Synthesis, Characterization, and Crystal Structure Refinement of Lanthanum and Yttrium Substituted Polycrystalline 2M Type Zirconolite Phases: Ca_1-xM_xZrTi₂O₇ (M = Y, La and x = 0.2)

Abstract

Solid phases of zirconolite-2M with composition Ca_0.8M_0.2ZrTi₂O₇ (M = La, Y) have been synthesized through ceramic route and their structures refined to a satisfactory convergence using Rietveld analysis. Zirconolites crystallize in space group C2/c with Z = 8. The powder diffraction data of Ca_0.8Y_0.2ZrTi₂O₇ (CZTY) and Ca_0.8La_0.2ZrTi₂O₇ (CZTLa) have been subjected to General Structural Analysis System software to arrive at a satisfactory structure fit with Rp = 0.1128 and Rwp = 0.1805 for CZTY and Rp = 0.1178 and Rwp = 0.1874 for CZTLa, respectively. The unit cell parameters are a = 10.1708 (6) Ǻ, b = 6.2711 (4), and c = 11.2779 (6) Ǻ for CZTY and a = 11.2548 (6) Ǻ, b = 6.2601 (4), and c=11.2606 (7) Ǻ for CZTLa. Calculated interatomic distances and bond angles are in good agreement with their standard values. Particle size along prominent reflecting planes calculated by Scherrer’s formula ranges between 67 and 107 nm. The polyhedral (CaO₈, ZrO₇, and TiO₆/TiO₅) distortions and valence calculation based on bond strength analysis have been reported. The compositions of the zirconolites were determined using energy dispersive X-ray (EDAX) analysis. Cation site occupancies were determined by applied compositional constraints which were found consistent with the expected zirconolite-2M cation site occupancies.

1. Introduction

Borosilicate glass has long been the first choice of material for the immobilization of high-level radioactive wastes (HLW) due to its good glass-forming ability, chemical durability, radiation stability, and so forth. However, there is a potential risk that the conventional borosilicate glass waste forms partially crystallize either during annealing or during long-term storage in deep geological environment [1, 2]. Another proposed method for the long-term storage of high-level radioactive waste is the encapsulation of the waste in a ceramic matrix, followed by storage in an underground repository. One such ceramic, which has many of the properties thought to be favorable for waste storage, is zirconolite (CaZrTi₂O₇). Zirconolite is a rare accessory mineral found in a wide range of rock types and geological environments. The chemical composition of natural zirconolite can vary extensively, with the main substitutions involving lanthanides (Ln), actinides (Act), Nb, and Fe. Zirconolite is one of the three major phases in the synthetic ceramic “Synroc” which is evaluated as a host for high-level radioactive waste [3]. Ceramics made from pyrochlore, titania, and zirconia have been suggested as potential waste forms because they seem to be more stable than nuclear waste forms made from glass [4, 5]. Zirconolite was proposed as a potential nuclear waste form for the incorporation of lanthanides, plutonium, and minor actinides such as Np, Am, and Cm. Solid solutions containing zirconolite appear to be very stable with respect to hydrothermal alteration [6–9]. Zirconolite was identified as anion deficient superstructure derived from eight fluorite subcells with a monoclinic distortion. Zirconolites of the type CaZr_xTi_3-xO₇ are monoclinic with two-layer repeat sequence [10–13]. These are referred to as 2M type zirconolites. In 2M zirconolites two-layer repeat sequence consists of TiO₆ and TiO₅ polyhedra that form a hexagonal bronze motif (HTB) as 0 0 1 plane. The Ca and Zr ions form in sheets between the HTB layers ordered in rows along (1 −1 0) plane [14–16]. The Ca and Zr sites are of particular importance in radwaste effluent cation substitution because these sites are used to accommodate trivalent rare earth lanthanides. Several polytypes of zirconolite have been observed in natural and synthetic systems, including 2M, 3T, 3O, 4 M, and 6T polytypes [17]. Zirconolite solid solutions with the general formula CaZr_xTi_3-xO₇ (where ) crystallize in the 2M polytype [18–20]. The other polytypes like 3T, 3O, and 4 M do not occur for ideal zirconolite but crystallize with increasing levels of substitution of Ca and Zr by lanthanides and actinides. The site occupancies of zirconolites-CaZr_xTi_3-xO₇ over different values of x are determined by crystal structure refinement [21]. This present communication is basically aimed at synthesis, solid state reactivity, and crystallographic characterization of substituted zirconolite based ceramic materials, which have been identified as potential matrix for process development in technology of radioactive waste management. Though their applications in this area are well documented, their structure-property relationship is not yet investigated to the desired level; therefore, it is necessary to understand thoroughly the structural complexities of the interactions of the effluent cations vis-à-vis ceramic precursors. Powder diffraction data and GSAS software based calculations of cell parameters, crystal symmetry, isotropic and thermal parameters, interatomic distances, and other structural factors provide a good database for process development. It is in this context that the author has selected substituted zirconolite for crystallochemical investigations. Herein we reported synthesis and Rietveld refinement of lanthanum and yttrium substituted 2M type zirconolites and their structure models evolved from powder diffraction data.

