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. 2018 Sep 18;3(9):11307–11316. doi: 10.1021/acsomega.8b01321

Optimization of the Synthesis and Physical Characterization of Praseodymium-Doped Type III KGd(PO3)4 Nanocrystals

Irina Adell 1, Rosa Maria Solé 1,*, Maria Cinta Pujol 1, Magdalena Aguiló 1, Francesc Díaz 1
PMCID: PMC6644752  PMID: 31459239

Abstract

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Scintillator materials are used as detectors in the ray imaging techniques for medical diagnosis. Because the ideal medical scintillator material does not exist, many efforts are being made to find new materials that satisfy a greater number of properties. Here, the synthesis conditions of Pr:KGd(PO3)4 nanocrystals by the modified Pechini method are optimized to obtain a single crystalline phase of those that form the polymorphism of KGd(PO3)4. The interest lies in the type III phase because less quenching by Pr3+ concentration is expected. By performing transmittance measurements and because of the wide transparency window of the type III KGd(PO3)4 host, the 3H4 → 5d1 absorption transition of Pr3+ has been observed in the vacuum ultraviolet spectral range. After creating electron–hole pairs in the host due to the excitation of the material by X-ray radiation, the bands corresponding to the 5d1 → 3H4, 3H5, 3H6 and 5d1 → 3F3, 3F4, 1G4 transitions of Pr3+ have been observed in the near-visible spectral range, being these 5d → 4f transitions interesting for scintillation applications. Therefore, the type III Pr:KGd(PO3)4 nanocrystals allow the conversion from high-energy radiation to visible or near-visible light.

1. Introduction

Potassium gadolinium phosphate, KGd(PO3)4, hereafter KGdP, is a monoclinic crystal that belongs to the wide family of condensed phosphates of double phosphates of alkali (M) and lanthanide (Ln) ions with the general formula MILnIII(PO3)4. These condensed phosphates are constituted by corner-sharing PO4 tetrahedra with variable O/P ratio (from 2.5 to 4) building phosphoric anions. Because of the different possibilities of condensation of the phosphoric anions (long chains, rings, and also three-dimensional networks), the condensed phosphates present multiple crystalline structures; so, KGdP can crystallize in three different crystalline structures: type III phase (space group: P21), type IV (space group: P21/n), and type B phase (space group: C2/c).1 The first two crystalline structures present long chains of polyphosphates, while the third crystalline structure belongs to the group of cyclophosphates, in which the PO4 forms rings. Besides this structural feature, the coordination of the cations presents differences among these three different crystalline phases.

KGdP can be easily doped with other lanthanide ions, such as praseodymium (Pr3+), because gadolinium (Gd3+) has a high capacity for substitution with lanthanides ions due to the proximity of the ionic radii and the common valence with the other lanthanides ions1 (ionic radii of Gd3+ and Pr3+ with coordination VIII are 1.053 and 1.126 Å, respectively2). Besides, KPr(PO3)4 and KGdP crystals are isostructural (in the polyphosphate form, with type III phase,3 and in the cyclophosphate form, with the type B phase4). To our knowledge, there is no evidence of the existence of the KPr(PO3)4 with the type IV crystalline structure.

The transparency window of KGdP extends from 160 nm to 4 μm,5 which covers the main absorption and emission wavelengths of praseodymium. This UV transparency is a great advantage of this compound to be used as a scintillator material based on the d–f electronic transitions of Pr3+ as the doping ion. Scintillator materials are extensively used in technological, industrial, and medical applications.6 One of its main applications is as a detector in the ray imaging techniques for medical diagnosis.7−9 In general, an ideal medical scintillator should satisfy the following eight properties to be considered a good scintillator: short decay time, high luminous efficacy, no afterglow, short radiation length, high density, low cost, good spectral match to photodetectors, and high light yield.10,11 However, until the time, the ideal scintillator does not exist, so each modality of medical imaging systems must select the most suitable combination of properties from the existing scintillators. For this reason, many efforts are being made to find the ideal one. Scintillators can be synthesized in different forms, such as single crystals, powder, thin films, and nanoparticles. Single-crystal sodium iodide doped with thallium ions (NaI (Tl)) is a conventional medical scintillator; however, it does not satisfy with some of the requirements. NaI (Tl) has a comparatively long decay time and a low density.10 Because the parity allowed f–d transitions in lanthanide ion-doped materials present high oscillator strengths and the 5d levels have short lifetimes (typically ns), the lanthanide ion-doped materials are an interesting alternative for scintillator materials.6 García-Murillo et al.12 proposed europium-doped Gd2O3 thin films as a possible good scintillator because of its middle light yield; nevertheless, later this type of films were not considered good as a scintillator for medical imaging because of its comparatively high afterglow. Pedrini et al.13 studied praseodymium-doped YAlO3, and they found that this material has fast UV fluorescence because of the allowed d–f transitions of Pr3+ but relatively low scintillation efficiency. Thus, praseodymium-doped KGdP crystals could be a novel alternative as a scintillator material because the photoluminescence generated by Pr3+ ions, after absorbing high-energetic photons by the crystal, in the visible and UV spectral regions through the 5d1 → 3H4 transition can be observed because of the low UV transparency cut-off of this material.

