CERAMICS Compositional control of tunnel features in hollandite- based ceramics: structure and stability of (Ba,Cs) 1.33 (Zn,Ti) 8 O 16 R. Grote 1 , M. Zhao 1 , L. Shuller-Nickles 2,6 , J. Amoroso 3 , W. Gong 4 , K. Lilova 4 , A. Navrotsky 4 , M. Tang 5 , and K. S. Brinkman 1,6, * 1 Department of Materials Science and Engineering, Clemson University, Clemson, SC, USA 2 Department of Environmental Engineering and Earth Sciences, Clemson, SC 29634, USA 3 Savannah River National Laboratory, Aiken, SC 29808, USA 4 University of California Davis, Davis, CA 95616, USA 5 Los Alamos National Laboratory, Los Alamos, NM 87545, USA 6 Center for Nuclear Environmental Engineering Sciences and Radioactive Waste Management (NEESRWM), Clemson University, Clemson, SC, USA Received: 1 August 2018 Accepted: 5 September 2018 Ó Springer Science+Business Media, LLC, part of Springer Nature 2018 ABSTRACT The impact of composition on the tunnel features of hollandite materials for the purpose of radioactive cesium (Cs) immobilization was evaluated. The barium (Ba) to cesium (Cs) ratio was varied in the tunnel sites referred to as the A-site of the hollandite structure. Zinc (Zn) was substituted for titanium (Ti) on the B-site to achieve the targeted stoichiometry with a general formula of Ba x Cs y Zn x ?y /2 Ti 8-x -y /2 O 16 (0 \ x \ 1.33; 0 \ y \ 1.33). The tunnel cross-section depended on the average A-site cation radius, while the tunnel length depended on the average B-site cation radius. Substitution of Cs resulted in a phase transition from a monoclinic to a tetragonal structure and an increase in unit cell volume of 1.8% across the compositional range. Cs loss due to thermal evaporation was found to decrease in compositions with higher Cs content. The enthalpies of formation from binary oxides of Zn-doped hollandite measured using high- temperature oxide melt solution calorimetry were strongly negative, indicating thermodynamic stability with respect to their parent oxides. The formation enthalpies became more negative, indicating hollandite formation is more energetically favorable, when Cs was substituted for Ba across the range of Zn- doped compositions investigated in this study. Compositions with high Cs content exhibited lower melting points of approximately 80 °C. In addition, high Cs content materials exhibited a significant reduction in Cs release from the solid to liquid phase by leaching or aqueous corrosion as compared to low Cs content materials. These property changes would be beneficial for applications in radioactive cesium immobilization in a multi-phase ceramic by allowing for Address correspondence to E-mail: [email protected]https://doi.org/10.1007/s10853-018-2904-1 J Mater Sci Ceramics
Transcript
CERAMICS
Compositional control of tunnel features in hollandite-
based ceramics: structure and stability
of (Ba,Cs)1.33(Zn,Ti)8O16
R. Grote1 , M. Zhao1 , L. Shuller-Nickles2,6 , J. Amoroso3 , W. Gong 4 , K. Lilova4 ,A. Navrotsky4 , M. Tang 5 , and K. S. Brinkman1,6,*
1Department of Materials Science and Engineering, Clemson University, Clemson, SC, USA2Department of Environmental Engineering and Earth Sciences, Clemson, SC 29634, USA3Savannah River National Laboratory, Aiken, SC 29808, USA4University of California Davis, Davis, CA 95616, USA5Los Alamos National Laboratory, Los Alamos, NM 87545, USA6Center for Nuclear Environmental Engineering Sciences and Radioactive Waste Management (NEESRWM), Clemson University,Clemson, SC, USA
Received: 1 August 2018
Accepted: 5 September 2018
! Springer Science+Business
Media, LLC, part of Springer
Nature 2018
ABSTRACT
The impact of composition on the tunnel features of hollandite materials for thepurpose of radioactive cesium (Cs) immobilization was evaluated. The barium(Ba) to cesium (Cs) ratio was varied in the tunnel sites referred to as the A-site ofthe hollandite structure. Zinc (Zn) was substituted for titanium (Ti) on the B-siteto achieve the targeted stoichiometry with a general formula of BaxCsyZnx?y/2
Ti8-x-y/2O16 (0\ x \ 1.33; 0\ y \1.33). The tunnel cross-section depended onthe average A-site cation radius, while the tunnel length depended on theaverage B-site cation radius. Substitution of Cs resulted in a phase transitionfrom a monoclinic to a tetragonal structure and an increase in unit cell volume of1.8% across the compositional range. Cs loss due to thermal evaporation wasfound to decrease in compositions with higher Cs content. The enthalpies offormation from binary oxides of Zn-doped hollandite measured using high-temperature oxide melt solution calorimetry were strongly negative, indicatingthermodynamic stability with respect to their parent oxides. The formationenthalpies became more negative, indicating hollandite formation is moreenergetically favorable, when Cs was substituted for Ba across the range of Zn-doped compositions investigated in this study. Compositions with high Cscontent exhibited lower melting points of approximately 80 "C. In addition, highCs content materials exhibited a significant reduction in Cs release from thesolid to liquid phase by leaching or aqueous corrosion as compared to low Cscontent materials. These property changes would be beneficial for applicationsin radioactive cesium immobilization in a multi-phase ceramic by allowing for
decreased processing temperatures and higher cesium weight loadings. Morebroadly, these results establish the link between composition, structural sym-metry, and thermodynamic stability for tunnel structured ceramics withimplications in the design of new energy conversion and storage materials.
