Features of formation and evolution of crystal and local structures in nanocrystalline Ln2Zr2O7 (Ln = La - Tb)

The formation of nanocrystal powders of Ln2Zr2O7 (Ln=La - Tb) compounds upon calcinations up to 1500°C has been investigated by using of X-ray absorption fine structure spectroscopy combined with X-ray powder diffraction and Raman spectroscopy. It was found that the fluorite-pyrochlore phase transition occurs at calcination temperature in the range of 900-1200 °C (except Tb, T~1500 °C). The appearance of the ordered pyrochlore structure at calcination temperatures above 900 °C was fixed as an observation of superlattice peaks in diffraction and splitting of the first oxygen coordination shell in the local lanthanides environment in EXAFS and Raman spectra. In addition the scheme of fluorite-pyrochlore transition temperature dependence on radii of lanthanide cations was constructed.

The aim of the present work is to study the crystallization processes of an amorphous precursor upon isothermal calcination and to study the specific features of fluorite -pyrochlore phase transition in Ln 2 Zr 2 O 7 (Ln=La-Tb) (γ=1.44-1.61), using combined X-ray diffraction (XRD), X-ray absorption fine structure spectroscopy (EXAFS) at synchrotron radiation and Raman spectroscopy.  [11][12][13][14]. X-ray diffraction investigations were carried out at the "Structural Materials Science" beamline of the Kurchatov synchrotron radiation source. The measurements were implemented in the Debye-Scherrer geometry (transmission mode) using an environment optimized for high quality experimental data: the distance the sample -detector -200 mm; beam size 200 200 mm; the exposure time of 15-30 minutes, the wavelength λ = 0.68886 Å. Six series of Ln 2 Zr 2 O 7 compounds with Ln=La-Tb were examined. Each series included 8-10 samples obtained by calcination of initial precursor at different temperatures in a temperature range 600 -1500 o C. The Rietveld full profile analysis of X-ray diffraction patterns was performed with the Jana2006 software [15].

Experimental section
EXAFS spectra of Ln 2 Zr 2 O 7 (Ln=La-Tb) above the L 3 -Ln (for La 5483 eV) and K-Zr (17998 eV) absorption edges, were measured at i811 beamline of MAXLab (Lund, Sweden) and P65 beamline of DESY (Gamburg, Germany) in a transmission mode at room temperature. Processing and modeling of XAFS-spectra were carried out using the VIPER [16] and Demeter [17] program packages. The amplitudes and phases of photoelectron back scattering in the local environment of zirconium, REE and hafnium atoms were calculated by program FEFF-8.20 [18] using the crystal structure parameters of pyrochlore and fluorite, taken from the diffraction data for Ln 2 Zr 2 O 7 (where Ln= La-Tb).

