Far-infrared spectra of synthetic [4][(Al2-xGax)(Si2-yGey)](OH,OD,F)2-kinoshitalite: Characterization and assignment of interlayer Ba-Oinner and Ba-Oouter stretching bands

Abstract

Far-infrared spectroscopy and X-ray diffraction Rietveld structure-refinement of synthetic kinoshitalite (Kn) solid solutions, BaMg3[(Al2−xGax)(Si2−yGey)]O10(OH,OD,F)2: (x = 0.0–2.0, y = 0.0–2.0), show that there is complete solid solution for all compositions in each (OH/OD)- and F-series: [4][Al2(Si2−yGey)]-, [4][(Al2−xGax)Si2]-, [4][Ga2(Si2−yGey)]-, [4][(Al2−xGax)Ge2]-Kn, and in OH/OD-for-F substituted [4](Al2Si2)-, [4](Ga2Si2)-, [4](Al2Ge2)-, [4](Ga2Ge2)-Kn end-member compositions. In the far-infrared region, 170–40 cm−1, three kind of bands are observed; an in-plane tetrahedral torsional mode, an interlayer Ba-Oinner stretching vibration and a Ba-Oouter stretching vibration. With increasing tetrahedral [4]Al-for-[4]Ga and Si-for-Ge substitution, the frequencies and intensities of the tetrahedral in-plane torsional bands decrease in both the (OH/OD)- and F-bearing phases, but in the [4](Al2Si2)-, [4](Ga2Si2)-, [4](Al2Ge2)-, [4](Ga2Ge2)-Kn end-member compositions, the frequencies are unaffected by (OH/OD)-for-F substitution. The frequencies of both the Ba-Oinner and Ba-Oouter stretching bands increase with increasing [4]Al-for-[4]Ga and Si-for-Ge substitution, but the frequencies of the Ba-Oinner stretching bands decrease with increasing (OH/OD)-for-F substitution in the [4](Al2Si2)-, [4](Ga2Si2)-, [4](Al2Ge2)-, [4](Ga2Ge2)-Kn end-member compositions. The frequency difference between the Ba-Oinner and Ba-Oouter stretching bands is linearly related to the tetrahedral rotation angles (α), and these differences are about 10 cm−1 larger in the (OH/OD)-bearing phases than in the corresponding F-bearing phases. The ranges of absorption frequencies and their corresponding deformation modes are as follows: (1) in-plane tetrahedral torsional mode, 105–150 cm−1; (2) Ba-Oinner stretching vibration, 105–140 cm−1; and (3) Ba-Oouter stretching vibration, 75–90 cm−1.