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Experimental phase diagram of the ZrO 2-Nd 2 O 3 pseudo-binary system. 1)
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The phase diagram of the ZrO2-Nd2O3 system has been characterized showing isolated two-phase regions for a cubic fluorite-type ZrO2 solid solution and Nd2Zr2O7 with a pyrochlore-type structure. A thermodynamic analysis was carried out to elucidate the origin of this interesting phase equilibrium. A compound energy model with the formula (Zr4+,Nd3+)...
Contexts in source publication
Context 1
... experimental studies on the phase relationships between ZrO 2 and representative sesquioxides have been carried out, but information on the ZrO 2 -Nd 2 O 3 phase diagram is rather limited. 1,2) Figure 1 shows an experimental phase diagram of the ZrO 2 -Nd 2 O 3 pseudo-binary system obtained using high- temperature X-ray analysis. 1) Cubic ZrO 2 is stabilized in the low-temperature range upon the addition of Nd, but further enrichment with Nd 2 O 3 leads to the appearance of the pyrochlore structure. ...
Context 2
... first detailed study of the ZrO 2 -Nd 2 O 3 system was carried out by Rouanet. 1) He described the high-temperature phase relationships above T ¼ 1800 C and the liquidus curve for this system using high-temperature X-ray measurements and thermal analysis. The phase diagram deduced by Rouanet is shown in Fig. 1. The liquidus curve was obtained from the cooling curves using thermal analysis, while the solidus curve was estimated. To prevent any contamination, the measurements were carried out in air without the use of crucibles, and the temperature was measured using optical pyrometry. The phase relationships above T ¼ 1800 C were observed ...
Citations
... The same phase transformations can also be achieved by doping ZrO 2 with trivalent lanthanides, Y 3 + , Al 3 + , Sc 3 + , and Ca 2 + , which often results in greatly increased ionic and electric conductivity [ 25 , 61-63 ]. For zirconia doped with Nd 2 O 3 and synthesized at 1500 °C ( Fig. 1 ), the tetragonal phase changes to the disordered defect fluorite phase at x ∼ 0.2 and ultimately to ordered pyrochlore at x ∼ 0.5 ( i.e. , Nd:Zr ratio is 1:1) [59] . The crystallographic study of this system has mostly been limited to XRD [ 25 , 33 , 36 , 40 ] and Raman spectroscopy [64] , which showed that the defect fluorite-to-pyrochlore transformation occurs over a narrow compositional range ( x ∼ 0.30 to 0.33) but gave limited information on the local atomic configuration. ...
... The pair distribution functions (PDFs), G ( r ), were obtained through the Fourier transform of S ( Q ) Fig. 1. Compositional slice at 1500 °C of the Nd 2 O 3 -ZrO 2 phase diagram adapted from Ohtani et al. [59] . The blue diamonds represent the compositions investigated in this study. ...
Pyrochlore complex oxides (A2B2O7) are an important class of fluorite-derivative ceramics with exceptional chemical and structural versatility which make them ideal model systems for studying disordering mechanisms over a range of spatial scales. Neutron total scattering methods were used to analyze the structural behavior in the non-stoichiometric series NdxZr1-xO2-0.5x (0.5 ≤ x ≤ 0.23) as a function of Nd content, x. Characterization of the structure functions using Rietveld refinement and the pair distribution functions with small-box refinement reveal complex disordering pathways; the average, long-range phase changes over a very narrow compositional range from pyrochlore (Fd3¯m) to defect fluorite (Fm3¯m) through full randomization of the cation and anion sublattices at x ∼ 0.31, while the local, short-range structure continuously adopts a weberite-type atomic arrangement (C2221). Comparison to a previously studied Ho2Ti2-xZrxO7 solid solution series reveals how changes in chemical composition and stoichiometry modify defect formation and determine how disordering progresses across different length scales in pyrochlore oxides.
