Beyond Rattling: Tetrahedrites as Incipient Ionic Conductors.

Materials with ultralow thermal conductivity are crucial to many technological applications, including thermoelectric energy harvesting, thermal barrier coatings, and optoelectronics. Liquid-like mobile ions are effective at disrupting phonon propagation, hence suppressing thermal conduction. However, high ionic mobility leads to the degradation of liquid-like thermoelectric materials under operating conditions due to ion migration and metal deposition at the cathode, hindering their practical application. Here, a new type of behavior, incipient ionic conduction, which leads to ultralow thermal conductivity, while overcoming the issues of degradation inherent in liquid-like materials, is identified. Using neutron spectroscopy and molecular dynamics (MD) simulations, it is demonstrated that in tetrahedrite, an established thermoelectric material with a remarkably low thermal conductivity, copper ions, although mobile above 200 K, are predominantly confined to cages within the crystal structure. Hence the undesirable migration of cations to the cathode can be avoided. These findings unveil a new approach for the design of materials with ultralow thermal conductivity, by exploring systems in which incipient ionic conduction may be present.

Lone Pair Rotation and Bond Heterogeneity Leading to Ultralow Thermal Conductivity in Aikinite.

Understanding the relationship between the crystal structure, chemical bonding, and lattice dynamics is crucial for the design of materials with low thermal conductivities, which are essential in fields as diverse as thermoelectrics, thermal barrier coatings, and optoelectronics. The bismuthinite-aikinite series, Cu1-x□xPb1-xBi1+xS3 (0 ≤ x ≤ 1, where □ represents a vacancy), has recently emerged as a family of n-type semiconductors with exceptionally low lattice thermal conductivities. We present a detailed investigation of the structure, electronic properties, and the vibrational spectrum of aikinite, CuPbBiS3 (x = 0), in order to elucidate the origin of its ultralow thermal conductivity (0.48 W m-1 K-1 at 573 K), which is close to the calculated minimum for amorphous and disordered materials, despite its polycrystalline nature. Inelastic neutron scattering data reveal an anharmonic optical phonon mode at ca. 30 cm-1, attributed mainly to the motion of Pb2+ cations. Analysis of neutron diffraction data, together with ab-initio molecular dynamics simulations, shows that the Pb2+ lone pairs are rotating and that, with increasing temperature, Cu+ and Pb2+ cations, which are separated at distances of ca. 3.3 Å, exhibit significantly larger displacements from their equilibrium positions than Bi3+ cations. In addition to bond heterogeneity, a temperature-dependent interaction between Cu+ and the rotating Pb2+ lone pair is a key contributor to the scattering effects that lower the thermal conductivity in aikinite. This work demonstrates that coupling of rotating lone pairs and the vibrational motion is an effective mechanism to achieve ultralow thermal conductivity in crystalline materials.

Tin-Substituted Chalcopyrite: An n-Type Sulfide with Enhanced Thermoelectric Performance.

The dearth of n-type sulfides with thermoelectric performance comparable to that of their p-type analogues presents a problem in the fabrication of all-sulfide devices. Chalcopyrite (CuFeS2) offers a rare example of an n-type sulfide. Chemical substitution has been used to enhance the thermoelectric performance of chalcopyrite through preparation of Cu1-x Sn x FeS2 (0 ≤ x ≤ 0.1). Substitution induces a high level of mass and strain field fluctuation, leading to lattice softening and enhanced point-defect scattering. Together with dislocations and twinning identified by transmission electron microscopy, this provides a mechanism for scattering phonons with a wide range of mean free paths. Substituted materials retain a large density-of-states effective mass and, hence, a high Seebeck coefficient. Combined with a high charge-carrier mobility and, thus, high electrical conductivity, a 3-fold improvement in power factor is achieved. Density functional theory (DFT) calculations reveal that substitution leads to the creation of small polarons, involving localized Fe2+ states, as confirmed by X-ray photoelectron spectroscopy. Small polaron formation limits the increase in carrier concentration to values that are lower than expected on electron-counting grounds. An improved power factor, coupled with substantial reductions (up to 40%) in lattice thermal conductivity, increases the maximum figure-of-merit by 300%, to zT ≈ 0.3 at 673 K for Cu0.96Sn0.04FeS2.

Improved Thermoelectric Performance through Double Substitution in Shandite-Type Mixed-Metal Sulfides.

