Rare-Earth Mixed-Valence Vanadium Oxides with Superlattice

Their three- dimensional V−O frameworks are similar to that of Fe−O one in the well-known orthorhombic CaFe2O4. Yb3+ cations are located in tunnel...
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Chem. Mater. 1997, 9, 141-147

141

Rare-Earth Mixed-Valence Vanadium Oxides with Superlattice Structures of Calcium Ferrite Yasushi Kanke* and Katsuo Kato National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan Received May 3, 1996. Revised Manuscript Received September 13, 1996X

New ternary ytterbium mixed valence vanadium oxides, R- and β-YbV4O8, were synthesized by solid-state reaction at 1473 and 1673 K, respectively. The crystal structure of R-YbV4O8 was determined by single-crystal X-ray diffraction; monoclinic, P121/n1, a ) 9.0648(3) Å, b ) 10.6215(4) Å, c ) 5.7607(1) Å, β ) 90.184(3)°, Z ) 4, R ) 0.030, and Rw ) 0.029. β-YbV4O8 was studied by Weissenberg photography and powder X-ray diffraction; monoclinic, P21/ n11, a ) 9.0625(7) Å, b ) 11.0086(9) Å, c ) 5.7655(5) Å, R ) 105.070(7)°. Its crystal structure was rerefined, for the sake of comparison between R- and β-form, adopting an A-centered pseudoorthorhombic cell dimension with relations ap-o ) am, bp-o ) 2bm + cm, and cp-o ) cm; monoclinic, A21/d11, a ) 9.030(5) Å, b ) 21.44(3) Å, c ) 5.752(2) Å, R ) 89.911(3)°, Z ) 8, R ) 0.029, and Rw ) 0.032. Their three- dimensional V-O frameworks are similar to that of Fe-O one in the well-known orthorhombic CaFe2O4. Yb3+ cations are located in tunnels running along the [001] direction as are Ca2+ cations in CaFe2O4. However, Yb3+ occupy half of the Ca2+ sites in ordered manners, resulting in superlattice structures of CaFe2O4. The shortest Yb-Yb distances, 5.4996(2) Å in R-YbV4O8 and 5.552(8) Å in β-YbV4O8, are much longer than the shortest Ln-Ln distances in well-known ternary rareearth oxides such as perovskites, pyrochlores, LnVO4 type phases, and garnets.

Introduction Vanadium oxides, V2O3, V2O4, and VnO2n-1 (n ) 3, 4-6, 8) show metal-insulator transitions as functions of temperature.1,2 3d-electrons in V3+-V4+ mixedvalence oxides are expected to be in intermediate states between localized and itinerant according mainly to electron correlation.3,4 An+-V3+-V4+ ternary oxides where An+ is alkaline, alkaline earth, or lanthanide cation represent interesting systems to study electron correlation.5-13 In these compounds, electric and magnetic properties are expected to be governed mainly by the character of 3d-electrons. A normal spinel, LiV2O4, shows Curie-Weiss paramagnetism and metallic conductivity.5 LiVO2 crystallizes in ordered rock salt type * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Morin, F. J. Phys. Rev. Lett. 1959, 3, 34. (2) Kachi, S.; Kosuge, K.; Okinaka, H. J. Solid State Chem. 1973, 6, 258. (3) Mott, N. F. Metal-Insulator Transition; Taylor & Francis: London, 1974. (4) Zaanen, J.; Sawatzky, G. A.; Allen, J. W. Phys. Rev. Lett. 1985, 55, 418. (5) Rogers, D. B.; Gillson, J. L.; Gier, I. E. Solid State Commun. 1967, 5, 263. (6) Hewston, T. A.; Chamberland, B. L. J. Solid State Chem. 1986, 65, 100. (7) Kanke, Y.; Takayama-Muromachi, E.; Kato, K.; Matsui, Y. J. Solid State Chem. 1990, 89, 130. (8) Uchida, Y.; Kanke, Y.; Takayama-Muromachi, E.; Kato, K. J. Phys. Soc. Jpn. 1991, 60, 2530. (9) Chamberland, B. L.; Danielson, P. S. J. Solid State Chem. 1971, 3, 243. (10) Uchida, Y.; Kanke, Y.; Onoda, Y. Proceedings 6th International Conference on Ferrites; Tokyo, 1992; p 722. (11) Dougier, P.; Hagenmuller, P. J. Solid State Chem. 1974, 11, 177. (12) Zubkov, V. G.; Bazuev, G. V.; Shveikin, G. P. Sov. Phys.-Solid State 1976, 18, 1165. (13) Bazuev, G. V.; Makarova, O. V.; Oboldin, V. Z.; Shveikin, G. P. Dokl. Akad. Nauk SSSR 1976, 230, 869.

