Rapid Synthesis and Chemical Durability of Gd2- x Nd x Zr2O7 (0.0 ≤ x ≤ 2.0) Sub-Micron Ceramics as Nuclear Waste Forms

In this work, Nd 3+ was used as a surrogate and it was incorporated into Gd 2 Zr 2 O 7 nanocrystalline ceramics to simulate the immobilization of trivalent actinide elements. A series of Gd 2 − x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0) nanocrystalline powders were fabricated by solvothermal method, and then Gd 2 − x Nd x Zr 2 O 7 sub-microncrystalline ceramics were prepared by sintering via self-propagating chemical furnace plus quick pressing (SCF/QP). All powders are in defective uorite structure, and Nd doping hardly change the powder grain size. After analyzing the sintered ceramics, it can be found that the transition from defective uorite structure to pyrochlore structure occurs when x ≥ 1.5. The sample density decreases with elevated Nd content, while the grain size gradually enlarges. Besides, the normalized release rates of Nd and Zr elements in the Nd 2 Zr 2 O 7 waste form are kept in low values (below 10 − 5 g•m − 2 •d − 1 ), which exhibits its excellent aqueous stability.


Introduction
Crystalline Gd 2 Zr 2 O 7 (GZO) oxide with pyrochlore or defective uorite structure has been widely used in many elds due to its excellent thermal stability, chemical stability and mechanical properties [1][2][3][4] . GZO ceramic exhibits the characteristics of immobilizing certain actinides and outstanding radiation resistance in term of nuclear waste immobilization, which makes it a promising host material for the safe disposal of high-level radioactive wastes [5][6][7][8][9] .
The GZO oxide can tolerate actinide elements into its crystal structure due to its exible lattice sites. GZO exhibits the A 2 B 2 O 7 stoichiometry, where the A site can immobilize trivalent nuclides (such as Am) while the B site can immobilize tetravalent nuclides (such as Pu) [10][11][12] . According to Subramanian theory, the phase stability is mainly determined by the cation radius ratio (r A /r B ). The phase structure of A 2 B 2 O 7 can be divided into three different types: disordered defective uorite structure (r A /r B < 1.46), ordered pyrochlore structure (1.46 ≤ r A /r B ≤ 1.78) and monoclinic crystal structure (r A /r B > 1.78) [13] . The r A /r B value of GZO is 1.46, which is exactly at the boundary between disordered defective uorite structure and ordered pyrochlore structure. This characteristic makes its structure unstable and can adapt to a range of structural defects [14,15] . Figure 1 depicts the crystal structure of pyrochlore and defective uorite. The pyrochlore structure is a superstructure derivative of simple uorite structure (AO 2 ). It can be written as A 2 B 2 O 6 O′ and belongs to the space group (Z = 8). A, B, O, O′ ions respectively occupy 16c, 16d, 48f and 8b four crystal sites, and the unoccupied 8a site corresponds to the oxygen vacancy. The defective uorite structure shows the (Z = 1) space group, in which the cation arrangement and oxygen vacancy distribution become disordered [16−18] . Of course, other factors may also cause disorder of cations and anions, such as sintering temperature, pressure, metal-oxygen bond covalentness and ion irradiation [19−21] . At present, research reports demonstrate that the pyrochlore structure formed in GZO at a temperature range of 1300-1500 °C and transformed to defective uorite structure at temperature higher than 1550 °C [22,23] .  In recent years, nuclear waste forms based on GZO ceramics have been widely investigated for high-level radioactive wastes [24][25][26] . Nd 3+ is usually selected as a surrogate of trivalent actinides to simulate the trivalent nuclide immobilization effect, which can be used for manufacturing or performance evaluation [27] . According to the isomorphism theory, Nd 3+ is designed to occupy the Gd 3+ site in the GZO crystal structure since the radius of Nd 3+ ( = 1.11 Å) is close to the equivalently charged Gd 3+ ( = 1.05 Å). Mandal et al. have investigated the phase relation and thermal expansion behavior of Gd 2 Zr 2 O 7 -Nd 2 Zr 2 O 7 system using conventional sintering methods [28] . Since then, Lu et al. have also prepared and characterized Gd 2 − x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0) ceramics using spark plasma sintering (SPS) technology [29] . The sample grain size prepared by the two methods are in the micron scale. Moreover, only the phase structure is characterized while the immobilizaiton properties (such as aqueous stability) are not studied.
The preparation and research of nanocrystalline ceramics have also been extensively carried out with the rise of nanocrystalline materials. Compared with large-grain ceramics, nanocrystalline ceramics dispaly more exceptional radiation stability in the eld of nuclear waste immobilization [30] . This conclusion is because nanocrystalline ceramics have massive crystal interfaces and grain boundaries. They can act as sinks for the annihilation of point defects (such as interstitials and vacancies) produced by radiation [31,32] . Therefore, it is of great value to prepare Gd 2−x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0) ceramics with grain size less than micron scale and to explore its immobilization properties.
In this study, the preparation process of Gd 2 − x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0) ceramics with grain size less than a micrometer and the aqueous stability with the maximum Nd doping were studied. In our previous work, GZO nanocrystalline ceramics have been successfully prepared [33] . Thus, Gd 2 − x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0) ceramics were also prepared by a two-step method in this work. The structure and performance evolution of these Nd-doped GZO waste forms were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and density measurement to understand the microstructural effects of Nd in place of Gd. The aqueous durability of Nd 2 Zr 2 O 7 ceramic was estimated using the static leaching experiment, and the crystal structure of the sample after leaching was characterized and analyzed as well. organics. Subsequently, the calcined powder was weighed 1 g and then uniaxially pressed with a steel mold to form a disk with diameter of 10 mm. Because the density of Gd 2 Zr 2 O 7 nanocrystalline ceramic obtained by the previous process is only 83.6%, a slight improvement was made to improve its density in this study. Cold isostatic pressing was performed on the pressed disk sample, thereby improving the compactness before sintering. The pressure of 200 MPa was adopted and the pressure was maintained for 200 s during cold isostatic pressing.

