Abstract
BaSr2Ge3O9 was prepared by high-temperature solid-state synthesis at 1100°C in a platinum crucible from barium carbonate, strontium carbonate, and germanium(IV) oxide. The compound crystallizes in the triclinic space group P1̅ (no. 2) isotypically to walstromite BaCa2Si3O9. The structure was refined from single-crystal X-ray diffraction data: a=7.104(5), b=10.060(7), c=7.099(5) Å, α=83.0(2), β=77.0(2), γ=70.2(2)°, V=464.3(6) Å3, R1=0.0230, and wR2=0.0602 for all data. BaSr2Ge3O9 is characterized by three-membered rings of germanate tetrahedra. There are three crystallographically different Ge sites (Ge1, Ge2, and Ge3) in each [Ge3O9]6− ring. The rings occur in layers with the apices of alternating rings pointing in opposite directions. The Sr2+ and Ba2+ ions are located in between. The Sr1 cation is eight-fold coordinated, while Sr2 is octahedrally surrounded by oxide anions, and the Ba cation again eight-fold coordinated.
1 Introduction
In the last few years, our research focused on the synthesis of novel alkaline earth metal borogermanates. These compounds are known as new potential materials for nonlinear optical devices, e.g. Rb2GeB4O9 and K2GeB4O9·2 H2O, in which the efficiency in generating the second harmonic (SHG) is two times higher than in the well-known reference material KDP [1], [2].
Up to now, a large variety of transition metal, alkaline earth, and alkali borogermanates has been synthesized, including Ni5GeB2O10 [3], Cd12Ge17B8O58 [4], Ca10Ge16B6O51 [4], SrGe2B2O8 [5], A3Ge2B6O16 (A=Sr, Ba) [5], [6], Ba3[Ge2B7O16(OH)2](OH)(H2O) [6], Rb4Ge3B6O17 [1], A2GeB4O9 (A=Rb, Cs) [1], [7], AGeB3O7 (A=Rb, Cs) [1], [8], and K2GeB4O9·2H2O [1]. These compounds show a number of interesting open-framework structures. For example, the two compounds Cd12Ge17B8O58 from Mao et al. [4] and Ca12Ge17B8O58 [9] from our group show three-dimensional [Ge17B8O58]24− frameworks. In our group, the compound Sr3−x/2B2−xGe4+xO14 (x=0.32) as the first boron containing member of the langasite family [10] was successfully synthesized. Our research efforts in the quaternary system SrCO3−BaCO3−GeO2−H3BO3 led us to a novel germanate with the composition BaSr2Ge3O9 being isotypic to walstromite BaCa2Si3O9 [11]. Walstromite was first described by Alfors et al. [12] as a mineral from Esquire No. 8 claim, Big Creek, Fresno County, California using powder X-ray diffraction data and optical properties. Later on, Dent-Glasser and Glasser in 1968 [11] used photographic X-ray diffraction data to characterize a synthetic walstromite BaCa2Si3O9. The first single crystal X-ray diffraction determination of walstromite was carried out by Barkley et al. in 2011 [13]. This article presents the synthesis of BaSr2Ge3O9 and its characterization through single crystal X-ray diffraction, IR and Raman spectroscopy.
2 Experimental section
2.1 Synthesis
As mentioned above, the first synthesis was based on the system SrCO3−BaCO3−GeO2−H3BO3, leading to the new compound BaSr2Ge3O9. The function of H3BO3 during this synthesis is still unclear; presumably it acts as a flux material being amorphous in the final product mixture as it could not be detected in the powder diffractogram any more.
According to Eq. (1), a stoichiometric mixture of the starting materials SrCO3 (99+%, Merck, Darmstadt, Germany), GeO2 (99.99%, ChemPur, Karlsruhe, Germany), and BaCO3 (99.95%, Alfa Aesar, Karlsruhe, Germany) was finely ground in an agate mortar and filled into a FKS 95/5 crucible (feinkornstabilisiert, 95% Pt, 5% Au, Ögussa, Wien, Austria).
The crucible with the mixture was positioned in an electric resistance furnace (Nabertherm muffle furnace) and heated up to 1100°C with a rate of 61°C h−1. The sample was maintained at that temperature for 2 h. After that, the temperature was lowered with a rate of 4°C h−1 to 500°C before switching off the furnace. Finally the product naturally cooled down to room temperature.
