Composition-Property-Structure Correlations of Scandium AluminosilicateGlasses Revealed by Multinuclear 45Sc, 27Al, and 29Si Solid-State NMR

Bholanath Pahari,‡ Shahriar Iftekhar,‡ Aleksander Jaworski,‡ Kirill Okhotnikov,‡ Kjell Jansson,§

Baltzar Stevensson,‡ Jekabs Grins,§ and Mattias Eden‡,†

‡Physical Chemistry Division, Department of Materials and Environmental Chemistry, Arrhenius Laboratory, StockholmUniversity, SE-106 91 Stockholm, Sweden

§Inorganic and Structural Chemistry Division, Department of Materials and Environmental Chemistry, Arrhenius Laboratory,Stockholm University, SE-106 91 Stockholm, Sweden

Many features of aluminosilicate glasses incorporating a rare-

earth (RE) ion are dictated by its mass and cation field strength

(CFS). Sc–Al–Si–O glasses are interesting because Sc3+

exhibitsthe highest CFS but the lowest mass of all RE3+ ions. We

explore relationships between the glass composition and several

physical properties, such as density, glass-transition tempera-

ture (Tg), Vickers hardness, and refractive index, over the glassforming region of the ternary Sc2O3–Al2O3–SiO2 system.

The glasses exhibit uniform and unexpectedly low Tg-values

(�875°C), but a high microhardness (�9.3 GPa) that corre-

lates with the Sc2O3 content.29Si magic-angle spinning (MAS)

nuclear magnetic resonance (NMR) spectroscopy shows

enhanced deshielding and a minor glass-network ordering as

either the Al or Sc content of the glass increases.27Al MAS

NMR reveals that besides the expected AlO4 tetrahedra,

substantial amounts of AlO5 (31%–35%) and AlO6 (�5%)

polyhedra are present in all Sc–Al–Si–O glass structures. 45Sc

isotropic chemical shifts (�92 ppm) derived from MAS and3QMAS (triple-quantum MAS) NMR experiments are

consistent with ScO6 environments.

I. Introduction

OWING to the high cation field strength (CFS) of trivalentrare-earth (RE) metal ions, aluminosilicate (AS) glasses

incorporating such cations differ in many physical, chemical,and structural aspects from their M–Al–Si–O counterpartsbased on alkali and alkaline earth metals. As compared tothe latter, a RE–Al–Si–O glass generally features a higherglass-transition temperature (Tg), Vickers hardness (HV),refractive index (nRI), elastic modulus, and chemical durabil-ity, whereas its electric conductivity, gas permeability, andthermal expansion coefficient are relatively low.1–18 Theseproperties render them beneficial for various technologicalapplications, such as in radiotherapy,3,4 optical devices,6,9

and storage matrices for radioactive waste.19

The field strength of a cation with valence z and radius ris given by z/r2. It is noteworthy that Sc3+ features the low-est mass but the highest CFS among all RE3+ ions. Explora-

tion of the Sc–Al–Si–O system is highly desirable,considering that studies of related RE AS glasses have con-cluded that both the mass and CFS of the RE3+ ion havemajor bearings on the glass properties and structure.3–9,12,14, 17

Up to very recently,17,18 however, scandium-bearing amor-phous silicates have only been reported with Sc as a minorcomponent in Na–Al–Si–O and Y–Al–Si–O oxide glasses2,20

or in vitreous Sc-based oxynitrides.21,22 By using a traditionalmelt-quench procedure, we recently reported the glass-form-ing region of the ternary Sc2O3–Al2O3–SiO2 system for thefirst time.17 For two members thereof, we examined severalphysical properties, as well as the network structures byusing solid-state nuclear magnetic resonance (NMR) spec-troscopy, with the primary aim of elucidating the property/structure alterations of the AS glass when the RE3+ CFSincreased across the series La3+ < Y3+ < Lu3+ < Sc3+.17

Our present study specifically targets all Sc–Al–Si–O glassesattainable by quenching melts from 1650°C.

The compositional range allowing for preparation of REAS glasses is known to contract for increasing RE3+

CFS.2,6,9,17 Indeed, as illustrated in Fig. 1(a), homogeneousSc–Al–Si–O glasses were only obtained over a range of15–21 mol% Sc2O3, 28–36 mol% Al2O3, and 49–55 mol%SiO2. Using a solar furnace and high power, Sadiki et al.very recently reported two Sc–Al–Si–O glasses incorporatingup to 8 mol% Sc2O3 at a 76 mol% SiO2 content;18 suchcompositions give residues of unmelted precursors andphase-separated specimens under our preparation conditions.

Several structural features differ between RE–Al–Si–Oglasses and their alkali or alkaline earth-based counterparts.The RE3+ ions balance the negative charges of both[AlO4]

� tetrahedra and the nonbridging oxygen (NBO) ions,as in any glass based on M+ or M2+ modifier ions.23–27

Yet, as opposed to the latter, RE–Al–Si–O glass networksmay comprise significant amounts of AlO5 and AlO6 poly-hedra,11,15–18,27–31 regardless of whether the condition nAl/nRE � 3 is obeyed, such that charge compensation of allAlO4 units is readily arranged (nE denotes the molaramount of element E). Also, the distribution of bridgingoxygen (BO) atoms and NBO ions among the AlO4 andSiO4 network building blocks tends to randomize for REAS glasses, as does the AlO4/SiO4 intermixing28,32 comparedwith the relatively strict alternation of SiO4 and AlO4 tetra-hedra in the networks of glasses solely incorporating mono-or divalent modifiers.23–27

Herein, we report the density (ρ), HV, nRI, molar volume

(MV), and compactness (C) for all glasses of the Sc2O3–Al2O3–SiO2 system shown in Fig. 1(a). We seek to unveiltheir physical-property/composition relationships, as well aslinking them to the local glass-network structures, the latterprobed by using 29Si and 27Al magic-angle spinning (MAS)NMR. Besides complementing the currently limited number

L. Klein—contributing editor

Manuscript No. 30691. Received November 18, 2011; approved April 24, 2012.This work was supported by the Swedish Research Council (contracts VR-NT

2009-7551 and 2010-4943) and the Faculty of Natural Sciences at Stockholm Univer-sity. B. P. was supported by a postdoctoral grant from the Wenner-Gren Foundations.We gratefully acknowledge NMR equipment grants from the Swedish Research Counciland the Knut and Alice Wallenberg Foundation.

