Thermochemistry of glass forming Y-substituted Sr-analogues
of titanite (SrTiSiO5)
Tae-Jin Park, Simon Li, and Alexandra Navrotskya)
Peter A. Rock Thermochemistry Laboratory, University of California at Davis, Davis,California 95616; and NEAT ORU, University of California at Davis, Davis, California 95616
(Received 10 June 2009; accepted 1 September 2009)
Strontium titanium silicates are possible oxide forms for immobilization of short livedfission products in radioactive waste. Through beta decay, strontium decays to yttrium,and then to zirconium. Therefore, not only the stability of Sr-loaded waste forms, but alsothat of a potential decay product series with charge-balance in a naturally occurringmineral or a ceramic is of fundamental importance. Strontium titanosilicate (SrTiSiO5) isthe Sr-analogue of titanite (CaTiSiO5). To incorporate the reaction 3Sr2+ = 2Y3+ +vacancy in the titanite composition, Y-substituted Sr-analogues of titanite, (Sr1–xY2/3x)TiSiO5 (x = 0, 0.25, 0.5, 0.75) were prepared by high temperature synthesis and werefound to form glass upon cooling. The Y-end-member (Y2/3TiSiO5, x = 1) crystallized toa mixture of Y2Ti2O7, TiO2, and SiO2 upon quenching in air. The enthalpies of formationof Y-substituted Sr-titanite glasses were obtained from drop solution calorimetry in amolten lead borate (2PbO�B2O3) solvent at 702
�C. The enthalpies of formation fromconstituent oxides are exothermic but become less so with increasing Y content. Thethermodynamic stability of the Y-substituted Sr-analogue of crystalline titanite maybecome marginal with increasing yttrium content.
I. INTRODUCTION
Nuclear waste from reactors contains varying quantitiesof radioactive nuclides with relatively short half lives of�30 yr, namely 137Cs and 90Sr, in addition to the actinides.Development of Cs/Sr-loaded waste forms is of currentinterest because of the high radiation levels and emissionof b and g–radiation associated with the decay of Cs andSr and because of proposed schemes to separate the shortlived isotopes and actinides into two different wastestreams.1–3 Titanosilicates (TS, also referred to as ST,silicotitanates) as waste forms for Cs and Sr are attractivedue to their successful function as selective ion exchangersand their chemical, mechanical, and structural stabilitycomparable to that of borosilicates or aluminosilicates.4–9
Processing schemes to utilize Cs/Sr-loaded titanosilicatewaste forms resemble those already well-developed toproduce Synroc.10 Most studies related to the Cs/Sr-loadedtitanosilicate waste forms have focused on their structureand selectivity.11–15 What is still lacking in this area is afundamental understanding of the thermodynamics of thetitanosilicate waste forms. Moreover, the relations amongcrystalline and amorphous or glassy states for waste formsare important due to the possibility of producing materialinstability through amorphization from radiation damageand during vitrification.16–19
Strontium titanosilicate (SrTiSiO5) is the Sr-analogueof titanite (CaTiSiO5). Titanite (also called sphene) hasbeen developed as a material for immobilizing high-level nuclear fuel recycling wastes because it is a com-mon accessory and leach-resistant mineral.20–22 It canincorporate up to 21.5 wt% Gd2O3 and 9.3 wt% UO2.
