Crystal Chemical Substitution at Ca and La Sites in CaLa4

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Crystal Chemical Substitution at Ca and La Sites in CaLa4(SiO4)3O To Design the Composition Ca1−xMxLa4‑xREx(SiO4)3O for Nuclear Waste Immobilization and Its Influence on the Thermal Expansion Behavior Ramya Ravikumar,† Buvaneswari Gopal,*,† and Hrudananda Jena‡ †

Department of Chemistry, School of Advanced Sciences, VIT University, Vellore 632 014, Tamil Nadu, India Materials Processing Chemistry Section, Materials Chemistry Division, MC & MFCG, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam 603 102, India

‡

S Supporting Information *

ABSTRACT: The oxysilicate apatite host CaLa4(SiO4)3O has been explored for immobilization of radioactive nuclides. Divalent ion, trivalent rare earth ion, and combined ionic substitutions in the silicate oxyapatite were carried out to optimize the simulated wasteform composition. The phases were characterized by powder X-ray diffraction, FT-IR, TGA, SEM-EDS, and HT-XRD techniques. The results revealed the effect of ionic substitutions on the structure and thermal expansion behavior. The investigation resulted in the formulation of simulated wasteforms such as La 3 . 4 Ce 0 . 1 Pr 0 . 1 Nd 0 . 1 Sm 0 . 1 Gd 0 . 1 Y 0 . 1 (SiO 4 ) 3 O (WF-1) and Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-2). In comparison to the average axial thermal expansion coefficients of the hexagonal unit cell of the parent CaLa4(SiO4)3O measured in the temperature range 298−1073 K (α′a = 9.74 × 10−6 K−1 and α′c = 10.10 × 10−6 K−1), rare earth ion substitution decreases the thermal expansion coefficients, as in the case of La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (α′a = 8.67 × 10−6 K−1 and α′c = 7.94 × 10−6 K−1). However, the phase Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O shows an increase in the values of thermal expansion coefficients: α′a = 11.74 × 10−6 K−1 and α′c = 11.70 × 10−6 K−1. wasteform was explored where monazite3 and NZP4 are potential matrices. Such matrices accommodate limited waste loading. Literature reports show the continued efforts in identifying an appropriate host matrix which fulfills the requirements and accommodates high waste loading. Apatite belongs to one such mineral family which is known for its structural versatility that provides space for the accommodation of a wide range of ions in its framework, and the members of the derived synthetic apatites exhibit thermal stability,5 chemical stability,6 and resistance toward radiation.7 The majority of compounds of the apatite family (general formula M(1)4M(2)6(XO4)6Y2) crystallize in a hexagonal system with the space group P63/m. The skeleton of the hexagonal structure is built up by XO4, M(1)−O(9), and M(2)−O(6) (Y) polyhedra. The XO4 moieties are isolated and are normally of the types SO42−, CO32−, PO43−, VO43−, SiO44−, GeO44−, etc. M(1) and M(2) represent two crystallographically nonequivalent cationic sites which are occupied by ions that are monovalent (Li+, Na+, K+, etc.), divalent (Ca2+, Sr2+, Pb2+, Ba2+, Cd2+, etc.), trivalent (Y3+, La3+, Ce3+, Nd3+, Sm3+, etc.), or tetravalent (U4+, Th4+ etc.). The 4f M(1) site is surrounded by

1. INTRODUCTION For the efficient containment of high-level nuclear waste that mostly contains harmful radionuclides, an appropriate host matrix needs to be identified. For long-term durability, a suitable matrix has to be selected on the basis of factors such as structural flexibility and thermal, chemical, and radiation stability, so that even if the wasteform fabricated that is based on the host is exposed to extreme conditions the host will remain intact, encapsulating the immobilized ions. These criteria form the basis for wasteform designing. Hence, a matrix based on existing minerals in a natural geological environment would be appropriate for a host.1 The crystal structure of the host matrix should possess multiple anion and cation sites to accommodate large numbers of radionuclides and also have a high melting point, so that any type of radiation from waste loadings will not lead to breakdown of the structural framework. Such hosts are expected to accommodate high waste loading. Synroc, a multiphasic ceramic host in which immobilizing ions preferentially partitioned into different phases, has been explored and studied. In this host system where high waste loading is possible, a disadvantage of differential thermal expansion is observed.2 To avoid such complications, an alternative route of formulating a single-phase ceramic © XXXX American Chemical Society

