Phase evolution and microstructural studies in CaZrTi2O7 (zirconolite)–Sm2Ti2O7 (pyrochlore) system

Available online at www.sciencedirect.com

ScienceDirect Journal of the European Ceramic Society 34 (2014) 4373–4381

Phase evolution and microstructural studies in CaZrTi2O7 (zirconolite)–Sm2Ti2O7 (pyrochlore) system M. Jafar a , P. Sengupta b , S.N. Achary a,∗ , A.K. Tyagi a a

b

Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India

Received 7 February 2014; received in revised form 30 June 2014; accepted 1 July 2014 Available online 19 July 2014

Abstract From the characterization of a series compositions with general stoichiometry as Ca1−x Zr1−x Sm2x Ti2 O7 (0.00 ≤ x ≤ 1.00), the phase evolution between zirconolite (CaZrTi2 O7 ) and pyrochlore type Sm2 Ti2 O7 has been elucidated. All the compositions were prepared by high temperature solid state reaction and characterized by powder X-ray diffraction (XRD) and electron probe for microanalyses (EPMA). Three major phase fields, namely two layer (2-M) or four layer (4-M) monoclinic zirconolite and cubic pyrochlore structure types were observed in this system. In addition, a feeble amount of perovskite type phase is found to coexist with zirconolite phase. 4-M zirconolite phase is observed as single phase field at the composition with x = 0.30 and 0.35, while cubic pyrochlore phase is observed as single phase at the compositions with x ≥ 0.60. Further, the composition and microstructure of coexisting phases are verified by back scattered electron image and EPMA studies. © 2014 Elsevier Ltd. All rights reserved. Keywords: Zirconolite; Pyrochlore; Nuclear waste immobilization; SYNROC; Phase relation

1. Introduction Immobilization of minor actinides like Np, Am etc., present within high level nuclear waste (HLW) is one of the major concerns of the nuclear industry due to their higher radiotoxicity and long half-life.1 Thus the separation of these minor actinides and their immobilization in durable and stable matrices has been of prime importance. Borosilicate glasses are considered as host matrices for immobilization of HLW.1,2 However, the low solubility of minor actinides in glass matrix and the metastable nature of glasses are the major limitations.3,4 In order to overcome such problems, Ringwood proposed assemblages or composites of mineral analogue titanates, named as SYNROC, as a potential alternate host matrix for HLW immobilization.5 Major components considered in SYNROC formulations are: zirconolite, perovskite, pyrochlore, hollandite and rutile structure type phases.5–9 Potentials of these phases and in particular

∗

Corresponding author. Tel.: +92 22 25592328; fax: +91 22 25505151. E-mail address: [email protected] (S.N. Achary).

http://dx.doi.org/10.1016/j.jeurceramsoc.2014.07.001 0955-2219/© 2014 Elsevier Ltd. All rights reserved.

zirconolite and pyrochlore type phases for such applications have been reported in several studies.5–9 The zirconolite and pyrochlore type phases form the major components in SYNROC compositions due to their abilities to accommodate ions having widely different oxidation states and ionic radii as well as other properties like higher leach resistance, higher radiation and thermal stabilities.6–14 Zirconolite (CaZrTi2 O7 ) has a layered structure, which is formed by stacking of layers of edge shared Ti O polyhedra (TiO6 and TiO5 ) and layers of Ca2+ and Zr4+ ions.15–17 Due to the isovalent nature, Zr4+ and Ti4+ ions can have mutual substitution and thus the structure can sustain a wider range of stoichiometry as CaZrx Ti(3−x) O7 (where x can vary from 0.80 to 1.37).15,16,18 Further the largely different coordination polyhedra around the cations render zirconolite structure amenable to accommodate larger cations like rare-earths, actinide and alkaline earth ions as well as smaller cations, like transition metal ions. Depending on the composition and nature of substitution, zirconolite structure can transform to different polytypes having closely similar structural arrangements.17,19,20 The well known polytype of zirconolite structure like zirconolite-3T (trigonal),

