Influence of Strontium Oxide on Structural Transformations in Diopside

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Influence of Strontium Oxide on Structural Transformations in Diopside-Based Glass-Ceramics Assessed by Diverse Structural Tools Allu Amarnath Reddy, Dilshat U Tulyaganov, Glenn Christopher Mather, Sonia Rodríguez-López, Subrata Das, María J. Pascual, Francisco Muñoz, Renée Siegel, Jürgen Senker, and José Maria da Fonte Ferreira J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02475 • Publication Date (Web): 05 May 2015 Downloaded from http://pubs.acs.org on May 9, 2015

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The Journal of Physical Chemistry

Influence of Strontium Oxide on Structural Transformations in Diopside-Based Glass-Ceramics Assessed by Diverse Structural Tools

Allu Amarnath Reddy,a,b,c Dilshat U. Tulyaganov,a,d Glenn C. Mather,b* Sonia RodríguezLópez,b Subrata Das,e Maria J. Pascual,b Francisco Muñoz,b Renée Siegel,c Jürgen Senker,c José M. F. Ferreira,a*

a

Department of Materials and Ceramic Engineering, CICECO, University of Aveiro, 3810–193 Aveiro, Portugal b

Instituto de Cerámica y Vidrio (CSIC), C/Kelsen 5, Campus de Cantoblanco, 28049 Madrid, Spain c

Department of Inorganic Chemistry III, University of Bayreuth, 95440 Bayreuth, Germany

d

Turin Polytechnic University in Tashkent, 17, Niyazova Str., 100095 Tashkent, Uzbekistan

e

Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C.

*

: Corresponding author Tel.: +351–234–370242; Fax: +351–234–370204. E–mail address: [email protected] (J.M.F. Ferreira) [email protected] (G.C. Mather)

1

ACS Paragon Plus Environment


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Abstract A full understanding of the structural glass variations and of the nucleation and growth of crystalline phases in glass-ceramics (GCs) induced by compositional changes is highly relevant for applications such as sealants for solid oxide fuel cells, bio- and dental materials, and photonics. Systematic substitution of Ca with Sr in diopside (Di-CaMgSi2O6) has a significant impact on the crystalline-phase assemblage after heat treating at 850 ºC for 500 h. A high percentage of Di crystallized from Sr-free glass; however, with increasing Sr content the tendency towards crystallization decreased, and Sr-akermanite (Sr-Ak) was preferentially crystallized. In this study, the structural transformations in the Di-based-GC system upon the substitution of calcium with strontium have been examined by X-ray powder diffraction (XRD), 29

Si and

27

Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR) and Raman

spectroscopies. XRD Rietveld refinement results indicate that up to a maximum of ~ 25 at.% of Sr occupies the Ca site of the Di structure.

27

Al MAS-NMR spectra reveal that most of the Al

exists in amorphous glassy phase after crystallization. Measured mean quadrupolar coupling constant values augment with increasing Sr content indicating an increase in order in the glassy phase, which is manifested by a strong preference of Mg2+ for non-bridging oxygens (NBO), resulting in the formation MgO4 tetrahedra. At higher Sr substitution levels (> 50%) the decrease in Mg2+ coordination from six to four is evident from the decrease in average Mg-O bond lengths and the systematic variation in intensity of characteristic Mg-O Raman vibrational bands.

Keywords: glass-ceramics; diopside; crystallization; bonding order.

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1. Introduction Diopside [CaO⋅MgO⋅2SiO2 − (Di)], a Ca-rich pyroxene, is one of the most essential meta-silicate systems in earth science. The general formula for diopside may be expressed as M2M1T2O6 where M1, M2 and T represent Mg2+, Ca2+ and Si4+ cations, respectively. The crystal structure of Di (space group C2/c), first described in detail by Cameron et al.,1 is characterized by octahedrally coordinated Mg2+ ions and tetrahedrally coordinated Si4+ ions, with Ca2+ in 8fold coordination. Many Di-based solid solutions with various alkali- or alkaline-earth cations exist due to the facility for ionic substitution on the three types of cation site.2-12 Such Di-based materials have been the focus of intensive study for different fields of application over the past few decades.13-20 Excellent thermo-physical properties have been achieved upon substituting Si4+ and Mg2+ with Ln3+ (Ln=La, Nd, Gd and Yb) in Di-based materials, demonstrating their suitability as sealants for solid oxide fuel cells.13,

