La(Ni_2/3Nb_1/3)O₃ by neutron powder diffraction

Lanthanum nickel niobium trioxide has been synthesized and its structure refined for the first time. It was found to be a member of the family of technologically important `double perovskites', crystallizing in the monoclinic space group P2₁/n. The structure is characterized by a strong ortho­rhom­bic pseudosymmetry and a concurrent exhibition of both 1:1 B-cation ordering and a^-a^-c⁺-type tilting of the (Ni/Nb)O₆ structural units. Trivalent lanthanum resides on the perovskite A site, which is strongly distorted owing to the tilting of the (Ni/Nb)O₆ sublattice. Ordering of divalent nickel and penta­valent niobium on the B sublattice is described in terms of two twofold special positions (2c and 2d), with nickel taking almost complete occupancy of the 2d site and the 2c position being occupied by a statistical distribution of nickel and niobium.

S1. Comment

Complex perovskites of the general stoichiometry A(M'_1-xM''_x)O₃ form a broad family of technologically and academically interesting materials. For convenience, the wider family can be split into the general compositions A²⁺(M_1/3²⁺M_2/3⁵⁺)O₃, A³⁺(M_1/2²⁺M_1/2⁴⁺)O₃ and A³⁺(M_2/3²⁺M_1/3⁵⁺)O₃, between which may exist regions of solid solubility. Although nominally all perovskite in structure, these may differ substantially in the exhibition of a number of structural modifications, most notably cation ordering and tilting of the MO₆ octahedral units. Members of the family that show B-site ordering are of particular technological importance, finding widespread application in microwave electronics owing to their favourable dielectric loss or quality factors Q (Reaney & Iddles, 2006). For this reason, the A²⁺(M_1/3²⁺M_2/3⁵⁺)O₃ and A³⁺(M_1/2²⁺M_1/2⁴⁺)O₃ systems have attracted greatest interest as they, respectively, correspond to stoichiometries ideal for 1:2 and 1:1 ordering of the B sublattice. It is generally the case that systems showing 1:2 ordering have the highest Q factors, making such materials extremely attractive for telecommunications applications. Unfortunately, this form of ordering is relatively rare and extremely sensitive to composition. Even small deviations from the ideal 1:2 B-cation ratio cause the breakdown of long-range ordering. The 1:1 rock-salt-type ordering is far more common and relatively insensitive to composition, and whilst these forms do not show the same magnitude of Q factor as the 1:2-ordered materials, their Q factors are still very high. In systems with low tolerance factors (Goldschmidt, 1926), such as La(Mg_1/2Ti_1/2)O₃ (Lee et al., 2000; Salak et al., 2008) and La(Mg_2/3Nb_1/3)O₃ (Paik et al., 1999, 2003), the tilting of the MO₆ octahedra is linked to a negative temperature coefficient of resonant frequency τ_f.

Excepting La(Mg_2/3Nb_1/3)O₃ and La(Mg_2/3Ta_1/3)O₃ (Kim & Woodward, 2007), the A³⁺(M_2/3²⁺M_1/3⁵⁺)O₃ general composition has been very rarely investigated. The synthesis of several compounds of this type has been reported by both Blasse (1965) and Bazuev et al. (1986), with the latter reporting the structure of La(Ni_2/3Nb_1/3)O₃ in a 2^1/2a_c × 2^1/2a_c × 2a_c orthorhombic supercell of cubic perovskite with unit cell a_c. As for La(Mg_2/3Nb_1/3)O₃, a 1:1 ordering of the B cations is suggested rather than the 2:1 form that the stoichiometry would suggest, but no further structural data were provided. In this communication, we present a detailed structural analysis of this compound.

A fitted neutron diffractogram of La(Ni_2/3Nb_1/3)O₃ is presented in Fig. 1, and its refined structure is depicted in Fig. 2. La(Ni_2/3Nb_1/3)O₃ is confirmed as an example of a B-site-ordered perovskite with concomitant a^-a^-c⁺-type tilting of the MO₆ octahedral units (Glazer, 1975). The refined unit-cell parameters are close to those described by Bazuev et al. (1986), and as with the isostructural La(Mg_2/3Nb_1/3)O₃ (Choi et al., 2000), β is close to 90° so the system can be regarded as pseudo-orthorhombic. A high degree of pseudosymmetry is normally observed in such systems, as the lowering of symmetry from orthorhombic to monoclinic does not arise from a distortion of the unit cell from orthogonality, but from the concurrent exhibition of both a^-a^-c⁺-type octahedral tilting and cation ordering, which cannot be described in orthorhombic symmetry (Howard et al., 2003).