Received: March 7, 2018

A

DOI: 10.1021/acs.inorgchem.8b00600 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry nine oxygen atoms with C3 symmetry. The 6h M(2) site is surrounded by six oxygen atoms and one “Y” anion (F−, Cl−, OH−, O2− etc.) with Cs symmetry. Tolerance to a wide range of ionic substitutions leads to extensive compositional variation; therefore, numerous reports on various types of apatite compounds such as hydroxyapatite (HA), fluorapatite (FA), chlorapatite (ClA), and oxyapatite (OA) having phosphate, silicate, vanadate, and arsenate moieties as interconnecting tetrahedra are available.8,9 Oxyapatites have been increasingly studied for various properties. Considering the ionic substitution in silicate oxyapatites, a wide variety of rare earth substitutions have been reported.10,11 In particular, actinides such as U-, Pu-, and Hf-incorporated silicate oxyapatites have been developed to study the extent of radiation damage.12 As mentioned, the thermal stability of the crystalline host is one of the important factors to be considered in the design of the wasteform composition, as it would encounter heat generated in situ, either by self-heating due to the decay heat of the fission products immobilized in the structural framework13 or from external factors in which a temperature rise might likely occur under repository conditions. In the geological repository, the rate of temperature rise with depth will differ significantly from site to site. It is observed that a more geologically stable area may increase in temperature by 1 °C for each 60−80 m of depth.14 Therefore, the combination of self-heating and geothermal heat would affect the durability, which may jeopardize the integrity of the waste package. To formulate a thermally intact wasteform, a matrix or crystalline host with high thermal conductivity and low thermal expansion properties has to be identified.15 The in situ heat generation causes lattice dilation, which certainly induces internal stress in the structural framework of the wasteform that results in microfractures. The high thermal stress experienced by the wasteform debilitates the integrity of the framework and thereby influences its ability to retain the radionuclides, which may lead to increased leaching of the immobilized radionuclides. Subsequently, understanding the fundamental thermophysical properties, mainly thermal expansion behavior in a wide temperature range, enables one to confirm the structural integrity of the wasteform at different temperatures. In the family of apatites, the thermal expansion behavior of oxysilicates such as X 9.33 (SiO 4 ) 6 O 2 (X = La, Nd), 16 La9ASi6O26.5 (A = Ca, Sr, Ba),17 CaLn4(SiO4)3O (Ln = Pr, Sm, Eu),18 Y4.69(SiO4)3O,19 Ca6Th4(SiO4)3O,20 La10Si6−xCuxO27,21 and SrPr4(SiO4)3O22 has been reported. Literature on oxysilicates of the formulas Ca3.05Ce2.38Fe 0.25 Gd 5.37 Si 4.88 O 26 and Ca 3.78 La 0.95 Ce 1.45 Zr 0.78 Fe 0.14 Nd2.15Eu0.50Si6.02O26 as wasteforms is available.23 The investigation of the thermal expansion behavior of oxysilicate-based wasteform has hitherto not been reported. Thus, the current work combines the study of phase stabilization to fabricate oxysilicate wasteforms and the thermal expansion behavior for practical applications in nuclear waste immobilization. The current work investigates ionic substitution in an oxysilicate apatite host of the formula CaLa4(SiO4)3O (CLSI) and its effect on the thermal expansion behavior. Such a study enables the fabrication of wasteforms containing complex nuclear waste compositions possessing alkalis, alkaline earths, rare earths, and other ions. The structural lattice of the oxysilicate is such that the 6h site is occupied by La3+ ions and 4f sites are occupied by both La3+ and Ca2+ ions,24 which provide ample room for accommodating ions of different sizes

and charges. Compositions containing rare earths and combined divalent and rare earth ions have been made, and their thermal expansion behavior has been studied to understand the compactness of the fabricated compositions at various temperatures. As the lanthanides mimic the chemical behavior of the actinides, the findings will be useful to predict the properties of actinides immobilized in an oxysilicate apatite system.

2. EXPERIMENTAL SECTION 2.1. Materials. The reagents used were La2O3 (99.9%, SigmaAldrich), Sr(CO3) (99%, SD. Fine), CaCO3 (99+%, Sigma-Aldrich), (NH4)2[Ce(NO3)6] (99.0%, SD. Fine), Pr6O11 (99.9%, Otto chemie), Nd2O3 (99.9%, Otto chemie), Sm2O3 (99.9%, Otto chemie), Gd2O3 (99.9%, Sigma-Aldrich), Y2O3 (99.9%, Sigma-Aldrich), PbO (99.9%, Sigma-Aldrich), and SiO2 (99.6%, Sigma-Aldrich). 2.2. Synthesis. Compositions of the formulas Ca1−xAxLa4(SiO4)3O (A = Sr2+, Pb2+, x = 0−0.9), CaLa4−xREx(SiO4)3O (RE = La, Ce, Pr, Nd, Sm, Gd, Y), CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF1), and Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF2) were synthesized by high-temperature solid-state reaction methods. Stoichiometric amounts of the reactants were ground well and heated at 750 °C for 12 h, at 950 °C for 24 h, and at 1050 °C for 24 h with intermittent grinding. 2.3. Characterization. The compounds synthesized were characterized by powder X-ray diffraction techniques (Bruker, D8 Advanced) using Cu Kα radiation at room temperature. The unit cell parameters were calculated by least-squares refinement methods. Fourier transform infrared spectra were recorded using a JASCO FTIR 4100 instrument in ATR mode in the frequency range 500−1200 cm−1. Scanning electron microscope images were recorded on a JEOL Model JSM-6390LV instrument, and EDS analysis was performed using a Zeiss-EVO 18 research apparatus. Thermal analysis was performed on a TA Instruments SDT Q 600 apparatus. 2.4. Thermal Expansion Studies. High-temperature X-ray diffraction (HT-XRD) studies were carried out using a Philips X’pert pro MPD instrument with a high-temperature attachment (temperature range 298−1073 K). The unit cell parameters (a, c) and unit cell volume (V) at room temperature as well as at elevated temperatures were calculated by least-squares refinement methods. The average coefficient of thermal expansion of the systems has been calculated using the formulas and methodology reported.25 The details are given below. The second-order polynomial of the formula lpT = x + yT + zT2 was employed to fit the variation in lattice parameters with temperature. The linear coefficient of thermal expansion (CTE), α, was determined using the formula