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zirconolite-3O (orthorhombic), zirconolite-4M (monoclinic), etc. are stabilized by substitution of rare-earth ions at Ca2+ and Zr4+ sites of zirconolite composition.17,19,20 The pyrochlore type structure has a general composition A2 B2 O7 , where the A and B cations have eight and six coordinated polyhedra, respectively.20 This structure is closely related to fluorite type structure except that it has anion deficiency and ordered arrangement of cations. Due to significant amount of unoccupied anion sites and wider choice of cations, the pyrochlore structure can also accommodate larger amounts diversified minor actinides of the HLW.8,13,14 Depending on the ionic radii of the A3+ ions (A = trivalent rare-earth ions), the A2 Ti2 O7 type titanates can have either cubic pyrochlore type structure or perovskite related monoclinic structure.8,20 The rare earth titanates with A3+ ions having radii larger than Sm3+ form perovskite related structures while with other rare-earth ions they form cubic pyrochlore type structures. The structures of zirconolite and pyrochlore phases have mutual compatibility as well as they are compatible with structures of other ceramic phases considered in SYNROC formulations. However, inter-diffusion of cations among different phases may occur under the SYNROC preparation conditions.5,8,20 In general the preparations of SYNROCs are carried out by high temperature reaction of precursor materials of the constituent elements. In order to prepare denser ensemble of ceramic phases, often hot iso-static (HIP) pressure conditions are adopted. Preparation temperature is usually varied from 1200 to 1400 ◦ C while the pressure is mostly maintained within 50–100 MPa. Although this pressure is not high enough to modify the pressure temperature phase diagrams, but useful for densification and faster reaction of the materials. The equilibrium phases formed by the high temperature reaction are retained in the HIP conditions. But the diffusion of cations in between the phases may occur in this preparation conditions. Thus the solid solubility limit and mutual exchange of ions among the component phases under preparative conditions are the important information desired for their practical applications. In view of this, phase relations between the component phases are being investigated to determine the solubility limits of lanthanide and actinide as well as to study structural stabilities of the lanthanide or actinide bearing phases. Several attempts to obtain monophasic lanthanide or actinide doped zirconolite and pyrochlore type phases have been reported in literature.6,8,9,18,19,21,22 Moreover lanthanides being non-radioactive and surrogates for minor actinides, they can be used to obtain phase and structural relations closely similar to the actinides. Caurant et al.21 have prepared single phase neodymium substituted zirconolite type CaHfTi2 O7 only by co-doping of Al3+ ions at Ti4+ sites. Jafar et al.22 have reported simultaneous substitution of Nd3+ at the Ca2+ and Zr4+ sites and indicated preferential substitution of Nd3+ at Ca2+ sites compared to Zr4+ sites. In the present study the phase evolution from zirconolite type CaZrTi2 O7 to pyrochlore type Sm2 Ti2 O7 structure has been investigated by XRD and EPMA analyses of a series of compositions in between them. The details of the structure and composition of phases formed in between CaZrTi2 O7 –Sm2 Ti2 O7 are explained in this manuscript.

2. Experimental Series of compositions with stoichiometry as Ca1−x Zr1−x Sm2x Ti2 O7 (0.0 ≤ x ≤ 1.0) were synthesized by solid state reaction of analytical grade CaCO3 (Sigma-Aldrich, St. Louis, MO, USA, purity >99.0%), ZrO2 (Sigma-Aldrich, purity 99.0%), Sm2 O3 (Indian Rare Earths Ltd., Mumbai, India, purity 99.9%) and TiO2 (Sigma-Aldrich, purity ≥99.5%). Apart from CaCO3 , all the oxides were heated overnight at 900 ◦ C prior to weighing. CaCO3 was dried at 200 ◦ C for 4 h. Appropriate amounts of reactants were homogenized in acetone media using agate mortar and pestle and then pressed into pellets of 1 in. diameter and 3–4 mm height. The pressed pellets were then heated at 1200 ◦ C for 24 h and then cooled to temperature. The obtained products were crushed into powder, rehomogenized, pelletized and then heated again at 1200 ◦ C for another 24 h. The same heating condition was repeated once more. All the pellets of the products were again crushed, homogenized and pressed into smaller pellets (1 cm diameter and 2–3 mm thickness). These pellets were heated again at 1300 ◦ C for 24 h where the formation of single phase CaZrTi2 O7 was observed. The typical heating and cooling rates for all heating schedules are 2 ◦ C/min. Sintered alumina plates were used as container for the reactions. The products obtained after each heat treatment and final sintering were characterized by powder X-ray diffraction technique. The XRD patterns of the samples were recorded on a PANalytical X-Pert Pro X-ray diffractometer using CuK␣ ˚ radiation, over the two theta range (λ = 1.5406 and 1.5444 A) ◦ of 10–70 with step width and time of 0.02◦ and 1.20 s, respectively. The observed powder XRD patterns for some representative compositions are shown in Fig. 1. The XRD patterns were analyzed by Rietveld refinement and Le Bail refinement methods using Fullprof-2k software package.23 Electron Probe Micro-Analyzer (CAMECA-SX 100) was used for imaging and to determine elemental composition of the phases. All the images were recorded by using the backscattered electrons. The sintered pellets of the product were polished by using different grades of emery papers (initially coarse and then finer) and then polished on lapping wheel by using 0.5 ␮m grain sized diamond paste. The polished samples were then cleaned in ultra˚ thick Au sonic bath. The cleaned pellets were coated with 100 A film to prevent charge accumulation under electron irradiation. The electron microscope was operated 20 kV for both imaging and X-ray analyses. Electron beam current of 4 nA and 20 nA, respectively were used for recording the images and X-ray analyses. The quantification of elemental compositions was carried out by using pure phase standards. The intensities of the emitted X-ray from standard phases and samples were corrected by Pouchou and Pichoir (PAP) depth distribution matrix correction method.24 3. Results and discussion The progress of the formation of phases in Ca1−x Zr1−x Sm2x Ti2 O7 (0.0 ≤ x ≤ 1.0) compositions was followed by analyzing their XRD patterns recorded after each