The new compound BaSr2Ge3O9 could be obtained in form of colorless, air- and water-resistant crystals. The powder diffraction pattern (Fig. 1) showed reflections of BaSr2Ge3O9 as the major phase. The reflection marked with a red asterisk could not be assigned until now and most likely derives from an impurity.
2.2 Crystal structure analysis
The powder diffraction pattern was obtained in transmission geometry, using a Stoe Stadi P powder diffractometer with Ge(111)-monochromatized MoKα1 (λ=70.93 pm) radiation. Figure 1 shows the experimental powder pattern of BaSr2Ge3O9 that matches well with the theoretical pattern simulated from the single-crystal data.
Small single crystals of BaSr2Ge3O9 were selected by mechanical fragmentation using a polarization microscope. A Bruker D8 Quest κ diffractometer with MoKα radiation (λ=71.073 pm) was used to collect the single-crystal intensity data at room temperature. A multi-scan absorption correction (Sadabs-2014/5 [14]) was applied to the intensity data sets. All relevant details of the data collection and the refinement are listed in Table 1. As higher symmetry was not observed, the triclinic space groups P1 (no. 1) and P1̅ (no. 2) were derived for the solution and the refinement of the crystal structure. Due to the fact that BaSr2Ge3O9 is isotypic to walstromite [13], the structural refinement was performed in the space group P1̅ (no. 2) by using the positional parameters of walstromite [13] as starting values (Shelxl-13 [15], [16]). All atoms were refined anisotropically, leading to residual values of 0.0230 and 0.0602 for R1 and wR2, respectively. The atomic coordinates, anisotropic displacement parameters, interatomic distances, and angles are listed in the Tables 2–5 . Graphical representations of the structure were produced with the program Diamond [17].
Empirical formula | BaSr2Ge3O9 |
Molar mass, g·mol−1 | 674.4 |
Crystal system | Triclinic |
Space group | P1̅ (no. 2) |
Powder data | |
Powder diffractometer | STOE Stadi P |
Radiation | MoKα1 (λ=70.93 pm) |
a, Å | 7.119(5) |
b, Å | 10.100(7) |
c, Å | 7.100(5) |
α, deg | 82.9(2) |
β, deg | 76.9(2) |
γ, deg | 70.1(2) |
V, Å3 | 464.6(6) |
Single crystal data | |
Single crystal diffractometer | Bruker D8 Quest κ |
Radiation | MoKα (λ=71.073 pm) |
a, Å | 7.104(5) |
b, Å | 10.060(7) |
c, Å | 7.099(5) |
α, deg | 83.0(2) |
β, deg | 77.0(2) |
γ, deg | 70.2(2) |
V, Å3 | 464.36(6) |
Formula units per cell | 2 |
Calculated density, g cm−3 | 2.41 |
Crystal size, mm3 | 0.04×0.02×0.02 |
Temperature, K | 293(2) |
F(000), e | 600 |
Absorption coefficient, mm−1 | 25.2 |
θ range, deg | 2.2–29.2 |
Range in hkl | ±9, ±13, ±9 |
Total no. of reflections | 16 513 |
Independent reflections | 2512 |
Reflections with I>2σ(I) | 2329 |
Absorption correction | Multi-scan [14] |
Data/ref. Parameters | 2329/136 |
Goodness-of-fit on F2 | 1.083 |
Final R1/wR2 indices [I>2 σ(I)] | 0.0230/0.059 |
R1/wR2 indices (all data) | 0.0260/0.0602 |
Largest diff. peak/hole, e Å−3 | 1.70/–1.82 |
Atom | x | y | z | Ueqa |
---|---|---|---|---|
Sr1 | 0.75451(6) | 0.00337(4) | 0.73472(6) | 0.00623(9) |
Sr2 | 0.93408(6) | 0.33131(4) | 0.56847(6) | 0.00891(9) |
Ba1 | 0.30945(4) | 0.35101(3) | 0.94015(4) | 0.01563(8) |
Ge1 | 0.48219(7) | 0.30271(5) | 0.44214(7) | 0.00794(10) |
Ge2 | 0.72414(7) | 0.01649(5) | 0.22814(7) | 0.00801(10) |
Ge3 | 0.84684(7) | 0.28617(5) | 0.09663(7) | 0.00834(10) |
O1 | 0.2366(5) | 0.4082(4) | 0.5218(5) | 0.0144(7) |
O2 | 0.6222(5) | 0.2636(4) | 0.6223(5) | 0.0123(6) |
O3 | 0.4903(5) | 0.1401(4) | 0.3530(5) | 0.0138(6) |
O4 | 0.6493(5) | 0.9074(4) | 0.1157(5) | 0.0131(6) |
O5 | 0.8986(5) | 0.9443(4) | 0.3747(5) | 0.0132(6) |
O6 | 0.8069(5) | 0.1326(4) | 0.0355(5) | 0.0131(6) |
O7 | 0.8981(5) | 0.3955(4) | 0.9013(5) | 0.0127(6) |
O8 | 0.0292(5) | 0.2498(4) | 0.2342(5) | 0.0126(6) |
O9 | 0.5982(5) | 0.3837(4) | 0.2278(5) | 0.0137(6) |
aUeq is defined as one third of the trace of the orthogonalized Uij tensor (standard deviations in parentheses).
