Kathleen Lee

Clathrate Ba8Au16P30: the "gold standard" for lattice thermal conductivity.

A novel clathrate phase, Ba8Au16P30, was synthesized from its elements. High-resolution powder X-ray diffraction and transmission electron microscopy were used to establish the crystal structure of the new compound. Ba8Au16P30 crystallizes in an orthorhombic superstructure of clathrate-I featuring a complete separation of gold and phosphorus atoms over different crystallographic positions, similar to the Cu-containing analogue, Ba8Cu16P30. Barium cations are trapped inside the large polyhedral cages of the gold-phosphorus tetrahedral framework. X-ray diffraction indicated that one out of 15 crystallographically independent phosphorus atoms appears to be three-coordinate. Probing the local structure and chemical bonding of phosphorus atoms with (31)P solid-state NMR spectroscopy confirmed the three-coordinate nature of one of the phosphorus atomic positions. High-resolution high-angle annular dark-field scanning transmission electron microscopy indicated that the clathrate Ba8Au16P30 is well-ordered on the atomic scale, although numerous twinning and intergrowth defects as well as antiphase boundaries were detected. The presence of such defects results in the pseudo-body-centered-cubic diffraction patterns observed in single-crystal X-ray diffraction experiments. NMR and resistivity characterization of Ba8Au16P30 indicated paramagnetic metallic properties with a room-temperature resistivity of 1.7 mΩ cm. Ba8Au16P30 exhibits a low total thermal conductivity (0.62 W m(-1) K(-1)) and an unprecedentedly low lattice thermal conductivity (0.18 W m(-1) K(-1)) at room temperature. The values of the thermal conductivity for Ba8Au16P30 are significantly lower than the typical values reported for solid crystalline compounds. We attribute such low thermal conductivity values to the presence of a large number of heavy atoms (Au) in the framework and the formation of multiple twinning interfaces and antiphase defects, which are effective scatterers of heat-carrying phonons.

BaAu2P4: Layered Zintl Polyphosphide with Infinite ∞1(P–) Chains

Barium gold polyphosphide BaAu2P4 was synthesized from elements and structurally characterized by single crystal X-ray diffraction. BaAu2P4 crystallizes in a new structure type, in the orthorhombic space group Fddd (No. 70) with a = 6.517(1) Å, b = 8.867(2) Å, c = 21.844(5) Å. The crystal structure of BaAu2P4 consists of Au–P layers separated by layers of Ba atoms. Each Au–P layer is composed of infinite ∞(1)(P–) chains of unique topology linked together by almost linearly coordinated Au atoms. According to Zintl–Klemm formalism, this compound is charge balanced assuming closed shell d10 configuration for Au: Ba2+(Au+)2(P–)4. Magnetic and solid state NMR measurements together with quantum-chemical calculations reveal diamagnetic and semiconducting behavior for the investigated polyphosphide, which is as expected for the charged balanced Zintl phase. Electron localization function and crystal orbital Hamilton population analyses reveal strong P–P and Au–P bonding and almost nonbonding Au–Au interactions in BaAu2P4.

Chemical Excision of Tetrahedral FeSe2 Chains from the Superconductor FeSe: Synthesis, Crystal Structure, and Magnetism of Fe3Se4(en)2

Fragments of the superconducting FeSe layer, FeSe2 tetrahedral chains, were stabilized in the crystal structure of a new mixed-valent compound Fe3Se4(en)2 (en = ethylenediamine) synthesized from elemental Fe and Se. The FeSe2 chains are separated from each other by means of Fe(en)2 linkers. Mössbauer spectroscopy and magnetometry reveal strong magnetic interactions within the FeSe2 chains which result in antiferromagnetic ordering below 170 K. According to DFT calculations, anisotropic transport and magnetic properties are expected for Fe3Se4(en)2. This compound offers a unique way to manipulate the properties of the Fe-Se infinite fragments by varying the topology and charge of the Fe-amino linkers.

Tellurium speciation, connectivity, and chemical order in As(x)Te(100-x) glasses: results from two-dimensional 125Te NMR spectroscopy.

