Ce or pr-doped type iii kgd(po3)4 crystalline materials. Growth and characterization as scintillators
- Autores: Irina Adell Barbarà
- Directores de la Tesis: M. Cinta Pujol Baiges (dir. tes.), Rosa Maria Solé Cartana (codir. tes.), Francesc Díaz i González (codir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Magdalena Aguiló Diaz (presid.), Alexandra Peña Revellez (secret.), María de la O Ramírez Herrero (voc.)
- Programa de doctorado: Programa de Doctorado en Nanociencia, Materiales e Ingeniería Química por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Scintillator materials, or simply scintillators, are materials able to convert a high-energy photon from the X-ray or gamma-ray range into a bunch of photons in the UV-visible range. Alternatively, scintillators can also detect the energy of accelerated charged particles (electrons, protons and heavier ions) or neutrons and convert it into UV-visible radiation. Scintillators are widely used as detectors in the detection systems of a variety of applications, such as medical imaging, astrophysics, non-destructive testing (airport security, industrial control, etc.), high energy physics and homeland security.
The scintillation property is observed in solids, liquids and gases. Among the solids, there are two big families: inorganic and organic scintillators, with inorganic scintillators being the most used in practical applications and the most studied in research. The history of inorganic scintillators and the state of the art taking into account the research works of the last approximately 20 years are described in the first chapter of the thesis. There are several important requirements to evaluate favourably the performance of any scintillator, which are: high light yield, high total absorption coefficient, efficient energy transfer, short emission lifetime, transmission of its own emitted light, no afterglow, radiation hardness, chemical stability, favourable mechanical properties, detection efficiency, temperature stability of the scintillation yield, proportionality of scintillation response, low radioactivity of the constituent elements of scintillator, availability to grow large and high crystalline quality samples, homogeneity or uniform distribution of impurities (activator ions), and low cost. It is worth mentioning that some requirements are more restrictive for some applications than for others, depending on the specific minimum needs of each application. Nevertheless, the ideal scintillator does not exist, so many efforts are dedicated to find new inorganic scintillators with better properties than those of the existing scintillators.
The classification of the inorganic scintillator materials based on the scintillation mechanism divides them into three classes. These classes are activated scintillators, based on crystalline compounds doped with activating ions; self-activated scintillators, where emitting centres are ions, anionic complexes or various excitonic states from the host itself; and cross-luminescent scintillators, which can be crystals with or without impurities.
In this Ph.D. Thesis, we focus on crystals containing lanthanide ions, i.e. lanthanide-based or lanthanide-doped crystals, which belong to the classes of activated and self-activated scintillators. In the final part of Chapter 1, the characteristics of Ce3+ and Pr3+ ions as doping elements for scintillator applications and of type III KGd(PO3)4 (KGdP) as a host have been provided, concluding that type III Ce:KGdP and type III Pr:KGdP are two promising combinations as scintillator materials. To our knowledge, type III Ce:KGdP and Pr:KGdP single crystals have never been reported in the current literature. For this reason, the first aim of this Thesis is to grow type III Ce:KGdP and Pr:KGdP bulk single crystals, with different concentrations of the corresponding doping ion, with high crystalline quality from high temperature solutions by the Top Seeded Solution Growth-Slow Cooling (TSSG-SC) technique. The incongruent melting of the KGdP compound is the main reason for using this high temperature solution technique to obtain the bulk single crystals. The solvent/flux used is an excess of K2O and P2O5, which are components that constitute the final desired crystals (self-flux), in order to avoid the possible introduction of foreign ions of the solvent into the crystalline structure. Besides, the characteristics of type B KYP4O12 and type IV KY(PO3)4 (type B and type IV KYP) as hosts for active lanthanide ions in scintillation applications have also been provided, being interesting candidates. Because of the incongruent melting of KYP, the growth of type B and IV KYP single crystals from solution will be investigated. Thus, the second aim of the Thesis is to determine the primary crystallization region of the type B KYP and type IV KYP phases in the K2O–Y2O3–P2O5 ternary system. They will be also grown from self-flux with an excess of K2O and P2O5. On the other hand, the third aim of this Thesis is to establish and optimize the synthesis conditions of type III Pr:KGdP nanocrystals, with different concentrations of praseodymium, by the modified Pechini method as a starting point to obtain it as a ceramic scintillator.
