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The structures of the 6H perovskites Ba3B2+Sb5+2O9, B = Ca and Sr, have been solved and refined using synchrotron X-ray and neutron powder diffraction data. Ba3CaSb2O9 and Ba3SrSb2O9 have monoclinic C2/c and triclinic P\bar 1 space-group symmetries, respectively, while Ba3MgSb2O9 has ideal hexagonal P63/mmc space-group symmetry. The symmetry-lowering distortions are a consequence of internal `chemical pressure' owing to the increasing effective ionic radius of the alkaline-earth cation in the perovskite B site from Mg2+ (0.72 Å) to Ca2+ (1.00 Å) to Sr2+ (1.18 Å). Increasing the effective ionic radius further to Ba2+ (1.35 Å) leads to decomposition at room temperature. The driving force behind the transition from P63/mmc to C2/c is the need to alleviate underbonding of Ba2+ cations in the perovskite A site via octahedral rotations, while the transition from C2/c to P\bar 1 is driven by the need to regularize the shape of the Sb2O9 face-sharing octahedral dimers. Ab initio geometry-optimization calculations were used to find a triclinic starting model for Ba3SrSb2O9.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768108005041/kd5019sup1.cif
Contains datablocks BA3CASB2O9, BA3SRSB2O9

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108768108005041/kd5019BA3CASB2O9sup2.rtv
Contains datablock BA3CASB2O9

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108768108005041/kd5019BA3CASB2O9sup3.hkl
Contains datablock BA3CASB2O9

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108768108005041/kd5019BA3SRSB2O9sup4.rtv
Contains datablock BA3SRSB2O9

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108768108005041/kd5019BA3SRSB2O9sup5.hkl
Contains datablock BA3SRSB2O9

Computing details top

For both compounds, program(s) used to refine structure: GSAS.