α(T ) =

d(lp) dT (lp)

where lp gives the lattice parameters at temperature T. The average CTE values α′a and α′c, along the a and c axes, respectively, were calculated using the expressions a 2 − a1 α′a = (T2 − T1)a1 α′c =

c 2 − c1 (T2 − T1)c1

Using the above expressions, the average linear thermal expansion coefficient for the hexagonal oxyapatite, αavg, was calculated as

αavg =

2α′a + α′c 3

Further, the calculated thermal expansion coefficients αa and αc at different temperatures were plotted to obtain thermal expansion diagrams. These diagrams explain the extent of thermal expansion along the given axis with respect to each step increase in temperature and priority direction of thermal expansion. B


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Inorganic Chemistry

3. RESULTS AND DISCUSSION With a view to fine-tune simulated wasteform compositions, systematic ionic substitutions in the silicate oxyapatite lattice CaLa4(SiO4)3O have been carried out. 3.1. Divalent Ion Substitution. Divalent ions such as Sr2+ and Pb2+ were substituted in CaLa4(SiO4)3O (CLSI) to derive a composition of the formula Ca1−xMxLa4(SiO4)3O (M = Sr2+, Pb2+). The powder X-ray diffraction analysis (Figures S1 and S2) shows the difference in the extent of substitution of the above two ions in the CLSI matrix. The phases of the formulas Ca1−xSrxLa4(SiO4)3O (x = 0−0.9) and Ca1−xPbxLa4(SiO4)3O (x = 0−0.9) were tried. Indexing the patterns on the basis of the JCPDS file of CaLa4(SiO4)3O (#71-1368) reveals singlephase formation for the complete range of substitution (x) in the former, whereas this formation was only up to x = 0.5 in the latter. The pure phases are isostructural and crystallize in a hexagonal system (space group P63/m); the lattice parameters are given in Tables S1 and S2. Pb2+ ion substitution beyond x = 0.5 resulted in the formation of a Pb2SiO4 as secondary phase (peaks at 2θ = 28.7, 29.4 and 29.45°). It is presumed that the stereochemical activity of the 6s2 lone pair of electrons plays a crucial role in restricting the substitution of Pb2+ ion with higher content.26 FT-IR spectra of the Sr2+- and Pb2+-substituted pure phases are shown in Figures S3 and S4, for which the peak assignments are given in Tables S3 and S4. The fundamental silicate anionic groups associated with apatite lattice can be identified in the spectrum of each of the respective phases. The features of the characteristic bands of SiO4 groups detected at 924−967 cm−1 are attributed to antisymmetric stretching modes (νas), those at 848−870 cm−1 to symmetric stretching modes (νs), and those at around 499−545 cm−1 to antisymmteric bending modes (δas). Substitution of Sr2+ in the oxysilicate apatite phase slightly influences the antisymmetric stretching (νas) mode and is indicated by the minor gradual decrease in νas(Si−O) values, which appear at around 925 cm−1. This result is consistent with earlier reports.27 In the case of Pb2+ ion substituted phases, the bands associated with antisymmetric stretching modes (ν3) shifted considerably to lower frequencies (Table S4.). These observations are in agreement with the difference in the cationic radii, the masses of Ca2+, Sr2+, and Pb2+ ions, and the greater covalent character induced by the Pb2+ ion.28 3.2. Rare Earth Ion Substitution. Powder X-ray diffraction patterns of trivalent rare earth ion substituted phases of the formula CaLa4‑xREx(SiO4)3O (where RE = Ce, Pr, Nd, Sm, Gd, Y and x = 0−0.5) are given in Figures S5−S10. The extent of substitution of RE3+ ions in CLSI is found to vary in each case. The parent CLSI provides space to accommodate Ce3+, Sm3+, Gd3+, and Y3+ ions up to x = 0.1, beyond which impure phases such as Ce2Si2O4 (JCPDS file #48-1588), Sm2Si2O7 (JCPDS file #24-0711), Gd2SiO5 (JCPDS file # 400287), and Y2SiO5 (JCPDS file #74-2011) are respectively formed. While Pr3+ and Nd3+ are accommodated up to x = 0.3 and 0.5, respectively, the former shows secondary phase formation of β-Pr2Si2O7 (JCPDS file #73-1154) at x = 0.5. To be precise, all of the rare earth ions mentioned are successfully embedded in the framework with the formula CaLa3.9RE0.1(SiO4)3O. These isostructural silicate phases crystallize in the hexagonal crystal system (space group P63/ m), and the calculated lattice parameters are given in Table S5. FT-IR spectra of the pure phases of rare earth substituted oxysilicate are shown in Figures S11−S16, and the spectral

