A series of new layered lithium europium(II) oxoniobates(V) and -tantalates(V)

2.1 Sample preparation

2.1.1 Synthesis of Li₂Eu₂Nb₃O₁₀ and Li₂Eu_1.5Ta₃O₁₀

The syntheses of Li₂Eu₂Nb₃O₁₀ and Li₂Eu_1.5Ta₃O₁₀ were performed by heating mixtures from stoichiometric quantities of Li₂[CO₃] (Acros Organics, Geel), Eu₂O₃ (ChemPur, 99.99%), Nb₂O₅ (Fluka, 99.9%) and Ta₂O₅ (ChemPur, 99.99%), respectively, and niobium (CIBA, reinst) and tantalum (Merck, 99.999%) metal powder, respectively, as described in the following equations:

5 Li2[CO3]+5 Eu2O3+6 Nb2O5+3 Nb→5 Li2Eu2Nb3O10 + 5 CO2↑

20 Li2[CO3]+15 Eu2O3+27 Ta2O5+6 Ta→20 Li2Eu1.5Ta3O10 +20 CO2↑

These reactions were performed in cramped, but not soldered niobium or tantalum capsules (Plansee, Reutte), which were heated in a high-frequency furnace above T=1600°C for 10 min under an argon atmosphere. The melting cakes were cooled to room temperature within 10 min and black, crystalline products (Fig. 1) were obtained. The crystals remain unchanged in air for months.

Fig. 1:

Samples of Li₂EuKNb₃O₁₀ (top left), Li₂Eu₂Nb₃O₁₀ (top right), Li₂Eu_1.5Ta₃O₁₀ (bottom left), and Li₂EuKTa₃O₁₀ (bottom right) under a light microscope.

2.2 Structure determinations

For the single-crystal X-ray diffraction studies, well-faceted crystals were selected under a light microscope, packed in 0.1 mm diameter glass capillaries and sealed. The crystals were measured at room temperature on a κ-CCD diffractometer (Bruker-Nonius) with MoKα radiation (λ=71.07 pm). The crystallographic data for Li₂EuKNb₃O₁₀, Li₂Eu₂Nb₃O₁₀, Li₂Eu_1.5Ta₃O₁₀, and Li₂EuKTa₃O₁₀ and the results of the final refinements are listed in Table 1, the atomic coordinates appear in Table 2.

Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49) 7247-808-666, http://www.fiz-karlsruhe.de/request_for_deposited _data.html, E-Mail: crys-data@fiz-karlsruhe.de), on quoting the depository numbers CSD-1949876 to CSD-1949879.

3.1 Crystal structures

Li₂Eu₂Nb₃O₁₀, Li₂Eu_1.5Ta₃O₁₀, Li₂EuKNb₃O₁₀, and Li₂EuKTa₃O₁₀ crystallize in the tetragonal space group I4/mmm typically found for Ruddlesden-Popper phases with lattice parameters ranging from a=393 to 396 pm and from c=2609 to 2688 pm (Fig. 2). The crystallographically independent Eu²⁺ cations are located in cuboctahedra formed by oxygen atoms. In all four compounds the Eu–O distances lie between 270 and 290 pm (Table 3), which is typical for europium atoms in the oxidation state +2 in agreement with the calculated bond-valence parameters (Table 4). These bond-valence parameters correspond well with the results from calculations of the Charge Distribution calculated with Chardi2015 and the Madelung Parts of the Lattice Energy (MAPLE) [30], [31], [32], [33], [34]. It is remarkable that in Li₂Eu₂Nb₃O₁₀ the position of the europium atoms is fully occupied, in contrast to Li₂Eu_1.5Ta₃O₁₀, where only ¾ of this position is filled with europium (Table 2). Simple charge counting thus results in the oxidation state +5 for tantalum in Li₂Eu_1.5Ta₃O₁₀, whereas an average oxidation state of +4.667 is obtained for niobium in Li₂Eu₂Nb₃O₁₀. For Li₂EuKNb₃O₁₀ and Li₂EuKTa₃O₁₀ a full occupation of the corresponding crystallographic sites with europium and potassium is found (Table 2). This appears reasonable, because of the comparable ionic radii of Eu²⁺ (r_i=148 pm) and K⁺ (r_i=164 pm) for a 12-fold coordination [35], [36].

Fig. 2:

Structural building units in Li₂Eu₂Nb₃O₁₀, Li₂Eu_1.5Ta₃O₁₀, Li₂EuKNb₃O₁₀, and Li₂EuKTa₃O₁₀ as extended unit cell viewed in b direction (top; RE=Eu or Eu and K, M=Nb and Ta) and a {[LiO4/2e]−}∞2 layer of edge-sharing [LiO₄]⁷⁻ tetrahedra (bottom).