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and is insulating.6 SrVO3 perovskite is metallic and exhibits anomalous paramagnetism,9 while LaVO3 and YVO3 perovskites are antiferromagnetic insulators with TN ≈ 150 and 114 K, respectively.11,12 Lu2V2O7 pyrochlore is ferromagnetic semiconductor with TC ) 80 K.13 NaV6O11 and SrV6O11 crystallize in magnetoplumbiterelated structures.14,15 They exhibit spontaneous magnetization, however, NaV6O11 (TC ) 64.2 K) is metallic while SrV6O11 (TC ) 70 K) is insulating below their TC.7,8,10 NaV6O11 shows two-step second-order structural phase transitions.16,17 To date, known ternary oxides in Ln3+-V3+-V4+ systems consist of LnVO3 perovskites (Ln ) La-Nd, Sm-Lu, and Y),18 Ln2V2O7 pyrochlores (Ln ) Tm, Yb, and Lu),13,19 LuV4O8,20 and Ln7V3O16 (Ln ) Tm, Yb, and Lu).20-22 Kitayama et al. have carried out a series of phase equilibrium studies on Ln2O3-V2O3-V2O5 systems at 1473 K (Ln ) La,23 Pr,24 Nd,25 Sm,26 Eu,22 Gd,27 (14) De Roy, M. E.; Besse, J. P.; Chevalier, R.; Gasperin, M. J. Solid State Chem. 1987, 67, 185. (15) Kanke, Y.; Kato, K.; Takayama-Muromachi, E.; Isobe, M. Acta Crystallogr. 1992, C48, 1376. (16) Kanke, Y.; Izumi, F.; Morii, Y.; Akiba, E.; Funahashi, S.; Kato, K.; Isobe, M.; Takayama-Muromachi, E.; Uchida, Y. J. Solid State Chem. 1994, 112, 429. (17) Kanke, Y.; Shigematsu, H.; Ohshima, K.; Kato, K. J. Appl. Cryst. 1995, 28, 599. (18) McCarthy, G. J.; Sipe, C. A.; McIlvried, K. E. Mater. Res. Bull. 1974, 9, 1279. (19) Kitayama, K.; Katsura, T. Chem. Lett. 1976, 815. (20) Kitayama, K.; Katsura, T. Bull. Chem. Soc. Jpn. 1978, 51, 1358. (21) Kitayama, K.; Katsura, T. Bull. Chem. Soc. Jpn. 1983, 56, 1084. (22) Kitayama, K.; Sou, H; Katsura, T. Bull. Chem. Soc. Jpn. 1983, 56, 3415. (23) Kitayama, K.; Zoshima, D; Katsura, T. Bull. Chem. Soc. Jpn. 1983, 56, 689. (24) Kitayama, K.; Katsura, T. Bull. Chem. Soc. Jpn. 1985, 58, 948. (25) Kitayama, K.; Mizokuchi, C; Katsura, T. Bull. Chem. Soc. Jpn. 1983, 56, 695. (26) Kitayama, K.; Katsura, T. Bull. Chem. Soc. Jpn. 1977, 50, 889.

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Tb,24 Dy,28 Ho,28 Er,29 Tm,21 Yb,22 Lu,20 and Y24) by controlling oxygen partial pressure. Their studies provide oxygen partial pressures in equilibrium for regions which consist of three solid phases. They discovered new phases, LuV4O8 and Ln7V3O16 (Ln ) Tm, Yb, and Lu)20-22 and reported powder X-ray diffraction data of them.20-22 None of the crystal structures of the new phases have been determined. Bazuev et al. prepared Lu2V2O7 by the ceramic technology from a mixture of Ln2O3 and V2O4,13 and synthesized Ln2V2O7 (Ln ) Tm, Yb, and Lu) by firing mixtures of Ln2O3 and V2O4 in vacuum at 1423-1523 K.30 Shin-ike et al. obtained the three Ln2V2O7 phases by heating mixtures of Ln2O3 and V2O4 in vacuum at 1673 K for 2 h.31 Greedan prepared the three phases by heating LnVO4 or a mixture of Ln2O3 and V2O5, at 1673 K in CO/CO2 atmosphere to control oxygen partial pressure.32 Kitayama et al. synthesized Lu2V2O7 by heating a mixture of Lu2O3 and V2O5 throughout in H2/ CO2 atmosphere, initially at 873 K for 24 h to reduce V2O5 to V2O3 and then at 1673 K for 6 h.19 However, neither of the remaining two, Tm2V2O7 and Yb2V2O7, was obtained by corresponding procedures.21,22 In our present study, we discovered new phases, R-YbV4O8 (low-temperature form) and β-YbV4O8 (hightemperature form). Crystal structures of R- and β-forms were determined by X-ray diffractometry using a singlecrystal specimen and a twinned one, respectively. Both phases are structurally related to the well-known calcium ferrite, CaFe2O4.33 We also describe the correct system YbVO4-YbVO3-V2O3-V3O5 at 1473 K. Experimental Section Starting materials, V2O5 (99.9%) and Yb2O3 (99.9%), were dried immediately before using at 873 and 1273 K, respectively. V2O3 was prepared by reducing V2O5 in H2/N2 atmosphere at 1073 K for 2 h. V2O4 was obtained by heating an equimolar mixture of V2O5 and V2O3 in a sealed silica tube at 1273 K for 3 days. YbVO4 was synthesized by heating an equimolar mixture of Yb2O3 and V2O5 at 1473 K for 3 days. YbVO3 was prepared by reducing YbVO4 in H2/N2 atmosphere at 1273 K for 1 day. The system YbVO4-YbVO3-V2O3-V3O5 at 1473 K was studied as follows. YbVO3, YbVO4, and V2O3, or YbVO3, V2O3, and V2O4 were mixed in a desired ratio using an agate mortar. About 1 g of the mixture was placed in a platinum crucible, sealed in an evacuated silica tube and heated at 1473 K for 1 day. The product was examined by powder X-ray diffraction using Cu KR radiation. This procedure with heating period of 2-3 days was repeated until the diffraction pattern changed no further. In general, one heating run was enough to reach equilibrium. However, the formation of Yb2V2O7 was rather slow so that one or two more heating runs were required for compositions in the Yb2V2O7-containing region. A new phase was found whose powder X-ray diffraction pattern is similar to that of LuV4O8 reported by Kitayama and Katsura20 and whose crystal structure has been so far unknown. (27) Kitayama, K.; Katsura, T. Bull. Chem. Soc. Jpn. 1982, 55, 1820. (28) Kitayama, K.; Katsura, T. Bull. Chem. Soc. Jpn. 1984, 57, 1222. (29) Kitayama, K.; Sugiura, T.; Katsura, T. Bull. Chem. Soc. Jpn. 1979, 52, 458. (30) Bazuev, G. V.; Samokhvalov, A. A.; Morozov, Y. N.; Matveenko, I. I.; Babushkin, V. S.; Arbuzova, T. I.; Shveikin, G. P. Sov. Phys. Solid State 1977, 19, 1913. (31) Shin-ike, T.; Adachi, G.; Shiokawa, J. Mater. Res. Bull. 1977, 12, 1149. (32) Greedan, J. E. Mater. Res. Bull. 1979, 14, 13. (33) Decker, B. F.; Kasper, J. S. Acta Crystallogr. 1957, 10, 332.

