Structural Response of Lithium Disilicate in Glass Crystallization

Article pubs.acs.org/crystal

Structural Response of Lithium Disilicate in Glass Crystallization Saifang Huang,†,‡ Zhaohui Huang,‡ Wei Gao,† and Peng Cao*,† †

Department of Chemical and Materials Engineering, The University of Auckland, PB 92019, Auckland 1142, New Zealand School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China

‡

ABSTRACT: The Li2O·SiO2 related glass systems have attracted extensive theoretical and experimental investigations for several decades. There are three different crystallization sequences commonly reported in lithium disilicate glasses although their contributing factors are unclear. Moreover, the effect of concurrent precipitation of crystalline phases during glass crystallization is rarely reported from a microscopic perspective. In this work, we observed the structural response of lithium disilicate (Li2Si2O5) to other crystalline phases, as probed in different types of lithium disilicate glasses by in situ synchrotron X-ray powder diffraction. The evolution of crystallographic structure of Li2Si2O5 along its c axis showed a silica-controlled fashion, viz., its evolution trend reverses whenever a crystalline silica phase precipitates. Besides, the evolution characteristics of the c axis of Li2Si2O5 at the nucleation stage differ if the crystallization sequence changes. These findings may initiate a novel crystallographic approach for analyzing the intrinsic mechanisms of reaction, nucleation, or crystallization in systems involving crystalline phases.

1. INTRODUCTION The discovery of glass-ceramics in the 1950s1 launched several decades of extensive work into the interrelations of their structure, processing, properties, and applications.2−5 The classical nucleation and crystal growth theories dedicated to glass6−8 have largely contributed to the advancement of glassceramics. Li2O·SiO2 glasses serve as an important system for investigating the nucleation theories of glass.8−10 The lithium disilicate (Li2O·2SiO2) glasses are of commercial significance, as the derived glass-ceramics have been widely used for dental restorative applications3,4,11 owing to their desirable performances for clinical use, such as biocompatibility, translucency, mild hardness, good chemical stability, high flexural strength, and relatively high toughness.12−16 In lithium disilicate glass systems, lithium disilicate (LS2: Li2Si2O5) and metasilicate (LS: Li2SiO3) are the main crystalline phases formed from base glasses through distinct crystallization sequences. The first (type I) phase transformation sequence involves the simultaneous nucleation of LS and LS2 phases at low temperatures.12−14,17−19 In the second (type II) sequence LS nucleates solely and then transforms to LS2 at a higher temperature.19−21 The temperatures for LS nucleation and its phase transformation to LS2 are composition dependent. In the third (type III) sequence only the LS2 phase nucleates from the beginning, while LS is completely absent.22,23 Owing to the attractive machinability of LS, the former two types of glass24 are usually preferred for the manufacturing of dental glass−ceramic products with desirable geometries using standard CAD/CAM systems.25 In situ experimentation, using either neutron or synchrotron X-rays, is crucial to monitoring the crystallographic structural changes in a material system that take place under certain © XXXX American Chemical Society

circumstances, such as precipitation in solutions or solidsolutions,26 crystallization of amorphous materials,27 decomposition of compounds,28 synthesis or sintering of polycrystalline materials,29 and electrochemical activity in lithium-ion batteries.30 In a simple system, structural evolution of a crystal could be a linear trend against temperature if there is no phase transition: a perovskite-type material La0.63(Ti0.92, Nb0.08)O3 is such an example.31 What about the case of a reacting system involving multiple phases? Will these phases still keep the original trend, or do they affect each other? Here we report an interesting structural response of LS2 to silica phases in complex lithium disilicate glasses with aforementioned different nucleation sequences, by investigating the crystallographic evolution of silicate phases.

2. EXPERIMENTAL PROCEDURES Several complex lithium disilicate glasses with main compositions are shown in Table 1. The major components and some minor additives (selected from Na2O, K2O, CaO, MnO, and Sb2O3) were melted in a Pt crucible at 1500 °C for 3 h and shaped in to glass rods 6−8 mm in diameter. To investigate the crystallographic evolution behavior of the lithium disilicate glasses, in situ high-temperature X-ray diffraction (HT-XRD) experiments of these glasses were conducted on the Powder Diffraction beamline at the Australian Synchrotron center. The glasses were cut and ground to thin samples with dimensions of approximately 5 mm × 5 mm × 0.4 mm for the in situ experiments. An Anton Paar HTK-2000 high-temperature chamber with a platinum strip heater was used for the annealing of glass samples. The ramp rate during heating was 400 K·min−1. In order to obtain high quality Received: June 10, 2014 Revised: August 14, 2014