Results and discussion
XRD study showed that precursors La 2 O 3 ·2ZrO 2 ·12,6H 2 O calcinated at temperatures below 800°C have the X-ray amorphous structure. An increase in the atomic number along the La-Tb series leads to a decrease in the crystallization temperature. However, structure can't be considered as completely disordered, since from one to several broad peaks were observed in the region 13-21 o of double Bragg angle. The increasing of the calcination temperature above 800 °C leads to appearance of about ten Xray reflections on XRD patterns for all samples, which corresponds to the fluorite structure formation. Besides at the annealing temperature above 1000°C we observed the gradual narrowing of the diffraction reflexes and the appearance of superlattice peaks (Figure 1), which point to the increase of the structure ordering. This is also correlated with increase of crystallite size and decrease of microstrain values obtained from the analysis of the diffraction patterns (see Figure 2).
These superstructure reflections are substantially wider than the main peaks of the fluorite-type structure. It suggests that cationic ordering with formation of the pyrochlore phase upon calcination occurs inside the separate regions (nanodomains) distributed over well-crystallized fluorite matrix.
Using EXAFS spectroscopy we have studied the evolution of the Ln 2 Zr 2 O 7 (Ln=La-Tb) local atomic structure upon the gradual heat treatment from initially amorphous precursors to well-crystallized powders. Figure 3 shows the Fourier transform moduli of L 3 -Nd EXAFS-functions χ(k)k 2 of Nd 2 Zr 2 O 7 powders calcinated at different temperatures. The first maximum at R~1.9 Å corresponds to the nearest oxygen coordination shell around Nd. For the sample calcination at 600 o C, which is still amorphous and for the samples calcination at temperatures <1000 o C with the distinct fluorite structure, the shoulder at ~2.2 Å on the right slope of the main peak is quite small. But with an increase of calcination temperature the oxygen coordination shell starts to split into two components.
The observed splitting peak corresponds to two shorter Nd-O(2) and six longer Nd-O(1) distances. The second large features at 3.1 Å and 3.5 Å include contributions from Nd-Zr and Nd-Nd interatomic bonds. Amplitude of this features increases significantly with calcination temperature and the position drifts to larger radius (Figure 3). This indicates gradual ordering of the overall crystal structure. At calcination temperature exceeding 1300 o C the peak shape and amplitude substantially stabilize, which corresponds to the end of the pyrochlore structure formation. Analysis of EXAFS data showed that spectra for samples prepared at calcination temperature <800°C are well reproduced by the model of fluorite structure (Figure 4). However, at higher calcination temperatures the fluorite structure model can no longer provide an adequate simulation of EXAFS-function and the analysis was conducted using the pyrochlore structure model. Thus above 1000 °C there is clearly observed splitting of the Nd-O bond length (Figure 4). This directly points to the restructuring of the local oxygen environment of rare-earth ions as a result of the observed fluorite-pyrochlore phase transition.    Also, we have conducted a Raman spectroscopy study for each series of samples Ln 2 Zr 2 O 7 . The effect of the type of rare-earth ions on the Raman spectra is represented in Figure 5. As seen in Figure 5 Raman mode peaks for compounds from La to Tb become wider and shift to higher wavenumbers. Thus the most intense F 2g mode (~305 cm -1 ) in the Raman spectra of Tb 2 Zr 2 O 7 is approximately one order of magnitude broader than that of La 2 Zr 2 O 7 .
The results of our complex study of the phase transition in the Ln 2 Zr 2 O 7 series using EXAFS, XRD and Raman spectroscopy methods are summarized in Figure 6. EXAFS curve shows the beginning of the fluorite-pyrochlore transition, while Raman data point to the end. The initial crystallization of amorphous precursors leads to the formation of nanocrystalline oxides with the fluorite structure, regardless the Ln 3+ cation type. The crystallization onset temperature is lower for Ln 2 Zr 2 O 7 with a smaller Ln 3+ cation radius (or γ ratio). In contrast, the fluorite-pyrochlore phase transition temperature becomes higher across the Ln 2 Zr 2 O 7 series.

Conclusion
Local and crystal structure evolution of Ln 2 Zr 2 O 7 (Ln = La-Tb) upon the initial X-ray amorphous precursors calcination in the temperature range 600 -1600 °C has been studied by using a combination of X-ray powder diffraction, XAFS-spectroscopy at synchrotron radiation and Raman spectroscopy. It was found that the fluorite-pyrochlore phase transition occurs at calcination temperature in the range of 900-1200 °C. The appearance of the ordered pyrochlore structure at calcination temperatures above 900 °C was fixed as an observation of superlattice peaks in diffraction and splitting of the first oxygen coordination shell in the local lanthanides environment in EXAFS and Raman spectra. Although X-ray diffraction data does not show crisp signs of superstructure in Tb 2 Zr 2 O 7 , the results of the EXAFS data analysis show traces of pyrochlore ordering also in this boundary compound calcinated at 1500 °C.
Characteristic temperatures for the ordering in the cationic and anionic sublattices are slightly different (lower for the anionic sublattice) but both temperatures increase with decreasing of Ln 3+ cation radius (or γ ratio). It was shown that the phase transition from fluorite to pyrochlore structure for Ln = La-Tb proceeds via the intermediate emergence and gradual growth of pyrochlore-type nanodomains in a fluorite matrix. In addition the scheme of fluorite-pyrochlore transition temperature dependence on radii of lanthanide cations was constructed.