Luminescence of rare-earth ions and intrinsic defects in Gd2O3 matrix

The X-ray luminescence and photoluminescence of the ultradispersed Gd2O3 powders with different purity were investigated in 90-360 K temperature range. Both impurity and intrinsic optical active centers were detected. The effect of energy transfer from Gd3+ to RE3+ ions was observed.

monochromators. Low-temperature measurements were carried out by using vacuum cryostat (P = 10 -4 Pa). The temperature of the sample was controlled by platinum resistance thermometer HRTS-5760-B-T-1-12 with accuracy of 1% and changed by the external heater inserted into a sample holder.

Results
In nominally pure Gd2O3 powders under X-ray excitation at room temperature a number of narrow lines of luminescence in blue (489 nm), green (550 nm), yellow (572-594 nm) and red (612-628 nm) regions of the spectrum has been observed (figure 1). Based on the Dieke diagram these emission bands were attributed to the intraconfiguration f-f transitions of Pr 3+ , Tb 3+ , Er 3+ and Eu 3+ ions existing in matrix as micro impurities. It should be noted that in the high-purity sample (GdО-G) quantum yield of impurity RE ions luminescence considerable decreases while 6 РJ → 8 S7/2 radiative transition of Gd 3+ ions with 314 nm characteristic line is firstly detected. In low-temperature XRL spectrum of the GdO-G sample the narrow emission lines of RE 3+ ions also appear but the most intense luminescence corresponds to the broad band centered at 430 nm with a full-width-half-maximum of about 130 nm (figure 2). The temperature dependence of the intensity of this band in the range 110-350 K is plotted in figure 3. The noticeable luminescence quenching starts at the 125 K approximately. At first, we tried to describe the obtained experimental data by the classic Mott's law that is valid for a single activation model. However, such approximation is satisfactory only at low temperatures up to 200 K (red line in figure 3). Therefore we have used the modified Mott's law expression that accounts for the realization of two-activated processes with different activation energies [5][6][7]: and are the preexponent factors reflected the non-radiative over radiative probability ratios; and are the activation energies, which characterize the height of the berries of non-radiative relaxation channels. Using the equation 1 allows adequately describing the ( ) dependence over the whole investigated temperature range (blue line in figure 3). The parameters of fit are listed in table 1.   (1)). The PL spectra recorded at room temperature under excitation corresponding to interband Gd2O3 transitions (5.6 eV) demonstrated the lines at the same wavelengths as in XRL spectra (not shown here). The PL excitation spectra of Pr 3+ , Er 3+ and Eu 3+ ions are represented in figure 4. It was found that f-f emission of RE 3+ can be efficiently excited by different ways: (i)under intracenter excitation; (ii)at energies corresponding to 6 PJ, 6 IJ, 6 DJ excited states of Gd 3+ ions; (iii)at energies more than 5.6 eV in the region of Gd2O3 fundamental absorption edge [8].

Discussion
The obtained results clearly indicate that, firstly, there is efficient energy transfer by band charge carriers to impurity centers as evidenced by high X-ray luminescence output ( figure 1). Secondly, the presence in excitation spectra the intense Gd 3+ transition lines (figure 4) points out to non-radiative energy transfer from gadolinium to Pr 3+ , Er 3+ and Eu 3+ ions. The reduction of impurity ions concentration leads to decreasing of Gd 3+ → RE 3+ energy transfer efficiency and simultaneous increasing the probability of radiative relaxation in Gd 3+ ion (see figure  1). This fact is confirmed by the appearance of 6 РJ → 8 S7/2 emission band in the XRL spectrum of greater purity sample (GdO-G) against the background of dramatically decreasing of luminescence intensity related to impurity RE 3+ ions. It denotes that the radiative 6 РJ → 8 S7/2 transition of Gd 3+ and energy transfer to impurity RE 3+ ions are competing processes.
The presence of local energy levels of Gd 3+ ions in the region of Gd2O3 optical transparency indicates the realization of the self-activation effect. In other words, matrix Gd 3+ ions behave as extrinsic activators that act as energy donors. In different host lattices the nature of the self-activation centers has been related to the point distortions of lattice caused by deviations from stoichiometry [9,10]. In our case, the most possible intrinsic defects involved in absorption and luminescence processes are either interstitial Gd 3+ ions or Gd 3+ at normal lattice sites associated with other nearest point defects. In any instances the local environment of the "defective" Gd 3+ ions is distorted by internal crystal field. It results in partly weakening of the parity selection rule for f-f transitions of Gd 3+ as confirmed by the appearance of corresponding bands in XRL and PL excitation spectra [11].
Low-temperature band at 430 nm manifested in XRL spectra of the high-purity Gd2O3 (figure 2), obviously, cannot be related to transition of any RE ions. The trivalent lanthanides are characterized by narrow emission lines originating from 4f intraconfiguration transitions while the divalent rareearth ions (for example, Eu 2+ ) show broad 5d-4f luminescence but in another spectrum region [12,13]. It is most likely that the nature of this band is associated with intrinsic defects of Gd2O3 lattice acting as optically active centers. In cognate La2O3 matrix the broad emission peak in the deep blue region has been attributed to single ionized oxygen vacancy [14]. Therefore we suggest that low-temperature luminescence at 430 nm also can be related with the presence of intrinsic lattice defects like F-centers.
The analysis of the temperature quenching curve of luminescence at 430 nm (figure 3) revealed that there are at least two non-radiative relaxation channels. The first activation process is realized at low temperatures (up to 200 K) and characterizes by energy barrier value of 0.141 eV. The low activation energy along with a small value of probability (table 1) indicates that the most likely this quenching process is intracenter. At the same time, at higher temperatures the another quenching channel with significantly higher barrier (1.033 eV) is dominant. For this case the non-radiative relaxation can also occurs within the center provided the complexity of configuration curves. However, it is possible that the second channel is due to the external quenching process with the transfer of excitation to other optically active centers. For detail information about luminescence quenching mechanisms the additional spectroscopic investigations are required.

Conclusion
The luminescence of impurity RE 3+ ions in Gd2O3 powders has been detected. We have found that so called "defective" Gd 3+ ions act as donors of excitation energy and presumably associated with intrinsic lattice defects. With decrease of impurity concentration the self-activated luminescence of Gd 3+ has been observed. The low-temperature luminescence at 430 nm was assigned to the host lattice defects like F-centers. The analysis of temperature quenching revealed two non-radiative relaxation channels: in temperature region up to 200 K the intracenter quenching process is realized while at higher temperatures the second activation channel with a higher energy barrier prevails.
The obtained results indicate that the processes of energy transport and efficiency of radiative transitions in ultradispersed Gd2O3 powders largely depend on intrinsic defects. The control of impurity composition and lattice disordering allows modifying the optical properties to improve the functional parameters of Gd2O3-based energy converters.