... Table 2. Summary of the reported structural phase transition temperatures and melting points of some A 2 Zr 2 O 7 oxides. The order to disorder transformation temperatures were either obtained experimentally [28,31] or predicted by calculations [30]. A large number of phase diagrams can be found in the literature for the ZrO 2 -A 2 O 3 systems and the published results vary considerably, however the melting temperatures in all cases are reported to be higher than 2000˝C for the A 2 Zr 2 O 7 oxides. ...
... Pyrochlore [28,31]. Therefore, it could prove difficult to prepare single crystals of these materials with a pyrochlore structure, due to the relatively small difference between the melting point and the structural transition temperature (see Table 2 and References [25,28]). ...
... Therefore, it could prove difficult to prepare single crystals of these materials with a pyrochlore structure, due to the relatively small difference between the melting point and the structural transition temperature (see Table 2 and References [25,28]). A key step in the growth of these materials is to determine their crystallographic structure, to customise and establish the optimum conditions of synthesis to obtain the desired crystal phase [22,31,35]. ...
This article reviews recent achievements on the crystal growth of a new series of pyrochlore oxides—lanthanide zirconates, which are frustrated magnets with exotic magnetic properties. Oxides of the type A 2 B 2 O 7 (where A = Rare Earth, B = Ti, Mo) have been successfully synthesised in single crystal form using the floating zone method. The main difficulty of employing this technique for the growth of rare earth zirconium oxides A 2 Zr 2 O 7 arises from the high melting point of these materials. This drawback has been recently overcome by the use of a high power Xenon arc lamp furnace for the growth of single crystals of Pr 2 Zr 2 O 7 . Subsequently, large, high quality single crystals of several members of the zirconate family of pyrochlore oxides A 2 Zr 2 O 7 (with A = La → Gd) have been grown by the floating zone technique. In this work, the authors give an overview of the crystal growth of lanthanide zirconates. The optimum conditions used for the floating zone growth of A 2 Zr 2 O 7 crystals are reported. The characterisation of the crystal boules and their crystal quality is also presented.
... A study of the phase diagram of Nd 2 O 3 -ZrO 2 shows that the pyrochlore oxide, Nd 2 Zr 2 O 7 , melts congruently above 2000 • C, although the melting point was not established [29]. The order-to-disorder transition in Nd 2 O 3 -ZrO 2 occurs at 2300 • C [15,30], raising the possibility that it may be difficult to prepare Nd 2 Zr 2 O 7 single crystals of the pyrochlore structure due to the relatively small difference between the melting point and the structural transition temperature. When preparing this material it is therefore essential to establish the crystal structure of the Nd 2 Zr 2 O 7 samples and to adjust the conditions of synthesis in order to prepare the desired phase, as has been emphasized in previous reports [30,31]. ...
... The order-to-disorder transition in Nd 2 O 3 -ZrO 2 occurs at 2300 • C [15,30], raising the possibility that it may be difficult to prepare Nd 2 Zr 2 O 7 single crystals of the pyrochlore structure due to the relatively small difference between the melting point and the structural transition temperature. When preparing this material it is therefore essential to establish the crystal structure of the Nd 2 Zr 2 O 7 samples and to adjust the conditions of synthesis in order to prepare the desired phase, as has been emphasized in previous reports [30,31]. ...
We report structural and magnetic properties studies of large high-quality single crystals of the frustrated magnet Nd2Zr2O7. Powder x-ray diffraction analysis confirms that Nd2Zr2O7 adopts the pyrochlore structure. Room-temperature x-ray diffraction and time-of-flight neutron-scattering experiments show that the crystals are stoichiometric in composition with no measurable site disorder. The temperature dependence of the magnetic susceptibility shows no magnetic ordering at temperatures down to 0.5 K. Fits to the magnetic susceptibility data using a Curie-Weiss law reveal a ferromagnetic coupling between the Nd moments. Magnetization versus field measurements show a local Ising anisotropy along the ⟨111⟩ axes of the Nd3+ ions in the ground state. Specific heat versus temperature measurements in zero applied magnetic field indicate the presence of a thermal anomaly below T∼7 K, but no evidence of magnetic ordering is observed down to 0.5 K. The experimental temperature dependence of the single-crystal bulk dc susceptibility and isothermal magnetization are analyzed using crystal field theory and the crystal field parameters and exchange coupling constants determined.