Moreover, nanocrystalline form allows the possibility to further prepare ceramic materials because of the larger activity in sintering from the nanoscale size14−16 and also the almost isometric nature of the KGdP structure. In this work, the Pechini method has been used to synthesize KGdP nanocrystals, which previously was successfully used to prepare KNP nanocrystals.17 This method yields inorganic oxides of excellent phase purity and well-controlled stoichiometry. It also allows obtaining ceramic powders with fine grain size at relatively low temperature through the formation of a polymeric organic net between a metallic acid chelate and a polyhydroxide alcohol by polyesterification. The polymeric network retains homogeneity on the atomic scale and reduces any segregation of the cations. Furthermore, the Pechini method allows to reach various compositions and to vary the nature and the concentration of the doping ion easily.18−22

Thus, the first aim of this work is the optimization of the synthesis conditions of type III KGdP nanocrystals doped with praseodymium by the modified Pechini method in order to obtain the nanocrystals in a single crystalline phase. The next goals are to achieve its physical characterization and then its optical spectroscopy to determine whether this material could be a new scintillator material.

2. Experimental Section

2.1. Synthesis of the Nanocrystals

A schematic diagram of the modified Pechini synthesis process used for the preparation of Pr:KGdP nanocrystals is shown in Scheme 1, and an illustration of the basic chemical reactions of the Pechini process19,23 is shown in Scheme 2.

Scheme 1. Diagram of the Synthesis Process of Pr:KGdP Nanocrystals by the Modified Pechini Method.

Scheme 1

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Scheme 2. Illustration of the Basic Chemical Reactions Involved in the Preparation of the Organic Precursor in the Modified Pechini Method.

Scheme 2

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Powders of Pr6O11 (Aldrich Chemical Company Inc., 99.9%), Gd2O3 (Aldrich, 99.9%), NH4H2PO4 (Fluka Analytical, ≥99.0%), (NH4)2HPO4 (Sigma-Aldrich, ≥98%), and K2CO3 anhydrous (Alfa Aesar, A Johnson Matthey Company, 99%) were used as starting materials. At first, stoichiometric amounts of Pr6O11 and Gd2O3 are converted to the lanthanide nitrate into a quartz crucible by dissolution in concentrated HNO3 (Merck, 65%) and heated at about 423 K under stirring to evaporate the liquid part. At the same time, stoichiometric amounts of K2CO3 and NH4H2PO4 or (NH4)2HPO4 are dissolved in distilled water into a glass beaker at 323 K.

After that, citric acid, CA, is added as a chelating agent to each container in a specific molar ratio (CM = [CA]/[METAL]) to prepare metal-chelated CA. CM describes the degree of the chelation process of the metal in the organic product.21 To give rise to good reaction between the metal cations and CA, each container is heated at 348 K for 10–20 h under stirring and covered with a crystallizer. On the contrary to other modified Pechini methods used, in this case, we had to use two different containers to avoid the precipitation of gadolinium phosphate.24 The quartz crucible with gadolinium and praseodymium nitrates is named container 1 and the glass beaker with K2CO3 and the phosphate precursor, NH4H2PO4 or (NH4)2PO4, is named container 2. Just after the incorporation of the CA, the pH in the chelation reaction in container 1 is 1 (pH1 = 1); it is 2 in container 2 with NH4H2PO4 as the phosphate precursor (pH2 = 2); and the pH in container 2 with (NH4)2PO4 as precursor is 3 (pH2 = 3). Then, in order to study the effect of pH change in the chelation reaction in container 2, the pH2 was varied by adding NH4OH just after the incorporation of the CA in Exp. 8, and afterward, the result from this experiment was compared with others.

Then, the content of the two containers are joined in the quartz crucible and they are mixed at 373 K for 4 h under stirring and covered with a crystallizer.

Afterward, ethylene glycol, EG, is added as an esterification agent in a specific molar ratio (CE = [CA]/[EG]) in order to obtain the polymeric resin. CE describes the degree of esterification between the hydroxyl groups of EG and the carboxylic acid groups of CA.21 For this purpose, the reagents are heated at 473 K in a sand bath without being covered and under stirring for 3–4 h. During this step, the viscosity of the product resin gradually increases until the resin gels;22 at this time, the stirrer is removed. To obtain a drier resin, it is heated at the same temperature of 473 K for a 20 h. This reaction is considered finished when a whitish gel is observed at the bottom of the crucible.

