3.1 Phase composition of the synthesized products
Yb2Si2O7 precursors with different Si/Yb molar ratios were calcined at a temperature range from 900 ℃ to 1400 ℃, the corresponding XRD patterns are shown in Fig. 1. It can be seen that no crystallization peak can be detected in the XRD patterns of obtained products calcined at 900 ℃, indicating that the calcined products still maintain amorphous state. After calcination at 1000 ℃, obvious crystallization behavior can be observed from the XRD patterns. However, the calcined products exhibit different phase compositions with each other. With the Si/Yb molar ratio of precursors less than or equal to the standard stoichiometry of Yb2Si2O7, the calcined products (Y1 − 1000 and Y2 − 1000) are composed of unsteady X1-Yb2SiO5 (JCPDS Card No.52-1187) and α-Yb2Si2O7 (JCPDS Card No.301439). Meanwhile, α-Yb2Si2O7 and β-Yb2Si2O7 (JCPDS Card No. 25-1345) coexist in the calcined product (Y3 − 1000) which was transformed from the precursor with Si/Yb molar ratio higher than the standard stoichiometry of Yb2Si2O7. Correlation analysis confirmed that the synthesis temperatures of X1-Yb2SiO5, α-Yb2Si2O7 and β-Yb2Si2O7 are between 900 ℃ and 1000 ℃. In addition. low-temperature phase X1-Yb2SiO5 has been identified with synthesis temperature lower than 950 ℃ in our previous research [38]. With the calcination temperature elevated to 1100 ℃, significant compositional change of the Y1 − 1100 and Y2 − 1100 can be observed in the XRD patterns. It can be seen from Fig. 1a that, the Y1 − 1100 is composed of X2-Yb2SiO5 (JCPDS Card No.40–0386), β-Yb2Si2O7 and trace of X1-Yb2SiO5, and the characteristic diffraction peaks of α-Yb2Si2O7 disappeared. In addition, only X2-Yb2SiO5 and β-Yb2Si2O7 can be identified in Y2 − 1100 (Fig. 1b). Compared with that calcined at 1000 ℃, X2-Yb2SiO5 and β-Yb2Si2O7 were formed in the Y1 − 1100 and Y2 − 1100. Differently, the Y3 − 1100 has the same phase compositions as Y3 − 1000, in which the peak intensity of β-Yb2Si2O7 increased significantly, as shown in Fig. 1c. Therefore, it can be inferred that the phase transformations from X1-Yb2SiO5 to X2-Yb2SiO5 and from α-Yb2Si2O7 to β-Yb2Si2O7 occurred during the calcination process within 1000 ℃ཞ1100 ℃. As the calcination temperature comes to 1200 ℃, the X1-Yb2SiO5 and α-Yb2Si2O7 transformed into X2-Yb2SiO5 and β-Yb2Si2O7 completely in the Y1 − 1200 and Y3 − 1200, respectively. Further raising the calcination temperatures to 1300 ℃ and 1400 ℃, the calcined products exhibit the constant phase composition as that after calcined at 1200 ℃, exhibiting excellent phase stability. Somewhat differently, the calcined products Y1 − 1200/1300/1400 and Y2 − 1200/1300/1400 are composed of β-Yb2Si2O7 and X2-Yb2SiO5, while the calcined products Y3 − 1200/1300/1400 consist of pure β-Yb2Si2O7 phase. There is no doubt that complex structural and phase evolutions occurred in Yb2Si2O7 products between 900 ~ 1200 ℃ in our present work. The calcination temperature plays a major role in determining the phase composition and structure of Yb2Si2O7 products, and thermally stable components can be obtained in Yb2Si2O7 powders with the calcination temperature above 1200 ℃.