Substitution of tin by indium in shandite-type phases, A3Sn2S2 with mixed Co/Fe occupancy of the A-sites is used to tune the Fermi level within a region of the density of states in which there are sharp, narrow bands of predominantly metal d-character. Materials of general formula Co2.5+x Fe0.5-x Sn2--yIn y S2 (x = 0, 0.167; 0.0 ≤ y ≤ 0.7) have been prepared by solid-state reaction and the products characterized by powder X-ray diffraction. Electrical-transport property data reveal that the progressive depopulation of the upper conduction band as tin is replaced by indium increases the electrical resistivity, and the weakly temperature-dependent ρ(T) becomes more semiconducting in character. Concomitant changes in the negative Seebeck coefficient, the temperature dependence of which becomes increasingly linear, suggests the more highly substituted materials are n-type degenerate semiconductors. The power factors of the substituted phases, while increased, exhibit a weak temperature dependence. The observed reductions in thermal conductivity are principally due to reductions in the charge-carrier contribution on hole doping. A maximum figure-of-merit of (ZT)max = 0.29 is obtained for the composition Co2.667Fe0.333Sn1.6In0.4S2 at 573 K: among the highest values for an n-type sulfide at this temperature.

Understanding the origin of disorder in kesterite-type chalcogenides A2ZnBQ4 (A = Cu, Ag; B = Sn, Ge; Q = S, Se): the influence of inter-layer interactions.

Semiconducting quaternary chalcogenides with A2ZnBQ4 stoichiometry, where A and B are monovalent and tetravalent metal ions and Q is a chalcogen (e.g. Cu2ZnSnS4 or CZTS) have recently attracted attention as potential solar-cell absorbers made from abundant and non-toxic elements. Unfortunately, they exhibit relatively poor sunlight conversion efficiencies, which has been linked to site disorder within the tetrahedral cation sub-lattice. In order to gain a better understanding of the factors controlling cation disorder in these chalcogenides, we have used powder neutron diffraction, coupled with Density Functional Theory (DFT) simulations, to investigate the detailed structure of A2ZnBQ4 phases, with A = Cu, Ag; B = Sn, Ge; and Q = S, Se. Both DFT calculations and powder neutron diffraction data demonstrate that the kesterite structure (space group: I4[combining macron]) is adopted in preference to the higher-energy stannite structure (space group: I4[combining macron]2m). The contrast between the constituent cations afforded by neutron diffraction reveals that copper and zinc cations are only partially ordered in the kesterites Cu2ZnBQ4 (B = Sn, Ge), whereas the silver-containing phases are fully ordered. The degree of cation order in the copper-containing phases shows a greater sensitivity to the identity of the B-cation than to the chalcogenide anion. DFT indicates that cation ordering minimises inter-planar Zn2+Zn2+ electrostatic interactions, while there is an additional intra-planar energy contribution associated with size mismatch. The complete Ag/Zn order in Ag2ZnBQ4 (B = Sn, Ge) phases can thus be related to the anisotropic expansion of the unit cell on replacing Cu with Ag.

The impact of charge transfer and structural disorder on the thermoelectric properties of cobalt intercalated TiS2.

A family of phases, Co x TiS2 (0 ≤ x ≤ 0.75) has been prepared and characterised by powder X-ray and neutron diffraction, electrical and thermal transport property measurements, thermal analysis and SQUID magnetometry. With increasing cobalt content, the structure evolves from a disordered arrangement of cobalt ions in octahedral sites located in the van der Waals' gap (x ≤ 0.2), through three different ordered vacancy phases, to a second disordered phase at x ≥ 0.67. Powder neutron diffraction reveals that both octahedral and tetrahedral inter-layer sites are occupied in Co0.67TiS2. Charge transfer from the cobalt guest to the TiS2 host affords a systematic tuning of the electrical and thermal transport properties. At low levels of cobalt intercalation (x < 0.1), the charge transfer increases the electrical conductivity sufficiently to offset the concomitant reduction in |S|. This, together with a reduction in the overall thermal conductivity leads to thermoelectric figures of merit that are 25% higher than that of TiS2, ZT reaching 0.30 at 573 K for Co x TiS2 with 0.04 ≤ x ≤ 0.08. Whilst the electrical conductivity is further increased at higher cobalt contents, the reduction in |S| is more marked due to the higher charge carrier concentration. Furthermore both the charge carrier and lattice contributions to the thermal conductivity are increased in the electrically conductive ordered-vacancy phases, with the result that the thermoelectric performance is significantly degraded. These results illustrate the competition between the effects of charge transfer from guest to host and the disorder generated when cobalt cations are incorporated in the inter-layer space.

Bridging silicon nanoparticles and thermoelectrics: phenylacetylene functionalization.

