**4. Results and discussion**

7 the regions of localized crystallinity.

calcined temperature (~1200<sup>o</sup>

**A. Structural studies through transmission electron microscopy (TEM) and X-ray diffraction (XRD):** Typical data are shown for Er2O3:SiO2 NPs-glass composite system having 0.5 mol% dopant Er2O3 concentration calcined at different temperatures, namely, 700, 800, 900, and 1200°C (henceforth referred as Er05-7, Er05-8, Er05-9 and Er05-12, respectively) [20]. Here, we discuss the TEM image of the powder specimen of typical Er05-8 (Figure 2(a)), showing nearly spherical NPs of Er2O3 embedded in the glass matrix in the range of 3–6 nm. The particle size distributions histogram (upper inset of Figure 2(a)) of the sample is calculated by counting onto the the micrograph at least 100 particles from Figure 2. The high-resolution TEM (HRTEM) image (lower inset of Figure 2(a)) of the respective sample also shows lattice fringes with interplanar spacing (3.04 Å). The selected area electron diffraction (SAED) pattern (Figure 2(b)) also shows spotted ring patterns, developing the regions of localized crystallinity. Colossal dielectric and MD response of RE2O3 nanoparticles in SiO2 glass matrix 5 1 TEM image of the powder specimen of typical Er05-8 (Figure 2(a)), showing nearly spherical NPs of Er2O3 2 embedded in the glass matrix in the range of 3–6 nm. The particle size distributions histogram 3 (upper inset of Figure 2(a)) of the sample is calculated by counting onto the the micrograph at least 4 100 particles from Figure 2. The high-resolution TEM (HRTEM) image (lower inset of Figure 2(a)) 5 of the respective sample also shows lattice fringes with interplanar spacing (3.04 Å). The selected 6 area electron diffraction (SAED) pattern (Figure 2(b)) also shows spotted ring patterns, developing

34 Figure 2. (Color online) (a) TEM image of Er05-8, upper inset: the particle size distribution histogram 35 and lower inset: the HRTEM image, (b) electron diffraction and (c) XRD patterns of all the Er05-7, 36 Er05-8 and Er05-12 samples. **Figure 2.** (Color online) (a) TEM image of Er05-8, upper inset: the particle size distribution histogram and lower inset: the HRTEM image, (b) electron diffraction and (c) XRD patterns of all the Er05-7, Er05-8 and Er05-12 samples.

~ 29.30° (222)

The XRD patterns of the Er2O3 oxide NPs doped SiO2 matrix calcined at 1200o 37 C (Er05-12) show crystalline nature with quite large Er2O3 38 NPs. It exhibits clearly in Figure 2(c) the most intense

40 corresponding the unit cell parameter *a* = 10.54 Å (space group: *Ia3*(206)). The sizes of Er-oxide NPs 41 in Er05-12 are also estimated (> 40 nm) by applying the well-known Scherrer's equation from the 42 integral breadths of the XRD lines. XRD patterns cannot be well resolved from feeble broad peaks 43 due to their amorphous-like character of Er05-7 and Er05-8 samples. However, it is significantly mentioned here that the sizes of Er2O3 44 NPs grow larger for samples calcined at higher temperatures. In the SiO2 glass matrix, low concentrations of Er3+ 45 ions are bound to non-bridging oxygen atoms with quite low solubility [21]. Consequently, at a higher Er2O3 concentration with SiO2 46 (1:2), the formation of an Er2Si2O7 mixture with Er2O3 may occur at 1200o 47 C [22]. However, at a higher

C and above), there is a possibility of the formation of Er2Si2O7 48 mixture with crystalline Er2O3 49 even with very low concentration (0.5 mol %) of dopant ions. Herein, the

characteristic line of single phase Er2O3 39 (JCPDF Card No. 43-1007) at 2

The XRD patterns of the Er2O3 oxide NPs doped SiO2 matrix calcined at 1200o C (Er05-12) show crystalline nature with quite large Er2O3 NPs. It exhibits clearly in Figure 2(c) the most intense characteristic line of single phase Er2O3 (JCPDF Card No. 43-1007) at 2*θ*~ 29.30° (222) corre‐ sponding the unit cell parameter *a* = 10.54 Å (space group: *Ia3*(206)). The sizes of Er-oxide NPs in Er05-12 are also estimated (> 40 nm) by applying the well-known Scherrer's equation from the integral breadths of the XRD lines. XRD patterns cannot be well resolved from feeble broad peaks due to their amorphous-like character of Er05-7 and Er05-8 samples. However, it is significantly mentioned here that the sizes of Er2O3 NPs grow larger for samples calcined at higher temperatures. In the SiO2 glass matrix, low concentrations of Er3+ ions are bound to nonbridging oxygen atoms with quite low solubility [21]. Consequently, at a higher Er2O3 concentration with SiO2 (1:2), the formation of an Er2Si2O7 mixture with Er2O3 may occur at 1200o C [22]. However, at a higher calcined temperature (~1200o C and above), there is a possibility of the formation of Er2Si2O7 mixture with crystalline Er2O3 even with very low concentration (0.5 mol %) of dopant ions. Herein, the sample obtained at 1200o C are not emphasized because of the observed dielectric behavior (discussed in the next section) is almost comparable with pure bulk Er2O3 (unsupported with SiO2).
