Structural evolution and synthesis mechanism of ytterbium disilicate powders prepared by cocurrent chemical coprecipitation method

Ytterbium disilicate powders were synthesized by cocurrent chemical coprecipitation method. The inuence of Si/Yb molar ratio and calcination temperature on compositions and structures of Yb 2 Si 2 O 7 products were investigated. The formation mechanism and thermal behavior of precursor as well as the phase evolution of Yb 2 Si 2 O 7 were also discussed in depth. Results show that pure β-Yb 2 Si 2 O 7 powders with nanoscale size can be obtained from the precursor with Si/Yb molar ratio of 1.1 after being calcinated at temperatures above 1200 ℃ . The Yb 2 Si 2 O 7 precursor is an amorphous polymer crosslinked with -[Si-O-Yb]- chain segments which are formed though Yb atoms embedding in the -[Si-O-Si]-network. After a continuous dihydroxylation and structural ordering, the amorphous precursor transformed to α-Yb 2 Si 2 O 7 crystals by atomic rearrangement. Elevated calcination temperature can induce to the coordination structures and environment evolutions of structural units and then converted to stable (Si 2 O 7 ) groups and (YbO 6 ) polyhedrons, which results in the formation of β-Yb 2 Si 2 O 7 .


Introduction
SiC ceramic matrix composites (CMCs) have been regarded as new generation hot section materials applied in gas turbine engines due to their excellent high-temperature capability, which can provide critical support for leading the revolutionize development of advanced vehicle technologies. However, SiC CMCs exhibit lower durability in high-velocity combustion resulting from the rapid surface recession in oxidizing and water vapor environment [1,2]. As a result, environmental barrier coatings (EBCs) emerged as the times require for protecting SiC CMCs substrate materials against corrosive media [3][4][5][6][7].
With the development of EBCs system and corresponding iteration of advanced materials,rare earth silicates have been focused on the new generation EBCs materials due to their excellent environment resistance [4,[7][8][9][10][11]. Among them, ytterbium disilicate (Yb 2 Si 2 O 7 ) and ytterbium monosilicate (Yb 2 SiO 5 ) are demonstrated as promising candidate materials for EBCs based on their superior high-temperature phase stability, excellent chemical and mechanical compatibility with SiC, lower oxygen permeability and so on [9,[12][13][14][15][16]. Especially, the Yb 2 Si 2 O 7 has a much closer coe cient of thermal expansion (CTE, 3.7-4.5×10 − 6 °C − 1 ) than that of Yb 2 SiO 5 (6.5-7×10 − 6 °C − 1 ) to SiC-CMCs substrate (ca. 4.5-5.5×10 − 6 °C − 1 ) [17], which is bene t to improve the bond strength between EBCs and substrate, as well as retard the thermal stress in EBC coating during thermal cycling. In addition, Yb 2 Si 2 O 7 has better thermochemically compatible with thermally grown SiO 2 than that of Yb 2 SiO 5 , which is highly competitive in ensuring system stability of EBC coating. Furthermore, the Yb 2 Si 2 O 7 materials also exhibit excellent corrosion resistance of water vapor and CaO-MgO-Al 2 O 3 -SiO 2 (CMAS). B.T. Richards et al. [18] prepared a bi-layer Yb 2 Si 2 O 7 /Si EBC coating on SiC substrates through an optimal design of CTE matching, which exhibited excellent steam-cycling durability at 1316 ℃ in a owing 90%H 2 O/10%O 2 environment. Zhong et al. [19] developed a tri-layer Yb 2 SiO 5 /Yb 2 Si 2 O 7 /Si EBC coating on SiC CMCs by atmospheric plasma spray technique. After 40-50 thermal cycles between room temperature and 1350 ℃, the Yb 2 Si 2 O 7 layer still maintained better structural integrity resulting from their outstanding thermal shock resistance. Zhao et al. [20] investigated the CMAS resistance of Yb 2 Si 2 O 7 and Yb 2 SiO 5 coatings. Results indicated that the Yb 2 Si 2 O 7 coating shows better CMAS corrosion resistance than that of Yb 2 SiO 5 coating at 1300 ℃, which is attributed to the reductive diffusion transport capacity of molten salt compositions. Stolzeburg et al. [21] studied the interaction of Yb 2 Si 2 O 7 and Yb 2 SiO 5 materials with CMAS engine deposits at 1300 ℃. After 96 hours exposure, no discernable reaction can be observed between Yb 2 Si 2 O 7 and CMAS, thus the Yb 2 Si 2 O 7 material displayed superior chemical stability in CMAS corrosion environment.
Comprehensive analysis shows that Yb 2 Si 2 O 7 materials exhibit great potential applying in EBCs system.