Atom | U11 | U22 | U33 | U12 | U13 | U23 |
---|---|---|---|---|---|---|
Sr1 | 0.00547(16) | 0.00554(17) | 0.00695(17) | –0.00039(13) | –0.00128(13) | –0.00077(13) |
Sr2 | 0.00872(18) | 0.01062(19) | 0.00850(18) | –0.00142(14) | –0.00176(14) | –0.00417(14) |
Ba1 | 0.01477(14) | 0.01133(13) | 0.01928(15) | –0.00426(10) | 0.00251(10) | –0.00487(10) |
Ge1 | 0.0058(2) | 0.0087(2) | 0.0087(2) | –0.00092(16) | –0.00149(15) | –0.00144(16) |
Ge2 | 0.0082(2) | 0.0081(2) | 0.0086(2) | –0.00101(16) | –0.00308(16) | –0.00255(16) |
Ge3 | 0.0089(2) | 0.0086(2) | 0.0077(2) | 0.00000(16) | –0.00188(16) | –0.00298(16) |
O1 | 0.0075(14) | 0.0118(15) | 0.0213(18) | –0.0035(13) | 0.0006(12) | –0.0012(12) |
O2 | 0.0102(14) | 0.0159(16) | 0.0120(15) | 0.0020(12) | –0.0056(12) | –0.0046(12) |
O3 | 0.0099(15) | 0.0123(15) | 0.0188(17) | –0.0057(13) | 0.0003(12) | –0.0032(12) |
O4 | 0.0138(15) | 0.0140(16) | 0.0146(16) | –0.0035(13) | –0.0035(13) | –0.0072(13) |
O5 | 0.0124(15) | 0.0155(16) | 0.0118(15) | 0.0018(12) | –0.0065(12) | –0.0031(13) |
O6 | 0.0184(16) | 0.0126(16) | 0.0099(15) | –0.0021(12) | –0.0017(12) | –0.0070(13) |
O7 | 0.0166(16) | 0.0116(15) | 0.0095(15) | 0.0011(12) | –0.0009(12) | –0.0058(13) |
O8 | 0.0118(15) | 0.0148(16) | 0.0120(15) | –0.0025(12) | –0.0060(12) | –0.0023(12) |
O9 | 0.0123(15) | 0.0111(15) | 0.0136(16) | 0.0004(12) | 0.0026(12) | –0.0024(12) |
Sr1–O8 | 2.501(3) | Sr2–O7 | 2.460(3) | Ba1–O4 | 2.585(3) |
Sr1–O6 | 2.556(3) | Sr2–O1a | 2.464(3) | Ba1–O7a | 2.710(3) |
Sr1–O5a | 2.591(3) | Sr2–O2 | 2.469(3) | Ba1–O2 | 2.775(3) |
Sr1–O5b | 2.622(3) | Sr2–O8 | 2.477(3) | Ba1–O7b | 2.874(3) |
Sr1–O4a | 2.697(3) | Sr2–O1b | 2.556(4) | Ba1–O8 | 2.889(3) |
Sr1–O6 | 2.784(3) | Sr2–O5 | 2.640(3) | Ba1–O9a | 3.005(3) |
Sr1–O4b | 2.784(4) | Ba1–O1 | 3.081(4) | ||
Sr1–O3 | 2.811(3) | Ba1–O9b | 3.316(4) | ||
Ø=2.668 | Ø=2.511 | Ø=2.904 | |||
Ge1–O1a | 1.713(3) | Ge2–O4a | 1.711(3) | Ge3–O8a | 1.711(3) |
Ge1–O2a | 1.722(3) | Ge2–O5a | 1.714(3) | Ge3–O7a | 1.712(3) |
Ø=1.718 | Ø=1.713 | Ø=1.712 | |||
Ge1–O9b | 1.802(3) | Ge2–O3b | 1.781(3) | Ge3–O6b | 1.815(3) |
Ge1–O3b | 1.804(3) | Ge2–O6b | 1.804(3) | Ge3–O9b | 1.815(3) |
Ø=1.803 | Ø=1.793 | Ø=1.815 |
aNon-bridging oxygen atoms; bbridging oxygen atoms.