21

Similarly, improved dielectric properties

along with a lower densification temperature have been achieved by replacing Mg2+ with Zn2+ rendering them potential candidates for low temperature co-fired ceramic applications.17 Persistent luminescence of manganese-doped, Di-based nanoparticles has been employed in a novel application of in-vivo optical imaging.20 Large ionic radii differences among the different atoms occupying a cation site may give rise to phase transformations as a function of composition with variations in occupancy associated with the ion-size requirements.22 Several papers have reported a change in the space group of clinopyroxene from C2/c to P21/c, associated with one and two crystallographically distinct silicon sites, respectively, on substituting Ca (M2 site) for smaller Mg or Fe cations.3, 9 According to Cameron et al.,1 the M2 site in the C2/c polymorph can expand up to 1.16 Å, which limits the substitution of Ca with Sr, considering the ionic radii of CaVIII (1.12 Å) and 3



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SrVIII (1.25 Å).23-24 Benna et. al.2, 4 have reported such a substitution in the clinopyroxene solid solution of Di100-Di70SrPx30 is limited to around 30 at %. Notwithstanding, the substitution or addition of the alkaline-earths Sr and Ca in a glassy matrix is generally facile. Accordingly, several studies on melt-quenched Sr-containing glasses showed that increasing the Sr/Ca ratio in the glasses did not significantly affect their structure.16, 25-30 Nevertheless, it is still unclear how the larger ionic radius of Sr compared to Ca24 in the glassy matrix may affect the glass structure, the silicate network, and its polymerization.31 Theoretical calculations based on a simplified model suggest that disordering the polymerized silicate units by randomly varying bond angles does not much alter the high intensity NMR peak of 29Si nuclei or the corresponding strong highand mid-frequency Raman/FTIR-active vibrations.32 In the case of alkali- and alkaline-earthsilicate glasses, it is well known that the dominant peaks of either NMR or high-frequency Raman/FTIR bands of a crystal frequently appear in the spectra of a glass of similar composition.32-37 As a result, comparison of the NMR and Raman/FTIR spectra of glasses and crystals has been used extensively to deduce structural information on glasses.38 We have previously reported structural transformations and retarded tendency towards crystallization in Di-based systems involving substitution of Ca with the larger Sr cation.39 The aim of the current work is to study the effects of this substitution on the silicate network structure and its propensity towards polymerization by correlating the structure and glass composition, employing X-ray diffraction, 29Si and 27Al magic angle spinning (MAS) NMR spectroscopy, and Raman vibration spectroscopy. To the best of our knowledge, this is the first systematic and detailed structural study of the effects of strontium content on the structure of melt-derived Srcontaining silicate glasses and GCs.

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2. Experimental The chemical compositions of the experimental glasses are presented in Table 1. Samples are labelled according to the ratio of Sr/Ca. The detailed glass and GCs synthesis procedure has been published elsewhere.39 X-ray powder diffraction data were collected with monchromatic Cu Kα1 radiation over the range 10 º ≤ 2θ ≤ ∼110 º in a step width of 0.02 º with a Bruker D8 diffractometer equipped with a solid-state LynxEye detector. The analyzed samples were prepared by grinding the GCs to a fine powder in an agate mortar with the addition of NaCl, which served as an internal standard for independent lattice-parameter refinement, prior to sieving through a 60 µm mesh. Rietveld refinement was performed with the Fullprof program40 using interpolation of points to model the background. 29

Si MAS-NMR spectra were recorded for all glasses and glass-ceramics on a Bruker

ASX 400 spectrometer operating at 79.52 MHz (9.4 T) using a 7 mm probe at a spinning rate of 5 kHz. The pulse length was 2 µs and a delay time of 60 s was used. Kaolinite was employed as the chemical-shift reference. 27

Al MAS-NMR spectra for all glasses and glass-ceramics were recorded on a Bruker

Avance III 400 spectrometer operating at a B0 field of 9.4 T with

27

Al Larmor frequencies of

104.3 MHz. Spectra were recorded using a 4 mm probe with a spinning rate of 14 kHz, 1 s recycle delay, and 0.5 µs RF excitation pulse (equivalent to a π/18 flip angle); the flip angle pulses were optimized using an aqueous solution of Al(NO3)3. Raman spectra were acquired under similar experimental conditions mentioned elsewhere41 using a Horiba LabRam HR 800 Evolution confocal Raman microscope. Infrared spectra of GCs were obtained using an Infrared Fourier spectrometer using FT-IR, Mattson Galaxy S7000, USA. The GC powder was mixed with KBr in the proportion of 1/150 (by 5



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weight) and pressed into a pellet using a hand press. Spectra were recorded with a resolution of 4 cm–1.

