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Oxygen ion conductivity in new oxygen deficient phases with scheelite related structures
Solid State Sciences 4 (2002) 329–333 www.elsevier.com/locate/ssscie
Oxygen ion conductivity in new oxygen deficient phases with scheelite related structures S. Uma, R. Bliesner, A.W. Sleight ∗ Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003, USA Dedicated to Martha Greenblatt
Abstract Phases of the type Bi1−2x A2x VO4−x have been prepared up to x ≈ 0.25 with A = Ca, Sr, and Cd. The monoclinic distortion of the ideal tetragonal scheelite structure disappears with increasing x. Single crystal X-ray diffraction studies were conducted for samples with A = Ca and Cd. Impedance measurements show conductivity values analogous to those observed for stabilized zirconia. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.
1. Introduction BiVO4 occurs naturally as the mineral pucherite, which has an orthorhombic structure with VO4 tetrahedra [1]. Apparently, laboratory syntheses have never given this modification. However, BiVO4 can be prepared in the tetragonal zircon structure or in a more stable form with the scheelite structure. Scheelite BiVO4 melts congruently at 940 ◦ C, and it has a phase transition at 255 ◦ C [2]. Above 255 ◦ C BiVO4 has the ideal scheelite structure in space group I41/a, below 255 ◦ C scheelite BiVO4 distorts to the monoclinic space group I2/a [2,3]. In Aizu’s terminology [4], the 255 ◦ C phase transition indicates that monoclinic BiVO4 is a pure ferroelastic. The zircon form of BiVO4 transforms irreversibly to the scheelite form on heating to about 400 ◦ C or upon prolonged grinding at room temperature [5]. Scheelite BiVO4 has been of considerable interest as a bright yellow pigment and as a selective oxidation catalyst, especially when doped to produce cation vacancies on the Bi site [6]. Significant oxygen ion conductivity is also reported for BiVO4 at high temperatures [7]. A recent study indicated mixed conductivity for BiVO4 at high temperatures with the electronic contribution being n-type [8]. Attempts have been made to increase the oxygen ion conductivity of BiVO4 by substitution of a few percent of Ca, Zr, or Ce for Bi [7–9]. Our initial interest in substitutions into BiVO4 was to explore the possibility that it might be made into a good p* Correspondence and reprints.
E-mail address: [email protected] (A.W. Sleight).
type electronic conductor by creating holes in the Bi 6s band. Compositions of the type Bi1−x Cax VO4 were attempted. We found that Ca would substitute for Bi at much higher levels than previously reported. However, both the color and the conductivity of such samples indicate that the substitution is of the type Bi1−2x Ca2x VO4−x , not Bi1−x Cax VO4 . Thus, the Ca substitution for Bi was not producing significant hole carrier concentration in the Bi 6s band. Annealing such samples under 300 bars of oxygen pressure at 500 ◦ C was not effective in changing this situation.
2. Experimental Reactants were Bi2 O3 (Cerac, 99.9%), V2 O5 (Johnson Matthey, 99.9%), CaCO3 (Mallinckrodt, 99.8%), SrCO3 (Aldrich, 98+%), CdO (Baker analyzed, 99.1%). Appropriate quantities of reactants were ground together and heated for 10 hours at 750 and 900 ◦ C. The products were examined by powder X-ray diffraction using a Siemens D5000 powder X-ray diffractometer with Cu Kα radiation. Crystals were mounted on glass fibers for collection of single crystal X-ray diffraction data. Details of the data collection are given in Tables 1 and 2. The data were collected using the ω–2θ -scan technique at a scan width of ω = (1.6 + 0.3 tan θ ). The intensities of three standard reflections measured every 150 reflections throughout data collection exhibited no significant fluctuations. The initial data processing and an absorption correction by φ-scan to the full data set were applied using the programs from the T EXSAN crystallographic software package [10]. The
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S. Uma et al. / Solid State Sciences 4 (2002) 329–333
