A Guide to Brighter Phosphors‐Linking Luminescence Properties to Doping Homogeneity Probed by NMR

Abstract Crystalline powders of Ln3+ doped LaPO4 (Ln=Nd, Gd, Dy, Ho, Er, Tm, Yb) have been synthesized to serve in a case study for linking doping homogeneity as determined by NMR to luminescent properties. Samples obtained via different synthesis methods act as examples of homo‐ and inhomogeneous doping. The sample quality was verified by X‐ray diffraction. The homogeneously doped samples show improved luminescent properties in terms of brightness and lifetime which is consistent with the interpretation that, NMR visibility curves probe the distribution of paramagnetic dopants on a similar length scale as necessary for an efficient energy transfer in crystalline phosphors i. e. between sensitizers and activators, and to killer sites. Thus “NMR homogeneity” as observed by visibility curves may serve as a tool to optimize luminescent materials.


Lattice parameters
Based on the Rietveld refinement results of the La1-xNdxPO4 series which were obtained by coprecipitation (Fig. S1), all the lattice parameters show linear correlation with doping level x.
The Rietveld refinements have been performed also on the La1-xDyxPO4 (Fig. S2), La1-xHoxPO4 ( Fig. S3), and La1-xYbxPO4 (Fig. S4) sample series, which were obtained by the co-precipitation method. The refinement results all show linear correlation with x. Note that the synthesized doping x ranges were different for different series, and at small doping level x, refinement data scattering is larger than at higher x. Figure S2. Lattice parameters as a function of the substitution degree x in the co-precipitated and 1000 °C sintered La1-xDyxPO4, as determined by Rietveld refinement based on X-ray powder diffraction data. The dotted lines represent linear fits resulting in a/Å = 6.5154 -0.2094·x, b/Å = 7.0780 -0.2890·x and c/Å = 8.2938 -0.3525·x, respectively. -5-

XRD diffractograms
For the 1000°C sintered solid state samples, at high doping concentration (x ≥ 0.2), phase separation of LnPO4 and LaPO4 becomes evident from diffractograms. Nd doped LaPO4 samples aimed at x = 0.5 were shown as examples. The XRD pattern of the solid state sample ( Fig. S5 left) shown phase separation of LaPO4 and NdPO4. No phase separation was observed for corresponding co-precipitated sample (Fig. S5 right).

NMR visibility curves
To establish the scale on which homogeneity is studied by NMR, the 31 P MAS NMR spectra were first obtained for the doped sample series La1-xLnxPO4 (Ln = Nd, Gd, Ho, Er, Tm, Yb).
Only one signal was observed (Fig. S6) and as the doping level x increases, the peak area gradually decreases as shown in the 31 P MAS NMR spectra stack plots ( Fig. S7-S12). Such data sets of the peak area of the homogeneous compounds La1-xLnxPO4 (Ln = Nd, Gd, Ho, Er, Tm, Yb) were fitted with the visibility function 2 f(x) = exp(-ar0 3 x) for the wipe-out radii r0 of blind spheres. The f(x) plots were shown next to the corresponding NMR stack plots in the Fig. S7-S12. NMR data from samples which were obtained from the solid state method were also compared along ( Fig. S7-S12), and the deviation from the NMR visibility function indicates heterogeneity. For all mentioned Ln 3+ dopants, homogeneously and heterogeneously doped samples can be distinguished. Therefore such method based on the -6-NMR visibility function serves as a nice tool for the evaluation of "NMR homogeneity". Based on the radii r0 of the blind spheres, 3 it may be concluded that "NMR homogeneity" relies on a length scale of about 1 nm, and the co-precipitated samples (annealed at 1000 °C) are more homogeneously doped on nm scale. Figure S6. The 31 P MAS NMR full spectrum (spectrum width 100 kHz) of La0.995Dy0.005PO4 obtained by coprecipitation method, which is shown as one example to demonstrate typical spectra of Ln doped LaPO4 series.
Only one P signal with its spinning side band were observed.

Lifetime of La1-xDyxPO4 doping series
The fluorescence lifetime of the La1-xDyxPO4 doping series has been obtained from the multiexponential (equation Fehler! Verweisquelle konnte nicht gefunden werden. and 2) fitting of the intensity decay curve (Fig. S13-S19). Ioffset was due to the dark counts and the corresponding parameter offset = offset / 0 is also fitted (Table S1 and S2). I0 is the initial intensity, which was set to be the intensity of the first data point in the decay curve. 1 , 2 and 3 are the lifetime and 1 , 2 and 3 are the fitted weight fractions, respectively. In particular, 1 is the mono-exponentiality factor.
The results shown in Tables S1 and S2 are the results of a single unconstrained non-linear least square fit of all slifetime curves with a self-written tcl-script including an error analysis based on a variance analysis. The fit model used different a1, a2, a3 and Ioffset / I0 parameters for the different lifetime curves but assumed the lifetime values 1 , 2 and 3 to be the same for both sample series. The fit converged consistently to the same minimal value independent of small changes in the starting values. -10- The values in Table S1 and S2 were used to produce Figure 6 in the manuscript. Table S1. The fitting parameters including weight fractions a1, a2, a3 and intensity offset coffset for the lifetime measurements of samples obtained from co-precipitation method. The lifetime values are 1 = 1.099 ± 0.003 ms, 2 = 0.583 ± 0.007 ms, 3      The measurements were recorded at emission wavelength λem = 477 nm and excitation wavelength λex = 350 nm.