3. Results 3.1 Rietveld refinement The powder X-ray diffraction patterns of the series were analyzed in accordance with the previously published phase assemblages, listed in Table 2.39 Rietveld analysis proceeded with refinement of CaMgSi2O6-based phase (diopside structure) and Sr2MgSi2O7-based phase (akermanite (Ak) structure) in space groups C2/c and P 4 21m, respectively, based on published structural data; minor phases of CaAl2Si2O8 (anorthite), SrSiO3 and MgSiO3 were included in the refinements of Sr/Ca=3/6, Sr/Ca=5/4, Sr/Ca=7/2 and Sr/Ca=9/0according to the earlier phase analyses,39 whereas a minor amount of La2Si2O7 (space group, P21/c) was included in the refinement of sample Sr/Ca=0/9. Quantitative Rietveld analysis previously indicated a higher amorphous fraction with increasing Sr content.39 The observed diffraction patterns and the difference patterns between observed and calculated data on termination of refinement for samples Sr/Ca=3/6 and Sr/Ca=9/0 are shown in Fig. 1. The structural parameters for the Dibased and Sr-Ak-based phases are listed in Tables 3(a) and (b) with selected bond lengths, calculated valence states and angles given in Table 4. The corresponding agreement factors for the series of compositions are shown in Table 5. The unit-cell parameters for the Di-based phase are lowest for the Sr/Ca=0/9 sample, whereas a slight maximum is observed for the Sr/Ca=5/4 composition (Table 3a); in contrast, the Ak unit-cell volume increases monotonously with increasing Sr content. On refining the Di structure according to the nominal stoichiometry of the parent phase, CaMgSi2O6, the thermal 6


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vibration factor (TVF) of the Ca site in the Sr-containing samples was found to be negative, indicating the possible presence of a heavier cation on this site as a result of the high correlation between occupancy and thermal parameters41. The TVFs of all sites were thus kept constant at Biso = 0.8 to avoid correlation with occupancy factors, background or other parameters. The refinement then proceeded with linear increments of Sr on the Ca site. A Sr content of 25 at% in  R the Sr/Ca=3/6 sample decreased the χ χ 2 =  wp R   exp  2

  

2

  of the refinement from 3.48 in the Ca  

stoichiometric composition to 2.74. The lower unit-cell volume of the Di-based phase of the Sr/Ca=0/9 sample can be understood to arise, therefore, from the occupation of the Ca position by Ca only, whereas the Sr-containing compositions with varying degrees of partial occupancy by the larger Sr2+ cation on this site exhibit a larger unit cell. We note that a similar treatment of site occupancies of the magnesium and silicon sites was not possible because of the very similar X-ray scattering lengths of Mg2+, Al3+ and Si4+. Nevertheless, the maximum in unit cell observed at Sr/Ca=5/4 may be due to a change in cation content with composition involving occupation of the Mg2+ and Si4+ positions with Al3+ (augite, Ca1-xSrxMg1-ySi2-yAl2yO6). The irregular unit-cell behavior of the Sr-containing Di-based series is unlikely to be attributable to experimental error due to the inclusion of NaCl as a standard in the lattice-parameter refinement. Similarly, on refinement of the Ak-based phases, better fit parameters were observed with a minor amount of Ca on the Sr site with the amount of Ca decreasing from the Sr/Ca=5/4 composition (20 at%) to the Ca-free Sr/Ca=9/0 composition (Table 3b). The linear increase in lattice parameters with increasing Sr content may be understood as arising from the greater proportion of the larger Sr2+ cation in comparison to the Ca2+ cation on progressing through the series. 7



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The bond-valence concept is often used to provide an indication of the correctness of the structure solution and to estimate the valence state of atoms. The valence of the atom is the sum of the individual bond valences surrounding the atom, which are, in turn, estimated from the calculated and “ideal” bond lengths. The bond-valence values for each cation and anion calculated with the Bond-Str program using parameters give in Ref.42 are shown in Table 4. In the present case, the calculated valence state was used as a possible indication of substitutions taking place on the smaller cation sites (Si, Al, Mg) given the difficulty of determining partial site occupancies of these cations by XRD. The calculated valence state of Mg is greater than the expected value +2 whereas that of Si is lower, 0.3 Å or the difference in field strength increases. For example, in K−Mg systems (∆r = 0.66 Å), Mg cations prefer to bond with NBOs (highly ordered in the NBO environment), whereas in Ba-Mg systems (∆r = 0.64 Å) the Ba cations show a stronger preference for Ba-NBO environments in comparison to Mg cations. In the case of Ca-Mg systems (highly disordered; ∆r = 0.27 Å) no such preference was observed. The ∆r values24 between Mg and Ca and Mg and Sr increase from 0.28 to 0.44 Å, respectively, with increasing substitution of Sr for Ca. This suggests that the extent of cation order should be high in Sr/Ca=9/0 glass, as in K−Mg systems,58-59 compared to Sr/Ca=0/9 glass, and this difference is likely to greatly influence the diffusivity of the Sr2+ cation with increasing Sr substitution for Ca, facilitating crystallization in the glass-ceramics.60 The Rietveld XRD results (Table 4) revealed that the average Mg−O bond length in the diopside structure decreases form 2.05 to 1.98 Å and the bond valence of the Mg cation increases 13