The short-range structure, connectivity, and chemical order in As(x)Te(100-x) (25 ≤ x ≤ 65) glasses are studied using high-resolution two-dimensional projection magic-angle-turning (pjMAT) (125)Te nuclear magnetic resonance (NMR) spectroscopy. The (125)Te pjMAT NMR results indicate that the coordination of Te atoms obeys the 8-N coordination rule over the entire composition range. However, in strong contrast with the analogous glass-forming As-S and As-Se chalcogenides, significant violation of chemical order is observed in As-Te glasses over the entire composition range in the form of homopolar As-As (Te-Te) bonds, even in severely As (Te)-deficient glasses. The speciation of the Te coordination environments can be explained with the dissociation reaction model As2Te3 → 2As + 3Te(II), characterized by a dissociation constant that is independent of glass composition. These structural characteristics can be attributed to the high metallicity of Te and the strong energetic similarity between the Te-Te, Te-As, and As-As bonds, and they are consistent with the monotonic and often nearly linear variation of physical properties observed in telluride glasses as a function of the Te content.

Distorted Phosphorus and Copper Square-Planar Layers in LaCu1+xP2 and LaCu4P3: Synthesis, Crystal Structure, and Physical Properties

Two new lanthanum copper phosphides, LaCu(1+x)P(2) and LaCu(4)P(3), were synthesized from elements. Their crystal structures were determined by means of single-crystal X-ray diffraction. LaCu(1+x)P(2) crystallizes in a complex crystal structure, a derivative of the HfCuSi(2) structure type, in the space group Cmmm (No. 65) with unit cell parameters of a = 5.564(3) Å, b = 19.96(1) Å, c = 5.563(3) Å, and Z = 8. Its crystal structure features disordered Cu(2x)P(2) layers alternated with fully ordered PbO-like Cu(2)P(2) layers. The Cu-P layers are separated by La counter-cations. The Cu(2x)P(2) layers are composed of rectangular P(4) polyphosphide rings connected by partially occupied Cu atoms. Investigations of the electrical resistivity and Seebeck thermopower for LaCu(1+x)P(2) reveal metallic-type behavior with holes as the main charge carriers. LaCu(1+x)P(2) exhibits unexpectedly low thermal conductivity presumably because of disorder in the Cu(2x)P(2) layers. LaCu(4)P(3) crystallizes in a new structure type, in the tetragonal space group P4/nmm (No. 129) with unit cell parameters of a = 5.788(2) Å, c = 7.353(2) Å, and Z = 2. Its crystal structure features distorted square nets of Cu atoms within the Cu(4)P(3) slabs. Electron localization function analysis indicates that both P atoms in LaCu(4)P(3) have 1 + 4 coordination involving multicenter Cu-P bonding. According to the density of states and band structure, LaCu(4)P(3) is predicted to be a metallic conductor.

Intricate Short-Range Ordering and Strongly Anisotropic Transport Properties of Li1–xSn2+xAs2

A new ternary compound, Li(1-x)Sn(2+x)As2, 0.2 < x < 0.4, was synthesized via solid-state reaction of elements. The compound crystallizes in a layered structure in the R3̅m space group (No. 166) with Sn-As layers separated by layers of jointly occupied Li/Sn atoms. The Sn-As layers are comprised of Sn3As3 puckered hexagons in a chair conformation that share all edges. Li/Sn atoms in the interlayer space are surrounded by a regular As6 octahedron. Thorough investigation by synchrotron X-ray and neutron powder diffraction indicate no long-range Li/Sn ordering. In contrast, the local Li/Sn ordering was revealed by synergistic investigations via solid-state (6,7)Li NMR spectroscopy, HRTEM, STEM, and neutron and X-ray pair distribution function analyses. Due to their different chemical natures, Li and Sn atoms tend to segregate into Li-rich and Sn-rich regions, creating substantial inhomogeneity on the nanoscale. The inhomogeneous local structure has a high impact on the physical properties of the synthesized compounds: the local Li/Sn ordering and multiple nanoscale interfaces result in unexpectedly low thermal conductivity and highly anisotropic resistivity in Li(1-x)Sn(2+x)As2.

