Synthesis, Crystal Structure, and Properties of La₄Zn₇P₁₀ and La₄Mg_1.5Zn_8.5P₁₂

Two new zinc phosphides, La₄Zn₇P₁₀ and La₄Mg_1.5Zn_8.5P₁₂, were synthesized via transport reactions, and their crystal structures were determined by single crystal X-ray diffraction. La₄Zn₇P₁₀ and La₄Mg_1.5Zn_8.5P₁₂ 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 La₄Mg_1.5Zn_8.5P₁₂. 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 La₄Mg_1.5Zn_8.5P₁₂. Investigation of the transport properties of a densified polycrystalline sample of La₄Zn₇P₁₀ demonstrates that it is an <i>n</i>-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

Distorted Phosphorus and Copper Square-Planar Layers in LaCu_1+<i>x</i>P₂ and LaCu₄P₃: Synthesis, Crystal Structure, and Physical Properties

Two new lanthanum copper phosphides, LaCu_1+<i>x</i>P₂ and LaCu₄P₃, were synthesized from elements. Their crystal structures were determined by means of single-crystal X-ray diffraction. LaCu_1+<i>x</i>P₂ crystallizes in a complex crystal structure, a derivative of the HfCuSi₂ structure type, in the space group <i>Cmmm</i> (No. 65) with unit cell parameters of <i>a</i> = 5.564(3) Å, <i>b</i> = 19.96(1) Å, <i>c</i> = 5.563(3) Å, and <i>Z</i> = 8. Its crystal structure features disordered Cu_2<i>x</i>P₂ layers alternated with fully ordered PbO-like Cu₂P₂ layers. The Cu–P layers are separated by La counter-cations. The Cu_2<i>x</i>P₂ layers are composed of rectangular P₄ polyphosphide rings connected by partially occupied Cu atoms. Investigations of the electrical resistivity and Seebeck thermopower for LaCu_1+<i>x</i>P₂ reveal metallic-type behavior with holes as the main charge carriers. LaCu_1+<i>x</i>P₂ exhibits unexpectedly low thermal conductivity presumably because of disorder in the Cu_2<i>x</i>P₂ layers. LaCu₄P₃ crystallizes in a new structure type, in the tetragonal space group <i>P</i>4/<i>nmm</i> (No. 129) with unit cell parameters of <i>a</i> = 5.788(2) Å, <i>c</i> = 7.353(2) Å, and <i>Z</i> = 2. Its crystal structure features distorted square nets of Cu atoms within the Cu₄P₃ slabs. Electron localization function analysis indicates that both P atoms in LaCu₄P₃ have 1 + 4 coordination involving multicenter Cu–P bonding. According to the density of states and band structure, LaCu₄P₃ is predicted to be a metallic conductor

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

A Solution for Solution-Produced β‑FeSe: Elucidating and Overcoming Factors that Prevent Superconductivity

A new low-temperature solvothermal synthesis of superconducting β-FeSe has been developed using elemental iron and selenium as starting materials. We have shown that syntheses performed in aerobic conditions resulted in the formation of nonsuperconducting antiferromagnetic β-FeSe, whereas syntheses performed in ultra-dry and oxygen-free conditions produced superconducting β-FeSe. Detailed characterization of both types of samples with magnetometry, resistivity, Mössbauer spectroscopy, synchrotron X-ray and neutron powder diffraction, and pair-distribution function analysis uncovered factors that trigger the loss of superconductivity in β-FeSe. Vacancies in the iron sublattice and the incorporation of disordered oxygen-containing species are typical for nonsuperconducting antiferromagnetic samples, whereas a pristine structure is required to preserve superconductivity. Exposure to ambient atmosphere resulted in the conversion of superconducting samples to antiferromagnetic ones. This synthetic method creates new possibilities for soft chemistry approaches to the synthesis and modification of iron-based superconductors

