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Crystal growth and structure refinement of hollandite-type K1.59Ga1.59Ti6.41O16
Solid State Ionics 184 (2011) 74–77
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Crystal growth and structure refinement of hollandite-type K1.59Ga1.59Ti6.41O16 Kenjiro Fujimoto ⁎, Chihiro Yamakawa, Shigeru Ito Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, Yamazaki 2641, Noda, Chiba 278-8510, Japan
a r t i c l e
i n f o
Article history: Received 17 May 2010 Received in revised form 4 October 2010 Accepted 7 October 2010 Available online 15 December 2010
a b s t r a c t Needle-like hollandite-type K1.59Ga1.59Ti6.41O16 single crystals were obtained by the flux slow-cooling method. The average dimensions of a single crystal were a length of about 2 mm and a diameter of about 250 μm. Structure refinement under a newly presented constraint condition was calculated by the JANA2006 program. The final reliability factors under an anisotropic constraint were R = 0.0153 and wR = 0.0481. © 2010 Elsevier B.V. All rights reserved.
Keywords: Hollandite Single crystal Crystal growth Structure refinement
1. Introduction Hollandite-type compounds can be described by the general chemical formula AαMβM′8 − βO16 (α b 2, β b 2); where, in most cases, A = alkali or alkaline earth ions, M = di- or trivalent ions such as Ga, Al, and Fe, and M′ = tetravalent ions such as Ti and Sn. The structure has tetragonal symmetry and one-dimensional (1-D) tunnels along the c-axis, as indicated by Fig. 1. The 1-D tunnels are constructed by double chains of (M, M′)O6 edge-shared octahedra. The A ions, meanwhile, are located in the vacancies of the 1-D tunnels. From these features, hollandite-type compounds have hitherto been studied as one-dimensional first ion conductors of alkali ions [1] and as nuclear waste immobilizers [2]. Watanabe et al. have reported that hollandite-type compounds have a promising NOx selective catalytic reduction property with the use of hydrocarbons [3]. They evaluated the catalytic property in a helium carrier gas flowing at a space velocity of 2500 h− 1 with constituent reactant species such as NO, C3H6, and O2. These reactant gases were diluted to 1200 ppm, 800 ppm and 4%, respectively. As shown in Table 1, the NOx conversion rates of KxGaxTi8 − xO16 (x ~ 1.6, KGTO) and KyGaySn8 − yO16(y ~ 2, KGSO) were 10% [3] and 40% [4], respectively, at 623 K. These catalytic properties were observed in the absence of doping noble metals. This suggests that it may be possible to design new materials once the reaction mechanism is clarified in greater detail. Our group has made headway in this direction by evaluating NO adsorption–desorption on the crystal surface [5] and the structure refinement in KGSO [6]. To better understand the NO
⁎ Corresponding author. Tel.: + 81 4 7124 1501x3648; fax: + 81 4 7123 9890. E-mail address: [email protected] (K. Fujimoto). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.10.018
absorption–desorption mechanism and the NOx selective catalytic reduction property, we need to examine the crystal growth and structure refinement of KGTO in comparison with KGSO. In this study, we grew single crystals of KxGaxTi8 − xO16 (KGTO) hollandite-type compound by the flux slow-cooling method and refined the crystal structure by single crystal X-ray diffraction.
2. Experimental Hollandite single crystals were grown by the conventional flux slow-cooling method in K2CO3–MoO3 melt. The starting materials were K2CO3 (3 N in purity, Rare Metallic Co.), Ga2O3 (4 N, High Purity Chemicals Co.), TiO2 (4 N, Rare Metallic Co.), and MoO3 (3 N, High Purity Chemicals Co.). The mixture of starting materials consisted of (K2CO3)1(Ga2O3)1(TiO2)8 as crystal composition “C” and (K2CO3)1 (MoO3)1 as flux composition “F.” The mixed molar ratio of C and F was C/F = 20/80, and the total weight of the mixture was 5 g. A 15 ml platinum crucible tightly covered with a fitted lid was used in all runs. The crucible was filled with starting materials, heated at 1623 K for 12, slowly cooled to 1023 K at a rate of 4 K, rapidly cooled to room temperature, and soaked in hot water at about 353 K to solve the flux composite into single crystal aggregates. The compounds obtained were identified by powder X-ray diffractometer (Rigaku Mini Flex). The shapes of single crystals were observed by scanning electron microscopy (SEM, JEOL JSM-5500). The chemical composition was evaluated by the ICP emission spectrometry method. The X-ray diffracted intensity of the KxGaxTi8 − xO16 single crystal identified (0.15 × 0.15 × 0.5 nm) was measured by Bruker AXS SMART APEX with applying MoKα radiation. The structure refinement was performed by the JANA2006 program [7].