†Author to whom correspondence should be addressed. e-mail: mattias.eden@mmk.su.seRe-use of this article is permitted in accordance with the Terms and Conditions set

out at http://wileyonlinelibrary.com/onlineopen#OnlineOpen_Terms.

2545

J. Am. Ceram. Soc., 95 [8] 2545–2553 (2012)

DOI: 10.1111/j.1551-2916.2012.05288.x

© 2012 The American Ceramic Society

Journal

of 29Si NMR reports from RE AS glasses,15–17,28,32 we alsopresent the first 45Sc NMR data from an AS glass incorpo-rating Sc3+ as the sole modifier ion.

II. Experimental Procedures

(1) Glass PreparationThe Sc–Al–Si–O glasses were prepared from ball-milled mix-tures (2.5 g) of precalcined (300°C) Sc2O3, Al2O3, and SiO2

precursors (99.99% purity; ChemPure, Karlsruhe, Germany).They were placed in Pt crucibles and heated sequentially insteps of 200°C, with successive holding intervals of 0.5 h upto 1650°C, where each melt was held for 2 h prior to itsquenching by dipping the crucible in water. The weight lossesduring synthesis were typically �2.5 wt%. Each glass wasdoped with 0.1 wt% Gd2O3 to enhance T1 relaxation for the29Si NMR experimentation.

Each sample was examined by powder X-ray diffraction,using a Panalytical X’pert PRO MPD diffractometer (Al-melo, The Netherlands) and CuKa1 radiation. The data col-lection lasted for 4 h per sample, with 2h ranging between10° and 70°. This gave a lower detection limit of ≲1% forcrystalline impurities. Samples marked by asterisks inFig. 1(a) comprise larger amounts of crystalline material.

(2) Electron MicroscopyPolished glasses were coated by a 10–20 nm carbon film andexamined by scanning electron microscopy (SEM), by usinga JSM 7000F (JEOL, Tokyo, Japan) microscope in back-scatter electron imaging mode at 15 kV acceleration voltage.Besides identifying minor amounts (≲2%) of poorly orderedSiO2-rich phases in most samples, SEM revealed amorphousphase separation in a few specimens: they are depicted byshaded boxes in Fig. 1(a) and are together with the previ-ously noted partially crystallized samples not considered fur-ther herein. The remaining (transparent) specimens representhomogeneous glasses: each is identified in Fig. 1(a) by anopen square and an index between 1 and 6. Cation contentswere estimated using a LINK INCA (Oxford Instruments,Abingdon, U.K.) energy-dispersive X-ray (EDX) detector.Each sample composition was obtained as an average overten analyses from different fragments. The batched andEDX-analyzed charge-balanced Sc–Al–Si–O glass composi-tions are listed in Table I; they display very good agreement,with a largest relative error of 3.5% observed for the Sc2O3

content of specimen #6.The glasses were ground under dry conditions, sprinkled

on a holey carbon film supported by a copper grid, andexamined using a field-emission gun transmission electronmicroscope (JEM 2100F; JEOL) equipped with an EDXdetector.

(3) Physical Property MeasurementsAll physical-property measurements of the present glasses fol-lowed the same procedures as described in detail in our previ-ous work;14,17 they are only recapitulated briefly. Densitieswere measured according to the Archimedes method in dis-tilled water. Tg-values were determined on powdered samplesusing differential thermal analysis with a heating rate of 40°C/min up to 1350°C in flowing air, by using a SETARAMLabsys TG-DTA 1600 instrument (Caluire, Lyon, France).Vickers hardness measurements employed a Matsuzawamicrohardness tester MXT-a1 equipped with a pyramidal dia-mond indenter, and an applied load of 0.982 N for 20 s. TheHV-values were calculated by the procedure of Anstis et al.33

The refractive index was determined from polished glass sur-faces and a laser light source operating at k = 640 nm, usingthe expression nRI = tan(hB), where hB is the estimated Brew-ster angle. Each value of ρ, nRI, and HV was obtained as theaverage over at least four, five, and seven measurements,respectively.

(4) Solid-State NMR29Si (spin S = 1/2) MAS NMR experiments were carried outat a magnetic field of 9.4 T (giving a 29Si Larmor frequency of79.5 MHz), using a Bruker Avance-III spectrometer (Karlsruhe,Germany). Finely ground glasses were filled in 7 mm zirconiarotors and spun at 7.00 kHz. The NMR data were recorded

(a)

(b)

(c)

(d)

Fig. 1. (a) Glass-forming region of the ternary Sc2O3–Al2O3–SiO2

system, where the analyzed glass compositions are indicated by whiteboxes and an index ranging between 1 and 6. Each gray boxcorresponds to the respective batched composition, whereas shadedsymbols and asterisks represent phase-separated and partially crystallineamorphous phases, respectively. Experimentally measured values of the(b) density, (c) Vickers hardness, and (d) refractive index, plottedagainst the as-indicated oxide mole fraction. The various symbolsidentify glasses associated with distinct groups of r-values [Eq. (1)], asspecified in Table I. The gray lines correspond to best-fit results.

