Acta Cryst. (2009). E65, i44 [ doi:10.1107/S1600536809018157 ]
Rubidium dicalcium triniobate(V), RbCa2Nb3O10, has been synthesized by solid-state reaction and its crystal structure refined from X-ray powder diffraction data using Rietveld analysis. The compound is a three-layer perovskite Dion-Jacobson phase with the perovskite-like slabs derived by termination of the three-dimensional CaNbO3 perovskite structure along the ab plane. The rubidium ions (4/mmm symmetry) are located in the interstitial space.
RbCa2Nb3O10 powders were prepared by a conventional solid state reaction described previously (Byeon et al., 2003). All starting materials were of analytical grade and were used without further purification. Stoichiometric amounts of CaCO3 and Nb2O5 with a 50% molar excess of Rb2CO3 were mixed together and heated in air at 1423 K for 24 h (heating rate 5 K /min). The calcination procedure was repeated one time after grinding to ensure a complete reaction. A 50% molar excess of Rb2CO3 was used in the reaction to offset the volatilization of the alkali oxides at the synthesis temperature. The products were washed thoroughly with distilled water to remove excess alkali oxides, and were then dried at 393 K overnight.
All peaks of the XRD pattern could be indexed on a tetragonal cell and the systematic absences show simple tetragonal symmetry. The P4/mmm crystal structure of RbSr2Nb3O10 (Thangadurai et al., 2001) was used as a starting model for the Rietveld refinement of the structure of RbCa2Nb3O10. The corresponding isotropic atomic displacement parameters of all oxygen atoms are constrained to be equal. The March-Dollase option in the EXPGUI program was applied to correct 00 l preferential orientation which were often observed in the Rietveld refinement of the layered perovskites.
Data collection: X'pert Data Collector (PANalytical, 2003); cell refinement: GSAS (Larson & Von Dreele, 2000) and EXPGUI (Toby, 2001); data reduction: X'pert Data Collector (PANalytical, 2003); program(s) used to solve structure: coordinates taken from an isotypic compound (Thangadurai et al., 2001); program(s) used to refine structure: GSAS (Larson & Von Dreele, 2000) and EXPGUI (Toby, 2001); molecular graphics: VESTA (Momma & Izumi, 2008); software used to prepare material for publication: publCIF (Westrip, 2009).
![[Figure 1]](./br2107fig1thm.gif)
| Fig. 1. Rietveld difference plot for the multi-phase refinement of RbCa2Nb3O10. The red crosses, and green and pink lines show respectively the observed, calculated and difference plots. Calculated Bragg reflection positions are indicated by black lines for the RbCa2Nb3O10 phase. |
![[Figure 2]](./br2107fig2thm.gif)
| Fig. 2. The crystal structure of RbCa2Nb3O10. blue octahedron show NbO3 units with Nb5+ cations as black spheres and O2- anions as red spheres. Large green spheres represent Ca2+ cations and large blue spheres Rb+ cations. |
| RbCa2Nb3O10 | V = 222.009 (7) Å3 |
| Mr = 604.34 | Z = 1 |
| Tetragonal, P4/mmm | Dx = 4.520 Mg m−3 |
| Hall symbol: -P 4 2 | Cu Kα radiation λ = 1.54178 Å |
| a = 3.85865 (6) Å | T = 298 K |
| b = 3.85865 (6) Å | Specimen shape: flat sheet |
| c = 14.9108 (3) Å | 10 × 15 × 1 mm |
| α = 90º | Specimen prepared at 1423 K |
| β = 90º | Particle morphology: plate-like, white |