Kanke and Kato

Figure 1. Phase diagram of the YbVO4-YbVO3-V2O3-V3O5 system at 1473 K. (A) R-YbV4O8, (B) Yb2V2O6.95. Solid and open circles show single-phase and multiphase compositions, respectively. Single crystals of the new phase were prepared as follows. YbVO3, V2O3, and V2O4 were mixed in a 1:1.1:0.4 molar ratio using an agate mortar. About 0.5 g of the mixture was sealed in a platinum tube and heated at 1623 K for 1 day. The wellsintered product consisted of prismatic black crystals. Quantitative X-ray microanalysis of these crystals was performed using YbFeO3 and NaV6O11 as reference materials. The analysis determined Yb/V ratio of (0.97 ( 0.01)/4, which supports the composition YbV4O8. Oscillation and Weissenberg photographs revealed the crystals to belong to monoclinic crystal system with a ) 9.1 Å, b ) 10.6 Å, c ) 5.8 Å, β ) 90.2°. Observed extinction rules of h + l ) 2n + 1 for h0l and k ) 2n + 1 for 0k0 lead to unique space group assignment, P121/n1. The size of the single-crystal specimen for diffraction study was 50, 30, and 75 µm along [100], [010], and [001], respectively. In what follows, we denote the new phase as R-YbV4O8. During our search for optimal conditions to prepare single crystals of R-YbV4O8, we discovered an additional phase of YbV4O8 at 1673 K as follows: YbVO3, V2O3, and V2O4 were mixed in a 2:2:1 molar ratio using an agate mortar. About 0.5 g of the mixture was sealed in a platinum tube and heated at 1673 K for 1 day. Microscopic observation indicated that the product passed through a partially molten state and contained needlelike black crystals instead of the prismatic ones. Oscillation and Weissenberg photographs, at a glance, suggested that the needle-like crystal has A-centered orthorhombic symmetry with a ) 9.0 Å, b ) 21.4 Å, c ) 5.8 Å. However, the oscillation photograph along [001] indicated that the intensity distributions in the even-numbered layer lines are symmetric but those in the odd-numbered ones are not. Kato and Kanke revealed that the specimen is twinned along (010) and that its true unit cell is monoclinic with relations, am ) ao, bm ) 1/2(bo - co), and cm ) co.34 None of the specimens studied by X-ray diffraction were free from twinning. In what follows, we denote this high-temperature phase as β-YbV4O8. The powder specimen was prepared by heating a mixture of YbVO3, YbVO4, and V2O3 in a 1.1:0.9:3 molar ratio at 1673 K for 1 day as described above. The partial melt was not observed in the powder specimen prepared from YbVO3, YbVO4, and V2O3. Intensity data for R-YbV4O8 were collected at 293 K on an Enraf-Nonius CAD4 diffractometer with graphite-monochromatized Mo KR radiation (λ ) 0.710 73 Å) using ω-θ scan with ∆ω ) (0.8 + 0.35 tan θ)°. Lattice parameters were determined from Bragg angles of 22 reflections in the region 61° < 2θ < 67°. Reflections for -14 e h e 14, -17 e k e 0, 0 e l e 9 with 2θ e 70° and those for -12 e h e 12, 1 e k e 14, -8 e l e 0 with 2θ e 60° were measured. Of the resulting 4306 (34) Kato, K.; Kanke, Y.; Friese, K. Z. Kristallogr., in press.

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Chem. Mater., Vol. 9, No. 1, 1997 143

