Rietveld refinement of KLaTiO₄ from X-ray powder data

Potassium lanthanum titanate(IV), KLaTiO₄, has been synthesized by conventional solid-state reaction. It crystallizes isotypically with the NaLnTiO₄ (Ln = La, Pr, Nd, Sm, Eu, Gd, Y and Lu) family. Five of the six atoms in the asymmetric unit (one K, one La, one Ti and two O atoms) are situated on sites with 4mm symmetry, whereas one O atom has 2mm. site symmetry. The crystal structure can be described as being composed of single layers of distorted corner-sharing TiO₆ octa­hedra extending parallel to (001). The layers are alternately separated by K⁺ and La³⁺ cations along [001]. The coordination number of both K⁺ and La³⁺ cations is nine, resulting in distorted KO₉ and LaO₉ polyhedra.

Related literature

For the isotypic NaLnTiO₄ (Ln = La, Pr, Nd, Sm, Eu, Gd, Y and Lu) family, see: Toda et al. (1996a). Ortho­rhom­bic symmetry for other members of this family has been reported by Nishimoto et al. (2006). Decomposition products of NaLnTiO₄ were investigated by Toda et al. (1996b). For preparation by ion-exchange and structure analysis of KLnTiO₄ (Ln = La, Nd, Sm, Eu, Gd, Dy) compounds, see: Schaak & Mallouk (2001). For hydro­thermal preparation of similar compounds, see: Dairong et al. (1999). For crystallographic background, see: Howard (1982); Thompson et al. (1987).

Experimental

Crystal data

• KLaTiO₄

• M_r = 289.90

• Tetragonal, P 4/n m m

• a = 3.84155 (10) Å

• c = 13.4695 (4) Å

• V = 198.78 (1) ų

• Z = 2

• Cu Kα radiation, λ = 1.54060, 1.54443 Å

• T = 298 K

• flat sheet, 20 × 20 mm

Data collection

• PANalytical X'pert PRO diffractometer

• Specimen mounting: packed powder pellet

• Data collection mode: reflection

• Scan method: continuous

• 2θ_min = 9.872°, 2θ_max = 109.815°, 2θ_step = 0.017°

Refinement

• R_p = 0.046

• R_wp = 0.068

• R_exp = 0.046

• R(F²) = 0.047

• χ² = 2.220

• 5880 data points

• 60 parameters

Table 1
Selected bond lengths (Å)

K1—O1	3.065 (4)
K1—O2	2.765 (9)
K1—O2^i	2.7242 (7)
La1—O1	2.530 (3)
La1—O3^ii	2.339 (7)
La1—O3^i	2.7628 (12)
Ti1—O1	1.9635 (12)
Ti1—O2^iii	1.775 (9)
Ti1—O3^iii	2.558 (7)
Symmetry codes: (i) -x, -y, -z+1; (ii) x, y, z+1; (iii) -x+1, -y+1, -z+1.

Data collection: X'pert Data Collector (PANalytical, 2003); cell refinement: GSAS (Larson & Von Dreele, 2004) and EXPGUI (Toby, 2001); data reduction: X'pert Data Collector; method used to solve structure: coordinates taken from an isotypic compound (Toda et al., 1996b); program(s) used to refine structure: GSAS and EXPGUI; molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Layered perovskites that belong to the Ruddlesden-Popper A'₂[A_n-1B₂O_3n+1] familiy (A' = alkali, A = alkaline earth or rare earth cation; B= transition metal cation) possess a variety of interesting properties, such as superconductivity, colossal magnetoresistance, ferroelectricity, as well as catalytic activity. The structure of KLaTiO₄ we report here is a n = 1 member of this familiy. Isotypic crystal structures have been reported for NaLnTiO₄ (Ln = La, Pr, Nd, Sm, Eu, Gd, Y and Lu; Toda et al., 1996a) in the space group P4/nmm.

Schaak & Mallouk (2001) reported the KLnTiO₄ (Ln= La, Nd, Sm, Eu, Gd, Dy) family of compounds to crystallize in space group Pbcm, as determined from Rietveld refinements of X-ray powder data. We tested both Pbcm and P4/nmm space groups with the underlying structures KLnTiO₄ (P4/nmm; Schaak & Mallouk, 2001) and NaLnTiO₄ (P4/nmm; Toda et al., 1996a) as starting models for Rietveld refinement of KLaTiO₄. The results revealed the P4/nmm model to be significantly better than the Pbcm model. It is well-know that different rare earth elements can affect the crystal structure dramatically. In single layer Ruddlesden-Popper phase perovskites some studies reported that NaLnTiO₄ compounds have tetragonal symmetry for Ln = La—Nd, while an orthorhombic symmetry is observed for Ln = Sm—Lu and Y (Nishimoto et al., 2006). We can infer that a similar situation might be present for KLnTiO₄ compounds. We ascribe the difference in symmetry between KLaTiO₄ obtained through solid state reactions (tetragonal) and through ion-exchange (orthorhombic) to the different temperature treatment (higher temperatures for the solid state reaction route).

