Crystal structure of the europium(II) aluminate Eu₃Al₂O₆

1 Introduction

Europium can adopt two different valence states, Eu²⁺ with a [Xe]4f⁷ and Eu³⁺ with a [Xe]4f⁶ configuration, leading to different magnetic ground states. This is already realized for the binary oxides. EuO with the simple rock salt structure contains purely divalent europium and shows ferromagnetic ordering at 70 K [1–3]. On the other hand, Van Vleck type paramagnetism occurs for Eu₂O₃ [4, 5] with exclusively trivalent europium. Mixed-valent Eu₃O₄ (≡ Eu^IIEu₂^IIIO₄) shows antiferromagnetic ordering only of the Eu²⁺ site at T_N=5 K. This is evident through magnetic hyperfine field splitting of the Eu(II) signal in ¹⁵¹Eu Mössbauer spectroscopy [6, 7].

The comparatively high Curie temperature of EuO initiated many investigations for ternary Eu(II)-based oxidic materials. The titanates EuTiO₃ (T_N=5.3 K) [8, 9], Eu₂TiO₄ (T_C=7.8 K), and Eu₃Ti₂O₇ (T_C=8 K) [9] were among the first examples, similar to the silicates EuSiO₃, Eu₂SiO₄, and Eu₃SiO₅ [10]. Especially the perovskite EuTiO₃ is broadly reinvestigated with respect to its structural instability and multiferroic behavior (e.g., [11–13]).

Phase analytical work in the EuO–Al₂O₃ system and thermochemical investigations of the reduction process of Eu₂O₃ with aluminium led to the aluminates EuAl₂O₄, Eu₃Al₂O₆, and Eu₅Al₂O₈ [10, 14]. These phases have intensively been studied with respect to their synthesis conditions, their crystal optical and magnetic properties, for their complex dielectric constants, or with respect to AlN/aluminate composite formation [10, 14–21].

Among these aluminates, Eu₃Al₂O₆ [15] is a ferromagnet with T_C=8.5 K. This phase has so far only been characterized with respect to its properties. Even lattice parameters are not available. Eu₃Al₂O₆ is isotypic with the C₃A clinker phase (an important component in Portland cement) Ca₃Al₂O₆ [22] and Sr₃Al₂O₆ [23]. During a synthesis attempt of an equiatomic compound ‘EuCdAl’ in a sealed niobium ampoule (strongly reducing atmosphere), we obtained small amounts of irregularly shaped single crystals of Eu₃Al₂O₆ as a byproduct. Surface-contaminated europium and/or cadmium were the oxygen source. A single crystal X-ray diffractometer study of Eu₃Al₂O₆ is reported herein.

2 Experimental

The starting materials for the preparation of the intermetallic phase were sublimed ingots of europium (Alfa-Aesar), cadmium tear drops (Johnson Matthey), and aluminium turnings (Koch Chemicals). Although the air- and moisture-sensitive europium ingots were kept in Schlenk tubes under argon prior to the reaction, they were most likely the oxygen source through surface oxidation. The elements were weighed in the ideal 1:1:1 atomic ratio and arc-welded [24] in a niobium tube under 800 mbar of argon. Argon was purified with titanium sponge (900 K), silica gel, and molecular sieves. The niobium ampoule was subsequently placed in the water-cooled sample chamber of an induction furnace (Typ TIG 1.5/300; Hüttinger Elektronik, Freiburg, Germany) [25]. The sample was first heated to 1323 K, maintained at that temperature for 5 min, and then annealed at 873 K for 2 h, followed by quenching. The temperature was controlled by a Sensor Therm Methis MS09 pyrometer with an accuracy of ±30 K. Afterward, the niobium tube was sealed in a silica tube under vacuum, rapidly heated to 1273 K, and cooled down to 673 K within 12 days in order to achieve a better crystallinity. No reaction with the container material was observed; however, within the reaction product, we obtained a small quantity of Eu₃Al₂O₆ single crystals.

Irregularly shaped crystal fragments with dark amber color were selected from the crushed ‘EuCdAl’ sample. The crystals were glued to thin quartz fibers using beeswax and their quality was checked by Laue photographs on a Buerger camera (white molybdenum radiation, image plate technique, BAS-1800, Fujifilm). The data set was collected using an IPDS II diffractometer (graphite monochromatized MoK_α radiation; oscillation mode, room temperature). A numerical absorption correction was applied to the data set. All relevant crystallographic data and details of the data collection and evaluation are listed in Table 1.

