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Temperature-induced phase transitions for stuffed tridymites SrGa2O4 and CaGa2O4
Journal of Solid State Chemistry 254 (2017) 195–199
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Temperature-induced phase transitions for stuffed tridymites SrGa2O4 and CaGa2O4
MARK
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Fuwei Jiang, Pengfei Jiang, Mufei Yue, Wenliang Gao, Rihong Cong, Tao Yang College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Stuffed tridymite Phase transition Rietveld refinements DFT calculations
Temperature-induced phase transitions for stuffed tridymite AGa2O4 (A = Sr, Ca) were investigated by experimental and theoretical calculations. A simple annealing of SrCO3 and Ga2O3 led to the formation of γSrGa2O4 (P21/n) below 1200 °C, and transform to β-SrGa2O4 (P21/c) when heated at 1200 °C. A similar phenomenon was found for CaGa2O4, and the temperature boundary between α-CaGa2O4 (Pna21) and the high temperature polymorph β-CaGa2O4 (P21/c) was about 1350 °C. Rietveld refinements provided detailed structural information for these polymorphs and suggest that the driving force of these phase transitions is the under-bonded nature of the alkaline earth cations. In other words, the need of larger space for Sr2+/Ca2+ in the high temperature β-phase forces the 6-membered-ring channel expand through increasing the Ga-O-Ga angles. Density functional theory calculations proved the formation energies for γ-SrGa2O4 and α-CaGa2O4 were both lower than their high temperature β-polymorphs, in accordance with the experimental observations.
1. Introduction Tridymite is one of the polymorphs for SiO2, where each silicon is coordinated by four oxygen atoms to form SiO4 [1], and these SiO4 tetrahedra corner-share with each other into an open-framework consisted of 6-membered-ring (see Fig. 1a). The open cavity inside the 6-membered-ring channel could be filled by large cations, like the alkaline earth cations, affording the stuffed tridymite structure, and it can be expressed using the common formula AB2O4 [2–7]. For example, BaFe2O4 is a typical stuffed tridymite compound, sharing the same space group P6322 with that of tridymite SiO2 as shown in Fig. 1b. All oxygen atoms are coordinated to two adjacent Fe3+, behaving as the bridge atoms, and Ba2+ cations locate at the channel center (the special position 2b) [8,9]. The case is quite complex that the stuffed tridymite family possesses the polymorphism characteristic, where the difference between polymorphs is the relative orientations of the BO4 tetrahedra in the 6-membered-ring, associated with different space groups. For instance, there is also an orthorhombic BaFe2O4 in the space group Pmcn, and it apparently comprises a different type of 6-membered-ring and a lower symmetry, compared with the hexagonal polymorph (please refer to Fig. 1b and c). SrGa2O4 and CaGa2O4 in the stuffed tridymite structure are quite interesting, because Ga3+ may have a flexibility of coordination to either 4, 5 or 6 oxygen atoms, as a consequence, it might adapt interstitial oxygen atoms within this open-framework structure (under
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Corresponding author. E-mail address: [email protected] (T. Yang).
http://dx.doi.org/10.1016/j.jssc.2017.07.024 Received 24 June 2017; Received in revised form 20 July 2017; Accepted 22 July 2017 Available online 24 July 2017 0022-4596/ © 2017 Elsevier Inc. All rights reserved.
some certain circumstances) and lead to the possible application in oxygen conductivity. Before that, people first need to understand the synthetic and structural chemistry of AGa2O4 (A = Sr, Ca), and it is exactly our motivation of this study. For SrGa2O4, it was first reported by Glasser in 1963 and suggested to be related to the stuffed tridymite BaAl2O4 [2]. To date, three polymorphs have been successively identified. In 1968, Plakthii reported that α-SrGa2O4 (in the hexagonal symmetry) was obtained by chilling a sample in liquid N2, which was previously heated at 1200 °C [10]; β-SrGa2O4 (in the space group P21/c) could be prepared by rapid quenching a melting mixture of 30% LiF and 70% SrGa2O4, which was previously heated at 1200 °C for 48 h [10,11]. In 2000, the third polymorph, γ-SrGa2O4, was discovered as a quenched highpressure product and it crystallized in the space group P21/n [12]. Please note that β- and γ-SrGa2O4 both belong to the stuffed tridymite family, and crystallize in the same space group (No. 14) and are just different in cell choice, however, their structures are completely different manifested by the different orientations of GaO4 tetrahedra in 6-membered-ring (see more details in later sections). α- and β-CaGa2O4 were first discovered by Jeevaratnam and Glasser in 1961 [13], and structurally characterized by Deiseroth and Müller-Buschbaum in 1973 [14,15]. They both belong to the stuffed tridymite structure and crystallize in the space group Pna21 and P21/c, respectively. In addition, a high-pressure polymorph of HP-CaGa2O4 was reported in 1980 and supposed to be the post-spinel structure (like
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Fig. 1. Crystal structure views for (a) tridymite-SiO2 (P6322), (b) BaFe2O4 (P6322) along the c-axis and (c) BaFe2O4 (Pmcn) along the a-axis.
CaFe2O4) [16,17], where the Ga3+ cations are all coordinated by 6 oxygen atoms to form GaO6 octahedra. SrGa2O4 and CaGa2O4 possess a high chemical stability and exhibit good performances as host materials for persistent luminescence, and in fact, they have been suggested to be potentially applicable in electroluminescence displays [18–21]. As mentioned above, there existed the polymorphism phenomenon in AGa2O4 (A = Sr, Ca), and of course, it is important to prepare the right phase for the property investigation or even the practical application. Hence we made our efforts to unravel which phases of SrGa2O4 or CaGa2O4 could be prepared by traditional solid state reactions under ambient pressure, and whether different structures could transform by changing the annealing temperatures, and what is the driving force for such phase transitions. Experiments and theoretical calculations were therefore performed, and were consistent with each other.