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from 2.28 to 2.79 throughout the series with increasing Sr content; concomitantly, the average Si−O bond length in the diopside-based phase increases (1.64 − 1.72 Å). In general, due to the more covalent character between oxygen and silicon, the bond strength within the SiO4 tetrahedra is much higher than that of the longer bond connecting oxygen to alkaline-earth cations. Consequently, the intensity of the vibrational Raman bands of the covalent bonds are stronger than those of the more ionic bonds.61 However, alkaline-earth cations still influence the intensity of the νs(Si−O−) band if the covalency of the bond between the metal ion and NBO increases and the degree of electron sharing in the Si−O− bond is reduced. Specifically, in pyroxenes,36 a decrease in the M1−O bond length (increasing covalent character) decreases the intensity of the νs(Si−O−) band. The Mg−O lattice vibration band at 387 cm−1 decreases in intensity whereas the covalently-bonded, tetrahedral, symmetric stretching vibration at 587 cm−1 increases, indicating that with increasing substitution of Ca with Sr, the covalent character between Mg and O increases as supported by the bond length decrease observed by XRD.36-37 Hence, the substitution of Ca with Sr in diopside-based glasses leads to the formation of Mgcontaining tetrahedral sites as attested by the formation of akermanite crystalline phase. The behavior concords with previous reports that a drastic change in the coordination number of the modifier cation leads to heterogeneous nucleation, and to the formation of different crystalline phases.62 In the Raman spectra of crystalline melilite37, the frequency of the νs(Si−O−Si) mode exhibited a linear relationship with the Si−O−Si bond angle. The decreasing bond angle in Si−O−Si linkages decreases the frequency of the νs(Si−O−Si) band. In contrast, in both metasilicate and disilicate structures, the frequency of the νs(Si−O−Si) decreases and diminishes 14


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in intensity with increasing Si−O−Si bond angle.63 As mentioned above, in Ca-rich pyroxene structures, the M2 cation shares four NBOs and four BOs with the chain units, such that the variation in Si−O bond length is directly associated with the strength of the M2−O bond length (i.e., ionic radius). Another factor that may also affect the νs(Si−O−Si) band is the mass of the cation at the M2 site. The dominant feature of the Raman spectra is the Si−O vibration due to the high covalency of the Si−O bond, in combination with Si−O−M2, compared to that of the M2−O bond. Thus, the M2 cation will not influence significantly the Si−O vibration characteristics. If we consider mainly the effective mass, we can suppose a combined mass of M2−O as an apparent mass of O in the Si−O−M2 stretching vibration. If the M2 cation is substituted by a heavier and larger cation, the apparent mass of O increases, thus increasing the reduced mass in the Si−OBO stretching vibration and decreasing the peak shift. An analogous result has been reported by Dowty,64 where the νs(Si−O−Si) mode shifted to higher frequency (from 664 cm−1 to ∼705 cm−1) upon substituting Na for Ca at the M2 diopside site. In the present scenario, the shift towards lower frequency with increasing Sr content is, therefore, possibly due to either an increase in Si−O−Si bond angle or a reduced mass effect. Since, with increasing Si−O−Si bond angle, the frequency of the asymmetric stretching vibrations νas(Si−O−Si) at 1007 and 1068 cm−1 in the FTIR spectra did not increase it may be expected that the reduced mass effect has a more dominant impact. Nevertheless, the influence of an increase in the Si−O−Si bond angle should not be discounted for the following reason. The width of the νs(Si−O−Si) band increases with increasing Sr content up to Sr/Ca=7/2 and then decreases with further addition, indicating a wide dispersion of Si−O−Si bond angles. For quadrilateral pyroxenes, the broadness or splitting of the ∼670 cm–1 mode indicates structural changes within the silicate chains.65 15



