Single-crystal growth, structure, and optical properties of a new non-centrosymmetric germanate, RbSbGe3O9

Abstract

The RbSbGe 3 O 9 compound, grown by the high-temperature solution technique from the Rb 2 Mo 4 O 13 flux, crystallizes in the trigonal non-centrosymmetric space group P31c (n o. 159). The structure was solved and refined from single-crystal X-ray diffraction data recorded at ambient temperature, a = 12.1378(2), c = 9.9553(2) Å, V = 1270.18(5) Å 3 , R1 = 0.0241 and wR2 = 0.0452 for all data. The unit cell contains six formula units. The three-dimensional RbSbGe 3 O 9 network is built with three-membered units [Ge 3 O 9 ] 6of regular germanate tetrahedra involving three independent Ge atoms. The Ge 3 O 3 central rings located in the ab plan deviate only slightly from the planarity. Antimony Sb V is octahedrally coordinated by oxygen, and rubidium (Rb +), which is surrounded by six oxygen atoms, is located in the channels of the 3-D network. The structure of RbSbGe 3 O 9 containing both isolated SbO 6 octahedra and Ge 3 O 9 cyclic units is comparable but not isostructural to the benitoite type. The almost flat character of the Ge 3 O 3 rings is also attested by the vibrational study at room temperature via non-polarized infrared and Raman spectroscopy.

Structural and Vibrational Study of CsSbGe3O9, a New Germanate in the ASbGe3O9 Family (A = Alkali Metal)

“…The compound was characterized using differential scanning calorimetry (DSC), powder Xray diffraction (XRD), and room-temperature infrared and non-polarized Raman spectroscopy. Details on the experimental procedures can be found in a previous paper 6 .…”

“…Details on the experimental procedures can be found in a previous paper. 6 The analysis by Energy Dispersive X-ray (EDX) spectroscopy of the grown crystals, using an Electron Scanning Environmental Microscope (ESEM) equipped with an Inca micro analyzer (Oxford Instruments), determined the content of Cs, Ge, and Sb elements. Three independent measures were made at the surface of the same asgrown crystal in order to check the homogeneity of the results.…”

“…Other significant changes can be expected based on the structural differences noted along with the ASbGe 3 O 9 series and related to both the arrangement and symmetry of the polyhedra. A broadening, or even a splitting, of the bands associated with bending or stretching vibration modes that involve Ge-O-Ge bridges, is expected for CsSbGe 3 O 9 as the dispersion of the angular values from 105 to 156° (average value of 133.5°) is slightly higher than in the trigonal structures with A = K and Rb 5,6 . Likewise, the Ge-O-Sb angles display a large dispersion in the range 121 to 136° and their average value of 124.6° greatly deviates from the average values of ~134° measured for the K-and Rb-compounds 5,6 .…”

“…It may therefore be inappropriate to speak here of a hierarchical relationship and preferable to see only a metric relationship between unit cells. structures via SbO 6 octahedra 5,6 , Figures 8e and 8f. This is the case of compounds A 2 GeGe 3 O 9 that are formed with the alkaline elements Na, K, and Rb 28 and crystallize in the trigonal P 3c1 symmetry.…”

“…5,13,14 In earlier crystallographic works dealing with X-ray diffraction studies of flux-grown single crystals, we have shown that KSbGe 3 O 9 is isostructural to Benitoite, while, despite many similarities, the compound RbSbGe 3 O 9 is not isostructural. 5,6 In this context, it seemed very attractive to establish the crystal structure of CsSbGe 3 O 9 , hitherto unknown. This is the first reason why attempts have been made to synthesize this phase by the same slow cooling method using the flux technique.…”