K. Fujimoto et al. / Solid State Ionics 184 (2011) 74–77
75
Fig. 3. Powder X-ray diffraction pattern of K1.59Ga1.59Ti6.41O16 single crystal aggregate.
has two sites when the x value in the chemical formula is less than two, as in the case of K1.59Ga1.59Ti6.41O16 (x ~ 1.59). Initially, the K1-
Fig. 1. Crystal structure of hollandite-type compounds.
3. Results and discussion 3.1. Crystal growth and structure refinement of KxGaxTi8 − xO16 The average size of the needle-like crystal was about 2 mm in length and 250 μm in diameter. Fig. 2 shows an SEM image of the obtained crystal. Fig. 3 plots the powder X-ray diffraction pattern of a ground single crystal aggregate. From these results, we identified the single crystal obtained as a hollandite-type structure. The chemical composition was calculated as K1.59Ga1.59Ti6.41O16. We used the constraint condition proposed by Michiue [8] to refine the structure (see Fig. 4(a)). This constraint condition suggests that the guest ion
Table 1 NO conversion rates of KGTO and KGSO.
Calcination temperature Specific surface NO conversion rate (at 623 K)
KGTO [3]
KGSO [4]
1573 K 1–2 m2g− 1 10%
1648 K 1 m2g− 1 40% Fig. 4. (a) Schematic diagram of the K1- and K2-sites in the hollandite tunnel. (b) Diagram of the K2-site located nearest the surface.
Table 2 Crystallographic data and data collection conditions for KxGaxTi8 − xO16 (x ~ 1.6). Molecular weight Crystal system Space group a/nm c/nm V/nm3 Z Dx/g cm− 3 μ(MoKα)(mm− 1) Crystal size/mm3 Color Radiation Range of h, k, l
Fig. 2. SEM image of hollandite-type K1.59Ga1.59Ti6.41O16.
Data collection temperature/K Time of exposure R/Rw
735.9 Tetragonal I4/m 1.0111(2) 0.29630(5) 0.3028(1) 1 4.0329 8.123 0.15 × 0.15 × 0.5 Yellow MoKα (0.071069 nm) − 13 ≤ h ≤ 13, − 13 ≤ k ≤ 12, −3≤l≤3 300 30 s/frame 0.0153/0.0481
76
K. Fujimoto et al. / Solid State Ionics 184 (2011) 74–77
Table 3 Site occupancies, atomic coordinates, and atomic displacement parameters of KxGaxTi8 − xO16 (x ~ 1.6). Atom
Occupancy
x
y
z
Ueq × 10− 3 (nm2)
K1 K2 Ga1 Ti1 O1 O2
0.44(2) 0.185(8) 0.204(2) 0.796(2) 1 1
0 0 0.167070(18) 0.167070(18) 0.20444(10) 0.16536(11)
0 0 0.351579(18) 0.351579(18) 0.15487(9) 0.54079(9)
0.5 0.725(6) 0 0 0 0
0.52(3) 0.40(2) 0.0742(18) 0.0742(18) 0.068(3) 0.075(3)
Atom
U11 × 10− 3 (nm2)
U22 × 10− 3 (nm2)
U33 × 10− 3 (nm2)
U12 × 10− 3 (nm2)
U13 × 0− 3 (nm2)
U23 × 10− 3 (nm2)
K1 K2 Ga1 Ti1 O1 O2
0.321(17) 0.104(13) 0.084(3) 0.084(3) 0.066(6) 0.093(5)
0.321(17) 0.104(13) 0.070(3) 0.070(3) 0.063(6) 0.060(6)
0.91(8) 0.99(6) 0.069(3) 0.069(3) 0.074(5) 0.073(6)
0 0 0.0095(6) 0.0095(6) 0.005(4) 0.005(3)
0 0 0 0 0 0
0 0 0 0 0 0