As expected from the stoichiometry, lanthanum completely occupies the larger A site of the perovskite structure. A calculated tolerance factor of 0.96 for the compound (Goldschmidt, 1926) agrees well with the observed symmetry and MO₆ tilting (Barnes et al., 2006), indicating that lanthanum is too small to take the ideal 12-fold coordination in La(Ni_2/3Nb_1/3)O₃. The position is thus distorted via the combination of MO₆ tilting and an antiparallel shift of the lanthanum 4e position approximately along the [010] monoclinic direction. Such shifts often accompany MO₆ octahedral tilting, and allow a more favourable A-cation coordination environment as A—O bond distances become shorter with increasing tilt angles (Woodward, 1997). Lanthanum in the title compound thus occupies a position with four of the 12 La—O bonds considerably shortened (Table 1). The average length of these La—O distances is 2.46 (7) Å, which accords well with values found for isostructural materials, such as La(Mg_2/3Nb_1/3)O₃ (Choi et al., 2000), Ca(Mg_1/2W_1/2)O₃ (Yang et al., 2003) and La(Mg_1/2Ti_1/2)O₃ (Lee et al., 2000).

The octahedrally coordinated B-cation position is split into two twofold sites (2c and 2d of the P2₁/n space group), giving a 1:1 rock-salt-type ordering. Nickel takes ~90% occupancy of the 2d position, with the stoichiometry then dictating mixed occupancy of the 2c site. In this way, the difference in formal charges between the sites is maximized. As would be expected from the relative sizes of Ni²⁺ (0.69 Å) and Nb⁵⁺ (0.64 Å) (Shannon, 1976), the 2d position is slightly larger than the 2c (Table 1), with average bond distances of 2.021 (9) and 2.038 (8) Å for 2c—O and 2d—O, respectively. Both octahedral positions show slight distortions due to the tilting phenomenon, with O—2c—O angles in the range 87.8–92.2° and O—2d—O angles in the range 88.1–91.9°. Given the similar sizes of Ni²⁺ and Nb⁵⁺ and similar 2c- and 2d-site environments, it can be concluded that the ordering is driven by electrostatic rather than steric considerations.

Related literature

For related literature, see: Barnes et al. (2006); Bazuev et al. (1986); Blasse (1965); Boultif & Louer (2004); Choi et al. (2000); Fontaine et al. (2006); Glazer (1975); Goldschmidt (1926); Howard et al. (2003); Kim & Woodward (2007); Larson & Von Dreele (2000); Laugier & Bochu (2001); Lee et al. (2000); Paik et al. (1999, 2003); Reaney & Iddles (2006); Roisnel & Rodríguez-Carvajal (2001); Salak et al. (2008); Shannon (1976); Stokes et al. (2002); Woodward (1997); Yang et al. (2003).

S2. Experimental

La(Ni_2/3Nb_1/3)O₃ powder was synthesized via a polymeric route derived from the work of Pecchini or Pechini (1967). Stoichiometric quantities of reagent grade La(NO₃)₃.6H₂O, Ni(NO₃)₂.6H₂O and NbCl₅ were dissolved in deionized water. Citric acid and ethylene glycol were then added, and the solution was heated at 393 K for 5 h with constant stirring. In accordance with previous studies (Fontaine et al., 2006), the cation/citric acid ratio was kept above 2 to obtain homogeneous sols. Burnout of organic components was performed in air at 673 K. The resulting grey powder was then fired in air at 1623 K for 2 h, with heating and cooling rates of 100 K h^-1.

S3. Refinement

X-ray powder diffraction data were collected in a 2θ range of 15–100° using a Siemens D5005 diffractometer equipped with a Cu Kα source and Si monochromator. Neutron powder diffractograms were collected using the GEM powder diffractometer at the ISIS facility, Rutherford Appleton Laboratory, UK. Reflection positions were calculated using the WinPLOTR (Roisnel & Rodríguez-Carvajal, 2001) software package. Observed reflections from both X-ray and neutron data were combined and indexed using the DICVOL software (Boultif & Louër, 2004), with candidate space groups searched both manually and with the Chekcell software (Laugier & Bochu, 2001).