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Table 1 Refined structural parameters of CaZrTi2 O7 composition. Atoms

Wyc.

x

y

Ca1 Zr1 Ti1 Ti2 Ti3 O1 O2 O3 O4 O5 O6 O7

8f 8f 8f 8f 4e 8f 8f 8f 8f 8f 8f 8f

0.3723(9) 0.1228(4) 0.2466(8) 0.4677(14) 0 0.302(2) 0.479(2) 0.214(2) 0.400(2) 0.701(2) 0.000(2) 0.117(2)

0.1264(14) 0.1262(6) 0.1329(17) 0.0556(16) 0.1376(19) 0.143(4) 0.152(3) 0.129(3) 0.155(3) 0.187(3) 0.149(3) 0.046(3)

z

Occ.

0.4989(7) −0.0248(3) 0.7448(6) 0.2537(24) 0.25 0.283(2) 0.111(2) 0.578(2) 0.718(2) 0.589(2) 0.426(2) 0.788(2)

Monoclinic, Space group: C2/c (No. 15). ˚ a = 12.4385(3), b = 7.2674(2), c = 11.3679(3) A, ˚ 3. V = 1010.22(5) A ˚ 2. Bov = 0.02(7) A Rp : 12.1%, Rwp : 16.4%, χ2 : 1.51.

Fig. 1. Representative XRD patterns of Ca1−x Zrx Sm2x Ti2 O7 compositions.

heat treatment. The XRD pattern of the composition with x = 0.00 (i.e. CaZrTi2 O7 ) obtained after heating two times at 1200 ◦ C (each for 24 h) shows weak reflections attributable to CaTiO3 type perovskite phase and fluorite type phase along with intense reflections due to zirconolite phase. Though the intensities of the peaks attributable to CaTiO3 decrease systematically with increasing soaking duration at this temperature but they persist even after 72 h heating. However, these weak peaks were completely absent in the XRD pattern of the sample obtained after final heating at 1300 ◦ C. Typical XRD patterns of x = 0.00 compositions recorded after different heating schedule are provided as supplementary data to this manuscript (Fig. A of Supplementary data). The comparison of the XRD patterns of the composition with x = 0.00 (i.e. CaZrTi2 O7 ) obtained after heating at different temperature confirm the formation of zirconolite phase through an intermediate CaTiO3 type perovskite phase. This observation is similar to the observation of the previous study on CaZrTi2 O7 –Nd2 Ti2 O7 system.22 Thus the completion of the reaction is ensured after heating the compositions at 1300 ◦ C. Similar analyses of the XRD patterns of the other end member, i.e. the composition with x = 1.00, show the complete formation of cubic pyrochlore type structure (Sm2 Ti2 O7 ) in 24 h at 1200 ◦ C. The XRD patterns of this sample remain unchanged in the subsequent heat treatments up to 1300 ◦ C. Thus the phase evolution between CaZrTi2 O7 and Sm2 Ti2 O7 is evaluated analyzing the phases in Ca1−x Zr1−x Sm2x Ti2 O7 (0.0 ≤ x ≤ 1.0) compositions subjected to identical heat treatments, i.e. as 1200 ◦ C (24 h) + 1200 ◦ C

1 1 1 0.5 1 1 1 1 1 1 1 1

β = 100.556(3)◦ ,

(24 h) + 1200 ◦ C (24 h) + 1300 ◦ C (24 h), and the details of phases are explained subsequently. Further detailed characterizations of the end member compositions (i.e. with x = 0.00 and 1.00) were carried out by Rietveld refinement of their corresponding powder XRD data. The reported structural details of the two layer monoclinic zirconolite CaZrTi2 O7 15–17,19,22 and pyrochlore type Sm2 Ti2 O7 20,25 phases were used as initial models for Rietveld refinements of the powder XRD data. Since the phases present in the studied system have a number of cations with wider differences in atomic number (Z) as well as a number of crystallographically distinguishable anion positions, no reliable values of isotropic thermal parameters for individual atoms could be obtained by Rietveld refinements. Thus only one overall thermal parameter is used in the refinement. Refined structural parameters for these two compositions are given in Tables 1 and 2, respectively. The refined structural parameters are close to the reported structural parameters for zirconolite type CaZrTi2 O7 16 and pyrochlore type Sm2 Ti2 O7 .20 The refined unit cell parame˚ ters for the CaZrTi2 O7 phase (at x = 0.00) are: a = 12.4385(3) A, ˚ and β=100.556(3)◦ ; for cubic b = 7.2674(2) Å, c = 11.3679(3) A ˚ pyrochlore type Sm2 Ti2 O7 (i.e. at x = 1.00) is: a = 10.2321(2) A, which are in agreement with earlier reported values [JCPDSPDF-84-0164, Refs. 16, 17, 22, 25]. The observed and calculated powder XRD patterns for the x = 0.00 and 1.00 compositions are shown in Fig. 2. Table 2 Refined structural parameters of Sm2 Ti2 O7 composition. Atoms

Wyc.

x

y

z

Occ.