Other methods used to prepare KLaTiO₄ have been reported previously, like an ion exchange method by Schaak & Mallouk (2001) and a hydrothermal method by Dairong et al. (1999). To our knowledge, a solid state route to synthesize this compound and its detailed structure analysis based on Rietveld refinement from X-ray powder diffraction data has not been reported. KLaTiO₄ easily decomposes at high temperature and is converted into the three-layer Ruddlesden-Popper phase K₂La₂Ti₃O₁₀. This phenomenon is also found in during preparation of NaLaTiO₄ reported by Toda et al. (1996b). Therefore we modified the reaction conditions on the basis of the preparation of NaLaTiO₄ and obtained a single phase product successfully.

Fig. 1 shows the observed difference plots (calculated, observed) of the Rietveld refinement.

Fig. 2 illustrates the structure of KLaTiO₄. It consists of a single layer of corner-sharing distorted TiO₆ octahedra extending parallel to (001). The layers are separated by alternating layers of K⁺ and La³⁺ cations along [001]. The TiO₆ octahedra (4mm symmetry) are considerably distorted. They have four equal equatorial Ti—O distances [1.9635 (12) Å], one very short Ti—O distance [1.775 (9) Å] toward the K layer and a significantly longer Ti—O distance [2.558 (7) Å] towards the La layer. The corresponding coordination polyhedra around the K⁺ and La³⁺ cations are distorted KO₉ and LaO₉ polyhedra, each with 4mm symmetry.

Related literature

For the isotypic NaLnTiO₄ (Ln = La, Pr, Nd, Sm, Eu, Gd, Y and Lu) family, see: Toda et al. (1996a). Orthorhombic symmetry for other members of this family has been reported by Nishimoto et al. (2006). Decomposition products of NaLnTiO₄ were investigated by Toda et al. (1996b). For preparation by ion-exchange and structure analysis of KLnTiO₄ (Ln = La, Nd, Sm, Eu, Gd, Dy) compounds, see: Schaak & Mallouk (2001). For hydrothermal preparation of similar compounds, see: Dairong et al. (1999). For crystallographic background, see: Howard (1982); Thompson et al. (1987).

S2. Experimental

The sample was prepared by conventional solid-state reaction. The starting materials were KNO₃, La₂O₃ and TiO₂ in a molar ratio of 2:1:2. An excess of KNO₃ (55 mol%) was added to compensate for the loss due to the volatilization of the potassium component. La₂O₃ was heated to 1173 K for 10 h prior to use to remove water and carbonate impurities. The mixture was then ground and calcined at 1223 K for 30 min.

S3. Refinement

The crystal structure of NaLaTiO₄ (Toda et al., 1996b) in the spacegroup P4/nmm was used as a starting model for the final Rietveld refinement of the KLaTiO₄ structure. Isotropic displacement parameters were used for all atoms. The March-Dollase option in the EXPGUI program (Toby, 2001) was applied to correct for preferential orientation along [00l] which is often observed for such layered perovskites.

Computing details

Data collection: X'pert Data Collector (PANalytical, 2003); cell refinement: GSAS (Larson & Von Dreele, 2004) and EXPGUI (Toby, 2001); data reduction: X'pert Data Collector (PANalytical, 2003); program(s) used to solve structure: coordinates taken from an isotypic compound (Toda et al., 1996b); program(s) used to refine structure: GSAS (Larson & Von Dreele, 2004) and EXPGUI (Toby, 2001); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. Rietveld difference plot for the refinement of KLaTiO4.
	Fig. 2. The crystal structure of KLaTiO4 in a projection along [010].