The optimized lattice parameters were in good agreement with experimental data. Subsequently, the formation energy Eformation for SrGa2O4 and CaGa2O4 polymorphs were calculated according to [23,24]:
Eformation = Etotal / Z − ∑ Ei / n Etotal is the total energy of each phase, Z is the number of molecules per unit cell, Ei is the energy of the most stable elementary substance, n is the number of atoms in the elementary substance. 3. Results and discussion 3.1. Temperature-induced phase transition from γ- to β-SrGa2O4 The direct annealing of SrCO3 and Ga2O3 in the molar ratio of 1:1 readily led to the phase-pure γ-SrGa2O4 in the temperature range of 950–1185 °C, and the resultant powder at 1200 °C was identified to be pure β-SrGa2O4 as shown in Fig. 2. In literature, γ-SrGa2O4 was first prepared by high-temperature (1250 °C) and high-pressure method (2.5 GPa) [12]. Herein our work, γ-SrGa2O4 is thermodynamically stable below 1200 °C under ambient pressure and it will transform to β-SrGa2O4 completely at 1200 °C. Such a phase transition is irreversible because the annealing of as-obtained β-SrGa2O4 at 1150, 1100, 1050, 1000, 950 °C (each step for 10 h) would not lead to any change of the powder XRD pattern (see Fig. S1 in the Electronic Supplementary Information). To verify the phase purity, Rietveld refinements were performed on the γ- and β-SrGa2O4 samples prepared at 950 °C and 1200 °C, respectively. Apparently, the final convergences for both samples were very good, indicated by the low R-factors and minor mismatch between the observed and calculated XRD data (see Fig. 3), and the so-obtained crystallographic parameters, atomic coordinates and selected bond distances were provided in Tables 1, S1 and S2. First, both β- and γ-SrGa2O4 crystallize in the same monoclinic space group (only different in the unit cell choice), but β-phase possesses a larger cell volume (a lower density). It is understandable
2. Experimental section Powder samples of SrGa2O4 and CaGa2O4 were prepared by conventional solid state reactions at high temperatures. Stoichiometric amounts of starting materials including SrCO3 (99.9%), CaCO3 (99.9%), Ga2O3 (99.99%) were weighed and carefully mixed in an agate mortar by hand. The mixture was first heated at 800 °C for 12 h and the resultant powder sample was then ground and further pressed into a compact pellet for the following annealing processes at higher temperatures. For instance, SrGa2O4 was successively heated from 850 to 1200 °C with the natural cooling procedure. After heating at a selected temperature for 10 h, powder X-ray diffraction (XRD) data was collected to identify the phase purity, followed with re-grinding, re-pressing and re-heating at a higher temperature. On the other hand, CaGa2O4 was successively heated from 900 to 1370 °C. Please note that the CaGa2O4 sample heated at 1350 °C was partially melted, and if annealed at 1370 °C, it was completely melted and no XRD data was collected. Powder XRD were performed on Panalytical Empyrean diffractometer equipped with a PXIcel 1D detector (Cu Kα radiation). The operational voltage and current were 40 kV and 40 mA, respectively. The data used for phase identification were collected with a setting of 45 s/0.0131°. High quality XRD data for Rietveld refinements were collected with the setting of 200 s/0.0131°. Rietveld refinements were performed with the TOPAS software package [22]. In this work, all the calculations were performed using the spinpolarized density functional theory (DFT) method with the VASP (Vienna ab initio simulation package) code. The generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerh (PBE) was employed to describe the exchange-correlation potential in the standard DFT calculations. The electron-ion interactions were described by the projector-augmented wave (PAW) method. Full relaxation of cell parameters were performed by using different Monkhorst-Pack k-point meshes: β-SrGa2O4, 13*13*3, γ-SrGa2O4, 8*4*6, α-CaGa2O4, 6*8*7, β-CaGa2O4, 8*7*6. The cutoff energy for the plane wave basis was set to 500 eV. The convergence criteria of energy and force were 1.0 × 10−6 eV and 1.0 × 10−2 eV/Å, respectively.
Fig. 2. XRD patterns for SrGa2O4 after annealed at different temperatures.
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Fig. 4. Projected structure views for (a) γ- and (b) β-SrGa2O4 along the a-axis. The orientations of the tetrahedra are presented as U or D in both red circles for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
perature-induced and reversible phase transitions P21 ↔ P63 (√3 A) and P63 (√3 A) ↔ P6322 at relatively low temperatures (~ 680 and 860 °C, respectively) [25]. The transitions involve the slight movements of oxygen atoms from ideal to off-center sites, and the sequences of the 6-membered-ring does not alert during both transitions. Herein the SrGa2O4 system, the temperature-induced transition is irreversible and involves a reconstruction of the lattice, therefore, there is no necessary correlation of the crystal symmetry between the low and high temperature polymorphs. For example, β- and γ-SrGa2O4 share the same space group and the low-temperature phase α-CaGa2O4 even has a higher symmetry than the high-temperature polymorph β-CaGa2O4, which will discussed later. DFT calculations on the formation energy for both phases were performed. γ-SrGa2O4 possesses a more negative formation energy (−56591.6978 eV) than that of β-SrGa2O4 (−56589.4436 eV), which supports our experimental observations. It is important to understand the nature of such a phase transition. The over-high temperature annealing on γ-SrGa2O4 (i.e. at 1200 °C) will readily lead to the over-active vibration of Sr2+ and it will naturally need a more open space to accommodate such an active thermal vibration. In GaO4-based stuffed tridymite structure, the rigid nature of GaO4 (almost unchanged Ga-O average bond distances) prohibits the expansion of GaO4 tetrahedra. Please note that the Ga-O-Ga angles between GaO4 tetrahedra are reasonably flexible, and we did observe the increase of the average Ga-O-Ga angle from 118.21° in γ-SrGa2O4 to 118.97° in β-SrGa2O4 (see Table 2). Under this circumstance, each tetrahedron of 6-membered-ring does not expand but the connections between the tetrahedra were forced to evolve, in order to allow the enlargement of the overall size of channel, thus having the coordination environment of Sr2+ satisfied in the high temperature β-phase. As discussed above, this transition is not a consequence of slight movements of oxygen atoms (or disorder-to-order transition) but involves the breaking of covalent Ga–O bonds, thus must be drastic and belong to the first-order transition. Based on the above analyses, we believe the driving force of such transition is the under-bonded nature of the alkaline earth cations.