Sm1 Ti1 O1 O2

16c 16d 8a 48f

0 0.5 0.125 0.432(2)

0 0.5 0.125 0.125

0 0.5 0.125 0.125

1 1 1 1

Cubic, Space group: Fd3m (No.227). ˚ V = 1071.26(4) A ˚ 3 ; Bov = 0.01(3) A ˚ 2/ a = 10.2321(2) A, 2 Rp : 16.8%, Rwp : 24.5%, χ : 1.46.

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Fig. 2. Final refinement plots of the powder XRD data of CaZrTi2 O7 (x = 0.00) and Sm2 Ti2 O7 (x = 1.00).

The analyses of XRD patterns of other compositions were carried out by comparing the XRD data of the end member compositions as well as related phases of Nd3+ substituted zirconolite system18,19,22 and subsequently either by Le Bail profile refinement or Rietveld refinement methods. In Le Bail refinement, the complete XRD profile is refined with unit cell parameters and space group using a suitable profile function but without considering any atomic positions. The advantage of this method is that the overlapping reflections of the contributing phases can be accounted and the unit cell parameters can be refined to obtain accurate values. The phases identified at different compositions and their unit cell parameters are summarized in Table 3. In the XRD pattern of composition with x = 0.05, weak additional peaks at 2θ = 33.2 and 47.6◦ are observed which persist in the XRD patterns of successive compositions up to x = 0.35. These peaks could be assigned to the cubic perovskite type structure. At the nominal composition with x = 0.15, the splitting of the intense peak at 2θ ∼ 30.5◦ due to the zirconolite phase is significantly reduced. Additionally, a new peak at 2θ ∼ 31.4◦ is observed at and beyond this composition. The decreasing trend in the splitting of intense peak and the appearance of new peak indicate the formation of a four layer monoclinic zirconolite (4-M) phase similar to that observed in the Nd3+ substituted zirconolite system.18,19,22 As mentioned in the introduction section, the zirconolite structure of CaZrTi2 O7 is formed by stacking of layers of edge-shared TiO6 octahedra and TiO5 trigonal pyramids and layers of Ca2+ and Zr4+ ions.16,19,21 These layers are stacked along the c-axis of the zirconolite structure. For the composition CaZrTi2 O7 , the alternate layers are identical and thus the structure is explained as 2-layer monoclinic (2-M) zirconolite

Table 3 Summary of phases identified in Ca1−x Zr1−x Sm2x Ti2 O7 (0.00 ≤ x ≤ 1.00) compositions and their unit cell parameters. x

Composition

Phases

Zirconolite (2M or 4M)

Pyrochlore (P2) (◦ )

a (Å)

b (Å)

c (Å)

β

100.556(3) 100.588(3) 100.596(4) 100.596 (9) 84.873(5) 100.604(11) 84.882(5) 100.637(25) 84.838(7) 84.860(6) 84.772(31) 84.852(16) 84.740(18) 84.710(21) – –

0.00 0.05 0.10 0.15

CaZrTi2 O7 Ca0.95 Zr0.95 Sm0.10 Ti2 O7 Ca0.90 Zr0.90 Sm0.20 Ti2 O7 Ca0.85 Zr0.85 Sm0.30 Ti2 O7

2M 2M + P1a 2M + P1 2M + 4M + P1

12.4385(3) 12.4607(4) 12.4888(4) 12.4888(8) 12.5248(7)

7.2674(2) 7.2722(2) 7.2813(2) 7.2813(6) 7.2293(5)

11.3679(3) 11.3700(4) 11.3804(4) 11.3804(7) 23.0315(6)

0.20

Ca0.80 Zr0.80 Sm0.40 Ti2 O7

2M + 4M + P1

0.25

Ca0.75 Zr0.75 Sm0.50 Ti2 O7

2M + 4M + P1

0.30 0.35 0.40 0.45 0.50 0.60 0.70 0.80 0.90 1.00

Ca0.70 Zr0.70 Sm0.60 Ti2 O7 Ca0.65 Zr0.65 Sm0.70 Ti2 O7 Ca0.60 Zr0.60 Sm0.80 Ti2 O7 Ca0.55 Zr0.55 Sm0.90 Ti2 O7 Ca0.50 Zr0.50 Sm1.00 Ti2 O7 Ca0.40 Zr0.40 Sm1.20 Ti2 O7 Ca0.30 Zr0.30 Sm1.40 Ti2 O7 Ca0.20 Zr0.20 Sm1.60 Ti2 O7 Ca0.10 Zr0.10 Sm1.80 Ti2 O7 Sm2 Ti2 O7

