CRYSTALLOGRAPHIC STUDY OF QUATERNARY PHASE
IN THE Eu-Mn-Ge-O SYSTEM BY TEM AND SEM 

E.A. Juárez-Arellano, G.U. Gamboa-Espinosa, J.A. Lara, L. Bucio and E. Orozco

 

Instituto de Física, UNAM, Apdo. Postal 20-364, 01000 México D.F., México

E-mail: erickj@fisica.unam.mx, bucio@fisica.unam.mx, eligio@fisica.unam.mx

Abstract

We have synthesized a new quaternary compound with stoichiometric formula EuMnGe₂O₇. The compound was grown up as micro-crystals into quartz tube under thermal treatment in vacuum. The structural characterization of these crystals was carried out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). After a careful analysis we found that the new quaternary compound crystallizes in the orthorhombic system with space group A222 (No.21) and cell parameters a = 4.6(2) Å, b = 8.5(4) Å, c = 12.7(6) Å and V = 495.6 ų.

Keywords: Europium manganese germanate; Crystal structure; TEM; SEM.

Resumen

Se sintetizó un nuevo compuesto cuaternario con formula estequiométrica EuMnGe₂O₇, en forma de una serie de micro cristales dentro de un tubo de cuarzo bajo tratamiento térmico en vacío. La caracterización estructural de los micro cristales se realizó utilizando microscopía electrónica de trasmisión (MET) y microscopía electrónica de barrido (MEB). Después de un cuidadoso análisis, se llegó a la conclusión de que el nuevo compuesto cuaternario EuMnGe₂O₇ cristaliza en el sistema ortorrómbico con grupo espacial A222 (No. 21) y parámetros reticulares a = 4.6(2) Å, b = 8.5(4) Å, c = 12.7(6) Å y V = 495.6 ų.

Palabras claves: Germanato de europio manganeso; estructura cristalina; MET; MEB.

1. Introduction

In the last years the quaternary oxide compounds have had an increasing interest due to its specific physical properties such as optical, electrical, magnetic, and others. Most of these quaternary compounds have in their composition a transition metal with covalent elements such as silicon or germanium giving a varied opened structure. Among this new compounds it has been reported many compounds having the thortveitite-type structure or very close to it such as FeInGe₂O₇, InYGe₂O₇, and FeTbGe₂O₇ for example [1, 2, 3]. These compounds belong to the monoclinic system and are very interesting because of its laminar structure. On the other hand if the transition metal is selected to be Mn in the stoichiometric formula, then the GdMnGe₂O₇ compound is obtained and crystallizes in the orthorhombic system having also a laminar structure. In this case, chains of MnO₆ octahedral altering with parallel chains of GdO₆ antiprisms form layers linked together through Ge₂O₇ groups [4]. This compound exhibits a novel quasi-one-dimensional lattice, which might be ascribed to the Jahn-Teller nature of the Mn³⁺ ion. The structural relationships between the low monoclinic symmetry exhibited by the MRGe₂O₇ family compounds (M = In, Fe; R = Y, Tb-Er) [2, 3] and the high orthorhombic symmetry in the GdMnGe₂O₇ compound, as well as the relationship with the magnetic and optical properties given specifically in the case of the trivalent rare earth ion (Eu³⁺) is the subject we are interested in the research of quaternary oxides having the nominal composition given before.
In this work we report the crystallographic study made in the new EuMnGe₂O₇ oxide by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

2. Experimental

The EuMnGe₂O₇ compound was prepared modifying the method reported by Taviot-Guéhe et al. [4]. For the
synthesis, MnO₂, GeO₂ and EuCl₂ (1:1:1) were mixed in stoichiometric proportions and heated for 3 days at 150 °C to dry it. After that, the sample was placed inside evacuated quartz tubes maintaining the temperature at 300 °C. The quartz tube was put inside the reflective furnace and the reaction was placed at 750 °C for 10 days. After the thermal treatment, many micro-crystals were formed on the wall of the quartz tube showing a perfect crystalline appearance. The resulting product of reaction was stirred using ethanol to dissolve the MnCl₂ sub-product. The schematic reaction is as follows:

The SEM and TEM experiments were carried out in a JEOL 5000 and JEOL 100CX electron microscopes respectively.

3. Results

Because of the small size of crystals (10-50 μm) and the low quantity of sample obtained, the structural characterization were carried out by SEM and TEM.

3.1. Scanning electron microscopy (SEM) analysis

The crystallographic study began with the analysis of the crystal shape observed in SEM images. In Fig. 1 (a, b and c) a view of micro-crystals of EuMnGe₂O₇ that were formed on the quartz tube wall can be observed. 

