High Temperature-high Pressure Synthesis And Photoluminescence Properties Of Lanthanide-containing Microporous Materials

Lanthanide-containing compounds have been extensively studied because they mayserve as optical, electronic, and magnetic materials. As a traditional material, mostsilicates have high thermal and hydrothermal stabilities. Many studies have focusedon the synthesis of microporous silicates of transition metals, main group elements,lanthanides, and uranium because of their versatile structural chemistry as well asinteresting physical and chemical properties. Among them, lanthanide silicates, whichcontain stoichiometric amounts of framework Ln3+ions, have high thermal stabilityand tunable optical properties, making them potential candidates as fast ionconductors and optical materials. Most of these compounds were synthesized undermild hydrothermal conditions in Teflon-lined stainless steel autoclaves in thetemperature range of100~240°C. Herein, we mainly focus on the preparation andcharacteristics of the lanthanide silicates/germanates with photoluminescenceproperties and novel structures under high-temperature and high-pressure conditions.The experiments were carried out at the range from0.5to5GPa and300to700°C inthe Quickpress3.0piston-cylinder and LECO Tem-Pres high pressure hightemperature instrument. The structures and photoluminescence properties of theseas-prepared compounds were also investigated.(1) A new europium silicates, Na15Eu3Si12O36was successfully synthesized under high temperature and high pressure conditions, and structurally characterizedby single-crystal and powder X-ray diffraction (XRD). The single-crystal XRDanalysis of Na15Eu3Si12O36reveals that its structure is based on [Si6O18]n12n cyclosilicate anions that are built from six SiO4tetrahedra sharing two of their four Ocorners with each other. Such [Si6O18]n12n cyclosilicate anions are linked via EuO6octahedra to form a three-dimensional (3D) framework containing6-membered ringchannels delimited by the SiO4tetrahedra and EuO6octahedra along the [010]direction. The Na+ions are located in the free void space to achieve the chargebalance.Single crystal X-ray analysis also reveals that the crystallographic positionsof Na(2) and Na(3) are shared with partially occupied Eu(2) and Eu(3) centers,respectively. This statistical disorder of Eu and Na centers have an importantinfluence on the photoluminescence properties of Na15Eu3Si12O36.(2) A new europium silicates, K2EuSi4O10F was successfully synthesized from aKF·2H2O–KOH flux under high temperature and high pressure conditions, andstructurally characterized by single-crystal and powder X-ray diffraction (XRD). Thesingle-crystal XRD analysis reveals that the structure of K2EuSi4O10F consists ofinfinite tubular chains of corner-sharing SiO4tetrahedra, which are further linkedtogether via corner sharing O atoms by infinite chains of EuO4F2octahedra forming a3-D framework that contains8-ring and6-ring channels along the [010] direction.Every EuO4F2octahedron has four equatorial O atoms that are shared by threeneighboring tubular silicate chains and two axial F atoms. These axial F atoms act ascommon vertices to other octahedra forming a one-dimensional chain of EuO4F2octahedra along the [010] direction. The photoluminescence study is consistent withthe crystallography result and show that K2EuSi4O10F exhibit strong red emission.(3) A new europium silicate, Na3EuSi6O15·1.47H2O, has been synthesized usinga high temperature and high pressure synthetic method. The structure ofNa3EuSi6O15·1.47H2O is based on corrugated silicate single layers with thecomposition [Si2O5] in the ab plane containing4,5,6, and8rings, which are linkedvia EuO7polyhedra to form a three-dimensional (3D) framework of Na3EuSi6O15·1.47H2O. It contains4,8-ring channels along the [100] direction and5,6ring along the [001] direction. The Na+ions are located in the free void space tobalance the charge. The photoluminescence studies are consistent with thecrystallographic results and show that Na3EuSi6O15·1.47H2O is a potential redphosphor.(4) A family of2D-layered lanthanide germanates K3[Gd1-xTbxGe3O8(OH)2](x=0,0.3,0.1, and1), have been synthesized by a high temperature, high pressurehydrothermal method and characterized by single-crystal X-ray diffraction,photoluminescence, IR spectra, and Energy-dispersive spectroscopy (EDS). TheX-ray powder diffraction patterns of these compounds reveal that they areisostructural. The single-crystal X-ray diffraction analysis of K3[GdGe3O8(OH)2]reveals that it is2D-layered [LnGe3O8(OH)2]n3n anionic framework which is built upfrom GeO4H/GeO4tetrahedra and GdO6octahedra by sharing vertex O atoms. K+ionslocate in the free void space to achieve the charge balance of the framework. Samplecontaining only Tb3+emit mainly from one transition,5D4â†'7F5(552nm). Mixedlanthanide samples, K3[Gd1-xTbxGe3O8(OH)2](x=0.3, and0.1), have also beenprepared and efficient Gdâ†'Tb energy transfer has been observed.