Table 1 Crystal data and intensity collection for Bi0.78 Cd0.22 VO3.89
Table 2 Crystal data and intensity collection for Bi0.71 Ca0.29 VO3.855
Color, habit Size (mm3 ) Diffractometer Radiation Monochromator Temperature Maximum 2θ (deg) Data collected
Color, habit Size (mm3 ) Diffractometer Radiation Monochromator Temperature Maximum 2θ (deg) Data collected
Scan type Scan speed Absorption correction Transmission factors, range Crystal system Space group Unit cell dimensions (Å) Volume (Å3 ) Z Formula weight Calculated density (g cm−3 ) Absorption coefficient (mm−1 ) No. of reflections collected in primitive setting No. of unique reflections No. of observed reflections (I > 4σ (I )) Refinement method No. of parameters refined Goodness of fit on F 2 Final R indices (I > 4σ (I )) R indices (all data) Extinction coefficient Largest difference peak and hole
Yellow, plate 0.04 × 0.04 × 0.06 Rigaku AFC6R Mo Kα (λ = 0.71069 Å) Graphite 23 ◦ C 60 −7 h 7, −7 k 7, −16 l 16 ω–2θ 8 ψ-scan 1.00–0.62 Tetragonal P4¯ (No. 81) 5.125(2), 11.672(2) 306.6(1) 4 298.4 6.464 49.176 3563 900, Rint = 10.49 455 Full-matrix least-squares on F 2 38 1.035 R = 8.27%, wR2 = 22.49% R = 9.43% 0.0044(5) 10.78 e Å−3 (0.79 Å from Bi3) and −6.55 e Å−3 (0.72 Å from Bi1)
structures were then solved by direct methods and refined using S HELXS 97 incorporated in the W INGX suite [11, 12]. Final refined parameters are given in Tables 3 and 4, interatomic distances in Tables 5 and 6, and anisotropic displacement factors in Tables 7 and 8. AC conductivity measurements were therefore performed on sintered pellets of varying thickness with a diameter of 1.1 cm. The pellets were sintered at 800–900 ◦ C. Both flat surfaces of the pellets were polished and coated with Engelhard 6926 platinum paste, and then heated to 400 ◦ C to remove the organic binder from the paste. Measurements were made on a Solatron 1260 impedance analyzer computer controlled over the frequency range of 10 MHz to 1 Hz using platinum electrodes. Conductivity data were obtained on samples on heating and cooling in the range 100– 600 ◦ C, after the samples were equilibrated at the measuring temperature for approximately two hours. 3. Results Products of attempts to substitute Sr, Cd, Pb, or Na for Bi at 750◦ gave X-ray diffraction patterns dominated by normal monoclinic BiVO4 plus some second phases. However
Scan type Scan speed Absorption correction Crystal system Space group Unit cell dimensions (Å) Volume (Å3 ) Z Formula weight Calculated density (g cm−3 ) Absorption coefficient (mm−1 ) No. of reflections collected in primitive setting No. of unique reflections No. of observed reflections (I > 2σ (I )) Refinement method No. of parameters refined Goodness of fit on F 2 Final R indices (I > 4σ (I )) Extinction coefficient
Yellow, plate 0.1 × 0.1 × 0.08 Rigaku AFC6R Mo Kα (λ = 0.71069 Å) Graphite 23 ◦ C 55 −1 h 6, −1 k 6, −1 l 15 ω–2θ 8 ψ-scan Tetragonal P4¯ (No. 81) 5.121(5), 11.701(5) 306.9(4) 4 272.62 5.901 30.918 708 590, Rint = 11.56 275 Full-matrix least-squares on F 2 58 1.506 R = 7.20%, wR2 = 13.65% 0.032(7)
for Bi0.75 Ca0.25 VO3.875, a single tetragonal phase formed with a = 5.147 Å and c = 11.7216 Å. Attempts to further increase the Ca substitution gave the same tetragonal phase plus peaks due to Ca2 V2 O7 . X-ray diffraction patterns of Bi1−2x Ca2x VO4−x samples with x intermediate between 0.0 and 0.25 showed a mixture of monoclinic BiVO4 and tetragonal Bi0.75 Ca0.25 VO3.875, suggesting that Bi0.75 Ca0.25 VO3.875 might be a discrete compound. The situation was very different for many of the compositions heated to 900 ◦ C. For Bi1−2x Ca2x VO4−x samples, single-phase tetragonal products were observed for values of x from 0.10 to 0.30. Furthermore, single-phase tetragonal products were also observed for Bi0.75 Sr0.25 VO3.875 (a = 5.1515 Å, c = 11.838 Å, V = 314.2 Å3 ) and for Bi0.75 Cd0.25VO3.875 (a = 5.1187 Å, c = 11.6594 Å, V = 305.5 Å3 ). These represent a substantial cell expansion and contraction relative to scheelite BiVO4 which has a cell volume of 