The effect of Si/Yb molar ratio on phase composition of as-synthesized Yb2Si2O7 powders were summarized and illustrated in Fig. 2a. It can be seen that both phase composition and crystal structure of the calcined products were strongly affected by the molar ratio of Si/Yb. With the molar ratio of 0.9 and 1.0, Yb2SiO5 impurities always exist in the final products, which cannot be eliminated by the elevated calcination temperature. In addition, the ending temperature for stable component are 1200 ℃ and 1100 ℃, respectively, indicating that the increased molar ratio of Si/Yb promoted the transformation from X1 to X2-Yb2SiO5, which has been confirmed in our previous work [38]. As the Si/Yb molar ratio increased to be 1.1, Yb2Si2O7 powders were synthesized without impurities in the temperature range of 1000 ℃ཞ1400 ℃. It is worth noticed that the ending temperature for obtaining thermally stable Yb2Si2O7 powders is 1200 ℃, which is higher than that prepared with Si/Yb molar ratio of 1.0. The phase transformation from α to β-Yb2Si2O7 might be inhibited by the increased Si/Yb molar ratio. In a word, the final calcination temperature as well as the Si/Yb molar ratio of starting materials have significantly influence on the components and crystal structures of Yb2Si2O7 powders. In our present work, desired β-Yb2Si2O7 powders were successfully synthesized through the optimization of calcination temperature and Si/Yb molar ratio. Figure 2b shows the XRD pattern of β-Yb2Si2O7 powders prepared at 1200 ℃. It can be seen that the as-prepared β-Yb2Si2O7 powders exhibit favorable crystallinity and are free of impurities.
3.2 Microstructure of the β-Yb2Si2O7 powders
The morphology and structural characteristics of β-Yb2Si2O7 powders were analyzed by TEM. Figure 3 shows the morphology and microstructure of the β-Yb2Si2O7 powders after calcination at 1200 ℃. It can be seen that the β-Yb2Si2O7 powders exhibit irregular shape and have a particle size of about 70 ~ 100 nm (Fig. 3a). In addition, there is obvious agglomeration between Yb2Si2O7 particles probably attributing to its high activity, which is different from that of Yb2SiO5 powders [31, 35, 38]. Figure 3b illustrates the high-resolution image and corresponding electron diffraction pattern of β-Yb2Si2O7 powders. A set of arranged lattice fringes are discriminated clearly in the HRTEM image, demonstrating that the β-Yb2Si2O7 powders were synthesized with high crystallinity. In addition, the measured interplanar spacing in Fig. 3b is 3.195 Å, which corresponds to the (021) crystal plane of β-Yb2Si2O7. The SAED (selected area electron diffraction) pattern of β-Yb2Si2O7 is present in top right corner of Fig. 3b, which can be indexed as [01-2] zone axis of β-Yb2Si2O7 with monoclinic symmetry and C2/m space group [42].
3.3 Formation mechanism of Yb2Si2O7 precursor
In this study, Yb2Si2O7 powders were synthesized by cocurrent chemical coprecipitation method whose mechanism is quite different from that of traditional ones. Therefore, fundamental studies on the structural characteristics and formation mechanism of the Yb2Si2O7 precursors were conducted to understand the synthesis process as well as to realize the controllable preparation of Yb2Si2O7 powders.
For the purpose of comparison, the precursors of ytterbium oxide (Yb2O3) and silicon oxide (SiO2) were synthesized through the precipitation reaction of YbCl3, ethyl orthosilicate with ammonia, respectively, via the same cocurrent coprecipitation method. Figure 4 shows the FTIR spectra of Yb2O3, SiO2 and Yb2Si2O7 precursors. It can be seen that the characteristic absorption peaks of the Yb2Si2O7 precursor are significantly different from the other two precursors, resulting from its special chain segment structure. In the FTIR spectra of the Yb2O3, SiO2 and Yb2Si2O7 precursors, the strong and broad absorption peak located at around 3380 cm-1 can be assigned to the O-H stretching modes of hydroxyl originated from residual water, ethanol molecules as well as intrinsic hydroxyl groups in precursors [45, 46]. The deformation vibration of H2O is also observed at 1622 cm-1 and 1634 cm-1, which can be attributed to the structural and residual water. In addition, the peaks at 1515 cm-1 and 1394 cm-1 for Yb2O3 precursor as well as 1538 cm-1 and 1404 cm-1 for Yb2Si2O7 precursor were identified to the bending vibration of -CH3/CH2 and stretching vibration of C-O in alkoxy groups of ethanol molecules [47], which has been confirmed in the synthesis process of ytterbium silicate powders [38]. The absence of relevant characteristic peaks in SiO2 precursor can be benefited to the trace of ethanol residue, which is also certificated by the weaker integral strength of peak at 3382 cm-1. Moreover, the insufficient dealkylation reaction of TEOS in hydrolysis process can also result in the existence of alkyl groups.