2. Experimental

2.1. Ceramic Route Synthesis of Ca_1-xM_xZrTi₂O₇ (M = Y, La and x = 0.2) Phases

Calculated quantities of AR grade CaCO₃, ZrO_2, TiO_2, Y₂O₃, and La₂O₃ for the stoichiometry Ca_0.8M_0.2ZrTi₂O₇ (, La) were thoroughly mixed with about 10 mL of 1,2,3-propanetriol to form a semisolid paste. The glycerol paste was gradually heated initially at 600°C for 4 hours in a crucible. The mixture was reground to micron size, pressed into pellets, and sintered in a platinum crucible at 1300°C for 12 hours. The process was repeated to get a polycrystalline dense material.

2.2. Characterization

The powder X-ray diffraction pattern has been recorded at 2θ = 10°–90° on a Pan Analytical diffractometer (XPERT-PRO) using CuKα radiation at step size of 2θ = 0.017° and a fixed counting time of 5 sec/step. Scanning electron microscopy (SEM) has been carried out on an electron microscope system (HITACHI S-3400) equipped with Thermonoran ultra dry detector facility for energy dispersive X-ray (EDAX) analysis.

3. Results and Discussion

3.1. Rietveld Refinement and Crystallographic Model of the Phases

Yttrium and lanthanum substituted zirconolite (Ca_0.8Y_0.2ZrTi₂O₇ or CZTY and Ca_0.8La_0.2ZrTi₂O₇ or CZTLa) synthesized at 1100°C give monoclinic zirconolite phase with some traces of unreacted compounds (Figures 1(a) and 1(b)), whereas samples sintered at 1300°C result in the formation of single phase denser solid solutions that give prominent reflections at 2θ = 10–80° with maxima at 30.46 and 30.44 for CZTY and CZTLa, respectively (Figures 1(c) and 1(d)). The reflections match in intensity and position with the standard pattern of synthetic zirconolite [22]. The powder diffraction data of each phase consisting of 4867 reflections has been subjected to processing and analysis on computer software GSAS package which is capable of handling such data simultaneously for completion of structure refinement. Atomic coordinates for Ca, Zr, Ti, and O atoms were obtained from international tables for crystallography [23]. Zirconolite framework, ideally CaZrTi₂O₇, has five cation acceptor sites, Ca in 8 coordination (M8), Zr in 7 coordination (M7), and three Ti sites (M5) in 5 coordination and a pair of 6 coordinated (M6) sites [24]. Y₂O₃ and La₂O₃ can be incorporated as a solid solution in the crystal structure of zirconolite. The Y³⁺ and La³⁺ cations (ionic radius) are largely accommodated on Ca site. It is this important property of this material which has led to its potential application in radioactive waste management using “Synroc” technology. In the CaO–ZrO₂–TiO₂ system, the existence of stable phase with molar ratio 1 : 1 : 2 was first established by Coughanour et al. [25], followed by crystallographic work on the single crystal of CaZrTi₂O₇ which is reported in monoclinic symmetry having space group C2/c (#15), Z = 8. The reflection statistics generated by GSAS showed that reflections hkl and hOl were absent for h + k = 2n + 1 and l = 2n + 1, respectively, which suggested the possible space group C2/c in the compounds. The structure was determined by method of general least squares. Rietveld plot of the synthetic phases shows satisfactory fit between the experimental and the theoretical values of intensities at most of the data points (Figure 2). The refinement converged satisfactorily to Rp = 0.1128 and Rwp = 0.1805 for CZTY and Rp = 0.1178 and Rwp = 0.1874 for CZTLa sample, with reasonable values for refined cell parameters (Table 1) as evident in the normal probability plot between and which reflects a satisfactory linear fit (Figure 3). Determination of atomic positions in the unit cell of the compounds was carried out by Rietveld analysis. Computation was started with a structure model based on the parent zirconolite structure. Y and La ions were assumed in the structure model to occupy the Ca atomic site with occupancies adopted from analytical data. The relevant atomic parameters, site occupation factors, and isotropic thermal parameters obtained from their refinement are presented in Table 2.