Up to date, EBCs system are generally built by thermal spraying technical, which are strongly dependent on the processing of coating and the quality of raw materials. For rare earth silicates, the chemical compositions and crystal structures have a signi cantly in uence on their mechanical and corrosion resistance [17,[22][23][24][25]. Rare earth silicates powders are mainly synthesized by solid-state reaction [19,[26][27][28][29][30], hydrothermal syntheses [31], sol-gel [32][33][34][35][36] and chemical coprecipitation methods [37]. For solid-state reaction method, rare earth silicates powders were synthesized through direct reaction between rare earth oxide and silicon oxide at relatively higher calcination temperature, however, the obtained powders usually have micron-scale and exhibit agglomeration to some extent. In addition, impurity phases will mingle in the nal powder products resulting from inadequate reaction and insu cient mixing of raw materials, which leads to uniform composition of the rare earth silicates materials. Hydrothermal synthesis method was also conducted to synthesize rare earth silicates in current research. However, higher speci c surface area of the ultra ne powders (20-30 nm) brings great di culties to the preparation of coating. Besides inherent impurities are di cult to eliminate through the optimization of synthesis process, which limits its further development. Comparatively, sol-gel and chemical coprecipitation methods are effective approaches to prepared moderate particle size, uniform composition and high purity rare earth silicates by controlling the stoichiometric molar ratio of starting materials. Up to date, the relative synthesis mechanism of rare earth silicates by sol-gel method have not been clari ed yet. In our previous studies, a novel cocurrent chemical coprecipitation method have been developed to synthesize Yb 2 SiO 5 [38] and dysprosia-stabilized zirconia (DySZ) powders [39], which has advantages of low cost, composition controllable, easy access to high purity and homogeneous powder materials.
According to the ionic and synthesis temperature [22,23,[40][41][42][43][44], rare earth disilicates (RE 2 Si 2 O 7 ) have seven different crystal types, which were denoted by capital letters A, B, C, D, E, F, and G in the original work of Felsche et al. [22,23,42]. However, only three crystal types of Yb 2 Si 2 O 7 can be identi ed by X-ray diffraction, α-Yb 2 Si 2 O 7 has triclinic structure with space groups P-1 or P1, meanwhile β-Yb 2 Si 2 O 7 and γ-Yb 2 Si 2 O 7 present monoclinic structure with space groups C2/m, and P 21 /c, respectively. In addition, β-Yb 2 Si 2 O 7 have been con rmed to be only stable phase in high temperature, which are endow with high expectations to satisfy the application requirements of EBC coating in extremely deteriorated environment of gas turbine engines.
In our present work, Yb 2 Si 2 O 7 powders were synthesized via a cocurrent chemical coprecipitation method for the rst time. The in uence of Si/Yb molar ratio in starting materials and calcination temperature on phase composition and crystal structure of Yb 2 Si 2 O 7 were investigated. The structural evolution as well as synthesis mechanism of Yb 2 Si 2 O 7 were also delineated in detail.

Synthesis of Yb 2 Si 2 O 7 powders
The Yb 2 Si 2 O 7 powders were synthesized by a cocurrent chemical coprecipitation method which has been developed in our previous work [38]. Different from traditional chemical coprecipitation method, the liquid reaction system of precursor formation can maintain a stable pH environment based on the welldesigned reaction mother liquor and controllable current speed of precipitant, which can promote the precipitation product formed more evenly.
In the rst instance, Yb 3+ containing solution was prepared through the chemical reaction of Yb 2 O 3 powders with boiled hydrochloric acid solution. Meanwhile, TEOS hydrolysate was prepared by mixing TEOS, ethyl alcohol and deionized water with volume ratio of 1:4:2. Subsequently, the two solutions were mixed and stirred, forming a homogeneous metal-ion solution. In addition, the precipitant was prepared by diluting the ammonia with isovolumetric deionized water. After that, the metal-ion solution and precipitant were respectively injected into a stirred ammonia solution which has a pH value of 9 10. The pH value of the reaction environment was controlled by adjusting the injection rate of precipitant. After the injection, the obtained precipitation solution was stirred for 30 min and then aged for 24 h. Then, the remaining Cl − and NH 4+ were removed sequentially with deionized water and ethanol to prepare a wet Yb 2 Si 2 O 7 precursor. Finally, the precursor powders was obtained after drying at 110 ℃ for 12 h, and then calcined for 10 h in ambient atmosphere with a temperature range of 400 1400 ℃. Based on the phase diagram of the Yb 2 O 3 -SiO 2 system [12,42], the Yb 2 Si 2 O 7 precursors were synthesized with different Si/Yb molar ratios (9:10, 10:10 and 11:10), which are noted as Y 1 , Y 2 and Y 3 respectively. The associated Yb 2 Si 2 O 7 materials calcined at various temperatures are labeled in form of Y n−c , in which n = 1, 2 or 3, c is the calcination temperature.

