RbCa₂Nb₃O₁₀ from X-ray powder data

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.

Related literature

For the synthesis of RbCa₂Nb₃O₁₀, see: Dion et al. (1981). For related three-layer Dion–Jacobson analogues, see: CsCa₂Nb₃O₁₀ (Dion et al., 1984); RbSr₂Nb₃O₁₀ (Thangadurai et al., 2001); KCa₂Nb₃O₁₀ (Fukuoka et al., 2000). For the application of Dion–Jacobson phases, see: Thangadurai et al. (2001); Li et al. (2007); Ida et al. (2008); Compton & Osterloh (2009). For properties of RbCa₂Nb₃O₁₀, see: Thangadurai & Weppner (2001, 2004); Byeon et al. (2003).

Experimental

Crystal data

• RbCa₂Nb₃O₁₀

• M_r = 604.34

• Tetragonal, P 4/m m m

• a = 3.85865 (6) Å

• c = 14.9108 (3) Å

• V = 222.01 (1) ų

• Z = 1

• Cu Kα radiation

• T = 298 K

• Specimen shape: flat sheet

• 10 × 15 × 1 mm

• Specimen prepared at 1423 K

• Particle morphology: plate-like, white

Data collection

• PANalytical X'pert PRO diffractometer

• Specimen mounting: packed powder pellet

• Specimen mounted in reflection mode

• Scan method: continuous

• 2θ_min = 10.0, 2θ_max = 110.0°

• Increment in 2θ = 0.02°

Refinement

• R_p = 0.035

• R_wp = 0.053

• R_exp = 0.008

• S = 2.54

• Wavelength of incident radiation: 1.54178 Å

• Profile function: pseudo-Voigt

• 238 reflections

• 26 parameters

• Preferred orientation correction: March–Dollase (Dollase, 1986) AXIS 1 Ratio = 0.95964, h = k = 0, l = 1; correction range: min = 0.94007, max = 1.13156

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; method used to solve structure: coordinates taken from an isotypic compound (Thangadurai et al., 2001); program(s) used to refine structure: GSAS and EXPGUI; molecular graphics: VESTA (Momma & Izumi, 2008); software used to prepare material for publication: publCIF (Westrip, 2009).

Supporting information

S1. Comment

The Dion-Jacobson phase which was first discovered by Dion et al. (1981), has a general formula A'[A_n-1B_nO_3n+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.

Related literature

For the synthesis of RbCa₂Nb₃O₁₀, see: Dion et al. (1981). For related three-layer Dion–Jacobson analogues, see: CsCa₂Nb₃O₁₀ (Dion et al., 1984); RbSr₂Nb₃O₁₀ (Thangadurai et al., 2001); KCa₂Nb₃O₁₀ (Fukuoka et al., 2000). For the application of Dion–Jacobson phases, see: Thangadurai et al. (2001); Li et al. (2007); Ida et al. (2008); Compton et al. (2009). For properties of RbCa₂Nb₃O₁₀, see: Thangadurai & Weppner (2001); Byeon et al. (2003); Thangadurai & Weppner (2004).

S2. Experimental

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.

S3. Refinement

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.

Computing details

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).

Figures

	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.
	Fig. 2. The crystal structure of RbCa2Nb3O10. blue octahedron show NbO3 units with Nb^5+ cations as black spheres and O^2- anions as red spheres. Large green spheres represent Ca^2+ cations and large blue spheres Rb^+ cations.

Rubidium dicalcium triniobate(V)

Crystal data

RbCa2Nb3O10	Dx = 4.520 Mg m^−3
Mr = 604.34	Cu Kα radiation, λ = 1.54178 Å
Tetragonal, P4/mmm	T = 298 K
Hall symbol: -P 4 2	Particle morphology: plate-like
a = 3.85865 (6) Å	white
c = 14.9108 (3) Å	flat sheet, 10 × 15 mm
V = 222.01 (1) Å^3	Specimen preparation: Prepared at 1423 K
Z = 1

Data collection

PANalytical X'pert PRO diffractometer	Data collection mode: reflection
Radiation source: sealed tube	Scan method: continuous
Graphite monochromator	2θmin = 10.008°, 2θmax = 109.985°, 2θstep = 0.017°
Specimen mounting: packed powder pellet

Refinement

Refinement on F^2	? data points
Least-squares matrix: full	Profile function: pseudo-Voigt
Rp = 0.035	26 parameters
Rwp = 0.053	0 restraints
Rexp = 0.008	w = 1/[σ^2(Fo^2) + (0.0677P)^2] where P = (Fo^2 + 2Fc^2)/3
R(F^2) = 0.08530	(Δ/σ)max = 0.020
χ^2 = 6.452	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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	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)*

Geometric parameters (Å, º)

Rb1—O4	3.138 (4)	Ca1—O3	2.560 (4)
Rb1—O4^i	3.138 (4)	Ca1—O3^ii	2.560 (4)
Rb1—O4^ii	3.138 (4)	Ca1—O3^iv	2.560 (4)
Rb1—O4^iii	3.138 (4)	Ca1—O3^v	2.560 (4)
Rb1—O4	3.138 (4)	Nb1—O1^vi	1.9293 (1)
Rb1—O4	3.138 (4)	Nb1—O1	1.9293 (1)
Rb1—O4	3.138 (4)	Nb1—O1^vii	1.9293 (1)
Rb1—O4	3.138 (4)	Nb1—O1^iv	1.9293 (1)
Ca1—O1	2.921 (2)	Nb1—O2	1.877 (7)
Ca1—O1^ii	2.921 (2)	Nb1—O2	1.877 (7)
Ca1—O1^iv	2.921 (2)	Nb2—O2	2.379 (7)
Ca1—O1^v	2.921 (2)	Nb2—O3^vi	1.9663 (11)
Ca1—O2	2.7468 (9)	Nb2—O3	1.9663 (11)
Ca1—O2^i	2.7468 (9)	Nb2—O3^vii	1.9663 (11)
Ca1—O2^ii	2.7468 (9)	Nb2—O3^iv	1.9663 (11)
Ca1—O2^iii	2.7468 (9)	Nb2—O4	1.650 (8)
O1^vi—Nb1—O1	180.0	O1^iv—Nb1—O2	90.0
O1^vi—Nb1—O1^vii	90.0	O2—Nb1—O2	180.0
O1^vi—Nb1—O1^iv	90.0	O3^vi—Nb2—O3	157.8 (3)
O1^vi—Nb1—O2	90.0	O3^vi—Nb2—O3^vii	87.87 (6)
O1^vi—Nb1—O2	90.0	O3^vi—Nb2—O3^iv	87.87 (6)
O1—Nb1—O1^vii	90.0	O3^vi—Nb2—O4	101.12 (16)
O1—Nb1—O1^iv	90.0	O3—Nb2—O3^vii	87.87 (6)
O1—Nb1—O2	90.0	O3—Nb2—O3^iv	87.87 (6)
O1—Nb1—O2	90.0	O3—Nb2—O4	101.12 (16)
O1^vii—Nb1—O1^iv	180.0	O3^vii—Nb2—O3^iv	157.8 (3)
O1^vii—Nb1—O2	90.0	O3^vii—Nb2—O4	101.12 (16)
O1^vii—Nb1—O2	90.0	O3^iv—Nb2—O4	101.12 (16)
O1^iv—Nb1—O2	90.0

Symmetry codes: (i) x, y+1, z; (ii) x+1, y, z; (iii) x+1, y+1, z; (iv) −y+1, x, z; (v) −y+1, x+1, z; (vi) x, y−1, z; (vii) −y, x, z.