Experimental
Finally, the SCF/QP technology was used to obtain Gd 2 − x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0) ceramics with higher density. The maximum temperature of SCF can reach to 1642 °C, the applied pressure was 50 MPa and the pressing time was 1 min. The entire sintering process was completed within 5 min. Speci c powder preparation and sintering processes have been described elsewhere [33] .

Characterizations
The calcined powders prepared by the solvothermal method were subjected to transmission electron microscopy (TEM; Zeiss Libra-200FE, Germany). The calcined powders and sintered ceramics were characterized by Cu Kα radiation X-ray diffraction (XRD; X'Pert PRO, Netherlands). Raman spectroscopy was performed on the sintered ceramics using a Raman spectrometer (Raman; InVia, Renishaw, Britain) with 785 nm excitation wavelength. Field-emission scanning electron microscopy (FESEM; Zeiss Ultra-55, Oberkochen, Germany) and grain size statistics software (Nano measurer; Fudan University, Shanghai, China) were used to determine the grain size of ceramic samples. The density of samples was tested based on the Archimedes drainage method. The leached samples were analyzed by grazing incidence Xray diffraction (GIXRD) using an X-ray diffractometer (Bruker; D8 Advance) with Cu Kα radiation (λ = 1.54056 Å).

Aqueous durability
The aqueous durability of Nd 2 Zr 2 O 7 ceramics was evaluated according to the standard MCC-1 method [34] . The selected sample was cut into 5.0 × 4.3 × 3.0 mm, which was polished and suspended with copper that they belong to defective uorite structure. According to Subramanian theory [13] , Gd 2 − x Nd x Zr 2 O 7 (0.5 ≤ x ≤ 2.0) should be a pyrochlore structure because r A /r B values are between 1.46 and 1.54. The cation radius ratio at the A and B sites is based on the following formula: Where r is the radius ratio of the cations at the A and B sites; r A is the average cation radius at the A site, consisting of Gd 3+ and Nd 3+ ; is 1.05 Å, is 1.11 Å and r B is equal to (0.72 Å); x is the matching value in Gd 2−x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0). The above phenomenon occurs due to the in uence of surface/interface energy on the Gibbs free energy, so nanocrystalline powders always exist in the form of a high-temperature phase (defective uorite structure) [35] . In addition, it can be discovered that the diffraction peaks gradually move to a smaller angle as the value of x increases. This occurrence is because the radius of Nd 3+ is larger than that of Gd 3+ , which causes the unit cell to expand. The XRD consequences also point out that it is feasible to introduce the simulated radionuclide Nd into the crystal structure using the solvothermal method.
The powders (at x = 0.0 and 2.0) were characterized by TEM to study the doping amount in uence on the micro-morphology. Figure 3 [23] . Interestingly, the structure of Gd 2-x Nd x Zr 2 O 7 ceramics starts to change from pyrochlore structure to defective uorite structure when x value goes beyond 1.0. With the doping of Nd, the cation radius ratio at the r A /r B is greater than 1.46 and its structure should theoretically be a pyrochlore structure. However, since the grain size of prepared ceramics is in sub-micron scale and the samples have more grain boundaries and crystal planes, which also affects the Gibbs free energy of the system. So it is still a defective uorite structure [35] . The cation radius ratio and the grain size gradually increases with the Nd content when x value goes beyond 1.0, which makes the ceramics form pyrochlore structure. Therefore, the phenomenon of structural change from defective uorite to pyrochlore appears.