Abbreviations
SYNROC Synthetic rockPCT Product consistency test
List of symbolsrA Average radius of atoms on A-siterB Average radius of atoms on B-siterO Radius of oxygentH Tolerance factorZB Valence of B cationdA Excess size of A cationdB Excess size of B cation
Introduction
Crystalline ceramics have been proposed as analternative to glass as an immobilization matrix (i.e.,waste form) for disposal of nuclear waste due to theenhanced retention of radionuclides in hydrothermalenvironments compared to current aluminoborosili-cate glasses [1–4]. Crystalline ceramic waste formswere pioneered by Ringwood and others in the late1970s [5]. Ceramic waste forms are generically ter-med SYNROC, a portmanteau of ‘‘synthetic rock’’ asthey are designed to mimic geologically stable min-erals assemblages. SYNROC typically consists of amulti-phase composite of distinct crystalline phases,each of which is compositionally tailored to immo-bilize specific elements or isotopes by chemicalincorporation into the crystalline lattice. Hollandite isthe mineral phase most often selected to immobilizeCs in the various SYNROC formulations. However,there are several key challenges associated withimmobilizing Cs. Firstly, Cs isotopes are a majorcontributor to the heat generated and ionizing radi-ation produced during the first few hundred years ofstorage, requiring thermally stable materials systems[6]. Secondly, Cs is a large cation requiring coordi-nation with eight nearest neighbors. This situationtypically occurs in model crystalline phases such as
the CsCl structure [7]. The atomic size and watersolubility of common Cs containing phases has dri-ven research toward more complex structures such ashollandites, pollucites, glass and glass ceramics forCs immobilization [3, 8–10].
In SYNROC, the hollandite phase is a limitingfactor for system performance due to its lower radi-ation tolerance, higher leaching rate, and lower sug-gested waste loading of 5% [11]. For these reasons,considerable work has been performed on the sta-bility, waste loading, and radiation tolerance of hol-landite for Cs immobilization [2, 4, 12, 13]. This studyexplores the impact of varying A-site Cs stoichiom-etry of Zn-doped hollandites with the formulaBaxCsyZnx?y/2Ti8-x-y/2O16 (0\ x \1.33; 0\ y \1.33).Specifically, this work examines the impact of Cscontent on the hollandite structure, the melting point,the thermodynamic of formation and the aqueousleaching rates. Additionally, these compositionsprovide an opportunity to explore the effects of Zn,which is a minor component of the waste stream andhas a larger ionic radius and lower valence (Zn2?) ascompared to other dopants in the hollandite system(M3?on the B-site such as Fe3?, Al3?, Ga3? [11, 13]).These properties can be used to optimize scalableprocessing methods, such as melt and crystallization,as well as aid in the compositional development ofresilient ceramic waste forms.
Background
Hollandite1 has the general formula A2B8O16, whereA is either mono- or divalent and B is di-, tri-, ortetravalent [5]. A wide range of substitutional com-positions are possible due to the structural flexibility
1 It is worth noting that for the purposes of this work,hollandite (BaMn8O16) refers to hollandite-type structureswhich, are generically classified under a mineralogical hollan-dite supergroup consisting of Mn(IV) and Ti(IV) oxidesdistinguished by tunneled structures with tetragonal orpseudo-tetragonal crystal symmetry.
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[4, 11, 14–21]. In Fig. 1, the two structural forms ofhollandite are shown: tetragonal (I4/m) and mono-clinic (I2/m) [22, 23]. The hollandite structure is basedon pairs of edge-sharing BO6 octahedra extendingalong the short crystallographic axis. These BO6 pairsshare corners with neighboring BO6 pairs forming atunnel framework, which accommodates the largerA-site cations. The tunnel size can be altered bychanging the A-site and/or B-site cations [13]. Sev-eral mechanisms can be used to compensate for thecharge imbalance induced by doping Cs1? on theBa2? site: including i) Ti4? reducing to Ti3? and ii)Ti4? partially replacing a lower valent B-site cation,M3?/2?. Substituting large cations on the B-site, suchas Zn, will increase the tunnel size and unit cellvolume providing more space for Cs storage.[11, 24–26]. Additionally, there is higher deformationof the a/b plane when doping larger cations on theA-site [24, 27, 28]. When a larger cation, such as Cs, isadded to the composition, rB decreases resulting inless B–B cation repulsion and smaller octahedra. Oneway to compensate for the increased rA is by a dis-tortion of the BO6 octahedra and expansion of the a/bplane. The causal relationship for a/b plane expan-sion is not fully understood; however, BO6 octahedraldistortion has been reported for hollandites analyzedunder pressure [24, 29].