Potassium lanthanum titanate

Crystal data

KLaTiO4	Z = 2
Mr = 289.90	Dx = 4.848 Mg m^−3
Tetragonal, P4/nmm	Cu Kα radiation, λ = 1.54060, 1.54443 Å
Hall symbol: -p 4a 2a	T = 298 K
a = 3.84155 (10) Å	white
c = 13.4695 (4) Å	flat sheet, 20 × 20 mm
V = 198.78 (1) Å^3	Specimen preparation: Prepared at 1223 K

Data collection

PANalytical X'pert PRO diffractometer	Data collection mode: reflection
Radiation source: sealed tube	Scan method: continuous
Graphite monochromator	2θmin = 9.872°, 2θmax = 109.815°, 2θstep = 0.017°
Specimen mounting: packed powder pellet

Refinement

Refinement on F^2	Excluded region(s): none
Least-squares matrix: full	Profile function: CW Profile function number 2 with 18 terms Profile coefficients for Simpson's rule integration of pseudovoigt function C.J. Howard (1982). J. Appl. Cryst.,15,615-620. P. Thompson, D.E. Cox & J.B. Hastings (1987). J. Appl. Cryst.,20,79-83. #1(GU) = 0.000 #2(GV) = -2.261 #3(GW) = -9.290 #4(LX) = 4.310 #5(LY) = 17.630 #6(trns) = 0.000 #7(asym) = 3.5282 #8(shft) = 0.0000 #9(GP) = 17.284 #10(stec)= 0.00 #11(ptec)= 0.00 #12(sfec)= 0.00 #13(L11) = 0.000 #14(L22) = 0.000 #15(L33) = 0.000 #16(L12) = 0.000 #17(L13) = 0.000 #18(L23) = 0.000 Peak tails are ignored where the intensity is below 0.0010 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rp = 0.046	60 parameters
Rwp = 0.068	0 restraints
Rexp = 0.046	w = 1/[σ^2(Fo^2) + (0.0677P)^2] where P = (Fo^2 + 2Fc^2)/3
R(F^2) = 0.04713	(Δ/σ)max = 0.01
χ^2 = 2.220	Background function: GSAS Background function number 1 with 36 terms. Shifted Chebyshev function of 1st kind 1: 353.285 2: -361.136 3: 220.846 4: -104.260 5: 61.8271 6: -33.1030 7: 19.7877 8: -5.01446 9: 3.42337 10: -3.14370 11: 0.340114 12: 2.15882 13: -0.130836 14: -1.88421 15: 5.08631 16: -1.48077 17: 4.42719 18: 2.91556 19: -3.924060E-0220: 0.679453 21: 5.77738 22: -2.47188 23: 3.81643 24: 3.21357 25: -4.71396 26: -1.63350 27: 0.665874 28: -7.16378 29: -7.040150E-0230: 3.04932 31: -2.36381 32: 0.787399 33: 4.27144 34: -2.96952 35: 4.90415 36: 1.54599
5880 data points	Preferred orientation correction: March-Dollase AXIS 1 Ratio= 0.96438 h= 0.000 k= 0.000 l= 1.000 Prefered orientation correction range: Min= 0.94706, Max= 1.11492

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

	x	y	z	Uiso*/Ueq
K1	0.25	0.25	0.5950 (2)	0.0278 (9)*
LA1	0.25	0.25	0.89446 (6)	0.0199 (4)*
TI1	0.75	0.75	0.74203 (19)	0.0151 (7)*
O1	0.75	0.25	0.7723 (4)	0.0189 (17)*
O2	0.25	0.25	0.3897 (6)	0.039 (2)*
O3	0.25	0.25	0.0681 (5)	0.018 (2)*

Geometric parameters (Å, º)