... Furthermore, even though the oxide compound is the isomeric pyrochlore, Nd 2 Zr 2 O 7 , the phase transition from pyrochlore to fluorite occurs above w2600 K (Wang et al., 2007). Such phase transitions depending on composition and temperature are different for different A cation (Wang et al., 2007;Ohtani et al., 2005) and are also related to the phase stability under the irradiation field (Sickafus et al., 2000). ...
... were shown in Fig. 5. According to the phase diagram of NdO 1.5 eZrO 2 (Wang et al., 2007;Ohtani et al., 2005), fluorite (ZrO 2 rich) þ pyrochlore coexists at y ¼ 0.3 and 0.4, pyrochlore solely exist at y ¼ 0.5, and fluorite (NdO 1.5 rich) þ pyrochlore coexists at y ¼ 0.6. For y ¼ 0.7, the phase diagram is different between them. ...
... For y ¼ 0.7, the phase diagram is different between them. The phase diagram evaluated by Ohtani et al. (2005) exhibits coexistence of fluorite and pyrochlore phases, whereas that obtained by Wang et al. (2007) indicates mono-phase fluorite. In the XRD measurement, we could not observe pyrochlore diffraction peaks, e.g. ...
For AO(1.5)-MoO2 and -ZrO2 equimolar systems (A = La, Nd, Sm, Gd, Dy, Y, Er, Lu), oxide compounds samples were prepared by solid-state sintering at 1673 K-1873 K in argon and air, respectively. In addition, oxide samples of yNdO(1.5) - (1 - y)ZrO2 with y = 0.3-0.7 were prepared to investigate the pyrochlore-fluorite (P-F) phase transition. X-ray diffraction measurements clarified that molybdenum oxides formed cubic-pyrochlore phase except for A = La, and zirconate pyrochlores formed for A = La, Nd, Sm, Gd, whereas other zirconium oxides exhibited the defect-fluorite structure. With decreasing ionic radius of A(3+), the broadening of Raman bands were observed for both pyrochlore systems. This might be due to the decrease of energy for defect cluster formation, i.e. cation antisite reaction and oxygen Frenkel defect formation. Especially for molybdate pyrochlores, it might be also attributed to the fact that the ionic charge of molybdenum was likely to be changeable. The P F phase transition of the yNdO(1.5) (1 - y)ZrO2 system was clearly confirmed by Raman spectroscopy, and the changes in band shape were influenced mainly by the oxygen environment around Zr, which was elucidated by molecular dynamics calculations.
... The equilibrium phase diagram of Ln 2 O 3 -ZrO 2 has been investigated by many researchers over a number of years. [1][2][3][4][5] The allied structures of the phases in these systems change from fluorite-type (BO 2 ) to the structure of the A 2 O 3 -type with increase in lanthanide content. At lower temperatures, (y1200 to 1500 uC) pyrochlore phases (Ln 2 Zr 2 O 7 ) form based on the ordered cubic fluorite. ...
