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.

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.

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The "Gold Standard" for Lattice Thermal Conductivity.-The title compound is prepared by solid state reaction of stoichiometric amounts of the elements (glassy carbon crucible in evacuated silica ampules, 1073 K, 12 d). Ba8Au16P30 crystallizes with an orthorhombic superstructure of clathrate-I type featuring a complete separation of Au and P atoms over different crystallographic positions (space group Pbcn, Z = 4, single crystal and high-resolution powder XRD). Ba is trapped inside the large polyhedral cages of the Au-P tetrahedral framework. NMR and resistivity measurements indicate paramagnetic metallic properties with a room-temperature resistivity of 1.7 mΩ cm. Ba8Au16P30 exhibits a low total thermal conductivity of 0.62 Wm -1 K -1 and a low lattice thermal conductivity of 0.18 Wm -1 K -1 at room temperature. The low thermal conductivity values are attributed 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 scattering centers of heat-carrying phonons. -(FULMER, J.; LEBEDEV, O. I.; RODDATIS, V. V.; KASEMAN, D. C.; SEN, S.; DOLYNIUK, J.-A.; LEE, K.; OLENEV, A. V.; KOVNIER*, K.; J. Am. Chem. Soc. 135 (2013) 33, 12313-12323, http://dx.doi.org/10.1021/ja4052679 ; Dep. Chem., Univ. Calif., Davis, CA 95616, USA; Eng.) -J. Schramke 51-003

The title compound is prepared by solid state reaction of the elements (glassy‐carbon crucible, 1173 K, 2 x 140 h).