Characterization
The crystallographic phases of the calcined powders were identi ed by an X-ray diffractometer (XRD, D8 advanced, Bruker AXS, Germany, Cu Kα radiation, λ = 0.15406 nm ) in a range of 2θ = 5 85˚. Thermal behavior of the precursor was analyzed by a thermal analyzer (STA 449C, Netzsch, Germany). Thermogravimetric (TG) and differential scanning calorimetry (DSC) curves were obtained through heating the precursor in air with 10°C/min from room temperature to 1200°C. The morphology and crystal structure of the as-prepared Yb 2 Si 2 O 7 powders were analyzed by transmission electron microscopy (TEM, Tecnai G2 F20, FEI CO., USA). The phase composition and structural evolution from precursor to nal product were studied by Fourier transform infrared spectroscopy (FTIR, Thermo Scienti c Nicolet iS50, USA) with wave number in range of 400 4000 cm − 1 .

Results And Discussion
3.1 Phase composition of the synthesized products Yb 2 Si 2 O 7 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 Yb 2 Si 2 O 7 , the calcined products (Y 1 − 1000 and Y 2 − 1000 ) are composed of unsteady X1- 1b). Compared with that calcined at 1000 ℃, X2-Yb 2 SiO 5 and β-Yb 2 Si 2 O 7 were formed in the Y 1 − 1100 and Y 2 − 1100 . Differently, the Y 3 − 1100 has the same phase compositions as Y 3 − 1000 , in which the peak intensity of β-Yb 2 Si 2 O 7 increased signi cantly, as shown in Fig. 1c. Therefore, it can be inferred that the phase transformations from X1-Yb 2 SiO 5 to X2-Yb 2 SiO 5 and from α-Yb 2 Si 2 O 7 to β-Yb 2 Si 2 O 7 occurred during the calcination process within 1000 ℃ 1100 ℃. As the calcination temperature comes to 1200 ℃, the X1-Yb 2 SiO 5 and α-Yb 2 Si 2 O 7 transformed into X2-Yb 2 SiO 5 and β-Yb 2 Si 2 O 7 completely in the Y 1 − 1200 and Y 3 − 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 Y 1 − 1200/1300/1400 and Y 2 − 1200/1300/1400 are composed of β-Yb 2 Si 2 O 7 and X2-Yb 2 SiO 5 , while the calcined products Y 3 − 1200/1300/1400 consist of pure β-Yb 2 Si 2 O 7 phase. There is no doubt that complex structural and phase evolutions occurred in Yb 2 Si 2 O 7 products between 900 ~ 1200 ℃ in our present work. The calcination temperature plays a major role in determining the phase composition and structure of Yb 2 Si 2 O 7 products, and thermally stable components can be obtained in Yb 2 Si 2 O 7 powders with the calcination temperature above 1200 ℃.
The effect of Si/Yb molar ratio on phase composition of as-synthesized Yb 2 Si 2 O 7 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, Yb 2 SiO 5 impurities always exist in the nal 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-Yb 2 SiO 5 , which has been con rmed in our previous work [38]. As the Si/Yb molar ratio increased to be 1.1, Yb 2 Si 2 O 7 powders were synthesized without impurities in the temperature range of 1000 ℃ 1400 ℃. It is worth noticed that the ending temperature for obtaining thermally stable Yb 2 Si 2 O 7 powders is 1200 ℃, which is higher than that prepared with Si/Yb molar ratio of 1.0. The phase transformation from α to β-Yb 2 Si 2 O 7 might be inhibited by the increased Si/Yb molar ratio. In a word, the nal calcination temperature as well as the Si/Yb molar ratio of starting materials have signi cantly in uence on the components and crystal structures of Yb 2 Si 2 O 7 powders. In our present work, desired β-Yb 2 Si 2 O 7 powders were successfully synthesized through the optimization of calcination temperature and Si/Yb molar ratio. Figure 2b shows the XRD pattern of β-Yb 2 Si 2 O 7 powders prepared at 1200 ℃. It can be seen that the asprepared β-Yb 2 Si 2 O 7 powders exhibit favorable crystallinity and are free of impurities.