2546 Journal of the American Ceramic Society—Pahari et al. Vol. 95, No. 8

using single-pulse (“Bloch-decay”) acquisitions employing�66° flip-angles at a 52 kHz 29Si nutation frequency, aspectral window of 49 kHz, and �350 accumulated signaltransients. The relaxation delays of �600 s were selectedfrom separate 29Si spin-lattice relaxation measurements, so asto provide quantitative NMR spectra. 29Si chemical shifts arequoted with respect to neat tetramethylsilane (TMS).

27Al (S = 5/2) and 45Sc (S = 7/2) MAS NMR experimen-tation was performed at 14.1 T (Bruker Avance-III), corre-sponding to Larmor frequencies of �156.4 and �145.8 MHzfor 27Al and 45Sc, respectively. The NMR data collectionemployed samples filled in 3.2 mm rotors and spinning at24.00 kHz. Short (� 0.3 ls) pulses were used at rf nutationfrequencies of 100 kHz for 27Al and 83 kHz for 45Sc. Theywere calibrated on 1M Al(NO3)3 and 0.02M ScCl3 aqueoussolutions, which also served for shift referencing of 27Al and45Sc, respectively. Typical values of the spectral windows,number of accumulated transients, and the relaxation delays,were as follows: 600 kHz, 8000–12 000, and 3.0 s for 27Al;1176 kHz, 12 000–16 000, and 4.0 s for 45Sc. Complementary45Sc MAS NMR spectra were acquired at 9.4 T(�97.19 MHz Larmor frequency) for the Sc1:040:62(2.20) andSc1:210:86(2.32) samples, using filled 2.5 mm rotors spinning at32.00 kHz, but otherwise using identical experimental condi-tions as those at the higher field.

A Z-filter triple-quantum (3Q) MAS (3QMAS34,35) 45Scexperiment was performed on the Sc1:040:62(2.20) glass at 14.1 Tand 24.00 kHz MAS, using 2.8 and 0.75 ls pulses for 3Qcoherence excitation and conversion, respectively, and a 12.0 lscentral-transition (CT) selective p/2-pulse. The strong and weakpulses operated at rf nutation frequencies of 104 kHz and5.2 kHz, respectively. The 32(t1) 9 1024(t2) time-points withdwell-times (Dt1, Dt2) = (13.88, 8.33) ls were recorded andzero-filled to (128 9 2048) points, with 4800 accumulated sig-nal transients per t1-value and 1.2 s relaxation delays.

The processing of 1D NMR experiments did not involvetime-signal apodization prior to Fourier transformation,

except for using a full width at half maximum (FWHM)height Gaussian broadening of 400 Hz for the 45Sc datarecorded at 9.4 T, and 700 Hz along the t2-dimension of the3QMAS 2D data.

III. Results and Discussion

(1) Physical PropertiesTo facilitate our discussions on structure-composition rela-tions, we follow the nomenclature of Iftekhar et al.32 anddenote each sample as Scab(r), where a and b represent thenAl/nSi and nSc/nSi molar ratios of the glass, respectively (seeTable I). The parameter r is defined by

r ¼ nOnSi þ nAl

; (1)

and is related to the glass-network connectivity, i.e. to theaverage number of BO atoms (�n) over all SiO4 and AlO4

tetrahedra: �n ¼ 2ð4� rÞ.25,32,36Table II lists the experimentally measured physical proper-

ties (ρ, Tg, HV, nRI, MV, and C) of the Sc–Al–Si–O glasses.

For each property, we sought to rationalize its underlyingrelationship to the glass composition by numerically fittingthe experimental values to the molar fractions of Sc2O3,Al2O3, and SiO2.

The glass-transition temperature of RE AS glasses isknown to increase with the CFS of the RE3+ ion, i.e. fordecreasing ion radius, whereas members within each RE–Al–Si–O system exhibit remarkably insensitive Tg values overlarge compositional ranges.4–9,11,14,17,31 Yet, contrary to theexpectations of a maximized transition temperature of Sc ASglasses relative to other RE-based analogs, we observed asurprisingly low Tg value of the Sc1:040:62(2.20) glass in a preli-minary report.17 As verified in Table II, relatively low Tg val-ues are indeed observed for all Sc–Al–Si–O glasses: theyrange between 873°C and 879°C, giving a mean value of

Table I. Sc–Al–Si–O Glass Compositions†

Index Label Formula aSc2O3 bAl2O3 cSiO2

1 Sc1:480:60(2.07) Sc0.60SiAl1.48O5.12 14.77(15.00) 36.21(36.00) 49.02(49.00)

2 Sc1:280:56(2.09) Sc0.56SiAl1.28O4.76 14.56(15.00) 33.37(33.00) 52.07(52.00)

3 Sc1:040:62(2.20) Sc0.62SiAl1.04O4.50 16.96(17.05) 28.48(27.70) 54.56(55.25)

4 Sc1:200:71(2.22) Sc0.71SiAl1.20O4.86 18.25(17.93) 30.59(30.31) 51.16(51.76)

5 Sc1:270:76(2.22) Sc0.76SiAl1.27O5.05 18.92(18.70) 31.49(32.05) 49.59(49.25)

6 Sc1:210:86(2.32) Sc0.86SiAl1.21O5.10 21.24(20.50) 29.62(29.30) 49.14(50.20)†Each specimen is assigned an index between 1 and 6 and is labeled ScnAl

nSc(r), where nAl and nSc represent the analyzed stoichiometric coefficients listed in the third

column (normalized such that nSi = 1). The three rightmost columns specify the analyzed oxide equivalents (in mol%) of the aSc2O3–bAl2O3–cSiO2 composition, with

a + b + c = 100 mol%. Each value within parenthesis represents the batched composition.