K1—O1^i	3.065 (4)	La1—O1^iii	2.530 (3)
K1—O1	3.065 (4)	La1—O3^viii	2.339 (7)
K1—O1^ii	3.065 (4)	La1—O3^iv	2.7628 (12)
K1—O1^iii	3.065 (4)	La1—O3^v	2.7628 (12)
K1—O2	2.765 (9)	La1—O3^vi	2.7628 (12)
K1—O2^iv	2.7242 (7)	La1—O3^vii	2.7628 (12)
K1—O2^v	2.7242 (7)	Ti1—O1	1.9635 (12)
K1—O2^vi	2.7242 (7)	Ti1—O1^ix	1.9635 (12)
K1—O2^vii	2.7242 (7)	Ti1—O1^iii	1.9635 (12)
La1—O1^i	2.530 (3)	Ti1—O1^x	1.9635 (12)
La1—O1	2.530 (3)	Ti1—O2^vii	1.775 (9)
La1—O1^ii	2.530 (3)	Ti1—O3^vii	2.558 (7)
O1^i—K1—O1	77.62 (13)	O1^i—La1—O3^v	65.85 (12)
O1^i—K1—O1^ii	52.61 (8)	O1^i—La1—O3^vi	130.30 (11)
O1^i—K1—O1^iii	52.61 (8)	O1^i—La1—O3^vii	130.30 (11)
O1^i—K1—O2	141.19 (6)	O1—La1—O1^ii	64.95 (9)
O1^i—K1—O2^iv	59.94 (15)	O1—La1—O1^iii	64.95 (9)
O1^i—K1—O2^v	59.94 (15)	O1—La1—O3^viii	130.59 (9)
O1^i—K1—O2^vi	112.52 (18)	O1—La1—O3^iv	130.30 (11)
O1^i—K1—O2^vii	112.52 (18)	O1—La1—O3^v	130.30 (11)
O1—K1—O1^ii	52.61 (8)	O1—La1—O3^vi	65.85 (12)
O1—K1—O1^iii	52.61 (8)	O1—La1—O3^vii	65.85 (12)
O1—K1—O2	141.19 (6)	O1^ii—La1—O1^iii	98.81 (17)
O1—K1—O2^iv	112.52 (18)	O1^ii—La1—O3^viii	130.59 (9)
O1—K1—O2^v	112.52 (18)	O1^ii—La1—O3^iv	65.85 (12)
O1—K1—O2^vi	59.94 (15)	O1^ii—La1—O3^v	130.30 (11)
O1—K1—O2^vii	59.94 (15)	O1^ii—La1—O3^vi	65.85 (12)
O1^ii—K1—O1^iii	77.62 (13)	O1^ii—La1—O3^vii	130.30 (11)
O1^ii—K1—O2	141.19 (6)	O1^iii—La1—O3^viii	130.59 (9)
O1^ii—K1—O2^iv	59.94 (15)	O1^iii—La1—O3^iv	130.30 (11)
O1^ii—K1—O2^v	112.52 (18)	O1^iii—La1—O3^v	65.85 (12)
O1^ii—K1—O2^vi	59.94 (15)	O1^iii—La1—O3^vi	130.30 (11)
O1^ii—K1—O2^vii	112.52 (18)	O1^iii—La1—O3^vii	65.85 (12)
O1^iii—K1—O2	141.19 (6)	O3^viii—La1—O3^iv	79.48 (14)
O1^iii—K1—O2^iv	112.52 (18)	O3^viii—La1—O3^v	79.48 (14)
O1^iii—K1—O2^v	59.94 (15)	O3^viii—La1—O3^vi	79.48 (14)
O1^iii—K1—O2^vi	112.52 (18)	O3^viii—La1—O3^vii	79.48 (14)
O1^iii—K1—O2^vii	59.94 (15)	O3^iv—La1—O3^v	88.09 (5)
O2—K1—O2^iv	94.34 (18)	O3^iv—La1—O3^vi	88.09 (5)
O2—K1—O2^v	94.34 (18)	O3^iv—La1—O3^vii	159.0 (3)
O2—K1—O2^vi	94.34 (18)	O3^v—La1—O3^vi	159.0 (3)
O2—K1—O2^vii	94.34 (18)	O3^v—La1—O3^vii	88.09 (5)
O2^iv—K1—O2^v	89.67 (3)	O3^vi—La1—O3^vii	88.09 (5)
O2^iv—K1—O2^vi	89.67 (3)	O1—Ti1—O1^ix	156.1 (3)
O2^iv—K1—O2^vii	171.3 (4)	O1—Ti1—O1^iii	87.53 (7)
O2^v—K1—O2^vi	171.3 (4)	O1—Ti1—O1^x	87.53 (7)
O2^v—K1—O2^vii	89.67 (3)	O1—Ti1—O2^vii	101.97 (16)
O2^vi—K1—O2^vii	89.67 (3)	O1^ix—Ti1—O1^iii	87.53 (7)
O1^i—La1—O1	98.81 (17)	O1^ix—Ti1—O1^x	87.53 (7)
O1^i—La1—O1^ii	64.95 (9)	O1^ix—Ti1—O2^vii	101.97 (16)
O1^i—La1—O1^iii	64.95 (9)	O1^iii—Ti1—O1^x	156.1 (3)
O1^i—La1—O3^viii	130.59 (9)	O1^iii—Ti1—O2^vii	101.97 (16)
O1^i—La1—O3^iv	65.85 (12)	O1^x—Ti1—O2^vii	101.97 (16)

Symmetry codes: (i) x−1, y, z; (ii) −y+1/2, x−1, z; (iii) −y+1/2, x, z; (iv) −x, −y, −z+1; (v) −x, −y+1, −z+1; (vi) −x+1, −y, −z+1; (vii) −x+1, −y+1, −z+1; (viii) x, y, z+1; (ix) x, y+1, z; (x) −y+3/2, x, z.