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dx.doi.org/10.1021/cg500841x | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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diffraction data, we designed a holding stage at each prescribed temperature from 500 to 1000 °C with a fixed interval of 20 °C. The X-ray was on for 4 min at each temperature to allow a high intensity of diffraction patterns. In this respect, heating from 500 to 1000 °C is indeed a quasi-continuous annealing process. The energy of X-ray beam is 11 keV, and the corresponding wavelength is 1.1273 Å. The detector modules have a 0.2° gap in every 5°. Owing to the active detection region is not continuous, two patterns (p1 and p2) are recorded for each data collection with a shift of 0.5°, i.e., with ranges of 6.2−86.0° and 6.7−86.5°, respectively.

Table 1. Main Composition of the Glasses (in mol %) glass

A

B

C

D

SiO2 Li2O MgO Al2O3 P2O5 ZrO2 others

67.09 27.86

63.27 24.77 2.49 2.48 1.63 0.90 4.46

66.04 26.36 5.32

66.08 26.38 5.32

0.74 0.43 1.11

1.11

1.89 1.29 1.88

1.11

Table 2. XRD Patterns, Evolution of Lattice Parameters of Lithium Disilicate (LS2), Crystalline Phases, and Their Temperature Ranges for Different Lithium Disilicate Glassesa

a Note: LS, lithium metasilicate; LS2, lithium disilicate; LP, lithium phosphate; CR, cristobalite (JCPDS 27-0605, cubic); TM, tridymite (JCPDS 501432, orthorhombic); QZ, quartz (JCPDS 11-0252, hexagonal); S, MgAl2Si4O12; U, unidentified phase (with peaks at d = 3.330, 2.483, and 1.405 Å). The standard deviations of lattice parameters of LS2 are not shown in the plots as they are considerably small ((0.2−3.5) × 10−4 Å).

B



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Figure 1. In situ synchrotron XRD patterns of lithium silicate glass-ceramics at high temperatures. (a1−d1) Main peaks of crystalline phases formed during the heat treatment process; (a2−d2) peak evolution of (002) plane of lithium disilicate, and (a3−d3) peak evolution of its (130), (020), and (111) planes. The letters a, b, c, and d represent the glasses A, B, C, and D, respectively. Red arrows show the events at the transition point of the evolution trend for the (002) diffraction peak of lithium disilicate phase. The letters A, C, D, M, P, Q, T, and U in the (a1−d1) patterns are denoting the crystalline phases of MgAl2Si4O12, cristobalite, lithium disilicate, lithium metasilicate, lithium phosphate, quartz, tridymite, and an unidentified phase, respectively. Structure refinement was performed with the aid of the software MAUD, which is based on the full-pattern analysis using the Rietveld method.32 The entire suite of data sets was refined sequentially via an iterative least-squares procedure by minimizing the residual parameters Rwp, RB, and Rexp. The evolution of lattice parameters (a, b, and c) of silicate phases at different temperatures were carefully examined.

corresponding in situ powder diffraction patterns, lattice parameters of lithium disilicate, and the phase assemblages of glasses at the temperature stages are listed in Table 2. The standard deviation of lattice parameters of LS2 phases during the refinements for each glass are less than 1.4 × 10−4 Å, which are too small to be shown in the plots (Table 2). The phase transformation process in glass A is summarized as follows. LS and LS2 nucleate at 580 °C simultaneously, and the LS phase subsequently transforms to LS2 at 820 °C and completes at 860 °C. This glass shows type I nucleation sequence. The cristobalite (CR) and lithium phosphate (LP) crystallize at 840 and 880 °C, respectively. The slight distortion of the sample was found at >960 °C. CR, LP, and LS2 phases remain in the glass-ceramics at 1000 °C.

3. RESULTS 3.1. Phase Evolution Process in Various Glasses. The four lithium disilicate glasses listed in Table 1 correspond to the three different phase evolution sequences of lithium silicates at elevated temperatures (Type I, glass A; Type II, glass B; Type III, glasses C and D, in Table 1). Each glass experienced a continuous annealing process from 500 to 1000 °C. The C