Table II. Physical Properties and 29Si NMR Parameters†

Glass Tg (°C) ρ (g/cm3) HV (GPa) nRI MV (cm3/mol) C �dCG (ppm) FWHM (ppm)

Sc1:480:60(2.07) 879 3.04 9.28 1.480 7.09 0.562 88.8 15.7

Sc1:280:56(2.09) 873 2.96 9.25 1.482 7.29 0.548 91.5 18.1

Sc1:040:62(2.20) 874 2.93 9.34 1.499 7.43 0.540 91.8 17.9

Sc1:200:71(2.22) 875 3.02 9.31 1.501 7.26 0.552 90.7 17.3

Sc1:270:76(2.22) 876 3.02 9.35 1.505 7.27 0.551 89.4 16.6

Sc1:210:86(2.32) 876 3.02 9.45 1.515 7.33 0.547 88.1 16.0†Experimentally determined physical properties with uncertainties of 1r as follows: Tg (±2°C); ρ (±0.01 g/cm3); HV (±0.03 GPa); nRI (±0.005); MV (±0.01 cm3/mol);

C (±0.001). The two rightmost columns list the center of gravity (dCG; uncertainty ±0.2 ppm) and full width at half maximum (FWHM; ±0.2 ppm) height of the 29Si

NMR peak.

August 2012 Scandium Aluminosilicate Glasses 2547

875°C, which is very similar to that (877°C) observed by usfrom the La2O3–Al2O3–SiO2 system.14 On the other hand,RE AS glasses exhibiting intermediate field strengths betweenthose of La3+(2.82 A�2) and Sc3+ (5.41 A�2) display ele-vated values of Tg ≳ 885°C3,8,12,14,17,31 (but may, as for manyother glass properties, differ between various reports on thesame RE AS system).

As the mass of the RE element is generally muchhigher than those of the other RE–Al–Si–O glass constitu-ents, nRE has hitherto predominantly dictated the glassdensity.3–6,8,9,14,17 However, due to the relatively low mass ofSc, this trend is broken for the case of Sc–Al–Si–O glasses.When first solely considering any single potential composi-tion variable, the ρ-values of Table II only correlate wellwith the molar fraction of SiO2,

q=ðg=cm3Þ¼3:97ð18Þ�1:90ð35Þ�xðSiO2Þ; ðR2¼0:88Þ;(2)

whereas bilinear fits also incorporating x(Al2O3) or x(Sc2O3)gave marginally improved correlations. Figure 1(b) plots theexperimentally determined densities together with the linearrelationship Eq. (2).

All our other results on physical properties, however, con-firm previously established observations that the trivalent RE3+

ion dictates most features of RE-based AS glasses.1,3–15,17 Forinstance, the HV-values of Table II consistently stay higherthan those of previous reports from RE–Al–Si–O glasses ofcomparable compositions,1,3,7,17 thereby underscoring thestrong influences on the microhardness from the enhancedRE–O bond strength accompanying the high RE3+

CFS.7,12,17,37 Figure 1(c) further shows that the Vickers hard-ness correlates reasonably well with the molar fraction ofSc2O3,

HV=GPa ¼ 8:9ð1Þ þ 2:6ð5Þ � xðSc2O3Þ; ðR2 ¼ 0:85Þ;(3)

in agreement with an analogous relationship establishedbetween HV and x(Y2O3) for Y–Al–Si–O glasses (S. Iftekhar,B. Pahari, K. Okhotnikov, A. Jaworski, B. Stevensson, J.Grins, and M. Eden, unpublished data). Linear fits to eitherof x(SiO2) or x(Al2O3) gave worse correlations.

The refractive index of an RE-based AS glass is expectedto depend primarily on the RE3+ content, i.e., on the mostpolarizable cation, whose polarizability generally decreases asthe ionic radius shrinks.5 Our experimentally observed nRI-values indeed relate linearly to x(Sc2O3):

nRI ¼ 1:405ð9Þ þ 0:531ð51Þ � xðSc2O3Þ; ðR2 ¼ 0:96Þ:(4)

Furthermore, members of the Sc2O3–Al2O3–SiO2 systemshould exhibit the lowest nRI-values of any RE-based coun-terpart, as indeed verified by the results of Table II com-pared with literature reports.1,3–5,8,14,17

We calculated the molar volume and compactness (i.e.,ion-packing efficiency) of each Sc–Al–Si–O specimen, asdescribed in detail by Iftekhar et al.17 The estimated C-valuesassumed the sole presence of Sc[6] coordinations, and variedinsignificantly (� 0.002) regardless of whether the calcula-tions assumed Al solely in tetrahedral coordination, or if theNMR-derived Al[5] and Al[6] populations (discussed below)were accounted for. The values of C scatter around 0.55,whereas MV varies slightly within 7.1–7.4 cm3/mol (Table II).These results confirm earlier inferences that MV decreaseswhile C increases for enhancing RE3+ CFS:1,5–7,10,17 consis-tent with the highest CFS of Sc3+, the packing efficiency ofthe constituents of RE AS glasses is maximized for the pres-ent Sc–Al–Si–O networks (cf. the results of Iftekhar et al.17),

thereby underlining the importance of the RE-O bondstrength for condensing the structure of RE ASglasses.7,12,14,17,37

(2) Transmission Electron MicroscopyGiven that SEM revealed glass-in-glass phase separation overa lm range in some of the silica-rich compositions initiallyattempted [marked by shaded boxes in Fig. 1(a)], and thatphase separation has been observed in related systems, suchas Y aluminate38,39 and even Ca AS40 glasses, transmissionelectron microscopy (TEM) was utilized to confirm thehomogeneous single-phase character of our six Sc AS glassesalso down to the lowest length-scale attainable by our instru-ment (≲10 nm). Figure 2 displays representative TEM imagesrecorded from four glass powders, each one examined across>30 fragments.