Silicon is a promising alternative to current thermoelectric materials (Bi(2)Te(3)). Silicon nanoparticle based materials show especially low thermal conductivities due to their high number of interfaces, which increases the observed phonon scattering. The major obstacle with these materials is maintaining high electrical conductivity. Surface functionalization with phenylacetylene shows an electrical conductivity of 18.1 S m(-1) and Seebeck coefficient of 3228.8 µV K(-1) as well as maintaining a thermal conductivity of 0.1 W K(-1) m(-1). This gives a ZT of 0.6 at 300 K which is significant for a bulk silicon based material and is similar to that of other thermoelectric materials such as Mg(2)Si, PbTe and SiGe alloys.

[C7H10N][In3Se5]: a layered selenide with two indium coordination environments.

A new layered indium selenide, [C(7)H(10)N][In(3)Se(5)] (1), has been prepared solvothermally using 3,5-dimethylpyridine as a solvent and structure-directing agent. This material, which was characterized by single-crystal and powder X-ray diffraction, thermogravimetric analysis, UV-vis diffuse-reflectance spectroscopy, IR spectroscopy, and elemental analysis, crystallizes in the monoclinic space group P2(1)/c [a = 3.9990(4) Å, b = 16.7858(15) Å, c = 23.930(2) Å, and ß = 94.728(4)°]. The crystal structure of 1 contains anionic layers of stoichiometry [In(3)Se(5)](-) in which indium atoms with octahedral and tetrahedral coordination coexist. The [In(3)Se(5)](-) layers are interspaced by monoprotonated 3,5-dimethylpyridinium cations. A closely related material, [C(7)H(10)N][In(3)Se(5)] (2), was obtained when using 2,6-dimethylpyridine instead of 3,5-dimethylpyridine.

A synchrotron powder X-ray diffraction study of the skutterudite-related phases AB1.5Te1.5 (A = Co, Rh, Ir; B = Ge, Sn).

X-Ray resonant scattering has been exploited to investigate the crystal structure of the AB(1.5)Te(1.5) phases (A = Co, Rh, Ir; B = Ge, Sn). Analysis of the diffraction data reveals that CoGe(1.5)Te(1.5) and ASn(1.5)Te(1.5) adopt a rhombohedral skutterudite-related structure, containing diamond-shape B(2)Te(2) rings, in which the B and Te atoms are ordered and trans to each other. Anion ordering is however incomplete, and with increasing the size of both cations and anions, the degree of anion ordering decreases. By contrast, the diffraction data of IrGe(1.5)Te(1.5) are consistent with an almost statistical distribution of the anions over the available sites, although some ordered domains may be present. The thermoelectric properties of these materials are discussed in light of these results.

Ternary erbium chromium sulfides: structural relationships and magnetic properties.

Single crystals of four erbium-chromium sulfides have been grown by chemical vapor transport using iodine as the transporting agent. Single-crystal X-ray diffraction reveals that in Er(3)CrS(6) octahedral sites are occupied exclusively by Cr(3+) cations, leading to one-dimensional CrS(4)(5-) chains of edge-sharing octahedra, while in Er(2)CrS(4), Er(3+), and Cr(2+) cations occupy the available octahedral sites in an ordered manner. By contrast, in Er(6)Cr(2)S(11) and Er(4)CrS(7), Er(3+) and Cr(2+) ions are disordered over the octahedral sites. In Er(2)CrS(4), Er(6)Cr(2)S(11), and Er(4)CrS(7), the network of octahedra generates an anionic framework constructed from M(2)S(5) slabs of varying thickness, linked by one-dimensional octahedral chains. This suggests that these three phases belong to a series in which the anionic framework may be described by the general formula [M(2n+1)S(4n+3)](x-), with charge balancing provided by Er(3+) cations located in sites of high-coordination number within one-dimensional channels defined by the framework. Er(4)CrS(7), Er(6)Cr(2)S(11), and Er(2)CrS(4) may thus be considered as the n = 1, 2, and infinity members of this series. While Er(4)CrS(7) is paramagnetic, successive magnetic transitions associated with ordering of the chromium and erbium sub-lattices are observed on cooling Er(3)CrS(6) (T(C)(Cr) = 30 K; T(C)(Er) = 11 K) and Er(2)CrS(4) (T(N)(Cr) = 42 K, T(N)(Er) = 10 K) whereas Er(6)Cr(2)S(11) exhibits ordering of the chromium sub-lattice only (T(N) = 11.4 K).

Bis(tetra-phenyl-phospho-nium) tetra-sulfido-tungstate(VI).