The following step is an initial calcination at 573 K for several hours to start the decomposition of the resin. Thereafter, a product of spongy texture of shiny black color is obtained. To end, this product is grinded in an agate mortar and the resulting precursor powder is calcined at different temperatures in the range from 673 to 1073 K and at air atmosphere for several hours. After each calcination, the resulting powder is grinded in an agate mortar to promote easier removal of the organic parts. These calcinations were performed in a vertical muffle controlled by an Eurotherm temperature controller/programmer.

2.2. Characterization Techniques

The crystalline phases of the nanocrystals were identified by X-ray powder diffraction using a Siemens D5000 diffractometer with a Bragg–Brentano parafocusing geometry and vertical θ–θ goniometer, fitted with a curved graphite diffracted-beam monochromator, incident and diffracted-beam Soller slits, a 0.06° receiving slit, and a scintillation counter as a detector. Cu Kα radiation was obtained from a copper X-ray tube operated at 40 kV and 30 mA. For conventional analysis, the X-ray powder diffraction patterns were collected from 5 to 70° of 2θ diffraction range with a step size (ss) of 0.05°, a step time (st) equal to 3 s, and sample rotation. For obtaining diffractograms with lower background noise and thus to be able to identify better the presence of undesirable phases or compounds, the data were collected in a 2θ diffraction range from 10 to 60° with a ss of 0.03°, a st equal to 7 s, and sample rotation. The unit cell parameters of the non-centrosymmetric phase were calculated using the Rietveld method.25

The chemical composition of the nanocrystals was measured quantitatively by electron probe microanalysis (EPMA) using a JEOL JXA-8230 electron microprobe. Before taking the measurements, the samples were prepared in tablet form and one of the two flat faces was sputtered with carbon, which acts as a conductor media to avoid the overcharge of the samples. The electron beam was generated at an intensity of 15 nA and an accelerating voltage of 15 kV. The concentrations of K, P, and O were determined using the Kα X-ray lines of K, P, and O, respectively, and the concentrations of Pr and Gd using the Lα X-ray lines of these elements. The measurement time for Pr was 60 s, while for the other elements was 10 s. In the case of Pr, the measurement time was higher because of its low atomic concentration in the KGdP host.

Differential thermal analysis (DTA) was used to study the thermal evolution of the nanocrystals with the temperature by using a TA Instruments SDT 2960 simultaneous differential scanning calorimetry–thermogravimetric analysis system. The heating rate was at 10 K/min with an air flux of 90 cm3/min and the Al2O3 was used as a reference.

The morphology, size distribution, and homogeneity of the nanocrystals were observed using a transmission electron microscopy (TEM) JEOL JEM-1011 system with a MegaView III (Soft Imaging System). Previous to the observation of the nanocrystals by TEM, the samples were mixed with ethanol and subsequently were placed in an ultrasound bath for 50 min to reduce and disperse the agglomerates. The TEM images were observed using an accelerating voltage of 80 and 100 kV.

The optical absorption of the praseodymium ion in KGdP nanocrystals was studied by transmittance measurements at room temperature using a Cary 5000 UV–vis–NIR spectrophotometer. The optical absorption spectrum at room temperature of a plate of undoped KGdP single crystal was also obtained in order to help us to label the electronic transitions belonging to praseodymium, using the same apparatus. The undoped KGdP single crystal was grown by the top seeded solution growth—slow cooling technique as reported in Parreu et al.26

The emission spectra at room temperature of the Pr:KGdP nanocrystals under 445 nm excitation were obtained using a Cary Eclipse fluorescence spectrophotometer with a xenon lamp as the excitation source. The emission spectra 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 of the Universitat Rovira i Virgili. The X-ray radiation reached the pellet sample with an incident angle of 63° with respect to the pellet sample plane. The luminescence emitted from the sample was collected at 90° with respect to the pellet plane, focused with an UV fused silica biconvex lens, guided with an optical fiber, and detected by an Ocean Optics FLAME-S-UV-VIS-ES spectrometer with a spectral resolution of ∼1.5 nm.