The energetics of the order-disorder phase transformation in the binary oxide system, Eu2O3–ZrO2, is studied by powder X-ray diffraction and high temperature drop solution calorimetry. The nanocrystalline defect fluorite phase of Eu2Zr2O7 is synthesized on crystallization of an amorphous precursor from aqueous precipitation. The defect fluorite transforms to an ordered pyrochlore above 1200 °C. Aerodynamic levitation combined with laser heating is used to prepare coarse defect fluorite, which is otherwise impossible by conventional synthesis techniques. Formation enthalpies from oxides are −62.4 ± 2.6 and −24.6 ± 3.7 kJ mol−1 for the pyrochlore and defect fluorite phase, respectively. The transformation enthalpy from pyrochlore to defect flourite in the coarse sample is 37.8 ± 3.1 kJ mol−1 at 25 °C. The enthalpy of water vapor adsorption on the surface of the nanocrystalline defect fluorite Eu2Zr2O7 is −75 ± 2.5 kJ mol−1 H2O for coverage of 9.5 ± 0.8 H2O/nm2. The calculated surface enthalpies for the anhydrous and hydrous surfaces of defect fluorite Eu2Zr2O7 are 1.47 ± 0.13 and 1.01 ± 0.15 J m−2, respectively.
... Stuffing more A cations onto the B-site in pyrochlores can lead to disordering of the separate corner-sharing tetrahedral networks. This disordering, or cation mixing, typically occurs only in pyrochlores with larger B cations, such as Zr 4+ .24252627 For the rare earth titanate pyrochlores, due to the large difference in size between Ln 3+ and Ti 4+ ions, mixing of cations in the Ln 2 Ti 2 O 7 stoichiometry has only been reported under conditions such as ball milling, ion irradiation, or reduction with CaH 2 .282930 ...
Synthesis and crystal structures are described for the compounds Ln2(Ti2-xLnx)O7-x/2, where Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu, and x ranges from 0 to 0.67. Rietveld refinements on X-ray powder diffraction data indicate that in Tb and Dy titanate pyrochlores, extra Ln3+ cations mix mainly on the Ti4+ site with little disorder on the original Ln3+ site. For the smaller rare earths (Ho-Lu), stuffing additional lanthanide ions results in a pyrochlore to defect fluorite transition, where the Ln3+ and Ti4+ ions become completely randomized at the maximum (x=0.67). In all of these Ln-Ti-O pyrochlores, the addition of magnetic Ln3+ in place of nonmagnetic Ti4+ adds edge sharing tetrahedral spin interactions to a normally corner sharing tetrahedral network of spins. The increase in spin connectivity in this family of solid solutions represents a new avenue for investigating geometrical magnetic frustration in the rare earth titanate pyrochlores. Comment: 25 pages, 7 figures, submitted to J. Solid State Chem
Rapid preparation of ceramic waste form at low temperature is of great engineering significance for immobilization of high-level radioactive waste. Herein, we demonstrated that Nd-doped zirconia ceramics can be rapidly prepared at low sintering temperature with the assistance of hydrothermal assisted sol-gel method. The effects of sintering temperature and Nd content on the phase and microstructure evolutions of the obtained ceramics were studied. With the increasing content of Nd, the phase of ZrO2 transformed from monoclinic to cubic at first, then the pyrochlore Nd2Zr2O7 phase appeared, and finally hexagonal neodymia solid solution structure formed. Moreover, the grain size and compactness of the obtained ceramics increased with the increment in sintering temperature. The successful accommodation of Nd within the ZrO2 lattice was revealed by the detected increase of cell parameters. This work demonstrated that the hydrothermal assisted sol-gel process can be employed to effectively synthesize the Nd-doped zirconia ceramics as potential hosts for HLW immobilization.
Since attempts were made to fabricate translucent polycrystalline alumina in the 1960s, considerable effort has been made to develop new transparent ceramics and improve their properties. The application of transparent ceramics nowadays is extensive, covering many areas. This review discusses the fundamentals of polycrystalline transparent ceramics, including a brief history, requirements for transparency, and important properties along with their feasible applications. In particular, details of γ-AlON are included as an example based on the constant anion lattice model and the authors’ results to help the readers’ understanding of transparent ceramics. Finally, the current research trend is explained to anticipate future research directions on transparent polycrystalline ceramics.
