Structures of 6H perovskites Ba₃CaSb₂O₉ and Ba₃SrSb₂O₉ determined by synchrotron X-ray diffraction, neutron powder diffraction and ab initio calculations

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 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 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.

Computing details

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

Figures

(BA3CASB2O9)

Crystal data

Ba3CaO9Sb2	V = 925.23 (3) Å^3
Mr = 839.56	Z = 4
Monoclinic, C2/c	Synchrotron radiation, λ = 1.182279 Å
a = 5.99898 (7) Å	T = 298 K
b = 10.37797 (19) Å	white
c = 14.8658 (3) Å	flat sheet, 10 × 10 mm
β = 91.384 (2)°

Data collection

Data collection mode: reflection	2θmin = 5.000°, 2θmax = 65.000°, 2θstep = 0.003°
Scan method: step

Refinement

Least-squares matrix: full	20001 data points
Rp = 0.156	Profile 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 = 0.204	49 parameters
Rexp = 0.104	0 restraints
R(F^2) = 0.07867	(Δ/σ)max = 0.05
χ^2 = 3.842	Background 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

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

	x	y	z	Uiso*/Ueq
Ba1	0.0	−0.0031 (5)	0.25	0.0080 (9)*
Ba2	0.0095 (2)	0.3327 (3)	0.09303 (9)	0.0221 (9)*
Ca1	0.0	0.0	0.0	0.0160 (9)*
Sb1	−0.0124 (2)	0.3370 (4)	0.84609 (8)	0.0160 (9)*
O1	0.0	0.511 (3)	0.25	0.013 (2)*
O2	0.300 (3)	0.236 (2)	0.2415 (8)	0.013 (2)*
O3	0.031 (3)	0.809 (2)	0.0875 (11)	0.013 (2)*
O4	0.323 (3)	0.0646 (18)	0.0777 (9)	0.013 (2)*
O5	0.768 (3)	0.074 (2)	0.1008 (10)	0.013 (2)*

(BA3SRSB2O9)

Crystal data

Ba3O9Sb2Sr	γ = 119.7431 (11)°
Mr = 887.10	V = 488.10 (2) Å^3
Triclinic, P1	Z = 2
a = 6.08467 (15) Å	Neutron radiation, λ = 1.597248 Å
b = 6.08608 (15) Å	µ = 0.00233 mm^−1
c = 15.1934 (3) Å	T = 298 K
α = 89.175 (2)°	white
β = 88.485 (2)°	cylinder, 20 × 5 mm

Data collection

Data collection mode: transmission	2θmin = 0.150°, 2θmax = 158.800°, 2θstep = 0.05°
Scan method: step

Refinement

Least-squares matrix: full	3174 data points
Rp = 0.051	Profile 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 = 0.067	76 parameters
Rexp = 0.167	0 restraints
R(F^2) = 0.07480	(Δ/σ)max = 0.04
χ^2 = 0.160	Background 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

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

	x	y	z	Uiso*/Ueq
Ba1	0.9851 (17)	0.0034 (18)	0.7502 (7)	0.0267 (14)*
Ba2	0.3469 (15)	0.6798 (17)	0.9008 (6)	0.0268 (17)*
Ba3	0.3194 (15)	0.6562 (14)	0.5962 (5)	0.0204 (14)*
O1	0.5313 (13)	0.4866 (13)	0.7518 (6)	0.0286 (14)*
O2	0.5176 (14)	0.0477 (13)	0.7644 (5)	0.0258 (14)*
O3	0.8774 (14)	0.2159 (15)	0.8982 (5)	0.0321 (17)*
O4	0.4256 (13)	0.2380 (14)	0.9308 (5)	0.0283 (16)*
O5	0.8926 (14)	0.6939 (14)	0.8898 (6)	0.0293 (15)*
O6	0.9526 (12)	0.4688 (13)	0.7325 (5)	0.0221 (13)*
O7	0.7512 (13)	0.1069 (14)	0.5942 (5)	0.0261 (15)*
O8	0.7823 (15)	0.5850 (14)	0.5730 (6)	0.0322 (17)*
O9	0.3062 (13)	0.1462 (14)	0.6116 (5)	0.0267 (15)*
Sb1	0.3089 (13)	0.6474 (14)	0.1562 (5)	0.0186 (11)*
Sb2	0.3616 (14)	0.6910 (14)	0.3422 (5)	0.0176 (11)*
Sr1	0.0	0.0	0.0	0.0219 (15)*
Sr2	0.0	0.0	0.5	0.0131 (12)*