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According to Fyfe,66 two atoms are mutually replaceable in ionic compounds if their sizes are similar and are replaceable in more covalent compounds only if the number and directional properties of their bonds are similar. Magnesium assumes a coordination environment of four in extremely covalent structures. However, it has been reported that, in framework silicates, Si prefers tetrahedral sites with the widest average T−O−T angles whilst Al-, B- and Mg-based tetrahedra prefer narrower average T−O−T angles.67 The average Si−O(3) (bridging oxygen) bond length in the Di-based crystal structure increased from 1.64 to 1.72 Å with increasing Sr substitution for Ca (Table 3). According to Brown et al.67 this indicates that the Si−O−Si bond angle is decreasing. This decrease, in conjunction with the 29Si MAS-NMR (Fig. 3) and Raman (Fig. 4) results, strongly supports the hypothesis that some of the Mg2+ cations in tetrahedral coordination are substituting for Si4+ tetrahedral sites in the Di-based glasses, leading to the formation of heterogeneous crystalline phases.

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5. Conclusions A detailed investigation of the influence of substituting Ca with Sr on the structure and devitrification behavior of diopside-based glasses has been carried out using XRD, MAS-NMR and Raman techniques. It is found that Sr has limited influence on the structure of the diopside glass, occupying up to ~ 25 at.% of the Ca site, but has a strong influence on the type of crystalline phase which is precipitated for a given composition. Diopside-based GCs crystallized homogeneously above the glass transition temperature at 850 ºC on 500 h heat treatment for up to 30 at.% of Sr substitution for Ca, but crystallized heterogeneously with the formation of akermanite for greater Sr contents. This can be understood to occur on the basis of the difference in field strength and ionic radius between Sr and Mg. The

29

Si MAS-NMR chemical shift towards higher positive values with increasing Sr

content compared to the characteristic chemical shift of diopside and the appearance of a Raman shift at 584 cm−1 indicates that four-fold coordinated Mg2+ cations substitute for silicon tetrahedral cations, in agreement with the shorter Mg-O bond lengths determined by Rietveld refinement of XRD data. The presence of Mg2+-O4 tetrahedra and highly mobile isolated Si4+O4 tetrahedral units is favorable for the precipitation of Sr-akermanite crystalline phase in the diopside-based glasses.

Acknowledgements This study was financially supported by the CICECO, University of Aveiro, by the JECS-trust frontiers of research (201242-2), and by the FCT, Portugal (PTDC/CTM–CER/114209/2009). A.A. Reddy thanks FCT for the doctoral grant (SFRH/BD/89915/2012). 17



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Glass

Page 26 of 39

Glass-ceramics at 850 ºC for 500 h Raman MAS-NMR

Sr2MgSi2O7 Sr0.9 MgAl0.1La0.1Si1.9O6

CaMgSi2O6

-60 -80 -100 -120 Si Chemical shift (ppm) Ca0.9 MgAl0.1La0.1Si1.9O6 29

29

-60 -80 -100 -120 Si Chemical shift (ppm)

400 800 –1 Raman shift (cm )

structural transformation in glass-ceramics with the substitution of Sr for Ca in diopside based glass composition Graphical Abstract

26


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Table 1: Nominal batch compositions of the glasses (mol%) Glass Sr/Ca=0/9 Sr/Ca=3/6 Sr/Ca=5/4 Sr/Ca=7/2 Sr/Ca=9/0

CaO 22.53 15.00 9.98 4.99 –

MgO 25.03 25.02 24.96 24.93 24.92

BaO 0.53 0.56 0.59 0.61 0.63

SrO 7.50 12.49 17.46 22.43

Al2O3 1.25 1.25 1.25 1.25 1.24

La2O3 1.25 1.25 1.25 1.25 1.25

SiO2 48.62 48.60 48.60 48.59 48.58

NiO 0.79 0.85 0.88 0.92 0.95

Table 2: Results of quantitative Rietveld refinement of glasses treated at 850 °C for 500 h (wt %) employing Al2O3 as weight standard (taken from ref. 39).