In addition, it can be found that the diffraction peaks gradually shift to a small angle with the Nd content increases. This outcome is in accordance with the XRD results of nanocrystalline powders, which can be attributed to lattice expansion caused by Nd doping. The phenomena of this study are somewhat different from previous report [29] . Lu et al. have used SPS (1700 °C sintering temperature) to prepare ceramics with a grain size of about 9 μm. Transition from defective uorite structure to pyrochlore structure appears when x ≥ 0.8. The reason for this difference is that the sub-micron ceramic samples have a greater in uence on the Gibbs free energy of the system, and there is also a certain difference in the sintering temperature between the two systems. Thus, the critical point of transition from defective uorite structure to pyrochlore structure is different between the two studies.
As a supplement and veri cation of XRD results, the Raman spectroscopy analysis was also conducted to further understand the ordered structure of Gd 2 − x Nd x Zr 2 O 7 ceramics as presented in Fig. 4(b). Firstly, the original matrix Gd 2 Zr 2 O 7 ceramic shows three Raman active modes of the defective uorite structure, including E g , F 2g and A 1g [36] . When x increases from 0 to 1.0, Gd 2 − x Nd x Zr 2 O 7 (0.5 ≤ x ≤ 1.0) ceramics also display the same three Raman active modes as above. When x = 1.5 and 2.0, Gd 0.5 Nd 1.5 Zr 2 O 7 and Nd 2 Zr 2 O 7 demonstrate four kinds of pyrochlore structure Raman active modes, which are E g + A 1g + 2F 2g [37] . In these Raman active modes, A 1g corresponds to O-Zr-O bending vibration type, E g corresponds to Zr-O bending vibration type and F 2g corresponds to Zr-O and Gd (Nd)-O stretching vibration type. This result is in line with XRD analysis that the pyrochlore structure appears at x = 1.5 and 2.0. Secondly, the Raman peaks shift to lower wavenumbers as x increases from 0 to 2.0 in Fig. 4 [38] . Hence, the Raman vibration peaks generated by these groups shift to lower frequencies as the average r A /r B ratio increases. technology. It can be discovered that the grain boundary of ceramics after SCF sintering is clear. The grain shape is approximately spherical and all display a dense microscopic state. The grain size distribution was counted by Nano measure software and the statistical results are depicted in the upper right corner of Fig. 5(a)-(e). The average grain size of the original matrix Gd 2 Zr 2 O 7 is 79 nm, which is in consistent with our previous work [33] . When the x values are 0.5, 1.0, 1.5, and 2.0, the average grain size is 116 nm, 164 nm, 205 nm, and 265 nm. Fig. 5(f) shows the tting result of XRD datas with Jade software to obtain the grain size. The results indicate that the calculated grain size is in accordance with the SEM grain statistics. The above results indicate that the average grain size of the ceramic samples also gradually enlarges with the Nd doping amount increases. This phenomenon is due to the fact that the atomic radius of Nd is larger than Gd, which leads to the grain size gradually enlarges after sintering. The density measured by the Archimedes drainage method is displayed in Fig. 6. At the same time, the results are compared with previous research [29] . It can be seen that the sample density prepared by the three sintering methods decreases with the increase of Nd content. Density is directly proportional to mass and inversely proportional to volume. As the radius of Nd atom is larger than Gd atom while Nd atom is lighter than Gd atom, the sample density gradually decreases in this system. In addition, the sample density after SCF/QP sintering is between SPS sintering and conventional sintering. However, the grain size of the samples sintered by SPS (1700 °C, 3 min) and conventional sintering (1500 °C, 72 h) is in micron scale. This consequence is due to SPS sintering temperature as high as 1700 °C and conventional sintering time as long as 72 hours. SCF sintering can produce dense samples with submicron grain sizes in just a few minutes. The density of Gd 2 Zr 2 O 7 after SCF sintering is 6.17 g·cm − 3 .

