The impact of anion ordering on octahedra distortion and phase transitions in SrTaO₂N and BaTaO₂N

In this work we synthesized BaTaO₂N and SrTaO₂N using a two-step high-temperature solid-state reaction method and analysed the structural distortions, relative to the ideal cubic perovskite structure, according to group theory. From a complete distortion analysis/refinement using high-resolution neutron diffraction data in the temperature range 8 to 613 K, we identified tetragonal structures for BaTaO₂N [P4/mmm (No. 123)] and SrTaO₂N [I4/mcm (No. 140)]. In contrast to an anion-disordered cubic perovskite ( No. 221) with Ta at the cell center, both systems show a site preference for oxygen anions in the two opposite corners (along the c axis) of the Ta–O/N octahedra rather than the four square corners in the ab plane (Γ₃⁺ occupancy distortion), which induces a tetragonal elongation of the unit cell with the c axis being longer than the a axis. A further Ta–O/N octahedra displacement [R₅⁻(a,0,0), rotation about the c axis] distortion was observed in SrTaO₂N. This distortion mode is accompanied by an increased unit-cell distortion that decreases as the temperature increases. Ultimately a second-order phase transition caused by the loss of the R₅⁻(a,0,0) mode was observed at 400–450 K.

Computing details

(I)

Crystal data

P4/mmm	β = 90°
a = 4.10502 (6) Å	γ = 90°
b = 4.10502 (6) Å	V = 69.24 (1) Å^3
c = 4.10874 (12) Å	T = 9 K
α = 90°

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

	x	y	z	Biso*/Beq	Occ. (<1)
Ba1	0	0	0	0.220 (17)
Ta1	0.5	0.5	0.5	0.671 (18)
O1	0	0.5	0.5	0.578 (15)	0.579 (8)
O2	0.5	0.5	0	0.578 (15)	0.844 (17)
N1	0	0.5	0.5	0.578 (15)	0.421 (8)
N2	0.5	0.5	0	0.578 (15)	0.156 (17)

(II)

Crystal data

P4/mmm	β = 90°
a = 4.11061 (6) Å	γ = 90°
b = 4.11061 (6) Å	V = 69.53 (1) Å^3
c = 4.11475 (11) Å	T = 300 K
α = 90°

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

	x	y	z	Biso*/Beq	Occ. (<1)
Ba1	0	0	0	0.48 (2)
Ta1	0.5	0.5	0.5	0.84 (2)
O1	0	0.5	0.5	0.752 (17)	0.565 (4)
O2	0.5	0.5	0	0.752 (17)	0.872 (9)
N1	0	0.5	0.5	0.752 (17)	0.435 (4)
N2	0.5	0.5	0	0.752 (17)	0.128 (9)

(III)

Crystal data

C12/m1	β = 135.09 (4)°
a = 8.0579 (12) Å	γ = 90°
b = 8.0305 (16) Å	V = 259.9 (3) Å^3
c = 5.690 (4) Å	T = 8 K
α = 90°

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

	x	y	z	Biso*/Beq	Occ. (<1)
Sr1	0.25	0	0	0.50 (3)
Ta1	0.25	0.25	0.5	0.41 (3)
O1	0	0.27283 (18)	0	1.26 (3)	0.21 (2)
O2	0	0.25	0.5	1.26 (3)	0.84 (3)
O3	0.22717 (18)	0	0.4543 (4)	1.26 (3)	0.95 (2)
N1	0	0.27283 (18)	0	1.26 (3)	0.79 (2)
N2	0	0.25	0.5	1.26 (3)	0.16 (3)
N3	0.22717 (18)	0	0.4543 (4)	1.26 (3)	0.05 (2)

(IV)

Crystal data

I4/mcm	β = 90°
a = 5.6837 (8) Å	γ = 90°
b = 5.6837 (8) Å	V = 260.47 (11) Å^3
c = 8.0630 (12) Å	T = 8 K
α = 90°

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

	x	y	z	Biso*/Beq	Occ. (<1)
Sr1	0	0.5	0.25	0.54 (3)
Ta1	0	0	0	0.34 (3)
O1	0.77213 (14)	0.27213 (14)	0	1.27 (3)	0.5
O2	0	0	0.25	1.27 (3)
N1	0.77213 (14)	0.27213 (14)	0	1.27 (3)	0.5

(V)

Crystal data

I4/mcm	β = 90°
a = 5.6949 (9) Å	γ = 90°
b = 5.6949 (9) Å	V = 261.66 (13) Å^3
c = 8.0681 (14) Å	T = 286 K
α = 90°

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

	x	y	z	Biso*/Beq	Occ. (<1)
Sr1	0	0.5	0.25	0.86 (4)
Ta1	0	0	0	0.55 (4)
O1	0.76655 (14)	0.26655 (14)	0	1.50 (4)	0.5
O2	0	0	0.25	1.50 (4)
N1	0.76655 (14)	0.26655 (14)	0	1.50 (4)	0.5

(VI)

Crystal data

I4/mcm	β = 90°
a = 5.7137 (2) Å	γ = 90°
b = 5.7137 (2) Å	V = 264.06 (5) Å^3
c = 8.0885 (10) Å	T = 418 K
α = 90°

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

	x	y	z	Biso*/Beq	Occ. (<1)
Sr1	0	0.5	0.25	0.95 (4)
Ta1	0	0	0	0.52 (4)
O1	0.75950 (18)	0.24050 (18)	0	1.61 (4)	0.5
O2	0	0	0.25	1.61 (4)
N1	0.75950 (18)	0.24050 (18)	0	1.61 (4)	0.5

(VII)

Crystal data

I4/mcm	β = 90°
a = 5.7224 (3) Å	γ = 90°
b = 5.7224 (3) Å	V = 265.27 (5) Å^3
c = 8.1010 (12) Å	T = 603 K
α = 90°

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

	x	y	z	Biso*/Beq	Occ. (<1)
Sr1	0	0.5	0.25	1.24 (5)
Ta1	0	0	0	0.72 (5)
O1	0.7551 (4)	0.2551 (4)	0	1.84 (5)	0.5
O2	0	0	0.25	1.84 (5)
N1	0.7551 (4)	0.2551 (4)	0	1.84 (5)	0.5

(VIII)

Crystal data

P4/mmm	β = 90°
a = 4.04633 (11) Å	γ = 90°
b = 4.04633 (11) Å	V = 66.32 (1) Å^3
c = 4.0506 (4) Å	T = 603 K
α = 90°

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

	x	y	z	Biso*/Beq	Occ. (<1)
Sr1	0.00000	0.00000	0.00000	1.26 (5)
Ta1	0.50000	0.50000	0.50000	0.73 (6)
O1	0.00000	0.50000	0.50000	1.89 (5)	0.5
O2	0.50000	0.50000	0.00000	1.89 (5)
N1	0.00000	0.50000	0.50000	1.89 (5)	0.5