Microstructure of the β-Yb 2 Si 2 O 7 powders
The morphology and structural characteristics of β-Yb 2 Si 2 O 7 powders were analyzed by TEM. Figure 3 shows the morphology and microstructure of the β-Yb 2 Si 2 O 7 powders after calcination at 1200 ℃. It can be seen that the β-Yb 2 Si 2 O 7 powders exhibit irregular shape and have a particle size of about 70 ~ 100 nm (Fig. 3a). In addition, there is obvious agglomeration between Yb 2 Si 2 O 7 particles probably attributing to its high activity, which is different from that of Yb 2 SiO 5 powders [31,35,38]. Figure 3b

Formation mechanism of Yb 2 Si 2 O 7 precursor
In this study, Yb 2 Si 2 O 7 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 Yb 2 Si 2 O 7 precursors were conducted to understand the synthesis process as well as to realize the controllable preparation of Yb 2 Si 2 O 7 powders.
For the purpose of comparison, the precursors of ytterbium oxide (Yb 2 O 3 ) and silicon oxide (SiO 2 ) were synthesized through the precipitation reaction of YbCl 3 , ethyl orthosilicate with ammonia, respectively, via the same cocurrent coprecipitation method. Figure 4 shows the FTIR spectra of Yb 2 O 3 , SiO 2 and Yb 2 Si 2 O 7 precursors. It can be seen that the characteristic absorption peaks of the Yb 2 Si 2 O 7 precursor are signi cantly different from the other two precursors, resulting from its special chain segment structure. In the FTIR spectra of the Yb 2 O 3 , SiO 2 and Yb 2 Si 2 O 7 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 H 2 O 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 Yb 2 O 3 precursor as well as 1538 cm -1 and 1404 cm -1 for Yb 2 Si 2 O 7 precursor were identi ed to the bending vibration of -CH 3 /CH 2 and stretching vibration of C-O in alkoxy groups of ethanol molecules [47], which has been con rmed in the synthesis process of ytterbium silicate powders [38]. The absence of relevant characteristic peaks in SiO 2 precursor can be bene ted to the trace of ethanol residue, which is also certi cated by the weaker integral strength of peak at 3382 cm -1 . Moreover, the insu cient 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 Yb 2 O 3 , SiO 2 and Yb 2 Si 2 O 7 precursors were re ected in characteristic absorption peaks ranged from 400 to 1100 cm -1 . In the FTIR spectrum of Yb 2 O 3 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 SiO 2 precursor presents complex chain segment structure.
Researches [45,[48][49][50][51][52][53][54][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 SiO 2 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 SiO 2 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 re ect in the corresponding peak at 959 cm -1 [49]. During the synthetic process of Yb 2 Si 2 O 7 precursor, Yb 3+ 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 SiO 2 precursor, which indicates that approximated cross-linked polymer with spatial-opened skeleton structure may be formed in the Yb 2 Si 2 O 7 precursor. In our previous work, Yb 2 SiO 5 precursor was synthesized with the same method. It is certain that the binding energies of both Si-OH and Yb-OH bonds were in uenced with each other, which con rmed the structural difference in chain segments between Yb 2 SiO 5 and SiO 2 precursor. Therefore, the obvious offset of characteristic absorption peaks in FTIR spectrum of Yb 2 Si 2 O 7 precursor can also con rm the formation of -  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 Yb 3+ 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 Yb 3+ 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 nally -[Si-O-Si]-network formed inevitably resulting from oxygen dimerization reaction. However, the reaction system of Yb 2 Si 2 O 7 precursor is totally different from that of SiO 2 due to the existence of Yb 3+ , in which the nucleophilic reaction was inhibited. Instead, the electronegative silicon oxygen ions were absorbed by Yb 3+ as ligands, which induced the formation of ion clusters with Yb 3+ as core. In the subsequent polycondensation process, the Yb may be embedded as bridging atoms in the -