Micromorphology and density analysis of
Compared with the previous result of 5.53 g·cm − 3 , the improved preparation process did improve the density [33] . Nd 2 Zr 2 O 7 with the largest amount of Nd doping has a density of 5.87 g·cm − 3 and relative density of 90.3%.

Aqueous durability analysis of Nd 2 Zr 2 O 7 ceramic
It is well known that nuclear waste forms will eventually be disposed in deep geological repositories. The waste forms will inevitably contact with groundwater and aqueous durability is a very important property to evaluate the safety of nuclear waste forms [39,40] . Standard MCC-1 measurements were performed at 90 °C for 1-42 days to estimate the leaching behavior of Nd 2 Zr 2 O 7 sample. The normalized elemantal leaching rates (LR i ) are depicted in Fig. 7. As the immersion time was extended from 1 day to 42 days, the degree of LR Nd value reduction during the rst 28 days is extremely large, which exceeds two orders of magnitude. But the value gradually stabilizes from the 28th to the 42nd day. The LR i value for Zr shows a different trend. The LR Zr value decreases signi cantly in the rst seven days. Then, there is a slight uctuation between the 7th and 42nd days. For short-term leaching tests, uctuations in LR i values on the same order of magnitude is a common phenomenon [41] . The leaching values of Nd and Zr on the 42nd day are 1.1 × 10 − 6 g•m − 2 •d − 1 and 2.5 × 10 − 7 g•m − 2 •d − 1 , respectively. Their values are lower than the leaching data of ordinary SYNROC (at the order of 10 − 4 to 10 − 3 g•m − 2 •d − 1 ) [42] . This result demonstrates that Gd 2 Zr 2 O 7 indicates great potential as immobilizaiton matrix of radioactive wastes.
The in uence of MMC-1 leaching experiment on the samples phase structure at different depths was further studied by the analysis of GIXRD, which has been used to evaluate the phase structure leaching stability of samples. The GIXRD patterns of Nd 2 Zr 2 O 7 after the MCC-1 leaching experiment are presented in Fig. 8. GIXRD analysis with different incident angles was carried out to achieve the purpose of detecting different depths. The incident angle γ was selected to be 4°, 2°, 1° and 0.5°, and the corresponding detection depths are 1440 nm, 720 nm, 360 nm and 180 nm. These depth values are calculated by the critical angle model, which has been described in the previous work [43] . The Nd 2 Zr 2 O 7 maintains the pyrochlore structure when the incident angle decreases from 4° to 1.0°. However, some characteristic peaks of pyrochlore structure begin to disappear (2θ = 60.3° and 70.9°) when the incident angle drops to 0.5°. Compared with the original Nd 2 Zr 2 O 7 sample, the characteristic peaks of pyrochlore structure do indeed weaken after leaching, and even some weaker characteristic peaks disappear. This phenomenon demonstrates that the leaching experiment mainly caused slight damage to the crystal structure in the depth range of 180 nm. There is basically no effect in the depth range of more than 360 nm. This conclusion proves that Gd 2 Zr 2 O 7 nanocrystalline ceramics possess excellent aqueous stability in immobilizaiton radioactive wastes.

Conclusions
In this study, Nd 3+ was successfully incorporated into Gd 2 Zr 2 O 7 nanocrystalline powder by solvothermal method, and Gd 2 − x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0) ceramics with sub-micron grain size were prepared by SCF/QP sintering technology. Gd 2 − x Nd x Zr 2 O 7 (0.0 ≤ x ≤ 2.0) nanocrystalline powders are in all defective uorite structure. In addition, the doping of Nd does not cause changes in the grain size of powders. The XRD and Raman analysis of the sintered ceramics demonstrate that the sample has a defective uorite structure when x < 1.5 while the pyrochlore structure appears when x ≥ 1.5. Furthermore, the sample density gradually decreases (6.17-5.87 g·cm − 3 ) and the grain size increases from nanometer to submicrometer (79-265 nm) as the amount of Nd doping elevates. The Nd 2 Zr 2 O 7 leaching experiment exhibits its promising aqueous stability as the LR i values are mostly in the range of 10 − 5 -10 − 7 g•m − 2 •d − 1 .
The sample after leaching displays a slight crystal structure damage in the depth range of 180 nm at the sample surface layer. These results indicate that Gd 2 Zr 2 O 7 nanocrystalline ceramics have great potential for immobilizing trivalent actinide elements.
Declarations Figure 1 Crystal structures of pyrochlore and defective uorite: (a) one-eighth unit cell of pyrochlore structure, (b) defective uorite structure.