Table 1 

Crystallographic data for type ZrTi₂O₇ (, La and ) ceramic phases at room temperature.

Table 2 

Refined atomic coordinate of ZrTi₂O₇ and ZrTi₂O₇ ceramic powders at room temperature.

(a)

(b)

(c)

(d)

Figure 1 

X-ray diffraction patterns of Ca1-xMxZrTi2O7 (M = Y, La and x = 0.2) samples sintered at 1100°C and 1300°C.

(a)

(b)

Figure 2 

Plots of the observed X-ray powder pattern from the zirconolite Ca1-xMxZrTi2O7 (M = Y, La and x = 0.2) together with the superimposed calculated pattern. A weighted difference plot is included to highlight the differences between observed and calculated patterns.

(a)

(b)

Figure 3 

Probability plots between and for polycrystalline (a) Ca0.8Y0.2ZrTi2O7 and (b) Ca0.8La0.2ZrTi2O7 ceramic samples.

The bond distances Ca–O, Zr–O, and Ti–O (Table 3) and bond angles O–Ca–O, O–Zr–O, and O–Ti–O (Table 4) have been calculated precisely. The M–O distances and O–M–O angles match with the earlier investigations on single crystal of zirconolite [26]. Figure 4 shows the PLATON projection of molecular structure depicting the interlinking of CaO₈, ZrO₇, and TiO₆/TiO₅ polyhedra in the framework of zirconolite structure. The Ca atoms are coordinated by 8 oxygen atoms at the corners of octahedra while Zr atoms are coordinated by oxygen at seven of the eight vertices of distorted cube and pair of Ti atoms by six oxygen atoms at the vertices of the octahedra of Ti(4) and Ti(6) and a trigonal prism Ti(5) (Figure 5). All these results are in accordance with crystallochemical expectations for zirconolite framework [27]. Table 5 lists the calculated valence of Ca, Zr, and Ti in the respective phase. Valence sum for Ca, Zr, and Ti shows that the Ca–O and Ti–O bonds are in relative compression while the Zr–O bonds are under tension in the large-cation layers. Particle size calculation using full wavelength at half maxima (FWHM) of major prominent reflections shows that the particle size of specimens ranges from 67 to 107 nm (Table 6). Compositional analysis of the materials by EDAX maps the elemental distribution of various elements (Figures 6(b) and 6(d)). The weight% and atomic% data of the elements have confirmed Y and La substitution at calcium site. The morphological examination of the specimens by scanning electron microscopy shows that the ceramic phases consist of crystals of ≈1–5 μm size (Figures 6(a) and 6(c)).

Table 3 

Interatomic distances () for ZrTi₂O₇ and ZrTi₂O₇ ceramic powders.

Table 4 

O–M–O bond angles in CZTY and CZTLa ceramic powders.

Table 5 

Bond valence calculation for substituted calcium zirconium titanate ceramic phases.

Table 6 

Particle size along prominent reflections of CZTY and CZTLa phases.

Figure 4 

PLATON projection of the molecular structure depicting the interlinking of CaO8, ZrO7 and TiO6/TiO5 polyhedra in the framework of zirconolite structure.

Figure 5 

DIAMOND view of stick and ball presentation of CZTY sample.

Figure 6 

(a) Scanning electron microphotograph of Ca0.8Y0.2ZrTi2O7 sample at magnifications of 7.5 k. (c) SEM of Ca0.8Y0.2ZrTi2O7 and (b) EDAX spectrums of Ca0.8Y0.2ZrTi2O7 and (d) EDAX spectrums of Ca0.8La0.2ZrTi2O7 phases.

4. Conclusions

Refinement of powder X-ray diffraction data shows that the solid solutions of Y and La zirconolite crystallize in the monoclinic (C12/c1 space group) system. Crystal data and structural parameters of the material have been refined to a satisfactory convergence with reasonable values of Rietveld parameters (Rp and Rwp). Calculations of Ca–O, Zr–O, and Ti–O bond distances and O–M–O bond angles quantify the extent of distortion in the CaO₈, ZrO₇, and TiO₆/TiO₅ polyhedron. Zirconolites have been identified as a potential material for immobilization and solidification of Y³⁺ and La³⁺ cations. Analytical evidence has been found to conclude that the yttrium and lanthanum are crystallochemically fixed in the ceramic matrix.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.