Glass Sr/Ca=3/6 Sr/Ca=5/4 Sr/Ca=7/2 Sr/Ca=9/0

CaMgSi2O6Anorthite based phase 68 4 62 – 29 – – –

SrSiO3 1 – – –

Sr2MgSi2O7 MgSiO3 Amorphous – 4 32 28

– – – 15

27 34 39 57

χ2 1.9 1.7 1.6 1.9

27



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Page 28 of 39

Table 3: (a) Structural parameters for the diopside-based phase (space group, C 2/c) obtained from XRD. a (Å) b (Å) c (Å) γ V (Å3) Ca/Sr position 4e y Occ Ca/Sr Mg position 4 e y Si position 8f x y z O(1) position 8f X Y Z O(2) position 8f x y z O(3) position 8f x y z

Sr/Ca=0/9 9.7180(4) 8.9309(3) 5.2616(2) 105.992(3) 438.99(3)

Sr/Ca=3/6 9.7648(5) 9.0131(4) 5.2674(3) 105.779(3) 446.12(4)

Sr/Ca=5/4 9.775(1) 9.038(1) 5.2739(6) 105.796(7) 448.33(8)

Sr/Ca=7/2 9.769(2) 9.015(2) 5.260(1) 105.67(2) 446.0(2)

0.3013(4) N/A

0.3045(3) 0.75/0.25

0.3049(7) 0.8/0.2

0.297(2) 0.9/0.1

0.9127(8)

0.9089(7)

0.902(2)

0.901(4)

0.2858(5) 0.0848(6) 0.2336(8)

0.2862(4) 0.0956(6) 0.2263(8)

0.2870(8) 0.010(1) 0.234(2)

0.288(2) 0.111(3) 0.234(5)

0.1144(8) 0.0824(8) 0.140(1)

0.1151(7) 0.0849(8) 0.137(1)

0.108(1) 0.088(2) 0.130(2)

0.094(3) 0.070(4) 0.126(6)

0.3553(9) 0.2345(8) 0.316(1)

0.3575(7) 0.2508(7) 0.311(1)

0.359(1) 0.251(2) 0.315(2)

0.368(3) 0.251(4) 0.330(6)

0.349(1) 0.0171(7) −0.008(2)

0.3437(8) 0.0187(7) -0.007(2)

0.342(1) 0.019(1) −0.023(4)

0.337(4) −0.007(4) 0.000(9)

28


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(b) Structural parameters for the akermanite-based phase (P 4 21m) obtained from XRD a (Å) c (Å) V (Å3) Sr/Ca position 4e x y z Occ Sr/Ca Si position 4e x y z O(1) position 2c z O(2) position 4e x y z O(3) position 8f x y z

Sr/Ca=5/4 7.9656(3) 5.1343(1) 325.77(2)

Sr/Ca=7/2 7.9899(1) 5.1543(1) 329.05(1)

Sr/Ca=9/0 8.0010(1) 5.1642(1) 330.59(1)

0.3342(3) 0.1658(3) 0.5079(9) 0.8/0.2

0.3341(2) 0.1659(2) 0.5072(6) 0.9/0.1

0.3339(1) 0.1661(2) 0.5089(6) N/A

0.1358(9) 0.3642(9) 0.945(2)

0.1369(6) 0.3631(6) 0.945(1)

0.1381(5) 0.3619(5) 0.945(1)

0.144(5)

0.147(3)

0.147(3)

1.149(2) 0.351(2) 0.220(3)

0.146(1) 0.354(1) 0.225(2)

0.143(1) 0.357(1) 0.228(2)

0.079(2) 0.181(2) 0.807(3)

0.083(1) 0.178(1) 0.813(2)

0.080(1) 0.185(1) 0.820(2)

29



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Page 30 of 39

Table 4: (a) Diopside-based phase bond lengths in (Å) and calculated valence states Ca/Sr−O(1) × 2 Ca/Sr−O(2) × 2 Ca/Sr−O(3) × 2 Ca/Sr−O(3) × 2 Ave. Ca/Sr-O Si−O(1) Si−O(2) Si−O(3) Si−O(3) Ave. Si−O Calculated Si valence state Mg−O(1) × 2 Mg−O(1) × 2 Mg−O(2) × 2 Ave. Mg-O Calculated Mg valence state Calculated O(1) valence state Calculated O(2) valence state Calculated O(3) valence state

Sr/Ca=0/9 2.397(8) 2.27(3) 2.57(5) 2.727(4) 2.49(1) 1.602(9) 1.70(3) 1.67(3) 1.61(4) 1.64(1) 3.8(1) 2.055(9) 2.05(5) 2.05(3) 2.051(8)

Sr/Ca=3/6 2.429(8) 2.403(5) 2.602(7) 2.750(9) 2.546(3) 1.611(8) 1.572(8) 1.64(1) 1.708(9) 1.632(4) 3.94(5) 2.122(9) 2.042(6) 2.077(8) 2.080(3)