O1–Ge1–O2 | 111.4(2) | O4–Ge2–O5 | 119.4(2) | O8–Ge3–O7 | 109.5(2) |
O1–Ge1–O9 | 110.0(2) | O4–Ge2–O3 | 105.7(2) | O8–Ge3–O6 | 113.7(2) |
O1–Ge1–O3 | 111.7(2) | O4–Ge2–O6 | 105.0(2) | O8–Ge3–O9 | 111.5(2) |
O2–Ge1–O9 | 112.5(2) | O5–Ge2–O3 | 112.1(2) | O7–Ge3–O6 | 114.0(2) |
O2–Ge1–O3 | 109.2(2) | O5–Ge2–O6 | 112.2(2) | O7–Ge3–O9 | 105.1(2) |
O9–Ge1–O3 | 101.7(2) | O3–Ge2–O6 | 100.5(2) | O6–Ge3–O9 | 102.6(2) |
Ø=109.4(2) | Ø=109.2(2) | Ø=109.4(2) |
Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-informationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition number CSD-431517.
2.3 Vibrational spectra
The transmission FT-IR spectra of single crystals of BaSr2Ge3O9 were measured in the spectral range of 600–4000 cm−1 with a Bruker Vertex 70 FT-IR spectrometer (spectral resolution 4 cm−1), equipped with a KBr beam splitter, an LN-MCT (mercury cadmium telluride) detector and attached to a Hyperion 3000 microscope. As mid-infrared source, a Globar (silicon carbide) rod was used. Three hundred and twenty scans of the sample were acquired with a 15×IR objective as focus. During the measurement, the sample was positioned on a BaF2 window. A correction for atmospheric influences using the Opus 6.5 software was performed.
The Raman spectrum of a single-crystal of BaSr2Ge3O9 was recorded with a Horiba Jobin Yvon LabRAM-HR 800 Raman micro-spectrometer in the spectral range of 100–4000 cm−1. The sample was excited using the 532 nm emission line of a frequency-doubled 100 mW Nd:YAG laser under an Olympus 100× objective lens. The diameter of the laser spot on the surface was approximately 1 μm. The scattered light was dispersed by an optical grating with 1800 lines mm−1 and collected by a 1024×256 open electrode CCD detector. The spectral resolution, determined by measuring the Rayleigh line, was <2 cm−1. The spectrum was recorded unpolarized. The accuracy of the Raman line shifts, calibrated by regularly measuring the Rayleigh line, was in the order of 0.5 cm−1.
3 Results and discussion
3.1 Crystal structure of BaSr2Ge3O9
The new compound BaSr2Ge3O9 crystallizes isotypically to the structure of walstromite BaCa2Si3O9 [13] in the triclinic space group P1̅ (no. 2) with two formula units per unit cell. In detail, the strontium cations occupy the crystallographic positions of the calcium cations, whereas the silicon atoms are substituted by germanium atoms. The structure is composed of rings of three corner-sharing GeO4 tetrahedra that occur in layers with the apices of alternating rings pointing in opposite directions (Fig. 2). The [Ge3O9]6− rings (Fig. 3) possess three crystallographically independent Ge sites (Ge1, Ge2, and Ge3) and the Ba and Sr cations occupy the space between these germanate ringlayers (Fig. 2).
