The strongly defective double perovskite Sr₁₁Mo₄O₂₃: crystal structure in relation to ionic conductivity

The crystal structure and ionic conductivity properties of a novel microcrystalline Sr₁₁Mo₄O₂₃ ceramic material are presented. This material has been prepared by thermal treatment up to 1473 K, in air, of previously decomposed citrate precursors. The complex crystal structure was refined from combined X-ray powder diffraction and neutron powder diffraction data. The formula of this phase can be rewritten as Sr_1.75□_0.25SrMoO_5.75, highlighting the relationship with double perovskites A₂B′B′′O₆. At room temperature, the crystal structure is tetragonal in space group I4₁/a, with a = 11.6107 (6) Å, c = 16.422 (1) Å and V = 2213.8 (2) ų. The crystal network contains O anion and Sr cation vacancies. The structure is complex, with Sr, Mo and O atoms distributed over four, two and six distinct Wyckoff sites, respectively. Only one of the Sr sites (SrO₆) corresponds to the octahedral network; one of the two MoO₆ types of octahedra is axially distorted. The three other Sr positions occupy the A site with higher coordination. There is an occupational deficit of O atoms of 22 (4)%. This defective framework material presents an interesting ionic mobility, enhanced above 773 K owing to a further reduction in the oxygen content.

Computing details

Program(s) used to refine structure: FULLPROF.

(I)

Crystal data

Mr = 1715.64	c = 16.4219 (13) Å
Tetragonal, I41/a	V = 2213.8 (2) Å^3
Hall symbol: -I 4ad	Constant Wavelength Neutron Diffraction radiation
a = 11.6107 (6) Å	T = 295 K

Data collection

Radiation source: nuclear reactor

2θmin = 4.119°, 2θmax = 164.769°, 2θstep = 0.050°

Refinement

Rp = 3.089	3214 data points
Rwp = 3.969	59 parameters
Rexp = 2.062	0 restraints
RBragg = 4.805

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sr1	0.00000	0.25000	0.12500	0.031 (6)*
Sr2	0.50000	0.25000	0.6112 (13)	0.044 (6)*
Sr3	0.2191 (11)	0.9791 (15)	0.8712 (11)	0.021 (3)*
Sr4	0.2164 (9)	0.2367 (12)	0.5263 (7)	0.018 (2)*
Mo1	0.00000	0.00000	0.00000	0.011 (2)*
Mo2	0.00000	0.00000	0.50000	0.0127 (19)*
O1	0.8716 (12)	0.1369 (11)	0.2591 (10)	0.006 (3)*
O2	0.6358 (16)	0.1329 (19)	0.2419 (16)	0.030 (5)*
O3	0.2754 (11)	0.2559 (15)	0.3648 (8)	0.003 (2)*
O4	0.8365 (14)	0.1719 (14)	0.6563 (11)	0.008 (4)*	0.79 (4)
O5	0.867 (2)	0.088 (2)	0.500 (2)	0.087 (10)*
O6	0.313 (2)	0.170 (3)	0.6621 (18)	0.076 (10)*

Geometric parameters (Å, º)

Sr1—O1	2.97 (2)	Sr3—O6	2.48 (3)
Sr1—O2	3.02 (2)	Sr4—O1	2.59 (2)
Sr1—O3	2.61 (1)	Sr4—O2	2.42 (2)
Sr2—O1	2.60 (2)	Sr4—O3	2.75 (2)
Sr2—O2	2.68 (3)	Sr4—O4	2.62 (2)
Sr2—O3	3.20 (2)	Sr4—O4	2.46 (2)
Sr2—O5	3.33 (3)	Sr4—O5	2.29 (3)
Sr2—O6	2.50 (3)	Sr4—O6	2.61 (3)
Sr3—O1	2.74 (2)	Sr4—O6	3.18 (3)
Sr3—O1	2.85 (2)	Mo1—O1	1.93 (1)
Sr3—O2	2.82 (3)	Mo1—O2	1.90 (2)
Sr3—O2	2.76 (3)	Mo1—O3	1.91 (1)
Sr3—O3	2.73 (2)	Mo2—O4	2.05 (2)
Sr3—O3	2.49 (2)	Mo2—O5	1.85 (2)
Sr3—O4	2.66 (2)	Mo2—O6	1.87 (3)
Sr3—O5	2.64 (3)
O1—Mo1—O2	92 (1)	O1—Mo2—O2	96 (2)
O1—Mo1—O3	91 (1)	O1—Mo2—O3	100 (2)
O2—Mo1—O3	94 (1)	O2—Mo2—O3	101 (2)