assignments are given in Table S6. As revealed in the table, substituted rare earth ions significantly influence the antisymmetric and symmetric stretching frequencies and have a relatively mild effect on the bending vibrational modes. The decrease in the frequency of the aforementioned vibrational modes are attributed to weakening of Si−O bonds with new RE−O bond formation.29 3.3. Formulation of Simulated Wasteform Composition. On the basis of the results obtained in the case of divalent and trivalent ion substitutions in the oxysilicate host, silicate oxyapatite compositions of the formula CaLa 3.4 Ce 0.1 Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-1) and Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-2) have been fabricated. Analysis of the powder XRD patterns given in Figure 1 reveals pure phase formation in both formulations, which are

Figure 1. Powder XRD patterns of (a) CaLa4(SiO4)3O, (b) CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O, and (c) Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O.

isostructural with CaLa4(SiO4)3O. Comparison of the FT-IR spectra of the compositions WF-1 and WF-2 with that of the parent phase (Figure S17) confirms the apatitic silicate tetrahedral groups. Such combined substitution of divalent and trivalent ions does not significantly influence the frequency of the Si−O vibrational modes (Table S7). The scanning electron micrographs of powder samples of the parent and the compositions WF-1 and WF-2 are given in Figure 2. In the case of CaLa4(SiO4)3O, CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O, and Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O, crystallite formation is found to be nonuniform in terms of size and shape. With rare earth ion substitution, the morphology of the crystallites appears to be similar to that of the parent phase along with agglomeration of particles being noticed additionally (Figure 2b). With the substitution of divalent ions along with rare earth ions, the agglomeration increases even more, which leads to particle growth to a larger size (Figure 2c). C


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Inorganic Chemistry

Figure 2. SEM micrographs of (a) CaLa4(SiO4)3O, (b) CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O, and (c) Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O.

2). The unit cell of the parent CLSI exhibits a contraction in all three crystallographic axes upon substituting La3+ ion by other rare earth ions such as Ce, Pr, Nd, Sm, Gd, and Y (WF-1), whereas partial replacement of Ca2+ by Sr2+ and Pb2+ ions along with the rare earth ions (WF-2) induces an enlargement of the unit cell dimensions owing to the larger ionic radii of the divalent ions. A similar trend is observed in the thermal expansion behavior. High-temperature powder X-ray diffraction (HT-XRD) analysis in the temperature range 298−1073 K shows that the average thermal expansion coefficient (αavg) of WF-1 (8.42 × 10−6 K−1) is lower than that of the parent (9.85 × 10−6 K−1) and the coefficient of WF-2 (11.72 × 10−6 K−1) is higher than that of the parent. The HT-XRD profiles at various temperatures in the range 298−1073 K are given in Figure S18. No phase transformation at any temperature in any of the three phases is noticed. The lattice parameters a and c are found to increase with an increase in temperature for all three systems, owing to the positive thermal expansion of such apatite-based silicates.18 The

Semiquantitative EDS composition analysis (Table 1) confirms the presence of elements as per the compositions CaLa4(SiO4)3O, CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O, and Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O. It is found that the experimental weight percentages of the elements are almost comparable to the calculated theoretical values, except that Pb is volatile in the latter composition. However, the powder XRD pattern (Figure 1c) confirms the structural stability of apatite lattice. 3.4. Thermal Expansion Behavior. As high-level nuclear waste disposal involves geological burial of the confined wasteforms, the leaching resistance of the wasteform under geological conditions depends on its thermal resistance. Hence, the intrinsic thermal expansion (linear or axial) behavior of the wasteforms WF-1 and WF-2 has beenstudied and compared with that of the parent. At room temperature, the response of the crystal lattice of CLSI upon trivalent rare earth and divalent ion substitutions is evidenced from the change in the unit cell parameters (Table D


Article

Inorganic Chemistry Table 1. Average Weight Percentages of the Elements Present As Obtained by EDS Analysis on Six Selected Spots weight percentage

