RE-doped CeO 2 pellets

4.1.1 Structural and morphological properties of CeO² pellets

The preparation of CeO2(:RE) PLD targets was carried out by solid state reaction of a stoichiometric mixture of the elemental oxide powders. The indicated concentrations are the nominal concentrations because the measurements of the concentration of elements that are so close in the periodic table is quite difficult by common techniques like RBS and EDX.

Figure 4.1 reports the XRD patterns of the pellets after sintering at 1400 °C and that of the CeO2

powder.

All peaks can be attributed to the cubic CeO2 structure. No spurious phases or dopant oxide peaks are observed, although the doped pellets are obtained by mixing CeO2 with the RE oxide. This is probably due to the low RE concentration.

Information about the crystal quality of the pellets can be obtained by calculating the crystallites size and lattice parameter. The values of the peak positions corrected by the zero shift caused by the different thickness of the pellets were obtained by least square method using all the peaks in the spectrum. The lattice parameter of the pellets calculated using Bragg’s law is 5.414(2) Å independently of the doping type and level. The values of the crystallites size calculated using Scherrer’s formula on the (111) peak are reported in Figure 4.2.

Due to the sintering process, the crystallites of the pellets are much larger than those in the powder. In particular, the crystallites in the undoped pellet seem to be slightly larger than those in the doped pellets.

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Figure 4.1 – XRD patterns of the CeO2 powder and of the pellets (doped and undoped) after sintering at 1400 °C.

Figure 4.2 – Calculated values of the crystallites size of CeO2 pellets after sintering at 1400 °C. The crystallite size of the powder is reported for reference.

Due to annealing at such a high temperature, some grains are expected to coalesce. An idea of the morphology of the target can be obtained by SEM (see Figure 4.3). Most pellets appear as a compact aggregate of particles with an average size of about 1 µm. Only the Yb-doped pellet appears more compact than the others. The different aspect of the Yb-doped pellet could be due to the lower melting point of Yb (824 °C against 935 °C for Pr and 1010 °C for Nd) that favors the diffusion. For comparison, the melting point of Ce is 795 °C. Unfortunately, due to the very similar atomic number between Ce and the other rare earths, it is not possible to determine if some RE oxide grains are still present after sintering.

After ablation, the appearance of flakes indicates that the surface of the pellet has melted under the highly energetic laser pulses.

Figure 4.3 – SEM images of several CeO2:RE pellets. The image of a typical pellet surface after ablation is also reported (in this case with 3 % of Nd).

4.1.2 Optical properties of CeO² pellets

The first optical observation on the pellets can be made by the naked eye. Contrarily to the snowy white CeO2 powder, after sintering all pellets acquire an intense copper-like color. This color is the characteristic color of Ce2O3 and is due to the small band gap (2.4 eV) of this oxide. However, no trace of Ce2O3 has been observed in the XRD spectra. Only the measurement of the diffuse reflectance will allow discriminating if this color is due to the presence of Ce³⁺ or to the emission from some impurities or intrinsic defects in CeO2. Figure 4.4 reports the absorbance curves calculated from the diffuse reflectance of the pellets and of the CeO2 powder. The absorbance of the powder is not sharp as expected from a direct band gap material and it extends below 3.6 eV. The high amount of surface in the powder and the associated high number of internal reflections might explain this behavior. For the pellets, the absorbance band extends well below the typical absorption edge of CeO2. In particular, three wide absorbance features are observed for wavelengths below 600 nm. This justifies the reddish color of the pellets. Besides the CeO2 band gap absorption, two additional absorbance bands can be distinguished at about 520 nm (2.4 eV) and 380 nm (3.26 eV). A closer look to the curves indicates that these new bands are already present in the powder, but very weak. Since 2.4 eV is the band gap value of Ce2O3 reported by Prokofiev et al. [201], this band could be related to this oxide. This means that some

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oxygen is lost during sintering even if the process occurs in air. However, if the sesquioxide is really present in the pellets, the absence of the relative XRD peaks must be explained. One possibility is that the amount of Ce2O3 is very small. The reason why a strong absorption band can be created despite the small amount of this oxide could be that CeO2 is transparent to visible light. The increased penetration depth in the pellet increases the amount of Ce2O3 material “seen” by this light.