3. Results and Discussion

3.1. Synthesis of KGdP Nanocrystals

Tables 1 and 2 show a summary of the experimental parameters used in the synthesis experiments and the phases obtained. The column entitled as “proportion of phases” refers to the ratio qualitatively estimated between the following two crystalline phases of KGdP: type III (monoclinic, space group: P21) and type B phase (monoclinic, space group: C2/c). The number of > or < symbols is related to the proportion between these two phases. Thus, P21 > C2/c means that the intensity of the X-ray diffraction peaks of the P21 phase is only slightly higher than the corresponding to the C2/c phase, while P21 ≫≫> C2/c means that the P21 is clearly the majority phase and the C2/c is a very minority phase. The column called “presence of other compounds” indicates the presence in the sample of compounds different from the KGdP type III and type B phases. Our interest lies in obtaining solely the type III phase (space group: P21) because the minor Gd–Gd interatomic distance in this crystalline phase is the longest one in comparison to the other two phases of KGdP (type IV and type B phases), resulting in a priori minor occurrence of the quenching by Ln3+ concentration in the process of emitting light by the optically active lanthanide ions (Ln3+). The minor Gd–Gd interatomic distance is 6.59,27 6.316,28 and 5.269 Å29 for the type III, type IV and type B phases of KGdP, respectively. Besides, given that the average Gd–O interatomic distance into the Gd coordination polyhedron and its distortion are similar in the three crystalline phases,27 the Ln3+ ions will experience similar interactions with the crystal field.30 In addition, a distinctive feature of the type III phase is that it is a non-centrosymmetric crystalline phase, which allows it to have nonlinear optical properties. Figure 1 shows a projection of the type III KGdP crystalline structure parallel to the b crystallographic direction, in which the Gd coordination polyhedron and the shortest Gd–Gd interatomic distance can be observed.

Table 1. Summary of the Experimental Parameters and the Observed Results in the Diffractogram Using NH4H2PO4 as a Source of Phosphate Groupsb.

Exp. CM CE calcination 1 [K-h] calcination 2 [K-h] phase P21 phase C2/c proportion of phases presence of other compounds
1 3 2 573-3 1073-3 ×     yes
2a 3 2 573-8 973-10 × × P21 ≪< C2/c yes↑
2b       1073-10 × × P21 ≪< C2/c yes↑
3a 3 1 573-10 973-10 × × P21 ≪< C2/c no
3b       1073-10 × × P21 ≪≪ C2/c no
4aa 6 2 573-10 773-10 × × P21 ≪≪ C2/c no
4ba       873-10 × × P21 ≪≪ C2/c no
4c       973-10 × × P21 ≪≪ C2/c no
4d       1073-10 × × P21 ≪≪< C2/c no
5aa 6 1 573-10 773-10 × × P21 ≪< C2/c no
5ba       873-10 × × P21 ≪< C2/c no
5c       973-10 × × P21 ≪< C2/c no
5d       1073-10 × × P21 ≪≪ C2/c no
6aa 3 2 573-10 773-10 × × P21 ≫≫ C2/c no
6b       873-10 × × P21 ≫≫> C2/c yes
6c       973-10 × × P21 ≫≫> C2/c yes↑
6d       1073-10 × × P21 ≫ C2/c yes↓
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a

Experiment whose diffractogram has a high background noise due to incomplete calcination.

b

In all cases, the atomic percentage of Gd substituted by Pr in KGdP is 1 at. % Pr, the pH in the chelation reaction in the quartz crucible with gadolinium and praseodymium nitrates is 1 (pH1 = 1), and the pH in the glass beaker with K2CO3 and NH4H2PO4 is 2 (pH2 = 2).

Table 2. Summary of the Experimental Parameters and the Observed Results in the Diffractogram Using (NH4)2HPO4 as a Source of Phosphate Groupsd.

Exp. amount of Pr in KGdP [at. %] pH2a calcination 2 [K-h] calcination 3/4 [K-h] phase P21 phase C2/c proportion of phases presence of other compounds
7a 1 3 773-10   ×     yes
7b     673-10 823-10 × × P21 ≫≫> C2/c yes↓
7c     873-10   ×     yes
7d     723-10 873-5 ×     yes↓
7e     973-10   ×     yes↑
7f     1073-10   × × P21 ≫> C2/c yes
8ab 1 5c 823-10   × × P21 ≫> C2/c yes↑
8b     873-10   × × P21 ≫≫ C2/c no
8c     723-10 873-5 × × P21 ≫≫ C2/c yes↓
8d     723-10 873-5/873-10 × × P21 ≫≫> C2/c yes
8e     973-10   × × P21 ≫≫ C2/c yes↑
8f     1073-10   × × P21 ≫ C2/c yes↑
9a 3 3 723-10 873-5 × × P21 ≫≫> C2/c no
9b     773-10 873-5 × × P21 ≫≫> C2/c no
10 5 3 773-10 873-7 ×     no
11 10 3 773-10 873-7 ×     no
12 0 3 773-10 873-7 ×     no
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a

The pH was measured at the beginning of the chelation reaction in the glass beaker with (NH4)2HPO4 and K2CO3 reagents.

b

Experiment whose diffractogram has a high background noise due to incomplete calcination.

c

This pH was achieved by adding NH4OH just after the incorporation of the CA.

d

In all cases, CM = 3, pH1 = 1, CE = 2, and a first calcination at 573 K for 10 h.

