The high temperature structure of ß-SnS and ß-SnSe and the B16-to-B33 type λ-transition path

Wiedemeier, H.

doi:10.1524/zkri.1981.156.14.143

Cited by 43 publications

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“…It must be noted that the crystal structure of a-SnS (the low-temperature form) is a much more deformed version of the NaCl-type structure, with each Sn having three pyramidal S neighbors, one at 2.627 A and two at 2.665 A; three other sulfur atoms are at rather larger distances [3.290 (2x), 3.388 ~ (Ix)] (Wiedemeier & yon Schnering, 1978). The fl-SnS structure (von Schnering & Wiedemeier, 1981) approximately in the ab plane and a fifth Sn-S bond along the c axis of 2.63 A, with bond order 1. The SnS slices in (SnS)~.17NbS are intermediate between those of a-and/3-SnS.…”

Section: Discussion Of the Structurementioning

confidence: 99%

The incommensurate misfit layer structure of (SnS)1.17NbS2, `SnNbS3'. I. A study by means of X-ray diffraction

Meetsma¹,

Wiegers²,

Haange³

et al. 1989

Acta Cryst Sect A

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Abstract(SnS)t.17NbS2, Mr=334"92, is a compound with misfit layer structure consisting of two-atom-thick layers of SnS and three-atom-thick sandwiches of NbS2 which alternate along the c axis. The lattices of SnS and NbS2 both have C-centered orthorhombic unit cells which match along the b and c axes but not along the a axes. The unit cells and space groups are a=5.673(1), b=5.750(1), c=11.760(1) A, space group C2mb (no. 39), Z=4, for SnS; a = 3.321 (1), b = 5.752 (1), c = 11-763 (1) ~, space group Cm2m (no. 38), Z = 2, for NbS2. From the ratio of the lengths of the a axes of the two parts of the complete structure (5.673/3.321= 1.708 being irrational) one obtains a composition (SnS)I.~7NbS2. The structure determination consisted of three parts: the structures of the SnS and NbS2 parts separately and their relative position. Intensities were measured with Mo Kd radiation (A = 0.71073/~) at T = 293 K, p, = 102.3 cm -1. For the SnS part RF=0.088 for 306 unique reflections; for the NbS2 part RF = 0.031 for 329 unique reflections (for both cases Okl reflections excluded). The relative position of these two lattices was determined from the common reflections Okl: RF =0"072 for 98 reflections. The SnS part of the structure consists of deformed slices of SnS with a thickness of half the cell edge of (hypothetical) NaC1-type SnS. Each Sn atom is in this way coordinated by five sulfur atoms; four sulfur atoms are in a plane perpendicular to the c axis with SnS distances 2.74 (1) (Ix), 2.913 (1) (2x) and 3.01 (1) ~ (Ix), whereas the fifth Sn-S bond with length 2.695 (9)~ is approximately along the c axis. The NbS2 part of the structure is that of NbS2-2H with Nb in trigonal prisms of sulfur; the Nb-S distances are 2.473 (1)A. From * Author to whom correspondence should be addressed.0108-7673/89/040285-07503.00 refinement of the common Okl reflections the relative y and z positions of the two sublattices were found; along the common a direction the lattices of SnS and NbS2 are incommensurate. IntroductionCompounds of assumed composition MTX 3 (M= Sn, Pb; T=Ti, V, Nb, Ta; X=S, Se) have been known for a considerable time (Schmidt, 1970; Sterzel & Horn, 1970;van Maaren, 1972;Donohue, 1975). The crystals obtained by syntheses from the elements have an appearance suggesting a layered structure but owing to very easy plastic deformation it has so far not been possible to determine the structure. X-ray powder diffraction revealed teteragonal or pseudotetragonal unit cells with a =4, c= 12/~. We succeeded in growing single crystals by vapor transport using chlorine as transport agent. In this paper the determination of the misfit layer structure of'SnNbS3' is described; the composition of the compound turned out to be (SnS)~.17NbS2; it will be designated for brevity as 'SnNbS3'. Experimental'SnNbS3' could be grown as thin platelets with diameters up to about 5 mm. The black crystals are mechanically very soft and have a brilliant luster. The starting material was SnNbS3 obtained by syntheses from the elements at 1123 K and the te...

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Section: Discussion Of the Structurementioning

confidence: 99%

The incommensurate misfit layer structure of (SnS)1.17NbS2, `SnNbS3'. I. A study by means of X-ray diffraction

Meetsma¹,

Wiegers²,

Haange³

et al. 1989

Acta Cryst Sect A

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“…3), which is in good agreement with the onset of the structural phase transition. 13 The C p fit reveals the presence of two Debye oscillators with different energy scales but almost equal weighting. It is therefore tempting to link these modes with Sn and Se atomic motions.…”

Section: à4mentioning

confidence: 97%

“…[7][8][9][10][11][12] The crystal structure can be understood as a highly distorted rock salt derivative. 13,14 At high-temperatures, adjacent two-atom thick rock salt layers are translated by a/2 so that Sn is coordinated by 5 Se atoms in the rocksalt layer and by 2 Se atoms in an adjacent layer. Upon cooling below 800 K, the structure undergoes a displacive phase transition, which leads to a strongly distorted Sn coordination polyhedron with three short and two long bonds within the rocksalt layer, and a single long bond connecting to an adjacent rocksalt layer (Fig.…”