Figure 2. X-ray powder diffraction profiles taken with Cu KR radiation. The upper is for R-YbV4O8: monoclinic, P121/n1, a ) 9.0660(7) Å, b ) 10.6244(10) Å, c ) 5.7622(6) Å, β ) 90.195(7)°. The lower is for β-YbV4O8: monoclinic, P21/n11, a ) 9.0625(7) Å, b ) 11.0086(9) Å, c ) 5.7655(5) Å, R ) 105.070(7)°. Asterisk indicate pyrochlore phase as impurity. reflections, 693 were unobserved. Three standard reflections 600, 04h 0, and 004 were measured every 4 h, and the decrease of intensity was 0.5% during the total exposure time of 327.1 h. A linear decay correction was applied. Absorption correction (µ ) 229.58 cm-1) was applied with a transmission correction factor for F from 1.458 to 2.208. The observed 3613 reflections were averaged into 2447 unique reflections with Rint ) 0.012 based on F. 2132 reflections with I >1.5σ(I) were used for refinement. Structural model was derived using Patterson function. Atomic scattering factors (f ) f0 + ∆f' + i∆f′′) for neutral atoms were employed.35 Structural parameters were refined by the least-squares method based on F, applying an extinction correction.36 Crystal structure of β-YbV4O8 has been determined using a twinned specimen as a commensurate composite structure including V2O4 framework and Yb cations as substructures.34 The total structure is monoclinic in P21/n11 with a ) 9.030 Å, b ) 11.097 Å, c ) 5.752 Å, R ) 104.94°, which corresponds to the above-mentioned true monoclinic unit cell. In this study, for the sake of comparison between R- and β-YbV4O8, the crystal structure of the latter was rerefined adopting the A-centered pseudoorthorhombic cell and the monoclinic space group A21/d11, applying the structure refinement technique for twinned specimens.37 The center of symmetry of the structure is located at (0,0,1/2) in ref 34, on the other hand, the center of symmetry is shifted to (0,0,0) in this study. The parameters including two scale factors and one free parameter for extinction correction were refined by the program FMLSM,38-40 based on the 1515 intensities (2θ e 60°, Mo KR) taken from ref 34. The intensities were weighted with 1/σ(Fo)2. Atomic scattering factors (f ) f0 + ∆f' + i∆f′′) for neutral atoms were employed.41 (35) International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, 1974; Vol. IV. (36) Fair, C. K. MolEN; Enraf-Nonius: Netherlands, 1990. (37) Kato, K. Z. Kristallogr., in press. (38) Kato, K. Acta Crystallogr. 1990, B46, 39. (39) Kato, K. Acta Crystallogr. 1994, A50, 351. (40) Friese, K.; Jarchow, O.; Kato, K. Z. Kristallogr., submitted. (41) International Tables for X-Ray Crystallography; Kluwer Academic Publishers: Dordrecht, 1992; Vol. C.

Table 1. Atomic Coordinates and Equivalent Thermal Parameters (Å2) in r-YbV4O8a atom position Yb V(1) V(2) V(3) V(4) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8)

4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e

x

y

z

Beqb

0.75808(2) 0.42637(7) 0.40816(7) 0.45610(8) 0.41875(7) 0.2026(3) 0.1202(3) 0.5399(3) 0.4138(3) 0.2084(3) 0.1143(3) 0.5135(3) 0.4157(3)

0.65759(2) 0.61880(6) 0.09693(6) 0.61089(6) 0.10578(6) 0.1592(3) 0.4731(3) 0.7764(3) 0.4306(3) 0.1581(3) 0.4753(3) 0.7890(3) 0.4271(3)

0.12667(3) 0.12348(11) 0.12463(11) 0.62686(11) 0.62574(11) 0.1127(5) 0.1167(5) 0.1212(5) 0.1158(5) 0.6389(5) 0.6340(5) 0.6283(5) 0.6329(5)

0.394(2) 0.411(9) 0.410(9) 0.428(9) 0.423(9) 0.56(4) 0.52(4) 0.49(4) 0.54(4) 0.58(4) 0.45(4) 0.50(4) 0.53(4)

a M ) 504.80, Monoclinic, P2 /n, a ) 9.0648(3) Å, b ) 10.6215(4) 1 Å, c ) 5.7607(1) Å, β ) 90.184(3)°, V ) 554.65 Å3, Z ) 4, dcalc ) 6.045 g‚cm-3, F(000) ) 904, R ) 0.030, Rw ) 0.029, w ) 1/σ2(F), ∆/σ < 0.005, and -4.988 < ∆F e 4.99 eÅ3. b Beq ) 8/3π2 (a2a*2U11 + b2b*2U22 + c2c*2U33 + 2aa*cc*U13 cos β).

X-ray powder diffraction data of R- and β-YbV4O8 were collected on a Phillips PD 1800 diffractometer with Cu KR radiation and a curved graphite monochromator on the counter side, using silicon as internal standard. The diffractometer is equipped with an auto divergence slit.

Results The phase diagram determined in the present study is shown in Figure 1. Kitayama et al. reported that LuV4Ox at 1473 K has a homogeneity range of 7.93 e x e 8.05.20 In the present study, YbV4O8 was the only single phase among five compositions for the R-phase, YbV4O7.9, YbV4O8, YbV4O8.1, Yb0.97V4O8, and Yb1.04V4O8. We did not detect any difference in the cell parameters

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Table 2. Interatomic Distances (Å) and Bond Angles (deg) in r-YbV4O8 V(1)-O(3)a O(4)a O(1i)b O(5i)b O(8ii)b O(4iii)b

1.965(3) 2.003(3) 1.968(3) 1.985(3) 2.060(3) 2.071(3)

V(3)-O(7)a O(8)a O(5vi)b O(1i)b O(4ii)b O(8ii)b

1.962(3) 1.987(3) 2.076(3) 2.055(3) 1.942(3) 1.940(3)

V(2)-O(6iv)a O(1)a O(7ii)b O(3iii)b O(2v)b O(6v)b

2.020(3) 1.978(3) 1.998(3) 2.011(3) 2.005(3) 1.982(3)

V(4)-O(2vii)a O(5)a O(3ii)b O(7ii)b O(6viii)b O(2v)b

2.010(3) 1.988(3) 1.957(3) 1.943(3) 1.982(3) 2.014(3)

V(1)-V(1iii) V(3ii) V(3) V(3x) V(3) -V(3ii) Ybe-Ybxiii, xiv Ybxv, xvi V(1)-O(4)-V(1iii) V(1)-O(4iii)-V(1iii) V(1)-O(4)-V(3ii) V(1)-O(8ii)-V(3ii) V(1)-O(1i)-V(3) V(1)-O(8ii)-V(3) V(1)-O(5i)-V(3x) V(1)-O(4iii)-V(3x) V(3)-O(8)-V(3ii) V(3)-O(8ii)-V(3ii) Yb-O(5iv)-Ybxiii