21
As a selective ion exchanger for Cs+, the Na-analogue oftitanite (Na2TiSiO5) was found to be the most commonphase formed under hydrothermal conditions.23 Accuratethermodynamic data are essential in calculations of thestability of titanite and in the thermodynamic modelingof its reactions. The enthalpy of formation and vitrifica-tion of titanite has been previously determined.24 Similardata for the Sr-analogue of titanite are needed.Further challenges relate to the incorporation of the
decay products (Ba, Y, Zr) into the waste forms initiallydesigned to hold Cs and Sr. 90Sr is a radioactive isotopewhich is the fission product of 235U and 239Pu (yield�6%). It has a half-life of 28.8 yr and it decays to 90Ywhich has a much shorter half-life, 64.1 h, decaying to90Zr. In nuclear physics, secular equilibrium is radioac-tive equilibrium where the half-life of the precursor (par-ent) isotope is so long that the change of its activity canbe ignored during the period of interest and all activitiesremain constant.25 The relative difference in half-life ofthe isotopes can lead to secular equilibrium when lA/lB� 10�4 (that is lA « lB), where lA and lB are the decayconstants of the parent and daughter isotopes, respec-tively.26 The decay constant lSr and lY are 6.60 � 10�5
a)Address all correspondence to this author.e-mail: [email protected]
for 90Sr and 2.57 � 10�1 day�1 for 90Y.26 These con-stants (lSr/lY = 2.57 � 10�4) satisfy the requirementfor secular equilibrium. Therefore, it is of interest tostudy phase equilibria and thermodynamics involvingthe intermediates in the decay process, since the activ-ities of Sr and Y in a sample become equal in secularequilibrium, although the concentrations differ on theorder of hours.
In the decay chain of Sr to Y to Zr, the steady stateconcentration of yttrium at secular equilibrium is low(�257 ppm per Sr). However every Zr that is finallyproduced from Sr has spent some time being Y, and thelocal environment of the waste form has presumablyresponded to the stepwise change in chemistry as well asthe radiation. To separate the effects of these differentfactors, it is appropriate to study the effects of yttrium aswell as zirconium substitution. Furthermore, yttriumserves as a model for other trivalent rare earths, whichare likely to be present in a real waste form. As a firststep, yttrium substitution in the absence of radioactivityis the focus of this paper. Because ionic substitutionsmust occur by charge balanced routes, this study focuseson such a reaction, 3Sr2+ = 2Y3+ + vacancy in the titanite,SrTiSiO5, composition. Future work will address zirconi-um substitution and simultaneous Y, Zr substitution.
There are few studies of the structure or stabilityof the ceramics containing the decay products, such asY- or Zr-substituted Sr-titanosilicate. Since Zr is not asbasic as Sr, Zr4+ would likely replace Ti4+ rather thanSr2+. For example, crystal structures and phase forma-tion kinetics in the SrTiO3-SrZrO3 solid solutions werereported.27,28 In the CaTi1–xZrxSiO5 series, the existenceof a Zr analogue of titanite in nature is considered un-likely.29 Therefore, it is of interest to study whether Y orZr can be substituted into SrTiSiO5 without leading tothe formation of one or more additional phases. Here, westudy the thermochemistry of a potential intermediate indecay product series with charge balance in the titanite.We have investigated the enthalpy of formation of theY-substituted Sr-analogue of titanite, Sr1–xY2/3xTiSiO5
(x = 0, 0.25, 0.5, 0.75, 1) using both oxide melt solutioncalorimetric and differential scanning calorimetric(DSC) techniques. We discuss the heat of crystallizationand formation of Sr-titanite in comparison to other sili-cates. However, it is important to note that Zr-substitutedSrTiSiO5 would accrue in significant proportions as timeproceeds, because of the short half-life of 90Y. The pres-ent studies are done with nonradioactive analogue mate-rials. This is necessary for safety reasons in a universityenvironment but it also provides an advantage, namelythe opportunity to consider separately the effects of in-trinsic chemistry and of radiation damage. Furthermore,the present studies add to more general understanding ofcomplex ceramic and glass forming systems, a topicbroader than radioactive waste management.
II. EXPERIMENTAL
Sr1–xY2/3xTiSiO5 [x = 0 (A), 0.25 (B), 0.5 (C), 0.75 (D),and 1 (E)] were made by high temperature synthesis.SrCO3, Y2O3, TiO2, and SiO2 with desired molar ratioswere ground thoroughly, and the mixture was heated in aplatinum crucible in air at 1550 �C for 30 h, followed byair quenching to room temperature. This produced glassysamples except for the Y end-member (Y2/3TiSiO5, x = 1),which, upon quenching in air, the Y-end-member crystal-lized to a mixture of Y2Ti2O7, TiO2 (rutile) and likelyglassy SiO2. The glassy samples were then heated at1100 �C for 3 h. Sample A crystallized to a mixture ofSr2TiSi2O8, SrTiO3, and TiO2, and sample E to a mixtureof Y2Ti2O7 and TiO2. Samples B, C, and D crystallized tomixtures of unknown phases. Samples were characterizedby a number of different methods including powder x-raydiffraction (XRD), microprobe analysis, differential scan-ning calorimetry (DSC), and high temperature oxide meltsolution calorimetry.