4M + P1 4M + P1 4M + P2 4M + P2 4M + P2 P2 P2 P2 P2 P2

12.5028(12) 12.5128(11) 12.4934(27) 12.5031(14) 12.5275(9) 12.5189(33) 12.5252(29) 12.5348(30) 12.5498(32) – –

7.2814(6) 7.2262(5) 7.2747(14) 7.2212(7) 7.2252(5) 7.2256(18) 7.2283(15) 7.2334(16) 7.2391(18) – –

11.3895(9) 22.8972(10) 11.3919(17) 22.9065(8) 22.9612(9) 22.9948(15) 23.0160(13) 23.0707(16) 23.1188(24) – –

a (Å)

V (Å)3

1010.22(5) 1012.77(5) 1017.23(6) 1017.23(12) 2077.0(2)

– – – –

– – – –

1019.2(2) 2062.1(3) 1017.6(3) 2059.8(3) 2069.9(2) 2071.4(8) 2075.3(6) 2082.9(7) 2091.4(8) – –

– – –

– – –

– – 10.1536(5) 10.1621(3) 10.17239 10.1696(3) 10.1891(3) 10.2016(3) 10.2183(3) 10.2321(2)

– – 1046.8(1) 1049.4(1) 1052.6(1) 1051.7(1) 1057.8(1) 1061.7(1) 1066.9(1) 1071.26(4)

V

(Å)3

˚ 3 Perovskite: (P1)*, P2: cubic 2M = two layer monoclinic zirconolite, 4M = four layer monoclinic zirconolite, P1: Perovskite (cubic, a −3.814 and V = 55.8 A pyrochlore. a The unit cell parameters are fixed.

M. Jafar et al. / Journal of the European Ceramic Society 34 (2014) 4373–4381

structure. However, on substitution as well as depending on the formation conditions or variation of Zr and Ti ratio, the periodicity of the layers vary which lead to different stacking variant polytypes. But the basic arrangements of cations and anions are closely similar to the basic framework of the parent zirconolite (2-M) structure and hence they have nearly identical XRD patterns, i.e., the intense peaks for these phases have nearly similar positions. The larger periodicity of the stacking layers is observed in such polytypes, viz. 4-M zirconolite has identical layers at the fourth stacked layer. Such longer periodicity are reflected in larger unit cell parameter along the staking direction (c-axis) and they are characterized by a weak low angle super structure peak (2θ ∼ 7◦ ).16,17,19,21,22 It can be mentioned here that the XRD pattern of 2-M zirconolite phase show two intense peaks at 2θ ∼ 30.4◦ (I/I0 = 100%) and 30.6◦ (I/I0 = 68%) and the separation of the peaks is about 0.2◦ (see Fig. A, Supplementary data). Similar to the studies on CaZrTi2 O7 –Nd2 Ti2 O7 system,22 with the increasing concentration of Sm3+ in the present studied system, a systematically decreasing trend in the separation of these two peaks is observed. In the same two theta region, 4-M zirconolite phase shows a single peak, but is a convolution of several planes.19,22 Additionally, a new peak at 2θ ∼ 31.4◦ is developed while the intensity of the peak observed at 2θ ∼ 32.2◦ is decreased with increasing Nd3+ or Sm3+ . A figure indicating these differences in XRD patterns is shown in Supplementary data (Fig. B). Thus the overall profile of the peaks at 2θ ∼ 30.5◦ (observed as doublet at 2θ ∼ 30.4 and 30.6◦ in 2-M phase and as singlet at 2θ ∼ 30.5◦ in 4-M phase) as well as intensities of the newly developing peak at 2θ ∼ 31.4◦ and disappearing peak at 2θ ∼ 32.2◦ can be used as a guide for the formation of 4-M phase or disappearance of 2-M phase. Such analyses revealed that the 2-M and 4-M phases coexist up to the nominal composition with x = 0.25. The final refined powder XRD pattern for the x = 0.20 compositions indicating the coexistence of these two phases are shown in Fig. 3a. With the increase in Sm3+ content in the nominal compositions, the fraction of the 4-M phase increases and eventually it exists as a single phase at the compositions with x = 0.30 and 0.35. Typical refined powder XRD pattern for x = 0.35 indicating single phase 4-M zirconolite structure is included in Fig. 3b. The existence of zirconolite phase either as 2-M or 4-M structure over a wide range of compositions suggests an appreciable solubility of rare-earth ions in the zirconolite type structures. It can be mentioned here that at higher Sm3+ ion concentrations, the 2-M phase has a lower stability compared to 4-M structure and hence transform to a single phase 4-M structure. It can be mentioned here that the XRD patterns of the compositions containing 4-M zirconolite phase could not be refined by Rietveld refinement method due a large number of cation and anion sites as well as distribution of cations in different sites. So the structural details of these phases are not obtained in this study. Similar observation has also been reported in the analogous Nd3+ doped zirconolite system.18,19,21 XRD patterns of compositions with x ≥ 0.40, show diminishing contribution from the 4-M zirconolite type phase. The intense peaks in these XRD patterns are closely similar to those of the cubic pyrochlore type Sm2 Ti2 O7 phase. Further the