309.7 Å3 at room temperature. The higher synthesis temperature of 900 ◦ C did not give indications of significant substitution of Pb or Na for Bi or Ge for V. Several crystals of tetragonal for Bi1−2x Ca2x VO4−x and for Bi1−2x Cd2x VO4−x compositions with x close to 0.25 were examined by X-ray diffraction. The tetragonal symmetry was confirmed with the 5 by 11 Å unit cell, but the space group was not the I41/a of the ideal scheelite structure. Reflections were observed that violate both the a glide and the body centering. For a Bi0.78 Cd0.22VO3.89 crystal 22 reflections violating body centering had F 2 val-
S. Uma et al. / Solid State Sciences 4 (2002) 329–333
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Table 3 Positional parameters and Ueq for Bi0.78 Cd0.22 VO3.89 Atom Bi1 Cd1 Bi2 Cd2 Bi3 V1 V2 V3 O1 O2 O3 O4
Site ¯4 1a 4¯ 1a 2 2g 2 2g 4¯ 1d 4¯ 1b 4¯ 1c 2 2g 1 4h 1 4h 1 4h 1 4h
x
y
z
Occupancy
Ueq 1 (Å2 )
0 0 0.5 0.5 0.5 0 0.5 0 0.863(3) 0.140(4) 0.256(4) 0.635(3)
0 0 0 0 0.5 0 0.5 0.5 0.247(3) 0.246(4) 0.640(3) 0.258(3)
0 0 0.2504(4) 0.2504(4) 0.5 0.5 0 0.249(1) 0.580(1) 0.168(2) 0.330(2) 0.076(1)
0.78(1) 0.22 0.67(1) 0.33 1.0 1.0 1.0 1.0 1.0 0.85(4) 0.87(3) 1.0
0.0169(5) 0.0169(5) 0.0208(8) 0.0208(8) 0.0102(4) 0.0120(4) 0.008(2) 0.012(2) 0.014(3) 0.018(4) 0.016(4) 0.015(3)
1 Equivalent isotropic U defined as one third of the trace of the orthogonalized U tensor. ij
Table 4 Positional parameters and Ueq for Bi0.71 Ca0.29 VO3.855 Atom Bi1 Ca1 Bi2 Ca2 Bi3 V1 V2 V3 O1 O2 O3 O4
Site ¯4 1a 4¯ 1a 2 2g 2 2g 4¯ 1d 4¯ 1b 4¯ 1c 2 2g 1 4h 1 4h 1 4h 1 4h
x
y
z
Occupancy
Ueq 1 (Å2 )
0 0 0.5 0.5 0.5 0 0.5 0 0.132(5) 0.360(5) 0.252(5) 0.738(9)
0 0 0 0 0.5 0 0.5 0.5 0.237(5) −0.248(4) 0.142(6) 0.379(6)
0 0 0.2501(5) 0.2501(5) 0.5 0.5 0 0.249(1) −0.578(2) 0.328(2) 0.167(2) 0.069(2)
0.73(1) 0.27 0.58(1) 0.42 1.0 1.0 1.0 1.0 0.92(4) 1.0 1.0 1.0
0.023(1) 0.023(1) 0.016(1) 0.016(1) 0.035(1) 0.048(8) 0.010(3) 0.009(2) 0.015(5) 0.014(6) 0.027(7) 0.009(14)
1 Equivalent isotropic U defined as one third of the trace of the orthogonalized U tensor. ij
Table 5 Anisotropic displacement parameters for Bi0.78 Cd0.22 VO3.89 Atom Bi1/Cd1 Bi2/Cd2 Bi3 V1 V2 V3
U11
U22
U33
U23
U13
U12
0.0136(9) 0.009(1) 0.0084(7) 0.009(2) 0.005(2) 0.017(4)
0.0136(9) 0.007(1) 0.0084(7) 0.009(2) 0.005(2) 0.009(3)
0.024(1) 0.045(1) 0.0139(7) 0.015(3) 0.014(2) 0.006(3)
0 0 0 0 0 0
0 0.0027(6) 0 0 0 0.008(2)
0 0.0208(8) 0 0 0 0.011(2)
Table 6 Anisotropic displacement parameters for Bi0.71 Ca0.29 VO3.855 Atom Bi1/Cd1 Bi2/Cd2 Bi3 V1 V2 V3 O1 O2 O3 O4
U11
U22
U33
U23
U13
U12
0.021(2) 0.015(2) 0.032(2) 0.060(12) 0.015(4) 0.016(13) 0.016(13) 0.022(15) 0.020(14) 0.024(40)
0.021(2) 0.014(2) 0.032(2) 0.060(12) 0.015(4) 0.009(5) 0.028(15) 0.004(11) 0.045(17) 0.047(18)
0.026(2) 0.019(2) 0.040(2) 0.022(10) 0.005(5) 0.001(3) 0.00(9) 0.015(12) 0.017(13) 0.008(13)
0 0 0 0 0 0 0.008(9) 0.009(9) −0.007(13) −0.028(12)
0 0 0 0 0 0 −0.001(9) −0.004(11) −0.017(9) 0.008(18)
0 −0.006(2) 0 0 0 −0.002(5) −0.011(13) −0.006(12) −0.026(13) −0.009(21)
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Table 7 Selected interatomic distances (Å) and bond angles (deg) for Bi0.78 Cd0.22 VO3.89 Bi1–O2 × 4 –O2 × 4
2.43(1) 2.45(1)
V1–O1 × 4 V2–O4 × 4
1.72(1) 1.67(1)
Bi2–O3 × 2 –O2 × 2 –O1 × 2 –O4 × 2
2.41(1) 2.43(1) 2.46(1) 2.52(1)
V3–O2 × 2 –O3 × 2 O1–V1–O1 O1–V1–O1
1.76(1) 1.76 107.2(8) 114.2(8)
Bi3–O3 × 4 –O1 × 4
2.45(1) 2.45(1)
O2–V2–O2 O2–V2–O3 O3–V2–O3 O3–V2–O3 O4–V3–O4 O4–V3–O4
114.8(11) 106.9(9) 106.7(9) 115.2(11) 116.1(8) 106.3(8)
Table 8 Selected interatomic distances (Å) and bond angles (deg) for Bi0.71 Ca0.29 VO3.855 Bi1–O3 × 4 –O4 × 4
2.45(2) 2.49(4)
V1–O1 × 4 V2–O4 × 4
1.59(4) 1.66(2)
Bi2–O1 × 2 –O2 × 2 –O3 × 2 –O4 × 2
2.52(2) 2.43(2) 2.44(2) 2.52(4)
V3–O2 × 2 –O3 × 2 O1–V1–O1 O1–V1–O1
1.73(2) 1.75(2) 106.44(1.26) 115.71(1.29)
Bi3–O1 × 4 –O2 × 4
2.48(2) 2.49(2)
O4–V2–O4 O4–V2–O4 O2–V3–O2 O2–V3–O3 O2–V3–O3 O2–V3–O3
114.62(1.65) 106.96(1.60) 116.08(1.50) 107.37(1.23) 105.98(1.24) 114.36(1.48)