Good figures of merit were obtained for indexing of 47 observed reflections on an orthorhombic cell (M20 = 140, F20 = 157) corresponding to that of a 2^1/2a_c × 2^1/2a_c × 2a_c perovskite supercell (where a_c is the cell edge of the cubic aristotype). Extinctions were found for h0l: h + l = 2n + 1, h00: h = 2n + 1, 0k0: k = 2n + 1, 00l: l = 2n + 1. We note here the importance of including both data sets in the indexing process, as an additional extinction was observed for 0kl: k + l = 2n + 1 in the X-ray data but not in the neutron data. The observed extinctions indicate Pmn2₁ or Pmnm as orthorhombic symmetry choices, though the extinction for 0k0: k = 2n + 1 is not required for this assignment. Indexing on a monoclinic cell gave lower figures of merit (M20 = 25, F20 = 31), with extinctions at h0l: h + l = 2n + 1 and 0k0: k = 2n + 1 indicating P2₁/n as a possible space group. From group theory considerations (Woodward, 1997), the assignment of Pmnm should correspond to a 2a_c × 2a_c × 2a_c perovskite supercell, and our data unambiguously index as 2^1/2a_c ×2^1/2a_c × 2a_c. Following the group theory work of Stokes et al. (2002), the assignment of Pmn2₁ corresponds to a 2^1/2a_c × 2^1/2a_c × 2a_c ferroelectric perovskite modification with possible A-site ordering. However, this is a very rarely reported perovskite form and is not expected given the composition of the material. Polymorphs with two distinct crystallographic A sites are expected to have two different A cations (Woodward, 1997). In general, double perovskites of the type A(M'_1-xM''_x)O₃ crystallize in one of two forms: an orthorhombic polymorph (space group Pbnm) with a^-a^-c⁺-type octahedral tilting, and a monoclinic (P2₁/n) polymorph, which additionally exhibits 1:1 rock-salt-type ordering of the B-sublattice cations. Both orthorhombic Pmn2₁ and monoclinic P2₁/n polymorphs were tested via Rietveld refinement, and the former was found to be unstable to refinement, with a strong correlation between O-atom positions and between La-atom positions. Accordingly, the monoclinic form was assigned.

Rietveld refinement was performed data using the GSAS software suite (Larson et al., 2000). Data from banks 3–6 (positioned at 34.96, 63.62, 91.3 and 154.4°, respectively) were included in a multi-histogram refinement. Two peak profile parameters, zero shift and an eight-parameter Chebyshev-type background function, were included in the refinement in addition to lattice parameters, atomic coordinates and isotropic displacement parameters for all sites. Cation ordering on the B sublattice was included in the refinement, though with the overall stoichiometry constrained to maintain the nominal La(Ni_2/3Nb_1/3)O₃ composition.

Computing details

Program(s) used to solve structure: WINPLOTR (Roisnel & Rodríguez-Carvajal, 2001), DICVOL (Boultif & Louër, 2004) and Chekcell (Laugier & Bochu, 2001); program(s) used to refine structure: GSAS (Larson & Von Dreele, 2000 ); molecular graphics: ATOMS (Dowty, 2002); software used to prepare material for publication: GSAS (Larson & Von Dreele, 2000 ).

Figures

	Fig. 1. : A fitted neutron powder diffractogram for La(Ni2/3Nb1/3)O3. The data shown are from the 91.3° bank. [Please confirm what the units of the y axis should be.]
	Fig. 2. : The refined structure of La(Ni2/3Nb1/3)O3 projected onto the (010) plane. La atoms are shown as spheres [the ordered 2c and 2d octahedrally coordinated cation sites, respectively, are shown in brown and grey in the electronic version of the paper].

Lanthanum nickel niobium trioxide

Crystal data

LaNb0.33Ni0.67O3	Z = 4
Mr = 257.00	Dx = 6.884 Mg m^−3
Monoclinic, P21/n	Neutron radiation
a = 5.5823 (2) Å	T = 295 K
b = 5.6194 (2) Å	Particle morphology: powder
c = 7.9045 (3) Å	grey
β = 90.000 (9)°	cylinder, ? × ? × ? mm
V = 247.96 (2) Å^3	Specimen preparation: Prepared at 1623 K and 101.3 kPa, cooled at 100 K min^−1

Data collection

GEM (ISIS, Didcot) diffractometer	Data collection mode: transmission
Radiation source: time-of-flight neutron, neutron	Scan method: time of flight
Specimen mounting: packed powder

Refinement

Least-squares matrix: full	Profile function: convolution of the Ikeda-Carpenter and Pseudo-Voight functions
Rp = 0.055	75 parameters
Rwp = 0.049	0 restraints
Rexp = ?	(Δ/σ)max = 0.05
χ^2 = 1.588	Background function: Shifted Chebyshev
? data points	Preferred orientation correction: none