A closer look at the metal cations shows that BaSr2Ge3O9 features two nonequivalent Sr sites. Sr1 is eight-fold coordinated by oxide anions with Sr1–O distances ranging from 2.051(3) to 2.811(3) Å with an average value of 2.668 Å. Sr2 shows an octahedral coordination by oxide anions with Sr2–O distances between 2.460(3) and 2.640(3) Å having a mean value of 2.511 Å. The Ba cations are eight-fold coordinated by oxygen atoms with Ba–O distances ranging from 2.585(3) to 3.316(4) Å with a mean value of 2.904 Å. These values tally well with the Ba–O distances in walstromite having an average value of 2.841 Å [13]. The Ge–O distances in BaSr2Ge3O9 range from 1.711(3) to 1.722(3) Å for the non-bridging oxygen atoms and from 1.781(3) to 1.815(3) Å for the bridging oxygen atoms. The mean value of all Ge–O distances in BaSr2Ge3O9 comes to 1.759(3) Å, fitting perfectly to values of other germanates containing rings built up from three GeO4 tetrahedra as found in SrGeO3 by Hilmer et al. (mean value 1.80 Å for Ge–O) [18] and in SrGeO3 by Nadezhina et al. (average value 1.76 Å for Ge–O) [19], [20]. The two SrGeO3 polymorphs show different structures. Hilmer et al. [18] found a hexagonal structure (a=7.29, c=31.64 Å) with alternating layers of [Ge3O9]6− rings and layers of Sr atoms. The SrGeO3 described by Nadezhina et al. [19], [20] showed a triclinic cell (a=8.699, b=9.935, c=11.148 Å, α=106.04, β=89.97, and γ=102.77°). The structure also contains rings of three GeO4 tetrahedra, but the configuration of the rings is different in comparison with the SrGeO3 studied by Hilmer et al. [18]. In the structure of Nadezhina et al. [19], [20], two out of three GeO4 tetrahedra in the [Ge3O9]6− rings possess the same orientation with apices pointing in one direction. In SrGeO3 by Hilmer et al. [18], all three GeO4 tetrahedra point in the same direction. The Ge–O–Ge angles within the germanium-oxygen tetrahedra of the here presented compound BaSr2Ge3O9 range between 100.5(2) and 114.0(2)°. The Tables 4 and 5 list all important interatomic distances and bond angles of BaSr2Ge3O9.
Walstromite (BaCa2Si3O9) is a Ba–Ca cyclosilicate belonging to the group of calcium silicates. The formal substitution of Ba by Ca leads to the related CaSiO3-walstromite [21] and high-pressure wollastonite-II (CaSiO3-II) [22]. Joswig et al. [21] pointed out that walstromite [11] and CaSiO3-walstromite [21] are isostructural. They also suggested a new kind of isomorphism between Ca-walstromite and high-pressure wollastonite-II, where the axiality of the Si3O9 rings is different. Later on, Barkley et al. [13] showed that the atomic coordinates from high-pressure wollastonite-II and walstromite (BaCa2Si3O9) can be converted into those of Ca-walstromite by a linear transformation showing that all three compounds are isostructural. This means that CaSiO3-walstromite and high-pressure wollastonite-II (CaSiO3-II) are identical compounds.
For purposes of comparison, we converted the atomic coordinates of walstromite and high-pressure wollastonite-II into the settings, which were used by Barkley et al. [13]. The required transformation matrix for walstromite is:
The following transformation matrix was used for high-pressure wollastonite-II:
Tables 6 and 7 show a comparison of the unit cell parameters and the atomic coordinates of BaSr2Ge3O9, walstromite (BaCa2Si3O9), and high-pressure wollastonite-II confirming that all three compounds are isostructural. For the standardized crystallographic data of walstromite and wollastonite-II, the reader is referred to the Pearson’s Crystal Database [23]. Therefrom it can be seen that the structure of walstromite is well known from other silicates like margarosanite PbCa2Si3O9 [24] or phosphates, e.g. KNa2P3O9 [25].