site was set in a special position in the center of the bottleneck. Next, the K2-site was set in a general position where it would shift along the c-axis from the bottleneck center. These two constraint conditions have often been used for normal structure refinement in hollanditetype compounds. Next, we used a third constraint condition proposed by Michiue, where there were two K2-sites per vacancy and each K2-site shifted slightly to a neighboring vacancy. Finally, we applied a fourth condition where the summation of the K1- and K2-site occupancy was equal to the Ga-site (M-site) occupancy. Tables 2 and 3 show the crystallographic data for data collection and the results of the structure refinement, respectively. The final reliability factors under an anisotropic constraint conditions were R = 0.0153 and wR= 0.0481. The final reliability factors under the earlier constraint conditions were almost the same as those under the new constraint conditions. However, the site occupancies of Ga and Ti yielded logical results under the new constraint conditions and corresponded to the results of the chemical composition analysis, calculated as K1.63Ga1.63Ti6.37O16. 3.2. Comparison of KGTO and KGSO As mentioned in the Introduction, the NO conversion rates of KGTO and KGSO were thought to be dependent on the position of the K-site near the surface. Table 4 shows the lattice parameters of KGTO and KGSO. The differences in the lattice parameters presumably resulted from the ionic radii of Sn and Ti at the center of the octahedral sites (Sn4+: 0.690 nm N Ti4+: 0.605 nm). The K-site located nearest the surface was therefore assumed to be the K2-site, as shown in Fig. 4(b). Under this condition, the distances between the K2-site and surface Table 4 Lattice parameters and potassium atomic coordinates of KGTO and KGSO.
b Lattice parameterN a/nm c/nm V/nm3 bK-siteN K1/K2
x y z
KGTO
KGSO [9]
1.0111(2) 0.29630(5) 0.3028(1)
1.0389(2) 0.3132(2) 0.33380(2)
0 0 0.5/0.725(6)
x y z
0 0 0.5/0.748(18)
Table 5 Dependence of the K2-site and NOx conversion rate. KGTO Distance between the K2-site and surface Probability that the K ion exists NO conversion rate
0.785 nm 75% 10% [3]
KGSO N b b
0.766 nm [9] 95% [9] 40% [4]
were calculated as 0.785 nm for KGTO and 0.766 nm for KGSO [9], respectively. And based on the chemical formula shown in Table 5, the probabilities that potassium ions existed at the K2-site were calculated as 75% (KGTO) and 90% (KGSO). In a discussion of the adsorption property of nitrogen oxide on the hollandite surface, Fujimoto et al. proposed that the amount of adsorption and the catalytic property of the NOx selective reduction were due to the potassium ions located nearest the surface [5]. In the present study, we found a correlation between the chemical composition and atomic coordinates in the hollandite structure with two K-sites. In the future we will need to prove the correlations among the crystal structure, NO absorption, and NOx selective catalytic reduction property through evaluations by temperature-programmed desorption and diffuse reflectance infrared spectrometry. Table 6 shows the bond lengths between the respective atoms in KGTO and KGSO. Every length for KGSO is 3–5.7% longer than that for KGTO, and element substitution reveals no local distortions in the crystal structure. The homogeneous lattice expansion was due to the ionic radii of Sn and Ti in the octahedral sites (Sn4+: 0.690 nm N Ti4+: 0.605 nm) and the number of potassium ions.
