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Flux-grown gadolinium aluminate perovskite, GdAlO3, was examined using single-crystal 0.7 Å-wavelength synchrotron X-ray diffraction. In the context of other well categorized rare earth aluminate (RAlO3) perovskite phases, the orthorhombic Pnma symmetry determined for the current compound is unsurprising. Corner-linked AlO6 octahedra form the structural backbone of RAlO3 perovskites and distort to accommodate the various rare earth ions in the structural voids. For GdAlO3, the octahedral distortion, characterized by tilting of the octahedra about the shortest R-Al-R vectors, and octahedral deformation, characterized by strain of the octahedra along those axes, are in accordance with trends in the RAlO3 series.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104025144/sq1172sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270104025144/sq1172Isup2.hkl
Contains datablock I

Comment top

Rare earth perovskite oxides (RMO3; R=Y, La, ···, Lu, and M is a metal) have drawn scientific interest for many decades (e.g. Geller & Bala, 1956) and found applications as laser hosts (Li et al., 1990), in recording media, in a variety of sensing applications and as high-temperature superconductors in the perovskite-like cuprates. Members of the RFeO3 (Rearick et al., 1993), RVO3 (Miyasaka et al., 2003), RMnO3 (Ramirez, 1997), RNiO3 (Piamonteze et al., 2002), RCoO3 (Samoilov et al., 1998) and RGaO3 (Ishihara et al., 1998) series display wide ranges of interesting magnetic and conduction properties. In contrast, the rare earth aluminate perovskites (RAlO3) are relatively less promising in these areas. Precise knowledge of their structural trends could, however, provide useful background information for resolving structural and electronic relationships in its technologically more important analogues.

The perovskite oxides are composed of relatively robust, corner-linked, M-centred MO6 octahedra, with nominally twelve coordinated rare earth (R) cations. Most rare earth perovskites adopt low-symmetry distortions from the ideal cubic perovskite $Pmοverline{3}m$ symmetry. This result is widely attributed to the cation size-mismatch effect, whereby small radii R cations induce structural collapse through mechanisms involving tilting of rigid octahedra, small octahedral deformations and cation displacements (Glazer, 1972; Howard & Stokes, 1998; Woodward, 1997; Thomas, 1996, 1998).

As the atomic number (Z) increases across the periodic table from R=La to R=Lu, the 4f electron shell is filled and the mean atomic radius decreases. This lanthanide contraction is associated with progressively larger-magnitude structural distortions in rare earth perovskites (Geller & Bala, 1956; Dernier & Maines, 1971). Whereas LaAlO3 is reportedly ideal cubic at 720 K (Geller & Bala, 1956), the symmetry reduces to $Rοverline{3}c$ at room temperature, with rhombohedral disortions increasing from La to Nd. Between Nd and Sm, the stable room-temperature phase shifts in favour of orthorhombic Pnma symmetry (Marezio, et al. 1972), the most common low-symmetry perovskite distortion. According to the Pnma symmetry model, the M atoms are located on inversion centres, the R atom and atom O1 lie on the y=1/4 and y=3/4 mirror planes perpendicular to the orthorhombic b axis, and atom O2 occupies a site of general symmetry. Higher-Z members of the RAlO3 series are all isomorphous with the Sm member, and the lightest rare earth analogue, YAlO3, adopts structural distortions comparable to those of HoAlO3 (Geller & Wood 1956; Diehl & Brandt, 1975).

Fig. 1 shows an AlO6 octahedron of GdAlO3 coupled to the shortest and therefore strongest Gd—O bonds in the lattice. The six short Gd—O bonds act on each octahedral vertex in a coherent manner, collectively leading to static tilting of the AlO6 unit through an angle of 14.3°, acting about a pseudo-threefold axis closely aligned with the shortest collinear Gd—Al—Gd contacts in the structure. This suggests that a model of the lattice in terms of linked Gd-bicapped octahedra, as shown in Fig. 2, could be as useful as the more customary representation in terms of corner-linked octahedra. Tilt angles of 5.8, 6.5, 9.5, 12.2 and 16.9° have been observed for La, Pr, Nd, Sm and Ho aluminate perovskites, respectively (du Boulay, 1997).

Megaw & Darlington (1975) characterized octahedral deformation in rhombohedrally distorted perovskites in terms of the octahedron strain, $ζeta$, describing flattening or elongation along a single preserved octahedral threefold axis. Specifically, $(1+ζeta) = {{c_{ρm hex}}οver {2σqrt{6} θuad οverline{l_{βot}}}}$ where $c_{ρm hex}$ is the hexagonal c axis length and $οverline{l_{βot}}$, is the mean length of the six octahedral O—O contact edges oriented normal to that threefold axis.