ues greater than four σ , and for a Bi0.71 Ca0.29 VO3.855 crystal 20 reflections violating body centering had F 2 values greater than 15 times σ . Apparently, the only other case of lowered tetragonal symmetry for a scheelite phase is in the case of NaBi(WO4 )2 where the space group drops from I41 /a to I4¯ due to partial ordering of Na and Bi [13]. Neither X-ray diffraction nor electron diffraction in an electron microscope give any indication of a superstructure for our For Bi1−2x A2x VO4−x phases with A = Ca or Cd. The space ¯ group for these tetragonal phases appears to be P4 or P4. In space group I41 /a there is just one crystallographic site for Bi and A; thus, an ordering of A and Bi cations would not be possible in this space group. However, the primitive tetragonal space groups could accommodate complete ordering of A and Bi for the Bi0.75 A0.25VO3.875 composition. Indeed our refinements of the intensity data from single crystals indicates that the A cations preferentially occupying two of the three Bi/A sites in both Bi0.78 Cd0.22 VO3.89 and Bi0.71 Ca0.29 VO3.855 (Tables 3 and 4). Evidence for oxygen deficiency was found for two of the four oxygen sites in Bi0.78 Cd0.22VO3.89 and one of the four oxygen sites in Bi0.71 Ca0.29 VO3.855. The high concentration of oxygen vacancies in Bi1−2x A2x VO4−x phases suggested that oxygen conductivity might be high in these phases. The Arrhenius plots for the AC
Fig. 1. Log conductivity vs. 1/T for BiVO4 , Bi0.75 Ca0.25 VO3.875 , and stabilized zirconia (YSZ).
conductivity for BiVO4 and Bi1−2x Ca2x VO4−x samples are shown in Fig. 1. Our own data for stabilized zirconia, which agree well with the literature [14], are shown for comparison. An increase in conductivity for Ca substituted BiVO4 was observed. This is consistent with the earlier observation on BiVO4 and Ca doped 1% [8] and 7% [9] BiVO4 samples. The total conductivity of Bi0.75 Ca0.25VO3.875 is almost of the same order of magnitude as that of YSZ in the temperature range 450–700 ◦ C. Conductivity measurements on different samples with varying thickness agreed very well. The composition, Bi0.80 Ca0.20 VO3.90 also gave higher conductivity than BiVO4 , and even higher than that of Bi0.75 Ca0.25 VO3.875 in the temperature range 200–400 ◦ C. Above 400 ◦ C, the conductivities of Bi0.80 Ca0.20 VO3.90 and Bi0.75 Ca0.25 VO3.875 are similar.
4. Discussion Three-fold coordination for V suggested by the formula Bi1−2x A2x VO4−x or Bi6 A2 (VO4 )7 (VO3 ) is chemically implausible. Presumably, the VO3 group would dimerize with a VO4 group creating Bi6 A2 ((VO4)6 (V2 O7 ). Such a formulation is also suggested when this is considered as a BiVO4 – Ca2 V2 O7 solid solution. One quarter of the vanadium would then be in V2 O7 groups with one such group for every two unit cells. Without a larger unit cell, there is no way to incorporate such a V2 O7 group in an ordered way, and there is no diffraction evidence for a larger cell. Thus, the assumed V2 O7 groups must be accommodated in a disordered manner. Therefore, it is not surprising that we are unable to obtain completely satisfactory refinements of our single crystal data even when oxygen occupation parameters are refined. The lack of order of the presumed V2 O7 groups may be due to an incomplete ordering of A2+ cations, to which the V2 O7 groups should be attracted. This situation may be the same as in recently reported Ce1−2x M2x VO4−x (M = Ca, Sr, or Pb) phases with the zircon structure [15–17]. In this case, the value x could be as high as 0.4 when M was Ca, but no mechanism to eliminate VO3 groups was proposed.
S. Uma et al. / Solid State Sciences 4 (2002) 329–333
Acknowledgements This work was supported by NSF Grant DMR-9802488.
References [1] [2] [3] [4] [5]
W.F. de Jong, J.J. Lange, Am. Mineral. 21 (1936) 809. J.D. Bierlein, A.W. Sleight, Solid State Commun. 16 (1975) 69. A.W. Sleight, H.-Y. Chen, A. Ferreti, Mater. Res. Bull. 14 (1979) 1571. K. Aizu, J. Phys. Soc. J. 27 (1969) 387. A.K. Bhattacharya, K.K. Mallick, A. Hartridge, Mater. Lett. 30 (1997) 7. [6] A.W. Sleight, K. Aykan, D.B. Rogers, J. Solid State Chem. 13 (1975) 231.