Between 2004 and 2006, a Cs-loaded Zn-dopedhollandite (Ba,Cs)1.2(Zn,Ti)8O16, was synthesized andfound to have relatively high Cs leach resistance;however, compositions with high Cs content werereported to be unstable and were unable to be syn-thesized [1, 30, 31]. Zn-based hollandites have alsobeen investigated as high dielectric constant
materials with a range of Ba/Sr substitutions on theA-site [32]. In recent work by Xu et al., a completesolid solution has been demonstrated for Ga-dopedhollandite of the form BaxCsyGa2x?yTi8-2x-yO16
(0\ x \ 1.33; y = 1.33 - x ), which exhibited tetrago-nal symmetry across the full solid solution. A solidsolution across the A-site compositional range hasnot been observed to date in Zn-doped hollandites.
Scalable processing techniques, such as meltingand crystallization require knowledge of the compo-sitionally dependent melting points of candidatematerials. To date, there have been limited studies onthe melting point of Ba-hollandite. Some studies haveinvestigated the B-site dopant effect on melting point,as well as the effluent gas composition and crystal-lization temperature [11, 12, 33]. The melting pointdata from the studies listed in Table 2 show that themelting point depends primarily on the B-site cationsin the compositional ranges studied. Samples byCarter et al. were melt processed and contained lessthan 65% hollandite with melting points that were100–200 "C lower than [33]. In work by Costa et al.,precursor powders formed single phase hollanditewith melting data close to values published by Aubinet al. [11, 12, 33]. Understanding the dopant effects onthe melting point and elemental volatility will aid indeveloping methods for efficient large-scale process-ing techniques.
The structural stability of hollandite is governed bythe interplay between the ionic radius, cation repul-sion, tunnel cation ordering, cation occupancy andcation the valence. Post et al. (1982) developed a simpleratio rule, where rB refers to the average B cation radiiand rA refers to the average A cation radii. Post’s rulestates that when rB= rA\0:48 the structure will betetragonal. However, this rule does not address thenumber (or valence) of cations and uses the averagecation size, which ought to be accounted for whendetermining structural stability. Kesson et al. (1986)showed that a tolerance factor, called tH was based onthe averageA-site cation radii, rA, average B-site cationradii, rB, and oxygen radius, rO, could be used to pre-dict the propensity for hollandite phase formation. Inthis framework, an ideal tetragonal structure wouldhave a tH = 1,while tH values ranging from 0.93 to 1.16were predicted to form a stable hollandite phase [23].Drawbacks of this model include not accounting foroctahedral distortions, ordering, or symmetry as itrelies entirely on ionic radii.
Figure 1 Hollandite unit cell viewed down the c-axis (tunnel-direction) where tetragonal is (a = b) = c and monoclinic is(a = b) = c. The dark balls represent the A-site (Ba, Cs), thegray balls represent the B-site (Zn,Ti) and oxygen is on eachcorner of the tetrahedrons.
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tH ¼rA þ rOð Þ2%1=2 rB þ rOð Þ2
h i1= 2
ffiffiffiffiffiffiffiffi3= 2
prB þ rOð Þ
ð1Þ
Building on the prior work of Post, Zhang et al. (1994)derived a more complete equation for predicting unitcell length and symmetry. Geometrical considera-tions and atomic level data including the valence ofthe B cation, Z
B, and the excess size of the cations, d
Aand d
B, were used to derive equations for the lattice
constant, a, and the c lattice constant, c. These cal-culations were based on 55 compositions collectedfrom literature, most of which had fully occupied A-site compositions containing Ba (no Cs).
aðA) ¼ 5:130ðrO þ rBÞ % 0:291ZB þ 0:441dA ð2Þ
cðA) ¼ffiffiffi2
pðrO þ rBÞ % 0:0366ZB þ 0:552dB ð3Þ
These equations present a correlation between thetunnel cation size and the structural symmetry of thecompound. Three distinct symmetry regions werealso identified: tetragonal symmetry, monoclinicsymmetry, or undetermined symmetry. Undeter-mined regions correspond to the space betweentetragonal and monoclinic symmetry; in this regionthe symmetry was determined by octahedral distor-tions and ordering effects [24]. The current work willshow that occupancy plays a role in symmetrystability.
The durability, or resistance to elemental release, isa measure of a waste form’s performance and utilityas a host for mobile radionuclides, such as Cs. Twomethods were used for quantifying elemental releasein this work: (1) the ASTM C 1285 product consis-tency test (PCT), which measures elemental releasefrom crushed powder at elevated temperature, and(2) the ASTM C 1220 leach test, which measureselemental release from a monolith at an elevatedtemperature. The PCT (and ASTM C 1220) have beenused globally as standardized leach tests for compo-sitional screening [34, 35].
Experimental methods
Hollandite samples of the general formula BaxCsyZn2x?yTi8-2x-yO16, where x ranged from 0 to 1.333and y = 1.333 - x , were synthesized from reagentgrade oxides and carbonates by solid state reaction inair. This study will discuss nine compositions, five of
which are denoted as Ba133, Cs029, Cs067, Cs100,and Cs133 and four which were used for calorimetry,Cs015, Cs043, Cs029 (ExCs), and Cs133 (ExCs) all ofwhich are listed in Table 1. Those listed with (ExCs)contained 15% excess Cs atoms prior to calcination.The precursor powders were ball milled in ethanoland calcined for 10 h at 1225 "C. The powders weresubsequently ball milled and calcined a second timefor 10 h at 1225 "C. Calcined powders were ballmilled and pressed into pellets (335 MPa) followedby sintering at 1300 "C for 1 h.