| γ = 90º |
| PANalytical X'pert PRO diffractometer | Scan method: continuous |
| Radiation source: sealed tube | T = 298 K |
| Monochromator: graphite | 2θmin = 10.01, 2θmax = 109.99º |
| Specimen mounting: packed powder pellet | Increment in 2θ = 0.02º |
| Specimen mounted in reflection mode |
| Refinement on F2 | Profile function: pseudo-Voigt |
| Least-squares matrix: full | 26 parameters |
| Rp = 0.035 | w = 1/[σ2(Fo2) + (0.0677P)2] where P = (Fo2 + 2Fc2)/3 |
| Rwp = 0.053 | (Δ/σ)max = 0.020 |
| Rexp = 0.008 | Extinction correction: none |
| S = 2.54 | Preferred orientation correction: March–Dollase (Dollase, 1986) AXIS 1 Ratio= 0.95964, h = k = 0, l = 1. Prefered orientation correction range: min = 0.94007, Max = 1.13156 |
| Wavelength of incident radiation: 1.54178 Å |
| RbCa2Nb3O10 | V = 222.009 (7) Å3 |
| Mr = 604.34 | Z = 1 |
| Tetragonal, P4/mmm | Cu Kα radiation λ = 1.54178 Å |
| a = 3.85865 (6) Å | T = 298 K |
| b = 3.85865 (6) Å | Specimen shape: flat sheet |
| c = 14.9108 (3) Å | 10 × 15 × 1 mm |
| α = 90º | Specimen prepared at ? kPa |
| β = 90º | Specimen prepared at 1423 K |
| γ = 90º | Particle morphology: plate-like, white |
| PANalytical X'pert PRO diffractometer | Scan method: continuous |
| Specimen mounting: packed powder pellet | 2θmin = 10.01, 2θmax = 109.99º |
| Specimen mounted in reflection mode | Increment in 2θ = 0.02º |
| Rp = 0.035 | Wavelength of incident radiation: 1.54178 Å |
| Rwp = 0.053 | Excluded region(s): ? |
| Rexp = 0.008 | Profile function: pseudo-Voigt |
| RB = ? | 26 parameters |
| S = 2.54 | Preferred orientation correction: March–Dollase (Dollase, 1986) AXIS 1 Ratio= 0.95964, h = k = 0, l = 1. Prefered orientation correction range: min = 0.94007, Max = 1.13156 |
| x | y | z | Uiso*/Ueq | ||
| Rb1 | 0.5 | 0.5 | 0.5 | 0.0433 (8)* | |
| Ca1 | 0.5 | 0.5 | 0.14706 (19) | 0.0281 (8)* | |
| Nb1 | 0.0 | 0.0 | 0.0 | 0.0127 (6)* | |
| Nb2 | 0.0 | 0.0 | 0.28537 (8) | 0.0134 (5)* | |
| O1 | 0.0 | 0.5 | 0.0 | 0.0716 (14)* | |
| O2 | 0.0 | 0.0 | 0.1258 (5) | 0.0716 (14)* | |
| O3 | 0.0 | 0.5 | 0.2599 (4) | 0.0716 (14)* | |
| O4 | 0.0 | 0.0 | 0.3960 (6) | 0.0716 (14)* |
| Rb1—O4 | 3.138 (4) | Ca1—O3 | 2.560 (4) |
| Rb1—O4i | 3.138 (4) | Ca1—O3ii | 2.560 (4) |
| Rb1—O4ii | 3.138 (4) | Ca1—O3viii | 2.560 (4) |
| Rb1—O4iii | 3.138 (4) | Ca1—O3ix | 2.560 (4) |
| Rb1—O4iv | 3.138 (4) | Nb1—O1x | 1.929320 (30) |
| Rb1—O4v | 3.138 (4) | Nb1—O1 | 1.929320 (30) |
| Rb1—O4vi | 3.138 (4) | Nb1—O1xi | 1.929320 (30) |
| Rb1—O4vii | 3.138 (4) | Nb1—O1viii | 1.929320 (30) |
| Ca1—O1 | 2.9207 (22) | Nb1—O2 | 1.877 (7) |
| Ca1—O1ii | 2.9207 (22) | Nb1—O2xii | 1.877 (7) |
| Ca1—O1viii | 2.9207 (22) | Nb2—O2 | 2.379 (7) |
| Ca1—O1ix | 2.9207 (22) | Nb2—O3x | 1.9663 (11) |
| Ca1—O2 | 2.7468 (9) | Nb2—O3 | 1.9663 (11) |
| Ca1—O2i | 2.7468 (9) | Nb2—O3xi | 1.9663 (11) |
| Ca1—O2ii | 2.7468 (9) | Nb2—O3viii | 1.9663 (11) |
| Ca1—O2iii | 2.7468 (9) | Nb2—O4 | 1.650 (8) |
| O1x—Nb1—O1 | 180.0 | O1viii—Nb1—O2xii | 90.0 |
| O1x—Nb1—O1xi | 90.0 | O2—Nb1—O2xii | 180.0 |
| O1x—Nb1—O1viii | 90.0 | O3x—Nb2—O3 | 157.75 (32) |
| O1x—Nb1—O2 | 90.0 | O3x—Nb2—O3xi | 87.87 (6) |
| O1x—Nb1—O2xii | 90.0 | O3x—Nb2—O3viii | 87.87 (6) |
| O1—Nb1—O1xi | 90.0 | O3x—Nb2—O4 | 101.12 (16) |
| O1—Nb1—O1viii | 90.0 | O3—Nb2—O3xi | 87.87 (6) |