4. Conclusion K1.59Ga1.59Ti6.41O16 hollandite single crystals were grown from a K2CO3–MoO3 flux melt by the slow-cooling method. The average dimensions of the needle-like crystals obtained were a length of about 2 mm and a diameter of about 250 μm. The structure was refined under the constraint condition proposed by Michiue [8]. The final reliability factors under an anisotropic condition were R = 0.0153 and wR = 0.0481. By comparing the structures of KGTO and KGSO, we found that the distance between the surface and K2-site was shorter in KGSO than in KGTO. This result may help to prove the correlations Table 6 Interatomic distance for octahedral coordination in KGTO and KGSO (M = Ga/Ti or Ga/Sn).
M–O1 M–O1′ M–O2 M–O2′ Average O1–O1′ O1–O2′ O2–O1′ O2–O2′ O1′–O1′ O1′–O2′ Average
KGTO (nm)
KGSO (nm) [9]
KGSO KGTO
0.20245(9) 0.19718(7) 0.19132(10) 0.19635(6) 0.19678 0.25972(10) 0.28701(12) 0.28004(11) 0.28508(12) 0.29636 0.25782(14) 0.28247
0.20854(6) 0.20754(10) 0.19947(10) 0.20286(6) 0.20496 0.27327(11) 0.29682(12) 0.29141(12) 0.29396(13) 0.31331(1) 0.26599(14) 0.28913
3.0 5.3 4.3 3.3 5.2 3.4 4.2 3.1 5.7 3.2
× 100 (%)
K. Fujimoto et al. / Solid State Ionics 184 (2011) 74–77
among the crystal structure, NO absorption, and NOx selective catalytic reduction property in the future. References [1] S. Yoshikado, I. Taniguchi, M. Watanabe, Y. Onoda, Y. Fujiki, Solid State Ionics 79 (1995) 34. [2] H. Abe, A. Sato, K. Nishida, E. Abe, T. Naka, M. Imai, H. Kitazawa, J. Solid State Chem. 179 (2006) 1521.
77
[3] M. Watanabe, T. Mori, S. Yamauchi, H. Yamamura, Solid State Ionics 79 (1995) 376. [4] T. Mori, S. Yamauchi, H. Yamamura, M. Watanabe, Appl. Catal. A 129 (1995)8 L1. [5] K. Fujimoto, J. Suzuki, M. Harada, S. Awatsu, T. Mori, M. Watanabe, Solid State Ionics 152–153 (2002) 769. [6] K. Fujimoto, S. Ito, M. Watanabe, Solid State Ionics 177 (2006) 1901. [7] V. Petricek, M. Dusek, L. Palatinus, The Crystallographic Computing System JANA2006, Praha, Czech Republic, Institute de Physics, 2006. [8] Y. Michiue, J. Solid State Chem. 180 (2007) 184. [9] K. Fujimoto, private communication.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Crystal growth and structure refinement of hollandite-type K1.59Ga1.59Ti6.41O16 Kenjiro Fujimoto ⁎, Chihiro Yamakawa, Shigeru Ito Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, Yamazaki 2641, Noda, Chiba 278-8510, Japan
a r t i c l e
i n f o
Article history: Received 17 May 2010 Received in revised form 4 October 2010 Accepted 7 October 2010 Available online 15 December 2010
a b s t r a c t Needle-like hollandite-type K1.59Ga1.59Ti6.41O16 single crystals were obtained by the flux slow-cooling method. The average dimensions of a single crystal were a length of about 2 mm and a diameter of about 250 μm. Structure refinement under a newly presented constraint condition was calculated by the JANA2006 program. The final reliability factors under an anisotropic constraint were R = 0.0153 and wR = 0.0481. © 2010 Elsevier B.V. All rights reserved.