Thermal behavior of the Yb 2 Si 2 O 7 precursors
The TG-DSC analysis was applied to investigate the thermal behavior of the Yb 2 Si 2 O 7 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 Yb 2 Si 2 O 7 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 Yb 2 Si 2 O 7 . Figure 7 shows the FTIR spectra of Yb 2 Si 2 O 7 precursors after calcination at 400 ℃~ 900 ℃. The Yb 2 Si 2 O 7 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 speci c 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 Yb 2 Si 2 O 7 precursors. However, the related characteristic peaks exhibit a signi cant 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-Ybstructure 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 Yb 2 O 3 and SiO 2 are also provided in Fig. 7. It is obvious that no characteristic peaks of both Yb 2 O 3 and SiO 2 can be observed in the FTIR spectra of the calcined precursors, which further con rmed that the structure form of -[Si-O-Yb]-network was relatively stable below 900 ℃.

Phase transformation and synthesis mechanism of Yb 2 Si 2 O 7 powders
The various phase compositions of Yb 2 Si 2 O 7 powders obtained at different calcination temperature con rmed that a phase transformation from α-Yb 2 Si 2 O 7 to β-Yb 2 Si 2 O 7 occurred with the elevated temperatures. Figure 8 [43,44,60], as shown in Fig. 8b and 8d. The (Si 2 O 7 ) groups can be considered as a structure with two (SiO 4 ) tetrahedrons connected through a Si-O-Si bridge with 180° bond angle. In this group, the (SiO 4 ) tetrahedrons show low degree of distortion. Except for the bridging oxygen, the others can be coordinated with adjacent Yb to form (YbO 6 ) polyhedral structural units. In despite of the fact that the Yb 2 Si 2 O 7 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-Yb 2 Si 2 O 7 , in which a twisted structure was transformed into an ideal stretched one due to the energy optimization. Figure 9 shows the FTIR spectra of Yb 2 Si 2 O 7 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-Yb 2 Si 2 O 7 and β-Yb 2 Si 2 O 7 , thus the characteristic absorption peaks of α and β-Yb 2 Si 2 O 7 are all present in the related FTIR spectra. Meanwhile, the typical FTIR spectrum of β-Yb 2 Si 2 O 7 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-Yb 2 Si 2 O 7 , which have also been con rmed 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 (Si 2 O 7 ) groups [45,61], respectively. While the 980 cm − 1 and 908 m − 1 peaks [32,62]  Based on the infrared spectrum shown in Fig. 9, it can be seen clearly that the characteristic peaks of α-Yb 2 Si 2 O 7 disappeared gradually with the elevated calcination temperature, a phase transformation from α to β-Yb 2 Si 2 O 7 happened exactly during the calcination process above 1000 ℃. As mentioned earlier, the structural units of β-Yb 2 Si 2 O 7 are composed of (Si 2 O 7 ) groups and (YbO 6 ) polyhedrons which is completely different from that of α-Yb 2 Si 2 O 7 . Therefore, it can be deduced that both of (SiO 4 ) tetrahedrons and (Si 3 O 10 ) groups experienced structural changes to form (Si 2 O 7 ) groups, as well as   Fig. 2a, it can be asserted that both increased temperature and extended time had a signi cant function in realizing the phase transformation from α to β-Yb 2 Si 2 O 7 . During the transformation process, β-Yb 2 Si 2 O 7 with nearstandard polyhedral structure generated accompanied by the evolution of structure and coordination environment for the structural units of α-Yb 2 Si 2 O 7 to (Si 2 O 7 ) groups and (YbO 6 ) polyhedrons.

Conclusions
In this work, pure β-Yb 2 Si 2 O 7 powders with nanoscale size were successfully synthesized by cocurrent chemical coprecipitation method through optimizing the Si/Yb molar ratio and calcination temperature.
With the Si/Yb molar ratio of precursor less than or equal to the standard stoichiometry of Yb 2 Si 2 O 7 , Yb 2 SiO 5 impurity forms in the powder products. In addition, elevated calcination temperature is bene cial to synthesis the desirable β-Yb 2 Si 2 O 7 powders. During the coprecipitation process, silicon oxygen ions can be absorbed around Yb 3+ and acted as ligands to form ion clusters with Yb 3+ core. Meanwhile, Yb is       FTIR spectra of Yb2Si2O7 precursors after calcination at 1000 ℃~ 1200 ℃ Figure 10 Schematic diagram of the synthesis mechanism for Yb2Si2O7