It is obviously that the structural differences among the Yb2O3, SiO2 and Yb2Si2O7 precursors were reflected in characteristic absorption peaks ranged from 400 to 1100 cm-1. In the FTIR spectrum of Yb2O3 precursor, the absorption peaks located at 667 cm-1 and 426 cm-1 corresponded to bending vibration of Yb-OH bond and the stretching vibration of Yb-O bonds [46], respectively, none of other vibration mode can be detected. Comparatively, the SiO2 precursor presents complex chain segment structure. Researches [45, 48–55] have shown that the cross-linked polymer with spatial-opened skeleton structure will be formed through the hydrolysis and polycondensation of TEOS, in which the -[Si-O-Si]- network was surrounded by coordinated hydroxyl. In our present work, the SiO2 precursor was synthesized in a strong alkali environment. Accelerated polycondensation can promote the connection of colloidal particles, resulting in the formation of large-scaled network structure. In the FTIR spectrum of SiO2 precursor, the 1085 cm-1 peak have been proven to be associated with the transverse optical (TO) mode of Si-O-Si asymmetric bond stretching vibration [45, 49]. Meanwhile, the 799 cm-1 and 464 cm-1 peaks were assigned to symmetric stretching and bending vibration of network Si-O-Si bonds [49, 50, 56], respectively. Moreover, the stretching vibration mode of typical Si-OH bonds reflect in the corresponding peak at 959 cm-1 [49]. During the synthetic process of Yb2Si2O7 precursor, Yb3+ and TEOS were coprecipitated in ammonia solution. It is worth noticed that the FTIR spectrum exhibits the same characteristics in spectral lines as that of SiO2 precursor, which indicates that approximated cross-linked polymer with spatial-opened skeleton structure may be formed in the Yb2Si2O7 precursor. In our previous work, Yb2SiO5 precursor was synthesized with the same method. It is certain that the binding energies of both Si-OH and Yb-OH bonds were influenced with each other, which confirmed the structural difference in chain segments between Yb2SiO5 and SiO2 precursor. Therefore, the obvious offset of characteristic absorption peaks in FTIR spectrum of Yb2Si2O7 precursor can also confirm the formation of -[Si-O-Yb]- network. Similar as the SiO2 precursor, the 989 cm-1 peak can be identified as TO mode of Si-O-Yb asymmetric bond stretching vibration, as well as the 920 cm-1 and 472 cm-1 peaks correspond to the stretching vibration of Si-OH bonds and bending vibrations of Si-O-Yb bonds, respectively. Moreover, the 688 cm-1 peak is assigned to bending vibrations of Yb-OH bond. In the FTIR spectrum of Yb2Si2O7 precursor, the Si-O-Si symmetric stretching disappeared significantly, which is also due to the formation of -[Si-O-Yb]- network.