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

GeAs: Highly Anisotropic van der Waals Thermoelectric Material

GeAs and Sn-doped GeAs were synthesized from elements. Both crystallize in a layered crystal structure in the <i>C</i>2/<i>m</i> space group (No. 12) in the GaTe structure type. The crystal structure consists of As-terminated layers separated by van der Waals gaps. ¹¹⁹Sn Mössbauer spectroscopy reveals that in the doped compound, Sn atoms are situated in a symmetric and homogeneous environment, most probably in the form of Sn₂ dumbbells. The anisotropic crystal structure of GeAs leads to highly anisotropic transport properties. High electrical and thermal conductivities were determined along the crystallographic layers. For the perpendicular direction across the layers, a sharp drop of more than an order of magnitude was observed for the transport properties of the GeAs single crystal. As a result, an order of magnitude difference in the figure of merit, <i>ZT</i>, was achieved. High-temperature thermoelectric characterization of the Sn-doped compound reveals a remarkable <i>ZT</i> with a maximum of 0.35 at 660 K

Enclathration of X@La₄ Tetrahedra in Channels of Zn–P Frameworks in La₃Zn₄P₆X (X = Cl, Br)

Two new quaternary lanthanum zinc phosphide-halides were synthesized via high-temperature solid-state reactions. Their complex crystal structures were determined by a combination of X-ray diffraction and advanced solid-state ³¹P NMR spectroscopy. La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8 share a common structural feature: a polyanionic Zn–P framework with large channels hosting complex one-dimensional cations. The cations are built from X@La₄ tetrahedral chains with X = Cl (La₃Zn₄P₆Cl) or Br_0.8P_0.2 (La₃Zn₄P_6.6Br_0.8). The X@La₄ tetrahedra share two vertices forming one-dimensional chains. To accommodate larger bromine-containing cations the Zn–P framework is rearranged by breaking and forming several Zn–P and P–P bonds. This results in the formation of a unique [P₃]^3– cycle, which is isoelectronic to cyclopropane. Analysis of the electron localization and orbital overlaps confirmed the presence of different chemical bonding in the Zn–P networks in the Cl- and Br-containing compounds. La₃Zn₄P₆Cl was predicted to be a narrow bandgap semiconductor, while the formation of the [P₃]^3– units in the structure of La₃Zn₄P_6.6Br_0.8 was shown to lead to a narrowing of the bandgap. Characterization of the transport properties confirmed both La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8 to be narrow bandgap semiconductors with electrons as dominating charge carriers at low temperatures. La₃Zn₄P₆Cl exhibits a <i>n-p</i> transition around 250 K. Due to the complex crystal structure and segregation of the areas of different chemical bonding, both title compounds exhibit ultralow thermal conductivities of 0.7 Wm^–1 K^–1 and 1.5 Wm^–1 K^–1 at 400 K for La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8, respectively

Enclathration of X@La₄ Tetrahedra in Channels of Zn–P Frameworks in La₃Zn₄P₆X (X = Cl, Br)

Two new quaternary lanthanum zinc phosphide-halides were synthesized via high-temperature solid-state reactions. Their complex crystal structures were determined by a combination of X-ray diffraction and advanced solid-state ³¹P NMR spectroscopy. La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8 share a common structural feature: a polyanionic Zn–P framework with large channels hosting complex one-dimensional cations. The cations are built from X@La₄ tetrahedral chains with X = Cl (La₃Zn₄P₆Cl) or Br_0.8P_0.2 (La₃Zn₄P_6.6Br_0.8). The X@La₄ tetrahedra share two vertices forming one-dimensional chains. To accommodate larger bromine-containing cations the Zn–P framework is rearranged by breaking and forming several Zn–P and P–P bonds. This results in the formation of a unique [P₃]^3– cycle, which is isoelectronic to cyclopropane. Analysis of the electron localization and orbital overlaps confirmed the presence of different chemical bonding in the Zn–P networks in the Cl- and Br-containing compounds. La₃Zn₄P₆Cl was predicted to be a narrow bandgap semiconductor, while the formation of the [P₃]^3– units in the structure of La₃Zn₄P_6.6Br_0.8 was shown to lead to a narrowing of the bandgap. Characterization of the transport properties confirmed both La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8 to be narrow bandgap semiconductors with electrons as dominating charge carriers at low temperatures. La₃Zn₄P₆Cl exhibits a <i>n-p</i> transition around 250 K. Due to the complex crystal structure and segregation of the areas of different chemical bonding, both title compounds exhibit ultralow thermal conductivities of 0.7 Wm^–1 K^–1 and 1.5 Wm^–1 K^–1 at 400 K for La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8, respectively

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

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