A. X-ray diffraction
Diffraction patterns were collected using a Bruker D8Advance x-ray diffractometer (Madison, WI) operated at40 kV and 40 mA using CuKa (l = 1.54056 A) radiationat two theta angles between 10 and 60� with a step sizeof 0.02�. Phase identification was performed using Jade6.1 software (Materials Data Inc., Livermore, CA) andthe PDF file version 2.0 (PDF-2 Database, JCPDS-ICDD, Newton Square, PA, 1999).
B. Differential scanning calorimetry
The glass transition and crystallization temperaturesof the glass samples were measured using a Netzsch449 thermal analysis system (Netzsch, Inc., Selb, Ger-many). The sample pellets (50�80 mg) were weighed andplaced in Pt pans and continuously heated from 40 �C to1100 �C with Ar flow. Ramp rates were 10� per min forheating and 20� per min for cooling. Continuous scanswere collected of the baseline (empty crucible), corundumstandard, and sample in sequence. Baseline correctionswere applied to both the standard and the sample runs.Calibration curves were obtained using accepted valuesof the heat capacity of corundum.30
C. Electron microprobe analysis
Chemical compositions were determined by wave-length dispersive electron probe microanalysis using aCameca SX-100 electron microprobe (Cameca Inc.,Courbevoie, France) operated at an accelerating voltageof 20 keV, a beam current of 10 nA, and a beam size of�1 mm. Analytic lines and crystals were YLa on LTAP,TiKa and SiKa on LPET, and AlKa and SrLa on TAP.The calibration standards used were Y3Al5O12 (YAG)
T-J. Park et al.: Thermochemistry of glass forming Y-substituted Sr-analogues of titanite (SrTiSiO5)
J. Mater. Res., Vol. 24, No. 11, Nov 2009 3381
for Y2O3 and Al2O3; SrTiO3 for SrO and TiO2; andquartz for SiO2. Before microprobe analysis, sampleswere polished and carbon coated.
D. High temperature oxide melt solutioncalorimetry
Calorimetric measurements were carried out employ-ing a custom-built Tian-Calvet microcalorimeter operat-ing at 702 �C with molten lead borate (2PbO�B2O3) assolvent. The instrument and experimental protocol havebeen described in detail by Navrotsky.31,32 A samplepellet of pressed powder weighing �5 mg was droppedfrom room temperature into the lead borate solvent in thehot calorimeter. The heat effect measured in drop solu-tion calorimetry includes the enthalpy associated withheating the sample from room temperature to 702 �C(heat content) in addition to the enthalpy of dissolutionof the sample in the lead borate melt at calorimetric tem-perature. The calorimeter was calibrated against theknown heat content of�5 mg corundum pellets. To facili-tate the dissolution of the sample as well as to ensurethe oxidation state of Ti (Ti4+), air was bubbled throughthe solvent at �5 cm3/min. The calorimetric assemblywas flushed with another flow of air at �80 cm3/minabove the solvent. Between 6 and 12 pellets were droppedfor each material. To minimize surface water, all sampleswere dried at �130 �C overnight immediately prior tocalorimetry and stored in the desiccator after drying. Allsamples dissolved reproducibly with a return to the cal-orimeter baseline signal within about 35 min, indicative ofproblem-free calorimetry.