Figure 5 is the schematic diagram showing the formation mechanism of the Yb2Si2O7 precursor. It is undoubtedly that the Yb2Si2O7 precursor gone through a series of complicated chemical reactions and structural evolution during the synthesis process. Firstly, the solution containing positive trivalent Yb ions was obtained through the reaction between Yb2O3 and hot hydrochloric acid. The solution has strong acidity due to the slightly excessive acid. Meanwhile, the hydrolysis reaction of TEOS was activated by the nucleophilic attack of water molecules on the Si atom when blending with alcohol and water [57]. Subsequently, the hydrolysis process was accelerated by the strong acid environment resulting from the mingle with Yb3+ solution. As a results, a lot of monosilicic acids were produced in the cation solution, which is also attributed to the inhibition in polymerization by acidic environment. Therefore, coprecipitation of Yb3+ and monosilicic acids was realized by the continuous injection of cation solution into a large volume solution rich in ammonia. During the coprecipitation, the polymerization of monosilicic acids became the dominated process. Researches [51, 58, 59] have shown that silicon oxygen ions will be generated through the dehydrogenation of monosilicic acids, then a nucleophilic attack to Si-OH groups of surrounding monosilicic acids occurred, and finally -[Si-O-Si]- network formed inevitably resulting from oxygen dimerization reaction. However, the reaction system of Yb2Si2O7 precursor is totally different from that of SiO2 due to the existence of Yb3+, in which the nucleophilic reaction was inhibited. Instead, the electronegative silicon oxygen ions were absorbed by Yb3+ as ligands, which induced the formation of ion clusters with Yb3+ as core. In the subsequent polycondensation process, the Yb may be embedded as bridging atoms in the -[Si-O-Si]- network, which resulted in the formation of -[Si-O-Yb]- network. Different form the SiO2 precursor, both Si and Yb act as crosslinked atom in -[Si-O-Yb]- chain links. So, it can be concluded that the Yb2Si2O7 precursor were synthesized in the form of cross-linked polymers with bimetallic crosslinking points in -[Si-O-Yb]- chain segments.
3.4 Thermal behavior of the Yb2Si2O7 precursors
The TG-DSC analysis was applied to investigate the thermal behavior of the Yb2Si2O7 precursor, and the corresponding curves are shown in Fig. 6. A continuous weight loss can be detected with the increased temperature, and the ceramic yield is about 83.6%. The weight loss can be attributed to the evaporation of adsorbed water and ethanol as well as the removal of bound water and constituent groups in Yb2Si2O7 precursor. Based on the mass lost rate, the TG-DSC curves can be divided into four sections (Fig. 6). In section A, the precursor undergone a rapid weightlessness of 13.2% below 300 ℃. Within this section, a broad endothermic peak can be observed in the DSC curve, which is attributed to the evaporation of adsorbed water, ethanol as well as to the removal of the bound water. With the temperature increasing from 300 to 500 ℃ (section B), the weight loss of precursor is about 2.7%. The appearance of exothermic peak can be corresponded to the decomposition of trace organic groups. In section C, the residual carbon was burn out in 500 ~ 800 ℃ with a minimal loss of weight (ca.1%). In section D, no obvious weight loss can be detected. However, a sharp exothermic peak at 1065°C was clearly observed in the DSC curve, which indicates the crystallization of Yb2Si2O7.
Figure 7 shows the FTIR spectra of Yb2Si2O7 precursors after calcination at 400 ℃~ 900 ℃. The Yb2Si2O7 precursors present similar characteristic spectra before and after calcination, indicating a high similarity in structure and composition. After calcination at 400 ~ 900 ℃, the precursors exist stably in the form of amorphous phase, which is consistent with the fact shown in Fig. 2. In the FTIR spectra, the characteristic absorption peaks located at about 3380 cm-1 are mainly associated with the O-H stretching modes of hydroxyl groups in the calcined precursors. The adsorbed water and ethanol molecules as well as the bound water have been removed in calcination process. In addition, the integral strength of 3380 cm-1 peaks [49] tends