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
La1	0.4915 (3)	0.53711 (11)	0.2496 (7)	0.01797 (16)*
Nb2	0.5	0.0	0.0	0.0057 (7)*	0.094 (21)
Ni3	0.5	0.0	0.0	0.0057 (7)*	0.906 (21)
Nb4	0.0	0.5	0.0	0.0042 (7)*	0.572 (21)
Ni5	0.0	0.5	0.0	0.0042 (7)*	0.428 (21)
O6	0.2898 (7)	0.2936 (6)	0.0346 (3)	0.0064 (5)*
O7	0.2119 (7)	0.7856 (7)	0.0471 (3)	0.0112 (7)*
O8	0.5783 (3)	0.9844 (2)	0.2503 (10)	0.0102 (3)*

Geometric parameters (Å, º)

La1—O6	2.455 (5)	Ni3—O6	2.043 (4)
La1—O6^i	2.730 (4)	Ni3—O6^viii	2.043 (4)
La1—O6^ii	2.728 (5)	Ni3—O7^vi	2.043 (4)
La1—O6^iii	3.362 (5)	Ni3—O7^ii	2.043 (4)
La1—O7	2.636 (5)	Ni3—O8^vi	2.028 (8)
La1—O7^iv	2.422 (5)	Ni3—O8^ii	2.028 (8)
La1—O7^ii	3.396 (5)	Nb4—La1^xi	3.463 (3)
La1—O7^v	2.835 (5)	Nb4—La1	3.386 (3)
La1—O8^vi	3.1436 (13)	Nb4—La1^iv	3.269 (3)
La1—O8	2.5597 (14)	Nb4—La1^i	3.610 (3)
La1—O8^iv	3.195 (2)	Nb4—La1^xii	3.386 (3)
La1—O8^vii	2.420 (2)	Nb4—La1^ii	3.463 (3)
Nb2—La1^vi	3.265 (3)	Nb4—La1^ix	3.610 (3)
Nb2—La1	3.606 (3)	Nb4—La1^xiii	3.269 (3)
Nb2—La1^iv	3.389 (3)	Nb4—O6	2.009 (4)
Nb2—La1^vii	3.467 (3)	Nb4—O6^xii	2.009 (4)
Nb2—La1^viii	3.606 (3)	Nb4—O7	2.029 (4)
Nb2—La1^ii	3.265 (3)	Nb4—O7^xii	2.029 (4)
Nb2—La1^ix	3.467 (3)	Nb4—O8^iv	2.024 (8)
Nb2—La1^x	3.389 (3)	Nb4—O8^xiii	2.024 (8)
Nb2—O6	2.043 (4)	Ni5—La1^xi	3.463 (3)
Nb2—O6^viii	2.043 (4)	Ni5—La1	3.386 (3)
Nb2—O7^vi	2.043 (4)	Ni5—La1^iv	3.269 (3)
Nb2—O7^ii	2.043 (4)	Ni5—La1^i	3.610 (3)
Nb2—O8^vi	2.028 (8)	Ni5—La1^xii	3.386 (3)
Nb2—O8^ii	2.028 (8)	Ni5—La1^ii	3.463 (3)
Ni3—La1^vi	3.265 (3)	Ni5—La1^ix	3.610 (3)
Ni3—La1	3.606 (3)	Ni5—La1^xiii	3.269 (3)
Ni3—La1^iv	3.389 (3)	Ni5—O6	2.009 (4)
Ni3—La1^vii	3.467 (3)	Ni5—O6^xii	2.009 (4)
Ni3—La1^viii	3.606 (3)	Ni5—O7	2.029 (4)
Ni3—La1^ii	3.265 (3)	Ni5—O7^xii	2.029 (4)
Ni3—La1^ix	3.467 (3)	Ni5—O8^iv	2.024 (8)
Ni3—La1^x	3.389 (3)	Ni5—O8^xiii	2.024 (8)
O6—La1—O6^i	117.59 (7)	O7^vi—Ni3—O8^vi	88.07 (11)
O6—La1—O6^ii	80.18 (19)	O7^vi—Ni3—O8^ii	91.93 (11)
O6—La1—O7	66.64 (19)	O7^ii—Ni3—O8^vi	91.93 (11)
O6—La1—O7^iv	85.36 (5)	O7^ii—Ni3—O8^ii	88.07 (11)
O6—La1—O8	129.5 (2)	O6—Nb4—O7	87.83 (18)
O6—La1—O8^vii	112.8 (3)	O6—Nb4—O7^xii	92.17 (18)