Fig. 3. Final convergences of Rietveld refinements for (a) γ- and (b) β-SrGa2O4. The blue circle ○ represents observed data and the red solid line is the calculated pattern; the purple marks below the diffraction patterns are the expected reflection positions, and the difference curve is also shown at the bottom. The refined cell parameters along with agreement factors are also given. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). Table 1 Crystallographic parameters for SrGa2O4 polymorphs from Rietveld refinements.
Space group a (Å) b (Å) c (Å) β (°) V (Å3) D (g/cm3) Z Formation energy (eV)
γ-SrGa2O4
β-SrGa2O4
P21/n (No. 14) 8.1111(2) 10.7553(2) 9.0518(2) 91.5424(7) 789.37(3) 4.90 8 −56591.6978
P21/c (No. 14) 8.3792(7) 8.9966(7) 10.6805(9) 93.9131(4) 803.27(2) 4.82 8 −56589.4436
that a high temperature annealing will drive the condensed solid into a polymorph with a lower density. Second, the Ga–O bond distances in GaO4 tetrahedra for both βand γ-SrGa2O4 are all in the usual range from 1.799(8) to 1.922(7) Å (see Table S2). In fact, the average bond distances of Ga-O remain the same between these two polymorphs of SrGa2O4 due to the rigid nature of GaO4 tetrahedra. On the contrast, the coordination of Sr2+ is supposed to be flexible, and indeed, the difference on the coordination environments of Sr2+ could be observed. The calculated BVS for Sr1 and Sr2 changed from 1.959/1.925 in γ-SrGa2O4 to 1.944/1.831 in βSrGa2O4 (see Table S1), which means the cavity inside the 6-membered-ring channel became more open during this phase transition, and it resulted in the expansion of cell volume macroscopically. Third, the orientations of the GaO4 tetrahedra in a single 6membered-ring are different in two structures of SrGa2O4 as shown in Fig. 4. For instance, the GaO4 tetrahedra point upwards (U) and downwards (D) in the sequence of UUUDDD (clockwise) in γ-SrGa2O4, and it changes to UUDUDD in β-SrGa2O4. This change in structure appears quite simple (just some modification of the orientations of tetrahedra), but in fact, the oxygen atoms should re-arrange to adapt the new coordination environments for Sr2+ cations, which involves the breaking and re-formation of Ga–O bonds and this is the reason why this transition was only observed by annealing the sample at a high temperature (1200 °C). For comparison, SrAl2O4 undergoes two tem-
Table 2 Selected bond angles (°) for SrGa2O4 polymorphs.
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γ-SrGa2O4
Angle (°)
β-SrGa2O4
Angle (°)
Ga3-O1-Ga2 Ga4-O2-Ga3 Ga2-O3-Ga1 Ga1-O4-Ga3 Ga1-O5-Ga4 Ga2-O6-Ga4 Ga4-O7-Ga2 Ga3-O8-Ga1 < Ga-O-Ga >
112.1(2) 117.37(9) 119.8(1) 124.40(6) 108.05(5) 115.23(7) 143.49(8) 105.31(9) 118.21
Ga4-O1-Ga1 Ga4-O2-Ga3 Ga1-O3-Ga3 Ga1-O4-Ga2 Ga2-O5-Ga1 Ga3-O6-Ga2 Ga2-O7-Ga4 Ga4-O8-Ga3 < Ga-O-Ga >
117.8(4) 116.5(3) 118.7(4) 131.3(4) 109.5(3) 112.7(4) 116.86(8) 128.5(4) 118.97
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Fig. 5. (a) XRD patterns for CaGa2O4 after annealed at different temperatures; (b) final convergence of Rietveld refinements for α-CaGa2O4.
3.2. Temperature-induced phase transition from α- to β-CaGa2O4 Under ambient pressure, the annealing of CaCO3 and Ga2O3 in the molar ratio of 1: 1 below 1350 °C will lead to the formation of αCaGa2O4 as shown in Fig. 5a. Once heated at 1350 °C for 10 h, the sample started to melt partially and the remaining solid was ground and characterized (by XRD) to be a mixture of α- and β-CaGa2O4 (together with a minor phase CaGa4O7). As shown in Fig. 5a, the major phase of the sample after heated at 1350 °C was β-CaGa2O4, however, no phase-pure sample could be obtained no matter how we changed the cooling process (either slow or fast quenching). It is not surprising because the synthetic temperature of β-CaGa2O4 is very close to its melting point [13]. Rietveld refinements on the phase-pure α-CaGa2O4 (heated at 1050 °C) were performed in order to obtain the structural parameters as listed in Table 3, S3 and S4. The good fitness of the XRD pattern (see Fig. 5b) and the chemically reasonable bond distances (see Table S4) for α-CaGa2O4 gave a solid evidence to the successful synthesis of αCaGa2O4. Multiphase Rietveld refinements on the XRD pattern (heated at 1350 °C) led to an unsatisfied structure for β-CaGa2O4 (containing too short or too long Ga–O bonds), thus we have to use the structure parameters from literature in the following [15]. Similar to the case of SrGa2O4, α- and β-CaGa2O4 also crystallize in the stuffed tridymite structure and exhibit different sequences of tetrahedra orientations. The projected structural views of α-CaGa2O4 (based on our Rietveld refinements) and β-CaGa2O4 (according to the reference [15]) are presented in Fig. 6, where the sequences of GaO4 orientations are UUUUDD and UUDUDD, respectively. Please note that β-CaGa2O4 and β-SrGa2O4 are isostructural. The α- to β-CaGa2O4 transition also involves the change of the Ga–O bonds, and eventually the 6-membered-ring was forced to expand, indicating by the increasing of the average Ga-O-Ga angles from 115.4° to 116.4°, in order to provide larger space for Ca2+ in β-CaGa2O4 (see Table 4). For example, the BVS of Ca2+ decrease significantly from low-temperature phase (αCaGa2O4, 1.965/1.945) to high-temperature phase (β-CaGa2O4, 1.796/ 1.582) as shown in Table S3. People may find that the average Ga-O-Ga
Fig. 6. Projected structure views for (a) α-CaGa2O4 along the b-axis and (b) β-CaGa2O4 along the a-axis. Table 4 Selected bond angles (°) for CaGa2O4 polymorphs.
