Structural phase transition in polycrystalline SnSe: a neutron diffraction study in correlation with thermoelectric properties

SnSe has been recently reported as a promising and highly efficient thermoelectric intermetallic alloy. The present material has been prepared by arc melting, as mechanically robust pellets, consisting of highly oriented polycrystals. The evolution of its orthorhombic GeS-type structure (space group Pnma) and phase transition to TlI-type structure (space group Cmcm) at high temperature has been studied in situ by neutron powder diffraction (NPD) in the temperature range 295–873 K. This transition has been identified by differential scanning calorimetry measurements, yielding sharp peaks at 795 K. In addition, thermal transport properties were measured in a similar temperature range, and large Seebeck coefficients, as high as 1050 µV K⁻¹ at 625 K, were found. The analysis from NPD data demonstrates an almost perfect stoichiometry, Sn_0.998(8)Se, that does not evolve with temperature, and a progressive decrease of the anharmonicity of the chemical bonds upon entering the domain of the Cmcm structure.

Computing details

For all compounds, program(s) used to refine structure: FULLPROF.

(SnSe-473K)

Crystal data

Orthorhombic, Pnma	c = 4.4261 (3) Å
Hall symbol: -P 2ac 2n	V = 213.62 (3) Å^3
a = 11.5524 (9) Å	Constant Wavelength Neutron Diffraction radiation
b = 4.1777 (3) Å

Data collection

D2B diffractometer	2θmin = 0.088°, 2θmax = 159.938°, 2θstep = 0.050°
Radiation source: nuclear reactor

Refinement

Rp = 2.280	3198 data points
Rwp = 3.048	72 parameters
Rexp = 1.817	0 restraints
RBragg = 3.036

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sn1	0.11839 (17)	0.25000	0.0957 (3)	0.0249 (13)	0.999 (8)
Se1	0.35525 (13)	0.25000	0.0191 (3)	0.0197 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.0254 (16)	0.0223 (12)	0.0269 (11)	0.00000	−0.0036 (8)	0.00000
Se1	0.0154 (9)	0.0185 (6)	0.0251 (7)	0.00000	−0.0016 (6)	0.00000

(SnSe-573K)

Crystal data

Orthorhombic, Pnma	c = 4.4081 (4) Å
Hall symbol: -P 2ac 2n	V = 214.65 (3) Å^3
a = 11.5925 (11) Å	Constant Wavelength Neutron Diffraction radiation
b = 4.2006 (3) Å

Data collection

Radiation source: nuclear reactor

2θmin = 0.099°, 2θmax = 159.949°, 2θstep = 0.050°

Refinement

Rp = 2.147	3198 data points
Rwp = 2.947	72 parameters
Rexp = 1.816	0 restraints
RBragg = 3.327

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sn1	0.11902 (19)	0.25000	0.0894 (4)	0.0312 (16)	0.997 (9)
Se1	0.35599 (16)	0.25000	0.0198 (3)	0.0248 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.0308 (19)	0.0283 (15)	0.0344 (13)	0.00000	−0.0009 (10)	0.00000
Se1	0.0195 (10)	0.0244 (7)	0.0305 (8)	0.00000	−0.0026 (7)	0.00000

(SnSe-673K)

Crystal data

Orthorhombic, Pnma	c = 4.3805 (4) Å
Hall symbol: -P 2ac 2n	V = 215.53 (3) Å^3
a = 11.6310 (12) Å	Constant Wavelength Neutron Diffraction radiation
b = 4.2302 (4) Å

Data collection

Radiation source: nuclear reactor

2θmin = 0.099°, 2θmax = 159.949°, 2θstep = 0.050°

Refinement

Rp = 2.106	3198 data points
Rwp = 2.879	72 parameters
Rexp = 1.822	0 restraints
RBragg = 3.523

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sn1	0.1205 (2)	0.25000	0.0784 (5)	0.0379 (19)	0.995 (10)
Se1	0.35653 (17)	0.25000	0.0175 (4)	0.0321 (10)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.039 (2)	0.0340 (17)	0.0409 (16)	0.00000	0.0008 (13)	0.00000
Se1	0.0293 (12)	0.0306 (10)	0.0364 (10)	0.00000	−0.0005 (10)	0.00000

(SnSe-773K-Pnma)

Crystal data

Orthorhombic, Pnma	c = 4.33321 (6) Å
Hall symbol: -P 2ac 2n	V = 216.66 (1) Å^3
a = 11.68526 (15) Å	Constant Wavelength Neutron Diffraction radiation
b = 4.27880 (5) Å

Data collection

Radiation source: nuclear reactor

2θmin = 0.090°, 2θmax = 159.940°, 2θstep = 0.050°

Refinement

Rp = 2.150	3198 data points
Rwp = 2.886	75 parameters
Rexp = 1.822	0 restraints
RBragg = 6.839

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

	x	y	z	Uiso*/Ueq
Sn1	0.1217 (3)	0.25000	0.0565 (8)	0.052 (2)
Se1	0.35675 (18)	0.25000	0.0093 (6)	0.0359 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.053 (3)	0.053 (2)	0.0500 (19)	0.00000	0.001 (2)	0.00000
Se1	0.0280 (16)	0.0379 (13)	0.0419 (11)	0.00000	−0.0002 (18)	0.00000

(SnSe-773K-Cmcm)

Crystal data

Orthorhombic, Cmcm	c = 4.29280 (19) Å
Hall symbol: -C 2c 2	V = 216.40 (2) Å^3
a = 4.3086 (2) Å	Constant Wavelength Neutron Diffraction radiation
b = 11.6997 (8) Å

Data collection

Radiation source: nuclear reactor

2θmin = 0.090°, 2θmax = 159.940°, 2θstep = 0.050°

Refinement

Rp = 2.150	0 data points
Rwp = 2.886	75 parameters
Rexp = 1.822	0 restraints
RBragg = 6.338

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

	x	y	z	Uiso*/Ueq
Sn1	0.00000	0.3726 (3)	0.25000	0.0671 (9)
Se1	0.00000	0.1427 (4)	0.25000	0.091 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.0823 (7)	0.0705 (13)	0.0484 (6)	0.00000	0.00000	0.00000
Se1	0.052 (2)	0.177 (14)	0.0435 (2)	0.00000	0.00000	0.00000

(SnSe-873K)

Crystal data

Orthorhombic, Cmcm	c = 4.3178 (5) Å
Hall symbol: -C 2c 2	V = 217.99 (4) Å^3
a = 4.3062 (5) Å	Constant Wavelength Neutron Diffraction radiation
b = 11.7243 (14) Å

Data collection

Radiation source: nuclear reactor

2θmin = 0.084°, 2θmax = 159.934°, 2θstep = 0.050°

Refinement

Rp = 2.162	3198 data points
Rwp = 3.003	83 parameters
Rexp = 1.825	0 restraints
RBragg = 4.068

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sn1	0.00000	0.3767 (3)	0.25000	0.069 (3)	0.998 (12)
Se1	0.00000	0.1429 (2)	0.25000	0.0481 (19)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.087 (3)	0.075 (4)	0.046 (3)	0.00000	0.00000	0.00000
Se1	0.0477 (15)	0.0526 (18)	0.044 (2)	0.00000	0.00000	0.00000