The next aim of this Ph.D. Thesis is to find out whether the presence of Ce3+ and Pr3+ in the crystalline lattice of KGdP implies significant changes in the physical properties of KGdP when it is doped. The structural characterization of the bulk single crystals previously obtained includes the crystal morphology, chemical composition and changes in the unit cell parameters with doping content of both compositions (Ce:KGdP and Pr:KGdP), as well as the discussion on thermal stability and thermal expansion of type III Pr:KGdP single crystals. The structural characterization of the nanocrystals previously obtained covers the same studies mentioned for the single crystals, but with the additional objective of finding the particle size distribution and the predominant particle size, since this has an effect on the sintering process to obtain the ceramic from the nanocrystals.
The last aim of this Ph.D. Thesis consists in an extensive spectroscopic characterization of type III Ce:KGdP and Pr:KGdP bulk single crystals and type III Pr:KGdP nanocrystals to study in detail the spectroscopy of the 4f–5d transitions, on which the scintillation effect is based. In addition to this, since Pr3+ also is an active ion in laser applications, the basic spectroscopic data of the type III Pr:KGdP crystal for lasing applications in the visible wavelength range based on the 4f–4f electronic transitions is of interest. The spectroscopic studies of these materials include unpolarized and polarized optical absorption at room temperature and at 6 K, luminescence and decay time measurements under synchrotron vacuum ultraviolet-ultraviolet (VUV-UV) excitation, decay time measurements under visible excitation and radioluminescence measurements after synchrotron X-ray irradiation.
Regarding the methodologies and experimental characterization techniques, X-ray powder diffraction (XRPD) technique was used to identify the crystalline phases obtained in the study on the determination of the primary crystallization region of type B and IV KYP, as well as those crystalline phases obtained in the experiments carried out to synthesize type III Pr:KGdP nanocrystals. The crystalline phases obtained were identified using the EVA software and the unit cell parameters refined using the TOPAS program and the Le Bail method. XRPD technique at higher temperatures was used for determining the thermal stability and thermal expansion of the type III Pr:KGdP bulk single crystal. Electron probe microanalysis (EPMA) technique was used to determine the chemical composition of the Ce:KGdP single crystals, Pr:KGdP single crystals and Pr:KGdP nanocrystals, especially their dopant concentrations. Reflection optical microscopy was used to visualize the crystal habit of the grown crystals during the determination of the crystallization region of type B and IV KYP in an immediate way, since no sample preparation is needed. An environmental scanning electron microscope (ESEM) combined with an energy dispersive spectrometer (EDX) detector system was used to obtain images and visualize the crystal habit of the grown crystals during the determination of the crystallization region of type B and IV KYP when this was not possible by reflection optical microscopy due to their small size, and also to carry out an elemental analysis of some grown crystals to determine roughly their chemical composition when this was impossible by X-ray powder diffraction analysis due to the low amount of crystals. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used to observe the morphology and to determine the particle size distribution of type III Pr:KGdP nanocrystals, respectively. The thermal evolution of the synthesized nanocrystals was studied by differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Besides, this technique was used to exactly determine the temperatures at which there are changes in weight, and temperature exchanges in a grinded Pr:KGdP crystal with the aim to complement the study of thermal stability obtained from XRPD technique. The unpolarized and polarized optical absorption at room temperature and at 6 K of undoped KGdP, Ce:KGdP and Pr:KGdP polished plates were studied to identify the absorption bands corresponding to the 4f → 5d and 4f → 4f electronic transitions of Gd3+, Ce3+ and Pr3+ in type III KGdP host, to observe whether there was a change in their optical absorption after exciting them with X-rays radiation, and to study the 4f → 4f electronic transitions of Pr3+ in function of the light polarization for possible future lasing applications. The unpolarized transmittance at room temperature was used in the case of the nanocrystals, which was suspended in distilled water.