Keywords: Hollandite Single crystal Crystal growth Structure refinement
1. Introduction Hollandite-type compounds can be described by the general chemical formula AαMβM′8 − βO16 (α b 2, β b 2); where, in most cases, A = alkali or alkaline earth ions, M = di- or trivalent ions such as Ga, Al, and Fe, and M′ = tetravalent ions such as Ti and Sn. The structure has tetragonal symmetry and one-dimensional (1-D) tunnels along the c-axis, as indicated by Fig. 1. The 1-D tunnels are constructed by double chains of (M, M′)O6 edge-shared octahedra. The A ions, meanwhile, are located in the vacancies of the 1-D tunnels. From these features, hollandite-type compounds have hitherto been studied as one-dimensional first ion conductors of alkali ions [1] and as nuclear waste immobilizers [2]. Watanabe et al. have reported that hollandite-type compounds have a promising NOx selective catalytic reduction property with the use of hydrocarbons [3]. They evaluated the catalytic property in a helium carrier gas flowing at a space velocity of 2500 h− 1 with constituent reactant species such as NO, C3H6, and O2. These reactant gases were diluted to 1200 ppm, 800 ppm and 4%, respectively. As shown in Table 1, the NOx conversion rates of KxGaxTi8 − xO16 (x ~ 1.6, KGTO) and KyGaySn8 − yO16(y ~ 2, KGSO) were 10% [3] and 40% [4], respectively, at 623 K. These catalytic properties were observed in the absence of doping noble metals. This suggests that it may be possible to design new materials once the reaction mechanism is clarified in greater detail. Our group has made headway in this direction by evaluating NO adsorption–desorption on the crystal surface [5] and the structure refinement in KGSO [6]. To better understand the NO
⁎ Corresponding author. Tel.: + 81 4 7124 1501x3648; fax: + 81 4 7123 9890. E-mail address: [email protected] (K. Fujimoto). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.10.018
absorption–desorption mechanism and the NOx selective catalytic reduction property, we need to examine the crystal growth and structure refinement of KGTO in comparison with KGSO. In this study, we grew single crystals of KxGaxTi8 − xO16 (KGTO) hollandite-type compound by the flux slow-cooling method and refined the crystal structure by single crystal X-ray diffraction.
2. Experimental Hollandite single crystals were grown by the conventional flux slow-cooling method in K2CO3–MoO3 melt. The starting materials were K2CO3 (3 N in purity, Rare Metallic Co.), Ga2O3 (4 N, High Purity Chemicals Co.), TiO2 (4 N, Rare Metallic Co.), and MoO3 (3 N, High Purity Chemicals Co.). The mixture of starting materials consisted of (K2CO3)1(Ga2O3)1(TiO2)8 as crystal composition “C” and (K2CO3)1 (MoO3)1 as flux composition “F.” The mixed molar ratio of C and F was C/F = 20/80, and the total weight of the mixture was 5 g. A 15 ml platinum crucible tightly covered with a fitted lid was used in all runs. The crucible was filled with starting materials, heated at 1623 K for 12, slowly cooled to 1023 K at a rate of 4 K, rapidly cooled to room temperature, and soaked in hot water at about 353 K to solve the flux composite into single crystal aggregates. The compounds obtained were identified by powder X-ray diffractometer (Rigaku Mini Flex). The shapes of single crystals were observed by scanning electron microscopy (SEM, JEOL JSM-5500). The chemical composition was evaluated by the ICP emission spectrometry method. The X-ray diffracted intensity of the KxGaxTi8 − xO16 single crystal identified (0.15 × 0.15 × 0.5 nm) was measured by Bruker AXS SMART APEX with applying MoKα radiation. The structure refinement was performed by the JANA2006 program [7].
K. Fujimoto et al. / Solid State Ionics 184 (2011) 74–77
75
Fig. 3. Powder X-ray diffraction pattern of K1.59Ga1.59Ti6.41O16 single crystal aggregate.
has two sites when the x value in the chemical formula is less than two, as in the case of K1.59Ga1.59Ti6.41O16 (x ~ 1.59). Initially, the K1-
Fig. 1. Crystal structure of hollandite-type compounds.
3. Results and discussion 3.1. Crystal growth and structure refinement of KxGaxTi8 − xO16 The average size of the needle-like crystal was about 2 mm in length and 250 μm in diameter. Fig. 2 shows an SEM image of the obtained crystal. Fig. 3 plots the powder X-ray diffraction pattern of a ground single crystal aggregate. From these results, we identified the single crystal obtained as a hollandite-type structure. The chemical composition was calculated as K1.59Ga1.59Ti6.41O16. We used the constraint condition proposed by Michiue [8] to refine the structure (see Fig. 4(a)). This constraint condition suggests that the guest ion
Table 1 NO conversion rates of KGTO and KGSO.