Figure 1.

Figure 1

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Projection of the type III KGd(PO3)4 structure parallel to the b crystallographic direction, showing the Gd coordination polyhedron and the shortest Gd–Gd interatomic distance.

The experimental parameters of the first experiment were as follows: an atomic percentage of praseodymium doping ions of 1 at. %, CM = 3, CE = 2, a first calcination at 573 K for 3 h, and a second one at 1073 K for 3 h. As it can be observed in Figure 2, the obtained product in Exp. 1 corresponds to a mixture of Gd2O3·2P2O5 (JCPDS 35-0078), KPO3 (JCPDS 35-0819), which have been identified as intermediate products, and KGdP (P21 phase). To provoke the complete synthesis of KGdP as a product and eliminate the presence of other compounds in the final product, the reaction times between the reactants in the chelating, esterification, and polymerization reactions (Scheme 2) under stirring were longer than in Exp. 1 and the time in the two calcinations was also elongated (Exp. 2, 3). The effect of the molar ratios CM and CE was also studied (Exp. 3–6). Almost in all these experiments, only KGdP product was obtained and its C2/c phase was in a higher proportion than the P21 phase, as can be seen in Figure 2. Because there is an overlap of many peaks of the type IV phase pattern with the patterns of the type III and B phases, it is difficult to affirm or discard the presence of a low amount of the type IV phase (monoclinic, space group: P21/n) in these experiments. The ratio of the two majoritarian crystalline phases of KGdP is reverted in Exp. 6 when CM = 3 and CE = 2 are used. Thus, CM = 3 and CE = 2 seem to be the most suitable molar ratios for obtaining mainly the non-centrosymmetric crystalline phase of KGdP (space group: P21). From now on, all synthesis experiments were performed using these two molar ratios and a first calcination at 573 K for 10 h.

Figure 2.

Figure 2

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X-ray powder diffractogram pattern for two crystalline phases of KGdP and one experimental diffractogram of Pr:KGdP nanocrystals for each experiment from Exp. 1 to Exp. 6.

By comparing Exp. 6, 7, and 8, the effect of the pH change in the chelation reaction can be observed. As explained in the Experimental Section, when NH4H2PO4 was used as the phosphate precursor (Exp. 6), the pH in this reaction was about pH2 = 2. In order to have the reaction in a less acid medium, ammonium phosphate dibasic, (NH4)2HPO4, was used as a source of phosphate groups, leading to an at least pH2 = 3 during this reaction in Exp. 7. Finally, in Exp. 8, ammonium hydroxide, NH4OH, was added just after the incorporation of CA in order to obtain a pH2 = 5. First, it should be noted that the carboxylate ion functions effectively to form metal chelates, whereas the non-ionized carboxylic acid group is not an efficient donor group.31 The concentration of the carboxylate ion is higher than the one of non-ionized carboxylic acid group when pH > pKa.32 Thus, because the logarithmic acid dissociation constant of the first proton of CA is pKa1 = 3.13 and of the second one is pKa2 = 4.76, CA is found with one carboxylate ion when pH is in the range 3.13–4.75 and with two carboxylate ions when pH is between 4.76 and 6.39.33 Besides, it should also be noted that the esterification reaction needs to be catalyzed by acid and the non-ionized carboxylic acid group is required in order to have the carbonyl activated toward nucleophilic attack. Thus, a pH2 of at least 3 seems to be the most suitable pH because pH2 = 3.13–4.75 results in a molecule of CA with one carboxylate ion to prepare one metal–CA chelate and with two non-ionized carboxylic acid groups to give rise to the esterification reaction in order to obtain the polymeric organic network. This homogeneous formation of the polymeric network could contribute to obtain a single crystalline phase of KGdP, as it can be observed in Figure 3. Then, the conditions of Exp. 7 are more appropriate to form type III KGdP than the ones for Exp. 6 and 8.

Figure 3.

Figure 3

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X-ray powder diffractogram pattern for two crystalline phases of KGdP and one experimental diffractogram of Pr:KGdP nanocrystals for each experiment from Exp. 7 to Exp. 12. The diffractograms 9b, 10, 11, and 12 belong to X-ray powder diffraction analysis with ss = 0.03° and st = 7 s.