CaLa4(SiO4)3O

CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O

Ca0.8Pb0.1Sr0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O

element

theor

exptl

theor

exptl

theor

exptl

La Ce Pr Nd Sm Gd Y Si Ca Sr Pb

62.57

65.18

9.49 4.51

7.70 4.18

53.25 1.57 1.58 1.62 1.69 1.77 1.00 9.50 4.52

54.46 2.03 1.13 3.35 0.90 3.09 0.59 7.50 3.61

51.99 1.54 1.55 1.58 1.65 1.73 0.97 9.27 3.53 0.96 2.28

58.30 1.73 0.69 1.85 2.36 1.23 0.85 7.61 3.45 1.10 0.53

coefficients of second-order polynomial equations x1, x2, and x3 and y1, y2, and y3 are obtained by fitting the variation of unit cell parameters with temperature for the three systems and the equations are given below. For the parent CLSI a(T ) = 9.645 + (7.424 × 10−5)T + (1.278 × 10−8)T 2 R2 = 0.9927 c(T ) = 7.140 + (4.242 × 10−5)T + (1.581 × 10−8)T 2 R2 = 0.9970

For WF-1 a(T ) = 9.643 + (7.223 × 10−5)T + (5.387 × 10−8)T 2 R2 = 0.9935 c(T ) = 7.136 + (5.007 × 10−5)T + (2.134 × 10−8)T 2 R2 = 0.9916

For WF-2 a(T ) = 9.655 + (3.929 × 10−5)T + (5.298 × 10−8)T 2 R2 = 0.9967 c(T ) = 7.146 + (5.073 × 10−5)T + (2.045 × 10−8)T 2 R2 = 0.9974 Figure 3. Variation of thermal expansion coefficients as a function of temperature for (a) CaLa4(SiO4)3O, (b) CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd 0 . 1 Y 0 . 1 (SiO 4 ) 3 O, and (c) Ca 0 . 8 Sr 0 . 1 Pb 0 . 1 La 3 . 4 Ce 0 . 1 Pr 0 . 1 Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O.

The calculated linear thermal expansion coefficients vary significantly with temperature (Figure 3). The axial and volume thermal expansion coefficients and anisotropic parameters are given in Tables S8−S10. The measured average thermal expansion coefficients for the hexagonal unit cell of CLSI are α′a = 9.74 × 10−6 K−1 and α′c = 10.10 × 10−6 K−1. Fractional substitution of La3+ by other rare earth ions, viz. Ce3+, Pr3+,

Nd3+, Sm3+, Gd3+, and Y3+, resulted in the reduction of thermal expansion coefficient values to α′a = 8.67 × 10−6 K−1 and α′c =

Table 2. Hexagonal Lattice Parameters of Parent, WF-1, and WF-2 composition

a (Å)

c (Å)

V (Å3)

CaLa4(SiO4)3O CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O

9.657(5) 9.571(5) 9.721(3)

7.152(7) 7.118(4) 7.190(7)

577.7(1) 564.8(1) 589.2(1)

E


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Inorganic Chemistry

Figure 4. Thermal expansion diagram of (a) CaLa4(SiO4)3O, (b) CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O, and (c) Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O.

7.94 × 10−6 K−1. It is to be noted here that, as per roomtemperature unit cell parameters, the partial replacement of La3+ in CaLa4(SiO4)3O by other smaller rare earth ions brings about a reduction in the unit cell dimensions of CLSI (Table 2). The smaller thermal expansion coefficient values could be attributed to the relatively stronger RE−O bond in comparison to the La−O bond30 due to their smaller size vs the charge (+3). Similar reports are available in the literature with the complete substitution of RE3+ ions such as Pr3+ and Sm3+ in systems such as CaPr4(SiO4)3O and CaSm4(SiO4)3O, showing marked decrease in the thermal expansion coefficient in the

temperature range 298−1173 K.18 Similarly, thermal expansion coefficients of the oxyapatite of the formula SrPr4(SiO4)3O are determined to be αa= 10 × 10−6 K−1 and αc= 6.7 × 10−6 K−1 22 at temperatures between 173 and 1173 K. On the other hand, partial substitution of Ca2+ by Sr2+ and Pb2+ as shown in Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O leads to a significant increase in the thermal expansion coefficient values α′a = 11.74 × 10−6 K−1 and α′c = 11.70 × 10−6 K−1 in comparison to those of CLSI. Such an observation of an increment in CTE has been reported in the oxyapatite La9ASi6O26.5 with an increase in ionic radii of substituting divalent ions A = Ca2+ → Sr2+ → Ba2+.31 The increase in the F