Calcination temperature Specific surface NO conversion rate (at 623 K)
KGTO [3]
KGSO [4]
1573 K 1–2 m2g− 1 10%
1648 K 1 m2g− 1 40% Fig. 4. (a) Schematic diagram of the K1- and K2-sites in the hollandite tunnel. (b) Diagram of the K2-site located nearest the surface.
Table 2 Crystallographic data and data collection conditions for KxGaxTi8 − xO16 (x ~ 1.6). Molecular weight Crystal system Space group a/nm c/nm V/nm3 Z Dx/g cm− 3 μ(MoKα)(mm− 1) Crystal size/mm3 Color Radiation Range of h, k, l
Fig. 2. SEM image of hollandite-type K1.59Ga1.59Ti6.41O16.
Data collection temperature/K Time of exposure R/Rw
735.9 Tetragonal I4/m 1.0111(2) 0.29630(5) 0.3028(1) 1 4.0329 8.123 0.15 × 0.15 × 0.5 Yellow MoKα (0.071069 nm) − 13 ≤ h ≤ 13, − 13 ≤ k ≤ 12, −3≤l≤3 300 30 s/frame 0.0153/0.0481
76
K. Fujimoto et al. / Solid State Ionics 184 (2011) 74–77
Table 3 Site occupancies, atomic coordinates, and atomic displacement parameters of KxGaxTi8 − xO16 (x ~ 1.6). Atom
Occupancy
x
y
z
Ueq × 10− 3 (nm2)
K1 K2 Ga1 Ti1 O1 O2
0.44(2) 0.185(8) 0.204(2) 0.796(2) 1 1
0 0 0.167070(18) 0.167070(18) 0.20444(10) 0.16536(11)
0 0 0.351579(18) 0.351579(18) 0.15487(9) 0.54079(9)
0.5 0.725(6) 0 0 0 0
0.52(3) 0.40(2) 0.0742(18) 0.0742(18) 0.068(3) 0.075(3)
Atom
U11 × 10− 3 (nm2)
U22 × 10− 3 (nm2)
U33 × 10− 3 (nm2)
U12 × 10− 3 (nm2)
U13 × 0− 3 (nm2)
U23 × 10− 3 (nm2)
K1 K2 Ga1 Ti1 O1 O2
0.321(17) 0.104(13) 0.084(3) 0.084(3) 0.066(6) 0.093(5)
0.321(17) 0.104(13) 0.070(3) 0.070(3) 0.063(6) 0.060(6)
0.91(8) 0.99(6) 0.069(3) 0.069(3) 0.074(5) 0.073(6)
0 0 0.0095(6) 0.0095(6) 0.005(4) 0.005(3)
0 0 0 0 0 0
0 0 0 0 0 0
site was set in a special position in the center of the bottleneck. Next, the K2-site was set in a general position where it would shift along the c-axis from the bottleneck center. These two constraint conditions have often been used for normal structure refinement in hollanditetype compounds. Next, we used a third constraint condition proposed by Michiue, where there were two K2-sites per vacancy and each K2-site shifted slightly to a neighboring vacancy. Finally, we applied a fourth condition where the summation of the K1- and K2-site occupancy was equal to the Ga-site (M-site) occupancy. Tables 2 and 3 show the crystallographic data for data collection and the results of the structure refinement, respectively. The final reliability factors under an anisotropic constraint conditions were R = 0.0153 and wR= 0.0481. The final reliability factors under the earlier constraint conditions were almost the same as those under the new constraint conditions. However, the site occupancies of Ga and Ti yielded logical results under the new constraint conditions and corresponded to the results of the chemical composition analysis, calculated as K1.63Ga1.63Ti6.37O16. 3.2. Comparison of KGTO and KGSO As mentioned in the Introduction, the NO conversion rates of KGTO and KGSO were thought to be dependent on the position of the K-site near the surface. Table 4 shows the lattice parameters of KGTO and KGSO. The differences in the lattice parameters presumably resulted from the ionic radii of Sn and Ti at the center of the octahedral sites (Sn4+: 0.690 nm N Ti4+: 0.605 nm). The K-site located nearest the surface was therefore assumed to be the K2-site, as shown in Fig. 4(b). Under this condition, the distances between the K2-site and surface Table 4 Lattice parameters and potassium atomic coordinates of KGTO and KGSO.