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[7] T. Lu, B.C.H. Steele, Solid State Ionics 21 (1986) 339. [8] I.C. Vinke, J. Diepgrond, B.A. Boukamp, K.J. de Vries, A.J. Burggraaf, Solid State Ionics 57 (1992) 83. [9] L. Hoffart, U. Heider, L. Jörissen, R.A. Huggins, W. Witschel, Solid State Ionics 72 (1994) 195. [10] TEXSAN, Single Crystal Structure Analysis Software, Version 5.0, Molecular Structure Corporation, The Woodlands, TX, 1989. [11] G.M. Sheldrick, SHELXS-97, Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997, release 97-2. [12] L.J. Farrugia, WinGX, Ver. 1.63.01, J. Appl. Crystallogr. 32 (1999) 837. [13] J. Hanuza, A. Benzar, A. Haznar, M. Maczka, A. Pietraszko, J.H. van der Maas, Vib. Spectrosc. 12 (1996) 25. [14] P. Abélard, J.F. Baumard, Pure Appl. Chem. 67 (1995) 1891. [15] A. Watanabe, J. Solid State Chem. 153 (2000) 174. [16] T. Hirata, A. Watanabe, J. Solid State Chem. 158 (2001) 254. [17] T. Hirata, A. Watanabe, J. Solid State Chem. 158 (2001) 264.
Oxygen ion conductivity in new oxygen deficient phases with scheelite related structures S. Uma, R. Bliesner, A.W. Sleight ∗ Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003, USA Dedicated to Martha Greenblatt
Abstract Phases of the type Bi1−2x A2x VO4−x have been prepared up to x ≈ 0.25 with A = Ca, Sr, and Cd. The monoclinic distortion of the ideal tetragonal scheelite structure disappears with increasing x. Single crystal X-ray diffraction studies were conducted for samples with A = Ca and Cd. Impedance measurements show conductivity values analogous to those observed for stabilized zirconia. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.
1. Introduction BiVO4 occurs naturally as the mineral pucherite, which has an orthorhombic structure with VO4 tetrahedra [1]. Apparently, laboratory syntheses have never given this modification. However, BiVO4 can be prepared in the tetragonal zircon structure or in a more stable form with the scheelite structure. Scheelite BiVO4 melts congruently at 940 ◦ C, and it has a phase transition at 255 ◦ C [2]. Above 255 ◦ C BiVO4 has the ideal scheelite structure in space group I41/a, below 255 ◦ C scheelite BiVO4 distorts to the monoclinic space group I2/a [2,3]. In Aizu’s terminology [4], the 255 ◦ C phase transition indicates that monoclinic BiVO4 is a pure ferroelastic. The zircon form of BiVO4 transforms irreversibly to the scheelite form on heating to about 400 ◦ C or upon prolonged grinding at room temperature [5]. Scheelite BiVO4 has been of considerable interest as a bright yellow pigment and as a selective oxidation catalyst, especially when doped to produce cation vacancies on the Bi site [6]. Significant oxygen ion conductivity is also reported for BiVO4 at high temperatures [7]. A recent study indicated mixed conductivity for BiVO4 at high temperatures with the electronic contribution being n-type [8]. Attempts have been made to increase the oxygen ion conductivity of BiVO4 by substitution of a few percent of Ca, Zr, or Ce for Bi [7–9]. Our initial interest in substitutions into BiVO4 was to explore the possibility that it might be made into a good p* Correspondence and reprints.
E-mail address: [email protected] (A.W. Sleight).
type electronic conductor by creating holes in the Bi 6s band. Compositions of the type Bi1−x Cax VO4 were attempted. We found that Ca would substitute for Bi at much higher levels than previously reported. However, both the color and the conductivity of such samples indicate that the substitution is of the type Bi1−2x Ca2x VO4−x , not Bi1−x Cax VO4 . Thus, the Ca substitution for Bi was not producing significant hole carrier concentration in the Bi 6s band. Annealing such samples under 300 bars of oxygen pressure at 500 ◦ C was not effective in changing this situation.