The luminescence and decay time curves measurements by exciting in the visible were measured using an fluorescence spectrophotometer. Unpolarized and polarized luminescence spectra of Pr:KGdP single crystals were investigated at room temperature and under visible light excitation to study the emission of these samples through the 4f → 4f electronic transitions of Pr3+ and find out whether there is an anisotropic behaviour in the optical emission. In addition, the long component of the luminescence decay curves of the 5d → 4f transitions of Ce3+, as well as the luminescence decay curves of the 4f → 4f transitions of Pr3+ and Gd3+, were studied in the Ce:KGdP and Pr:KGdP single crystals by the same fluorimeter. The luminescence and decay time curves of Ce:KGdP and Pr:KGdP polished single crystalline plates under vacuum ultraviolet-ultraviolet (VUV-UV) excitation were carried out at room temperature in the DESIRS beamline at SOLEIL Synchrotron (Saint-Aubin, France) in order to study the luminescence of these samples under direct 4f → 5d excitation of Ce3+ and of Pr3+, and observe the fast component of the decay time curves of their 5d levels. The luminescence measurements of Ce:KGdP and Pr:KGdP polished single crystalline plates using different excitation energies within the spectral range of X-ray radiation were performed at room temperature in the BL22-CLAESS beamline at ALBA Synchrotron (Cerdanyola del Vallès, Spain). In the case of Pr:KGdP nanocrystals, the luminescence measurements at room temperature under X-ray excitation were performed using an X-ray tube with a copper target operating at 40 kV and 30 mA, available at Servei de Recursos Científics i Tècnics (SRCiT) of the Universitat Rovira i Virgili. By last, Raman spectroscopy was used to observe whether undoped KGdP, Ce:KGdP and Pr:KGdP polished plates had undergone some change in their chemical structure after being excited with X-rays radiation.
The most relevant conclusions extracted from this Doctoral Thesis are as follows. (1) Type III Ce3+-doped KGd(PO3)4 bulk single crystals of up to 2.6 atomic % of Ce3+ and type III Pr3+-doped KGd(PO3)4 bulk single crystals of up to 5.8 atomic % of Pr3+ substituting Gd3+ can be grown with high crystalline quality by the Top Seeded Solution Growth-Slow Cooling (TSSG-SC) technique from self-flux solutions. (2) Through the determination of the primary crystallization region of the type B KYP4O12 and type IV KY(PO3)4 in the K2O–Y2O3–P2O5 ternary system, it has been found that the optimal composition range to grow type B and IV KYP bulk single crystals is with a K2O/P2O5 molar ratio of around 38/62 and an Y2O3 percentage in the range 4-5 mol%. (3) The modified Pechini method allows to synthesize type III Pr:KGd(PO3)4 nanocrystals with a single crystalline phase.
(4) KGd0.942Pr0.058(PO3)4 crystal is thermally stable up to 1140 K, where it suffers an irreversible decomposition into a unique crystalline compound, GdPO4, and a liquid phase. (5) The exhaustive spectroscopic characterization of type III Ce:KGdP and Pr:KGdP bulk single crystals allowed to find out the energy position of the 5d levels of Ce3+ and Pr3+ ions with respect to their corresponding ground states in type III KGdP, the lifetime of the 5d1 level of Ce3+ and that of Pr3+ in this host, the emission mechanisms under VUV-UV excitation and ionizing radiation, among others. Summarizing, both compositions allow the conversion from X-ray radiation to near-visible light. (6) The polarized optical absorption cross sections of the 3H4 → 4f electronic transitions of Pr3+ in Pr:KGdP, the optical emission spectra of these samples under 445 nm excitation (3H4 → 3P2 of Pr3+) and the lifetime of the 1D2 level of Pr3+ in this host determined along the Thesis are the basic spectroscopic data of the type III Pr:KGdP crystal for lasing applications in the visible wavelength range based on 4f → 4f electronic transitions.
(7) Radiation damage of the type III undoped, Ce3+- and Pr3+-doped KGdP single crystals after prolongated synchrotron X-ray irradiation was observed. The results indicate that the Ce3+-doped KGdP samples might not suffer from radiation damage and, in addition, could allow us to conclude that the Pr3+-doped KGdP samples had already recovered 10 days after synchrotron X-ray irradiation. By last, (8) the type III Pr:KGd(PO3)4 nanocrystals also allow the conversion from X-ray radiation to near-visible light.