By changing the calcination time, calcination temperature, and number of calcinations, the product obtained presents different ratios of the P21 and C2/c crystalline phases and different quantity of subproducts. In the Exp. 7, it is observed by these changes in the calcination procedure than a lower temperature in the calcination 2, and the addition of a third calcination favors the obtaining of the P21 crystalline phase and the reduction of the presence of other phases in the final product. In the Exp. 9, 10, 11, and 12, besides change in the concentration of praseodymium of the obtained product, the temperatures and times of calcination 2 and calcination 3 were optimized till obtaining no presence of other compounds and only obtaining the P21 crystalline KGdP phase in the product. Thus, summarizing, the optimal conditions of calcination are a first calcination at 573 K for 10 h, a second calcination at 773 K for 10 h, and a third calcination at 873 K for 7 h.

Thus summarizing, the optimal parameters to synthesize KGd1–xPrx(PO3)4 nanocrystals with a unique crystalline P21 phase by the modified Pechini method are CM = 1 with a pH2 = 3, CE = 2, a reaction time between metal cations and CA of 10–20 h at 348 K under stirring (when the reagents are in two separate containers) followed by 4 h more at 373 K after mixing the content of the containers, a reaction time between metal–CA complexes and EG of around 4 h at 473 K in a sand bath under stirring followed by about 20 h more at the same temperature without stirring, a first calcination at 573 K for 10 h, a second one at 773 K for 10 h, and a third one at 873 K for 7 h.

3.2. Morphological Characterization and Particle Size Distribution

The morphology, size distribution, and homogeneity of the nanocrystals were observed by TEM. The particle size distribution was estimated using multiple TEM images as the pictures shown in Figure 4. In the same figure, the histogram of the particle size distribution of 5 at. % Pr:KGdP nanocrystals is also shown. From the histogram, a predominance of small particles with sizes ranging from 10 to 20 nm can be observed. The distribution of the particle size can be fitted by a lognormal function, as expected in particles prepared by the Pechini method, in which their constitution follows the steps of nucleation, condensation, and crystal particle growth.34,35

Figure 4.

Figure 4

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Representative TEM images of 5 at. % Pr:KGdP nanocrystals from Exp. 10 and the total particle size distribution.

3.3. Effects of Doping: Changes in Unit Cell Parameters with Praseodymium Doping

The unit cell parameters and unit cell volume of Pr:KGdP nanocrystals were refined by the Rietveld method.25Figure 5 shows the observed, calculated and differential X-ray powder diffraction profile obtained after Rietveld refinement of the 10 at. % Pr:KGdP nanocrystals, as an example. In Table 3, it can be seen how as the praseodymium content increases, the a, b, and c unit cell parameters and the unit cell volume V increase. However, the increase of the a unit cell parameter is very slight. This increase in the unit cell volume when Pr3+ is introduced as the doping ion in the KGdP structure was expected because the ionic radius of Pr3+ is higher than that of Gd3+ (ionic radii of Gd3+ and Pr3+ with coordination VIII are 1.053 and 1.126 Å, respectively2).

Figure 5.

Figure 5

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Observed, calculated and differential X-ray powder diffraction profile obtained after Rietveld refinement at room temperature of 10 at. % Pr:KGdP nanocrystals.

Table 3. Unit Cell Parameters and Unit Cell Volume V of Undoped KGdP and Pr-Doped KGdP Nanocrystals.

real at. % Pr in KGdP a [Å] b [Å] c [Å] β [deg] V [Å3]
0 7.2486(3) 8.3505(3) 7.9204(3) 91.828(3) 479.17(3)
2.8 7.2488(4) 8.3517(4) 7.9215(4) 91.832(3) 479.32(4)
4.9 7.2509(4) 8.3553(5) 7.9247(4) 91.827(4) 479.86(5)
8.9 7.2512(3) 8.3575(4) 7.9285(4) 91.847(4) 480.23(4)
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The atomic percentage of praseodymium with respect to gadolinium obtained by EPMA for the sample labeled as 3 at. % Pr:KGdP was 2.8 at. %, for 5 at. % Pr:KGdP was 4.9 at. %, and 8.9 at. % for the sample labeled as 10 at. % Pr:KGdP. The sample labeled as 1 at. % Pr:KGdP was not measured by EPMA because the amount of praseodymium is quite low; however, its atomic percentage with respect to gadolinium is expected to be similar to 1 at. %.

3.4. Thermal Analysis of the Nanocrystals

Figure 6 shows the DTA thermogram of type III 0, 5, and 10 at. % Pr:KGdP nanocrystals. By DTA of type III KGdP nanocrystals and considering a previous study26 about how type III KGdP single crystal evolves with temperature between room temperature and 1273 K using DTA and X-ray powder diffraction, it was determined that type III KGdP nanocrystals melt incongruently at 1121 K. The incongruent melting point of type III KGdP single crystal is 1142 K, as reported in ref (26). Because the first ones are nanocrystals, a decrease of its incongruent melting point in relation to the bulk material was already expected, given that a higher surface to volume ratio results in a lower melting temperature.36,37 From the same Figure 6, it could be observed that as praseodymium content in the nanocrystals increases, the incongruent melting point slightly decreases.