2. Experimental Reactants were Bi2 O3 (Cerac, 99.9%), V2 O5 (Johnson Matthey, 99.9%), CaCO3 (Mallinckrodt, 99.8%), SrCO3 (Aldrich, 98+%), CdO (Baker analyzed, 99.1%). Appropriate quantities of reactants were ground together and heated for 10 hours at 750 and 900 ◦ C. The products were examined by powder X-ray diffraction using a Siemens D5000 powder X-ray diffractometer with Cu Kα radiation. Crystals were mounted on glass fibers for collection of single crystal X-ray diffraction data. Details of the data collection are given in Tables 1 and 2. The data were collected using the ω–2θ -scan technique at a scan width of ω = (1.6 + 0.3 tan θ ). The intensities of three standard reflections measured every 150 reflections throughout data collection exhibited no significant fluctuations. The initial data processing and an absorption correction by φ-scan to the full data set were applied using the programs from the T EXSAN crystallographic software package [10]. The
1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 1 ) 0 1 2 6 0 - 2
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S. Uma et al. / Solid State Sciences 4 (2002) 329–333
Table 1 Crystal data and intensity collection for Bi0.78 Cd0.22 VO3.89
Table 2 Crystal data and intensity collection for Bi0.71 Ca0.29 VO3.855
Color, habit Size (mm3 ) Diffractometer Radiation Monochromator Temperature Maximum 2θ (deg) Data collected
Color, habit Size (mm3 ) Diffractometer Radiation Monochromator Temperature Maximum 2θ (deg) Data collected
Scan type Scan speed Absorption correction Transmission factors, range Crystal system Space group Unit cell dimensions (Å) Volume (Å3 ) Z Formula weight Calculated density (g cm−3 ) Absorption coefficient (mm−1 ) No. of reflections collected in primitive setting No. of unique reflections No. of observed reflections (I > 4σ (I )) Refinement method No. of parameters refined Goodness of fit on F 2 Final R indices (I > 4σ (I )) R indices (all data) Extinction coefficient Largest difference peak and hole
Yellow, plate 0.04 × 0.04 × 0.06 Rigaku AFC6R Mo Kα (λ = 0.71069 Å) Graphite 23 ◦ C 60 −7 h 7, −7 k 7, −16 l 16 ω–2θ 8 ψ-scan 1.00–0.62 Tetragonal P4¯ (No. 81) 5.125(2), 11.672(2) 306.6(1) 4 298.4 6.464 49.176 3563 900, Rint = 10.49 455 Full-matrix least-squares on F 2 38 1.035 R = 8.27%, wR2 = 22.49% R = 9.43% 0.0044(5) 10.78 e Å−3 (0.79 Å from Bi3) and −6.55 e Å−3 (0.72 Å from Bi1)
structures were then solved by direct methods and refined using S HELXS 97 incorporated in the W INGX suite [11, 12]. Final refined parameters are given in Tables 3 and 4, interatomic distances in Tables 5 and 6, and anisotropic displacement factors in Tables 7 and 8. AC conductivity measurements were therefore performed on sintered pellets of varying thickness with a diameter of 1.1 cm. The pellets were sintered at 800–900 ◦ C. Both flat surfaces of the pellets were polished and coated with Engelhard 6926 platinum paste, and then heated to 400 ◦ C to remove the organic binder from the paste. Measurements were made on a Solatron 1260 impedance analyzer computer controlled over the frequency range of 10 MHz to 1 Hz using platinum electrodes. Conductivity data were obtained on samples on heating and cooling in the range 100– 600 ◦ C, after the samples were equilibrated at the measuring temperature for approximately two hours. 3. Results Products of attempts to substitute Sr, Cd, Pb, or Na for Bi at 750◦ gave X-ray diffraction patterns dominated by normal monoclinic BiVO4 plus some second phases. However
Scan type Scan speed Absorption correction Crystal system Space group Unit cell dimensions (Å) Volume (Å3 ) Z Formula weight Calculated density (g cm−3 ) Absorption coefficient (mm−1 ) No. of reflections collected in primitive setting No. of unique reflections No. of observed reflections (I > 2σ (I )) Refinement method No. of parameters refined Goodness of fit on F 2 Final R indices (I > 4σ (I )) Extinction coefficient
Yellow, plate 0.1 × 0.1 × 0.08 Rigaku AFC6R Mo Kα (λ = 0.71069 Å) Graphite 23 ◦ C 55 −1 h 6, −1 k 6, −1 l 15 ω–2θ 8 ψ-scan Tetragonal P4¯ (No. 81) 5.121(5), 11.701(5) 306.9(4) 4 272.62 5.901 30.918 708 590, Rint = 11.56 275 Full-matrix least-squares on F 2 58 1.506 R = 7.20%, wR2 = 13.65% 0.032(7)
