High-pressure phase transitions in the rare-earth orthoferrite LaFeO₃

Sequential Rietveld refinements were applied on high-pressure synchrotron powder X-ray diffraction measurements of lanthanum ferrite (LaFeO₃) revealing two phase transitions on the room-temperature isotherm up to a pressure of 48 GPa. The first structural phase transition of second order occurs at a pressure of 21.1 GPa, changing the space group from Pbnm to Ibmm. The second transition, involving a isostructural first-order phase transition, occurs at approximately 38 GPa, indicating a high-spin to low-spin transition of the Fe³⁺ ion. Following the behavior of the volume up to the hydrostatic limit of methanol-ethanol it was possible to use inverted equations of state (EoS) to determine a bulk modulus of B₀ = 172 GPa and a corresponding pressure derivative of B'₀ = 4.3. In addition, the linearized version of the inverted EoS were used to determine the corresponding moduli and pressure derivatives for each lattice direction.

Computing details

(I)

Crystal data

FeLaO3	c = 7.85490 (17) Å
Mr = 242.75	V = 242.88 (1) Å^3
Orthorhombic, Pbnm	Z = 4.0
a = 5.55494 (12) Å	Dx = 6.639 (1) Mg m^−3
b = 5.56627 (9) Å	T = 293 K

Data collection

ID09 beamline, ESRF, Grenoble, France diffractometer	Data collection mode: transmission
Radiation source: synchrotron radiation	2θmin = 5°, 2θmax = 37.4°, 2θstep = 0.014°
Water-cooled Si-(111) monochromator

Refinement

Rp = 0.026	2774 data points
Rwp = 3.761	Profile function: fundamental parameter
Rexp = 3.843	Background function: Chebyshev polynom of 11th order
RBragg = 1.107

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

	x	y	z	Biso*/Beq
La1	−0.00641 (17)	0.02932 (7)	0.25	0.318 (8)
Fe1	0	0.5	0	0.318 (8)
O1	0.0765 (19)	0.4875 (9)	0.25	0.318 (8)
O2	−0.2817 (13)	0.2853 (12)	0.0366 (10)	0.318 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?

(II)

Crystal data

FeLaO3	c = 7.543 (3) Å
Mr = 242.75	V = 219.38 (9) Å^3
Orthorhombic, Ibmm	Z = 4.0
a = 5.4290 (3) Å	Dx = 7.350 (3) Mg m^−3
b = 5.3573 (6) Å	T = 293 K

Data collection

ID09 beamline, ESRF, Grenoble, France diffractometer	Data collection mode: transmission
Radiation source: synchrotron radiation	2θmin = 5°, 2θmax = 20.8°, 2θstep = 0.014°
Water-cooled Si-(111) monochromator

Refinement

Rp = 0.021	2371 data points
Rwp = 3.302	Profile function: fundamental parameter
Rexp = 1.418	Background function: Chebyshev polynom of 11th order
RBragg = 0.427

Special details

Refinement. Spherical Harmonics of 4t h order

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

	x	y	z	Biso*/Beq
La1	−0.0040 (13)	0.0	0.25	0.31 (8)
Fe1	0	0.5	0	0.31 (8)
O1	0.068 (4)	0.5	0.25	0.31 (8)
O2	−0.25	0.25	0.033 (5)	0.31 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?

(III)

Crystal data

FeLaO3	c = 7.3063 (16) Å
Mr = 242.75	V = 201.95 (1) Å^3
Orthorhombic, Ibmm	Z = 4.0
a = 5.2621 (19) Å	Dx = 7.984 (4) Mg m^−3
b = 5.2528 (16) Å	T = 293 K

Data collection

ID09 beamline, ESRF, Grenoble, France diffractometer	Data collection mode: transmission
Radiation source: synchrotron radiation	2θmin = 5°, 2θmax = 20.8°, 2θstep = 0.014°
Water-cooled Si-(111) monochromator

Refinement

Rp = 0.020	2372 data points
Rwp = 3.162	Profile function: fundamental parameter
Rexp = 1.291	Background function: Chebyshev polynom of 11th order
RBragg = 0.676

Special details

Refinement. Spherical Harmonics of 4t h order

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

	x	y	z	Biso*/Beq
La1	−0..0082 (14)	0.0	0.25	0.16 (9)
Fe1	0	0.5	0	0.16 (9)
O1	0.024 (9)	0.5	0.25	0.16 (9)
O2	−0.25	0.25	0.035 (3)	0.16 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?