(3) 29Si NMRAn AS glass network consists of interlinked AlO4 and SiO4

tetrahedra.15–18,25,26,28–32 Such a silicate group will be labeledQn

Si(mAl), where n denotes the number of BO atoms at thetetrahedron, whereas m represents the number of Si–O–Allinkages, consequently giving n–m Si–O–Si bridges.25 The29Si chemical shift of the Qn

Si(mAl) unit is primarily (but notsolely) dictated by the parameters n and m,15–17,23–26,28,32

where QnSi(mAl)?Qn�1

Si (mAl) and QnSi(mAl)?Qn

Si([m + 1]Al)

conversions typically increase the 29Si shift by �10 and�5 ppm, respectively. However, a large number of Qn

Si(mAl)units may coexist in an AS glass network, which maypotentially encompass all integers 0 � m � n � 4.Whereas 29Si NMR does not generally allow for directlyquantifying their populations, the 29Si center-of-gravity shift(dCG) correlates well with both the RE2O3 and Al2O3 con-tents of a RE–Al–Si–O glass,16,17,28,32 in analogy with well-established observations from AS glasses based on M+ orM2+ modifiers.23–27 Such a trend is also confirmed by the29Si NMR spectra of the Sc-based glasses shown in Fig. 3:both dCG (see Table II) and the shift at the peak maximum(dmax) displace toward less negative values when either thenSc/nSi or the nAl/nSi molar ratio of the sample increases. Werecently highlighted an anomalously high 29Si shielding

(a) (b)

(c) (d)

Fig. 2. Transmission electron microscopy images recorded from the(a) Sc1:480:60(2.07), (b) Sc1:040:62(2.20), (c) Sc1:270:76(2.22), and (d) Sc1:210:86(2.32)glasses at a 200 kV acceleration voltage. Particle-thickness variationsaccount for the darker areas in the images.

2548 Journal of the American Ceramic Society—Pahari et al. Vol. 95, No. 8

observed from the Sc1:040:62(2.20) specimen17 compared to La, Y,or Lu AS glasses of similar compositions.16,17,28,32 This featureis verified by all other 29Si NMR spectra in Fig. 3. The under-lying reason is unknown, but a plausible explanation invokesthe presence of free O2� ions in the Sc AS glass structure.17

The FWHM height of the 29Si NMR peak conveys quali-tative insight into the glass network (dis)order,17,23–26,28,32 as

partly dictated by the distribution-widths of the Si–O–T(T = Si, Al) bond angles, but primarily by the parameters{n, m} associated with the set {Qn

Si(mAl)}, as recentlyobserved for the sister RE AS glass systems based on Y andLu (S. Iftekhar, B. Pahari, K. Okhotnikov, A. Jaworski, B.Stevensson, J. Grins, and M. Eden, unpublished data).Figure 4(a) evidences a good correlation between the respec-tive values of dCG and FWHM for each glass: the peak widthdecreases when dCG increases, thereby indicating a slightnetwork ordering as either of the nSc/nSi or nAl/nSi ratiosgrow. Identical trends have been reported for Y,28 Lu, andLa32 AS glasses, as will be discussed elsewhere.

Each set of dCG and FWHM values was fitted to eitherthe molar fraction of each oxide constituent, or to the respectivecation molar fraction defined by xM = nM/(nSi + nAl + nSc),with M = (Si, Al, Sc). Owing to the established linear rela-tionship between the 29Si FWHM and dCG, we primarily dis-cuss the dependence of the latter; the FWHM data revealsnearly identical (degrees of) correlations, only displaying dif-ferent best-fit parameters. The average number of NBO ionsat the SiO4 tetrahedra,25,32,36 which constitutes one main fac-tor controlling the average 29Si NMR shift, grows forincreasing r-value of the glass; the latter correlates roughlywith xSc. However, the exact functional form of the NMRshift dependence on the xSc or nSc/nSi parameters is not apriori known. Nevertheless, as previously established forLa–Al–Si–O glasses,32 provided that the glass-network poly-merization (i.e., the r-value25,32,36) remains constant, a linearrelationship is expected between dCG and the amount of Alin the glass. The specimens #3, #4, and #5 (see Tables I andII) exhibit a nearly constant r � 2.21; their dCG and FWHMvalues indeed scale linearly with the ratio nAl/nSi, both givingR2� 0.94 (not shown). Next, considering the entire set {dCG}listed in Table II, the ratios nSc/nSi and nAl/nSi provided thebest fit (out of several tested parameter pairs), thus verifyingthe anticipated roles of Sc and Al for controlling the net 29SiNMR shift:

dCG=ppm ¼ �107ð3Þ þ 11ð2Þ nScnSi

þ 7ð1Þ nAl

nSi;

ðR2 ¼ 0:90Þ:(5)

When solely considering single-parameter fits, however,reasonable correlations were only observed for either of xSior x(SiO2), e.g.

dCG=ppm ¼ �57ð7Þ � 64ð12Þ � xðSiO2Þ; ðR2 ¼ 0:84Þ:(6)

Figure 4(b) plots the experimental and best-fit results. Hence,when both the average numbers of BO atoms and AlO4

neighbors alter among the QnSi(mAl) network-units due to

simultaneous changes of the glass constituents, the net 29SiNMR deshielding is best captured from the accompanyingvariations of the SiO2 content.