Space group a (Å) b (Å) c (Å) β (°) V (Å3) D (g/cm3) Z Formation energy (eV)
β-CaGa2O4 [15]
Pna21 (No. 33) 10.3442(2) 7.7434(1) 9.1174(2) 90 730.29(2) 4.43 8 −58353.4356
P21/c (No. 14) 7.992 8.830 10.585 94.72 744.44 4.34 8 −58352.1442
Angle (°)
β-CaGa2O4 [15]
Angle (°)
Ga4-O1-Ga1 Ga3-O2-Ga2 Ga1-O3-Ga3 Ga2-O4-Ga1 Ga4-O5-Ga1 Ga2-O6-Ga3 Ga4-O7-Ga2 Ga3-O8-Ga4 < Ga-O-Ga >
113.5(6) 111.6(6) 109.5(4) 118.7(5) 107.3(6) 115.5(7) 108.3(1) 139.31(8) 115.4
Ga1-O1-Ga4 Ga4-O2-Ga3 Ga3-O3-Ga1 Ga2-O4-Ga1 Ga2-O5-Ga1 Ga3-O6-Ga2 Ga2-O7-Ga4 Ga4-O8-Ga3 < Ga-O-Ga >
115.44 115.86 116.75 126.41 108.36 110.31 113.43 124.91 116.4
angles are smaller in the case of CaGa2O4 compared to the values in SrGa2O4, which is another evidence that the expansion of 6-membered rings is through adjusting the GaO4 connections. In addition, DFT calculations suggested that the low temperature phase should be αCaGa2O4, which possesses a lower formation energy (−58353.4356 eV) (see Table 3). Though both α-CaGa2O4 and β-CaGa2O4 belong to the stuffed tridymite structures, it is interesting to observe the symmetry lowering down during this temperature-induced phase transition from Pna21 (No. 33) to P21/c (No. 14). It is somehow different with the case in cubic-type perovskites, where the higher temperature phase usually possesses a higher symmetry and there also exists the principle of the symmetry evolution [26,27]. It is well known that the symmetry evolution versus temperature in cubic-type (not hexagonal-family) perovskites are mostly due to the deviation of some oxygen atoms from special to general positions (sometimes disorder-to-order transition), indicated by the rotation or tilting of octahedra, herein the case of CaGa2O4 (as well as SrGa2O4), the phase transition is not the same type, but involves a big change (even reconstruction) of the oxygen lattice, because the sequence of the GaO4 orientations have been reordered. This is also a support that this phase transition is more likely a first-order type. In fact, such transitions are similar to those in metal borates, where phase transitions are usually induced by the change of polyborate ionic frameworks through the breaking and/or formation of
Table 3 Crystallographic parameters for CaGa2O4 polymorphs from Rietveld refinements or from reference [15]. α-CaGa2O4
α-CaGa2O4
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B-O covalent bonds. For example, when elevating the heating temperature, δ-BiB3O6 (Pca21, No. 29) first transforms to γ-BiB3O6 (P21/n, No. 14), and to α-BiB3O6 (C2, No. 5) thereafter. No group-subgroup correlation could be found between these polymorphs [28]. 4. Conclusion In conclusion, stuffed tridymite compounds AGa2O4 (A = Sr, Ca) were prepared by traditional solid state reactions under ambient pressure. γ-SrGa2O4 (P21/n) was the thermodynamically stable phase below 1200 °C, and would readily transform to β-SrGa2O4 (P21/c) at 1200 °C. Similar phase transition from α-CaGa2O4 (Pna21) to βCaGa2O4 (P21/c) was also observed at about 1350 °C, however no pure phase of β-CaGa2O4 can be obtained probably due to the melting issue. Rietveld refinements on high quality data provided a great detail of both structures, and the comparison of these polymorphs suggest that the driving force of such phase transitions is the under-bonded nature of the alkaline earth cations. In other words, the need of larger space for Sr2+/Ca2+ in the high temperature β-phase forces the 6-memberedring channel expand through increasing the Ga-O-Ga angles. This transition involves the breaking and re-formation of Ga–O bonds, thus supposed to be the first-order in nature. Our results provide necessary information about the thermodynamic behaviors of AGa2O4 at ambient pressure, and is expected to be helpful to the preparation of these materials. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21171178, 21671028). We also acknowledge the support from the sharing fund of large-scale equipment of Chongqing University. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jssc.2017.07.024. References [1] R.E. Gibbs, The polymorphism of silicon dioxide and the structure of trtidymite, Proc. R. Soc. Lond. Ser. A Contain. Pap. Math. Phys. Charact. 113 (1926) 351–368. [2] F.P. Glasser, L.S. Dent Glasser, Crystal chemistry of some AB2O4 compounds, J. Am. Ceram. Soc. 46 (1963) 377–380. [3] V. Kahlenberg, C. Weidenthaler, High temperature single crystal diffraction study on monobarium gallate – the crystal structure of β-BaGa2O4, Solid State Sci. 4 (2002) 963–968. [4] V. Kahlenberg, R.X. Fischer, J.B. Parise, The stuffed framework structure of
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Temperature-induced phase transitions for stuffed tridymites SrGa2O4 and CaGa2O4
MARK
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Fuwei Jiang, Pengfei Jiang, Mufei Yue, Wenliang Gao, Rihong Cong, Tao Yang College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Stuffed tridymite Phase transition Rietveld refinements DFT calculations
Temperature-induced phase transitions for stuffed tridymite AGa2O4 (A = Sr, Ca) were investigated by experimental and theoretical calculations. A simple annealing of SrCO3 and Ga2O3 led to the formation of γSrGa2O4 (P21/n) below 1200 °C, and transform to β-SrGa2O4 (P21/c) when heated at 1200 °C. A similar phenomenon was found for CaGa2O4, and the temperature boundary between α-CaGa2O4 (Pna21) and the high temperature polymorph β-CaGa2O4 (P21/c) was about 1350 °C. Rietveld refinements provided detailed structural information for these polymorphs and suggest that the driving force of these phase transitions is the under-bonded nature of the alkaline earth cations. In other words, the need of larger space for Sr2+/Ca2+ in the high temperature β-phase forces the 6-membered-ring channel expand through increasing the Ga-O-Ga angles. Density functional theory calculations proved the formation energies for γ-SrGa2O4 and α-CaGa2O4 were both lower than their high temperature β-polymorphs, in accordance with the experimental observations.