Figures top
[Figure 1]
[Figure 2]
[Figure 3]
[Figure 4]
(BA3CASB2O9) top
Crystal data top
Ba3CaO9Sb2V = 925.23 (3) Å3
Mr = 839.56Z = 4
Monoclinic, C2/cSynchrotron radiation, λ = 1.182279 Å
a = 5.99898 (7) ÅT = 298 K
b = 10.37797 (19) Åwhite
c = 14.8658 (3) Åflat sheet, 10 × 10 mm
β = 91.384 (2)°
Data collection top
Data collection mode: reflection2θmin = 5.000°, 2θmax = 65.000°, 2θstep = 0.003°
Scan method: step
Refinement top
Least-squares matrix: full20001 data points
Rp = 0.156Profile function: CW Profile function number 4 with 21 terms Pseudovoigt profile coefficients as parameterized in P. Thompson, D.E. Cox & J.B. Hastings (1987). J. Appl. Cryst.,20,79-83. Asymmetry correction of L.W. Finger, D.E. Cox & A. P. Jephcoat (1994). J. Appl. Cryst.,27,892-900. Microstrain broadening by P.W. Stephens, (1999). J. Appl. Cryst.,32,281-289. #1(GU) = 0.000 #2(GV) = 0.000 #3(GW) = 0.000 #4(GP) = 0.012 #5(LX) = 0.012 #6(ptec) = 0.00 #7(trns) = 0.00 #8(shft) = 0.0000 #9(sfec) = 0.00 #10(S/L) = 0.0005 #11(H/L) = 0.0005 #12(eta) = 0.4900 #13(S400 ) = 6.6E-02 #14(S040 ) = 1.6E-01 #15(S004 ) = 1.1E-02 #16(S220 ) = 1.3E-02 #17(S202 ) = 9.4E-02 #18(S022 ) = 7.2E-01 #19(S301 ) = -1.2E-02 #20(S103 ) = 5.7E-03 #21(S121 ) = -3.2E-01 Peak tails are ignored where the intensity is below 0.0010 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rwp = 0.20449 parameters
Rexp = 0.1040 restraints
R(F2) = 0.07867(Δ/σ)max = 0.05
χ2 = 3.842Background function: GSAS Background function number 1 with 8 terms. Shifted Chebyshev function of 1st kind 1: 17.7610 2: -12.5477 3: 12.4164 4: -4.17240 5: 4.03662 6: -1.22665 7: 1.92542 8: 7.976240E-02
Crystal data top
Ba3CaO9Sb2β = 91.384 (2)°
Mr = 839.56V = 925.23 (3) Å3
Monoclinic, C2/cZ = 4
a = 5.99898 (7) ÅSynchrotron radiation, λ = 1.182279 Å
b = 10.37797 (19) ÅT = 298 K
c = 14.8658 (3) Åflat sheet, 10 × 10 mm
Data collection top
Data collection mode: reflection2θmin = 5.000°, 2θmax = 65.000°, 2θstep = 0.003°
Scan method: step
Refinement top
Rp = 0.156χ2 = 3.842
Rwp = 0.20420001 data points
Rexp = 0.10449 parameters
R(F2) = 0.078670 restraints
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Ba10.00.0031 (5)0.250.0080 (9)*
Ba20.0095 (2)0.3327 (3)0.09303 (9)0.0221 (9)*
Ca10.00.00.00.0160 (9)*
Sb10.0124 (2)0.3370 (4)0.84609 (8)0.0160 (9)*
O10.00.511 (3)0.250.013 (2)*
O20.300 (3)0.236 (2)0.2415 (8)0.013 (2)*
O30.031 (3)0.809 (2)0.0875 (11)0.013 (2)*
O40.323 (3)0.0646 (18)0.0777 (9)0.013 (2)*
O50.768 (3)0.074 (2)0.1008 (10)0.013 (2)*
(BA3SRSB2O9) top
Crystal data top
Ba3O9Sb2Srγ = 119.7431 (11)°
Mr = 887.10V = 488.10 (2) Å3
Triclinic, P1Z = 2
a = 6.08467 (15) ÅNeutron radiation, λ = 1.597248 Å
b = 6.08608 (15) ŵ = 0.00233 mm1
c = 15.1934 (3) ÅT = 298 K
α = 89.175 (2)°white
β = 88.485 (2)°cylinder, 20 × 5 mm
Data collection top
Data collection mode: transmission2θmin = 0.150°, 2θmax = 158.800°, 2θstep = 0.05°
Scan method: step
Refinement top
Least-squares matrix: full3174 data points
Rp = 0.051Profile function: CW Profile function number 4 with 27 terms Pseudovoigt profile coefficients as parameterized in P. Thompson, D.E. Cox & J.B. Hastings (1987). J. Appl. Cryst.,20,79-83. Asymmetry correction of L.W. Finger, D.E. Cox & A. P. Jephcoat (1994). J. Appl. Cryst.,27,892-900. Microstrain broadening by P.W. Stephens, (1999). J. Appl. Cryst.,32,281-289. #1(GU) = 28.186 #2(GV) = -42.405 #3(GW) = 47.957 #4(GP) = 0.000 #5(LX) = 8.685 #6(ptec) = 0.00 #7(trns) = 0.00 #8(shft) = 0.0000 #9(sfec) = 0.00 #10(S/L) = 0.0005 #11(H/L) = 0.0005 #12(eta) = 0.0000 Peak tails are ignored where the intensity is below 0.0100 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rwp = 0.06776 parameters
Rexp = 0.1670 restraints
R(F2) = 0.07480(Δ/σ)max = 0.04
χ2 = 0.160Background function: GSAS Background function number 1 with 6 terms. Shifted Chebyshev function of 1st kind 1: 25.8423 2: 3.28057 3: 4.07127 4: 0.734216 5: 0.344875 6: -0.144124
Crystal data top
Ba3O9Sb2Srγ = 119.7431 (11)°
Mr = 887.10V = 488.10 (2) Å3
Triclinic, P1Z = 2
a = 6.08467 (15) ÅNeutron radiation, λ = 1.597248 Å
b = 6.08608 (15) ŵ = 0.00233 mm1
c = 15.1934 (3) ÅT = 298 K
α = 89.175 (2)°cylinder, 20 × 5 mm
β = 88.485 (2)°
Data collection top
Data collection mode: transmission2θmin = 0.150°, 2θmax = 158.800°, 2θstep = 0.05°
Scan method: step
Refinement top
Rp = 0.051χ2 = 0.160
Rwp = 0.0673174 data points
Rexp = 0.16776 parameters
R(F2) = 0.074800 restraints
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Ba10.9851 (17)0.0034 (18)0.7502 (7)0.0267 (14)*
Ba20.3469 (15)0.6798 (17)0.9008 (6)0.0268 (17)*
Ba30.3194 (15)0.6562 (14)0.5962 (5)0.0204 (14)*
O10.5313 (13)0.4866 (13)0.7518 (6)0.0286 (14)*
O20.5176 (14)0.0477 (13)0.7644 (5)0.0258 (14)*
O30.8774 (14)0.2159 (15)0.8982 (5)0.0321 (17)*
O40.4256 (13)0.2380 (14)0.9308 (5)0.0283 (16)*
O50.8926 (14)0.6939 (14)0.8898 (6)0.0293 (15)*
O60.9526 (12)0.4688 (13)0.7325 (5)0.0221 (13)*
O70.7512 (13)0.1069 (14)0.5942 (5)0.0261 (15)*
O80.7823 (15)0.5850 (14)0.5730 (6)0.0322 (17)*
O90.3062 (13)0.1462 (14)0.6116 (5)0.0267 (15)*
Sb10.3089 (13)0.6474 (14)0.1562 (5)0.0186 (11)*
Sb20.3616 (14)0.6910 (14)0.3422 (5)0.0176 (11)*
Sr10.00.00.00.0219 (15)*
Sr20.00.00.50.0131 (12)*

Experimental details

(BA3CASB2O9)(BA3SRSB2O9)
Crystal data
Chemical formulaBa3CaO9Sb2Ba3O9Sb2Sr
Mr839.56887.10
Crystal system, space groupMonoclinic, C2/cTriclinic, P1
Temperature (K)298298
a, b, c (Å)5.99898 (7), 10.37797 (19), 14.8658 (3)6.08467 (15), 6.08608 (15), 15.1934 (3)
α, β, γ (°)90, 91.384 (2), 9089.175 (2), 88.485 (2), 119.7431 (11)
V3)925.23 (3)488.10 (2)
Z42
Radiation typeSynchrotron, λ = 1.182279 ÅNeutron, λ = 1.597248 Å
µ (mm1)0.00233
Specimen shape, size (mm)Flat sheet, 10 × 10Cylinder, 20 × 5
Data collection
Diffractometer??
Specimen mounting??
Data collection modeReflectionTransmission
Scan methodStepStep
Absorption correction?
GSAS Absorption/surface roughness correction: function number 2 Surface roughness ABS. correction (Pitschke, et al.) Terms = 0.68087E-01 0.35062E-01 Correction is refined.
Tmin, Tmax0.561, 0.949
2θ values (°)2θmin = 5.000 2θmax = 65.000 2θstep = 0.0032θmin = 0.150 2θmax = 158.800 2θstep = 0.05
Refinement
R factors and goodness of fitRp = 0.156, Rwp = 0.204, Rexp = 0.104, R(F2) = 0.07867, χ2 = 3.842Rp = 0.051, Rwp = 0.067, Rexp = 0.167, R(F2) = 0.07480, χ2 = 0.160
No. of data points200013174
No. of parameters4976

Computer programs: GSAS.

 

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