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confidence: 99%

Evidence for hard and soft substructures in thermoelectric SnSe

Popuri

Pollet

Decourt

et al. 2017

Applied Physics Letters

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SnSe is a topical thermoelectric material with a low thermal conductivity which is linked to its unique crystal structure. We use low-temperature heat capacity measurements to demonstrate the presence of two characteristic vibrational energy scales in SnSe with Debye temperatures thetaD1 = 345(9) K and thetaD2 = 154(2) K. These hard and soft substructures are quantitatively linked to the strong and weak Sn-Se bonds in the crystal structure. The heat capacity model predicts the temperature evolution of the unit cell volume, confirming that this two-substructure model captures the basic thermal properties. Comparison with phonon calculations reveals that the soft substructure is associated with the low energy phonon modes that are responsible for the thermal transport. This suggests that searching for materials containing highly divergent bond distances should be a fruitful route for discovering low thermal conductivity materials.Comment: Accepted by Applied Physics Letter

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“…We clarify the complex phase transition pathway between Pnma and Cmcm, and show that the high-T Cmcm phase is stabilized only when anharmonicity is included. Using harmonic phonon calculations and metadynamics simulations, we identify the sequence of elastic and phononic distortions which was suggested over 30 years ago by von Schnering and Wiedemeier [3]. The carrier concentrations are derived from intrinsic defect calculations within density functional theory.…”

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confidence: 99%

“…Bulk SnSe is a narrowband-gap semiconductor that undergoes a phase transition spanning the temperature range from 600 to 807 K, from a Pnma low-temperature phase as illustrated in Fig. 1 (space group 62) to a Cmcm high-temperature phase (space group 63) [3]. Both are distorted phases of rocksalt Fm3m (the isoelectronic structure of PbTe and SnTe).…”

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confidence: 99%

Two-Step Phase Transition in SnSe and the Origins of its High Power Factor from First Principles

et al. 2016

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The interest in improving the thermoelectric response of bulk materials has received a boost after it has been recognized that layered materials, in particular SnSe, show a very large thermoelectric figure of merit. This result has received great attention while it is now possible to conceive other similar materials or experimental methods to improve this value. Before we can now think of engineering this material it is important we understand the basic mechanism that explains this unusual behavior, where very low thermal conductivity and a high thermopower result from a delicate balance between the crystal and electronic structure. In this Letter, we present a complete temperature evolution of the Seebeck coefficient as the material undergoes a soft crystal transformation and its consequences on other properties within SnSe by means of first-principles calculations. Our results are able to explain the full range of considered experimental temperatures. DOI: 10.1103/PhysRevLett.117.276601 Thermoelectric (TE) materials and the thermoelectric effect are an interesting alternative energy source, harvesting waste heat from power production and other thermal engines. Despite their vast potential impact, only few materials are used in practice: most thermoelectric materials are highly toxic, expensive, and the devices present too low efficiencies to compete with other forms of power generation in industry. The main concern in this field is to discover or design thermoelectric materials that deal with these issues. The efficiency of a TE material is quantified by the thermoelectric figure of merit zT ¼ S 2 σT=ðκ el þ κ l Þ, which is the ratio of the electrical conductivity (σ), multiplied by the Seebeck coefficient (S) squared and the absolute temperature (T), over the thermal conductivity, which has both ionic (κ l ) and electronic (κ el ) contributions. The recent demonstration of zT ¼ 2.6 in monocrystalline tin selenide [1] or zT ¼ 1.34 in device form [2] has given a new breath to the field of thermoelectrics. By more than doubling the efficiency record for intrinsic bulk systems, SnSe has shown that economically competitive, nontoxic TE devices are within reach. The microscopic mechanism responsible for the performance is, however, not fully established, in particular due to sublimation effects in the high-T phase. Bulk SnSe is a narrowband-gap semiconductor that undergoes a phase transition spanning the temperature range from 600 to 807 K, from a Pnma low-temperature phase as illustrated in Fig. 1 (space group 62) to a Cmcm high-temperature phase (space group 63) [3]. Both are distorted phases of rocksalt Fm3m (the isoelectronic structure of PbTe and SnTe). Exceptional values of zT are obtained for two main reasons: the intrinsically low thermal conductivity (in both phases) and the strong enhancement of the carrier concentration and conductivity in the Cmcm phase. This intricate interplay opens perspectives for many other layered or heterostructure materials, and calls for a profound understanding of the mecha...

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The high temperature structure of ß-SnS and ß-SnSe and the B16-to-B33 type λ-transition path

Cited by 43 publications

References 0 publications

The incommensurate misfit layer structure of (SnS)1.17NbS2, `SnNbS3'. I. A study by means of X-ray diffraction

The incommensurate misfit layer structure of (SnS)1.17NbS2, `SnNbS3'. I. A study by means of X-ray diffraction

Evidence for hard and soft substructures in thermoelectric SnSe

Two-Step Phase Transition in SnSe and the Origins of its High Power Factor from First Principles

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