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Yin Guangchao, Yin Hong, Sun Meiling, Gao Wei. Pressure-induced structural evolution of apatite-type La9.33Si6O26 . Chinese Physics B, 2018, 27(1): 018202  
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Pressure-induced structural evolution of apatite-type La9.33Si6O26
Yin Guangchao1, Yin Hong2, Sun Meiling1, Gao Wei2, †
Functional Molecular Materials Laboratory, School of Physics and Optoelectronic Engineering, Shandong University of Technology, Zibo 255000, China
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

 

† Corresponding author. E-mail: gwei@jlu.edu.cn

Abstract

The pressure-induced structural evolution of apatite-type La9.33Si6O26 was systematically studied using in situ synchrotron x-ray diffraction (XRD). The XRD spectra indicated that a subtly reversible phase transition from P63/m to P63 symmetry occurred at ∼13.6 GPa because of the tilting of the SiO4 tetrahedra under compression. Furthermore, the La9.33Si6O26 exhibited a higher axial compression ratio for the a-axis than the c-axis, owing to the different axial arrangement of the SiO4 tetrahedra. Interestingly, the high-pressure phase showed compressibility unusually higher than that of the initial phase, suggesting that the low P63 symmetry provided more degrees of freedom. Moreover, the La9.33Si6O26 exhibited a lower phase transition pressure ( ) and a higher lattice compression than La10Si6O27. Comparisons between La9.33Si6O26 and La10Si6O27 provided a deeper understanding of the effect of interstitial oxygen atoms on the structural evolution of apatite-type lanthanum silicates (ATLSs).

PACS: 82.45.Gj;61.05.cp;81.40.Vw
Keyword:lanthanum silicates;structural evolution;interstitial oxygen atoms;compressibility
1. Introduction

Apatite-type lanthanum silicates (ATLSs) with the general formula La9.33 +x(SiO4)6O2 + 1.5x ( ) have been gaining widespread attention owing to their potential applications in gas-sensing and fuel cell devices.[1–16] ATLSs have an especially hexagonal apatite structure (Fig. 1), which is beneficial for conducting oxygen ions.[17–19] In this hexagonal structure, isolated SiO4 tetrahedra are piled up along the c-axis to form unique oxide ion channels, and there are four oxygen sites (O1, O2, O3, and O4) in the structure when the La composition is stoichiometric (i.e., x = 0). When , a fifth oxygen site (i.e., interstitial oxygen, O5) emerges and is located close to the SiO4 tetrahedra, which can also play a crucial role in conducting oxygen ions and improving the structural properties.[10,16] Prior research studies have indicated that the apatite-type structure is very flexible and sensitive to external forces.[20–27] However, most investigations to date have focused on the electrical properties of ATLSs; hence, the lack of systematic studies on the structural evolution of ATLSs under applied external forces.

Fig. 1. (color online) Illustration of the “microporous” structure of ATLS.

Recently, we used in situ synchrotron x-ray diffraction (XRD) to investigate ATLS under high pressure to investigate the structural evolution and corresponding structural formation/alteration mechanism of ATLS under external forces.[28,29] We found that the flexibility of the apatite structure was mainly dominated by the isolated SiO4 tetrahedra and that the compressibility was mainly dominated by the La–O polyhedral environment.[29] Moreover, we found that for the ATLS, similar to what happens in scheelite-type ABO4 compounds, doping the Si sites with a larger cation (e.g., Mg2+) increased the ratio, thus enhancing the and lowering the axial compressibility of the ATLS.[28] However, previous studies mainly focused on ATLS with interstitial oxygen atoms, so the detailed structural alteration mechanism of ATLS has not been rigorously determined because of the lack of comparative data to illuminate the effect of interstitial oxygen atoms on the structural evolution of ATLS under compression. Therefore, it is crucial to study in detail the behaviors of stoichiometric oxygen-interstitial-free ATLS (La9.33Si6O26) under high pressure to gain insight into the structural properties of ATLS.

In the present study, the pressure-induced structural evolution of apatite-type La9.33Si6O26 was systematically investigated using in situ synchrotron XRD. A subtly reversible phase transition was observed at ∼13.6 GPa. The pressure dependence of the cell parameters a, c, V, and the bulk modulus were systematically analyzed. Moreover, the behavior of La9.33Si6O26 under a high pressure was compared with those of La10Si6O27.