Structure and elemental composition were charac-terized by XRD with Cu Ka wavelength (k = 1.54 A)with a Rigaku Ultima IV and refined using the Full-Prof software. SEM and EDX were collected with aS-3400N VP-SEM. TEM micrographs were imaged atANL with a 300 keV Hitachi 9000. Weight percent ofsecondary phases were sufficiently low as to bewithin the XRD error. Therefore, the secondary phaseweight percent was calculated by averaging the areaof rutile per total area of SEM images.
Thermogravimetric analysis (TGA) was performedusing a Perkin Elmer TGA 7 to record from 25 to1000 "C at 10 "C/min and 1000 "C to 1250 "C at 5 "C/min. Differential Scanning Calorimetry (DSC) wasperformed using a Netzsch DSC 404 C to measurefrom 25 to 1450 "C on 6–10 mg samples in yttrium-coated alumina crucibles at 10 "C/min with 60 mL/min argon flow. Infrared (IR-spectroscopy) was per-formed with a Thermo-Nicolet Magna 550 FTIR aftersamples were calcined at four different temperatures;350 "C, 650 "C, 800 "C and 1050 "C. The meltingpoints were calculated from the onset of theendothermic peak.
High-temperature oxide melt solution calorimetrywas performed using the AlexSYS 1000 calorimeter(SETARAM) operating at 702 "C with sodiummolybdate (3Na2O&4MoO3) solvent to measureenthalpies of formation of six different Zn-dopedhollandites. Detailed instrument and experimentalprocedure can be found in reviews and reports byNavrotsky [36, 37]. Approximately 5–10 mg sampleswere weighted, loosely pressed into pellets anddropped from room temperature into the moltenoxide solvent contained in a platinum crucible in thecalorimeter. The calorimeter assembly was flushedwith gas at an average of 50 mL/min rate. Gas wasalso bubbled through the solvent at 5 mL/min inorder to stir the melt and enhance the dissolution rateof the samples. Since there is no change in the
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oxidation state of the ions after dissolution, either airor oxygen can be used for flushing and bubbling.Measurements were repeated at least six times(should be 8 successful drops or 6 only if the error isbelow 1%) to achieve statistically reliable data. Thereal composition based on EDX measurements wereused for all calculations. The calorimeter was cali-brated using the heat content of a-alumina.
Two methods were used for quantifying elementalrelease: (1) the ASTM C 1285 product consistency test(PCT) which uses particles and (2) the ASTM C 1220leach test, which uses a monolithic specimen. For theASTM C 1285 tests, the PCT-A procedure was fol-lowed. Each composition was measured in triplicateusing * 1.5 g of washed and sieved (- 100/? 200mesh) powder in * 15 mL of de-ionized water. Forthe ASTM C 1220 tests, each composition was mea-sured in duplicate using monolithic samplesapproximately 8 mm x 8 mm x 2 mm in de-ionizedwater (* 15 mL) to maintain a constant surface areato volume (S/V) ratio of 13 mm2/mL between allsamples. In addition to the samples, two blanks and astandard reference material referred to as theapproved reference material (ARM) was prepared intriplicate for the PCT testing. All samples and stan-dards were loaded into sealed stainless steel pressurevessels, sealed with threaded enclosures, and heatedat * 90 "C for * 7 days. Subsequently, the vesselswere cooled and the leachates were filtered, acidified,and measured for elemental composition usinginductively coupled plasma mass spectrometry (ICP-MS). The concentration of elements in the leachateswere compared to the elemental concentrations in the
un-leached samples that were measured using ICP-MS from excess washed and sieved powder (fromPCT preparation) to obtain a fraction loss for eachelement.
