Modeling Luminescent Properties of HfO₂:Eu Powders with Li, Ta, Nb, and V Codopants

Abstract

Monoclinic HfO 2 :Eu,M (M = Li, Ta, Nb, V) powders were prepared with the Pechini method. The M-dopants were added for charge compensation of Eu 3+ positioned in Hf (IV) site. This was hoped to enhance the host-to-activator energy transfer and consequently the overall efficiency of X-ray excited luminescence. Only in the case of codoping with Nb, the final phosphor did not contained any foreign phases indicating that (Eu 3+ ,Nb (V) ) pairs easily dissolved in the host. In the HfO 2 :Eu,V composition, the phase purity was unsatisfying for higher concentrations. However, in the case of HfO 2 :Eu,Ta and HfO 2 :Eu,Li a foreign phase/phases could be easily detected even for quite low concentrations. The addition of Ta, Nb, or V allowed to create additional absorption/excitation bands located in UV around 225− 250 nm, 240−300 nm, and 240−320 nm, respectively. However, only in the presence of Nb the red Eu 3+ photoluminescence could be efficiently excited and strongly enhanced upon stimulation into this extrinsic excitation structure compared to other investigated compositions as well as to singly doped HfO 2 :Eu reported previously. While XRDs showed that HfO 2 :Eu,Li contained the highest fraction of a foreign phase, this composition was found to produce the most efficient red emission upon irradiation with X-rays. It reached about 30% of the commercial Gd 2 O 2 S:Eu powder performance. This was achieved despite that a significant portion (roughly additional 30−35%) of the HfO 2 :Eu,Li radioluminescence was contained in a broad band covering the 400−600 nm range of wavelengths. Unfortunately, all the other codopants, Ta, Nb, and V, only suppressed the X-ray excited emission yield. Especially surprising was that the phase pure HfO 2 Eu,Nb powders, that presented the most efficient, truly bright photoluminescence, performed the least upon X-rays. ■ INTRODUCTIONWith one of the highest densities among all compounds (∼9.7 g/cm 3 ), high effective atomic number (Z eff = 67.2), and transparency over the whole visible and almost all UV part of electromagnetic spectru,m monoclinic hafnia, HfO 2 , is an extremely attractive host for modern X-ray phosphors and scintillators. 1,2 It ensures an efficient absorption of X-rays and gamma particles and warrants a high photofraction (an appreciated property in many applications), an ability to absorb a whole energy of the incoming ionizing particle in a single position rather instead of multiple distant locations (due to Compton scattering). 3,4 The literature on hafnia-based X-ray phosphors and scintillators is still rather scarce. Yet, the published results on spectroscopic properties of Eu-activated monoclinic HfO 2 , including excitation with synchrotron radiation over its energy gap, showed that the host was able to transfer the acquired energy to Eu 3+ ions. Hence, the activator could get excited through host simulation, and the characteristic red luminescence of Eu 3+ could be then produced. 5−10 The early reports on the performance of HfO 2 :Eu under stimulation with X-rays we...

Charge Compensation in Europium-Doped Hafnia Nanoparticles: Solvothermal Synthesis and Colloidal Dispersion

“…The origin of these structural modifications are attributed to the charge mismatch at the doping lattice site, and the consequent creation of defects by the substitutional doping. One proposed approach to avoid the effect of this defective dopant implies the simultaneous doping with elements with higher oxidation state than hafnium or zirconium, in order to compensate for the charge mismatch induced by the substitution of Hf 4+ or Zr 4+ by only trivalent RE 3+ ions [2,7,9,11,12]. This strategy was successfully applied to hafnia and zirconia by using Nb 5+ or Ta 5+ as co-dopants and a significant improvement of the Eu 3+ related emission was reported with increase ranging from 5 to 50 times [2,7,12].…”

Luminescence emission features of Nd 3+ ions in PbO–Sb 2 O 3 glasses mixed with Sc 2 O 3 /Y 2 O 3 /HfO 2

“…The phonon losses of the host glass are also reported to have been reduced due to the addition of HfO 2 [22]. In some other studies on HfO 2 mixed glasses, it was found that these glasses are efficient absorbers of the incident gamma/X-rays [23,24]. In view of such special characteristics, HfO 2 containing glasses are being used as optical coatings for astronomical charge coupled, devices, IR optical devices, antireflective multi-layer coatings for night vision devices and for the applications in the inorganic scintillators [25e27].…”

Phase Control in Hafnia: New Synthesis Approach and Convergence of Average and Local Structure Properties

“…10 Though much less studied than the ZrO 2 homologue, several reports evidenced remarkable photoluminescence (PL) and scintillator properties of monoclinic and tetragonal/cubic phases of HfO 2 when doped with lanthanide (Ln) ions such as Ce, Sm, Eu, Tb, and Dy. 5−7,12−15 As Zr and Hf are regarded as the two chemically most similar homogenesis elements, 16 HfO 2 and ZrO 2 are called twin oxides 17,18 because of their similar crystal structure, with close ionic radii of 0.78 Å (Hf 4+ ) and 0.79 Å (Zr 4+ ), 19 despite their large difference in the atomic number, Z, 72 to 40, respectively. In the bulk, hafnia and zirconia can each adopt three different, albeit related, crystal structures at ambient pressures, that is, monoclinic, tetragonal, and cubic.…”