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Glass B shows type II nucleation sequence. The LS phase nucleates exclusively at 560 °C. A very weak peak of the LS2 phase is detected at 700 °C indicating its formation at this temperature. The content of LS2 crystals dramatically increases to over 60 wt % at 760 °C according to the quantitative phase analysis, whereas the LS phase disappears. It indicates that the transformation of LS2 crystals from LS crystals and base glass completes at 760 °C. In this glass, the LP phase crystallizes at 720 °C and reaches a maximum of ∼6 wt % at 820 °C. Quartz (QZ) is formed at 840 °C as a trace phase with a maximum amount of ∼0.4 wt %. Another trace phase MgAl2Si4O12 with a similar content (∼0.5 wt %) appears in this glass at 820 °C and dissolves completely at 900 °C. The sample slightly distorted at above 860 °C. The glasses C and D demonstrate the type III nucleation sequence, i.e., only the LS2 phase nucleates throughout the annealing process, while the LS phase does not exist at all. In fact, these two glasses have similar compositions except for 0.43 mol % ZrO2 in glass C (Table 1). In glass C, the LS2 and LP phases nucleate at 560 and 800 °C, respectively; silica phase including CR and tridymite (TM) precipitate at 780 and 860 °C, respectively. Dramatic melting of these crystalline phases in glass C occurs at 980 °C and completes at 1000 °C. In glass D, the LS2 and LP phases nucleate at 600 and 800 °C, respectively; three types of silica phases including CR, QZ, and TM are formed in this glass at 780, 840, and 960 °C, respectively. The quartz phase melts at 980 °C, while other crystalline phases still remain in the sample at 1000 °C. This glass showed slight distortion at 960 °C during the annealing process. 3.2. Lattice Parameters of Silicate Phases. Figure 1 illustrates the relationship of phase evolution and peak shift in all glasses. Figure 1a1−d1 displays the XRD patterns of lithium disilicate glasses A, B, C, and D, respectively. The intensity (in counts) of the powder diffraction patterns is indicated by colors. The expanded patterns of lithium disilicate phase at the selected 2θ ranges are given in Figure 1a2−d2/a3−d3, demonstrating the evolution trend of (002) plane and other main peaks of (130), (020), and (111) planes. Interestingly, the trends of lattice parameter c of LS2 are different in the three glass systems (types I, II, and III), as shown in Table 2. With careful examination on the diffraction patterns of LS2 phase forming in the glass samples, we found that the (002) plane changes in different fashions, as shown in Figure 1a2−d2. This observation confirms that the c axis of the LS2 phase changes in the trends as presented in Table 2. The red arrows, for the ease of reading, are drawn in Figure 1 for each glass to show the corresponding events at each inflection point of the (002) peak evolution, i.e., lattice parameter c, of lithium disilicate (Table 2). There is no similar phenomenon for a and b axes. These reflection points of the trends may be associated with the disappearance of LS and the formation of silica phases. In glass A, the lattice parameters of the LS phase as a function of temperature (600−860 °C) are present in Figure 2a; the diffraction pattern of (002) plane is shown in Figure 2b confirming the trend of the lattice parameter c. The critical point at 760 °C implies that the transformation of LS to LS2 is initiating at this temperature. In glass B, the evolution of lattice parameters of the LS phase are shown in Figure 3a, and closeup diffraction patterns are illustrated in Figure 3b−d for (200), (020), and (002) planes, respectively. The evolution trend of c axis of the LS phase is identical to that previously reported in a

Figure 2. (a) Lattice parameters of the LS phase in glass A as a function of temperature and (b) the diffraction patterns for (002) plane of LS whose trend is consistent with the evolution trend of the c axis as shown in panel a.

glass, i.e., decreasing nonlinearly against temperature.33 While in glasses C and D, there is no LS phase forming from the base glass during the entire annealing process. In type I glass (A) where LS2 and LS phases nucleate simultaneously, the parameter c of LS2 phase presents a typical “V”-shaped fashion which consists of a nonlinear decreasing curve in the LS−LS2 coexisting range (600−860 °C) and a straight increasing line in the LS2-only range (860−1000 °C). Such a phenomenon was also observed in another lithium disilicate glass that has the same nucleation sequence.33 In contrast, both parameters a and b have an increasing trend, although there is a jump at 860 °C (shown in Table 2). The lattice parameters of the LS phase over the temperature range of 600−860 °C are present in Figure 2a; the diffraction pattern of (002) plane is shown in Figure 2b confirming the trend of the lattice parameter c. The critical point at 760 °C may suggest that the transformation of LS to LS2 is initiated at this temperature. Nevertheless, the suggested influence of the precipitation of silica phase (cristobalite), i.e., reversing the evolution trend of c axis of LS2, at 840 °C may be associated with the similar effect of LS on it. In type II glass (B) where LS nucleates solely and then transforms to LS2, the declining trend of lattice parameter c of LS2 terminates and shows a slight increase at 840−860 °C where quartz is detected. In comparison, the lattice parameter c of LS in glass B decreases nonlinearly against temperature upon heating, whereas parameters a and b increase (Figure 3a). Correspondingly, the diffraction patterns of (200), (020), and (002) planes are shown in Figures 3b−d. Such a trend for the LS phase was also observed in type I glasses such as glass A in D



