Octahedral deformations and cationic displacements in the ferroelectric PbHf_0.8Ti_0.2O₃: a neutron powder diffraction study from 10 to 770 K

Neutron powder diffraction data, collected over the temperature range 10–770 K, have been analysed in order to make a detailed characterization of the sequence of phase transitions occurring in the Hf-rich ferroelectric PbHf_0.8Ti_0.2O₃, titanium hafnium lead oxide. Over the whole temperature range this compound undergoes two phase transitions, which involve cationic displacements and octahedral deformations (tilt and/or distortion) leading to strongly distorted perovskite-type structures. The first transition appears around 415 K between two ferroelectric rhombohedral phases: a low-temperature nonzero-tilt phase F_RL (space group R3c) and an intermediate zero-tilt phase F_RH (space group R3m). The second one, detected around 520 K, is associated with a ferroelectric to-paraelectric transition between the F_RH phase and the P_C cubic phase (space group Pm3m). From high-resolution neutron powder diffraction data (diffractometer 3T2-LLB, Saclay, France, λ = 1.2251 Å), the crystallographic structure of the three successive phases has been accurately determined at the following temperatures: T = 10 K (F_RL): space group R3c, Z = 6, a_hex = 5.7827 (1), c_hex = 14.2702 (4) Å, V_hex = 413.26 (2) ų; T = 150 K (F_RL): space group R3c, Z = 6, a_hex = 5.7871 (1), c_hex = 14.2735 (4) Å, V_hex = 413.98 (3) ų; T = 290 K (F_RL): space group R3c, Z = 6, a_hex = 5.7943 (1), c_hex = 14.2742 (5) Å, V_hex = 415.04 (3) ų; T = 440 K (F_RH): space group R3c, Z = 6, a_hex = 5.8025 (1), c_hex = 14.2648 (4) Å, V_hex = 415.94 (3) ų; T = 520 K (P_C): space group Pm3m, Z = 1, a_cub = 4.1072 (2) Å, V_cub = 69.29 (1) ų. In addition, a neutron powder thermodiffractometry experiment, performed between 290 and 770 K (diffractometer D1B-ILL, Grenoble, France, λ = 2.533 Å), has been used to study in situ the temperature-induced phase transitions. From sequential Rietveld refinements, the temperature dependence of the cation displacements and the rotation and/or distortion of oxygen octahedra was derived.

Computing details

For all compounds, cell refinement: Fullprof; program(s) used to refine structure: Fullprof; molecular graphics: CaRIne, version 3.0.

(1) lead hafnate titanate

Crystal data

Hf0.8O3PbTi0.2	γ = 120°
Mr = 407.52	V = 413.26 (2) Å^3
Rhombohedral, R3c	Z = 6
a = 5.7827 (1) Å	Neutron radiation, λ = 1.2251 Å
c = 14.2702 (4) Å	T = 10 K
α = 90°

Data collection

Diffractometer in Debye-Scherrer geometry	k = →
h = →	l = →

Refinement

S = 2.02	39 parameters
158 reflections	Weighting scheme based on measured s.u.'s w = 1/(σ^2), σ^2 = y

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Pb	0	0	0.2835 (1)	0.0073 (3)
Hf	0	0	0.0126 (1)	0.0004 (4)*	0.80
Ti	0	0	0.0126 (1)	0.0004 (4)*	0.20
O	0.1427 (2)	0.3461 (2)	0.0833	0.0085 (3)*

Bond lengths (Å)

Hf—O	2.01	O—O	2.92
Hf—O	2.12	Pb—O	2.49
Ti—O	2.01	Pb—O	2.76
Ti—O	2.12	Pb—O	3.10
O—O	2.80	Pb—O	3.35
O—O	3.02

(2) lead hafnate titanate

Crystal data

Hf0.8O3PbTi0.2	γ = 120°
Mr = 407.52	V = 413.98 (3) Å^3
Rhombohedral, R3c	Z = 6
a = 5.7871 (1) Å	Neutron radiation, λ = 1.2251 Å
c = 14.2735 (4) Å	T = 150 K
α = 90°