2. Experimental

Apatite-type La9.33Si6O26 powders were synthesized by the simple NaCl molten-salt method outlined in Ref. [10]. Figure 2 shows that the La9.33Si6O26 powders used here showed a pure apatite-type structure (Fig. 2(a)), consisting of homogeneous 100–200-nm-diameter single crystals (Figs. 2(b)–2(d)). The high-pressure synchrotron XRD experiments were performed at the high-pressure station of the Beijing Synchrotron Radiation Facility (BSRF) with a synchrotron x-ray source of λ = 0.6199 Å. A symmetric diamond anvil cell (DAC) with 400- -culet diamond anvils was utilized to generate high pressure. A 120- -diameter hole was drilled in the center of T-301 stainless steel to prepare the sample chamber. Small ruby balls were inserted into the sample for in situ pressure calibration, and silicone grease was used as the pressure medium. The high-pressure data were collected and analyzed by a MAR165 CCD detector and FIT2D software, respectively.[30]

Fig. 2. (color online) (a) XRD, (b) SEM, (c) TEM, (d) HRTEM, and SAED (the inset of panel (d)) patterns of the La9.33Si6O26 powders.
3. Results and discussion

Representative high-pressure XRD patterns for the La9.33Si6O26 are displayed in Fig. 3. All the diffraction peaks typically weaken and shift toward higher angles with increasing pressure.[31–33] When the La9.33Si6O26 is compressed to ∼13.6 GPa, two new diffraction peaks emerge, indicating a phase transition occurs. When the pressure is increased to ∼15.9 GPa, the new peaks completely replace the original ones, implying that the phase transition is complete. Notably, the phase transition is reversible; that is, the initial phase is recovered upon decompression. Compared with La10Si6O27 ( ),[29] the La9.33Si6O26 exhibits a slightly lower , which could be related to the absence of interstitial oxygen atoms in the La9.33Si6O26. According to previous studies,[28,34] increases with increasing ratio, where and represent the radii of polyhedral SiOx units and La cations, respectively. The La10Si6O27 contains the interstitial oxygen (O5) sites close to the SiO4 tetrahedra, facilitating the formation of hexahedral SiO5 units distinctly larger than the tetrahedral SiO4 units.[35,36] As a result, the La9.33Si6O26 exhibits a slightly lower than the La10Si6O27, indicating that the interstitial oxygen atoms could increase the of the ATLS.

Fig. 3. (color online) Representative high-pressure XRD patterns of La9.33Si6O26 at various pressures.

To confirm the phase transformation mechanism, both the initial and high-pressure phases are refined by Rietveld analysis. According to previous studies, a refined structural model showing the symmetry and containing seven independent sites in the unit cell is used to fit the initial phase, and a refined structural model showing the P6 3 symmetry and containing nine independent sites in the unit cell is used to fit the high-pressure phase[28,29,37] (see Table 1). The refined structural models both significantly refine the initial (1.8 GPa) and high-pressure (15.9 GPa) phases, respectively, as shown in Figs. 4(a) and 4(b). Furthermore, the relevant Rietveld refinement parameters shown in Table 1 are reasonable,[38] confirming that the phase transition is attributed to the reduction in symmetry during the transition from the phase to the P6 3 one. In addition, the refined crystal structures of the La9.33Ge6O26 at 1.8 and 15.9 GPa are shown in the insets of Figs. 4(a) and 4(b), respectively. The SiO4 tetrahedra displays obvious tilting, and the O3 oxygen sites split into two different 6c sites (O3a and O3b) during the phase transition, which could account for the reduction in symmetry during the transition from the phase to the P6 3 one. The detailed structural data are listed in Table 1.

Fig. 4. (color online) Refined XRD patterns of La9.33Si6O26 at (a) 1.8 and (b) 15.9 GPa and the corresponding refined crystal structures (the insets of (a) and (b)).
Table 1.

Refined structural data of La9.33Si6O26 at 1.8 and 15.9 GPa.

.