Results and discussion
Initial screening experiments for Zn-doped samplesshowed that an A-site occupancy ranges from 63 to83% readily formed hollandite with no secondaryphases which was in agreement with the existingknowledge for stability ranges. A compositionalrange was designed to avoid A-site occupancyeffects; therefore, the occupancy was held constant at67% to allow for comparisons with published data insimilar systems [11, 27, 38]. The 67% occupancy wasselected to create solid solutions with integer valuesof A-site dopants for a limited supercell which wasamenable to DFT modeling performed in prior work[22, 38, 39]. Five compositions, referred to as, Ba133,Cs029, Cs067, Cs100, and Cs133, were prepared totest the range of hollandite stability with corre-sponding variations in Zn/Ti ratio in order to chargebalance the structure. The XRD patterns of the syn-thesized hollandite samples are shown in Fig. 2. Allcompositions were identified as I4/m (tetragonal)hollandite and found good agreement with publishedpowder diffraction data for Ba0.41Cs0.82Ti8O16 (PDF#080-2264) except for the pure Ba composition (Ba133),which was I2/m (monoclinic) and agreed well withBa1.31Ti8O16 (PDF# 080-0910) [40]. Overall, the pat-terns indicate the samples were single phase and
Table 1 Target, EDX and ICP-MS compositions with refined lattice constants
Target composition EDX composition ICP-MS Acronym Lattice constantsa; (b); c
Cs029 (ExCs) and Cs133 (ExCs) contained 15% excess Cs to counteract Cs loss during processing. *Samples used for calorimetry allother tests used the first five compositions
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crystalline. However, a minor peak at about 27.3"corresponding to rutile was visible in the Cs133 pat-tern. In addition, a broad peak in the Ba133 pattern at26.5" has been attributed to tunnel ordering which isalso observed in existing hollandite literature [41, 42].A minor amount of rutile in the otherwise singlephase hollandite has been observed in prior workand likely results from excess Ti, possibly due to Csvolatilization [1]. The XRD patterns exhibited a shiftin the primary hollandite peak near 28" as a functionof composition. The primary peak shifted towardlower 2h (Fig. 2) with increasing Cs content, whichindicates an increased unit cell volume. The observedvolume expansion was primarily attributed to dif-ferences in the A-site cation size of Cs (rCs = 1.74 A)versus Ba (rBa = 1.42 A). Structural data showing thevolume expansion increased linearly with the aver-age A-site radius indicated this effect was due totunnel expansion rather than B-site octahedralexpansion. Figure 3a compares the a and c latticeparameters as a function of Cs concentration andshows that the tunnel cross-section expansion is anorder of magnitude greater than the tunnel lengthexpansion. The trends in the calculated values fora and c lattice parameters as a function of Cs con-centration based on Eq. 2 generally agree with mea-sured results. The differences which do exist betweenthe experimental and model predictions could be dueto partial occupancy of the current samples rangingfrom 62 to 65%. Model predictions outlined in Eq. 2
assume full tunnel occupancy with a tetragonalstructure [24].
The decrease observed in the c lattice parameterwith increased Cs content was attributed to thereduction in the average B-site cation radius (Ti4?
replaces Zn2? as Cs content increases). After fullysubstituting Cs on the tunnel sites, the latticeexpanded 1.2% along the a-axis and contracted 0.6%along the c-axis, resulting in a total volume expansionof 1.8%. The unit cell deformation results agree withpreviously reported trends in a Ga hollandite system[24, 27, 28]. Figure 3b compares the samples from thisstudy to the Post and Zhang structural equations. Itshows that the Ba end member, which formed mon-oclinic, was within the undetermined zone, while theother compositions formed tetragonal as predicted.The 67% partial A-site occupancy and large Zn sub-stitutions are probably what allowed for a monoclinicsymmetry to form. In addition to confirming the lat-tice expansion trends for Zn-doped hollandite overthe A-site solid solution previously seen in the Gasystem, the current work demonstrates how compo-sition can control the symmetry.
Figure 4 displays the SEM determinedmicrostructure of the sintered hollandites. MinorTiO2 formation, identified by XRD and EDX, andincreased grain size were observed with increasingCs content. No rutile was observed by SEM (ormeasured by XRD) in Cs029, whereas the rutilecontent increased to 1 and 4 weight percent for Cs067and Cs133, respectively. TEM selected area electrondiffraction patterns (SAED) were collected to confirmmonoclinic symmetry in the Ba133 composition andtetragonal symmetry in the Cs029, Cs067 and Cs133compositions as observed in XRD. Overall, theseresults indicate a trend that increasing Cs contentresults in the stabilization of the tetragonal symmetryand an expansion of the unit cell. Stabilization of thetetragonal symmetry occurred below Cs029, with67% occupancy, for the Zn-doped hollandite.
Thermal analysis
Cs loss upon volatilization is a known phenomenonwhen processing hollandite at high temperatures andhas been quantified ex-situ through pre- and post-processing compositional analysis; however, it hasnot been well quantified for hollandite materials[13, 17, 43, 44]. The effect of targeted Cs stoichiometryon Cs retention during high temperature processing
Figure 2 XRD patterns of BaxCsyZnx-y /2Ti8-x?y /2O16
synthesized by oxide route after sintering to 1300 "C for 1 hwith Cs = 0.00, 0.29, 0.67, 1.00, 1.33. PDF# 080-2264 and020-0910 were used to identify the tetragonal and monoclinicsymmetry * denotes rutile (TiO2) and ? denotes tunnel ordering.
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is of practical and theoretical interest. TGA mea-surements were performed on samples of precursorpowder to quantify the mass loss of Cs for eachcomposition during heating. There should be nosignificant difference between Ba116, Ba1.16Zn1.16Ti6.84O16, and Ba133 as both form the same structureand neither contain Cs. Four distinct temperaturezones in which weight loss occurred are shown in
Fig. 5. The first zone from 0 to 300 "C was associatedwith desorption of water and CO2 [26]. The secondzone near 550 "C was attributed to the decompositionof Cs2CO3 to Cs2O. The third zone starting at 800 "Ccan be attributed to the loss of carbonate from theBaCO3 precursor. As a reference for this analysis,pure Ba compositions exhibited no mass loss in thesecond zone, while the pure Cs compositions
Figure 3 a Lattice constants dependence on Cs stoichiometry. The vertical dashed line at Cs 0.29 denotes the approximate monoclinic totetragonal boundary; b stability predictions for experimental data based on geometrical calculations by Zhang and Post.