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Figure 3. (a) Lattice parameters of LS phase against temperature in glass B, and close-up diffraction patterns for (b) (200), (c) (020), and (d) (002) planes of Li2SiO3 (M).

this study and others.33 It suggests that the LS phase has intrinsic behavior of compression in c axis upon heating, as no interaction with other crystalline phases (e.g., LS2) is present in the glass. In type III glasses (C and D), the trends of parameter c of LS2 phase are “N”-shaped with two inflection points, namely, an increase−drop−increase trend. At each inflection point, the change of trends could be ascribed to the formation of silica phases in the glasses (Table 2 and Figure 1). For the first section of crystallographic evolution (below 760 °C), the lattice parameter c of LS2 shows an increasing trend against annealing temperature. It could be the intrinsic nature of LS2 phase because no other phases nucleate/crystallize. Then, it changes to a decreasing mode since cristobalite appears at 760 °C (glass C) or 780 °C (glass D), until the crystallization of tridymite phase (glass C) or quartz phase (glass D) at 860 °C occurs. Thus, the lattice parameter c of LS2 phase shows a reversed trend when a silica phase forms. 3.3. Unit Cell Volume of Silicate Phases. The unit cell volumes of lithium disilicate phase in glasses A, B, C, and D as a function of temperature are shown in Figures 4−6, respectively. In glass A, the unit cell volume of the LS phase may be expressed with a linear equation during the crystallization; while that of LS2 phase can be fitted into two equations that separate at 860 °C, a temperature at which LS disappears (Figure 4). It is consistent with the modeling in our previous work.33 Thus, we conclude that such a common feature can readily exist in the glass system with simultaneous nucleation of LS and LS2. In glass B, the unit cell volume of both LS and LS2 phases can be well fitted using a straight line. The fitting results

Figure 4. Unit cell volume of (a) LS2 and (b) LS phases in glass A as a function of temperature.

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are displayed in Figure 5. The influence of both LS and QZ phases on the fitting for LS2 is marginal and can be due to the

Figure 6. Unit cell volume of LS2 phase in (a) glass C and (b) glass D as a function of temperature. Figure 5. Unit cell volume of (a) LS2 and (b) LS phases in glass B as a function of temperature.

4. DISCUSSION In glass A (type I), a significant reversal in the trend can be observed for the lattice parameter c of LS2 when the LS phase disappears at 860 °C. It is evident that during the crystallization process of silicate phases, the LS phase has a strong constraint on the crystallographic structure of LS2.33 In other types of glasses, there is a silica precipitation event taking place at each inflection point. More specifically, in glass B (type II), the decreasing trend of c terminates at 820−840 °C where QZ forms; in glass D (type III), the “N”-shaped trend of c has inflection points at 760 and 860 °C, which correspond to the precipitation temperatures of CR and QZ, respectively. Such a phenomenon indicates the close relationship between the crystallographic change along c axis of lithium disilicate and the precipitation of silica phases from the base glass. Thus, the refined lattice parameters of lithium disilicate phase (Table 2) are somehow correlated with the silicon-related phases including LS, cristobalite (CR), quartz (QZ), and tridymite (TM). In comparison, cell parameters a and b of LS2 in all glasses have in common the normal increasing trend without significant change. The mechanism for the silica-controlled crystallographic evolution of lithium disilicate is unclear so far and would be a remaining scientific question for more future investigations using in situ experiments and/or first-principle simulations. We anticipate our report will initiate more experimental or modeling studies in either the glass systems or other chemical reaction systems involving crystalline phases. For instance, it may be insightful to study the crystallographic change of phases to understand the intrinsic mechanisms of the reaction, nucleation, precipitation, and/or transformation in polycrystalline materials, glasses, sol−gels, solid solutions, and solvent solutions.