3.1917(9) 3.0241(9) 2.9127(9) 2.8756(9) 2.8855(9) 5.4996(2) 5.7104(2) 103.16(13) 103.16(13) 100.08(13) 96.69(13) 92.73(13) 93.40(13) 90.14(13) 91.49(12) 94.56(13) 94.56(13) 143.55(12)

V-V Sharing Two O Atoms r zigzag f r zigzag f r along [001] f r along [001] f r zigzag f -Ybxvii 5.7108(2) Ybix, xviii 5.7251(2) V(2)-(6iv)-V(2xi) 97.75(13) V(2)-O(6v)-V(2xi) 97.75(13) V(2)-O(6iv)-V(4xii) 98.20(13) V(2)-O(2v)-V(4xii) 97.80(13) V(2)-O(7ii)-V(4) 94.31(13) V(2)-O(2v)-V(4) 91.94(13) iii x V(2)-O(3 )-V(4 ) 92.98(12) V(2)-O(6v)-V(4x) 93.10(13) V(4)-O(2vii)-V(4xii) 98.78(13) V(4)-O(2v)-V(4xii) 98.78(13) ii xiv Yb-O(5 )-Yb 143.55(12)

Yb-O(5iv)c O(3)c O(7ix)c O(5ii)d O(1iii)d O(6ii)d O(2iii)d O(8ii)d O(4iii)d

3.384(3) 2.346(3) 2.384(3) 2.397(3) 2.412(3) 2.285(3) 2.263(3) 2.285(3) 2.289(3)

V(2)-V(2xi) V(4xii) V(4) V(4x) V(4)-V(4xii) -Ybx, xix Ybiii

3.0149(9) 3.0251(9) 2.8896(9) 2.8774(9) 3.0548(9) 5.7607(2) 5.9317(2)

a Axial O atoms. b Equatorial O atoms. c Planar-triangular type O atoms. d Trigonal-prism type O atoms. e Only Ybxiii and Ybxiv share one O atom with Yb, respectively. Symmetry codes: (i) 1/2 - x, 1/2 + y, 1/2 - z; (ii) 1 - x, 1 - y, 1 - z; (iii) 1 - x, 1 - y, - z; (iv) 1/2 + x, 1/ - y, -1/ + z; (v) 1/ - x, -1/ + y, 1/ - z; (vi) 1/ - x, 1/ + y, 3/ - z; (vii) 1/ + x, 1/ - y, 1/ + z; (viii) 1/ - x, -1/ + y, 3/ - z; (ix) 1/ 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 + x, 3/2 - y, -1/2 + z; (x) x, y, -1 + z; (xi) 1 - x, - y, - z; (xii) 1 - x, - y, 1 - z; (xiii) 3/2 - x, -1/2 + y, 1/2 - z; (xiv) 3/2 - x, 1/2 + y, 1/2 z; (xv) -1/2 + x, 3/2 - y, -1/2 + z; (xvi) 1/2 + x, 3/2 - y, 1/2 + z; (xvii) 2 - x, 1 - y, - z; (xviii) -1/2 + x, 3/2 - y, 1/2 + z; (xix) x, y, 1 + z.

of the R-phase among the five compositions. Yb2V2O6.95 resulted in pure pyrochlore, but Yb2V2O6.90 and Yb2V2O7.00 contained impurities YbVO3 and YbVO4, respectively. The pyrochlore phase showed no detectable differences in cell parameters among the three compositions. On the basis of nominal compositions, we conclude that the compositions of the R- and pyrochlore phases at 1473 K are respectively YbV4O8 and Yb2V2O6.95, and are free from solid solution, if any, small. X-ray powder diffraction profiles of R- and β-YbV4O8 are shown in Figure 2. A pure powder specimen of the R-phase was synthesized from the ideal nominal composition. However, we could not obtain the β-phase as a pure powder specimen. The best sample was synthesized from a nominal composition of YbV4O7.95, still including the pyrochlore phase as impurity (Figure 2). Though precise composition of the β-phase is an open issue, we conclude that the composition is stoichiometric YbV4O8 from analogy with that of the R-phase. Atomic fractional coordinates and equivalent isotropic thermal parameters, and interatomic distances and bond angles of R-YbV4O8 are listed in Tables 1 and 2, respectively. Those of β-YbV4O8 are shown in Tables 3 and 4. Discussion Pyrochlore phase in Yb-V-O system can be prepared by heating a mixture of Yb2O3 and V2O4 in vacuum at 1423-152330 or at 1673 K,31 by firing YbVO4, or a mixture of Yb2O3 and V2O5, at 1673 K in CO/CO2

Table 3. Atomic Coordinates and Equivalent Thermal Parameters (Å2) in β-YbV4O8 in A-Centered Pseudoorthorhombic Notationa atom

x

y

z

Beqb

Yb V(1) V(2) V(3) V(4) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8)

0.75786(4) 0.4261(2) 0.4089(2) 0.4558(2) 0.4179(2) 0.2030(7) 0.1135(7) 0.5387(7) 0.4157(7) 0.2080(7) 0.1200(7) 0.5138(7) 0.4139(7)

0.32880(2) 0.30927(7) 0.54857(7) 0.30550(7) 0.55291(7) 0.5793(3) 0.7377(3) 0.3879(3) 0.7135(3) 0.5785(3) 0.7370(3) 0.3951(3) 0.7148(3)

