutile chains of the type [(Ge,Al)O 2] connected by pairs of [(Ge,Al)O 5] pyramids, arranged in the manner to delimit infinite tunnels with pentagonal sections, running parallel to [100] and filled by erbium ions. The close coordination spheres of Er(III) cations can be viewed as a bicapped trigonal prism. Due to the cationic distribution of both Al 3+ and Ge 4+ ions on the same octahedral and pyramidal centres, the crystal chemical formula of the title compound can be written as: Er(Al 0.73Ge 0.27)(Ge 0.73Al 0.27)O 5. The crystallographic data of the present study have evidenced two original characteristics of the resolved structure: (i) pentavalent coordination of both Al 3+ and Ge 4+ ions; (ii) location of Ge 4+ ion in an octahedral environment, which is unusual for this cation under normal pressure.