Figure 6.

Figure 6

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DTA thermogram of type III undoped, 5 and 10 at. % Pr:KGdP nanocrystals.

3.5. Optical Spectroscopy of Pr3+ in Pr:KGdP Nanocrystals

Figure 7a shows the transmittance spectra at room temperature of type III 5 at. % Pr:KGdP nanocrystals suspended in distilled water. In order to help us identify the absorption peaks observed in the transmittance spectra, the information obtained from the optical absorption spectrum at room temperature of a plate of undoped KGdP single crystal from 190 to 320 nm (Figure 7b) was used. In Figure 7b, by using the Dieke’s diagram,38,39 all the 4f → 4f electronic transitions of Gd3+ at wavelengths higher than 190 nm have been labeled. No other absorption belonging to Gd3+ can be observed at longer wavelengths than at 320 nm.

Figure 7.

Figure 7

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(a) Transmittance spectra of type III 5 at. % Pr:KGdP nanocrystals suspended in distilled water at room temperature. (b) Optical absorption spectrum of a plate of undoped KGdP single crystal at room temperature. In green, the labels corresponding to the electronic transitions of Pr3+ assigned to each observed band, and in purple, those corresponding to the electronic transitions of Gd3+.

In Figure 7a, a broad and intense band centered at 218 nm and two small peaks centered at 445 and 482 nm can be seen. With regard to the intense broad band, it has been proposed that it belongs to the 3H4 → 5d1 transition of Pr3+ because an absorption transition of Gd3+ is not expected in this wavelength, as seen in Figure 7b. These 4f → 5d absorption transitions are electric dipole allowed transitions exhibiting very intense and broad bands, and besides, they appear in the spectral range of the vacuum ultraviolet and ultraviolet.40,41 Thus, the wide transparency window of type III KGdP host allows us to observe the 3H4 → 5d1 transition of Pr3+. By using the Dieke’s diagram,38 the peaks which appear at 445 and 482 nm have been labeled as 3H4 → 3P2 and 3H4 → 3P0 electronic transitions of Pr3+, respectively. The 4f → 4f absorption peak centered at 445 nm has the highest intensity of these two absorption transitions because of its quite large electric dipole oscillator strength because of its pseudo-hypersensitive nature.41

On the other hand, it is worth commenting that the 4f → 5d absorption transitions of Pr4+ in KGdP host are not observed in the spectral range of 180–325 nm. That is, because Pr4+ is isoelectronic with Ce3+, similar results can be expected in the case of having the same host doped with Pr4+ or Ce3+.42 Hence, taking into account the work made by Zhong et al.,43 the 4f → 5d absorption transitions of Ce3+ in KGdP host are found at 193, 209, 221, 245, and 307 nm for 4f → 5d5, 4f → 5d4, 4f → 5d3, 4f → 5d2, and 4f → 5d1 transitions of Ce3+, respectively, so that it can be concluded that the 4f → 5d absorption transitions of Pr4+ are not observed in Figure 7. During the calcination process, the polymeric network is decomposed, leading to the formation of carbon oxides in addition to other calcination products, among them Pr:KGdP. The formation of CO as a calcinations product is expected.44 According to the literature, CO can act as a reducing agent resulting in its oxidation by forming CO2,45 and in this case, its oxidation could promote the reduction of praseodymium from Pr4+ to Pr3+ because Pr4+ is a very powerful oxidizing agent.46,47 Therefore, it is reasonable to assume that praseodymium ions are, in the final product, basically in Pr3+ form. Thus, the partial substitution of Gd3+ by Pr3+ in the crystalline network of KGdP will result in an electrically neutral structure, with higher stability than if Pr4+ was present.

Figure 8 shows the emission spectrum at room temperature of type III 5 at. % Pr:KGdP nanocrystals suspended in EG under 445 nm excitation. After exciting them at 445 nm (3H4 → 3P2 of Pr3+), five emission peaks can be observed in the wavelength range from 590 to 850 nm. Such emission transitions belong to 4f → 4f transitions of Pr3+. By comparing the obtained emission spectrum with the emission spectra of Pr3+ doped in LaAlO3,48 NaGd(WO4)2,49 and La1/2Na1/2TiO350 hosts as well as Bi3+ and Pr3+ co-doped lead silicate glasses51 under Pr3+ 4f → 4f excitation and by using the Dieke’s diagram,38 the emission peaks have been labeled as 1D2 → 3H4, 3P0 → 3H6, 3P0 → 3F3, 3P0 → 3F4, and 1D2 → 3F2. Because its photoluminescence under Pr3+ 4f → 4f excitation occurs from both 3P0 and 1D2 levels of Pr3+, this means that the emission from the 3P0 level is not quenched because of the presence of an intervalence charge transfer state between these two levels, as happens in perovskite titanates R1/2Na1/2TiO3:Pr3+ (R = La, Gd, Y, Lu)50 and in Pr3+- and Tb3+-doped double tungstate crystals KRE(WO4)2 (RE = Y, Gd, Yb, Lu).52

Figure 8.