Fig 1 (a) A view of micro-crystals of EuMnGe₂O₇ formed on the quartz tubes wall, SEM-500X. (b) Crystals with morphology y compatible with the disphenoidal shape in the point group 222, SEM-1000X. (c) A single crystal exhibiting the same morphology characteristic in orthorhombic symmetry, SEM-1500X.

In the first image, Fig. 1 (a), a homogeneous micro-crystal size with perfectly crystalline appearance can be appreciated. The Fig. 1 (b) shows some details of the micro-crystals at 1000X amplification. The typical morphology is clearly exhibited in Fig. 1 (c). In fact, this crystal shape is in perfect agreement with the point group symmetry 222 (D_2h according to Schoenflies) and the orthorhombic symmetry reported for the GdMnGe₂O₇ compound [4]. It is well know that crystals with symmetry disphenoidal morphology [5, 6], Fig. 2.

Fig. 2 Schematic view of the crystal shape belonging to the point group 222 (D_2h). The point group 222 has three two-fold axes perpendicular each one to other. Because its appearance is similar to a monoclinic sphenoid, it is know as disphenoidal [6].

3.2. Transmission electron microscopy (TEM) analysis

Assuming that our compound crystallize in the orthorhombic system, we analyze the electron diffraction patterns of EuMnGe₂O₇ using the GdMnGe₂O₇ cell parameters and its hkl Bragg reflections. From the inorganic crystal structure database ICSD [7] we obtained the crystallographic data given by the space group A222 (No. 21), the orthorhombic cell parameters a = 4.735(1) Å, b = 7.839(2) Å, c = 13.500(3) Å, V = 501.09 ų and Z = 4 corresponding to the GdMnGe₂O₇ compound. The space group A222 (No.21) have the following systematic conditions observed for the Bragg reflections hkl: k+l; 0kl: k+l; h0l: l; hk0:k; 0k0: k and 00l: l. Fig. 3 (a) shows a plane a*b* in the reciprocal space with [001] as the zone axis. In this image it can be observed clearly the high symmetry of the EuMnGe₂O₇ so the consideration of the orthorhombic symmetry appear to be reasonable. After a carefully measurement and making some calculus in this electron diffraction pattern, the following cell parameters were obtained: a = 4.6(2) Å and b = 8.5(4) Å. Fig. 3 (b) shows a reciprocal plane in the [111] zone axis. With this pattern and after a simple geometric considerations we found that the cell parameter c = 12.7(6) Å and in consequence V = 495.6 ų. The patterns showed in Fig. 3 (c and d) could be indexed having the [001] and [312] zone axis respectively. These results allow us to establish that the new compound EuMnGe₂O₇ has symmetry compatible with the orthorhombic symmetry (space group A222) reported for the GdMnGe₂O₇ compound. Table 1 shows the cell parameters of two compounds (EuMnGe₂O₇ and GdMnGe₂O₇).

From the analysis of the crystal data given in table 1, it can be noted that if the ionic radio values of 1.078 and 1.087 Å are used for the six-co-ordinated Gd³⁺ and Eu³⁺ ions respectively [8]; then it should be expected no significant differences between the cell volumes. It is in complete agreement with the cell volumes given in table 1. A more detailed characterization by X-ray diffraction measurements will be necessary to establish the precise lattice parameters as well as the crystal structure data.

          

Fig. 3 The images show electron diffraction patterns of the new EuMnGe₂O₇ compound with zone axis (a) [001], (b) [111], (c) [001] and (d) [312].

Table 1 Lattice parameters for EuMnGe2O7 and GdMnGe2O7, space group A222 (No. 21).

^aCalculated values from the indexed electron diffraction patterns.
^bFrom single crystal X-ray diffraction data at room temperature.

 4. Conclusions

It can be concluded that the substitution of europium in the GdMnGe₂O₇ structure type is possible for the stoichiometric formula EuMnGe₂O₇. The carried out by TEM and SEM seems to be very powerful tools to confirm the characterization described before when it is impossible to do it by mean of X-ray diffraction measurements. The results about EuMnGe₂O₇ crystal structure will be useful to understand the magnetic and optical properties of this compound, which are now under progress.

Acknowledgements

The authors are grateful to José Reyes, Pedro Mexia, Carlos Flores, Jacqueline Cañetas, Roberto Hernandez, Mercedes Fonseca, Gabriela Campuzano, Angel Osornio and Miguel Bustos for the technical support. The financial aid from DGAPA-UNAM under contract PAPIIT IN 113199 are also acknowledged.

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