Article

Inorganic Chemistry

Table 3. Comparison of Thermal Expansion Coefficients of Some Reported Oxysilicate Apatite Systems with Those of Current Systems composition La9.33(SiO4)6O2 Nd9.33(SiO4)6O2 La10Si6O27 La9CaSi6O26.5 La9SrSi6O26.5 La9BaSi6O26.5 La10Si6O27 La10Si5.5Cu0.5O26.5 La10Si5Cu1O26 La10Si4.5Cu1.5O25.5 La10Si5Cu2O2 SrPr4(SiO4)3O CaLa4(SiO4)3O CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O

av CTE, 10−6 K−1

temp range

9.4

295−1073 K

16

9.6 9.8 10.7 10.9 8.9 8.8 8.8 9.0 9.1 αa = 10.0, αc = 6.7 9.85 8.42 11.72

25−900 °C

17

25−800 °C

21

173−1173 K 298−1073 K

22 this work

ref

WF-1 throughout the temperature range of the measurements. However, the deformation is more along the c axis up to 573 K and from 673 K onward the deformation direction switches to the a axis in the case of WF-2. The average thermal expansion coefficient (αavg) values of the oxysilicate systems studied are found to be lower in comparison to that of Ca hydroxyapatite (16 × 10−6 K−1).42 The values are significantly low in comparison to the rare earth containing phosphate−silicate apatite Ca9Nd(PO4)5(SiO4)F2, whose average thermal expansion coefficient is reported to be 21 × 10−6 K−1 in the temperature range 20−1000 °C.43 The compilation of the CTE values of various oxyapatites (Table 3)16,17,21,22 shows that the thermal expansion coefficients of the current systems are comparable. For a potential wasteform low thermal expansion behavior is expected.15 Synroc, out of the reported ceramic wasteforms, shows intermediate to high thermal expansion.15 Calcium silicate, titania, and monazite based wasteforms have been reported as ceramics with intermediate thermal expansion.15 The average CTE values obtained for the current system specify that wasteform CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-1) at 8.42 × 10−6 K−1 falls under the class of an intermediate group of expanding materials ((5−10) × 10−6 K−1). However, the wasteform Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-2) is a high thermal expansion material (>10 × 10−6 K−1) with a value of 11.72 × 10−6 K−1. Figure 5 compares the CTE values of the three compositions at each temperature. It is noted that at all elevated temperatures the composition CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-1) exhibits lower thermal expansion coefficient values in comparison to those of Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF2). This provides an insight into fabricating a potential candidate wasteform for immobilizing various elements of a high-level waste, considering geological burial of the nuclear waste. To avoid the difficulty of handling real nuclear waste for conversion into a wasteform involving a multistep process of heating and grinding, an attempt was made to heat treat the starting materials in a single step at 1050 °C. The powder X-ray diffraction pattern of the final product shown in Figure S19b confirms the formation of pure oxysilicate wasteform CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O (WF-1), and the

CTE could be correlated to the simultaneous presence of divalent Ca, Sr, and Pb ions in the apatite lattice and the role of the 6s2 lone pair of electrons of the Pb2+ ion, which induce greater distortion of the SiO4 polyhedra that in turn in a combined effect leads to an increase in thermal expansivity.32,33 The plots of αa (αc) versus temperature for the formulated compositions confirm the anisotropic nature of the oxyapatite system (Figure 3). The thermal expansion coefficients αa > αc in the parent CLSI and WF-1 throughout the temperature range studied. This could be attributed to the fact that the ratio of distribution of substituting RE3+ ions between 4f and 6h sites in WF-1 is expected to be the same as that of La3+ in CLSI.24,34,35 However, the CTE of WF-1 at each temperature is lower than that of CLSI, and this could be due to the effect of smaller rare earth ions in strengthening the M−O bonding.30 The composition WF-2 exhibits a different behavior, showing αa < αc in the temperature range 298−573 K, αa ≈ αc at 673 K, and αa > αc with further increase in temperature (673 K < T K < 1073 K) (Figure 3c). The probable reason for the above observation is given below. As noted in the synthesis of other lead-containing apatite systems,36 there is a possibility of vaporization of PbO during the formation of the apatite phase in the high-temperature synthesis process (the percent weight loss of Pb is evidenced from EDS results given in Table 1). The Pb2+ ion is presumed to be present in the 4f site as per the description of the CLSI structure. Partial removal of Pb2+ ions creates vacancies in 4f sites. Smaller RE ions are expected to occupy 6h sites preferentially.37 As is observed in apatite systems, the ionic distribution in 4f sites governs the change in c lattice parameter, whereas the occupancy of 6h sites by ions governs the a lattice parameter.38 The presence of vacancies in 4f sites contributes to higher αc values up to 673 K; with an increase in temperature RE ions from 6h sites migrate to the vacancies existing in 4f sites, and as a result new vacancies appear in 6h sites.39−41 Due to the contribution of these vacancies toward an increase in thermal expansion41 αa becomes higher than αc from 673 K onward. The thermal expansion diagrams (Figure 4) depict the relationship of M−O bond strength, the distribution of cations in the two crystallographically nonequivalent sites (6h and 4f), and the preferred axis of thermal deformation. The deformation is more along the crystallographic axis a in the case of CLSI and G


Article

Inorganic Chemistry

Figure 6. TGA curves of (a) CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y 0.1 (SiO 4 ) 3 O and (b) Ca 0.8 Sr 0.1 Pb 0.1 La 3.4 Ce 0.1 Pr 0.1 Nd 0.1 Sm 0.1 Gd0.1Y0.1(SiO4)3O.

given temperature range is observed to be 1.24%, which is almost the same as that of WF-1. This suggests that there is minimal weight loss during TGA analysis, as observed in the case of WF-1. It is to be noted here that, once Pb becomes partially removed as PbO at high temperature during the synthesis process of WF-2 as revealed by the EDS data, there is no further weight loss during TGA analysis.