In summary, the narrowed FWHM of the 29Si NMR peakevidences a network ordering as the Al2O3 and Sc2O3 con-tents of the glass increases. This trend is mirrored by a 29SiNMR deshielding trend, stemming from the concurrentBO?NBO [for increasing x(Sc2O3)] and Si–O–Si?Si–O–Al(as x(Al2O3) increases) substitutions at the local {Qn(mAl)}environments [Eqs. (5) and (6)]. The monotonic alterationsof the FWHM and dCG values as the glass composition var-ies further confirm the homogeneous character of the glasses,as inferred from SEM and TEM.

(4) 27Al NMRFigure 5(a) shows the 27Al MAS NMR spectrum recordedfrom the Sc1:040:62(2.20) sample. It reveals three main CT reso-nances �50 ppm, �30 ppm, and � 0 ppm, evidencing the

Fig. 3. 29Si NMR spectra recorded at 9.4 T and 7.0 kHz MASfrequency from the as-indicated samples. The peak maximum (dmax)is specified at the lower left region of each NMR spectrum.

(a)

(b)

Fig. 4. (a) Correlation (R2 = 0.92) between the center of gravity(dCG) and FWHM of the 29Si NMR peaks, where different symbolsgroup the specimens according to (nearly) constant r-value. (b) Plotof dCG versus the mole fraction of silica in the glass. The gray line isthe best-fit according to Eq. (6).

August 2012 Scandium Aluminosilicate Glasses 2549

coexistence of AlO4, AlO5 and AlO6 polyhedra, respectively,in the structure.15–18,23–25,28–31 The (zoomed) NMR spectrashown in Fig. 5(b) displays relatively minor alterations whenthe Sc–Al–Si–O glass composition changes.

Using a software developed in-house, each 27Al NMR spec-trum was deconvoluted into three signal contributions, onefrom each Al[p] coordination and including its fractional popu-lation (x[p]), average isotropic 27Al chemical shift (d½p�iso), androot-mean-square (rms) quadrupolar product C

½p�Qg ¼

C½p�Q ð1þ g2=3Þ1=2, where C

½p�Q and g denote the quadrupolar

coupling constant and asymmetry parameter of the electric

field gradient tensor, respectively.27,41 Czjzek42 and Gaussian(with W

½p�iso denoting the FWHM in ppm) distributions were

assumed for the quadrupolar products and isotropic 27Alchemical shifts, respectively. During fitting, the parametersC

½p�Qg and W

½p�iso (p = 5, 6) were kept restricted within

±0.2 MHz and ±0.5 ppm, respectively. No bounds were usedon any parameter associated with Al[4], nor on any isotropicchemical shift. The best-fit data are listed in Table III, andone example of a deconvolution result is shown in Fig. 5(a).The uncertainties of the data (see Table III) were obtained asthe respective range over which each parameter could varyfreely, whereas the rms deviation between the experimentand simulation remained within 20% of its global minimumvalue. The W

½p�iso values are 13–14 ppm, �12 ppm and

10–11 ppm for the 27AlO4,27AlO5, and 27AlO6 polyhedra,

respectively, whereas the fractional populations stayed within±0.02 even in the absence of any parameter-restriction.

None of the resulting 27Al NMR parameters reveal anysignificant variations across the glass series. As observed forrelated La and Y AS glasses,16–18,29,31 the quadrupolar prod-ucts of the 27AlO4 tetrahedra (�11.0 MHz) are slightly higherthan those of the 27AlO5 (�10.3 MHz) and 27AlO6 (8.5–8.7 MHz) polyhedra. Furthermore, each AlOp populationremains fairly constant: {x[4], x[5], x[6]}� {0.60–0.65, 0.31–0.35, 0.04–0.05}. Remarkably large amounts of Al[5] coordi-nations are present in all structures, considering that the sam-ples exhibit ratios nAl/nSc < 3 throughout (see Table III),allowing for complete charge compensation even if the entireAl speciation would remain solely as Al[4]. Sadiki et al.18 alsorecently observed a high fraction x[5] � 0.35, however, for aSc AS glass featuring a low Sc content and high ratio nAl/nSc�11, necessarily implying large amounts of AlO5 units dueto insufficient charge-balance. Our complementary resultssuggest that the high abundance of Al[5] coordinations(x[5]� 0.31) is an inherent property of Sc–Al–Si–O glasses.Note that because x[5] � 0.31 is obtained also from theSc1:040:62(2.20) specimen displaying nAl/nSi � 1, the elevatedamounts of high-coordination polyhedra may not simply berationalized as stemming from violations of the LoewensteinAl[4]–O–Al[4] avoidance principle;43 this “rule” has no strongbearing on Sc AS glass structures (see Section I).

The enhanced abundance of AlO5 (and AlO6) polyhedrafor increasing CFS of the RE3+ ion is attributed to equilib-ria of the type AlO4 + REO7 ⇌ AlO5+REO6,

29,31 which tendto shift to the right as the RE3+ CFS grows. This naturallyexplains the significantly higher {AlO5, AlO6} populations ofSc–Al–Si–O glasses relative to those based on RE3+ionsassociated with lower CFS.17,29–31 The present x[5]-values of0.31–0.35 may for instance be compared with the correspond-ing ranges of 0.15–0.23 and 0.23–0.27 observed by us fromglasses based on Y3+ and Lu3+, respectively (S. Iftekhar, B.Pahari, K. Okhotnikov, A. Jaworski, B. Stevensson, J. Grins,and M. Eden, unpublished data), whose CFS values amountto 68% and 75% relative to that of Sc3+.