1. Introduction Tridymite is one of the polymorphs for SiO2, where each silicon is coordinated by four oxygen atoms to form SiO4 [1], and these SiO4 tetrahedra corner-share with each other into an open-framework consisted of 6-membered-ring (see Fig. 1a). The open cavity inside the 6-membered-ring channel could be filled by large cations, like the alkaline earth cations, affording the stuffed tridymite structure, and it can be expressed using the common formula AB2O4 [2–7]. For example, BaFe2O4 is a typical stuffed tridymite compound, sharing the same space group P6322 with that of tridymite SiO2 as shown in Fig. 1b. All oxygen atoms are coordinated to two adjacent Fe3+, behaving as the bridge atoms, and Ba2+ cations locate at the channel center (the special position 2b) [8,9]. The case is quite complex that the stuffed tridymite family possesses the polymorphism characteristic, where the difference between polymorphs is the relative orientations of the BO4 tetrahedra in the 6-membered-ring, associated with different space groups. For instance, there is also an orthorhombic BaFe2O4 in the space group Pmcn, and it apparently comprises a different type of 6-membered-ring and a lower symmetry, compared with the hexagonal polymorph (please refer to Fig. 1b and c). SrGa2O4 and CaGa2O4 in the stuffed tridymite structure are quite interesting, because Ga3+ may have a flexibility of coordination to either 4, 5 or 6 oxygen atoms, as a consequence, it might adapt interstitial oxygen atoms within this open-framework structure (under
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Corresponding author. E-mail address: [email protected] (T. Yang).
http://dx.doi.org/10.1016/j.jssc.2017.07.024 Received 24 June 2017; Received in revised form 20 July 2017; Accepted 22 July 2017 Available online 24 July 2017 0022-4596/ © 2017 Elsevier Inc. All rights reserved.
some certain circumstances) and lead to the possible application in oxygen conductivity. Before that, people first need to understand the synthetic and structural chemistry of AGa2O4 (A = Sr, Ca), and it is exactly our motivation of this study. For SrGa2O4, it was first reported by Glasser in 1963 and suggested to be related to the stuffed tridymite BaAl2O4 [2]. To date, three polymorphs have been successively identified. In 1968, Plakthii reported that α-SrGa2O4 (in the hexagonal symmetry) was obtained by chilling a sample in liquid N2, which was previously heated at 1200 °C [10]; β-SrGa2O4 (in the space group P21/c) could be prepared by rapid quenching a melting mixture of 30% LiF and 70% SrGa2O4, which was previously heated at 1200 °C for 48 h [10,11]. In 2000, the third polymorph, γ-SrGa2O4, was discovered as a quenched highpressure product and it crystallized in the space group P21/n [12]. Please note that β- and γ-SrGa2O4 both belong to the stuffed tridymite family, and crystallize in the same space group (No. 14) and are just different in cell choice, however, their structures are completely different manifested by the different orientations of GaO4 tetrahedra in 6-membered-ring (see more details in later sections). α- and β-CaGa2O4 were first discovered by Jeevaratnam and Glasser in 1961 [13], and structurally characterized by Deiseroth and Müller-Buschbaum in 1973 [14,15]. They both belong to the stuffed tridymite structure and crystallize in the space group Pna21 and P21/c, respectively. In addition, a high-pressure polymorph of HP-CaGa2O4 was reported in 1980 and supposed to be the post-spinel structure (like
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Fig. 1. Crystal structure views for (a) tridymite-SiO2 (P6322), (b) BaFe2O4 (P6322) along the c-axis and (c) BaFe2O4 (Pmcn) along the a-axis.
CaFe2O4) [16,17], where the Ga3+ cations are all coordinated by 6 oxygen atoms to form GaO6 octahedra. SrGa2O4 and CaGa2O4 possess a high chemical stability and exhibit good performances as host materials for persistent luminescence, and in fact, they have been suggested to be potentially applicable in electroluminescence displays [18–21]. As mentioned above, there existed the polymorphism phenomenon in AGa2O4 (A = Sr, Ca), and of course, it is important to prepare the right phase for the property investigation or even the practical application. Hence we made our efforts to unravel which phases of SrGa2O4 or CaGa2O4 could be prepared by traditional solid state reactions under ambient pressure, and whether different structures could transform by changing the annealing temperatures, and what is the driving force for such phase transitions. Experiments and theoretical calculations were therefore performed, and were consistent with each other.