to weaken with the increased temperature, suggesting that the dihydroxylation is in continuous progress during the calcination process in the temperature range of 400 ~ 900 ℃. Similarly, the intensities of the characteristic peaks shown in the blue dotted box display a downward tendency except for the peak at 1634 cm-1. Due to the exposure to atmosphere, trace of water will be absorbed on the surface of tested powders with high specific surface area, which is responsible to the constant visible of the peaks at 1634 cm-1 [45] and 3380 cm-1. Meanwhile, the other peaks at 1538 cm-1 and 1404 cm-1 can be attributed to the gradual dealkylation [48]. Previous analysis has shown that the 989 cm-1 and 472 cm-1 peaks represent the stretching and bending vibrations of Si-O-Yb bonds [45], concurrently the 920 cm-1 [48] and 688 cm-1 [45] peaks can be assigned to the bending vibrations of Si-OH and Yb-OH bonds in Yb2Si2O7 precursors. However, the related characteristic peaks exhibit a significant shift although the calcined precursors remain amorphous. It is visible that the peak position for stretching vibration of Si-O-Yb bonds has shifted from 989 cm-1 to 973 cm-1, and those for bending vibration of Si-O-Yb bonds have a deviation to about 500 cm-1. Meanwhile, there are also slight shifts in peak positions for the bending vibrations of Si-OH and Yb-OH bonds. It is worth noting that the intensities of peaks around 970 cm-1 and 500 cm-1 become stronger with the increased temperature, while that of the peak at around 700 cm-1 displayed a decreased trend, which indicated that the polarity of Si-O bond and the symmetry of Si-O-Yb bond are strengthened. Therefore, it can be inferred combing with the TG-DSC analysis that -Si-O-Yb- structure consisted of Yb, Si and O atoms only was formed by the removal of hydroxyl and continuous ordering. The removal of hydroxyl and the continuous ordering and are the dominant factors in resulting to the change of peak position and peak intensity in FTIR spectra of the calcined precursors. For comparison, the FTIR spectra of Yb2O3 and SiO2 are also provided in Fig. 7. It is obvious that no characteristic peaks of both Yb2O3 and SiO2 can be observed in the FTIR spectra of the calcined precursors, which further confirmed that the structure form of -[Si-O-Yb]- network was relatively stable below 900 ℃.
3.5 Phase transformation and synthesis mechanism of Yb2Si2O7 powders
The various phase compositions of Yb2Si2O7 powders obtained at different calcination temperature confirmed that a phase transformation from α-Yb2Si2O7 to β-Yb2Si2O7 occurred with the elevated temperatures. Figure 8 displays the crystal structure diagrams of α-Yb2Si2O7 and β-Yb2Si2O7. As shown in Fig. 8a and 8c, the α-polymorph Yb2Si2O7 includes 22 independent constituent atoms which build isolated (Si3O10) chain-like groups, (SiO4) tetrahedra, (YbO6) and (YbO8) polyhedral structural units [22, 40, 42]. Among them, the (Si3O10) groups are constituted by three (SiO4) tetrahedrons with two Si-O-Si bridges, and the adjacent bridges exhibit a significant difference in bond angles. In addition, isolated (SiO4) tetrahedrons possess high distortion in length and angle of Si-O bonds compared with that of ideal tetrahedrons. In the (YbO6) and (YbO8) polyhedral structural units, the Yb atoms were coordinated separately by six and eight oxygens which belong to the (Si3O10) groups or (SiO4) tetrahedrons. It is distinctly different that only (Si2O7) groups and (YbO6) polyhedral structural units exist in the monoclinic β-Yb2Si2O7 crystals [43, 44, 60], as shown in Fig. 8b and 8d. The (Si2O7) groups can be considered as a structure with two (SiO4) tetrahedrons connected through a Si-O-Si bridge with 180° bond angle. In this group, the (SiO4) tetrahedrons show low degree of distortion. Except for the bridging oxygen, the others can be coordinated with adjacent Yb to form (YbO6) polyhedral structural units. In despite of the fact that the Yb2Si2O7 can exist steadily as α-polymorph in room temperature, it is still a metastable phase in terms of thermal stability [44]. Enough heat input from nearby surroundings can induce the phase transformation towards β-Yb2Si2O7, in which a twisted structure was transformed into an ideal stretched one due to the energy optimization.