The crystal structure of the title compound, (C(24)H(20)P)(2)[WS(4)], which was prepared under hydro-thermal conditions, contains tetra-phenyl-phospho-nium cations linked by supra-molecular inter-actions into chains running along the [110] and [10] directions. The [WS(4)](2-)anions, which lie on twofold axes, are located in the cavities created between the cation chains.

1,4,8,11-Tetraazacyclotetradecane antimony(III) sulfide.

Poly[1,4,8,11-tetraazacyclotetradecane(2+) [hepta-mu-sulfido-trisulfidohexaantimony(III)]], {(C10H26N4)[Sb6S10]}n, consists of novel [Sb6S10]2- layers containing Sb2S2, Sb4S4 and Sb7S7 hetero-rings, which are separated by macrocyclic amine molecules. The macrocyclic amine molecules are disordered over two crystallographically distinct positions and are diprotonated in order to balance the charge of the anionic layers.

Macrocyclic amines as structure-directing agents for the synthesis of three-dimensional antimony-sulfide frameworks.

A new family of antimony sulfides, incorporating the macrocyclic tetramine 1,4,8,11-tetraazacyclotetradecane (cyclam), has been prepared by a hydrothermal method. [C10N4H26][Sb4S7] (1), [Ni(C10N4H24)][Sb4S7] (2), and [Co(C10N4H24)]x[C10N4H26](1-x)[Sb4S7] (0.08 < or = x < or = 0.74) (3) have been characterized by single-crystal X-ray diffraction, elemental analysis, thermogravimetry, and analytical electron microscopy. All three materials possess the same novel three-dimensional Sb4S7(2-) framework, constructed from layers of parallel arrays of Sb4S8(4-) chains stacked at 90 degrees to one another. In 1, doubly protonated macrocyclic cations reside in the channel structure of the antimony-sulfide framework. In 2 and 3, the cyclam acts as a ligand, chelating the divalent transition-metal cation. Analytical and X-ray diffraction data indicate that the level of metal incorporation in 2 is effectively complete, whereas in 3, both metalated and nonmetalated forms of the macrocycle coexist within the structure.

Methylammonium antimony sulfide.

Bis(methylammonium) octaantimony(III) dodecasulfide persulfide, (CH3NH3)2[Sb8S12(S2)], contains pairs of [Sb4S7]2- chains joined through an unusual persulfide bond to create infinite double [Sb8S14]2- chains. The double chains are interlocked by longer Sb...S interactions to form sheets approximately parallel to the (101) crystallographic plane. Methylammonium cations, formed by decomposition of 2-methylpropane-1,2-diamine during the synthesis, are located in large (Sb8S10) hetero-ring apertures created within the double chains.

Topotactic oxidation of TiGaPO-1, a pyridine-templated titanium gallophosphate with a new octahedral-tetrahedral 3-D framework structure containing Ti(III)/Ti(IV).

The first 3-D open-framework TiGaPO complex, constructed from Ti(III)O(6), Ti(IV)O(6), GaO(4), and PO(4) polyhedra, contains pyridinium cations in a 1-D pore network and can be oxidized in air at 543 K with retention of the original framework structure.

[Co(en)3][Sb12S19]: a new antimony sulfide with a zeolite-like structure containing one-dimensional channels.

Solvothermal synthesis affords access to the first truly three-dimensional antimony-sulfide framework which contains one-dimensional circular channels.

Templated synthesis of the novel layered silver-antimony sulfides [H3NCH2CH2NH2][Ag2SbS3 and [H3NCH2CH2NH2]2[Ag5Sb3S8].

Two new silver-antimony sulfides, [C(2)H(9)N(2)][Ag(2)SbS(3)] (1) and [C(2)H(9)N(2)](2)[Ag(5)Sb(3)S(8)] (2), have been prepared solvothermally in the presence of ethylenediamine and characterized by single-crystal X-ray diffraction, thermogravimetry, and elemental analysis. Compound 1 crystallizes in the space group Pn (a = 6.1781(1) A, b =11.9491(3) A, c = 6.9239(2) A, beta =111.164(1) degrees ) and 2 in the space group Pm (a = 6.2215(2) A, b = 15.7707(7) A, c = 11.6478(5) A, beta = 92.645(2) degrees ). The structure of 1 consists of chains of fused five-membered Ag(2)SbS(2) rings linked to form layers, between which the template molecules reside. Compound 2 contains honeycomb-like sheets of fused silver-antimony-sulfide six-membered rings linked to form double layers. The idealized structure can be considered to be an ordered defect derivative of that of lithium bismuthide, Li(3)Bi, and represents a new solid-state structure type.