0.12127(8) 0.1259(3) 0.1247(3) 0.6232(3) 0.6261(3) 0.113(1) 0.118(2) 0.126(1) 0.117(1) 0.637(1) 0.635(1) 0.627(1) 0.634(1)

0.60(1) 0.61(5) 0.57(5) 0.63(5) 0.64(6) 0.8(3) 0.8(3) 0.7(2) 0.7(2) 0.9(3) 0.8(3) 0.6(2) 0.7(2)

a M ) 504.80, Monoclinic, A2 /d, a ) 9.030(5) Å, b ) 21.44(3) 1 Å, c ) 5.752(2) Å, R ) 89.911(3)°, V ) 1113.3 Å3, Z ) 8, dcalc ) 6.024 g‚cm-3, F(000) ) 1808, R ) 0.029, Rw ) 0.032, w ) 1/σ2(F), and ∆/σ < 0.0003. Symmetry operations: x, y, z; - x, - y, - z; x, 1/ + y, 1/ + z; - x, 1/ - y, 1/ - z; 1/ + x, 1/ - y, 1/ - z; 1/ - x, 2 2 2 2 2 4 4 2 1/ + y, 1/ + z; 1/ + x, 3/ - y, 3/ - z; 1/ - x, 3/ + y, 3/ + z. b B 4 4 2 4 4 2 4 4 eq ) 8/3π2 (a2a*2U11 + b2b*2U22 + c2c*2U33 + 2bb*cc*U23 cos R).

atmosphere,32 or by heating a mixture of YbVO3, YbVO4, and V2O3 (or YbVO3, V2O3, and V2O4) in an evacuated and sealed tube at 1473 K (this work). Kitayama et al. have studied a series of Ln2O3-V2O3-V2O5 systems precisely.20-29 In their work, however, the pyrochlore phase was not found by heating a mixture of Yb2O3 and V2O3 in H2/CO2 atmosphere at 1473 K.22 The difference may be explained by the kinetic point of view. At 1473 K, the reduction from YbVO3 to YbVO4 would be much faster than the competing one from YbVO3 to Yb2V2O6.95. In addition, the formation from a mixture of YbVO3 and

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Table 4. Interatomic Distances (Å) and Bond Angles (deg) in β-YbV4O8 V(1)-O(3)a

a

O(8i)a O(5ii)b O(1ii)b O(8iii)b O(4iv)b

1.969(8) 2.029(8) 1.974(7) 1.959(7) 2.065(8) 2.057(8)

V(3)-O(7)a O(4vi)a O(1vii)b O(5ii)b O(4iii)b O(8iii)b

1.991(7) 2.005(8) 2.053(7) 2.067(7) 1.935(8) 1.938(8)

V(2)-O(1)a O(2v)a O(7iii)b O(3iv)b O(6ii)b O(2ii)b

1.973(6) 2.005(7) 1.997(8) 2.038(8) 2.012(8) 1.984(9)

V(4)-O(5)a O(6viii)a O(3iii)b O(7iii)b O(2vii)b O(6ii)b

1.974(7) 2.017(7) 1.949(8) 1.933(7) 1.990(8) 2.011(8)

V(1)-V(1x) V(3x) V(3) V(3xi) V(3)-V(3xii) Ybe-Ybxiii, xiv Ybxv, xvi V(1)-O(8i)-V(1x) V(1)-O(8iii)-V(1x) V(1)-O(8i)-V(3x) V(1)-O(4iv)-V(3x) V(1)-O(5ii)-V(3) V(1)-O(8iii)-V(3) V(1)-O(1ii)-V(3xi) V(1)-O(4iv)-V(3xi) V(3)-O(4vi)-V(3xii) V(3)-O(4iii)-V(3xii)

3.203(4) 3.042(4) 2.873(3) 2.905(3) 2.901(4) 5.552(8) 5.685(3) 103.0(3) 103.0(3) 100.1(3) 97.0(3) 90.6(3) 91.7(3) 92.8(3) 93.4(3) 94.8(3) 94.8(3)

V-V Sharing Two O Atoms r zigzag f r zigzag f r along [001] f r along [001] f r zigzag f -Ybxvii Ybix, xviii V(2)-O(2v)-V(2iv) V(2)-O(2ii)-V(2iv) V(2)-O(2v)-V(4iii) V(2)-O(6ii)-V(4iii) V(2)-O(7iii)-V(4) V(2)-O(6ii)-V(4) V(2)-O(3iv)-V(4xi) V(2)-O(2ii)-V(4xi) V(4)-O(6viii)-V(4iii) V(4)-O(6ii)-V(4iii)

5.720(4) 5.731(2) 98.3(3) 98.3(3) 99.0(3) 97.9(3) 94.5(3) 91.7(3) 92.1(3) 92.5(3) 99.6(3) 99.6(3)

Yb-O(5ix)-Ybxiii

142.9(3)

Yb-O(5iii)-Ybxiv

142.9(3)

b

c

Yb-O(5ix)c O(3)c O(7ix)c O(5iii)d O(1iv)d O(6iii)d O(2iv)d O(8iii)d O(4iv)d

3.402(8) 2.350(6) 2.378(6) 2.446(7) 2.410(7) 2.272(7) 2.296(8) 2.294(8) 2.272(7)

V(2)-V(2iv) V(4iii) V(4) V(4xi) V(4)-V(4iii) -Ybxi, xix Ybx

3.018(4) 3.037(3) 2.887(3) 2.870(3) 3.076(4) 5.752(2) 5.939(4)