following two factors: (1) the LS phase diminishes when the LS2 phase forms, and (2) quartz forms at the end of crystallization period of the LS2 phase. In glasses C and D, the unit cell volume of the LS2 phase cannot be well fitted by one linear equation, which is ascribed to complex evolution trends (i.e., increase−drop−increase) of c axis of the LS2 phase. We therefore applied two linear equations for fitting in each glass as given in Figure 6. Figure 7 shows the XRD patterns of the splitting (111) peak of CR and the affected (101) peak evolution of LP (glass D). The peak splitting is related to the sample distortion caused by the melting phenomenon of the sample at high temperature. The crystalline silica phases have near zero expansion rate, which is also evidenced in our previous study in a lithium disilicate glass.33 This characteristic helps us check the reliability of crystallographic evolution trends of crystalline phases in all glasses. Owing to the reversed trend of lattice parameter c, the unit cell volume for lithium disilicate does not show an overall linear relationship in glasses A, C, and D. However, it does show a linear trend for each period, separated by the precipitation events of silica phases, if there were sufficient data points. For glass B, one linear equation can be well applied to the data points in the temperature range of 700−860 °C. The influence of the QZ phase on the unit cell volume of LS2 in glass B (Figure 5) and that of the TM phase in glass C (Figure 6) are obscure due to their weak effects on crystal lattices of LS2 (Figure 2). The difference in the fitted coefficients of the linear equations can be ascribed to the evolution-dependent lattice parameters of lithium disilicate in each glass. F



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was added (as reported by Fernandes et al.), 36 the crystallization sequence of lithium disilicate glasses shifted from type I to type II when increasing the addition of K2O in the composition. Therefore, the crystallization behavior of lithium disilicate glasses is probably controlled by a complex mechanism that involves several factors, including the SiO2/ Li2O ratio, the type and content of the nucleating agents as well as other additives. It requires further systematic investigations in the future, but it is beyond the objective of this work.

5. CONCLUSIONS The results obtained from the synchrotron XRD study of several glasses confirm a strong correlation between the crystallographic c axis of lithium disilicate and the precipitation of silica phases. Besides, the crystallization sequence of silicate phases has an impact on the crystallographic structure of lithium disilicate. As a consequence, the evolution trend of its c axis at early stages differs in the glasses A, B, C, and D. The first ever observation of such a structural response of lithium disilicate suggests the mutual interaction of crystalline phases that crystallize from base glasses. These findings would give insight into the nucleation mechanism of crystalline phases in the related glasses and the structural change in crystal lattice.

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AUTHOR INFORMATION

Corresponding Author

*(P.C.) E-mail: [email protected]. Tel: +64 9 3737 599, ext. 86924. Notes

The authors declare no competing financial interest.

Figure 7. (a) The (101) peak of LP phase and (b) the (111) peak of CR phase in glass D. As indicated by arrows, the glass is deformed at temperatures above 960 °C. (According to ref 33, the CR phase has near zero expansion at the relevant temperature range.)

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ACKNOWLEDGMENTS This research was undertaken on the Powder Diffraction beamline at the Australian Synchrotron, Victoria, Australia (project ID: AS121/PDFI/5330). We thank the beamline scientists Dr. Qinfen Gu and Dr. Justin Kimpton for their technical assistance. S.F.H. would like to acknowledge the China Scholarship Council (CSC) for providing a doctoral scholarship for his Ph.D. study at the University of Auckland.

The glasses reported in this work possess different crystallization behaviors, representing the three types of crystallization sequence of silicate phases previously overviewed in the Introduction section. Although these phenomena have been extensively reported in the literature,12−14,17−23 the decisive factors that regulate the crystallization behavior of a lithium disilicate glass are unknown yet. Possible important factors include the SiO2/Li2O ratio and the addition of the nucleating agents.34,35 In the present work, the investigated glasses have different molar ratios of SiO2/Li2O (2.41−2.56) and various additions of MgO and/or Al 2 O 3 in the compositions, thus it is postulated that, apart from the molar ratio of SiO2/Li2O, the type and content of minor additives also contribute to the crystallization type of lithium disilicate glasses. The observed phenomenon from the present study is that the LS phase tends to nucleate first (i.e., type II) in Al2O3-rich compositions (e.g., glass B), while both LS and LS2 nucleate simultaneously (i.e., type I) in Al2O3-lean compositions (e.g., glass A), and the LS2 phase tends to nucleate solely (type III) if the multicomponent glass is MgO-rich (over 5 mol % in glasses C and D). However, we cannot draw a regular pattern with regard to the effects of the above-mentioned factors on the crystallization sequence of silicate phases from parent glasses. For instance, one may consider that the higher the molar ratio of SiO2/Li2O (>2) is, the less tendency the crystallization of LS phase has; while this is not the case as reported in several other studies.36,37 Besides, even though a constant amount of Al2O3

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Structural Response of Lithium Disilicate in Glass Crystallization

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