Taking the refined occupation factors of the crystallographic sites for europium and potassium in Li₂EuKNb₃O₁₀ and Li₂EuKTa₃O₁₀ into account, an oxidation of +5 results for both octahedrally coordinated atoms, niobium and tantalum, respectively. The polyhedra are vertex-connected forming {[MO4/2vO2/1t]3−}∞2 layers in the ab plane. Along the c direction, these layers form triple blocks (Fig. 2, top), which are separated by {[LiO4/2e]−}∞2 layers (Fig. 2, bottom), where the Li⁺ cations are tetrahedrally coordinated (Fig. 3), but exhibit two extra oxygen contacts along [001].

Fig. 3:

Coordination polyhedra in Li₂Eu₂Nb₃O₁₀, Li₂Eu_1.5Ta₃O₁₀, Li₂EuKNb₃O₁₀, and Li₂EuKTa₃O₁₀ of M1 (top left) and M2 (bottom left, M=Nb or Ta) as octahedra, the cuboctahedral coordination sphere of the europium (or potassium) site (=RE, top right) and the lithium-centered oxygen tetrahedron (bottom right) with two extra oxygen neighbors along [001].

3.2 Physical properties

The individual magnetic susceptibilities of Li₂Eu₂Nb₃O₁₀, Li₂Eu_1.5Ta₃O₁₀, and Li₂EuKNb₃O₁₀ were measured between T=5 and 300 K in a SQUID magnetometer MPMS3 from Quantum Design (San Diego) (Fig. 4). The susceptibilities follow a paramagnetic behavior in the measured temperature range (Table 5). Applying the Curie-Weiss law, the magnetic moments (μ_eff) were calculated to 6.31 μ_B (Li₂Eu₂Nb₃O₁₀), 7.89 μ_B (Li₂Eu_1.5Ta₃O₁₀), and 8.01 μ_B (Li₂EuKTa₃O₁₀). The theoretical moment for Eu²⁺ cations is 7.94 μ_B, so for Li₂Eu_1.5Ta₃O₁₀ and Li₂EuKTa₃O₁₀ it fits well, but for Li₂Eu₂Nb₃O₁₀ the moment is too small. As it is expected from the nearly full occupation of the europium site, niobium has to be partly reduced from +5 to +4. According to the findings from X-ray photo emission spectroscopy (XPS) studies, there are Nb⁴⁺ cations in the structure, which show van Vleck paramagnetism for a 4d¹ configuration, leading to a calculated moment of 6.47 μ_B:

Fig. 4:

Magnetic susceptibilities (χ_mol) and their inverse values (χmol−1) as a function of temperature (T) for Li₂Eu_1.86(2)□_0.14(2)Nb₃O₁₀ (top), Li₂Eu_1.5Ta₃O₁₀ (mid), and Li₂EuKTa₃O₁₀ (bottom).

μtheo=3.725.72(μeff, Eu2+)2+25.72(μeff, Nb4+)2  =3.725.72(7.94 μB)2+25.72(1.73 μB)2=6.47 μB

The cation-defect variant Li₂Eu_1.86(2)□_0.14(2)Nb₃O₁₀ has been characterized by a single-crystal structure determination based on X-ray diffraction (Section 3.1). Such a deficiency at the europium site must be accompanied by the presence of niobium atoms which have an average oxidation state lower than +5. For an additional analysis of this oxoniobate, XPS spectra have been measured on a commercial Kratos Axis Ultra DLD system (Shimadzu, Kyoto) with a monochromatized AlKα source (1486.6 eV) with a base pressure in the lower 10⁻¹⁰ mbar range. The binding energy (BE) was calibrated by setting the C 1s BE to 284.8 eV with respect to the Fermi level. High-resolution spectra were acquired with an analyzer pass energy of 20 eV. Analysis of the XPS data was performed with CasaXPS software. The measured polycrystalline samples were pressed into an indium foil. In their XPS spectra (Fig. 5) only europium atoms with the oxidation state +2 are observed and just minor impurities of Eu³⁺ can be detected, which probably stem from partial oxidation on the surface. However, as expected, besides Nb⁵⁺ a significant amount of Nb⁴⁺ is found, confirming the results from the structure refinement. The peak around 1200 eV in Fig. 5 (top) originates from small impurities of carbon arising from transportation processes of CO₂ during the high-temperature syntheses.

Fig. 5:

a) XPS spectrum of Li₂Eu₂Nb₃O₁₀ in the range from E=0 to 1400 eV, b) XPS spectrum of Li₂Eu₂Nb₃O₁₀ together with a simulation (red) in the range between 200 and 213 eV for the niobium transitions, c) transitions for europium together with simulations (red) in the range from 1127 to 1170 eV.