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

Novel ternary Zintl phosphide halides Ba3P5X (X = Cl, Br) with 1D helical phosphorus chains: synthesis, crystal and electronic structure

Black single crystals of two novel ternary phosphide halides, Ba3P5Cl and Ba3P5Br, were grown using molten metal Pb-flux high-temperature reactions. These compounds were structurally characterized with the aid of the single-crystal X-ray diffraction (SCXRD) method at 100(2) K. The SCXRD shows that both compounds are isostructural and adopt a new structure type (space group R3̄c, No. 167, Z = 6) with unit cell parameters a = 14.9481(16) Å, c = 7.3954(11) Å and a = 15.045(4) Å, c = 7.537(3) Å for Ba3P5Cl and Ba3P5Br, respectively. Cl- and Br- anions are octahedrally coordinated by Ba2+ cations, thus composing a face-sharing 1D infinite chain 1∞[XBa3]5+ running along the [001] direction. Moreover, the crystal structures feature peculiar one-dimensional disordered infinite helical chains of 1∞P-, composed of partially occupied phosphorous atoms, each being a superposition of three symmetrical copies of the ordered phosphorus chain, with continuity along the c-axis. Ba3P5X (X = Cl, Br) compounds are charge-balanced heteroanionic Zintl phases according to the charge-partitioning scheme (Ba2+)3[P-]5X-. The presumed semiconducting behavior of both compounds corroborates well with the results of the electronic structure calculations performed with the aid of the TB-LMTO-ASA code.

Type-1 Pentagonal Tiling Realized in 2D Penta-SrP2 Sheet

According to the systematic classification of pentagon-based two-dimensional (2D) materials [ Phys. Rep. 2022, 964, 1], only type-2 and type-4 out of the 15 pentagonal tiling patterns have been realized in 2D materials so far. Here, we propose the first stable pentagon-based 2D material characterized by the type-1 pentagonal tiling pattern named penta-SrP2. We find that penta-SrP2 is not only thermally and mechanically stable but also dynamically stable when the temperature is above 200 K derived from the calculations by taking both phonon renormalization and thermal expansion into consideration. Moreover, the penta-SrP2 sheet is semiconducting with an indirect band gap of 0.96 eV. These findings expand the family of pentagon-based 2D materials in morphology and provide a new perspective to explore the dynamical stability of high-temperature phases.

Metal-Rich Phosphides Obtained from the Lead Flux: Synthesis, Crystal, and Electronic Structure of Sr5Pt12P9 and BaPt3P2

Using a high-temperature ampoule technique and lead metal as a flux, we have grown single crystals and determined crystal structures from single-crystal X-ray diffraction data of two metal-rich phosphides, Sr₅Pt₁₂P₉ (P 2₁/m, a = 6.1472(3) Å, b = 25.1713(13) Å, c = 6.4635(3) Å, β = 99.604(2)°, Z = 2, R₁ = 0.0326, wR₂ = 0.0786) and BaPt₃P₂ (P 2₁2₁2₁, a = 6.3605(6) Å, b = 6.8541(7) Å, c = 11.3493(12) Å, Z = 4, R₁ = 0.0231, wR₂ = 0.0501). Both compounds belong to their own structure types and feature 3D networks of Pt and P atoms, with the channels occupied by alkaline earth metal cations. Density functional theory calculations reveal Sr₅Pt₁₂P₉ to be a metal, while BaPt₃P₂ is a narrow-gap semiconductor with a band gap of 0.24 eV. Bonding analysis shows that both compounds feature networks of prominent covalent localized Pt-P bonds, responsible for their structural stability, as well as additional weaker and, likely, less localized Pt-Pt interactions.

Metavalent bonding in chalcogenides: DFT-chemical pressure approach

Understanding the chemical bond nature has attracted considerable attention as it is crucial to analyze and comprehend the different physical and chemical properties of materials. This work is considered a complementary part of our previous work in studying the nature of different types of bonding interactions in a wide variety of molecules and materials using the DFT Chemical Pressure (CP) approach. Recently, a new type of chemical bond, the metavalent bond (MVB), has been defined. We show how the CP formalism can be used to analyze and study the establishment of MVB in two chalcogenides, GeSe and PbSe, in a similar fashion as the electron localization function (ELF) profiles. This is accomplished by analyzing the CP maps of these two chalcogenides at different pressures (up to 40 GPa for GeSe and 10 GPa for PbSe). The CP maps show distinctive features related to the MVB, providing insights into the existence of such chemical interaction in the crystal structure of the two compounds. Similar to ELF profiles, CP maps can visualize and track the strength of the MVB in GeSe and PbSe under pressure.

