Crystal structure of ErAlGeO5 and evidence of a peculiar double coordination sphere of Al(III) and Ge(IV) cations

Single crystals of a new erbium alumino-germanate with gross formula ErAlGeO5 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=4Z=4, Dcalc=3.326g⋅cm−1, R1=4.29%R1=4.29%; wR2=11.76%wR2=11.76%). When regarded along [100], the structure can be seen as made of rutile chains of the type [(Ge,Al)O2] connected by pairs of [(Ge,Al)O5] 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 Al3+ and Ge4+ ions on the same octahedral and pyramidal centres, the crystal chemical formula of the title compound can be written as: Er(Al0.73Ge0.27)(Ge0.73Al0.27)O5. The crystallographic data of the present study have evidenced two original characteristics of the resolved structure: (i) pentavalent coordination of both Al3+ and Ge4+ ions; (ii) location of Ge4+ ion in an octahedral environment, which is unusual for this cation under normal pressure.

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