b Lattice parameterN a/nm c/nm V/nm3 bK-siteN K1/K2
x y z
KGTO
KGSO [9]
1.0111(2) 0.29630(5) 0.3028(1)
1.0389(2) 0.3132(2) 0.33380(2)
0 0 0.5/0.725(6)
x y z
0 0 0.5/0.748(18)
Table 5 Dependence of the K2-site and NOx conversion rate. KGTO Distance between the K2-site and surface Probability that the K ion exists NO conversion rate
0.785 nm 75% 10% [3]
KGSO N b b
0.766 nm [9] 95% [9] 40% [4]
were calculated as 0.785 nm for KGTO and 0.766 nm for KGSO [9], respectively. And based on the chemical formula shown in Table 5, the probabilities that potassium ions existed at the K2-site were calculated as 75% (KGTO) and 90% (KGSO). In a discussion of the adsorption property of nitrogen oxide on the hollandite surface, Fujimoto et al. proposed that the amount of adsorption and the catalytic property of the NOx selective reduction were due to the potassium ions located nearest the surface [5]. In the present study, we found a correlation between the chemical composition and atomic coordinates in the hollandite structure with two K-sites. In the future we will need to prove the correlations among the crystal structure, NO absorption, and NOx selective catalytic reduction property through evaluations by temperature-programmed desorption and diffuse reflectance infrared spectrometry. Table 6 shows the bond lengths between the respective atoms in KGTO and KGSO. Every length for KGSO is 3–5.7% longer than that for KGTO, and element substitution reveals no local distortions in the crystal structure. The homogeneous lattice expansion was due to the ionic radii of Sn and Ti in the octahedral sites (Sn4+: 0.690 nm N Ti4+: 0.605 nm) and the number of potassium ions.
4. Conclusion K1.59Ga1.59Ti6.41O16 hollandite single crystals were grown from a K2CO3–MoO3 flux melt by the slow-cooling method. The average dimensions of the needle-like crystals obtained were a length of about 2 mm and a diameter of about 250 μm. The structure was refined under the constraint condition proposed by Michiue [8]. The final reliability factors under an anisotropic condition were R = 0.0153 and wR = 0.0481. By comparing the structures of KGTO and KGSO, we found that the distance between the surface and K2-site was shorter in KGSO than in KGTO. This result may help to prove the correlations Table 6 Interatomic distance for octahedral coordination in KGTO and KGSO (M = Ga/Ti or Ga/Sn).
M–O1 M–O1′ M–O2 M–O2′ Average O1–O1′ O1–O2′ O2–O1′ O2–O2′ O1′–O1′ O1′–O2′ Average
KGTO (nm)
KGSO (nm) [9]
KGSO KGTO
0.20245(9) 0.19718(7) 0.19132(10) 0.19635(6) 0.19678 0.25972(10) 0.28701(12) 0.28004(11) 0.28508(12) 0.29636 0.25782(14) 0.28247
0.20854(6) 0.20754(10) 0.19947(10) 0.20286(6) 0.20496 0.27327(11) 0.29682(12) 0.29141(12) 0.29396(13) 0.31331(1) 0.26599(14) 0.28913
3.0 5.3 4.3 3.3 5.2 3.4 4.2 3.1 5.7 3.2
× 100 (%)
K. Fujimoto et al. / Solid State Ionics 184 (2011) 74–77
among the crystal structure, NO absorption, and NOx selective catalytic reduction property in the future. References [1] S. Yoshikado, I. Taniguchi, M. Watanabe, Y. Onoda, Y. Fujiki, Solid State Ionics 79 (1995) 34. [2] H. Abe, A. Sato, K. Nishida, E. Abe, T. Naka, M. Imai, H. Kitazawa, J. Solid State Chem. 179 (2006) 1521.
77
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