Emerging Yttrium Phosphides with Tetrahedron Phosphorus and Superconductivity under High Pressures

Metal phosphides have triggered growing interest for their exotic structures and striking properties. Hence, within advanced structure search and first-principle calculations, several unprecedented Y-P compounds (e. g., Y₃ P, Y₂ P, Y₃ P₂ , Y₂ P₃ , YP₂ , and YP₃ ) were identified under compression. Interestingly, as phosphorus content increases, P atoms exhibit diverse behaviors corresponding to standalone anion, dumbbell, zigzag chain, planar sheet, crossing chain-like network, buckled layer, three-dimensional framework, and wrinkled layer. Particularly, Fd-3m YP₂ can be viewed as assemblage of diamond-like Y structure and rare vertex-sharing tetrahedral P₄  units. Impressively, electron-phonon coupling (EPC) calculations elucidate that Pm-3m Y₃ P possesses the highest superconducting critical temperature T_c of 10.2 K among binary transition metal phosphides. Remarkably, the EPC of Pm-3m Y₃ P mainly arises from the contribution of low-frequency soft phonon modes, whereas mid-frequency phonon modes of Fd-3m YP₂ dominate. These results strengthen knowledge of metal phosphides and pave a way for seeking superconductive transition metal phosphides.

s-Block metal ions induce structural transformations between figure-eight and double trefoil knots

The presence or absence of s-block metal ions induces reversible structural transformation of molecular knots.

Ba2Si3P6: 1D Nonlinear Optical Material with Thermal Barrier Chains

A novel barium silicon phosphide was synthesized and characterized. Ba₂Si₃P₆ crystallizes in the noncentrosymmetric space group Pna2₁ (No. 33) and exhibits a unique bonding connectivity in the Si-P polyanion not found in other compounds. The crystal structure is composed of SiP₄ tetrahedra connected into one-dimensional double-tetrahedra chains through corner sharing, edge sharing, and covalent P-P bonds. Chains are surrounded by Ba cations to achieve an electron balance. The novel compound exhibits semiconducting properties with a calculated bandgap of 1.6 eV and experimental optical bandgap of 1.88 eV. The complex pseudo-one-dimensional structure manifests itself in the transport and optical properties of Ba₂Si₃P₆, demonstrating ultralow thermal conductivity (0.56 W m^-1 K^-1 at 300 K), promising second harmonic generation signal (0.9 × AgGaS₂), as well as high laser damage threshold (1.6 × AgGaS₂, 48.5 MW/cm²) when compared to the benchmark material AgGaS₂. Differential scanning calorimetry reveals that Ba₂Si₃P₆ melts congruently at 1373 K, suggesting that large single crystal growth may be possible.

Synthesis and Characterization of K and Eu Binary Phosphides

The synthesis, structural characterization, and optical properties of the binary Zintl phases of α-EuP₃, β-EuP₃, EuP₂, and α-K₄P₆ are reported in this study. These crystal structures demonstrate the versatility of P fragments with dimensionality varying from 0D (P₆ rings in α-K₄P₆) to 1D chains (EuP₂) to 2D layers (both EuP₃). EuP₂ is isostructural to previously reported SrP₂ and BaP₂ compounds. The thermal stabilities of the EuP₂ and both EuP₃ phases were determined using differential scanning calorimetry (DSC), with melting temperatures of 1086 K for the diphosphide and 1143 K for the triphosphides. Diffuse reflectance spectroscopy indicated that EuP₂ is an indirect semiconductor with a direct bandgap of 1.12(5) eV and a smaller indirect one, less than 1 eV. Both EuP₃ compounds had bandgaps smaller than 1 eV.

Chemical Pressure Maps of Molecules and Materials: Merging the Visual and Physical in Bonding Analysis

The characterization of bonding interactions in molecules and materials is one of the major applications of quantum mechanical calculations. Numerous schemes have been devised to identify and visualize chemical bonds, including the electron localization function, quantum theory of atoms in molecules, and natural bond orbital analysis, whereas the energetics of bond formation are generally analyzed in qualitative terms through various forms of energy partitioning schemes. In this Article, we illustrate how the chemical pressure (CP) approach recently developed for analyzing atomic size effects in solid state compounds provides a basis for merging these two approaches, in which bonds are revealed through the forces of attraction and repulsion acting between the atoms. Using a series of model systems that include simple molecules (H₂, CO₂, and S₈), extended structures (graphene and diamond), and systems exhibiting intermolecular interactions (ice and graphite), as well as simple representatives of metallic and ionic bonding (Na and NaH, respectively), we show how CP maps can differentiate a range of bonding phenomena. The approach also allows for the partitioning of the potential and kinetic contributions to the interatomic interactions, yielding schemes that capture the physical model for the chemical bond offered by Ruedenberg and co-workers.