for Bi0.75 Ca0.25 VO3.875, a single tetragonal phase formed with a = 5.147 Å and c = 11.7216 Å. Attempts to further increase the Ca substitution gave the same tetragonal phase plus peaks due to Ca2 V2 O7 . X-ray diffraction patterns of Bi1−2x Ca2x VO4−x samples with x intermediate between 0.0 and 0.25 showed a mixture of monoclinic BiVO4 and tetragonal Bi0.75 Ca0.25 VO3.875, suggesting that Bi0.75 Ca0.25 VO3.875 might be a discrete compound. The situation was very different for many of the compositions heated to 900 ◦ C. For Bi1−2x Ca2x VO4−x samples, single-phase tetragonal products were observed for values of x from 0.10 to 0.30. Furthermore, single-phase tetragonal products were also observed for Bi0.75 Sr0.25 VO3.875 (a = 5.1515 Å, c = 11.838 Å, V = 314.2 Å3 ) and for Bi0.75 Cd0.25VO3.875 (a = 5.1187 Å, c = 11.6594 Å, V = 305.5 Å3 ). These represent a substantial cell expansion and contraction relative to scheelite BiVO4 which has a cell volume of 309.7 Å3 at room temperature. The higher synthesis temperature of 900 ◦ C did not give indications of significant substitution of Pb or Na for Bi or Ge for V. Several crystals of tetragonal for Bi1−2x Ca2x VO4−x and for Bi1−2x Cd2x VO4−x compositions with x close to 0.25 were examined by X-ray diffraction. The tetragonal symmetry was confirmed with the 5 by 11 Å unit cell, but the space group was not the I41/a of the ideal scheelite structure. Reflections were observed that violate both the a glide and the body centering. For a Bi0.78 Cd0.22VO3.89 crystal 22 reflections violating body centering had F 2 val-
S. Uma et al. / Solid State Sciences 4 (2002) 329–333
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Table 3 Positional parameters and Ueq for Bi0.78 Cd0.22 VO3.89 Atom Bi1 Cd1 Bi2 Cd2 Bi3 V1 V2 V3 O1 O2 O3 O4
Site ¯4 1a 4¯ 1a 2 2g 2 2g 4¯ 1d 4¯ 1b 4¯ 1c 2 2g 1 4h 1 4h 1 4h 1 4h
x
y
z
Occupancy
Ueq 1 (Å2 )
0 0 0.5 0.5 0.5 0 0.5 0 0.863(3) 0.140(4) 0.256(4) 0.635(3)
0 0 0 0 0.5 0 0.5 0.5 0.247(3) 0.246(4) 0.640(3) 0.258(3)
0 0 0.2504(4) 0.2504(4) 0.5 0.5 0 0.249(1) 0.580(1) 0.168(2) 0.330(2) 0.076(1)
0.78(1) 0.22 0.67(1) 0.33 1.0 1.0 1.0 1.0 1.0 0.85(4) 0.87(3) 1.0
0.0169(5) 0.0169(5) 0.0208(8) 0.0208(8) 0.0102(4) 0.0120(4) 0.008(2) 0.012(2) 0.014(3) 0.018(4) 0.016(4) 0.015(3)
1 Equivalent isotropic U defined as one third of the trace of the orthogonalized U tensor. ij
Table 4 Positional parameters and Ueq for Bi0.71 Ca0.29 VO3.855 Atom Bi1 Ca1 Bi2 Ca2 Bi3 V1 V2 V3 O1 O2 O3 O4
Site ¯4 1a 4¯ 1a 2 2g 2 2g 4¯ 1d 4¯ 1b 4¯ 1c 2 2g 1 4h 1 4h 1 4h 1 4h
x
y
z
Occupancy
Ueq 1 (Å2 )
0 0 0.5 0.5 0.5 0 0.5 0 0.132(5) 0.360(5) 0.252(5) 0.738(9)
0 0 0 0 0.5 0 0.5 0.5 0.237(5) −0.248(4) 0.142(6) 0.379(6)
0 0 0.2501(5) 0.2501(5) 0.5 0.5 0 0.249(1) −0.578(2) 0.328(2) 0.167(2) 0.069(2)
0.73(1) 0.27 0.58(1) 0.42 1.0 1.0 1.0 1.0 0.92(4) 1.0 1.0 1.0
0.023(1) 0.023(1) 0.016(1) 0.016(1) 0.035(1) 0.048(8) 0.010(3) 0.009(2) 0.015(5) 0.014(6) 0.027(7) 0.009(14)
1 Equivalent isotropic U defined as one third of the trace of the orthogonalized U tensor. ij
Table 5 Anisotropic displacement parameters for Bi0.78 Cd0.22 VO3.89 Atom Bi1/Cd1 Bi2/Cd2 Bi3 V1 V2 V3
U11
U22
U33
U23
U13
U12
0.0136(9) 0.009(1) 0.0084(7) 0.009(2) 0.005(2) 0.017(4)
0.0136(9) 0.007(1) 0.0084(7) 0.009(2) 0.005(2) 0.009(3)
0.024(1) 0.045(1) 0.0139(7) 0.015(3) 0.014(2) 0.006(3)
0 0 0 0 0 0
0 0.0027(6) 0 0 0 0.008(2)
0 0.0208(8) 0 0 0 0.011(2)
Table 6 Anisotropic displacement parameters for Bi0.71 Ca0.29 VO3.855 Atom Bi1/Cd1 Bi2/Cd2 Bi3 V1 V2 V3 O1 O2 O3 O4
U11
U22
U33
U23
U13
U12
0.021(2) 0.015(2) 0.032(2) 0.060(12) 0.015(4) 0.016(13) 0.016(13) 0.022(15) 0.020(14) 0.024(40)
0.021(2) 0.014(2) 0.032(2) 0.060(12) 0.015(4) 0.009(5) 0.028(15) 0.004(11) 0.045(17) 0.047(18)
0.026(2) 0.019(2) 0.040(2) 0.022(10) 0.005(5) 0.001(3) 0.00(9) 0.015(12) 0.017(13) 0.008(13)
0 0 0 0 0 0 0.008(9) 0.009(9) −0.007(13) −0.028(12)
0 0 0 0 0 0 −0.001(9) −0.004(11) −0.017(9) 0.008(18)
0 −0.006(2) 0 0 0 −0.002(5) −0.011(13) −0.006(12) −0.026(13) −0.009(21)
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Table 7 Selected interatomic distances (Å) and bond angles (deg) for Bi0.78 Cd0.22 VO3.89 Bi1–O2 × 4 –O2 × 4
2.43(1) 2.45(1)
V1–O1 × 4 V2–O4 × 4
1.72(1) 1.67(1)
Bi2–O3 × 2 –O2 × 2 –O1 × 2 –O4 × 2
2.41(1) 2.43(1) 2.46(1) 2.52(1)
V3–O2 × 2 –O3 × 2 O1–V1–O1 O1–V1–O1
1.76(1) 1.76 107.2(8) 114.2(8)
Bi3–O3 × 4 –O1 × 4
2.45(1) 2.45(1)
O2–V2–O2 O2–V2–O3 O3–V2–O3 O3–V2–O3 O4–V3–O4 O4–V3–O4