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Fig. 3. Final refinement plots of the powder XRD data of compositions with x = 0.20 (a), 0.35 (b), 0.50 (c) and 0.60 (d). The vertical marks indicate Bragg positions. (a) 2-M zirconolite, 4-M zirconolite and perovskite phases (top to bottom), (b) 4-M zirconolite and perovskite phases (upper and lower), (c) 4-M zirconolite and pyrochlore phase (upper and lower) and (d) pyrochlore phase.

appearance of additional peaks at 2θ ∼ 15.1 and 29.8◦ confirms the formation of a cubic pyrochlore type phase. Presence of 4M zirconolite (minor) and cubic pyrochlore type (major) phases is observed in XRD patterns of the nominal compositions with 0.40 ≤ x ≤ 0.50. Refined powder XRD pattern for x = 0.50 composition indicating the coexistence of these two phases is shown in Fig. 3c. All the observed peaks in the XRD patterns of compositions with x ≥ 0.60 could be assigned to cubic pyrochlore type structure similar to Sm2 Ti2 O7 . Typical powder XRD pattern for the composition with x = 0.60 indicating single phase pyrochlore type structure is shown in Fig. 3d. The structural data for the single phase pyrochlore systems have been obtained by Rietveld refinement of their corresponding powder XRD patterns. In the structure, the Ca2+ , Sm3+ and Zr4+ are placed in the 16c sites while the Ti4+ are placed in 16d sites. The occupancies of the cations are assumed as per their nominal composition and no further refinements have been carried out. Also, only one overall thermal parameter is refined for these phases also. The refined structural details of the cubic pyrochlore type solid solution phases are compared in Table 4. It is observed that the oxygen parameters are almost similar in these cubic pyrochlore type solid solution compositions. Details of the unit cell parameters of the pyrochlore type phase observed in different compositions

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Table 4 Comparison of the structural parameters of cubic pyrochlore type solid solution phases in Ca1−x Zr1−x Sm2x Ti2 O7 (0.60 ≤ x ≤ 1.00). x

r[A] Å

r[A]/r[B]

0.60

1.04

1.719

0.70

1.05

1.736

0.80

1.06

1.752

0.90

1.07

1.769

1.00

1.08

1.785

Ca/Zr/Sm: 16c: (xyz)

Ti: 16d: (xyz)

O1: 8a: (xyz)

O2: 48f: (xyz)

Rp , Rwp , χ2

(0, 0, 0) [0.20:0.20:0.60]a (0, 0, 0) [0.15:0.15:0.70]a (0, 0, 0) [0.10:0.10:0.80]a (0, 0, 0) [0.05:0.05:0.90]a (0, 0, 0) [0.00:0.00:1.00]a

(1/2, 1/2, 1/2)

(1/8, 1/8, 1/8)

17.9, 25.0, 1.50

(1/2, 1/2, 1/2)

(1/8, 1/8, 1/8)

(1/2, 1/2, 1/2)

(1/8, 1/8, 1/8)

(1/2, 1/2, 1/2)

(1/8, 1/8, 1/8)

(1/2, 1/2, 1/2)

(1/8, 1/8, 1/8)

(0.430(2), 1/8,1/8) (0.429(4), 1/8,1/8) (0.430(2), 1/8,1/8) (0.432(2), 1/8,1/8) (0.432(2), 1/8,1/8)

15.9, 22.6, 1.42 17.1, 23.5, 1.48 18.2, 25.5, 1.43 16.8, 24.5, 1.46

a Occupancy of metal ions of A site, Occupancy of all others atoms are unit. r[A] = average ionic radii of the A site, = [occ.of Ca2+ × rCa2+ ] + [occ.of Zr4+ × rZr4+ ] + [occ.of Sm3+ × rSm3+ ] Cubic, Space group: Fd3m (No.227) ˚ rZr4+ = 0.84 A ˚ and rSm3+ = 1.08 A ˚ (all in coordination number 8) Ionic radii: rCa2+ = 1.12 A, ˚ (in coordination number 6) rTi4+ = 0.605 A