Sr/Ca=5/4 2.40(2) 2.39(1) 2.64(1) 2.71(2) 2.53(5) 1.69(1) 1.54(2) 1.75(2) 1.65(2) 1.656(9) 3.74(9) 2.16(2) 2.00(1) 2.04(2) 2.066(6)

Sr/Ca=7/2 2.40(4) 2.28(3) 2.50(4) 2.99(5) 2.54(1) 1.86(3) 1.50(4) 1.80(5) 1.64(5) 1.70(2) 3.52(2) 1.98(4) 1.96(3) 1.99(4) 1.98(2)

2.28(5)

2.11(2)

2.23(4)

2.79(1)

−2.13(3)

−2.06(2)

−1.91(4)

−1.80(9)

−1.64(9)

−1.85(3)

−1.99(6)

−2.30(2)

−2.2(1)

−2.10(3)

−1.98(6)

−1.85(2)

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(b) Akermanite-based phase bond lengths (in Å) and calculated valence states Sr/Ca−O(1) Sr/Ca−O(2) Sr/Ca−O(2) × 2 Sr/Ca−O(3) × 2 Sr/Ca−O(3) × 2 Ave. Sr/Ca−O Si−O(1) Si−O(2) Si−O(3) × 2 Ave. Si−O Calculated Si valence state Mg−O(3) × 2 Mg−O(3) × 2 Ave. Mg−O Calculated Mg valence state Calculated O(1) valence state Calculated O(2) valence state Calculated O(3) valence state

Sr/Ca=5/4 2.64(2) 2.56(2) 2.87(2) 2.55(2) 2.81(2) 2.709(6) 1.60(1) 1.42(2) 1.68(2) 1.596(8) 4.5(1) 1.86(2) 1.86(2) 1.859(8) 2.55(5) −2.61(4) −2.23(9) −1.92(5)

Sr/Ca=7/2 2.64(1) 2.57(1) 2.86(1) 2.55(1) 2.87(1) 2.721(4) 1.619(7) 1.45(1) 1.68(1) 1.607(5) 4.34(7) 1.844(9) 1.84(1) 1.844(5) 2.66(3) −2.50(3) −2.17(5) −1.94(3)

Sr/Ca=9/0 2.65(1) 2.599(9) 2.830(9) 2.593(8) 2.860(8) 2.727(3) 1.634(6) 1.46(1) 1.624(9) 1.586(5) 4.53(6) 1.863(8) 1.863(8) 1.863(4) 2.53(3) −2.42(2) −2.12(5) −2.04(3)

(c) Selected bond angles in degrees for the diopside-based phase Angle

O−Mg−O

O−Si−O

Sr/Ca=0/9 85.0(6) 90(2) 83.6(5) × 2 94.6(5) × 2 92.5(2) × 2 91.3(2) × 2 90.4(2) × 2 174(2) × 2 177.6(4) 118(2) 109.3(1) 112.0(2) 108(4) 102(4) 107.7(2)

Sr/Ca=3/6 83.2(5) 93.4 (5) 83.6(4) × 2 94.0(5) × 2 92.3(5) × 2 87.9(5) × 2 94.2(4) × 2 170.1(6) × 2 176.9(5) 118.7(7) 107.1(8) 106.7(7) 112.0(7) 106.3(7) 105.0(9)

Sr/Ca=5/4 78.7(9) 95.6(1) 80.7(8) × 2 95.3(9) × 2 93.6(1) × 2 90.1(9) × 2 93.4(9) × 2 167.3(1) × 2 174.8(9) 120.0(1) 103.4(1) 107.5(1) 111.9(2) 109.4(2) 103.1(2)

Sr/Ca=7/2 79(3) 94 (3) 72(2) x 2 96(3) x 2 94(3) x 2 99(3) x 2 92 (2) x 2 168(3) x2 165(3) 132(3) 96(3) 101(3) 121(4) 101(4) 100(4)

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Page 32 of 39

(c) Selected bond angles in degrees for the akermanite-based phase Angle

Sr/Ca=5/4 106.5(1) × 4 115.6 (1) × 2 106.8(1) 106.5(1) × 2 112.0(2) × 2 112.6(2)

O−Mg−O

O−Si−O

Sr/Ca=7/2 105.9(8) × 4 117.0(8) × 2 111.4(1) 107.6(7) × 2 111.8(1) × 2 106.3(9)

Sr/Ca=9/0 104.5(7) × 4 120.1(7) × 2 109.9(8) 106.2(6) × 2 112.5(1) × 2 109.3(1)