Compound Reference Empirical formular | BaSr2Ge3O9 [This work] BaSr2Ge3O9 | Walstromite [13] BaCa2Si3O9 | HP-wollastonite-II [22] Ca3Si3O9 |
---|---|---|---|
a, Å | 7.014(5) | 6.686(2) | 6.659(5) |
b, Å | 10.060(7) | 9.614(3) | 9.429(7) |
c, Å | 7.099(5) | 6.734(2) | 6.666(6) |
α, deg | 83.0(2) | 83.1(2) | 83.7(5) |
β, deg | 77.0(2) | 77.7(2) | 76.1(5) |
γ, deg | 70.2(6) | 69.6(2) | 67.6(5) |
Atom | x | y | z | ||||||
---|---|---|---|---|---|---|---|---|---|
BaSr2Ge3O9 [this work] | Walstromite [13] | HP-wollast.-II [22] | BaSr2Ge3O9 [this work] | Walstromite [13] | HP-wollast.-II [22] | BaSr2Ge3O9 [this work] | Walstromite [13] | HP-wollast.-II [22] | |
Sr1, Ca1, Ca1 | 0.75451(6) | 0.76296(2) | 0.7557(3) | 0.00337(4) | 0.00942(7) | 0.0002(3) | 0.73472(6) | 0.72436(2) | 0.7377(3) |
Sr2, Ca2, Ca3 | 0.93408(6) | 0.94439(2) | 0.9201(3) | 0.33131(4) | 0.32810(7) | 0.3356(9) | 0.56847(6) | 0.56296(9) | 0.5755(3) |
Ba, Ba, Ca2 | 0.30945(4) | 0.32147(3) | 0.2698(3) | 0.35101(3) | 0.34865(2) | 0.3513(3) | 0.94015(4) | 0.95270(3) | 0.9154(3) |
Ge1, Si3, Si3 | 0.48219(7) | 0.48668(2) | 0.4773(4) | 0.30271(5) | 0.30378(2) | 0.3009(4) | 0.44214(7) | 0.44128(2) | 0.4581(4) |
Ge2, Si2, Si2 | 0.72414(7) | 0.71385(2) | 0.7255(4) | 0.01649(5) | 0.01866(2) | 0.0162(4) | 0.22814(7) | 0.23397(2) | 0.2299(4) |
Ge3, Si1, Si1 | 0.84684(7) | 0.84747(2) | 0.8365(4) | 0.28617(5) | 0.27788(2) | 0.2951(4) | 0.09663(7) | 0.09661(2) | 0.1084(4) |
O1, O8, O8 | 0.2366(5) | 0.2430(5) | 0.2319(2) | 0.4082(4) | 0.4083(4) | 0.4031(9) | 0.5218(5) | 0.50803(5) | 0.5460(2) |
O2, O7, O7 | 0.6222(5) | 0.6314(5) | 0.6089(2) | 0.2636(4) | 0.2658(4) | 0.2629(9) | 0.6223(5) | 0.6141(4) | 0.6409(9) |
O3, O6, O6 | 0.4903(5) | 0.4940(5) | 0.4953(2) | 0.1401(4) | 0.1440(3) | 0.1386(9) | 0.3530(5) | 0.3598(5) | 0.3647(2) |
O4, O5, O5 | 0.6493(5) | 0.6303(5) | 0.6561(2) | 0.9074(4) | 0.9139(4) | 0.9068(9) | 0.1157(5) | 0.1349(5) | 0.1140(2) |
O5, O4, O4 | 0.8986(5) | 0.8929(5) | 0.9003(2) | 0.9443(4) | 0.9437(4) | 0.9388(9) | 0.3747(5) | 0.3730(4) | 0.3715(2) |
O6, O3, O3 | 0.8069(5) | 0.7995(5) | 0.8085(2) | 0.1326(4) | 0.1286(4) | 0.1359(8) | 0.0355(5) | 0.0457(4) | 0.0482(1) |
O7, O2, O2 | 0.8981(5) | 0.8959(5) | 0.8805(1) | 0.3955(4) | 0.3772(4) | 0.4055(9) | 0.9013(5) | 0.8991(4) | 0.9157(1) |
O8, O1, O1 | 0.0292(5) | 0.0293(5) | 0.0286(2) | 0.2498(4) | 0.2388(4) | 0.2564(9) | 0.2342(5) | 0.2334(4) | 0.2344(1) |
O9, O9, O9 | 0.5982(5) | 0.6067(5) | 0.5993(2) | 0.3837(4) | 0.3755(4) | 0.3853(9) | 0.2278(5) | 0.2305(4) | 0.2574(1) |
The bond valence sums for all atoms in BaSr2Ge3O9 were calculated from the crystal structure using the bond-length/bond-strength concept (ΣV) [26], [27] and the Chardi concept (charge distribution in solids, ΣQ) [28]. The results of these calculations are listed in Table 8 and correspond well with the expected values of the formal ionic charge of each specific atom.