Synthesis, Crystal Structure, and Magnetic Properties of R2Mg3SiPn6 (R = La, Ce; Pn = P, As)

Four new quaternary pnictides, R₂Mg₃SiPn₆ (R = La, Ce; Pn = P, As), were synthesized via high-temperature solid-state reactions and gas-phase transport reactions with iodine. Their crystal structures were determined by single crystal X-ray diffraction. All four compounds are isostructural and crystallize in a new structure type in the orthorhombic space group Pnma (No. 62, Z = 4), Pearson symbol oP48. The crystal structures of R₂Mg₃SiPn₆ are composed of two-dimensional puckered MgP₃ layers, which are connected in a three-dimensional framework by P-P dimers and MgSiP₄ double-tetrahedral chains. Rare-earth cations are encapsulated inside the channels of the framework running along [010]. Quantum-chemical calculations predict that La₂Mg₃SiP₆ is an indirect narrow bandgap semiconductor. The Mg-P bonding in MgP₄ tetrahedra and MgP₆ octahedra was analyzed by means of crystal orbital Hamilton population (COHP) analysis. Magnetic characterization of Ce-containing compounds confirmed the trivalent nature of cerium atoms and revealed complex ferrimagnetic ordering at low temperatures.

Controlling superstructural ordering in the clathrate-I Ba8M16P30 (M = Cu, Zn) through the formation of metal-metal bonds

Order-disorder-order phase transitions in the clathrate-I Ba8Cu16P30 were induced and controlled by aliovalent substitutions of Zn into the framework. Unaltered Ba8Cu16P30 crystallizes in an ordered orthorhombic (Pbcn) clathrate-I superstructure that maintains complete segregation of metal and phosphorus atoms over 23 different crystallographic positions in the clathrate framework. The driving force for the formation of this Pbcn superstructure is the avoidance of Cu-Cu bonds. This superstructure is preserved upon aliovalent substitution of Zn for Cu in Ba8Cu16-x Zn x P30 with 0 < x < 1.6 (10% Zn/Mtotal), but vanishes at greater substitution concentrations. Higher Zn concentrations (up to 35% Zn/Mtotal) resulted in the additional substitution of Zn for P in Ba8M16+y P30-y (M = Cu, Zn) with 0 ≤ y ≤ 1. This causes the formation of Cu-Zn bonds in the framework, leading to a collapse of the orthorhombic superstructure into the more common cubic subcell of clathrate-I (Pm3n). In the resulting cubic phases, each clathrate framework position is jointly occupied by three different elements: Cu, Zn, and P. Detailed structural characterization of the Ba-Cu-Zn-P clathrates-I via single crystal X-ray diffraction, joint synchrotron X-ray and neutron powder diffractions, pair distribution function analysis, electron diffraction and high-resolution electron microscopy, along with elemental analysis, indicates that local ordering is present in the cubic clathrate framework, suggesting the evolution of Cu-Zn bonds. For the compounds with the highest Zn content, a disorder-order transformation is detected due to the formation of another superstructure with trigonal symmetry and Cu-Zn bonds in the clathrate-I framework. It is shown that small changes in the composition, synthesis, and crystal structure have significant impacts on the structural and transport properties of Zn-substituted Ba8Cu16P30.

Synthesis, crystal and electronic structures, and physical properties of a new quaternary phosphide Ba4Mg2+δCu12−δP10(0 < δ < 2)

A new quaternary phosphide composed of a Cu–Mg–P framework hosting Ba and Mg cations is synthesized and characterized.

Nanoscale stabilization of zintl compounds: 1D ionic Li-P double helix confined inside a carbon nanotube

One-dimensional (1D) ionic nanowires are extremely rare materials due to the difficulty in stabilizing 1D chains of ions under ambient conditions. We demonstrate here a theoretical prediction of a novel hybrid material, a nanotube encapsulated 1D ionic lithium monophosphide (LiP) chain, featuring a unique double-helix structure, which is very unusual in inorganic chemistry. This nanocomposite has been investigated with density functional theory, including molecular dynamics simulations and electronic structure calculations. We find that the formation of the LiP double-helical nanowire is facilitated by strong interactions between LiP and CNTs resulting in a charge transfer. This work suggests that nanostructured confinement may be used to stabilize other polyphosphide 1D chains, thus opening new ways to study the chemistry of zintl compounds at the nanoscale.