| O1—Nb1—O2 | 90.0 | O3—Nb2—O3viii | 87.87 (6) |
| O1—Nb1—O2xii | 90.0 | O3—Nb2—O4 | 101.12 (16) |
| O1xi—Nb1—O1viii | 180.0 | O3xi—Nb2—O3viii | 157.75 (32) |
| O1xi—Nb1—O2 | 90.0 | O3xi—Nb2—O4 | 101.12 (16) |
| O1xi—Nb1—O2xii | 90.0 | O3viii—Nb2—O4 | 101.12 (16) |
| O1viii—Nb1—O2 | 90.0 |
| Symmetry codes: (i) x, y+1, z; (ii) x+1, y, z; (iii) x+1, y+1, z; (iv) , , +1; (v) , +1, +1; (vi) +1, , +1; (vii) +1, +1, +1; (viii) −y+1, x, z; (ix) −y+1, x+1, z; (x) x, y−1, z; (xi) −y, x, z; (xii) , , . |
The authors are grateful for financial support by the National Natural Science Foundation of China, the 973 Projects of China and the Program for New Century Excellent Talents in Universities (NCET).
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The Dion-Jacobson phase which was first discovered by Dion et al. (1981), has a general formula A'[An-1BnO3n+1], where A'is a monovalent ion, A is a divalent alkaline earth metal ion and B is a tetravalent or pentavalent transition metal ion. Related crystal structures of three-layer Dion-Jacobson phase have been reported for KCa2Nb3O10 (Dion et al., 1984), RbSr2Nb3O10 (Thangadurai et al., 2001), and KCa2Nb3O10 (Fukuoka et al., 2000). Although the three-layer Dion-Jacobson phase RbCa2Nb3O10 was first synthesized by Dion et al. (1981), its crystal structure has not yet been reported. The structure of RbCa2Nb3O10 has now been refined by the Rietveld method from powder diffraction data in the present communication.
The observed, calculated and intensities difference plots of the Rietveld refinement are shown in Fig. 1. There are some 00 l preferential orientation which were often observed in the Rietveld refinement of the layered perovskites. Then we applied the March-Dollase option for a correction in the EXPGUI program and obtain the best result finally.
The structure of the compound is illustrated in Fig. 2. The structure consists of three layers of corner-sharing NbO6 octahedra that run perpendicular to the c axis; adjacent sets of layers are staggered.
Table 1 shows refined interatomic distances and angles for the RbCa2Nb3O10 structure. The octahedra forming the inner layer are less distorted with Nb—O distances ranging from 1.876 (7) to 1.92932 (3)Å (Table 2), which is typical for layered perovskites involving Nb(V). As it is well known in layered perovskites, the NbO6 octahedra forming the outer layer of the slabs are characterized by off-centering of the Nb atoms, leading to four equal equatorial Nb—O distances within the perovskite layers [1.9663 (11) Å], a short Nb—O bond toward the interlayer spacing [1.650 (8) Å], and a long opposite Nb—O bond [2.379 (7) Å]. Such a distortion is quite similar to that encountered in homologous niobates and tantalates where the niobium shows an out-of-plane distortion, moving away from the more positively charged calcium towards the rubidium layer. Similar behavior has been observed in a number of d0 systems containing niobium, tantalum, and titanium. This has been attributed to a second-order Jahn-Teller effect. Concerning the interlayer, the rubidium ions are coordinated with eight terminal oxygen atoms to form the same eight Rb—O bonds [3.318 (4) Å]. These distances, as for the Ca—O bonds [2.560 (4)–2.9207 (22) Å] are close to those commonly observed in layered perovskites.