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Analysis of The Properties of Lanthanides: The Case of Europium

The aim of this paper is to explore lanthanides, with a keen consideration of europium. According to Binnemans (2015), lanthanides are silvery-white metals that tarnish on exposure to air to form oxides, comparatively soft metals, have high boiling and melting points, react readily with water and dilute acids, burn easily in air, and are strong reducing agents. Alongside other properties, these elements are used as catalysts in the production of synthetic products and petroleum, used in lasers, magnets, lamps, motion picture projectors, phosphors, and X-ray. There are several lanthanides. However, this paper focuses on europium.

Lanthanide and Europium

The lanthanides refer to a group of fifteen chemical elements whose atomic numbers running from 57 to 71 (Binnemans, 2015). All these elements have single valence electron within their 5d shell (Naiman et al., 2018). One of the lanthanides is the Europium, an element symbolized by Eu and having atomic number 63 (Khuyen et al., 2019).  It is the most reactive lanthanide, necessitating that it be stored under an inert fluid for reasons of protecting it from moisture or atmospheric oxygen (Hajimazdarani et al., 2020). Similarly, Lobacheva and Dzhevaga (2017) note that Europium is equally the softest lanthanide since it can be dented by a fingernail and simply cut using a knife. When oxidation is scaled off, a shiny-white metal is exposed. Eu assumes an oxidation state of +3.

Electronic Configuration

Eu’s electronic configuration is [1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶] 4f⁷ 6s², which is as distributed as shown below.

Fig. 1. Periodic table (Gökçe et al., 2017).

Fig. 2. Schematic electronic configuration of Eu (Al-Shehri et al., 2019).

Eu’s Electronic Properties and Applications

Eu has several electronic properties. Eu has an electrochemical equivalent of 1.8899g/amp-hr, electron work function of 2.5eV, electronegativity of 1.01 (Allrod Rochow) or 1.2 (Pailing), heat fusion of 9.21kJ/mol, Ionization potential of 5.67 for first ionization, 11.245 for the second ionization, and 24.926 for the third one. Due to these properties, Europium is used differently. As aforementioned, the element is a less expensive, soft silvery metal that is the most reactive of the members of the lanthanide. It quickly tarnishes in air at room temperature and burns at approximately 150-180^oC and readily react with water (Binnemans, 2015). Europium is a neutron adsorber, making it be used in nuclear reactors regulatory rods (Naiman et al., 2018). Khuyen et al. (2019) add that Europium phosphors are employed in TV tubes to produce bright red colour as well as an activator for yttrium-based phosphors. A little europium is normally added to mercury vapour lamps to produce extra natural light when powerful street lighting is needed (Lobacheva & Dzhevaga, 2017). Europium salt is employed for newer phosphorescent paints and powder. It is equally employed in making thin-film superconductor alloys. Europium is used as a dopant in certain kinds of glasses in lasers along with other optoelectronic devices. Whereas trivalent Eu produces red phosphors, Al-Shehri et al. (2019) note that the luminescence of divalent Eu relies robustly upon the composition of the host structure. Lately, Eu is employed in quantum memory chips that can reliably keep data and information for days, allowing sensitive quantum information to be kept to some hard disk-like tools and shipped around (Hajimazdarani et al., 2020).

Eu’s Optical Properties and Applications

The reflective index of europium is unclear to date. However, a study by Naiman et al. (2018) revealed that Eu3+ function as the emitting centre in doped samples with only Eu as well as with both Ti and Eu with the transition from ⁵D₀ to ⁷F₂ being the most intense, suggesting that Eu3+ in a non-centrosymmetric site. The use of europium in UV light is, thus, explained by Eu’s ability to glow red. Similarly, red-green-blue luminescence is obtainable through the use of a single-type host structure. 

Energy Transitions

Europium undergoes three ionizations in its chemical reactions. This is because it has four valence electrons. With an electron affinity of 50 kJ/mol, Eu’s first ionization energy is 547.1 kJ/mol, the second one is 1085 kJ/mol, and the third one is 2404, and finally 4120 kJ/mol (Lobacheva and Dzhevaga (2017). A summary of the element’s energy transitions is as shown below.

Fig. 3. Eu’s energy transition (Lobacheva & Dzhevaga, 2017). 

Energy Level of Atomic Orbitals

Eu has 63 electrons, with an electronic shell structure of [2, 8, 18, 25, 8, 2]. Its number of protons is 63 as well with a mass number of 152. Its electron configuration is [Xe] 4f⁷ 6s². Eu’s atomic orbitals as shown in the figure below.

Fig. 4. Eu’s shell structure (Hajimazdarani et al., 2020).

Eu Luminescent Properties

Eu complexes having β-diketone ligands have, according to Gökçe et al. (2017), been characterized and synthesized using luminescence spectroscopy and fluorescence showing high florescence in sharp emission peaks (Khuyen et al., 2019). Al-Shehri et al. (2019) show that a 273 ± 9 μs lifetime for transition on excitation with laser lights. Remarkably, red europium emissions can equally be noted using a high-power microscope. These results imply that self-assembled gels, according to Hajimazdarani et al. (2020), can be doped with Eu salts producing smart soft materials for use in time-gated microscopy and spectroscopy. Gökçe et al. (2017) add that complexes have superb emission intensities, high quantum efficiencies, and long emission lifetimes, properties which can be associated with the fact that the triplet energy level of the ligands aptly match with the lowest excitation energy level state of Eu (3+)

Emission of EU Magnetic and Electrical

Eu (3+) compounds, according to Al-Shehri et al. (2019), usually show intense red photoluminescence. Gökçe et al. (2017) reason that Eu (3+) is a spectroscopic probe for the symmetry seen in lanthanide site. Similarly, Eu (3+) has excited state (⁵D₀) and non- degenerate ground state. The figure below summarizes Eu’s electrical and magnetic emission capabilities.

Fig. 5. Eu’s magnetic and electrical emissions (Khuyen et al., 2019).

The focus of this paper was to explore the various properties of europium. The same has comprehensively been covered in the paper.