Table 5: Agreement factors for Rietveld refinements of X-ray diffraction data. Composition Sr/Ca=0/9 Sr/Ca=3/6 Sr/Ca=5/4 Sr/Ca=7/2 Sr/Ca=9/0

Rp 9.17 7.51 5.75 6.08 6.71

Rwp 12.2 9.63 7.55 7.88 8.67

RB diopside 8.81 4.14 2.05 2.70 -

RB akermanite 4.39 4.19 5.32

χ2 4.18 2.74 1.59 1.81 2.33

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Table 6: Comparisons of Raman and infrared vibration spectral frequencies (cm−1) of Sr/Ca=0/9 and Sr/Ca=9/0 glass-ceramics produced after 850 °C for 500 h with pure synthetic diopside and Sr-akermanite crystalline materials.

Sr/Ca=0/9 glass-ceramic Sr/Ca=9/0 glass-ceramic Raman Infrared Raman Infrared Present Richet Present Omiri et al., Present Hanuza Present Hanuza et al., 1068 1070 1073 − − − − − 1045 1045 1070 1025 − − − − 1011 1011 1005 1004 − − − − 968 965 966 965 − − − − 912 920 983 985 926 926 − − 866 865 918 841 841 − − − 850 898 901 − − − − − 702 734 − − − − − − 671 670 680 − − − − − 666 675 669 671 − − − − 634 630 651 652 − − − − 586 619 622 − − − − − 559 566 598 599 − − − − 526 537 587 588 − − − − 520 511 510 575 566 − − − 509 514 565 565 − − − − 471 478 470 510 − − − − 466 474 474 − − − − − 391 397 395 468 474 − − − 372 385 447 450 − − − − 354 363 366 391 392 − − − 317 329 335 356 − − − − 311 316 − − − − − −

33



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Figure captions: Fig 1: (a) Observed and difference X-ray powder diffraction profiles of the Sr/Ca=3/6 glassceramic obtained on heat treatment at 850 ºC for 500 h with NaCl as internal standard. The vertical bars represent Bragg peaks of diopside-based phase (blue), NaCl (red), anorthite (green) and SrSiO3 (magenta). (b) Observed and difference X-ray powder diffraction profiles of the Sr/Ca=9/0 glass-ceramic obtained on heat treatment at 850 ºC for 500 h with NaCl as internal standard. The vertical bars represent Bragg peaks of akermanite-based phase (blue), NaCl (red) and MgSiO3 (green). Fig 2: 29Si (left) and 27Al (right) MAS NMR spectra for glasses together with the deconvolution (individual peaks in grey lines and sum of all peaks in dashed grey lines). Fig 3: 29Si (left) and 27Al (right) MAS NMR spectra for glass-ceramics heat treated at 850 ºC for 500 h together with the deconvolution (individual peaks in grey lines and sum of all peaks in dashed grey lines). * indicates spinning sidebands. Fig 4: Micro-Raman spectra for glass-ceramics heat treated at 850 ºC for 500 h. Fig 5: FTIR spectra for glass-ceramics heat treated at 850 ºC for 500 h.

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Fig 1: (a) Observed and difference X-ray powder diffraction profiles of the Sr/Ca=3/6 glass-ceramic obtained on heat treatment at 850 ºC for 500 h with NaCl as internal standard. The vertical bars represent Bragg peaks of diopside-based phase (blue), NaCl (red), anorthite (green) and SrSiO3 (magenta). (b) Observed and difference X-ray powder diffraction profiles of the Sr/Ca=9/0 glass-ceramic obtained on heat treatment at 850 ºC for 500 h with NaCl as internal standard. The vertical bars represent Bragg peaks of akermanite-based phase (blue), NaCl (red) and MgSiO3 (green). 199x283mm (300 x 300 DPI)



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Fig 2: 29Si (left) and 27Al (right) MAS NMR spectra for glasses together with the deconvolution (individual peaks in grey lines and sum of all peaks in dashed grey lines). 141x192mm (300 x 300 DPI)


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Fig 3: 29Si (left) and 27Al (right) MAS NMR spectra for glass-ceramics heat treated at 850 ºC for 500 h together with the deconvolution (individual peaks in grey lines and sum of all peaks in dashed grey lines). * indicates spinning sidebands. 142x195mm (300 x 300 DPI)



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Fig 4: Micro-Raman spectra for glass-ceramics heat treated at 850 ºC for 500 h. 144x183mm (300 x 300 DPI)


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Fig 5: FTIR spectra for glass-ceramics heat treated at 850 ºC for 500 h. 134x167mm (300 x 300 DPI)


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