The pseudo-threefold axes of the Gd-bicapped octahedra shown in Fig. 2 suggest that an equivalent parameterization of octahedral strain in orthorhombically distorted octahedra may be relevant. Although there are four approximate threefold axes, only the bicapping axis identified in Figs. 1 and 2 demonstrates marked structural continuity across the phase transition. Quantifying the strain along that axis involves a more intimate description in terms of the distance, $h$, separating the upper and lower triangular faces oriented normal to the pseudo-threefold axis or, $οverline{l_{\|}}$, the mean of the six O—O contact lengths connecting those two faces. $(1+ζeta) = σqrt{{3}οver{2}}{{h}οver{οverline{l_{βot}}}} = σqrt{{3οverline{l_{\|}}2οver {2οverline{l_{βot}}2}} −1/2} $

For GdAlO3, $ζeta=0.0012 (2)$, contrasting with values of −0.003, −0.014, −0.022, −0.004 and 0.012 for La, Pr, Nd, Sm and Ho, respectively (du Boulay, 1997). A discontinuity in the strain tendency between Nd and Sm suggests that alleviating the initial rapid increase in octahedral compression as the octahedral tilt angle increases smoothly across the series may be the prime motivating factor behind the rhombohedral to orthorhombic RAlO3 phase change. This is not to say that strain is the fundamental order parameter, but presumably it correlates strongly with some pertinent attributes of the Al atom's bonding and electronic configuration. The octahedron strain and tilt in GdAlO3 itself place it comfortably within the orthorhombic perovskite distortion regime.

Experimental top

Crystals of GdAlO3 were grown using the high-temperature flux growth technique of Wanklyn (1969), by dissolving Gd2O3 and Al2O3 in a PbO, PbF2 and B2O3 flux. The mixture was heated rapidly to 1580 K in a 10 ml platinum crucible and then cooled slowly to 1280 K at 10 K h−1. The crystals were liberated by dissolving the residual flux in hot dilute nitric acid. GdAlO3 crystals were easily identified by their rectangular prismatic habit with pronounced (101), (10–1) and (010) faces.

Refinement top

Measured structure factors were optimized against calculated structure factors, derived from neutral atomic form factors combined with the wavelength-dependent dispersion corrections of Sasaki (1989). Least-squares agreement indices wR(F) support the notion of this as a high-precision diffraction experiment. Modest electron density differences were encountered. The largest of these were two independent depletions, −3.7 (2) and −3.3 (2) e Å−3 in magnitude, both located 0.4 Å away from the Gd atom and diametrically disposed. They maximize within the mirror plane normal to [010] and their respective Gd vectors were directed along [100], normal to the shortest Gd—O1 bonds.

Computing details top

Data collection: reference?; cell refinement: reference?; data reduction: DIFDAT, ADDREF, SORTRF and ABSORB in Xtal3.7.2 (Hall et al., 2003); program(s) used to solve structure: Xtal3.7.2; program(s) used to refine structure: CRYLSQ in Xtal3.7.2; molecular graphics: Xtal3.7.2 and ATOMS (Dowty, 2002); software used to prepare material for publication: BONDLA and CIFIO in Xtal3.7.2.

Figures top
[Figure 1] Fig. 1. Pseudo-threefold symmetry of the AlO6 octahedra in GdAlO3. Foreshortened Gd—O1 [2.3069 (7) Å] and Gd—O2 [2.3391 (5) Å] bonds (all symmetry combinations) induce static tilting of the octahedra about a Gd—Al—Gdix [2× 3.0793 (3) Å] axis. [Symmetry codes: (i) 1/2 − x,-y,1/2 + z; (ii) −1 + x,+y,+z; (iii) 1/2 − x,-1/2 + y,1/2 + z; (iv) 1/2 − x,-y,-1/2 + z; (v) −1/2 + x,1/2 − y,3/2 − z; (vi) −1/2 + x,+y,3/2 − z; (vii) −x,-y,2 − z; (viii) −1/2 + x,1/2 − y,5/2 − z; (ix) −x,-1/2 + y,2 − z; (x) 1 − x,-1/2 + y,2 − z.]
[Figure 2] Fig. 2. ATOMS (Dowty, 2002) extended GdAlO3 unit cell, demonstrating shared-edge connectivity between mirror-related Gd-bicapped octahedra.
(I) top
Crystal data top
GdAlO3F(000) = 404
Mr = 232.23Dx = 7.43 Mg m3
Orthorhombic, PnmaSynchrotron radiation, λ = 0.7 Å
Hall symbol: -p 2ac 2nCell parameters from 12 reflections
a = 5.3049 (7) Åθ = 20–30°
b = 7.4485 (9) ŵ = 29.65 mm1
c = 5.2537 (6) ÅT = 295 K
V = 207.59 (4) Å3Rectangular prism, colourless
Z = 40.03 × 0.03 × 0.02 mm
Data collection top
Rigaku BL14A four-circle
diffractometer
Rint = 0.034
ω/2θ scansθmax = 50.0°, θmin = 4.7°
Absorption correction: analytical
(de Meulenaer & Tompa, 1965)
h = 1111
Tmin = 0.412, Tmax = 0.586k = 1616
8547 measured reflectionsl = 1111
1193 independent reflections6 standard reflections every 94 reflections
1193 reflections with F > .00 sig(F ) intensity decay: 0.
Refinement top
Refinement on F0 restraints
Least-squares matrix: full0 constraints
R[F2 > 2σ(F2)] = 0.012σ w = 1/[σ2(Fo)]
wR(F2) = 0.009(Δ/σ)max = 0.001
S = 2.06Δρmax = 1.25 e Å3
1193 reflectionsΔρmin = 3.68 e Å3
29 parameters
Crystal data top
GdAlO3V = 207.59 (4) Å3
Mr = 232.23Z = 4
Orthorhombic, PnmaSynchrotron radiation, λ = 0.7 Å
a = 5.3049 (7) ŵ = 29.65 mm1
b = 7.4485 (9) ÅT = 295 K
c = 5.2537 (6) Å0.03 × 0.03 × 0.02 mm
Data collection top
Rigaku BL14A four-circle
diffractometer
1193 reflections with F > .00 sig(F )
Absorption correction: analytical
(de Meulenaer & Tompa, 1965)
Rint = 0.034
Tmin = 0.412, Tmax = 0.5866 standard reflections every 94 reflections
8547 measured reflections intensity decay: 0.
1193 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.01229 parameters
wR(F2) = 0.0090 restraints
S = 2.06Δρmax = 1.25 e Å3
1193 reflectionsΔρmin = 3.68 e Å3
Special details top