Figure 9 shows the FTIR spectra of Yb2Si2O7 precursors after calcination at 1000 ℃~ 1200 ℃. It was clearly observed that the calcined products exhibit a completely different characteristic peak compared with that after calcination at 900 ℃. As we known that the powder products both calcined at 1000 ℃ and 1100 ℃ are composed of α-Yb2Si2O7 and β-Yb2Si2O7, thus the characteristic absorption peaks of α and β-Yb2Si2O7 are all present in the related FTIR spectra. Meanwhile, the typical FTIR spectrum of β-Yb2Si2O7 was obtained from the pure-phase powder product after calcination at 1200 ℃. Therefore, it can be concluded that the peaks located at 1096 cm− 1, 980 cm− 1, 908 cm− 1, 849 cm− 1, 565 cm− 1, 537 cm− 1, 499 cm− 1 and 472 cm− 1 are all belong to β-Yb2Si2O7, which have also been confirmed in the related research works [32]. In the FTIR spectrum, the peaks at 1096 cm− 1 and 849 cm− 1 are ascribed to the stretching vibration of silicon atoms against bridging and non-bridging oxygen atoms in (Si2O7) groups [45, 61], respectively. While the 980 cm− 1 and 908 m− 1 peaks [32, 62] correspond to the stretching modes of ytterbium atoms against oxygen atoms in Yb-O-Yb chains normal and subparallel to Si-O-Si chain, respectively. In addition, the 565 cm− 1 and 537 m− 1 peaks can be assigned to the stretching and bending vibrations of Yb-O bonds in (YbO6), and the peaks at 499 cm− 1 and 472 cm− 1 [62, 63] are all associated with the bending vibration of various categories of Si-O bonds. Consequently, it is logical that these peaks located at 1129 cm− 1, 1042 cm− 1, 1000 cm− 1, 946 cm− 1, 881 cm− 1, 722 cm− 1, 693 cm− 1, 557 cm− 1, 514 cm− 1 and 429 cm− 1 could be identified to the characteristic absorption peaks of α-Yb2Si2O7, the complexity of spectrum can be attributed to the diverse structural units as shown in Fig. 8c.
Unfortunately, there are few reports on the FTIR study of α-Yb2Si2O7. Research showed that the FTIR characteristic peaks at 1000–1100 cm− 1 are mainly from the stretching vibrations of Si-O bonds in Si-O-Si chains for rare earth disilicates [61]. As for α-Yb2Si2O7, (Si3O7) chain-like groups are the unique structural unit containing Si-O-Si chain. Thus, the 1042 cm− 1 and 1000 cm− 1 peaks should be associated with the stretching vibrations of Si atoms against O atoms in Si-O-Si chains, and the double-peak distribution may be attributed to the difference in bond length and bond angle of various types of Si-O-Si chains in (Si3O10) groups. In addition, it has been confirmed that the bending vibrations of bridging oxygen bond in Si-O-Si usually give expression to the peaks in the range of 600–800 cm− 1[61, 62], which can provide a tenable inference that the 722 cm− 1 and 693 cm− 1 peaks can also trace to the (Si3O10) groups and correspond to Si-O-Si with difference bond angles. In particular, there is no relative characteristic peak can be detected in the FTIR spectrum of β-Yb2Si2O7, which is resulted from the D3d symmetrical structure of (Si2O7) groups (180° bond angle in Si-O-Si) [61]. Wen et al. [35] investigated the FTIR spectrum of Yb2SiO5 and have affirmed the ascription of 930 cm− 1 and 870 cm− 1 to asymmetric and symmetric stretching vibrations of Si-O bonds for isolated (SiO4) tetrahedrons. As the similar structural units in α-Yb2Si2O7, the corresponding vibration modes of that should be identified to the peaks located at 946 cm− 1 and 881 cm− 1. In which, the 946 cm− 1 peak may be associated with the asymmetric stretching vibrations of Si-O bonds and the 881 cm− 1 peak should be assigned to the symmetric stretching vibrations of Si-O bonds in highly distorted (SiO4) tetrahedrons. Moreover, correlativity researches [34, 44, 61, 62] have shown that the FTIR characteristic peaks located at 500–600 cm− 1 and 400–500 cm− 1 are usually originated from the stretching vibrations of Yb-O bonds and bending vibrations of Si-O bonds, respectively. So, it can be inferred that the 557 cm− 1 and 514 cm− 1 peaks should be assigned to the Yb-O stretching vibrations in (YbO6) and (YbO8) groups. Meanwhile, the 429 cm− 1 peak corresponds to the low-frequency bending vibrations of Si-O bonds from isolated (SiO4) tetrahedrons or (Si3O10) groups. Echoed with that the 1129 cm− 1 peak can be assigned to the high-frequency bending vibrations of the Si-O bonds [34].