Axial O atoms. Equatorial O atoms. Planar-triangular type O atoms. Trigonal-prism type O atoms. e Only Ybxiii and Ybxiv share one O atom with Yb, respectively. Symmetry codes: (i) x, -1/2 + y, -1/2 + z, (ii) 1/2 - x, -1/4 + y, -1/4 + z, (iii) 1 - x, 1 - y, 1 - z, (iv) 1 - x, 1 - y, - z, (v) 1/2 + x, 5/4 - y, 1/4 - z, (vi) x, -1/2 + y, 1/2 + z, (vii) 1/2 - x, - 1/4 + y, 3/4 + z, (viii) 1/2 + x, 5/4 - y, 5/4 - z, (ix) 1/2 + x, 3/4 - y, 3/4 - z (x) 1 - x, 1/2 - y, 1/2 - z, (xi) x, y, -1 + z, (xii) 1 - x, 1/2 - y, 3/2 - z, (xiii) 3/2 - x, -1/4 + y, -1/4 + z, (xiv) 3/2 - x, 1/4 + y, 1/4 + z, (xv) -1/2 + x, 3/4 - y, -1/4 - z, (xvi) 1/2 + x, 3/4 - y, -1/4 - z, (xvii) 2 - x, 1/2 - y, 1/2 - z, (xviii) -1/2 + x, 3/4 - y, 3/4 - z, (xix) x, y, 1 + z.

YbVO4 to Yb2V2O6.95 would be sluggish; indeed, at least two heating runs (1 and 2-3 days) were required in our study at 1473 K for the pyrochlore-containing region to reach equilibria. This idea may also explain why neither Tm2V2O7 nor R-YbV4O8 was found in Kitayama et al.’s study.21,22 Probably, the kinetics does not affect the formation of Lu2V2O7 and LuV4O8.20 The true unit cell dimension for β-YbV4O8 is monoclinic, P21/n11 (Figure 2). In what follows, we discuss β-YbV4O8 in monoclinic A21/d11 setting (Table 3 and Figure 3) for the sake of comparison with R-YbV4O8. If we calculate the P-lattice parameters of the β-phase from the A-lattice parameters given in Table 3, we obtain the values a ) 9.030 Å, b ) 11.097 Å, c ) 5.752 Å, R ) 104.94° which are significantly deviating from the corresponding values determined for the powder specimen (Figure 2), a ) 9.0625(7) Å, b ) 11.0086(9) Å, c ) 5.7655(5) Å, R ) 105.070(7)°. The rather large discrepancies are due to the twinning and the ambiguity of indexing caused by the pseudoorthorhombic character of the β-phase. Figure 3 shows the crystal structures of R- and β-YbV4O8 viewed along [001]. Both structures can be interpreted as superlattice structures of orthorhombic CaFe2O433 with lattice relationship c(R-YbV4O8) ≈ c(β-YbV4O8) ≈ 2c(CaFe2O4). In CaFe2O4, all atoms are located on mirror planes of Pnam at z ) 1/4 and 3/4, while in R- and β-YbV4O8, atomic positions are slightly displaced from the corresponding planes. Atoms in Tables 1 and 3 are numbered in such a way that V(m)/ V(m + 2) and O(n)/O(n + 4) (m ) 1, 2; n ) 1-4)

d

correspond to Fe(m) and O(n) in CaFe2O4, respectively. Fe-O and V-O frameworks are essentially the same in CaFe2O4,33 R-YbV4O8, and β-YbV4O8. V atoms in Rand β-YbV4O8 are surrounded by six O atoms in a slightly distorted octahedral coordination. VO6 octahedra share edges to form zig-zag chains along the [001] direction. Adjacent chains are linked to each other through common octahedral corners, forming a threedimensional framework with tunnels along [001]. Yb3+ cations in both phases are accommodated within these tunnels; each of them surrounded by six O atoms in trigonal-prismatic coordination, with additional three O atoms capping the prism faces. These three O atoms are located within the same plane as Yb3+. However, one of them is distant from Yb3+; Yb-O(5iv) ) 3.384(3) Å in R-YbV4O8 and Yb-O(5ix) ) 3.402(8) Å in β-YbV4O8. The same situation is also encountered in other CaFe2O4type phases.33,42,43 The other Yb-O and V-O distances in R- and β-YbV4O8 (Tables 2 and 4) are consistent with r(VIIIYb3+) + r(IVO2-) ) 2.37 Å, and r(VIV3+) + r(IVO2-) ) 2.02 Å or r(VIV4+) + r(IVO2-) ) 1.96 Å.44 The three structures are distinguished by the arrangement of Yb3+ and Ca2+. Figure 4 illustrates the arrangements of Yb and Ca coordination polyhedra in the three structures. In CaFe2O4, a Ca coordination polyhedra neighbors eight Ca ones with which shares (42) Reid, A. F.; Wadsley, A. D.; Sienko, M. J. Inorg. Chem. 1968, 7, 112. (43) Akimoto, J; Takei, H. J. Solid State Chem. 1989, 79, 212. (44) Shannon, A. D. Acta Crystallogr. 1976, A32, 751.