Sr1 | Sr2 | Ba | Ge1 | Ge2 | Ge3 | ||||
---|---|---|---|---|---|---|---|---|---|
∑V | 1.89 | 2.11 | 1.77 | 3.9 | 3.88 | 3.98 | |||
∑Q | 1.98 | 1.95 | 1.93 | 4.07 | 4.10 | 3.97 | |||
O1 | O2 | O3 | O4 | O5 | O6 | O7 | O8 | O9 | |
∑V | –1.92 | –2.04 | –1.85 | –1.93 | –1.88 | –1.91 | –2.02 | –1.84 | –1.93 |
∑Q | –1.96 | –2.14 | –1.83 | –2.06 | –1.98 | –1.90 | –2.13 | –2.10 | –1.89 |
Furthermore, the Maple values (Madelung Part of Lattice Energy) [29], [30], [31] of BaSr2Ge3O9 were calculated to compare them with the Maple values received from the summation of the starting compounds SrO [32], BaO [32], and GeO2 [33]. A value of 12646 kcal mol−1 was obtained in comparison to 12769 kcal mol−1 (deviation=0.97%), starting from the oxides [2 SrO (1810 kcal mol−1)+BaO (840 kcal mol−1)+3 GeO2 (10119 kcal mol−1)].
3.2 IR spectroscopy
In Fig. 4, the IR spectrum is displayed which was performed on a single crystal of BaSr2Ge3O9. It can be divided in two sections, one with a series of different absorption bands with frequencies below 1500 cm−1 and one from 2800–3000 cm−1. The absorption bands between 645 and 900 cm−1 can be related to various modes within the GeO4 tetrahedra [34], [35], [36], [37], [38]. The region between 1350 and 1450 cm−1 presumably represents combination tones of two vibration modes of Ge–O–Ge bonds [39]. A precise assignment of the individual bands is difficult because of the overlap of various bands. The absorption bands at 2800–3000 cm−1 belong to the grease, which was used to fix the crystal on the glass fiber.
3.3 Raman spectroscopy
The Raman spectroscopic measurements were also performed on a single crystal of BaSr2Ge3O9 (Fig. 5). The bands between 750 and 850 cm−1 can be attributed to the symmetric and antisymmetric stretching vibrations of the Ge–O bonds in the GeO4 tetrahedra [40]. The two bands at 550 cm−1 can be assigned to Ge–O–Ge bending vibrations [40]. The bands in the range of 300 to 400 cm−1 can probably also be assigned to the stretching and bending modes of the GeO4 tetrahedra [40], [41]. The bands below 300 cm−1 can be associated with the rotational and translational modes of the GeO4 tetrahedra [40], [41], [42].
4 Conclusions
With the syntheses of BaSr2Ge3O9, the list of known alkaline earth germanates could be extended by an additional compound. It crystallizes in the triclinic space group P1̅ (no. 2) being isostructural to walstromite [13], Ca-walstromite [21], and high-pressure wollastonite-II [22]. The main structural characteristics are [Ge3O9]6− rings, which are composed of three different GeO4 tetrahedra. The rings occur in layers with the apices of alternating rings pointing in opposite directions. The interlayer space is occupied by the Sr2+ and Ba2+ cations.
Acknowledgements
Special thanks go to Dr. G. Heymann for collecting the single-crystal diffraction data, to D. Vitzthum for the measurements of the single-crystal IR spectra, and to Univ.-Prof. Dr. R. Stalder, Institute for Mineralogy and Petrography, University of Innsbruck, for the access to the FTIR microscope.
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