(a)

(b)

Fig. 5. 27Al NMR results obtained at 14.1 T and 24.0 kHz MAS.(a) Experimental spectrum (black trace) obtained from theSc1:040:60(2.20) glass, deconvoluted into three component peaks (graytraces) stemming from the 27Al[p] (p = 4, 5, 6) coordinations. Thecurve beneath the NMR spectrum represents the difference betweenthe experiment and best-fit. Satellite-transition27 spinning sidebandsare marked by asterisks. (b) Zooms around the CT centerbandregion of NMR spectra obtained from the as-indicated samples.

Table III.27Al NMR Parameters

†

Glass nAl/nSc d½4�iso (ppm) d½5�iso (ppm) d½6�iso (ppm) C½4�Qg (MHz) C

½5�Qg (MHz) C

½6�Qg (MHz) x[4] x[5] x[6]

Sc1:480:60(2.07) 2.45 66.0 38.3 8.1 11.02 10.32 8.63 0.598 0.351 0.051

Sc1:280:56(2.09) 2.29 64.4 37.5 8.0 10.99 10.26 8.65 0.625 0.327 0.048

Sc1:040:62(2.20) 1.68 63.9 37.0 7.9 11.02 10.27 8.60 0.649 0.307 0.044

Sc1:200:71(2.22) 1.68 64.4 37.1 7.7 11.00 10.29 8.59 0.637 0.316 0.047

Sc1:270:76(2.22) 1.66 65.4 37.5 7.6 11.08 10.33 8.62 0.630 0.324 0.046

Sc1:210:86(2.32) 1.39 65.9 38.0 8.0 11.05 10.35 8.50 0.615 0.331 0.054†Best-fit 27Al NMR data for each 27Al[p] coordination. d½4�max denotes the peak maximum of the 27AlO4 NMR signal. The parameters are associated with the follow-

ing uncertainties: d½p�iso (±0.5 ppm); C½p�Qg (±0.2 MHz); x[p] (±0.02).

2550 Journal of the American Ceramic Society—Pahari et al. Vol. 95, No. 8

(5) 45Sc NMRFigure 6(a) displays 45Sc MAS NMR spectra recorded at 9.4and 14.1 T from the Sc1:040:62(2.20) glass. We focus most of ourdiscussion on this specimen, as all Sc–Al–Si–O glasses revealnearly identical spectral features. Figure 6(b) shows the three“most extreme” variations observed across the series of NMRspectra recorded at 14.1 T: all exhibit a broad and Gaussian-shaped CT centerband signal, with the peak maxima appear-ing between 61 and 65 ppm (at this magnetic field coincidingwith their respective center of gravities) and associated withFWHM values between 112 and 114 ppm (�16.5 kHz). Thecorresponding data from the Sc1:040:62(2.20) sample at 9.4 T aredmax = 45 ppm, dCG=22 ppm, and FWHM = 165 ppm(�16.0 kHz); they are within experimental error the same asthose of Sc1:480:60(2.07) (spectrum not shown).

We report the 45Sc NMR peakwidths both in ppm and infrequency units of Hz. For the latter case, the FWHM (W) isdictated both by distributions of 45Sc chemical shifts thatscale linearly with the external magnetic field (B0), whereasthe broadening from second-order quadrupolar interactions isinversely proportional to B0.

27 However, when expressed inppm (�W/B0), the chemical-shift dispersion becomes B0-independent, whereas the quadrupolar broadening scales asB�20 . The almost equal peakwidths (in Hz) obtained at the

magnetic fields of 9.4 and 14.1 T implies that the increasedpeakwidth stemming from the elevated shift distribution atthe higher B0 value is offset exactly by an equal FWHMdecrease associated with the diminished quadrupolar broad-ening. The latter contribution dominates at the lower field, aswitnessed by the asymmetric tail toward lower ppm-values inthe NMR spectrum [Fig. 6(a)], whereas the Gaussian-shapedNMR peak at 14.1 T evidences a significant shift-dispersioncontribution. Also, the more intense CT spinning sidebandformation observed in the MAS NMR spectrum obtained at14.1 T compared to 9.4 T originates from the chemical-shiftanisotropy (CSA) that scales linearly with B0.

41

None of the MAS NMR spectra in Figs. 6(a) and (b) per-mitted signal-separation from multiple 45Sc sites. This obser-vation also applies to a 3QMAS NMR spectrum recordedfrom the Sc1:040:62(2.20) sample at 14.1 T [Fig. 6(c)]. The indirect(vertical) dimension of such 2D NMR spectra is devoid ofquadrupolar broadenings,34 thereby underlining that the 45ScNMR peakwidths are mainly limited by distributions in iso-tropic chemical shifts at B0 � 14.1 T. We conservativelyestimated dCSA = dzz–diso = (110 ± 30) ppm from numericalsimulations (see Eden41 for parameter definitions). This rangeof values conforms to the upper regime of 45Sc CSA datareported from various Sc-bearing complexes,44 therebyreflecting strong distortions from perfectly symmetric 45Scelectronic environments in the present glasses.