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Figure 3. Crystal structures of R-YbV4O8 (upper) and β-YbV4O8 (lower, in A21/d notation) viewed along [001] drawn by ATOMS.49 Octahedra and circles indicate VO6 coordination octahedra and Yb atoms, respectively. The numbers out of parentheses show types of the V atoms. Odd numbers in parentheses indicate approximate z coordinates of the V and Yb atoms multiplied by 8.

at least one O atom; two of them in the tunnel, two in [100] direction, and four along [011] and [011h ] directions. Adjacent Ca atoms within the tunnels share basal faces of the trigonal prism. Neighboring Ca atoms form a chain in [100] direction by sharing two O atoms of planar-triangular type which cap the trigonal prism faces. Along [011] and [011 h ] directions, however, Ca cations are connected in such a way that the O atom with the largest distance to Ca2+ forms a corner of the trigonal prism around the adjacent Ca2+. In contrast to CaFe2O4 where Ca cations fully occupy all available 9-fold coordination sites, in R- and β-YbV4O8, Yb3+ cations are located in every other site so that occupied and vacant sites alternate along [001] and [100] to form a corrugated, pseudocentered rectangular net of Yb3+ cations parallel to (010). Within the net, the coordination polyhedra of adjacent Yb3+ cations have no common O atoms. The two vanadate phases are distinguished by the stacking of the Yb3+ nets. In β-YbV4O8, the nets are stacked with a stacking vector of (b + c)/4 or (b-c)/4, while in R-YbV4O8, the stacking vectors are b/2 + c/4 and b/2-c/4 alternating in a zigzag sequence. Adjacent Yb3+ cations which belong to

different nets share an O atom in the same way as the Ca cations along [011] and [011h ] directions in CaFe2O4. The shortest Yb-Yb distances in R- and β-YbV4O8 are as long as Yb-Ybxiii, xiv ) 5.4996(2) Å and Yb-Ybxiii, xiv ) 5.552(8) Å, respectively, although each pair of atoms share an O atom (Tables 2 and 4). There are no large differences between the O-atom-sharing Yb-Yb distances and the other Yb-Yb ones. Two pairs of atoms in each phase, Yb-Ybxvii and Yb-Ybiii in R-YbV4O8, and Yb-Ybxvii and Yb-Ybx in β-YbV4O8, are separated by a “wall” which consist of the zig-zag chain of the V octahedra. Nevertheless interatomic distances of these pairs are not so different from the O-atom-sharing YbYb distances. The maximum differences are 0.4321 Å in R-YbV4O8 and 0.387 Å in β-YbV4O8. In both phases, occupied and vacant sites are ordered as to achieve, in a sense, three-dimensionally uniform distributions of Yb3+ cations to minimize the electrostatic repulsion among trivalent Yb3+ cations. With the restriction of the V-O framework, such a uniform distribution is achievable exclusively in R- and β-YbV4O8 structures and their composite structures. The shortest Yb-Yb distances in R- and β-YbV4O8, 5.4996(2) and 5.552(8) Å, respectively, are much longer than the shortest Ln-

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Chem. Mater., Vol. 9, No. 1, 1997 147

Conclusion R- and β-YbV4O8 were synthesized and were characterized by X-ray diffraction. Their V-O frameworks are similar to that of Fe-O in the well-known CaFe2O4.33 Yb3+ occupy half of Ca2+ sites in ordered manners, resulting in superlattice structures of CaFe2O4. LuV4O8, whose powder X-ray diffraction data were previously reported by Kitayama et al. without indexing,20 seems to be isostructural with R-YbV4O8. A series of phase equilibrium studies of Ln2O3-V2O3-V2O5 systems have been carried out by Kitayama et al. at 1473 K by controlling oxygen partial pressure.20-29 R-YbV4O8, as well as Yb2V2O6.95 and Tm2V2O7 pyrochlore phases, did not appear under their experimental conditions.21,22 These suggest possible existence of R- and/or β-LnV4O8 with another Ln3+ cations. The shortest Yb-Yb distances in both R- and β-YbV4O8 are much longer than the shortest Ln-Ln distances in well-known ternary rare-earth oxides.31,45-48 If V cations of R- and/or β-YbV4O8 are fully replaced by cations free from unpaired electron, resulting phases appear to be good candidates for laser host materials, because long LnLn distance can decrease concentration quenching.

Figure 4. Locations of Ca or Yb coordination polyhedra in CaFe2O4 (upper), R-YbV4O8 (middle), and β-YbV4O8 (lower). Fractions express z coordinates of Ca atoms in terms of R-YbV4O8 cell dimension (upper), approximate z coordinates of Yb atoms (middle), and those defined by the A21/d notation (lower). Trigonal-prism type O atoms are located at the corner of triangles. Circles show planar-triangular type O atoms. The planar-triangular type O atoms are shared by two Ca atoms in CaFe2O4 but are not shared by two Yb atoms in R- and β-YbV4O8. Broken lines indicate bonding between distant O atom and Ca or Yb cation.

Ln distances in well-known ternary rare-earth oxides such as perovskites (GdFeO3:45 3.898 Å), pyrochlores (Yb2V2O7:31 3.512 Å), LnVO446 type phases (YbVO4:47 3.852 Å), and garnets (Yb3Fe5O12:48 3.076 Å).

Acknowledgment. We thank Mr. K. Kosuda for X-ray microanalysis, Mr. A. Sato for help in operation of the 4-circle X-ray diffractometer, and Prof. A. Lachgar for critical reading of the manuscript. Supporting Information Available: Listings of X-ray powder diffraction data of R- and β-YbV4O8 and anisotropic thermal parameters in R-YbV4O8 (25 pages); observed and calculated structure factors of R-YbV4O8 (11 pages). Ordering information is given on any current masthead page. CM960259+ (45) Geller, S. J. Chem. Phys. 1956, 24, 1236. (46) Baglio, J. A.; Gashurov, G. Acta Crystallogr. 1968, B24, 292. (47) Chakoumakos, B, C.; Abraham, M. M.; Boatner, L. A. J. Solid State Chem. 1994, 109, 197. (48) Euler, F.; Bruce, J. A. Acta Crystallogr. 1965, 19, 971. (49) Dowty, E. ATOMS; Shape Software: Kingsport, TN, 1993.