Data collection

Diffractometer in Debye-Scherrer geometry	k = →
h = →	l = →

Refinement

S = 2.00	39 parameters
158 reflections	Weighting scheme based on measured s.u.'s w = 1/(σ^2), σ^2 = y

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Pb	0	0	0.2821 (1)	0.0127 (3)
Hf	0	0	0.0121 (1)	0.0008 (2)*	0.80
Ti	0	0	0.0121 (1)	0.0008 (2)*	0.20
O	0.1458 (2)	0.3456 (2)	0.0833	0.0105 (3)*

Bond lengths (Å)

Hf—O	2.01	O—O	2.91
Hf—O	2.11	Pb—O	2.51
Ti—O	2.01	Pb—O	2.78
Ti—O	2.11	Pb—O	3.08
O—O	2.80	Pb—O	3.33
O—O	3.01

(3) lead hafnate titanate

Crystal data

Hf0.8O3PbTi0.2	γ = 120°
Mr = 407.52	V = 415.04 (3) Å^3
Rhombohedral, R3c	Z = 6
a = 5.7943 (1) Å	Neutron radiation, λ = 1.2251 Å
c = 14.2742 (5) Å	T = 290 K
α = 90°

Data collection

Diffractometer in Debye-Scherrer geometry	k = →
h = →	l = →

Refinement

S = 2.28	39 parameters
158 reflections	Weighting scheme based on measured s.u.'s w = 1/(σ^2), σ^2 = y

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Pb	0	0	0.2793 (1)	0.0243 (4)
Hf	0	0	0.0114 (2)	0.0023 (2)*	0.80
Ti	0	0	0.0114 (2)	0.0023 (2)*	0.20
O	0.1525 (2)	0.3448 (3)	0.0833	0.0162 (3)*

Bond lengths (Å)

Hf—O	2.02	O—O	2.91
Hf—O	2.11	Pb—O	2.54
Ti—O	2.02	Pb—O	2.81
Ti—O	2.11	Pb—O	3.04
O—O	2.80	Pb—O	3.29
O—O	3.00

(4) lead hafnate titanate

Crystal data

Hf0.8O3PbTi0.2	γ = 120°
Mr = 407.52	V = 415.94 (3) Å^3
Rhombohedral, R3c	Z = 6
a = 5.8025 (2) Å	Neutron radiation, λ = 1.2251 Å
c = 14.2648 (4) Å	T = 440 K
α = 90°

Data collection

Diffractometer in Debye-Scherrer geometry	k = →
h = →	l = →

Refinement

S = 3.00	38 parameters
117 reflections	Weighting scheme based on measured s.u.'s w = 1/(σ^2), σ^2 = y

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Pb	0	0	0.2734 (2)	0.0447 (8)
Hf	0	0	0.0105 (2)	0.0041 (4)*	0.80
Ti	0	0	0.0105 (2)	0.0041 (4)*	0.20
O	0.1722 (4)	0.3444 (8)	0.0833	0.0248 (3)*

Bond lengths (Å)

Hf—O	2.02	O—O	3.00
Hf—O	2.10	O—O	2.91
Ti—O	2.02	Pb—O	2.61
Ti—O	2.10	Pb—O	2.92
O—O	2.80	Pb—O	3.22

(5) lead hafnate titanate

Crystal data

Hf0.8O3PbTi0.2	V = 69.28 (1) Å^3
Mr = 407.52	Z = 1
Cubic, Pm3m	Neutron radiation, λ = 1.2251 Å
Hall symbol: -P 4 2 3	T = 520 K
a = 4.1072 (1) Å

Data collection

Diffractometer in Debye-Scherrer geometry	k = →
h = →	l = →

Refinement

S = 2.37	33 parameters
39 reflections	Weighting scheme based on measured s.u.'s w = 1/(σ^2), σ^2 = y

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Pb	0	0	0	0.0553 (5)*
Hf	0.5	0.5	0.5	0.0037 (4)*	0.8
Ti	0.5	0.5	0.5	0.0037 (4)*	0.2
O	0.5	0.5	0	0.0346 (4)

Bond lengths (Å)

Hf—O	2.05	O—O	2.90
Ti—O	2.05	Pb—O	2.90