Refinement. X-ray diffraction data were measured from a 24.5× 18.5× 33 Å Gd AlO3 crystal using the BL14A horizontally mounted four-circle diffractometer at the Photon Factory, Tsukuba, Japan (Satow & Iitaka, 1989). An NaI scintillation detector was used for photon counting and Cu foil attenuators were inserted to reduce deadtime errors on strong reflections. Data measurement was driven by purpose-built software executed on a Mitsubishi Melcom computer.

A full sphere of diffraction data to 2θ=100° was measured at 16 °/min in ω/2θ scanning mode. The absorption coefficient µ=29.6 interpolated from the tables of Sasaki (1990) helped to reduce the merging R-factor from Rint=0.052 to 0.034.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Al0.000000.000000.000000.00318 (8)
Gd0.462223 (7)0.250000.008091 (7)0.003969 (17)
O10.01389 (12)0.250000.07210 (12)0.00495 (16)
O20.28504 (8)0.03823 (6)0.21534 (8)0.00479 (12)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Al0.00342 (8)0.00306 (8)0.00305 (8)0.00007 (7)0.00012 (6)0.00006 (5)
Gd0.004376 (17)0.003646 (16)0.003885 (17)0.000000.000505 (8)0.00000
O10.00661 (18)0.00312 (15)0.00510 (17)0.000000.00044 (14)0.00000
O20.00434 (12)0.00536 (12)0.00467 (12)0.00040 (9)0.00127 (9)0.00035 (9)
Geometric parameters (Å, º) top
Al—O11.9017 (3)Gd—O12.9758 (7)
Al—O11.9017 (3)Gd—O2iv3.1476 (5)
Al—O21.9021 (5)Gd—O2ii3.1476 (5)
Al—O21.9021 (5)O1—O22.6718 (7)
Al—O2i1.9099 (5)O1—O22.6718 (7)
Al—O21.9099 (5)O1—O2iii2.6916 (6)
Gd—O1ii2.3069 (7)O1—O22.6916 (6)
Gd—O22.3391 (5)O1—O2i2.6988 (7)
Gd—O22.3391 (5)O1—O2v2.6988 (7)
Gd—O12.4154 (7)O1—O22.7074 (6)
Gd—O22.5139 (5)O1—O22.7074 (6)
Gd—O22.5139 (5)O2—O2iv2.6773 (7)
Gd—O2iii2.6212 (5)O2—O22.6773 (7)
Gd—O22.6212 (5)O2—O2vi2.7135 (7)
Gd—O12.9567 (8)O2—O22.7135 (7)
Al···Gd3.0793 (3)Al···Gd3.2608 (3)
Al···Gd3.0793 (3)Al···Gd3.4071 (3)
O2···O23.1547 (7)Al···Gd3.4071 (3)
Al···Gdvi3.1916 (3)Gd···Gd3.6737 (3)
Al···Gd3.1916 (3)Gd···Gd3.6737 (3)
Al···Gd3.2608 (3)
O1—Al—O1180.0000O2—Gd—O159.933 (14)
O1—Al—O289.24 (2)O2—Gd—O158.914 (14)
O1—Al—O290.76 (2)O2—Gd—O2iv112.059 (16)
O1—Al—O2i89.85 (2)O2—Gd—O2ii57.019 (15)
O1—Al—O290.15 (2)O1—Gd—O265.609 (16)
O1—Al—O290.76 (2)O1—Gd—O265.609 (16)
O1—Al—O289.24 (2)O1—Gd—O2iii64.456 (13)
O1—Al—O2i90.15 (2)O1—Gd—O264.456 (12)
O1—Al—O289.85 (2)O1—Gd—O1161.763 (18)
O2—Al—O2180.0000O1—Gd—O1105.33 (2)