III. RESULTS AND DISCUSSION
The phase purity and crystallinity were initially char-acterized using powder x-ray diffraction (XRD, Fig. 1).The glass samples (A, B, C, D) showed typical broadXRD background signals at low angles confirming nosignificant crystallization upon quenching. A trace inten-sity of corundum peaks for the glass samples was ob-served due to possible contamination during grinding.Upon quenching in air, the Y-end-member Y2/3TiSiO5
(E) crystallized to a mixture of Y2Ti2O7, TiO2 (rutile),and possibly leaving glassy SiO2. This observation sug-gests that there might be a narrow window in tempera-ture in the formation of glass for this composition. It isalso consistent with the slightly positive formation en-thalpy of Y2/3TiSiO5 from the constituent oxides, seebelow. The Y-substituted Sr-titanite glass system canhold a realistic extent of substitution of Sr by Y up to atleast 50% Sr in the charge-balanced titanite composition(D, Sr0.5Y0.33TiSiO5). In the context of waste loading onan oxide basis, this is 16.4 wt% Y2O3 and 22.6 wt% SrOloaded in this glass. The initial SrTiSiO5 holds 42.5 wt%
SrO. Thus this system is indeed promising as a wasteform with high levels of loading.Although most characterization has focused on glassy
samples in this study, the crystallization behavior of theglasses was also examined by heating A, B, C, and Dat 1100 �C for 3 h. We note that this treatment wasnot intended to ensure maximum crystallization of theglasses, but to correlate with the major crystallizationpeaks observed in TG/DSC measurements (see below).In the crystallized samples, fresnoite Sr2TiSi2O8
(PDF #39-0228), perovskite SrTiO3 (PDF #64-0615),and rutile TiO2 (PDF #65-1835) were identified in sam-ple A; unknown phases were observed in samples B andC; and pyrochlore Y2Ti2O7 (PDF #85-1584) and rutileTiO2 were identified in sample D. XRD patterns of thecrystallized samples and Sr2TiSi2O8 as well as sampleE (Y2/3TiSiO5) are found in supplementary information(Fig. A1). From the XRD peak intensities, the crystal-lization behavior of Y-substituted Sr-titanite can be sum-marized as follows: Sr-rich samples crystallize mainlyto the Sr-analogue of fresnoite (Sr2TiSi2O8) and Y-richsamples mainly to pyrochlore Y2Ti2O7.The chemical compositions measured by electron mi-
croprobe are listed in Table I. For each sample, between8 and 12 points were analyzed and compositions pre-sented are the average value with errors of two standarddeviations of the mean. The analyzed compositionsagree with the nominal stoichiometry within experimen-tal accuracy. In addition, backscattered electron (BSE)images (Fig. 2) for the samples show no anomalous
FIG. 1. The powder x-ray diffraction (XRD) patterns of as-prepared
Y-substituted Sr-analogue of titanite (Sr1–xY0.67xTiSiO5, x = 0, 0.25,
0.5, 0.75, and 1). Peaks corresponding to the pyrochlore Y2Ti2O7
phase are marked with a P and those for the rutile TiO2 are marked
with an R. Trace corundum impurity peaks are observed for the glass
samples (x = 0, 0.25, 0.5, and 0.75) and are marked with an asterisk.
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J. Mater. Res., Vol. 24, No. 11, Nov 20093382
grains or secondary phases, and no heterogeneities incomposition for glassy samples. For Sr0.25Y0.5TiSiO5
(D, x = 0.75), a Ti-rich phase was exsolved as �1 mmuniform dots embedded in the glass matrix. Y0.67TiSiO5
(E, x = 1) showed larger (�20 mm) Ti-rich (rutile asconfirmed by XRD) phase in snowflake-like structureswithin the crystallized Y2Ti2O7 phase. We preparedY1.33TiSiO6 (F) for comparison in glass forming behav-ior with our titanosilicate system. Surprisingly, the moreY-rich titanosilicate formed a homogeneous glass [Fig. 2(f)].This suggests that sample F (51.8 wt% Y2O3) hasmuch better glass forming ability than the Y-analogueof titanite (E, 35.0 wt% Y2O3).