The B0-dependent center of gravity of the NMRpeak relates to the isotropic chemical shift (diso) asdCG = diso + dQ, where the isotropic second-order quadrupo-lar shift scales as dQ �C2

Qg=B20; see, for example Kim et al.45

for the exact expression. These relationships were used toderive the 45Sc mean isotropic chemical shift and quadrupo-lar product by using the two MAS NMR spectra recordedfrom Sc1:040:62(2.20) at each external magnetic field. Thisresulted in diso = (92 ± 3) ppm and CQg = (16.0 ± 0.6) MHz,in reasonable agreement with the corresponding parameters(diso/ppm, CQg/MHz) = (91 ± 3, 14.7 ± 0.6) extracted fromthe 3QMAS NMR spectrum by using standard procedures.34

The relatively few 45Sc solid-state NMR reports to dateencompass studies of simple Sc salts/complexes,44,46 interme-tallic compounds,47 ceramics,45,48,49 crystalline silicates,50

aluminophosphate glasses,51 and one Na3AlSi3O9 glass incor-porating 2 wt% Sc2O3.

20 The chemical shift of a cationdepends foremost on the coordination number.25,27 45Sc[6]

chemical shifts are typically observed in the range of0–130 ppm, whereas 45Sc[8] coordinations resonate in theregion of diso ≲ �10 ppm.44–46 Given that we present the first45Sc NMR data from any crystalline or amorphous AS phasewhere Sc constitutes the sole (or even dominating) cationmodifier, it is difficult to make unambiguous interpretations,but the average isotropic chemical shifts �92 ppm stronglysuggest Sc[6] coordinations in our Sc–Al–Si–O specimens.The shifts conform well to the range 78–86 ppm observed forScO6 groups in crystalline silicates,50 and slightly higher than

the value d½6�iso � 50 ppm reported from Y3ScxAl5�xO12 cera-

mic phases.48 However, besides the Sc coordination number,the literature also indicates a nonnegligible dependence of the45Sc chemical shift on geometrical factors (e.g., Sc–O bonddistances and O–Sc–O bond angles), as well as the identity ofthe cations in the second coordination sphere of Sc,44,48,50 inanalogy with previous observations for Y.52 A large numberof distinct 45Sc environments with slightly different chemical

(a)

(b)

(c)

Fig. 6. (a) 45Sc NMR spectra recorded at 9.4 T (top) and 14.1 T(bottom) from the Sc1:040:60(2.20) specimen, where asterisks mark CTspinning sidebands originating primarily from chemical shiftanisotropy. All other sidebands derive from the satellite transitions.(b) NMR spectra (14.1 T) zoomed around the CT centerbandregion; they were selected to convey the largest differences observedwithin the set of samples. (c) 45Sc 3QMAS NMR spectrum obtainedat 14.1 T and 24.00 kHz MAS from Sc1:040:60(2.20), shown togetherwith projections along each spectral dimension.

August 2012 Scandium Aluminosilicate Glasses 2551

shifts is likely the source of the large chemical shift disper-sion observed from these Sc AS samples.

IV. Conclusions

We have presented the first comprehensive investigation ofphysical properties (ρ, Tg, HV, nRI, MV, and C) of glassesfrom the Sc2O3–Al2O3–SiO2 system, as well as their localstructures as probed by 45Sc, 27Al, and 29Si MAS NMR.Our results confirm several expectations, but also offersome surprises. As anticipated, we found that (i) both therefractive index and Vickers hardness depend linearlyon the molar fraction of Sc2O3. (ii) Silicon is exclusivelypresent in tetrahedral coordination. When either the Sc orAl content of the glass increases, the 29Si center-of-gravityshift displaces toward more positive values, while the peakconcurrently narrows, which suggests a glass-network order-ing due to a reduction in the numbers {n, m} of distinct{Qn

Si(mAl)} network-building units. (iii) Aluminum mainlyassumes tetrahedral coordination, but significant amountsof AlO5 (31–35%) and AlO6 (�5%) polyhedra are presentin all Sc–Al–Si–O glass structures. The elevated amounts ofAl[5] and Al[6] coordinations compared to other RE ASglasses16–18,28–31 stems from the maximized CFS associatedwith Sc3+.

More remarkable features of Sc–Al–Si–O glasses include(i) their comparatively modest Tg values � 875°C, which arelower than those typically observed from other RE ASglasses of similar compositions,3,8,11,12,14,17,31 thereby break-ing the hitherto observed trend that Tg increases concurrentlywith the RE3+ CFS. (ii) As opposed to other RE–Al–Si–Osystems, where the mass of the RE element outweighs anyother constituent, the density of Sc AS glasses correlatespoorly with their molar fraction of Sc2O3, but well with thatof SiO2. This suggests that volume effects overcome the rela-tively minor mass differences among the cations, as also veri-fied from the high compactness (�0.55) observed from the ScAS glass structures. (iii) Over the series of samples, both 27Aland 45Sc MAS NMR suggest insignificant changes in thelocal Al and Sc environments, respectively, which partlyreflects the relatively narrow glass-forming region of theSc2O3–Al2O3–SiO2 system, whose members apparently mustobey the stoichiometry nAl=nSiJ1 (Table I).

Based on the observed 45Sc isotropic NMR chemical shiftsof �92 ppm, we attribute Sc to be present (primarily) inoctahedral coordination.44,48,50,51 Given that our work repre-sents the first 45Sc NMR report from an AS glass where Sc3+

constitutes the sole/main modifier ion, the present average45Sc isotropic chemical shifts (�92 ppm), quadrupolar prod-ucts (�16 MHz), and chemical shift anisotropies (�110 ppm)will serve as benchmark values for future investigations of ScAS glasses.

The combination of relatively high glass transition temper-atures (�875°C) and microhardness (�9.3 GPa) at a modestdensity (�3.0 g/cm3) makes Sc–Al–Si–O glasses attractivecandidates for engineering applications that demand reason-ably light, but hard and durable materials.

Acknowledgments

We thank P.-O. Soderholm (Royal Institute of Technology, Stockholm) foraccess to the microhardness tester and the reviewers for helpful comments.

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