Several Eu₃Al₂O₆ crystal fragments were analyzed by semiquantitative EDX analysis using a Zeiss EVO MA10 scanning electron microscope with EuF₃ and Al₂O₃ as standards. No impurity elements (especially from the crucible material and cadmium) heavier than sodium (detection limit of the instrument) were observed. The experimentally determined composition (23 ± 3 at.-% Eu:18 ± 3 at.-% Al:59 ± 3 at.-% O) was close to that obtained from the structure refinement (27.3:18.2:54.5). The standard deviations account for the irregular shape of the crystals (typical conchoidal fracture; see Fig. 1).

Fig. 1

A single crystal of Eu₃Al₂O₆. The maximum edge length is 80 μm.

2.2 Crystal chemistry

The aluminate Eu₃Al₂O₆ crystallizes with the cubic Ca₃Al₂O₆-type structure [22]. The calcium compound is one of the important so-called clinker phases in Portland cement [29]. Other isotypic phases are Sr₃Al₂O₆ [23] and Sr₃Ga₂O₆ [30]. In the case of the strontium phases, a homogeneous solid solution Sr₃Al_2–xGa_xO₆ occurs [31]. Crystallization of these aluminate phases is not simple. Only the prototype structure has been refined from X-ray film data [22], whereas the structures of Sr₃Al₂O₆ [23] and Sr₃Ga₂O₆ [30] were deduced from Rietveld refinements based on X-ray and neutron powder diffraction data.

Herein we report on the first crystallographic study of Eu₃Al₂O₆. This aluminate crystallizes with a comparatively large unit cell (a=1584.82 pm). The best approach for description of the structure relies on the metal–oxygen polyhedra. The Eu₃Al₂O₆ structure has two crystallographically independent aluminium sites, both in tetrahedral oxygen coordination with Al–O distances ranging from 173 to 178 pm. For both sites, the bond-valence sums (BVSs) (Table 3) are close to the ideal value. Three Al1@O₄ and three Al2@O₄ tetrahedra share common corners in –up–down–up–down–up–down– sequence, leading to hexagonal rings (Fig. 2) perpendicular to the threefold axis. These networks are condensed in the third direction via the europium atoms.

Fig. 2

Condensation of the aluminium–oxygen tetrahedra in the structure of Eu₃Al₂O₆ (view along the threefold axis).

The Eu₃Al₂O₆ structure contains six crystallographically independent europium sites, which can be assigned to two groups. The Eu1, Eu2, and Eu3 atoms have distorted octahedral oxygen coordination with a small range of Eu–O distances (247–250 pm). These EuO₆ octahedra are linked to the network of AlO₄ tetrahedra via common corners (Fig. 3). This network still leaves large voids that are filled with the Eu4, Eu5, and Eu6 atoms. These three europium sites have higher coordination numbers of 9, 8, and 7, respectively, with a much broader range of Eu–O distances (241–297 pm). The irregularly shaped Eu4@O₉, Eu5@O₈, and Eu6@O₇ polyhedra are condensed to the octahedra via common triangular faces (Fig. 4), leading to a dense packing.

Fig. 3

The crystal structure of Eu₃Al₂O₆. The octahedral (in magenta) and tetrahedral (in light gray) oxygen coordination around the Eu1, Eu2, Eu3, Al1, and Al2 atoms is emphasized. Voids left by this complex network are filled by the Eu4, Eu5, and Eu6 atoms.

Fig. 4

Cutout of the Eu₃Al₂O₆ structure, emphasizing the condensation of the europium–oxygen polyhedra.

Although a simple ionic formula splitting 3Eu²⁺2Al³⁺6O^2– leads to an electron-precise description of this transparent salt-like material, the course of the calculated BVSs (Table 3) reveals some differences for the six europium sites. The BVSs of Eu1, Eu2, and Eu3 are slightly higher than 2, and we observe average Eu–O distances that are shorter than those for Eu4, Eu5, and Eu6. This is consistent with stronger covalent Eu–O bonding for the octahedrally coordinated europium atoms, whereas Eu4, Eu5, and Eu6 show higher ionicity for the Eu–O bonds (with an almost optimal value for Eu6). A similar range of BVSs has been reported for the isotypic strontium compound [23].