are given in Table 3. A comparison of the unit cell parameters of pyrochlore type phases (Table 3) observed at different compositions shows an increasing trend with the increase in the values of x. The observed increasing trend of the unit cell parameter is in accordance with the increasing average ionic radius of the A site.26 The observation of single phase region in the compositions with 0.60 ≤ x ≤ 1.00 suggests that about 50% of the A site cation of pyrochlore (A2 B2 O7 ) structure can be substituted by proper combination cations of equivalent oxidation states. Interestingly, the weak peaks attributable to perovskite structure types phase observed along with the zirconolite type phase are not observed in the XRD patterns of compositions with x ≥ 0.40. This indicates that equivalent amounts Ca2+ and Zr4+ together can replace the Sm3+ ions of the pyrochlore structure of Sm2 Ti2 O7 , while the Sm3+ has a non equivalent substitution in Ca2+ and Zr4+ sites of zirconolite structure. It is known that the formation of cubic pyrochlore type phase of A2 B2 O7 type composition is mainly governed by the ionic radii of the A and B site cations.20 For rare-earth zirconates (A2 Zr2 O7 ), the radius ratio (rA3+ /rB4+ ) for lighter rare-earth ions as A3+ ion is suitable for fluorite type structure while that for heavier rareearth ions as A3+ ion (beyond Eu3+ ) is suitable for pyrochlore structure. However in case of rare-earth titanates (A2 Ti2 O7 ), for the lighter rare-earths (up to Nd3+ ), the radius ratio exceeds the limit for pyrochlore structure. Thus they form perovskite related structures. The cubic pyrochlore structures are generally observed for A2 Ti2 O7 , with A = Sm3+ to Lu3+ . Thus as the ˚ in coordination number 8) ions are replaced Sm3+ (r = 1.08 A, ˚ in coordination number 8), the by Ca2+ and Zr4+ (rav = 0.98 A, radius ratio decreases but it remains within the pyrochlore structure limit (Table 4).20 The formation of cubic pyrochlore type phase in Ca2+ and Zr4+ substituted Nd2 Ti2 O7 has been explained by the same reason.22 From the powder XRD studies of the Ca1−x Zr1−x Sm2x Ti2 O7 , for 0.00 ≤ x ≤ 1.00, compositions, the phase relation between the CaZrTi2 O7 and Sm2 Ti2 O7 can be explained by three major phase fields, namely monoclinic 2-M and 4-M zirconolite

and cubic pyrochlore types. In addition, a minor amount of perovskite related phase is observed in the Sm3+ substituted zirconolite rich compositions. This observation is similar to the earlier Nd3+ or Gd3+ substituted zirconolite systems.21,22,27 The formation of perovskite type phase at the zirconolite rich region can be attributed to the preferential substitution of rareearth ions in the Ca2+ site of zirconolite compared to the Zr4+ site.19,22 It is observed that the 2-M structure of zirconolite phase becomes unstable on Sm3+ substitution and it eventually transforms to four layer (4-M) hettotype of zirconolite structures. The phase fields in the rare-earth rich compositions in Ca1−x Zr1−x A2x Ti2 O7 (A = Sm3+ and Nd3+ ) are found to be primarily governed by the ionic radii of the rare-earth ions. Cubic pyrochlore type single phase region is observed with Sm3+ ion while cubic pyrochlore and monoclinic Nd2 Ti2 O7 type phases are observed for Nd3+ ion.22 In the intermediate compositions in Ca1−x Zr1−x A2x Ti2 O7 (A = Sm3+ and Nd3+ ), the zirconolite and pyrochlore phases are found to coexist together for both Nd3+ and Sm3+ ions. In order to further understand the phase evolutions, the molar volume of the major phases are compared. Typical variation of molar volume (V/Z, where V = volume of unit cell and Z = number of formula units in the unit cell) of the different phases with x is shown in Fig. 4. The major phase fields observed in this system are shown as inset in Fig. 4. The structural transitions are appeared as discontinuities in the variation in the molar volume with composition. Such variations can be attributed to the differences in arrangement of ions as well as to the differences in solubility of rare-earth ions or Ca2+ + Zr4+ ions in different structure types. The rearrangement of cations in the 4M-zirconolite structure appears as the sole guiding factor towards the formation of cubic pyrochlore phase from 4-M zirconolite phase. The formation of zirconolite or cubic pyrochlore type phases with wider concentration of alkaline earth and rare-earth ions and coexistence of these phases suggest that the present studied system can form a suitable matrix for immobilization of analogous radioactive ions.

M. Jafar et al. / Journal of the European Ceramic Society 34 (2014) 4373–4381

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Fig. 4. Variations of normalized volume (V/Z) of different observed phases with nominal compositions (2M-Zirconolite (Z = 8); 4M-zirconolite (Z = 16); Py: cubic pyrochlore (Z = 8). The minor perovskite phase observed is not included in the plot. The major phase fields are shown as inset.

