Juli-Anna Dolyniuk

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.

mP‐BaP3: A New Phase from an Old Binary System

A polyphosphide, mP-BaP3, with a unique two-dimensional phosphorus layer has been discovered and characterized. It crystallizes in the monoclinic space group P2₁/c with unit-cell parameters a=6.486(1), b=7.710(1), c=8.172(2) Å; β=104.72(3)°; Z=4. Its phosphorus polyanion can be derived from the strong elongation of 2/3 of the P-P bonds present in the layers of black phosphorus. The unit-cell volume of the mP-BaP3 phase is 1.4% larger than the volume of another polymorph, mS-BaP3, reported more than 40 years ago. The latter phase features the presence of one-dimensional phosphorus chains separated by Ba atoms. The differences in the structures of the phosphorus fragments in both polymorphs of barium triphosphide result in large differences in both the thermal stability of these materials as well as in their properties as evidenced by DSC, (31)P solid-state MAS NMR, UV/Vis, and surface photovoltage spectroscopies, alongside quantum-chemical calculations.

Breaking the Tetra-Coordinated Framework Rule: New Clathrate Ba8M24P28+δ (M=Cu/Zn)

A new clathrate type has been discovered in the Ba/Cu/Zn/P system. The crystal structure of the Ba8 M24 P28+δ (M=Cu/Zn) clathrate is composed of the pentagonal dodecahedra common to clathrates along with a unique 22-vertex polyhedron with two hexagonal faces capped by additional partially occupied phosphorus sites. This is the first example of a clathrate compound where the framework atoms are not in tetrahedral or trigonal-pyramidal coordination. In Ba8 M24 P28+δ a majority of the framework atoms are five- and six-coordinated, a feature more common to electron-rich intermetallics. The crystal structure of this new clathrate was determined by a combination of X-ray and neutron diffraction and was confirmed with solid-state 31 P NMR spectroscopy. Based on chemical bonding analysis, the driving force for the formation of this new clathrate is the excess of electrons generated by a high concentration of Zn atoms in the framework. The rattling of guest atoms in the large cages results in a very low thermal conductivity, a unique feature of the clathrate family of compounds.

Ba and Sr Binary Phosphides: Synthesis, Crystal Structures, and Bonding Analysis

Synthesis, crystal structures, and chemical bonding are reported for four binary phosphides with different degrees of phosphorus oligomerization, ranging from isolated P atoms to infinite phosphorus chains. Ba3P2 = Ba4P(2.67)□(0.33) (□ = vacancy) crystallizes in the anti-Th3P4 structure type with the cubic space group I4̅3d (no. 220), Z = 6, a = 9.7520(7) Å. In the Ba3P2 crystal structure, isolated P(3-) anions form distorted octahedra around the Ba(2+) cations. β-Ba5P4 crystallizes in the Eu5As4 structure type with the orthorhombic space group Cmce (no. 64), Z = 4, a = 16.521(2) Å, b = 8.3422(9) Å, c = 8.4216(9) Å. In the crystal structure of β-Ba5P4, one-half of the phosphorus atoms are condensed into P2(4-) dumbbells. SrP2 and BaP2 are isostructural and crystallize in the monoclinic space group P2₁/c (no. 14), Z = 6, a = 6.120(2)/6.368(1) Å, b = 11.818(3)/12.133(2) Å, c = 7.441(2)/7.687(2) Å, β = 126.681(4)/126.766(2)° for SrP2/BaP2. In the crystal structures of SrP2 and BaP2, all phosphorus atoms are condensed into ∞(1)P(1-) cis-trans helical chains. Electronic structure calculations, chemical bonding analysis via the recently developed solid-state adaptive natural density partitioning (SSAdNDP) method, and UV-vis spectroscopy reveal that SrP2 and BaP2 are electron-balanced semiconductors.