Based on the infrared spectrum shown in Fig. 9, it can be seen clearly that the characteristic peaks of α-Yb2Si2O7 disappeared gradually with the elevated calcination temperature, a phase transformation from α to β-Yb2Si2O7 happened exactly during the calcination process above 1000 ℃. As mentioned earlier, the structural units of β-Yb2Si2O7 are composed of (Si2O7) groups and (YbO6) polyhedrons which is completely different from that of α-Yb2Si2O7. Therefore, it can be deduced that both of (SiO4) tetrahedrons and (Si3O10) groups experienced structural changes to form (Si2O7) groups, as well as (YbO8) to (YbO6) polyhedrons. During the process of phase transformation, the external energy plays a role in inducing the breaking of Si-O bonds in (SiO4) tetrahedrons and (Si3O10) groups and then promoted the formation of stable (Si2O7) groups, which is responsible to the disappearance of (SiO4) tetrahedrons and (Si3O10) groups characteristic peaks for stretching vibrations in the FTIR spectrum of Yb2Si2O7 calcined at 1200 ℃. In addition, the characteristic peaks of bending vibration modes were also affected by the structural evolution of (SixOy) groups, including the vanishing of 1129 cm− 1 and the shift of 429 cm− 1 peak. It is worth mentioning that the Si-O stretching vibration of Si-O-Si chains in (Si3O10) groups disappeared in the FTIR spectrum of Yb2Si2O7 calcined at 1100 ℃, while the bending vibration of the corresponding Si-O bond are greatly weakened. In addition, the peak of the symmetrical stretching vibration of the (SiO4) tetrahedron exhibits a significant shape change. It illustrated that the polarity of bridging oxygen bond in Si-O-Si chains decreased and then the bridging oxygen bond broke as well as the peak shape change of the highly distorted (SiO4) tetrahedron shows an improvement in the symmetry of the isolated (SiO4) tetrahedron. Therefore, it can be reasonably inferred that (Si3O10) groups were divided into (SiO4) units through the fracture of Si-O-Si and then formed (Si2O7) groups of β-Yb2Si2O7. As a kind of units owned by both α and β-Yb2Si2O7, the characteristic peaks of (YbO6) polyhedrons can be regarded as located at 565 cm− 1, 557 cm− 1 and 537 m− 1, thus the deviation of 565 cm− 1 and 557 cm− 1 peaks as well as the decrease in peak intensity of 537 cm− 1 peaks can be attributed to the changed structural parameters and coordination environments of (YbO6) polyhedrons. Similarly, the 514 cm− 1 peak is regarded as the characteristic vibration modes of Yb-O bonds in (YbO8) polyhedrons according to the obvious absence in β-Yb2Si2O7 resulted from the structural transformation from (YbO8) to (YbO6) polyhedrons. Overall, the metastable α-Yb2Si2O7 converted to a stable β-Yb2Si2O7 through a reconstructive progress.
Figure 10 shows the synthesis mechanism diagram of Yb2Si2O7 powders. There have two clear periods during the entire synthetic process of β-Yb2Si2O7 powders. In the period of precursor synthesis, a cross-linked polymer with Yb and Si as crosslinking atoms in -[Si-O-Yb]- chain segment was obtained though the embedding of Yb atoms in -[Si-O-Si]- network during the polycondensation process of monosilicic acids, which give the credit to the coprecipitation of Yb ions and monosilicic acids. Subsequently, the amorphous precursor experienced a structural conversion towards to α-Yb2Si2O7 through the removal of hydroxyl of -[Si-O-Yb]- network and the continuous ordering of -Si-O-Yb- structure in calcination process, in which the -Si-O-Yb- structure was broken and restructured gradually, and then resulted in the formation of (Si3O10) groups, (SiO4) tetrahedra, (YbO6) and (YbO8) polyhedral structural units. According to the effects of calcination parameters on phase composition of Yb2Si2O7 powders, as shown in Fig. 2a, it can be asserted that both increased temperature and extended time had a significant function in realizing the phase transformation from α to β-Yb2Si2O7. During the transformation process, β-Yb2Si2O7 with near-standard polyhedral structure generated accompanied by the evolution of structure and coordination environment for the structural units of α-Yb2Si2O7 to (Si2O7) groups and (YbO6) polyhedrons.