The optimized lattice parameters were in good agreement with experimental data. Subsequently, the formation energy Eformation for SrGa2O4 and CaGa2O4 polymorphs were calculated according to [23,24]:
Eformation = Etotal / Z − ∑ Ei / n Etotal is the total energy of each phase, Z is the number of molecules per unit cell, Ei is the energy of the most stable elementary substance, n is the number of atoms in the elementary substance. 3. Results and discussion 3.1. Temperature-induced phase transition from γ- to β-SrGa2O4 The direct annealing of SrCO3 and Ga2O3 in the molar ratio of 1:1 readily led to the phase-pure γ-SrGa2O4 in the temperature range of 950–1185 °C, and the resultant powder at 1200 °C was identified to be pure β-SrGa2O4 as shown in Fig. 2. In literature, γ-SrGa2O4 was first prepared by high-temperature (1250 °C) and high-pressure method (2.5 GPa) [12]. Herein our work, γ-SrGa2O4 is thermodynamically stable below 1200 °C under ambient pressure and it will transform to β-SrGa2O4 completely at 1200 °C. Such a phase transition is irreversible because the annealing of as-obtained β-SrGa2O4 at 1150, 1100, 1050, 1000, 950 °C (each step for 10 h) would not lead to any change of the powder XRD pattern (see Fig. S1 in the Electronic Supplementary Information). To verify the phase purity, Rietveld refinements were performed on the γ- and β-SrGa2O4 samples prepared at 950 °C and 1200 °C, respectively. Apparently, the final convergences for both samples were very good, indicated by the low R-factors and minor mismatch between the observed and calculated XRD data (see Fig. 3), and the so-obtained crystallographic parameters, atomic coordinates and selected bond distances were provided in Tables 1, S1 and S2. First, both β- and γ-SrGa2O4 crystallize in the same monoclinic space group (only different in the unit cell choice), but β-phase possesses a larger cell volume (a lower density). It is understandable
2. Experimental section Powder samples of SrGa2O4 and CaGa2O4 were prepared by conventional solid state reactions at high temperatures. Stoichiometric amounts of starting materials including SrCO3 (99.9%), CaCO3 (99.9%), Ga2O3 (99.99%) were weighed and carefully mixed in an agate mortar by hand. The mixture was first heated at 800 °C for 12 h and the resultant powder sample was then ground and further pressed into a compact pellet for the following annealing processes at higher temperatures. For instance, SrGa2O4 was successively heated from 850 to 1200 °C with the natural cooling procedure. After heating at a selected temperature for 10 h, powder X-ray diffraction (XRD) data was collected to identify the phase purity, followed with re-grinding, re-pressing and re-heating at a higher temperature. On the other hand, CaGa2O4 was successively heated from 900 to 1370 °C. Please note that the CaGa2O4 sample heated at 1350 °C was partially melted, and if annealed at 1370 °C, it was completely melted and no XRD data was collected. Powder XRD were performed on Panalytical Empyrean diffractometer equipped with a PXIcel 1D detector (Cu Kα radiation). The operational voltage and current were 40 kV and 40 mA, respectively. The data used for phase identification were collected with a setting of 45 s/0.0131°. High quality XRD data for Rietveld refinements were collected with the setting of 200 s/0.0131°. Rietveld refinements were performed with the TOPAS software package [22]. In this work, all the calculations were performed using the spinpolarized density functional theory (DFT) method with the VASP (Vienna ab initio simulation package) code. The generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerh (PBE) was employed to describe the exchange-correlation potential in the standard DFT calculations. The electron-ion interactions were described by the projector-augmented wave (PAW) method. Full relaxation of cell parameters were performed by using different Monkhorst-Pack k-point meshes: β-SrGa2O4, 13*13*3, γ-SrGa2O4, 8*4*6, α-CaGa2O4, 6*8*7, β-CaGa2O4, 8*7*6. The cutoff energy for the plane wave basis was set to 500 eV. The convergence criteria of energy and force were 1.0 × 10−6 eV and 1.0 × 10−2 eV/Å, respectively.
Fig. 2. XRD patterns for SrGa2O4 after annealed at different temperatures.
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Fig. 4. Projected structure views for (a) γ- and (b) β-SrGa2O4 along the a-axis. The orientations of the tetrahedra are presented as U or D in both red circles for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
perature-induced and reversible phase transitions P21 ↔ P63 (√3 A) and P63 (√3 A) ↔ P6322 at relatively low temperatures (~ 680 and 860 °C, respectively) [25]. The transitions involve the slight movements of oxygen atoms from ideal to off-center sites, and the sequences of the 6-membered-ring does not alert during both transitions. Herein the SrGa2O4 system, the temperature-induced transition is irreversible and involves a reconstruction of the lattice, therefore, there is no necessary correlation of the crystal symmetry between the low and high temperature polymorphs. For example, β- and γ-SrGa2O4 share the same space group and the low-temperature phase α-CaGa2O4 even has a higher symmetry than the high-temperature polymorph β-CaGa2O4, which will discussed later. DFT calculations on the formation energy for both phases were performed. γ-SrGa2O4 possesses a more negative formation energy (−56591.6978 eV) than that of β-SrGa2O4 (−56589.4436 eV), which supports our experimental observations. It is important to understand the nature of such a phase transition. The over-high temperature annealing on γ-SrGa2O4 (i.e. at 1200 °C) will readily lead to the over-active vibration of Sr2+ and it will naturally need a more open space to accommodate such an active thermal vibration. In GaO4-based stuffed tridymite structure, the rigid nature of GaO4 (almost unchanged Ga-O average bond distances) prohibits the expansion of GaO4 tetrahedra. Please note that the Ga-O-Ga angles between GaO4 tetrahedra are reasonably flexible, and we did observe the increase of the average Ga-O-Ga angle from 118.21° in γ-SrGa2O4 to 118.97° in β-SrGa2O4 (see Table 2). Under this circumstance, each tetrahedron of 6-membered-ring does not expand but the connections between the tetrahedra were forced to evolve, in order to allow the enlargement of the overall size of channel, thus having the coordination environment of Sr2+ satisfied in the high temperature β-phase. As discussed above, this transition is not a consequence of slight movements of oxygen atoms (or disorder-to-order transition) but involves the breaking of covalent Ga–O bonds, thus must be drastic and belong to the first-order transition. Based on the above analyses, we believe the driving force of such transition is the under-bonded nature of the alkaline earth cations.