As for the band centered at 380 nm (3.26 eV), it matches very well with the f-d transitions of isolated Ce³⁺ ions in CeO2, as shown by Askrabic et al. [214].

Figure 4.4 also shows that strong absorption bands related to 4f transitions of RE³⁺ ions are observed in the case of Nd- and Yb-doped pellets. The absence of such absorption bands in the Pr-doped pellet indicates that this rare earth must be in its 4+ state, favored by the presence of Ce⁴⁺.

Figure 4.4 - Absorbance spectra calculated from the diffuse reflectance of the pellets and of the CeO2 powder. The transitions excited state relative to the RE³⁺ absorption bands are indicated for Nd and Yb.

PL and PLE spectra have been recorded in order to see if the dopant is already optically active in the pellets. Figure 4.5a-b show the PL emission spectra of the CeO2 pellets under 325 nm laser excitation.

Several strong emission bands can be observed, composed of a large number of narrow peaks. Since none of these peaks can be attributed to Ce ions or to CeO2, indirect excitation of the dopants by energy transfer from the host must occur. The two emission bands in the infrared range can be easily attributed to Nd³⁺ and Yb³⁺, but the emission band in the visible region cannot be attributed to these rare earths.

This band is attributed to Sm³⁺ [259], which must exist in the CeO2 powder as impurity. Nd³⁺ must also be present as impurity in the powder, as its signal is visible in all spectra. Given the high purity of the powder (99.95 %), their concentration must be very small (< 0.05 %). Still, intense PL signals are recorded, indicating that almost all ions must be optically active and that the energy transfer from CeO2

is very efficient.

The fact that the peaks are so narrow indicates a small distortion of the lattice sites and that the phonon broadening is weak. The large number of peaks indicates the existence of several active sites.

For instance, the greatest part of the Nd³⁺ ions could still be in Nd2O3 particles and only a small fraction has migrated in CeO2.

More insight in the transfer mechanisms can be obtained by the analysis of the PLE spectra of the RE³⁺

emission. Figure 4.6 reports the PLE spectra of the strongest emission lines of Yb³⁺ and Nd³⁺. In the spectrum of Nd-doped CeO2, several absorption bands are observed.

a) b)

Figure 4.5 - PL emission spectra of the doped and undoped CeO2 pellets under 325 nm laser excitation in a) linear and b) logarithmic scales.

The PLE bands around 600 nm and 800 nm correspond to direct excitation of Nd³⁺ ions, while the intense peak at 380 nm is attributed to indirect excitation of the host. Curiously, indirect excitation only occurs by exciting the absorption feature observed at 3.26 eV, which had been ascribed to isolated Ce³⁺

ions. This suggests that Nd³⁺ and Yb³⁺ ions are situated close to isolated Ce³⁺ ions, from which efficient transfer occurs. An oxygen vacancy created to balance the charge when trivalent Nd and Yb ions replace Ce⁴⁺ ions might explain the presence of Ce³⁺ in the vicinity of the RE.

Figure 4.6 - PLE spectra of the main emission line of Yb³⁺ and Nd³⁺. The absorbance spectra of the pellets and the powder are reported for reference.

Concluding remarks

The structural analysis on the pellets showed that the rare earths do not modify the typical structure of CeO2. They have, however, a small effect on the crystallites size.

The different color observed for the pellets after sintering is due to the appearance of two absorption bands below the band gap of CeO2. One of these bands, centered at 3.26 eV, revealed to be the band from which the energy transfer to the rare earth impurities occurs. This band has been attributed to isolated Ce³⁺ ions in CeO2. The other band is probably related to Ce2O3.

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Strong absorption bands have been also observed arising from direct absorption of the trivalent dopant ions.

Since the pellets proved to be of good quality and the presence of the dopants has been confirmed, they have been used as targets for PLD from which RE-doped CeO2 films have been deposited. The structural and optical properties of these films will be the topic of the next sections.