4. CONCLUSION The two oxysilicate apatite compositions CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O and Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O based on ionic substitutions at Ca and La sites in CaLa4(SiO4)3O were synthesized by hightemperature solid-state reactions. The compositions possess 62.51 and 61.04 wt % rare earth ion loadings, respectively. High-temperature XRD studies showed that the oxyapatite phases are characterized by anisotropic thermal expansion, as observed in similar systems. As shown by the average thermal expansion coefficients (αav) of CLSI (9.85 × 10−6 K−1), WF-1 (8.42 × 10−6 K−1), and WF-2 (11.72 × 10−6 K−1), the rare earth ion substitution for La3+ decreases the extent of thermal expansion of CaLa4(SiO4)3O.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00600. Powder XRD patterns and FT-IR spectra of Ca 1−x Sr x La 4 (SiO 4 ) 3 O, Ca 1−x Pb x La 4 (SiO 4 ) 3 O, CaLa4‑xREx(SiO4)3O (RE = Ce, Pr, Nd, Sm, Gd, Y), assignments (cm−1) of FT-IR spectra of CaLa4(SiO4)3O, CaLa 3.4 Ce 0.1 Pr 0.1 Nd 0.1 Sm 0.1 Gd 0.1 Y 0.1 (SiO 4 ) 3 O, and Ca0.8Sr0.1Pb0.1La3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO4)3O, lattice parameters of all pure phase compounds, FT-IR spectral assignments and high-temperature XRD patterns of CaLa4(SiO4)3O, CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y0.1(SiO 4 ) 3 O, and Ca 0.8 Sr 0.1 Pb 0.1 La 3.4 Ce 0.1 Pr 0.1 Nd 0.1 Sm0.1Gd0.1Y0.1(SiO4)3O, axial and volume thermal expansion coefficients of CLSI, WF-1, and WF-2, and powder XRD pattern of WF-1 after direct heating at 1050 °C and lattice parameters (PDF)

Figure 5. Comparison of coefficients of thermal expansion (a) αa (b) αc, and (c) αvol for CaLa4(SiO4)3O, CaLa3.4Ce0.1Pr0.1Nd0.1Sm0.1Gd0.1Y 0.1 (SiO 4 ) 3 O, and Ca 0.8 Sr 0.1 Pb 0.1 La 3.4 Ce 0.1 Pr 0.1 Nd 0.1 Sm 0.1 Gd 0.1 Y0.1(SiO4)3O.

calculated lattice parameters are given in Table S11. Thus, the current investigation results in the formulation of an oxyapatite wasteform exhibiting intermediate thermal expansion in a single heat treatment. The thermogravimetric analysis of WF-1 (Figure 6a) shows a weight loss of 1.2% in the temperature range 300−973 K, indicating the thermal stability of the wasteform as a bulk material. The TGA curve of WF-2 (Figure 6b) is compared with that of WF-1 to note the thermal stability difference between the phases (WF-1, WF-2) due to the different compositions. Interestingly, the weight loss in the H