Figure 8

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Emission spectrum of type III 5 at. % Pr:KGdP nanocrystals suspended in EG at room temperature under 445 nm excitation (3H4 → 3P2 of Pr3+). In green, the labels corresponding to the electronic transitions of Pr3+ assigned to each observed band.

Figure 9 shows the emission spectra at room temperature under X-ray excitation of type III Pr:KGdP nanocrystals pressed in a pellet. The most intense band centered at 313 nm corresponds to the 6P3/2, 6P5/2, 6P7/2 → 8S7/2 transitions of Gd3+. The low intensity bands centered at around 239 and 261 nm belong to the 5d1 → 3H4, 3H5, 3H6 and 5d1 → 3F3, 3F4, 1G4 transitions of Pr3+, respectively, and as mentioned earlier, the 5d → 4f transitions of lanthanide ions are of special interest for scintillation applications. The level transition assignation has been done using the Dieke’s diagram38 and taking into account that the energy difference from the 5d1 level of Pr3+ in type III KGdP host to the 3H4 level is about 45 872 cm–1 (5.69 eV, 218 nm). As can be seen in the same figure, there is a different behavior depending on the praseodymium content. Hence, the composition that seems to be the most interesting for scintillation applications is type III KGdP nanocrystals doped with 5 at. % of Pr because the intensity of the bands corresponding to the 5d1 → 4f transitions of Pr3+ is the highest and at the same time that the band corresponding to the 4f → 4f transitions of Gd3+ is not so intense.

Figure 9.

Figure 9

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Emission spectra under X-ray excitation of type III 1, 3, and 5 at. % Pr:KGdP nanocrystals as a pellet at room temperature. In purple, the labels corresponding to the electronic transitions of Gd3+ assigned to the observed band, and in green, the labels corresponding to the electronic transitions of Pr3+ assigned to each observed band.

4. Conclusions

The present paper reports on obtaining type III Pr:KGd(PO3)4 nanocrystals by the modified Pechini method for first time up to now. As previously stated, the optimal parameters to synthesize Pr:KGd(PO3)4 nanocrystals with a unique crystalline phase, type III phase (space group: P21), by the modified Pechini method are CM = 3 with a pH2 = 3, CE = 2, a reaction time between metal cations and CA of 10–20 h at 348 K under stirring followed by 4 h more at 373 K after mixing the content of the two containers, a reaction time between metal–CA complexes and EG of around 4 h at 473 K in a sand bath under stirring followed by about 20 h more at the same temperature without stirring, a first calcination at 573 K for 10 h, a second one at 773 K for 10 h, and a third one at 873 K for 7 h. By using TEM, a predominance of the nanocrystals with sizes ranging from 10 to 20 nm has been observed.

From the transmittance measurements, it has been found that the 3H4 → 5d1 transition of Pr3+ in type III Pr:KGd(PO3)4 nanocrystals appears as a broad and intense absorption band centered at 218 nm. In the emission spectra of these nanocrystals under X-ray excitation, several 5d1 → 4f transitions of Pr3+ and the 6P3/2, 6P5/2, 6P7/2 → 8S7/2 transitions of Gd3+ have been observed. Therefore, the 5d1 level of Pr3+ is located within the forbidden band gap of type III KGd(PO3)4, a fact that was expected due to the wide transparency window of the host.

Despite the lower intensity of the bands corresponding to the 5d1 → 4f transitions of Pr3+ in comparison with the band corresponding to the 4f → 4f transitions of Gd3+, it would be interesting to excite the nanocrystals under direct f–d excitation to obtain the decay time curves from the 5d1 level as well as to excite once again with high-energy radiation to widely study their scintillation properties.

Acknowledgments

The authors acknowledge the financial support from the Spanish Government under Projects MAT2016-75716-C2-1-R (AEI/FEDER, UE) and TEC 2014-55948-R and from the Catalan Government under Projects 2014 SGR 1358 and 2017 SGR 755. I.A. thanks the Catalan Government for the financial support under grants 2015 FI_B 00711, 2016 FI_B1 00113, and 2017 FI_B2 00017 as well.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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