Crystallization and phase transitions were explored bythermogravimetry and differential scanning calorimetry[TG/DSC, Fig. 3(a)]. TG of all the samples showed nosignificant weight change (less than 0.3%). The onsetglass transition temperatures (Tg) of samples A, B, C,and D were 767, 768, 770, and 789 �C, respectively. Tgof the glassy samples (A, B, and C) were almost thesame. The onset crystallization temperature (Tc) of A, B,C, and D were 842, 883, 912, and 884 �C, respectively. Tcincreased with the Y content except for sample D, whichalready had a trace of rutile nuclei and showed differentbehavior. Its Tg (789
�C) is higher than other glasses, andits Tc (884
�C) is lower than other glasses [Fig. 3(b)]. Theenthalpies of crystallization (DHcryst) for A, B, C, and Dwere measured by DSC to be –34.7, –43.8, –31.1, and–20.6 kJ/mol, respectively. For A (SrTiSiO5), the majorexothermic peak at 861 �C was due to crystallizationto Sr2TiSi2O8, confirmed by XRD. In addition, a weakexothermic peak was observed at 1007 �C, suggestingthat an additional phase formed. Assuming that the peak
TABLE I. Electron microprobe analyzed chemical compositions of
for Y-substituted Sr1–x,Y0.67xTiSiO5 (titanite) samples.
the glass transition temperatures (Tg) and the enthalpies of crystalliza-
tion (DHcryst) for Y-substituted Sr-analogue of titanite glass samples.
(b) Tg and the onset crystallization (Tc) are summarized as a function
of Y content.
T-J. Park et al.: Thermochemistry of glass forming Y-substituted Sr-analogues of titanite (SrTiSiO5)
J. Mater. Res., Vol. 24, No. 11, Nov 2009 3383
at 1007 �C is due to the crystallization of TiO2 (rutile),we can write the reaction: SrTiSiO5 (glass) ! ½Sr2Ti-Si2O8 + ½TiO2. The DHcryst calculated is –34.7 kJ/moland this value is comparable to half of DHvit of Sr2Ti-Si2O8 (68.5 � 6.2 kJ/mol) determined from drop solutionenthalpies for Sr2TiSi2O8 glass and crystal.33 This resultindicates that there might be only a narrow window intemperature for the synthesis of the crystalline Sr-ana-logue of titanite (SrTiSiO5), because the energeticallymore stable fresnoite phase would form and leavenon-stoichiometric residues prior to the crystallization ofSrTiSiO5. This glass-crystal behavior is analogous to thatof BaTiSiO5 reported in our previous study.
34
We have obtainedDT = Tc – Tg to further understand theglass stability of our samples.DT for A, B, C, and D are 75,115, 142, and 95 �C, respectively. This suggests that theglass stability increases in the order: C > B > A. Crystal-lization includes both nucleation and crystal growth.Crystallization can happen quickly (bulk crystallization)or slowly (surface crystallization). In the latter case, nucle-ation starts on the surface first, then the crystal growthcontinues into the bulk. DT less than�100 �C is often usedas the criterion for bulk crystallization for similar titanosi-licate materials.35 In our samples, by this definition,A undergoes bulk crystallization, whereas B and C under-go surface crystallization. DT (95 �C) for D suggeststhat the preexisting rutile crystal nuclei likely act as seedsto reduce the resistance to crystallization during heating.
Measured drop solution enthalpies (DHds) of Sr1–xY2/3x
TiSiO5 (x = 0, 0.25, 0.5) samples as well as referenceDHds values of binary oxides, SrO,36 YO1.5,
37 TiO2,38
and SiO239 (Table II), were used in a thermodynamic
cycle to establish the enthalpy of formation (DHof,ox) of
Sr1–xY2/3xTiSiO5 from the constituent oxides (Table III).The standard enthalpies of formation of titanite fromthe elements (DHo
f,el) can be derived from the calculatedDHo
f,ox values and the DHof,el values of the constituent
oxides (Table II).36,37,40 DHof,ox of CaTiSiO5 and SrTiSiO5
glass are –38.8 � 3.424 and –162.9 � 2.4 kJ/mol, respec-tively. Thus, SrTiSiO5 glass is more stable than titanite(CaTiSiO5) with respect to its oxides reflecting the greaterbasicity of SrO than that of CaO.Figure 4 shows a linear relation between DHo
f,ox andx (YO1.5 content), suggesting zero enthalpy of mixingfor Sr1–xY0.67xTiSiO5 glasses, within 0 � x � 0.5. Inaddition, DHo
f,ox becomes less exothermic with substitu-tion of Sr2+ by Y3+. This behavior indicates a destabilizingeffect of the charge-coupled substitution 3Sr2+ = 2Y3+
on the titanite composition. Since Y2O3 is less basic thanSrO, this trend is expected. However, we consider anadditional factor, the concentration of non-tetravalentcations, to explain better glass forming ability forY1.33TiSiO6 (F) than for Y0.67TiSiO5 (E). Although tet-ravalent cations (Ti and Si) do not explicitly form frame-works at titanite composition, we believe their functionis distinguishable from that of non-tetravalent cations.