To further understand the structural evolution of the La9.33Si6O26 under high pressure, P–a, P–c, P–c/a, and P–V are depicted in Fig. 5. The cell parameter a continuously varies during the phase transition (Fig. 5(a)), but the cell parameter c displays discontinuous variation with ∼1.9% c-axis shrinkage (Fig. 5(b)) during the phase transition, leading to a % collapse in volume (Fig. 5(d)). Furthermore, the c/a ratios of both phases gradually increase with increasing pressure (Fig. 5(c)), indicating that the compressibility of the a-axis is higher than that of the c-axis and is related to the different arrangements of rigid SiO4 tetrahedra along the a- and c-axes. In ATLS, the rigid SiO4 units are directly piled along the c-axis, whereas there is a La–O polyhedral environment between the rigid SiO4 tetrahedra along the a-axis. As a result, the c-axis is less compressible than the a-axis. In addition, compared with the axial compressible ratios of La10Si6O27, those of La9.33Si6O26 in both phases are slightly higher (as shown in Table 2), which could be related to the absence of interstitial oxygen atoms in the La9.33Si6O26. According to previous studies,[29,39] compressibility is mainly dominated by the La–O polyhedral environment. In La10Si6O27, the interstitial oxygen atoms could form larger hexahedral SiO5 units,[35,36] compressing the La–O polyhedral environment thereby making it less compressible and further indicating that both the a- and c-axes of the La10Si6O27 are slightly less compressive than those of the La9.33Si6O26.

Fig. 5. (color online) Refined (a) P–a, (b) P–c, (c) P–c/a, and (d) P–V of La9.33Si6O26.
Table 2.

Axial compressibility ratios of La9.33Si6O26 and La10Si6O27 in the initial and high-pressure phases.

.

In addition, we fit the experimental pressure-volume data with the third-order Birch–Murnaghan equation of state (EOS) where V0 is the volume per formula unit under normal pressure, V is the volume per formula unit at pressure P (in GPa), B0 is the isothermal bulk modulus, and is the first pressure derivative of the bulk modulus. Here, = 4.74 is obtained from the – data of the initial phase (where is the normalized stress and is the finite strain).[40,41] As shown in Fig. 5(d), the bulk modulus, B0, of the initial phase (120.7±5.7 GPa) is unusually higher than that of high-pressure phase (107.8±1.6 GPa), implying that the high-pressure phase becomes more compressive than the initial phase and could be owing to the reduction in symmetry that occurs during the transition from the high-symmetry phase to the low-symmetry P6 3 one. According to previous studies of ATLS under high pressure,[28,29] the low symmetry ( phase could afford more degrees of freedom to accommodate for the tilting of the SiO4 tetrahedra than the high symmetry ( phase could. Moreover, the bulk moduli of both the initial (120.7±5.7GPa) and high-pressure (107.8±1.6 GPa) phases for La9.33Si6O26 are lower than those of both the initial (128.3±1.7 GPa) and high-pressure (117.2±6.6 GPa) phases for La10Si6O27,[29] which also could be attributed to the absence of interstitial oxygen atoms in the La9.33Si6O26. Similar to how interstitial oxygen atoms affect the compressive behaviors of the a- and c-axes, interstitial oxygen atoms could make the La–O polyhedral environment less compressible; thus, both phases of the La9.33Si6O26 display a lower B0 than the La10Si6O27.

4. Conclusions

The structural evolutions of apatite-type La9.33Si6O26 under high pressure were systematically investigated up to 31.8 GPa, using in situ synchrotron XRD. A subtly reversible phase transition occurred at ∼13.6 GPa, which was ascribed to the reduction in symmetry during the transition from the phase to the P6 3 phase, induced by the tilting of the SiO4 tetrahedra. Furthermore, the high-pressure phase became more compressive than the initial phase, because its lower symmetry affords more degrees of freedom to accommodate for the tilting of the SiO4 tetrahedra. Moreover, the La9.33Si6O26 exhibited a lower and a higher compressibility than the La10Si6O27, which could be attributed to the absence of interstitial oxygen atoms in the La9.33Si6O26. In other words, interstitial oxygen atoms could increase the and lower the compressibility of the ATLS.

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