The structures of (Sr,Ba)[(Al,Ga)2(Si,Ge)2O8]. IV. The crystal structure of the synthetic feldspar BaGa2Ge2O8

Abstract

The structure of the feldspar modification of BaGa2Ge2Oa has been found from diffractometer intensities and refined by Fourier and least-squares methods. The structure is similar to that of celsian:space group I2/c, Z = 8. The mean values for the four independent tetrahedra of the asymmetric unit are:(Ge(1) (0)-O)= 1.74, (Ge(2) (z)-O)= 1.74, (Ga(1) (z)-O)= 1"81, (Ga(2) (0)-O)= 1.82 A. These figures are compatible with the postulate of nearly complete Ga/Ge order. The strains in the tetrahedral bond angles are very marked, the angles varying between 96 and 118 °. The Ba ion can be considered sevencoordinated. The influence of different T and M cations on the framework of the 14 A monoclinic feldspars is examined. IntroductionThe structural investigation of the feldspar form of BaGa2Ge2Os has been performed as part of a programme of research on the phases of compounds with the general formula (Sr, Ba)[(A1, Ga)2(Si, Ge)208]. The present modification is metastable and was obtained only by cooling of the melt (Gazzoni, 1973); the structure of the stable, paracelsian-like, modification was described in part III of this series (Calleri & Gazzoni, 1976).The structures of a few 14 A monoclinic feldspars are known: synthetic SrA12Si208 (Chiari, Calleri, Bruno & Ribbe, 1975); synthetic SrGa2Si208 and BaGa2Si208 (Calleri & Gazzoni, 1975a); synthetic BaGa2Ge208 (present work); natural celsian, BaA12Si208 (Newnham & Megaw, 1960). We intend therefore to compare some dimensional features of these compounds, as has been done for three paracelsian modifications (Calleri & Gazzoni, 1976), looking for a correlation with the dimensions of the cations. Structure determination Experimental resultsWeissenberg and precession photographs showed only 'a-type' (h + k even, l even) and 'b-type' (h + k odd, l odd) reflexions and permitted the assignment of space group I2/e, assuming the presence of a centre of inversion at (000) by analogy with other 14 A feldspars (for example see Newnham & Megaw, 1960). The cell parameters were refined by least squares from 0 values measured on powder spectra taken with a focusing camera (Cu Kc~ radiation).The results are: BaGazGezOs, M=549.98, monoclinic, a=8.896 (6), b=13.528 (6), e=14.906 (6) A, fl= 114.87 (6)°; U= 1627.5 Aa; De =4"49 g cm-3; Z=8; space group I2/c; F(O00) --1968. Crystal dimensions: 0.160 mm across the opposite faces of {100}, 0-055 mm across the opposite faces of {010}, 0.115 mm between (001) and (104); p(Mo K~)= 195.5 cm -1.The intensities accessible to Mo Kc~ radiation, within 20=60 °, were collected at room temperature with a Philips four-circle diffractometer, equipped with a graphite monochromator, by the 0-20 step-scanning technique. The integration speed was 0.025 ° s -1 and the integration interval 1.5°; 1405 reflexions, out of 2164 explored, had I>_2a(1) and were included in the analysis; there were 420 b-type reflexions. The weaker reflexions were scanned four times and the measurements were averaged. The reflexions with I>_4a (1) were measured also with the 09 step-scann...

Ba5Ga2SiO4S6: a Phase-Matching Nonlinear Optical Oxychalcogenide Design via Structural Regulation Originated from Heteroanion Introduction

“…BaGa 2 Si 2 O 8 adopts the monoclinic space group of I 2/ c (no. 15), and it contains 13 crystallographically independent atoms, including 1 Ba atom, 2 Ga atoms, 1 Si atom, and 8 O atoms, all of which occupy Wyckoff sites of 8 f . As illustrated in Figure a, the asymmetric unit [Ga 4 Si 4 O 23 ] is composed of 2 [Ga1O 4 ], 2 [Ga2O 4 ], 2 [Si1O 4 ], and 2 [Si2O 4 ] tetrahedra via corner-sharing O1 atoms with normal Ga–O (1.768–1.837 Å) or Si–O (1.614–1.666 Å) bond distance; furthermore, each [Ga 4 Si 4 O 23 ] group is interlinked with two adjacent groups to form a single 1D chain along the b direction (Figure b).…”

“…Some successful examples include melilite-type Sr 2 MGe 2 OS 6 (maternal structure: Sr 2 MGe 2 O 7 ) and fresnoite-type Ba 2 SnSSi 2 O 7 (maternal structure: Ba 2 TiOSi 2 O 7 ) . When searching for potential maternal structures through the ICSD database, a silicate, BaGa 2 Si 2 O 8 , aroused our attention . As a famous mineral celsian-type structure, it possesses the three-dimensional (3D) [Ga 2 Si 2 O 8 ] 2+ framework that is constructed by tetrahedral [SiO 4 ] and [GaO 4 ] ABBs.…”

The structures of (Sr,Ba)[(Al,Ga)2(SiGe)2O8]. V. The crystal structure of the synthetic feldspar BaAl2Ge2O8

“…It is possible that the M cations are so far apart in all feldspars that the repulsion effect is of secondary importance in determining the stability conditions and that the mentioned trend is a consequence of the 'collapse' of the larger feldspar frameworks for accommodating the M cation even when it is as large as Ba. At any rate, the present compound fits the overall trend of the O(A 1)...O(A 1) edge (Table 6 and Caileri & Gazzoni, 1976b). However, the distance is here THE STRUCTURES OF (Sr,Ba)[(AI,Ga)2(Si,Ge)208].…”