PhD Seminar, Sergei Novikov, Thursday, December 1, 1:30 - ABB 165
Title: Synthesis, Structure and LuminescentProperties of New Germanate Materials
Date: Thursday,December 1, 2022
Room: ABB 165
Host: Yurij Mozharivskyj
Abstract: In this work aseries of new germanates was prepared and their structures were characterizedwith X-ray diffraction (XRD). We employed solid-state synthesis, flux growth,and crystallization from melts to obtain crystals of the new materials. Thecrystals were studied by means of single crystal XRD. New germanates suitableto accommodate Mn4+ – a well-known activatorion for the preparation of the rare-earth-free red phosphors – wereof a particular interest. The Ge4+ substitution for Mn4+is possible if the crystal structure features octahedrally coordinatedgermanium atoms, and we indeed were able to prepare such germanates.
The crystal structures of thefollowing phases were characterized: Mg3Ge1-dO4(1-d)F2(1+2d) (d≈ 0.1), Mg14Ge4O20F4,Mg2Pb2Ge2O7F2, Sr3GeO4Cl2,Ba3GeO4Br2, Sr6Ge2O7Cl6,Ba5GeO4Br6, Na2BaGe8O18,Rb2BaGe8O18, Na0.36Sr0.82Ge4O9,Na2SrGe6O14, and K2SrGe8O18.Two phases, Mg3Ge1-dO4(1-d)F2(1+2d) and Na0.36Sr0.82Ge4O9demonstrate deficiency on certain crystallographic sites. Despite thesimilar stoichiometry, the connectivity of GeOn polyhedra isdifferent in RI2Ge4O9, RIIGe4O9and RIRIIGe8O18germanates (RI = alkali, RII = alkalineearth metal).
The Ge4+ substitutionfor Mn4+ was successfully done for the Rb2BaGe8O18,Na2SrGe6O14, and K2SrGe8O18phases yielding new red phosphors. The optimal synthetic strategies weredeveloped yielding high purity (≈ 99 wt. %) samples. The photoluminescentexcitation and emission spectra were collected for the new phosphors. Strongabsorption of the UV light and emission in the far-red region of the visiblespectra were confirmed. Temperature-dependent luminescence spectra werecollected for the Na2SrGe6O14 : Mn4+and K2SrGe8O18: Mn4+,and the K2SrGe8O18: Mn4+ showedhigher resistance to temperature quenching.