Date/Time
Date(s) - 01/12/2022
1:30 pm - 2:30 pm

Title: Synthesis, Structure and LuminescentProperties of New Germanate Materials

Date: Thursday,December 1, 2022

Time: 1:30-2:30

 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: Mg₃Ge_1-dO_4(1-d)F_2(1+2d) (d? 0.1), Mg₁₄Ge₄O₂₀F₄,Mg₂Pb₂Ge₂O₇F₂, Sr₃GeO₄Cl₂,Ba₃GeO₄Br₂, Sr₆Ge₂O₇Cl₆,Ba₅GeO₄Br₆, Na₂BaGe₈O₁₈,Rb₂BaGe₈O₁₈, Na_0.36Sr_0.82Ge₄O_9,Na₂SrGe₆O_14, and K₂SrGe₈O₁₈.Two phases, Mg₃Ge_1-dO_4(1-d)F_2(1+2d) and Na_0.36Sr_0.82Ge₄O₉demonstrate deficiency on certain crystallographic sites. Despite thesimilar stoichiometry, the connectivity of GeO_n polyhedra isdifferent in R^I₂Ge₄O₉, R^IIGe₄O₉and R^IR^IIGe₈O₁₈germanates (R^I = alkali, R^II = 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.