Article

Inorganic Chemistry

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(16) Iwata, T.; Fukuda, K.; Béchade, E.; Masson, O.; Julien, I.; Champion, E.; Thomas, P. Structural Change of Oxide-IonConducting Lanthanum Silicate on Heating from 295 to 1073K. Solid State Ionics 2007, 178, 1523−1529. (17) Zhang, L.; He, H. Q.; Wu, H.; Li, C. Z.; Jiang, S. P. Synthesis and Characterization of Doped La9ASi6O26.5 (A = Ca, Sr, Ba) Oxyapatite Electrolyte by a Water-Based Gel-Casting Route. Int. J. Hydrogen Energy 2011, 36, 6862−6874. (18) Knyazev, A. V.; Bulanov, E. N.; Korshunov, A. O.; Krasheninnikova, O. V. Synthesis and thermal expansion of some lanthanide-containing apatites. Inorg. Mater. 2013, 49, 1133−1137. (19) Key, T. S.; Presley, K. F.; Hay, R. S.; Boakye, E. E. Total Thermal Expansion Coefficients of the Yttrium Silicate Apatite Phase Y4.69(SiO4)3O. J. Am. Ceram. Soc. 2014, 97, 28−31. (20) Bulanov, E. N.; Wang, J.; Knyazev, A. V.; White, T.; Manyakina, M. E.; Baikie, T.; Lapshin, A. N.; Dong, Z. Structure and Thermal Expansion of Calcium−Thorium Apatite, [Ca4]F[Ca2Th4]T[(SiO4)6]O2. Inorg. Chem. 2015, 54, 11356−11361. (21) Ding, X.; Hua, G.; Ding, D.; Zhu, W.; Wang, H. Enhanced ionic conductivity of apatite-type lanthanum silicate electrolyte for ITSOFCs through copper doping. J. Power Sources 2016, 306, 630−635. (22) Knyazev, A. V.; Bulanov, E. N.; Smirnova, N. N.; Korokin, V. Z.; Shushunov, A. N.; Blokhina, A. G.; Ziyan, X. Thermodynamic and thermophysics properties of synthetic britholite SrPr4(SiO4)3O. J. Chem. Thermodyn. 2017, 108, 38−44. (23) Utsunomiya, S.; Yudintsev, S.; Wang, L. M.; Ewing, R. C. IonBeam and Electron-Beam Irradiation of Synthetic Britholite. J. Nucl. Mater. 2003, 322, 180−188. (24) Schroeder, L. W.; Mathew, M. Cation Ordering in Ca2La8(SiO4)6O2. J. Solid State Chem. 1978, 26, 383−387. (25) Halvarsson, M.; Vuorinen, S. Determination of the Thermal Expansion of K- Al2O3 by High Temperature XRD. Surf. Coat. Technol. 1995, 76−77, 358−362. (26) Baikie, T.; Schreyer, M.; Wei, F.; Herrin, J. S.; Ferraris, C.; Brink, F.; Topolska, J.; Piltz, R. O.; Price, J.; White, T. J. The Influence of Stereochemically Active lone-pair electrons on crystal symmetry and twist angles in lead apatite-2H type Structures. Mineral. Mag. 2014, 78, 325−345. (27) Lakshminarasimhan, N.; Varadaraju, U. V. Synthesis and Eu3+ Luminescence in new oxysilicates, ALa 3 Bi(SiO 4 ) 3 O and ALa2Bi2(SiO4)3O [A= Ca, Sr and Ba] with Apatite-related Structure. J. Solid State Chem. 2005, 178, 3284−3292. (28) Bigi, A.; Gandolfi, M.; Gazzano, M.; Ripamonti, A.; Roveri, N.; Thomas, S. A. Structural modifications of hydroxyapatite induced by lead substitution for calcium. J. Chem. Soc., Dalton Trans. 1991, 11, 2883−2886. (29) Kim, D.; Park, D.; Oh, N.; Kim, J.; Jeong, E. D.; Kim, S. J.; Kim, S.; Park, J. C. Luminescent Properties of rare earth fully activated apatites, LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb): Site selective crystal field effect. Inorg. Chem. 2015, 54, 1325−1336. (30) Manu, K. M.; Karthik, C.; Leu, L. C.; Lazar, K. A.; Ubic, R.; Sebastian, M. T. Crystal structure and microwave dielectric properties of LiRE9(SiO4)6O2 Ceramics (RE = La, Pr, Nd, Sm, Eu, Gd, and Er). J. Am. Ceram. Soc. 2013, 96, 1504−1511. (31) Zhang, L.; He, H. Q.; Wu, H.; Li, C. Z.; Jiang, S. P. Synthesis and characterization of doped La9ASi6O 26.5 (A = Ca, Sr, Ba) oxyapatite electrolyte by a water-based gel-casting Route. Int. J. Hydrogen Energy 2011, 36, 6862−6874. (32) Filatov, S. K. Vysokotemperaturnaya kristallokhimiya. Teoriya, metody I rezul’taty issledovanii (High-Temperature crystal chemistry: Theory, methods, and results) Nedra: Leningrad, 1990. (33) Chernorukov, N. G.; Knyazev, A. V.; Bulanov, E. N. Phase transitions and thermal expansion of apatite-structured compounds. Inorg. Mater. 2011, 47, 172−177. (34) Skakle, J. M. S.; Dickson, C. L.; Glasser, F. P. The crystal structures of CeSiO4 and Ca2Ce8(SiO4)6O2. Powder Diffr. 2000, 15, 234−238.

AUTHOR INFORMATION

Corresponding Author

*B.G.: e-mail, [email protected], [email protected]; tel, 91-0416-2202338/2202393; fax, 91-0416-2243092. ORCID

Ramya Ravikumar: 0000-0002-2349-6673 Buvaneswari Gopal: 0000-0002-9578-6020 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the UGCDAE-CSR of India and thank VIT, Vellore, India, for providing all required facilities to carry out the experiments.

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ABBREVIATIONS WF, wasteform; CTE, coefficient of thermal expansion REFERENCES

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Crystal Chemical Substitution at Ca and La Sites in CaLa4

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