The observed phase fields in the CaZrTi2 O7 –Sm2 Ti2 O7 system were further supported by the electron imaging and quantitative elemental analyses by electron probe for micro analyses (EPMA) technique. Typical back-scattering electron images (BSE) of representative compositions are shown in Fig. 5(a–c). The X-ray mapping for a representative composition Ca0.50 Zr0.50 Sm1.00 Ti2 O7 (x = 0.50) is shown in Fig. 6(a–d). In any of the BSE images, the presence of minor segregated perovskite type phase could not be observed. The BSE image (Fig. 5a) of the nominal composition Ca0.90 Zr0.90 Sm0.20 Ti2 O7 (i.e. x = 0.10) show homogeneous single phase zirconolite type phase only. Similarly the existence of a uniform single phase is conformed from the BSE image of the Ca0.40 Zr0.40 Sm1.20 Ti2 O7 composition (Fig. 5c). The coexistence of two phases (4-M zirconolite and cubic pyrochlore) at the composition with x = 0.50 (i.e. Ca0.50 Zr0.50 Sm1.00 Ti2 O7 ) is reflected in the phase contrast BSE images (Fig. 5b). The minor zirconolite-4M phase is observed as with darker contrast while the cubic pyrochlore phase (matrix) appeared with lighter contrast within the BSE images. Well developed crystallites as observed by the increasing sharpness of peaks of XRD patterns are reflected in BSE images. Besides, homogeneous continuous grains are observed in single phase compositions compared to the two phase compositions. The analyses of the compositions of the phases from the emitted X-rays (Fig. 6) support the coexistence of zirconolite

Fig. 5. Representative BSE images for Ca1−x Zr1-x Sm2x Ti2 O7 compositions. (x = 0.10 (a). 0.50 (b) and 0.60 (c)). Dark black regions are polishing grain pull-outs defect.

and pyrochlore type phase. Both BSE and X-ray images show that the minor 4-M zirconolite phase is distributed within the matrix of cubic pyrochlore phase. Elemental stoichiometries of the observed phases in samples of other nominal compositions

Table 5 Phase and compositions (at %) observed at various representative compositions (after 1300 ◦ C). x

Composition

XRD

Phase

Zirconolite

Perovskite

Cubic pyrochlore

0.10 0.50

Ca0.90 Zr0.90 Sm0.20 Ti2 O7 Ca0.50 Zr0.50 Sm1.00 Ti2 O7

M1 P2 M2

Matrix Matrix

Ca0.73 Zr0.76 Sm0.15 Ti2 O6.48

–

– Ca0.38 Zr0.38 Sm1.04 Ti2 O6.70

Gray

Ca0.49 Zr0.53 Sm0.72 Ti2 O6.63

–

0.60

Ca0.40 Zr0.40 Nd1.20 Ti2 O7

P2

Matrix

–

–

M1 = 2-M zirconolite; M2 = 4-M zirconolite; P1 = perovskite; P2 = cubic pyrochlore.

Ca0.35 Zr0.35 Sm1.22 Ti2 O6.88

4380

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Fig. 6. X-ray images of Ca0.50 Zr0.50 Sm1.00 Ti2 O7 (x = 0.50) (Two phase system consisting of zirconolite and pyrochlore).

are given in Table 5. Elemental compositions of different phases are also in close agreement with the nominal compositions of single phase compositions and expected compositions of the biphasic system. Both XRD and EPMA studies on CaZrTi2 O7 and Sm2 Ti2 O7 system confirm the formation of wider range of cation substituted phases with either zirconolite or pyrochlore-type structure. Also it is observed that with increasing Sm3+ concentration in zirconolite phase, the structure transforms to stacking variant hettotypes and then to cubic pyrochlore type structure. The pyrochlore type phase accommodates divalent and tetravalent ions simultaneously without any change in the crystal structure.

phase show a gradually increasing trend with increasing Sm3+ contents. Incorporation of Sm3+ into zirconolite structure leads to preferential segregation of Ca2+ ions leading to the formation of a minute amount of cubic perovskite phase. At higher concentration of Sm3+ ions, the zirconolite 2-M structure transforms to 4-M structure without much alteration in the framework of the crystal structure. Existence of a cubic pyrochlore type structure over a wider domain indicates a promising phase region to accommodate larger amount of heterovalent cation. The crystallochemical findings of this study suggest CaZrTi2 O7 –Sm2 Ti2 O7 system can form a potential ceramic composite ensemble for HLW immobilization.

4. Conclusions

Acknowledgements

From the foregoing XRD and EPMA analyses of Ca1−x Zr1−x Sm2x Ti2 O7 (0.00 ≤ x ≤ 1.00) compositions equilibrated at 1300 ◦ C, the phase relations between zirconolite-type CaZrTi2 O7 and cubic pyrochlore Sm2 Ti2 O7 have been established. The CaZrTi2 O7 and Sm2 Ti2 O7 systems consists of three major phase fields namely, 2-M zirconolite, 4-M zirconolite and cubic pyrochlore types. Additionally, a minor amount of perovskite type phase coexists in the zirconolite rich compositions. Molar volumes of all the phases except the perovskite

Dr.V.K. Jain, Head, Chemistry Division and Dr. G.K. Dey, Associate Director, Materials Group are thanked for their keen interest in this work. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jeurceramsoc. 2014.07.001.

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