Self-reduction And Valence Control Of Mn In Na₂ZnSiO₄:Mn And Li₂ZnSiO₄:Mn

Transition metal Mn doped luminescence materials are vital for lighting and displays due to their unique luminescence properties.The investigations of new luminescent materials and the application based on Mn has become one of the research hotspots in inorganic luminescence field.Among the various valence states of Mn,the Mn²⁺and Mn⁴⁺ions are the most used as activators in inorganic luminescence materials,and the luminescence behavior of the Mn doped luminescence materials depend strongly on the valence state of Mn ions.Therefore,it is of great importance to study and control the valence of Mn not only in fundamental research but also the applications.The Mn²⁺activated luminescent materials are generally prepared in a reducing atmosphere.However,in some special compounds,the Mn can maintain as Mn²⁺,or the reduction of the higher valence Mn to Mn²⁺spontaneously in even an air or oxidation atmosphere.The self-reduction phenomenon of Mn has been observed in some cases,while reduction conditions and mechanisme are still some unclear.It is of great significance to further investigate and elucidate the self-reduction mechanism and valence regulation of high valence Mn to Mn²⁺.In view of this,the self-reduction characteristics,valence regulation and luminescence of Mn in silicates have been studied in detail in this dissertation.The dissertation consists of five main parts.The first part is a review on the luminescent characters of Mn²⁺and Mn⁴⁺and the relevant applications,the topics,methods about the valence control and self-reduction of Mn,and the features of silicates applied as hosts in inorganic luminescence materials.The research significance,contents of research and project background were also included.In the second part,the sample synthesis and characterization,the raw materials,instruments,synthesis,characterization and theory calculation method were introduced.The experimental results were presented in detail in the next two parts.In third part,the self-reduction processes and mechanism of Mn in Na₂ZnSiO₄:Mn were carefully investigated.Then,the process and ability of self-reduction of Mn in Li₂ZnSiO₄:Mn system were discussed in the fourth part.In addition,the tunable luminescence in Li₂ZnSiO₄:Mn was realized by controlling doping concentration and temperature,then the temperature sensing of the materials was studied as well.Finally,the achieved results were concluded in the fifth part.In summary,the self-reduction of high valence Mn to Mn²⁺was realized in some selected silicates,and the reduction conditions and mechanism were studied in the dissertation.Moreover,the luminescence behavior of Mn²⁺was also tuned by varying the doping concentration,and the main research results of this dissertation are as follows:?1?A series of Na₂ZnSiO₄:Mn²⁺phosphors were synthesized by a high-temperature solid-state reaction method under air atmosphere by using different Mn-sources.The self-reduction phenomenon of Mn⁷⁺/Mn⁴⁺/Mn³⁺to Mn²⁺has been confirmed in this system.Based on the measurements and analysis on crystal structure,the redox reactions of Mn-sources and defects,the self-reduction process of Mn was discussed,then a zinc vacancy(V'_Zn)induced self-reduction mechanism was provided.Besides to the photoluminescence,the long persistent phosphorescence?LPL?has also been investigated,and a possible relevant mechanism was proposed based on the LPL spectra and thermoluminescence curves.?2?A series of Na₂ZnSiO₄:Mn²⁺phosphors were synthesized by a high-temperature solid-state reaction method under air atmosphere by using different Mn-sources.The self-reduction phenomenon of Mn⁷⁺/Mn⁴⁺/Mn³⁺to Mn²⁺can be realized in this system.Then,the reduction process and ability of Mn have been analysed according to the various valences and spectroscopy characterizations.By tuning the doping concentration,single to dual luminescence from isolated Mn²⁺and Mn²⁺-Mn²⁺pair can be realized in continuous solid solution samples Li₂Zn_1-xMn_xSiO₄?x=0.01-1.0?.The formation mechanism of the Mn²⁺-Mn²⁺pair and luminescence tuning of the isolated Mn²⁺and Mn²⁺-Mn²⁺have been carefully investigated.The unique dual-band emitting phosphor luminescence have good stability and repeatability during a certain temperature and cycling number range,suggesting high potential for tenpreture sensing application.