Fig. 3. Final convergences of Rietveld refinements for (a) γ- and (b) β-SrGa2O4. The blue circle ○ represents observed data and the red solid line is the calculated pattern; the purple marks below the diffraction patterns are the expected reflection positions, and the difference curve is also shown at the bottom. The refined cell parameters along with agreement factors are also given. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). Table 1 Crystallographic parameters for SrGa2O4 polymorphs from Rietveld refinements.
Space group a (Å) b (Å) c (Å) β (°) V (Å3) D (g/cm3) Z Formation energy (eV)
γ-SrGa2O4
β-SrGa2O4
P21/n (No. 14) 8.1111(2) 10.7553(2) 9.0518(2) 91.5424(7) 789.37(3) 4.90 8 −56591.6978
P21/c (No. 14) 8.3792(7) 8.9966(7) 10.6805(9) 93.9131(4) 803.27(2) 4.82 8 −56589.4436
that a high temperature annealing will drive the condensed solid into a polymorph with a lower density. Second, the Ga–O bond distances in GaO4 tetrahedra for both βand γ-SrGa2O4 are all in the usual range from 1.799(8) to 1.922(7) Å (see Table S2). In fact, the average bond distances of Ga-O remain the same between these two polymorphs of SrGa2O4 due to the rigid nature of GaO4 tetrahedra. On the contrast, the coordination of Sr2+ is supposed to be flexible, and indeed, the difference on the coordination environments of Sr2+ could be observed. The calculated BVS for Sr1 and Sr2 changed from 1.959/1.925 in γ-SrGa2O4 to 1.944/1.831 in βSrGa2O4 (see Table S1), which means the cavity inside the 6-membered-ring channel became more open during this phase transition, and it resulted in the expansion of cell volume macroscopically. Third, the orientations of the GaO4 tetrahedra in a single 6membered-ring are different in two structures of SrGa2O4 as shown in Fig. 4. For instance, the GaO4 tetrahedra point upwards (U) and downwards (D) in the sequence of UUUDDD (clockwise) in γ-SrGa2O4, and it changes to UUDUDD in β-SrGa2O4. This change in structure appears quite simple (just some modification of the orientations of tetrahedra), but in fact, the oxygen atoms should re-arrange to adapt the new coordination environments for Sr2+ cations, which involves the breaking and re-formation of Ga–O bonds and this is the reason why this transition was only observed by annealing the sample at a high temperature (1200 °C). For comparison, SrAl2O4 undergoes two tem-
Table 2 Selected bond angles (°) for SrGa2O4 polymorphs.
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γ-SrGa2O4
Angle (°)
β-SrGa2O4
Angle (°)
Ga3-O1-Ga2 Ga4-O2-Ga3 Ga2-O3-Ga1 Ga1-O4-Ga3 Ga1-O5-Ga4 Ga2-O6-Ga4 Ga4-O7-Ga2 Ga3-O8-Ga1 < Ga-O-Ga >
112.1(2) 117.37(9) 119.8(1) 124.40(6) 108.05(5) 115.23(7) 143.49(8) 105.31(9) 118.21
Ga4-O1-Ga1 Ga4-O2-Ga3 Ga1-O3-Ga3 Ga1-O4-Ga2 Ga2-O5-Ga1 Ga3-O6-Ga2 Ga2-O7-Ga4 Ga4-O8-Ga3 < Ga-O-Ga >
117.8(4) 116.5(3) 118.7(4) 131.3(4) 109.5(3) 112.7(4) 116.86(8) 128.5(4) 118.97
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Fig. 5. (a) XRD patterns for CaGa2O4 after annealed at different temperatures; (b) final convergence of Rietveld refinements for α-CaGa2O4.
3.2. Temperature-induced phase transition from α- to β-CaGa2O4 Under ambient pressure, the annealing of CaCO3 and Ga2O3 in the molar ratio of 1: 1 below 1350 °C will lead to the formation of αCaGa2O4 as shown in Fig. 5a. Once heated at 1350 °C for 10 h, the sample started to melt partially and the remaining solid was ground and characterized (by XRD) to be a mixture of α- and β-CaGa2O4 (together with a minor phase CaGa4O7). As shown in Fig. 5a, the major phase of the sample after heated at 1350 °C was β-CaGa2O4, however, no phase-pure sample could be obtained no matter how we changed the cooling process (either slow or fast quenching). It is not surprising because the synthetic temperature of β-CaGa2O4 is very close to its melting point [13]. Rietveld refinements on the phase-pure α-CaGa2O4 (heated at 1050 °C) were performed in order to obtain the structural parameters as listed in Table 3, S3 and S4. The good fitness of the XRD pattern (see Fig. 5b) and the chemically reasonable bond distances (see Table S4) for α-CaGa2O4 gave a solid evidence to the successful synthesis of αCaGa2O4. Multiphase Rietveld refinements on the XRD pattern (heated at 1350 °C) led to an unsatisfied structure for β-CaGa2O4 (containing too short or too long Ga–O bonds), thus we have to use the structure parameters from literature in the following [15]. Similar to the case of SrGa2O4, α- and β-CaGa2O4 also crystallize in the stuffed tridymite structure and exhibit different sequences of tetrahedra orientations. The projected structural views of α-CaGa2O4 (based on our Rietveld refinements) and β-CaGa2O4 (according to the reference [15]) are presented in Fig. 6, where the sequences of GaO4 orientations are UUUUDD and UUDUDD, respectively. Please note that β-CaGa2O4 and β-SrGa2O4 are isostructural. The α- to β-CaGa2O4 transition also involves the change of the Ga–O bonds, and eventually the 6-membered-ring was forced to expand, indicating by the increasing of the average Ga-O-Ga angles from 115.4° to 116.4°, in order to provide larger space for Ca2+ in β-CaGa2O4 (see Table 4). For example, the BVS of Ca2+ decrease significantly from low-temperature phase (αCaGa2O4, 1.965/1.945) to high-temperature phase (β-CaGa2O4, 1.796/ 1.582) as shown in Table S3. People may find that the average Ga-O-Ga
Fig. 6. Projected structure views for (a) α-CaGa2O4 along the b-axis and (b) β-CaGa2O4 along the a-axis. Table 4 Selected bond angles (°) for CaGa2O4 polymorphs.
