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Missing Member in the M^II M^III Si₄ N₇ Compound Class: Carbothermal Reduction and Nitridation Synthesis Reveal Substitution of Nitrogen by Carbon and Oxygen in CaLu[Si₄ N_7-2x C_x O_x ]:Eu²⁺ /Ce³⁺ (x≈0.3).

ABSTRACT: The nitridosilicate CaLu[Si₄ N_7-2x C_x O_x ] (x≈0.3) was synthesized by carbothermal reduction and nitridation starting from CaH₂ , Lu₂ O₃ , graphite and amorphous Si₃ N₄ at 1550 °C in a radiofrequency furnace. CaLu[Si₄ N_7-2x C_x O_x ] (x≈0.3) crystallizes isotypically to many previously known M^II M^III Si₄ N₇ compounds in the space group P6₃ mc, as was confirmed by Rietveld refinement based on powder X-ray diffraction data. Incorporation of carbon into the crystal structure as a result of the carbothermal synthesis route was confirmed by ¹³ C and ²⁹ Si MAS NMR spectroscopy. For the first time in the M^II M^III Si₄ N₇ compound class, complementary EDX measurements suggest that simultaneous incorporation of oxygen compensates for the negative charge excess induced by carbon, resulting in an adjusted sum formula, CaLu[Si₄ N_7-2x C_x O_x ] (x≈0.3). When excited with UV-to-blue light, CaLu[Si₄ N_7-2x C_x O_x ] (x≈0.3) shows an emission maximum in the blue spectral region (λ_em =484 nm; fwhm=4531 cm^-1 ) upon doping with Ce³⁺ , whereas Eu²⁺ -doped CaLu[Si₄ N_7-2x C_x O_x ] (x≈0.3) exhibits a yellow-green emission (λ_em =546 nm; fwhm=3999 cm^-1 ).

SUBMITTER: Gamperl L

PROVIDER: S-EPMC9300086 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:We have investigated the incorporation of the luminescent Eu3+ cation in different LnPO4 (Ln = Tb, Gd1-xLu x , x = 0.3, 0.5, 0.7, 1) host phases. All samples were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, and site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS) directly after synthesis and after an aging time of one year at ambient conditions. The PXRD investigations demonstrate the formation of a TbPO4 phase in an uncommon anhydrite-like crystal structure evoked by a pressure-induced preparation step (grinding). In the Gd1-xLu x PO4 solid solution series, several different crystal structures are observed depending on the composition. The TRLFS emission spectra of LuPO4, Gd0.3Lu0.7PO4, and Gd0.5Lu0.5PO4 indicate Eu3+-incorporation within a xenotime-type crystal structure. TRLFS and PXRD investigations of the Gd0.7Lu0.3PO4 composition show the presence of anhydrite, xenotime, and monazite phases, implying that xenotime no longer is the favored crystal structure due to the predominance of the substantially larger Gd3+-cation in this solid phase. Eu3+-incorporation occurs predominantly in the anhydrite-like structure with smaller contributions of Eu3+ incorporated in monazite and xenotime. The electronic levels of the Eu3+-dopant in Gd0.3Lu0.7PO4 and Gd0.5Lu0.5PO4 xenotime hosts are strongly coupled to external lattice vibrations, giving rise to high-energy peaks in the obtained excitation spectra. The coupling becomes stronger after aging to such an extent that direct excitation of Eu3+ in the xenotime structure is strongly suppressed. This phenomenon, however, is only visible for materials where Eu3+ was predominantly incorporated within the xenotime structure. Single crystals of Eu3+-doped LuPO4 show no changes upon aging despite the presence of vibronically coupled excitation peaks in the excitation spectra measured directly after synthesis. Based on this observation, we propose a lattice relaxation process occurring in the powder samples during aging, resulting in Eu3+ migration within the crystal structure and Eu3+ accumulation at grain boundaries or xenotime surface sites.

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Missing Member in the MII MIII Si4 N7 Compound Class: Carbothermal Reduction and Nitridation Synthesis Reveal Substitution of Nitrogen by Carbon and Oxygen in CaLu[Si4 N7-2x Cx Ox ]:Eu2+ /Ce3+ (x≈0.3).

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Missing Member in the M^II M^III Si₄ N₇ Compound Class: Carbothermal Reduction and Nitridation Synthesis Reveal Substitution of Nitrogen by Carbon and Oxygen in CaLu[Si₄ N_7-2x C_x O_x ]:Eu²⁺ /Ce³⁺ (x≈0.3).