Synthesis, crystal structure, and thermoelectric properties of two new barium antimony selenides: Ba2Sb2Se5 and Ba6Sb7Se16.11

Two new antimony selenides, Ba2Sb2Se5 and Ba6Sb7Se16.11, were synthesized via high-temperature solid-state reactions and their structures were determined by single crystal X-ray diffraction. Both of the title compounds crystallize in two new structure types in orthorhombic space groups, Ba2Sb2Se5: Pbam (No. 55) Z = 4, a = 8.403(2) A; b = 27.567(5) A c = 4.6422(8) A; and Ba6Sb7Se16: Pnnm (No. 58) Z = 4, a = 12.469(2) A; b = 62.421(7) A; c = 4.6305(5) A. The crystal structures of both phases contain Sb–Se slabs and one-dimensional Ba–Se chains that are not commonly found in alkaline-earth antimony selenides. Ba2Sb2Se5 is an n-type semiconductor with a bandgap close to 1 eV as revealed by resistivity measurements and UV-visible spectroscopy. Due to the complex crystal structure and disorder in the Se sublattice, Ba2Sb2Se5 exhibits extremely low thermal conductivity (0.4 W m−1 K−1) from room temperature to 800 K.

BP: synthesis and properties of boron phosphide

Cubic boron phosphide, BP, is notorious for its difficult synthesis, thus preventing it from being a widely used material in spite of having numerous favorable technological properties. In the current work, three different methods of synthesis are developed and compared: from the high temperature reaction of elements, Sn flux assisted synthesis, and a solid state metathesis reaction. Structural and optical properties of the products synthesized from the three methods were thoroughly characterized. Solid state metathesis is shown to be the cleanest and most efficient method in terms of reaction temperature and time. Synthesis by Sn flux resulted in a novel Sn-doped BP compound. Undoped BP samples exhibit an optical bandgap of ~2.2 eV while Sn-doped BP exhibits a significantly smaller bandgap of 1.74 eV. All synthesized samples show high stability in concentrated hydrochloric acid, saturated sodium hydroxide solutions, and fresh aqua regia.

Synthesis, Crystal Structure, and Properties of La4Zn7P10 and La4Mg1.5Zn8.5P12

Two new zinc phosphides, La4Zn7P10 and La4Mg1.5Zn8.5P12, were synthesized via transport reactions, and their crystal structures were determined by single crystal X-ray diffraction. La4Zn7P10 and La4Mg1.5Zn8.5P12 are built from three-dimensional Zn-P and Zn-Mg-P anionic frameworks that encapsulate lanthanum atoms. The anionic framework of La4Zn7P10 is constructed from one-dimensional Zn4P6, Zn2P4, and ZnP4 chains. The Zn4P6 chains are also the main building units in La4Mg1.5Zn8.5P12. In La4Zn7P10, the displacement of a zinc atom from the origin of the unit cell causes the Zn4 position to split into two equivalent atomic sites, each with 50% occupancy. The splitting of the atomic position substantially modifies the electronic properties, as suggested by theoretical calculations. The necessity of splitting can be overcome by replacement of zinc with magnesium in La4Mg1.5Zn8.5P12. Investigation of the transport properties of a densified polycrystalline sample of La4Zn7P10 demonstrates that it is an n-type semiconductor with a small bandgap of ∼0.04 eV at 300 K. La4Zn7P10 also exhibits low thermal conductivity, 1.3 Wm-1 K-1 at 300 K, which mainly originates from the lattice thermal conductivity. La4Zn7P10 is stable in a sealed evacuated ampule up to 1123 K as revealed by differential scanning calorimetry.

Synthesis, crystal structure, and advanced NMR characterization of a low temperature polymorph of SiSe2

A low temperature polymorphic modification of silicon diselenide was synthesized via solid-state reaction and its crystal structure was determined by single crystal X-ray diffraction. In contrast to the well-characterized high-temperature form of SiSe2, where edge-sharing SiSe4 tetrahedra form one-dimensional chains, in the crystal structure of the low-temperature polymorph two-dimensional puckered layers are formed via edge- and corner-sharing SiSe4 tetrahedra. Such rearrangements have a significant impact on the electronic structure of SiSe2 as revealed by quantum chemical calculations and optical spectroscopy. The experimentally determined indirect bandgap for the low-temperature modification of SiSe2 is 2.00(5) eV. Multinuclear (77Se, 29Si) NMR spectroscopy combined with first principles calculations is applied to characterize the NMR chemical shift tensor parameters for the Si and Se sites in the crystal structure. These results will enable quantitative application of combined 29Si and 77Se NMR spectroscopy in future structural studies of Si–Se glasses, whose structures consist of both corner- and edge-sharing SiSe4/2 tetrahedra.