Space group a (Å) b (Å) c (Å) β (°) V (Å3) D (g/cm3) Z Formation energy (eV)
β-CaGa2O4 [15]
Pna21 (No. 33) 10.3442(2) 7.7434(1) 9.1174(2) 90 730.29(2) 4.43 8 −58353.4356
P21/c (No. 14) 7.992 8.830 10.585 94.72 744.44 4.34 8 −58352.1442
Angle (°)
β-CaGa2O4 [15]
Angle (°)
Ga4-O1-Ga1 Ga3-O2-Ga2 Ga1-O3-Ga3 Ga2-O4-Ga1 Ga4-O5-Ga1 Ga2-O6-Ga3 Ga4-O7-Ga2 Ga3-O8-Ga4 < Ga-O-Ga >
113.5(6) 111.6(6) 109.5(4) 118.7(5) 107.3(6) 115.5(7) 108.3(1) 139.31(8) 115.4
Ga1-O1-Ga4 Ga4-O2-Ga3 Ga3-O3-Ga1 Ga2-O4-Ga1 Ga2-O5-Ga1 Ga3-O6-Ga2 Ga2-O7-Ga4 Ga4-O8-Ga3 < Ga-O-Ga >
115.44 115.86 116.75 126.41 108.36 110.31 113.43 124.91 116.4
angles are smaller in the case of CaGa2O4 compared to the values in SrGa2O4, which is another evidence that the expansion of 6-membered rings is through adjusting the GaO4 connections. In addition, DFT calculations suggested that the low temperature phase should be αCaGa2O4, which possesses a lower formation energy (−58353.4356 eV) (see Table 3). Though both α-CaGa2O4 and β-CaGa2O4 belong to the stuffed tridymite structures, it is interesting to observe the symmetry lowering down during this temperature-induced phase transition from Pna21 (No. 33) to P21/c (No. 14). It is somehow different with the case in cubic-type perovskites, where the higher temperature phase usually possesses a higher symmetry and there also exists the principle of the symmetry evolution [26,27]. It is well known that the symmetry evolution versus temperature in cubic-type (not hexagonal-family) perovskites are mostly due to the deviation of some oxygen atoms from special to general positions (sometimes disorder-to-order transition), indicated by the rotation or tilting of octahedra, herein the case of CaGa2O4 (as well as SrGa2O4), the phase transition is not the same type, but involves a big change (even reconstruction) of the oxygen lattice, because the sequence of the GaO4 orientations have been reordered. This is also a support that this phase transition is more likely a first-order type. In fact, such transitions are similar to those in metal borates, where phase transitions are usually induced by the change of polyborate ionic frameworks through the breaking and/or formation of
Table 3 Crystallographic parameters for CaGa2O4 polymorphs from Rietveld refinements or from reference [15]. α-CaGa2O4
α-CaGa2O4
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B-O covalent bonds. For example, when elevating the heating temperature, δ-BiB3O6 (Pca21, No. 29) first transforms to γ-BiB3O6 (P21/n, No. 14), and to α-BiB3O6 (C2, No. 5) thereafter. No group-subgroup correlation could be found between these polymorphs [28]. 4. Conclusion In conclusion, stuffed tridymite compounds AGa2O4 (A = Sr, Ca) were prepared by traditional solid state reactions under ambient pressure. γ-SrGa2O4 (P21/n) was the thermodynamically stable phase below 1200 °C, and would readily transform to β-SrGa2O4 (P21/c) at 1200 °C. Similar phase transition from α-CaGa2O4 (Pna21) to βCaGa2O4 (P21/c) was also observed at about 1350 °C, however no pure phase of β-CaGa2O4 can be obtained probably due to the melting issue. Rietveld refinements on high quality data provided a great detail of both structures, and the comparison of these polymorphs suggest that the driving force of such phase transitions is the under-bonded nature of the alkaline earth cations. In other words, the need of larger space for Sr2+/Ca2+ in the high temperature β-phase forces the 6-memberedring channel expand through increasing the Ga-O-Ga angles. This transition involves the breaking and re-formation of Ga–O bonds, thus supposed to be the first-order in nature. Our results provide necessary information about the thermodynamic behaviors of AGa2O4 at ambient pressure, and is expected to be helpful to the preparation of these materials. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21171178, 21671028). We also acknowledge the support from the sharing fund of large-scale equipment of Chongqing University. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jssc.2017.07.024. References [1] R.E. Gibbs, The polymorphism of silicon dioxide and the structure of trtidymite, Proc. R. Soc. Lond. Ser. A Contain. Pap. Math. Phys. Charact. 113 (1926) 351–368. [2] F.P. Glasser, L.S. Dent Glasser, Crystal chemistry of some AB2O4 compounds, J. Am. Ceram. Soc. 46 (1963) 377–380. [3] V. Kahlenberg, C. Weidenthaler, High temperature single crystal diffraction study on monobarium gallate – the crystal structure of β-BaGa2O4, Solid State Sci. 4 (2002) 963–968. [4] V. Kahlenberg, R.X. Fischer, J.B. Parise, The stuffed framework structure of
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