Figure 4 Backscatter electron micrographs (BSE) of unpolishedhollandite grains a Ba133; b Cs029; c Cs067; d Cs133. The darkgrains are TiO2, as indicated by EDX, and the light grains are the
hollandite. The TEM patterns for Ba133. Cs029, Cs067, andCs133 were along the [100], [100], [100], and [111] zone axesrespectively.
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exhibited no mass loss across the third zone. Thefourth zone at 1250 "C, the sintering temperature,exhibited mass loss attributed to Cs volatilization.Cs2O has the lowest decomposition temperature andSEM–EDX elemental analysis indicated Cs exhibitedthe greatest deviation from target stoichiometry.
The decomposition reactions for each temperatureregime seen in TGA were confirmed by ATR-FTIR.Results from the FTIR characterization revealed sig-nificant differences between the three samples Ba133,Cs067 and Cs133 after 350, 650 and 800 "C heattreatments. The primary feature used to identifystructural changes was the Cs2CO3 major peak at
1334 cm-1 and minor peak at 1440 cm-1. The relativeintensity of BaCO3 peaks was also used; however, theTiO2 peaks overlapped with the BaCO3 peak at1421 cm-1 and all the ZnO vibrational modes werebelow the measured range. After the 350 "C heattreatment, a significant difference in peak intensitywas observed for the BaCO3 and Cs2CO3 peaks asseen in Fig. 6a. Figure 6b displays the spectra at800 "C, indicating the Cs2CO3 fully decomposed asthe vibrations associated with BaCO3 remained in thespectra. All the precursors were reacted at tempera-tures in excess of 1050 "C as seen in Fig. S2. Theremaining peaks were associated with hollanditevibrational modes below 800 cm-1. The comparisonof the 350, 800 and 1050 "C data showed that Cs2CO3
was more reactive than BaCO3 since it decomposed ata lower temperature and confirms Cs2CO3 as the firstcarbonate precursor to decompose during the TGAexperiment.
Analysis of the isothermal hold at 1250 "C indi-cated that mass loss increased with increasing Cscontent as shown in Fig. 7. However, normalized Csmass loss, defined by dividing the Cs mass loss bythe initial Cs mass in the sample calculated frommaterial stoichiometry exhibited a different trend.The normalized Cs mass loss decreased withincreasing initial Cs content. The decreased relativeCs mass loss indicates that hollandite formation wasmore favorable with higher Cs content, and that theresulting structures were more stable with regard tohigh-temperature Cs loss. This conclusion agreeswith the XRD results indicating that the tetragonal
Figure 5 TGA scan of ZnTi hollandite from 20 to 1250 "Cweight loss from each zone was attributed to a different process,Zone 1: H2O evaporation, Zone 2: Cs2CO3 decomposition andZone 3: BaCO3 decomposition.
Figure 6 a FTIR after 350 "C calcination; b FTIR after 800 "Ccalcination. The TiO2 peaks overlap with the BaCO3 peak at1421 cm-1, while the Cs2CO3 has a major peak at 1334 cm-1 and
a minor peak at 1440 cm-1. ZnO vibration modes do not havepeaks within the 1000 cm-1 to 1800 cm-1 range.
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phase stability increased with additional Cs contentand supports the hypothesis that increased rutileformation is due to higher amounts of Csevaporation.
DSC
Table 2 displays the melting points measured byDSC, which were observed to decrease withincreasing Cs content and average B-site radii. Themelting point depression has been previously asso-ciated with Cs doping [11]. However, in this work, Cssubstitution was shown to result in melting pointdepressions of about 80 "C, which is comparable tomelting point changes between different B-site dopedsystems [11, 12, 33]. However, there are a number ofother factors that affect the melting point thereforethe average B-site radii is not a reliable predictor formelting point changes. Zn hollandite had meltingpoints below Cr-hollandite and similar to Fe- and Al-
doped hollandite. There were also endothermic peaksnear 550 "C and 800 "C that agree with the TGAdecomposition analysis.
Calorimetry
Table 3 gives a summary of the measured enthalpiesof drop solution (DHds) for six different hollandites.Using these DHds values (shown in Table 4 andFig. 8a) and previous reported DHds and DHf;el data
for BaO, Cs2O, ZnO and TiO2 (shown in Table 3),enthalpies of formation for six hollandite phases fromtheir constituent oxides (DHf;ox) and from elements
(DHf;el) were calculated via thermochemical cyclesusing the real (EDX) composition. The thermochem-ical cycle for a general compound BaxCsyZnx?y/2
Ti8-x-y/2O16 is shown in Table 5. The obtained DHf;ox
and DHf;el are also listed in Table 3. As shown inTable 3 and Fig. 8b, the values of DHf;ox of all sixhollandite compositions were strongly exothermic,indicating that they are thermodynamicallystable relative to their constituent oxides. Moreover,these values become more negative (more exother-mic) with increasing Cs content. This decreasingtrend agreed with the DFT modeling results for Znhollandite [45].