114.8(11) 106.9(9) 106.7(9) 115.2(11) 116.1(8) 106.3(8)
Table 8 Selected interatomic distances (Å) and bond angles (deg) for Bi0.71 Ca0.29 VO3.855 Bi1–O3 × 4 –O4 × 4
2.45(2) 2.49(4)
V1–O1 × 4 V2–O4 × 4
1.59(4) 1.66(2)
Bi2–O1 × 2 –O2 × 2 –O3 × 2 –O4 × 2
2.52(2) 2.43(2) 2.44(2) 2.52(4)
V3–O2 × 2 –O3 × 2 O1–V1–O1 O1–V1–O1
1.73(2) 1.75(2) 106.44(1.26) 115.71(1.29)
Bi3–O1 × 4 –O2 × 4
2.48(2) 2.49(2)
O4–V2–O4 O4–V2–O4 O2–V3–O2 O2–V3–O3 O2–V3–O3 O2–V3–O3
114.62(1.65) 106.96(1.60) 116.08(1.50) 107.37(1.23) 105.98(1.24) 114.36(1.48)
ues greater than four σ , and for a Bi0.71 Ca0.29 VO3.855 crystal 20 reflections violating body centering had F 2 values greater than 15 times σ . Apparently, the only other case of lowered tetragonal symmetry for a scheelite phase is in the case of NaBi(WO4 )2 where the space group drops from I41 /a to I4¯ due to partial ordering of Na and Bi [13]. Neither X-ray diffraction nor electron diffraction in an electron microscope give any indication of a superstructure for our For Bi1−2x A2x VO4−x phases with A = Ca or Cd. The space ¯ group for these tetragonal phases appears to be P4 or P4. In space group I41 /a there is just one crystallographic site for Bi and A; thus, an ordering of A and Bi cations would not be possible in this space group. However, the primitive tetragonal space groups could accommodate complete ordering of A and Bi for the Bi0.75 A0.25VO3.875 composition. Indeed our refinements of the intensity data from single crystals indicates that the A cations preferentially occupying two of the three Bi/A sites in both Bi0.78 Cd0.22 VO3.89 and Bi0.71 Ca0.29 VO3.855 (Tables 3 and 4). Evidence for oxygen deficiency was found for two of the four oxygen sites in Bi0.78 Cd0.22VO3.89 and one of the four oxygen sites in Bi0.71 Ca0.29 VO3.855. The high concentration of oxygen vacancies in Bi1−2x A2x VO4−x phases suggested that oxygen conductivity might be high in these phases. The Arrhenius plots for the AC
Fig. 1. Log conductivity vs. 1/T for BiVO4 , Bi0.75 Ca0.25 VO3.875 , and stabilized zirconia (YSZ).
conductivity for BiVO4 and Bi1−2x Ca2x VO4−x samples are shown in Fig. 1. Our own data for stabilized zirconia, which agree well with the literature [14], are shown for comparison. An increase in conductivity for Ca substituted BiVO4 was observed. This is consistent with the earlier observation on BiVO4 and Ca doped 1% [8] and 7% [9] BiVO4 samples. The total conductivity of Bi0.75 Ca0.25VO3.875 is almost of the same order of magnitude as that of YSZ in the temperature range 450–700 ◦ C. Conductivity measurements on different samples with varying thickness agreed very well. The composition, Bi0.80 Ca0.20 VO3.90 also gave higher conductivity than BiVO4 , and even higher than that of Bi0.75 Ca0.25 VO3.875 in the temperature range 200–400 ◦ C. Above 400 ◦ C, the conductivities of Bi0.80 Ca0.20 VO3.90 and Bi0.75 Ca0.25 VO3.875 are similar.
4. Discussion Three-fold coordination for V suggested by the formula Bi1−2x A2x VO4−x or Bi6 A2 (VO4 )7 (VO3 ) is chemically implausible. Presumably, the VO3 group would dimerize with a VO4 group creating Bi6 A2 ((VO4)6 (V2 O7 ). Such a formulation is also suggested when this is considered as a BiVO4 – Ca2 V2 O7 solid solution. One quarter of the vanadium would then be in V2 O7 groups with one such group for every two unit cells. Without a larger unit cell, there is no way to incorporate such a V2 O7 group in an ordered way, and there is no diffraction evidence for a larger cell. Thus, the assumed V2 O7 groups must be accommodated in a disordered manner. Therefore, it is not surprising that we are unable to obtain completely satisfactory refinements of our single crystal data even when oxygen occupation parameters are refined. The lack of order of the presumed V2 O7 groups may be due to an incomplete ordering of A2+ cations, to which the V2 O7 groups should be attracted. This situation may be the same as in recently reported Ce1−2x M2x VO4−x (M = Ca, Sr, or Pb) phases with the zircon structure [15–17]. In this case, the value x could be as high as 0.4 when M was Ca, but no mechanism to eliminate VO3 groups was proposed.
S. Uma et al. / Solid State Sciences 4 (2002) 329–333
Acknowledgements This work was supported by NSF Grant DMR-9802488.
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