A series of iso-structural La5(Si2+xB1–x)(O13–xNx):Ce3+ phosphors with apatite structure have been prepared. A combination of powder X-ray diffraction and neutron scattering technique was employed to explore the crystal structural evolution and the rigid nature from oxy- to oxynitride-based apatites, and some local structures were also characterized by HRTEM and 29Si NMR data, respectively. The new La5(Si2+xB1–x)(O13–xNx):Ce3+ solid solution phosphors gave continuously controlled emission from 421 nm [La5Si2BO13:Ce3+, end-member (x = 0)] to 463 nm (La5Si3O12N:Ce3+, end-member (x = 1)). Substitution of B3+ and O2– by Si4+ and N3– in La5(Si2+xB1–x)(O13–xNx):Ce3+ phosphors produced more covalency into the crystal field environment around the Ce3+ ions inducing the red-shifted emission, further improving the thermal stability of the oxynitride-based apatite phosphors. The proposed approach from oxy- to oxynitride based iso-structural phases could significantly contribute to future research in designing complex solid solution phosphors.
Bond valence method illustrates the relation between valence and length of a particular bond type. This theory has been used to predict structure information, but the effect is very limited. In this paper, two indexes, i.e., global instability index (GII) and bond strain index (BSI), are adopted as a judgment of a search-match program for prediction. The results show that with GII and BSI combined as judgment, the predicted atom positions are very close to real ones. The mechanism and validity of this searching program are also discussed. The GII & BSI distribution contour map reveals that the predicted function is a reflection of exponential feature of bond valence formula. This combined searching method may be integrated with other structure-determination method, and may be helpful in refining and testifying light atom positions.
The union of structural and spectroscopic modeling can accelerate the discovery and improvement of phosphor materials if guided by an appropriate principle. Herein, we describe the concept of “chemical unit cosubstitution” as one such potential design scheme. We corroborate this strategy experimentally and computationally by applying it to the Ca2(Al1–xMgx)(Al1–xSi1+x)O7:Eu2+ solid solution phosphor. The cosubstitution is shown to be restricted to tetrahedral sites, which enables the tuning of luminescent properties. The emission peaks shift from 513 to 538 nm with a decreasing Stokes shift, which has been simulated by a crystal-field model. The correlation between the 5d crystal-field splitting of Eu2+ ions and the local geometry structure of the substituted sites is also revealed. Moreover, an energy decrease of the electron–phonon coupling effect is explained on the basis of the configurational coordinate model.
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