TABLE II. Enthalpies of drop solution in lead borate at 702 �C (DHds) and enthalpies of formation from the oxides (DHof,ox) and from the
elements (DHof,el) at 25
�C for Y-substituted Sr-analogue of titanite samples Sr1–x,Y0.67xTiSiO5 (x = 0, 0.25, 0.5) and reference values of binary
DHof,ox (DH6) = �DH1 + (1 – x)DH2 + 0.67xDH3 + DH4 + DH5; DHds values are found in Table II.
T-J. Park et al.: Thermochemistry of glass forming Y-substituted Sr-analogues of titanite (SrTiSiO5)
J. Mater. Res., Vol. 24, No. 11, Nov 20093384
Sample F has twice the non-tetravalent cations withrespect to the tetravalent cations compared to sample E.The ratios of non-tetravalent cations to tetravalentcations [Rcat = (Sr + Y)/(Ti + Si)] for the samples A, B,C, D, and E are 0.50, 0.46, 0.42, 0.38, and 0.33, res-pectively. Rcat for F, which forms a homogeneous glass,is 0.67. This suggests that the easy crystallization startswhen Rcat � 0.38 (D, as confirmed by microscopy).Rcat also shows a linear correlation [Fig. 4(b)]. We notethat extrapolation of the linearity in DHo
f,ox and Rcat
shows an endothermic formation enthalpy for E, whichis indeed observed.
IV. CONCLUSION
A series of Sr-analogues of titanite glass waste formfor radioactive Sr and its potential intermediates in betadecay product has been prepared between SrTiSiO5 andY0.67TiSiO5 by high temperature synthesis. The Y-sub-stituted Sr-titanite glass system can hold an extent ofsubstitution of Sr by Y up to at least 50% Sr in thecharge-balanced titanite composition. Crystallization be-havior of the Sr1–xY0.67xTiSiO5 glass system can be sum-marized as follows: Sr-rich samples crystallize mainly tothe Sr-analogue of fresnoite (Sr2TiSi2O8) and Y-richsamples mainly to pyrochlore Y2Ti2O7. The enthalpy offormation (DHo
f,ox) of SrTiSiO5 glass is measured tobe –162.9 � 2.4 kJ/mol as compared with that of titanite(CaTiSiO5) glass, –38.78 � 3.4 kJ/mol. DHo
f,ox forY-substituted Sr-titanite samples from constituent oxidesis exothermic but becomes less so with increasing Y content.The thermodynamic stability of the Y-substituted Sr-analogue of crystalline titanite may become marginalwith increasing Y content. The destabilizing effect intitanite compositions with increasing Y can be under-stood in terms of the basicity difference between SrOand YO1.5 as well as the ratios of non-tetravalent cationsto tetravalent cations [Rcat = (Sr+Y)/(Ti+Si)]. Leachingtests to investigate the durability of SrTiSiO5 andY-substituted SrTiSiO5 should be done to test the stabil-ity to the environment. The role of Zr4+ (produced byfurther decay of Y3+) needs to be investigated.
ACKNOWLEDGMENTS
We acknowledge the U.S. Department of Energy(NERI Program Grant: DE-FC07-07ID14830) for sup-port. We thank S. Roeske (UCD) for help with electronmicroprobe analysis.
