# Enhancement of the Luminescence Properties of Eu (III) Containing Paramagnetic Saponite Clays
## 1 Introduction  
Following the definition of the International Union of Pure and Applied Chemistry (IUPAC), lanthanides are a unique series of 15 chemical elements with metallic properties, ranging from lanthanum (atomic number 57) to lutetium (71) [1, 2]. They are historically known as rare-earth elements (REEs) along with the chemically similar scandium and yttrium elements [1]. Lanthanides are usually found in the trivalent oxidation state as ions (_Ln\({}^{3+}\)_), while divalent (Sm\({}^{2+}\), Eu\({}^{2+}\), Tm\({}^{2+}\), Yb\({}^{2+}\)) and tetravalent (Ce\({}^{4+}\), Pr\({}^{4+}\), Nd\({}^{4+}\), Tb\({}^{4+}\), Dy\({}^{4+}\)) forms are rare [3, 4, 5]. As it is well-known, their chemistry is largely determined by the ionic radius which decreases along the series due to the lanthanide contraction effect [3]. The generic electron configuration for _Ln\({}^{3+}\)_ ions is [Xe]_4_p_-[14], in which the electrons of 4\(p\) orbitals are shielded from the external environment by outer-core 5s\({}^{2}\)5p\({}^{6}\) sub-shells, which gives rise to a very abundant amount of electronic levels with well-defined energies [3]. This effect gives the lanthanides some very fascinating features, such as paramagnetic, luminescent and catalytic properties [6, 7, 8]. Indeed, the 4\(p\) electron configurations determined several magnetic and electronic characteristics, such as considerable spin-orbit couplings and noteworthy paramagnetic character due to the unpaired electrons in 4\(p\) orbitals (excluding La\({}^{3+}\) and Lu\({}^{3+}\)) [3, 6]. Moreover, magnetic moments, susceptibilities and anisotropy and electron relaxation times greatly differ through the series [3, 6]. Owing to these properties, lanthanides were successfully employed in numerous magnetic applications: in the molecular magnetism as single-molecule magnets (SMM) and for magnetic refrigeration [9, 10, 11]; in biomedical fields, as in the magnetic resonance imaging (MRI), with more than a few clinically-approved Gd\({}^{3+}\)-chelates contrast agents (CAS), or with Gd\({}^{3+}\) (as ion or complex) embedded in nanosized biocompatible matrixes such as oxides, hydroxides, fluorides, oxysalts and silica based compounds [6, 12, 13, 14]; and in novel superconductors, battery and REE-magnets [15, 16]. Alongside their interesting magnetic features, lanthanides possessed very attractive optical properties and unique spectroscopic characteristics: sharp absorption bands with narrow and very recognizable emissions peaks of practically monochromatic radiations (in the NIR-UV-Vis range), long-lived excited state lifetimes and excellent photostability [17]. As a result of these unique features, many rare earth elements, i.e., Eu\({}^{3+}\), Tb\({}^{3+}\), Er\({}^{3+}\), Yb\({}^{3+}\), Sm\({}^{3+}\), have been extensively used in numerous optical applications, from laboratory scale [18] to a more commercial/industrial level [16; 19]. Notable applications include optical bioimaging (OI) and therapy [6; 14; 20; 21]; energy-efficient and photonic devices such as phosphors for lighting and displays (i.e., light-emitting diodes (LEDs)), photoluminescence complexes and materials (i.e., \(\beta\)-diketones, metal-organic frameworks (MOFs)), luminescent sensors and probes (i.e., for NIR up-conversion), security inks and tags, photocatalysts, solar energy panels, fiber-optics and lasers [20; 21; 22; 23; 24; 25; 26]; luminescent nano-thermometry [27]; optical refrigeration [28] and quantum information processing [29]. The direct excitation of 4\(\prime\) levels of luminescent Ln\({}^{3+}\) ions generally produces a weak luminescence due to the parity-forbidden \(f\)-\(f\) transitions which lead to weak light absorption (most molar absorption coefficients are \(<\) 10 L\(\cdot\)mol\({}^{-1}\cdot\)cm\({}^{-1}\)) [3].