High-temperature And High-pressure Synthesis And Photoluminescence Propetries Of Lanthanide Silicates And Germanates

Lanthanide-containing compounds have been extensively studied because theymay serve as optical, electronic, and magnetic materials. Most silicates have highthermal and hydrothermal stabilities. Also the silicates with micropore can be used inmany fields such as catalysis, ion-exchange, absorption and host-guest materials.Combining the photoluminescence properties of lanthanides and the microporosity ofsilicates is an interesting research field.Herein, we mainly focus on the preparation and characteristics of the lanthanidesilicates with photoluminescence properties and novel structures underhigh-temperature and high-pressure conditions. Then in situ synchrotron X-raydiffraction analysis and photoluminescence spectroscopic measurement in a diamondanvil cell (DAC) under high pressure were performed, in order to establish therelationship between the structure of the framework and the photoluminescenceproperties of the lanthanide ions. So the rare-earth ion may be a useful and efficientprobe in the future research of structural phase transition of lanthanide-containingcompounds under high pressure. Also we may find the stable photoluminescencematerials under high pressure conditions.â… . Under high-temperature and high-pressure conditions, we synthesized threelanthanide silicates with novel structures by changing the reaction ratio, temperature,pressure and time. The detailed results are as follows:(â…°) The gadolinium silicate GdSiO-CJ7was synthesized under400°C and80MPa. The structure of GdSiO-CJ7consists of complex zigzag silicate chains[Si₅O₁5]10-containing4-and10-rings running along the [010] direction, and a chain ofGdO6and GdO8polyhedra along the [001] direction. The structure has10-ringchannels along the [100] and [001] directions.(â…±) The terbium silicate TbSiO-CJ8was synthesized under600°C and220MPa.The structure of TbSiO-CJ8consists of undulated silicate double layer with4-and8-rings parallel to the (001) plane, which are built up from SiO4tetrahedra via cornersharing. Such double silicate layers are linked together by TbO6octahedra via vertexoxygen atoms resulting in the3-D open-framework. The structure has8-ring channelsalong the [100] and [010] directions. (â…²) The dysprosium silicate DySiO-CJ9was synthesized under600°C and220MPa. The structure of DySiO-CJ9possesses a3-D silicate framework [Si6O15]6-constructed from SiO₄tetrahedra. This framework is a relatively dense phasecontaining only6-rings.â…¡. The in situ synchrotron X-ray diffraction analysis and photoluminescencespectroscopic measurement in a diamond anvil cell (DAC) under high pressure wereperformed. The detailed results are as follows:(â…°) The europium germanate NaEuGeO₄was prepared under hydrothermalconditions. The in-situ photoluminescence spectroscopic measurement and thesynchrotron X-ray diffraction analysis in a diamond anvil cell (DAC) under highpressure were performed to study the pressure-induced phase transition of NaEuGeO₄as well as changes of the luminescent properties of Eu3+ions. Photoluminescencespectroscopic studies revealed that a phase transition occurred at a pressure range of6.5-10GPa and the high-pressure phase of NaEuGeO₄(NaEuGeO₄-HP) was stillstable when the pressure raised up to about20GPa. As the pressure was released, thespectra returned to the original state, revealing that the pressure-induced phasetransition is a completely reversible process. The synchrotron X-ray diffractionanalysis demonstrated that the structure of NaEuGeO₄transformed to an unidentifiedphase NaEuGeO₄-HP at a pressure higher than10GPa, and the phase transition isreversible. This result is consisted with that obtained by photoluminescence spectraanalysis.(â…±) The europium germanate NaEu3(GeO₄)₂(OH)₂was prepared under mildhydrothermal conditions. In situ synchrotron X-ray diffraction analysis andphotoluminescence spectroscopic measurement in a diamond anvil cell (DAC) underhigh pressure were performed to study its structural stability as well as theluminescent properties of the Eu3+ions. The synchrotron X-ray diffraction analysisrevealed that the structure of NaEu3(GeO₄)₂(OH)₂was stable under pressure thatranged from1atm to20GPa with only a little distortion, and the unit-cellcompression occurred more readily along the a axis than along the c axis. When thepressure was reduced to ambient pressure, the distortion and the unit cell recovered tothe original state. Photoluminescence spectroscopic studies under high pressureshowed that the redshift of the5D0→7FJ(J=0,1,2) transitions and the pressureimposed on the sample had a good linear relationship. Also, the intensity of the redemission lines was independent of the pressure in the range from9to about20GPa.â…¢. The in situ photoluminescence spectroscopic measurement in a diamondanvil cell (DAC) under high pressure was performed. The detailed results are asfollows:(â…°) The in-situ high-pressure photoluminescence measurement was taken on themesoporous MCM-41and SBA-15loaded with rhodamine B. The luminescenceemission could be found until the pressure reaches10GPa, which indicates that MCM-41and SBA-15can prevent the aggregation of rhodamine B under highpressures. Meanwhile obvious peak shifts have a broader band comparing with that inthe aqueous solution and that loaded on amorphous SiO2.(â…±) The in-situ high-pressure photoluminescence measurement was employed onthe zeolite Y loaded with CdSe quantum dots. The results show that the peaks areblue-shifted with the pressure increased while the intensity of the luminescence isdecreased. The blue-shift indicates the shrinking of CdSe quantum dots, which isascribed to the shrinking of the cages of zeolite Y. The decrease in the intensity isprobably owing to the increase of surface defects with the increasing pressure, whichaugments the possibility of the recombination of the electrons and the holes, thusresulting in the decrease of quantum yield.