Even though large exothermic formation enthal-pies of six Zn-doped hollandites indicate their strongthermodynamic stability relative to their binary con-stituent oxides, one must consider their stability withrespect to other competing phase assemblages,including ternary constituent oxides, when evaluat-ing the hollandite suitability as a nuclear waste form[12]. A potential phase assemblage may containBaTiO3 (perovskite) and other constituent oxides,which can be assessed by thermodynamics using thefollowing reactions (analyzed EDX stoichiometriesare used as examples) [12]:
Ba133 ¼ 1:306BaTiO3 þ 1:36ZnOþ 5:361TiO2 ð4Þ
Figure 7 Cesium (Cs) mass loss and Normalized Cesium (Cs)mass loss as determined from TGA experiments. Normalized Csmass loss was calculated by dividing the total Cs mass loss by theinitial Cs mass in the sample determined by stoichiometry.
Table 2 Reported melting points for different M2?/3? doped hollandite and ZnTi hollandite melting points calculated from onsetmeasurements of the endothermic peak
Cs–Zn–O, Cs–Ti–O and Zn–Ti–O could theoreticallyform additional phases, but they have not beenreported in SYNROC. In addition, enthalpies of for-mation are not available for many of these potentialphases and will not be considered in the subsequentanalysis. The enthalpies of reaction (DHrxn) at stan-dard conditions are calculated as follows [12, 15]:
Figure 8 a Drop solution enthalpy, DHds, across the Znhollandite compositional range increased with increasing Cscontent; b experimental enthalpies of formation and DFTcalculated enthalpies versus Cs stoichiometry from the
component oxides at 25 "C (DHf;ox) for Zn hollandite decreased
(formation became more favorable) with increasing Cs content[45].
Table 3 Enthalpies of dropsolution (DHds) in sodiummolybdate solvent at 702 "Cand enthalpies of formationfrom component oxides(DHf;ox) and from the elements
Uncertainty is two standard deviation of the mean and value in parentheses is the number ofexperiments
Table 4 Enthalpies of drop solution in sodium molybdate solventat 702 "C (DHds) and enthalpies of formation from the elements(DHf;el) at 25 "C of related component binary oxides
where DHf,ox values of hollandite are from Table 3,DHf,ox (BaTiO3) = - 152.3 ± 4.0 kJ/mol.
The Cs end member was not calculated as it con-tains no Ba to form BaTiO3. Therefore based on theDHrxn of five hollandites it can be concluded thatCs029 (ExCs), Cs043, and Cs067 are energeticallystable at room temperature with respect to BaTiO3,Cs2O, ZnO, and TiO2, as their DHrxn are endothermic.In contrast, Ba133 is not stable with respect to BaTiO3,ZnO, and TiO2, as determined by the exothermicDHrxn. Moreover, the stability of Cs015 was compa-rable to that of BaTiO3, ZnO, and TiO2, because itsreaction enthalpy was close to zero consideringexperimental uncertainties. The thermochemicalanalysis further supports that increased Cs substitu-tion stabilizes the hollandite phase from the per-spective of formation enthalpy of the targeted phase,as well as from possible decomposition to otherphases.
Leaching measurements
The calculated fractional release of Cs for individualreplicates from the particle and monolith leach test-ing is summarized in Fig. 9. For the PCT (particle)samples, the fraction of Cs released decreased as Csstoichiometry (concentration) was increased. Thisbehavior indicates that the Cs stoichiometry in hol-landite affects its release, which is supported byreported favorable formation enthalpies and struc-tural stabilization caused by increasing Cs stoi-chiometry [13]. Similar behavior was observed for themonolith samples although there was greater vari-ability among replicates with lower Cs stoichiometry.More striking, however, was the similar magnitudein the calculated fraction release of Cs from themonoliths compared to the particles. Both the vari-ance in replicate response and the nominal fraction ofCs release could be attributed to several factorsincluding pH, microstructure, and surface area. Themeasured pH values after testing were measured andalthough variation was seen, experience indicatesthat the effect would not be significant. Therefore,T
able
5Therm
ochemical
cycleforthegeneralcompo
undBa xCs yZn x
?y/2Ti 8-x-y/2O16that
was
used
forcalculationof
enthalpies
form
ationho
lland
itefrom
constituent
oxides
(DH
f;ox)andfrom
theelem
ents(DH
f;el)at
25"C
with
correctio
nbasedon
EDX
analyzed
compo
sitio
nin
Table1
Enthalpyof
form
ationof
Ba xCs yZn x
?y/2Ti 8-2-y/2O16from
theox
ides
at25
"C(DH
f;ox)
Ba xCs yZn2x
þyTi 8%2x
%yO
16s,25
( Cð
Þ!xB
aOsln,702
( Cð
Þþy=2C
s 2O(sln,702
( C)þ2xþyZ
nO
sln,975
Kð
Þþ8%
2x%yT
iO2ðsln,702
( C)
DH
ds
BaO
(s,298
K)!
BaO
sln,702
( Cð
ÞDH
1
Cs 2O
s,29
8K
ðÞ!
Cs 2O
sln,702
( Cð
ÞDH
2
ZnO
s,29
8K
ðÞ!
ZnO
sln,702
( Cð
ÞDH
3
TiO
2s,29
8K
ðÞ!
