Twisted Kelvin Cells and Truncated Octahedral Cages in the Crystal Structures of Unconventional Clathrates, AM₂P₄ (A = Sr, Ba; M = Cu, Ni)

A new strontium nickel polyphosphide, SrNi₂P₄, was synthesized from elements and structurally characterized by single-crystal X-ray diffraction. It crystallizes in the orthorhombic space group <i>Fddd</i> (No. 70), with <i>Z</i> = 8. The crystal structure is that of a clathrate type, composed of Ni₈P₁₆, 14-faced polyhedral cages that encapsulate Sr atoms. Together with the previously reported but unrecognized clathrate VII, BaNi₂P₄, and another previously reported clathrate, BaCu₂P₄, which is isostructural to SrNi₂P₄, a family of transition metal–phosphorus clathrates is represented. The crystal structures of each of the discussed transition metal-based clathrates are composed of unique polyhedra containing square faces. These structural fragments were predicted to be unstable for the conventional clathrates based on Si, Ge, and Sn. In this work, we report the crystal and electronic structures, chemical bonding, as well as the thermoelectric properties of this novel class of unconventional clathrates

Twisted Kelvin Cells and Truncated Octahedral Cages in the Crystal Structures of Unconventional Clathrates, AM₂P₄ (A = Sr, Ba; M = Cu, Ni)

A new strontium nickel polyphosphide, SrNi₂P₄, was synthesized from elements and structurally characterized by single-crystal X-ray diffraction. It crystallizes in the orthorhombic space group <i>Fddd</i> (No. 70), with <i>Z</i> = 8. The crystal structure is that of a clathrate type, composed of Ni₈P₁₆, 14-faced polyhedral cages that encapsulate Sr atoms. Together with the previously reported but unrecognized clathrate VII, BaNi₂P₄, and another previously reported clathrate, BaCu₂P₄, which is isostructural to SrNi₂P₄, a family of transition metal–phosphorus clathrates is represented. The crystal structures of each of the discussed transition metal-based clathrates are composed of unique polyhedra containing square faces. These structural fragments were predicted to be unstable for the conventional clathrates based on Si, Ge, and Sn. In this work, we report the crystal and electronic structures, chemical bonding, as well as the thermoelectric properties of this novel class of unconventional clathrates

BaAu₂P₄: Layered Zintl Polyphosphide with Infinite _∞¹(<i>P</i>^–) 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 <i>Fddd</i> (No. 70) with <i>a</i> = 6.517(1) Å, <i>b</i> = 8.867(2) Å, <i>c</i> = 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(<i>P</i>^–) 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

BaAu₂P₄: Layered Zintl Polyphosphide with Infinite _∞¹(<i>P</i>^–) 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 <i>Fddd</i> (No. 70) with <i>a</i> = 6.517(1) Å, <i>b</i> = 8.867(2) Å, <i>c</i> = 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(<i>P</i>^–) 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

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 <i>I</i>4̅3<i>d</i> (no. 220), <i>Z</i> = 6, <i>a</i> = 9.7520(7) Å. In the Ba₃P₂ crystal structure, isolated P^3– anions form distorted octahedra around the Ba²⁺ cations. β-Ba₅P₄ crystallizes in the Eu₅As₄ structure type with the orthorhombic space group <i>Cmce</i> (no. 64), <i>Z</i> = 4, <i>a</i> = 16.521(2) Å, <i>b</i> = 8.3422(9) Å, <i>c</i> = 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 <i>P</i>2₁<i>/c</i> (no. 14), <i>Z</i> = 6, <i>a</i> = 6.120(2)/6.368(1) Å, <i>b</i> = 11.818(3)/12.133(2) Å, <i>c</i> = 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<i>P</i>^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

Chemical Bonding and Transport Properties in Clathrates‑I with Cu–Zn–P Frameworks

Quaternary clathrate-I phases with an overall composition of Ba₈<i>M</i>_16+yP_30‑y (M = Cu,Zn) exhibit complex structural chemistry. Characterization of the electronic structures and chemical bonding using quantum-chemical calculations and ³¹P solid state NMR spectroscopy demonstrated that the Cu–Zn–P framework is flexible and able to accommodate up to six Zn atoms per formula unit via bonding rearrangements, such as partial Zn/P substitution and the formation of Cu–Zn bonds. Such perturbations of the framework’s bonding affect the thermal and charge transport properties. The overall thermoelectric figure-of-merit, <i>ZT</i>, of Ba₈Cu₁₄Zn₂P₃₀ is 0.62 at 800 K, which is 9 times higher than the thermoelectric performance of the ternary parent phase Ba₈Cu₁₆P₃₀. Through a combination of inelastic neutron scattering and single crystal X-ray diffraction experiments at 10 K, low-energy rattling of the Ba guest atoms inside the large tetrakaidecahedral cages are shown to be the reason for the low thermal conductivities observed for the studied clathrates

Chemical Bonding and Transport Properties in Clathrates‑I with Cu–Zn–P Frameworks

Quaternary clathrate-I phases with an overall composition of Ba₈<i>M</i>_16+yP_30‑y (M = Cu,Zn) exhibit complex structural chemistry. Characterization of the electronic structures and chemical bonding using quantum-chemical calculations and ³¹P solid state NMR spectroscopy demonstrated that the Cu–Zn–P framework is flexible and able to accommodate up to six Zn atoms per formula unit via bonding rearrangements, such as partial Zn/P substitution and the formation of Cu–Zn bonds. Such perturbations of the framework’s bonding affect the thermal and charge transport properties. The overall thermoelectric figure-of-merit, <i>ZT</i>, of Ba₈Cu₁₄Zn₂P₃₀ is 0.62 at 800 K, which is 9 times higher than the thermoelectric performance of the ternary parent phase Ba₈Cu₁₆P₃₀. Through a combination of inelastic neutron scattering and single crystal X-ray diffraction experiments at 10 K, low-energy rattling of the Ba guest atoms inside the large tetrakaidecahedral cages are shown to be the reason for the low thermal conductivities observed for the studied clathrates

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 ³¹P 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