O2—Al—O2i90.77 (2)O1—Gd—O2iv116.776 (14)
O2—Al—O289.23 (2)O1—Gd—O2ii116.776 (14)
O2—Al—O2i89.23 (2)O2—Gd—O277.724 (16)
O2—Al—O290.77 (2)O2—Gd—O2iii63.757 (16)
O2i—Al—O2180.0000O2—Gd—O2125.849 (15)
O1ii—Gd—O2113.210 (16)O2—Gd—O1126.785 (14)
O1ii—Gd—O2113.210 (17)O2—Gd—O158.166 (15)
O1ii—Gd—O186.78 (2)O2—Gd—O2iv173.862 (11)
O1ii—Gd—O2129.485 (14)O2—Gd—O2ii97.951 (15)
O1ii—Gd—O2129.485 (14)O2—Gd—O2iii125.849 (15)
O1ii—Gd—O2iii66.293 (13)O2—Gd—O263.757 (15)
O1ii—Gd—O266.293 (13)O2—Gd—O1126.785 (14)
O1ii—Gd—O174.98 (2)O2—Gd—O158.166 (14)
O1ii—Gd—O1167.89 (2)O2—Gd—O2iv97.951 (15)
O1ii—Gd—O2iv56.646 (13)O2—Gd—O2ii173.862 (13)
O1ii—Gd—O2ii56.646 (13)O2iii—Gd—O2109.981 (16)
O2—Gd—O284.807 (17)O2iii—Gd—O1106.925 (12)
O2—Gd—O1130.211 (15)O2iii—Gd—O1118.618 (11)
O2—Gd—O2116.946 (17)O2iii—Gd—O2iv122.340 (14)
O2—Gd—O266.853 (17)O2iii—Gd—O2ii54.381 (14)
O2—Gd—O2iii165.093 (15)O2—Gd—O1106.925 (13)
O2—Gd—O282.050 (16)O2—Gd—O1118.618 (12)
O2—Gd—O159.933 (14)O2—Gd—O2iv54.381 (14)
O2—Gd—O158.914 (14)O2—Gd—O2ii122.340 (14)
O2—Gd—O2iv57.019 (15)O1—Gd—O192.907 (18)
O2—Gd—O2ii112.059 (16)O1—Gd—O2iv52.545 (11)
O2—Gd—O1130.211 (14)O1—Gd—O2ii52.545 (11)
O2—Gd—O266.853 (17)O1—Gd—O2iv115.877 (13)
O2—Gd—O2116.946 (17)O1—Gd—O2ii115.877 (13)
O2—Gd—O2iii82.050 (17)O2iv—Gd—O2ii86.013 (14)
O2—Gd—O2165.093 (15)
Symmetry codes: (i) x, y, z; (ii) x+1/2, y+1/2, z+1/2; (iii) x, y+1/2, z; (iv) x+1/2, y, z+1/2; (v) x, y+1/2, z; (vi) x+1/2, y, z+1/2.

Experimental details

Crystal data
Chemical formulaGdAlO3
Mr232.23
Crystal system, space groupOrthorhombic, Pnma
Temperature (K)295
a, b, c (Å)5.3049 (7), 7.4485 (9), 5.2537 (6)
V3)207.59 (4)
Z4
Radiation typeSynchrotron, λ = 0.7 Å
µ (mm1)29.65
Crystal size (mm)0.03 × 0.03 × 0.02
Data collection
DiffractometerRigaku BL14A four-circle
diffractometer
Absorption correctionAnalytical
(de Meulenaer & Tompa, 1965)
Tmin, Tmax0.412, 0.586
No. of measured, independent and
observed [F > .00 sig(F )] reflections
8547, 1193, 1193
Rint0.034
(sin θ/λ)max1)1.094
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.012, 0.009, 2.06
No. of reflections1193
No. of parameters29
Δρmax, Δρmin (e Å3)1.25, 3.68

Computer programs: reference?, DIFDAT, ADDREF, SORTRF and ABSORB in Xtal3.7.2 (Hall et al., 2003), CRYLSQ in Xtal3.7.2, Xtal3.7.2 and ATOMS (Dowty, 2002), BONDLA and CIFIO in Xtal3.7.2.

 

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