Synthesis and characterization of PbBaGeO₄

2.1 Synthesis of PbBaGeO₄

Starting materials for the synthesis of PbBaGeO₄ were PbO (99.7 %, Acros, Geel, Belgium) BaCO₃ (99.95 %, Alfa Aesar, Karlsruhe, Germany) and GeO₂ (99.99 %, ChemPur, Karlsruhe, Germany). The stoichiometric mixture of the carbonate and the oxides according to eq. 1 was thoroughly hand milled using an agate mortar, and placed in a FKS 95/5 crucible (Feinkornstabilisiert, 95 % Pt, 5 % Au, Ögussa, Wien, Austria) for the heating procedure.

(1)PbO + BaCO3 + GeO2 →900 °C PbBaGeO4 + CO2 (1)

Calcination was performed in an electric resistance furnace (Nabertherm muffle furnace) with a heating rate of 3 °C min⁻¹. The heating temperature of 900 °C was maintained for 48 h before switching off the furnace. The sample cooled down to room temperature by natural rate. The colorless crystals are air and water resistant. PbBaGeO₄ represents the major phase in the powder diffraction pattern (Fig. 1). One tiny reflection marked with an asterisk in the powder pattern could not be assigned up to now.

Fig. 1:

Experimental powder pattern (top) of PbBaGeO₄ compared with the theoretical powder pattern (bottom) simulated from the single-crystal data.

2.2 Crystal structure analysis

The powder diffraction pattern was obtained from a flat sample of the reaction product in transmission geometry, using a Stoe Stadi P powder diffractometer with Ge(111)-monochromatized MoK_α1 (λ = 70.93 pm) radiation. Fig. 1 shows the experimental powder pattern of PbBaGeO₄ that matches well the theoretical pattern simulated from the single-crystal data.

Small single-crystals of PbBaGeO₄ were isolated by mechanical fragmentation. The single crystal intensity data were collected at room temperature using a Nonius Kappa-CCD diffractometer with graphite-monochromatized MoK_α radiation (λ = 71.073 pm). A semiempirical absorption correction based on equivalent and redundant intensities (Scalepack [8]) was applied to the intensity data. All relevant details of the data collection and evaluation are listed in Table 1. According to the systematic extinctions, the orthorhombic space group P2₁2₁2₁ was derived. Due to the fact that PbBaGeO₄ is isotypic to PbSrGeO₄ [3], the structural refinement was performed by taking the positional parameters of PbSrGeO₄ as starting values (Shelxl-13 [9, 10]). All atoms were refined with anisotropic displacement parameters, and the final difference Fourier synthesis did not reveal any significant residual peaks. The refinement exhibited that the measured crystal was an inversion twin, where the twin domains showed a ratio of 1:2. Positional parameters, anisotropic displacement parameters and interatomic distances are listed in the Tables 2–5. Graphical representations of the structure were produced with the program Diamond [11].

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 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-429369.

2.3 Vibrational spectra

The Raman spectrum of a single crystal of PbBaGeO₄ was recorded with a Horiba Jobin Yvon LabRAM-HR 800 Raman micro-spectrometer in the spectral range of 100–1200 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. Second-order polynomial and convoluted Gaussian–Lorentzian functions were fitted to the background and Raman bands, respectively, using the built-in spectrometer software Labspec.

3.1 Crystal structure of PbBaGeO₄

PbBaGeO₄ crystallizes in the BaNdGaO₄ structure type isotypic to the lead strontium germanate PbSrGeO₄ in the orthorhombic space group P2₁2₁2₁ (No. 19) with four formula units per unit cell. Beside the above mentioned compounds, there exist further more substances that crystallize in the BaNdGaO₄ structure type, e.g., BaLaGaO₄ [4], α-NaCuPO₄ [12], SrTlVO₄ [13], KSrVO₄ [14], and BaLaAlO₄ [15]. The isolated [GeO₄]⁴⁻ tetrahedra are the fundamental building units that are arranged in columns along the c axis building pairs with oppositely orientated tetrahedra. In the space between the columns, the lead and the barium cations form chains that are also oriented along the crystallographic c axis. Figure 2 shows the crystal structure of PbBaGeO₄ with a view along [001]. The barium ions are eightfold coordinated by the oxygen anions and the coordination sphere of the lead cation consists of three oxygen atoms and a stereochemically active lone-pair of electrons forming a pseudo-tetrahedral coordination (Fig. 3). The interatomic lead-oxygen distances in the coordination sphere range from 224.5(9) to 234.7(11) pm (mean value: 231.2 pm) whereby the Pb²⁺ cation is shifted out of the central position in the opposite direction of the lone-pair. The behavior of the lead cation in the presence of a stereochemically active electron lone-pair is well known from PbSrGeO₄ [3] where even the electron localization function (ELF) was calculated. The interatomic barium–oxygen distances are between 261.6(11) and 293.7(11) pm with a mean value of 277.4 pm. The germanium–oxygen bond lengths inside the tetrahedra vary from 173.6(11) to 177.2(10) pm with a mean value of 176.1 pm, and the O–Ge–O bond angles range from 106.2(5) to 116.2(7)° (average value: 109.5°). Bond lengths and angles are comparable with other compounds containing GeO₄ tetrahedra [3, 16, 17].

Fig. 2:

Crystal structure of PbBaGeO₄ (space group: P2₁2₁2₁) showing pairs of [GeO₄]⁴⁻ tetrahedra-columns forming infinite tubes along [001] filled with Ba²⁺ ions (green) and Pb²⁺ ions (orange).

3.2 Raman spectroscopy

The Raman spectroscopic measurements were performed on a single crystal of PbBaGeO₄ (Fig. 4). The bands below 200 cm⁻¹ may arise from mixed vibrations of the GeO₄ tetrahedron [18]. Presumably, these vibrations overlap with vibrations of the pseudo-tetrahedrally coordinated lead cation [19]. Bands between 680 and 800 cm⁻¹ can be interpreted as symmetric stretching vibrations [18, 20] of the GeO₄ tetrahedron, and bands in the range of 300–400 cm⁻¹ may be assigned to additional stretching and bending modes of the GeO₄ tetrahedron [19].

Fig. 3:

Comparison of the coordination spheres of the Pb²⁺ ion (left) with the pseudo-tetrahedral coordination (lone pair not shown) and the Ba²⁺ cation (right) with the distorted eightfold coordination sphere.

Fig. 4:

Single-crystal Raman spectrum of PbBaGeO₄ in the range of 100–1200 cm⁻¹.