TiO
2sln,702
( Cð
ÞDH
4
xBaO
s,29
8K
ðÞþ
y=2C
s 2O(s,25( C
)þ2xþyZ
nO
s,25
( Cð
Þþ8%
2x%yT
iO2ðs,25( C
)!
Ba xCs yZn2x
þyTi 8%2x
%yO
16s,25
( Cð
ÞDH
f;ox
Enthalpyof
form
ationof
Ba xCs yZn x
?y/2Ti 8-2-y/2O16from
theelem
entsat
25"C
(DH
f;el)
xBaO
s,29
8K
ðÞ þ
y=2C
s 2O(s,25( C
)þ2xþyZ
nO
s,25
( Cð
Þ þ8%
2x%yT
iO2ðs,25( C
)!
Ba xCs yZn2x
þyTi 8%2x
%yO
16s,25
( Cð
ÞDH
f;ox
Ba(s,298
K)þ1=2O
2g,25( C
"#!
BaO
s,25
( Cð
ÞDH
5
1=2C
ss,29
8K
ðÞþ
1=2O
2g,25( C
"#!
Cs 2O
s,70
2( C
ðÞ
DH
6
Zn
s,29
8K
ðÞþ
1=2O
2g,25( C
"#!
ZnO
s,70
2( C
ðÞ
DH
7
Tis,29
8K
ðÞþ
O2g,25( C
"#!
TiO
2s,70
2( C
ðÞ
DH
8
xBas,29
8K
ðÞþ
yCs(s,25
( C)þ2x
þyZ
ns,25
( Cð
Þþ8%
2x%yT
i(s,25
( C)þ8O
2g,25( C
"#!
Ba xCs yZn2x
þyTi 8%2x
%yO
16s,25
( Cð
ÞDH
f;el
J Mater Sci
surface area measurements using the Brunauer–Em-mett–Teller method (Micromeritics) were performedon the Cs067 monolith samples post-leaching andconfirmed that the surface area was several hundredtimes greater than if geometrically estimated, whichcould explain the data. The variance in the leach datafor the monolith replicates also appeared to increasewith decreasing Cs stoichiometry. Although XRDand SEM analysis indicated the samples were phasehomogeneous, the replicate variance could indicatethat surface area is not the only contributing factor,but phase/structure formation also impacts the Csleaching behavior, further supporting the supposi-tion that increasing Cs stoichiometry in hollandite isfavorable for formation and stability. The fractionrelease of Ba and Zn elements were on the order of10-4 and 10-5, respectively, for all replicates.
Conclusion
Solid state processing was utilized to synthesizeBaxCsyZnx?y/2Ti8-x-y/2O16 in order to examine theeffect of Cs and Zn substitutions on the structure,thermal properties, and resistance to aqueous corro-sion of the resulting hollandite materials. Cs addi-tions led to a structural shift from a monoclinic to amore stable tetragonal phase which was accompa-nied by unit cell expansion. Cs was also found tofurther stabilize the tetragonal structure across thesolid solution as seen by reductions in Cs loss duringheating, a decrease in Cs released during aqueous
leaching tests, and lower formation enthalpies. HighCs content hollandites resulted in lowered meltingpoints which could limit Cs evaporation and facilitatelarge-scale processing. In summary, Cs-rich hollan-dite compositions are more favorable to form,resulting in a stable structure with enhanced Csretention. More broadly, these results establish thelink between composition, structural symmetry, andthermodynamic stability for tunnel structuredceramics with implications in the design of newenergy conversion and storage materials.
Acknowledgements
KSB andMZ acknowledge support of thermodynamicmeasurements as part of the Center for HierarchicalWaste Form Materials, an Energy Frontier ResearchCenter funded by the U.S. Department of Energy,Office of Science, Basic Energy Sciences under AwardNo. DE-SC0016574. LSN gratefully acknowledgesfinancial support from the DOE-EPSCoR ProjectNumber: DE-SC0012530, ‘‘Radionuclide Waste Dis-posal: Development of Multi-scale Experimental andModeling Capabilities’’ for support of modeling. Thecalorimetric experiments carried out at University ofCalifornia, Calorimetry at Davis were supported aspart of the Materials Science of Actinides, an EnergyFrontier Research Center funded by the U.S. Depart-ment of Energy, Office of Science, Basic Energy Sci-ences under Award Number DE-SC0001089. JAacknowledges the support of durability testing by theU.S. Department of Energy, Office of Nuclear Energy,Fuel Cycle Technology,Materials Recovery andWasteForm Development Campaign. Work conducted atSavannah River National Laboratory was supportedby the U.S. Department of Energy under contractnumber DE-AC09-08SR22470.
Compliance with ethical standards
Conflict of interest K. Brinkman is a member of theeditorial board for Journal of Materials Science. Theauthorsdeclare that theyhavenoother conflict of interest.
Electronic supplementary material: The onlineversion of this article (https://doi.org/10.1007/s10853-018-2904-1) contains supplementary material,which is available to authorized users.
Figure 9 Fraction of Cs released from PCT and C 1220 leachingmeasurements for Cs029, Cs067 and Cs133 compositions; Circle:PCT measurements of Cs leaching Diamond: C 1220measurements of Cs leaching.
J Mater Sci
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