O6^i—La1—O6^ii	126.00 (6)	O6—Nb4—O8^iv	90.93 (9)
O6^i—La1—O7	76.06 (5)	O6—Nb4—O8^xiv	89.07 (9)
O6^i—La1—O7^iv	67.91 (18)	O6^xii—Nb4—O7	92.17 (18)
O6^i—La1—O8	65.75 (13)	O6^xii—Nb4—O7^xii	87.83 (18)
O6^i—La1—O8^vii	129.4 (3)	O6^xii—Nb4—O8^iv	89.07 (9)
O6^ii—La1—O7	65.17 (19)	O6^xii—Nb4—O8^xiv	90.93 (9)
O6^ii—La1—O7^iv	163.62 (18)	O7—Nb4—O8^iv	88.93 (10)
O6^ii—La1—O8	64.82 (16)	O7—Nb4—O8^xiv	91.07 (10)
O6^ii—La1—O8^vii	66.35 (19)	O7^xii—Nb4—O8^iv	91.07 (10)
O7—La1—O7^iv	115.74 (8)	O7^xii—Nb4—O8^xiv	88.93 (10)
O7—La1—O8	65.99 (12)	O6—Ni5—O7	87.83 (18)
O7—La1—O8^vii	130.8 (3)	O6—Ni5—O7^xii	92.17 (18)
O7^iv—La1—O8	131.3 (2)	O6—Ni5—O8^iv	90.93 (9)
O7^iv—La1—O8^vii	113.2 (3)	O6—Ni5—O8^xiv	89.07 (9)
O8—La1—O8^vii	86.11 (6)	O6^xii—Ni5—O7	92.17 (18)
O6—Nb2—O7^vi	89.99 (18)	O6^xii—Ni5—O7^xii	87.83 (18)
O6—Nb2—O7^ii	90.01 (18)	O6^xii—Ni5—O8^iv	89.07 (9)
O6—Nb2—O8^vi	91.62 (10)	O6^xii—Ni5—O8^xiv	90.93 (9)
O6—Nb2—O8^ii	88.38 (10)	O7—Ni5—O8^iv	88.93 (10)
O6^viii—Nb2—O7^vi	90.01 (18)	O7—Ni5—O8^xiv	91.07 (10)
O6^viii—Nb2—O7^ii	89.99 (18)	O7^xii—Ni5—O8^iv	91.07 (10)
O6^viii—Nb2—O8^vi	88.38 (10)	O7^xii—Ni5—O8^xiv	88.93 (10)
O6^viii—Nb2—O8^ii	91.62 (10)	Nb2—O6—Nb4	155.55 (16)
O7^vi—Nb2—O8^vi	88.07 (11)	Nb2—O6—Ni5	155.55 (16)
O7^vi—Nb2—O8^ii	91.93 (11)	Ni3—O6—Nb4	155.55 (16)
O7^ii—Nb2—O8^vi	91.93 (11)	Ni3—O6—Ni5	155.55 (16)
O7^ii—Nb2—O8^ii	88.07 (11)	Nb2^xv—O7—Nb4	153.12 (16)
O6—Ni3—O7^vi	89.99 (18)	Nb2^xv—O7—Ni5	153.12 (16)
O6—Ni3—O7^ii	90.01 (18)	Ni3^xv—O7—Nb4	153.12 (16)
O6—Ni3—O8^vi	91.62 (10)	Ni3^xv—O7—Ni5	153.12 (16)
O6—Ni3—O8^ii	88.38 (10)	Nb2^xv—O8—Nb4^i	154.56 (8)
O6^viii—Ni3—O7^vi	90.01 (18)	Nb2^xv—O8—Ni5^i	154.56 (8)
O6^viii—Ni3—O7^ii	89.99 (18)	Ni3^xv—O8—Nb4^i	154.56 (8)
O6^viii—Ni3—O8^vi	88.38 (10)	Ni3^xv—O8—Ni5^i	154.56 (8)
O6^viii—Ni3—O8^ii	91.62 (10)

Symmetry codes: (i) −x+1/2, y+1/2, −z+1/2; (ii) −x+1, −y+1, −z; (iii) x+1/2, −y+1/2, z+1/2; (iv) −x+1/2, y−1/2, −z+1/2; (v) x+1/2, −y+3/2, z+1/2; (vi) x, y−1, z; (vii) −x+3/2, y−1/2, −z+1/2; (viii) −x+1, −y, −z; (ix) x−1/2, −y+1/2, z−1/2; (x) x+1/2, −y+1/2, z−1/2; (xi) x−1, y, z; (xii) −x, −y+1, −z; (xiii) x−1/2, −y+3/2, z−1/2; (xiv) x+1/2, −y+5/2, z+1/2; (xv) x, y+1, z.