Estudo anatômico e palinológico de Antônia ovata Pohl (Loganiaceae)

Nesta comunicação o autor considera a anatomia do caule, pecíolo, lâmina foliar e madeira, além dos aspectos morfológicos externo e palinológico, de espécimes de Antonia ovata, ocorrentes na floresta da região do rio Jarí (Estado do Pará) e nos cerrados da Amazônia e do Brasil Central; nomeia os espécimes da mata como sendo uma variedade nova para a ciência: Antonia ovata Pohl var. excelsa Paula.In this paper the author studies extern morphological, palinological and anatomical aspects, aiming to put an end to the doubts in the taxonomic studies of the specimens of Antonia ovata Pohl (or aiming make clear the taxonomy of the specimens of Antonia ovata. Specimens of Antonia ovata from the woods of the region of Jarí river (Amazônia) are considered by the author as a new variety. With its description, the number of varieties of Antonia ovata rose to three: pilosa, ovata and excelsa (new variety). The extern morphological aspect is found among the individuals from three habitats: "cerrados" of Amazônia, Brasil Central and forest of the region Jarí river. The identification of the three varieties is based on the following characteristic. Presence or lack of hairs on the leaves and branches; microscopic structure of wood (see comparative table); height and diameter of the specimens; and finally the habitat. Pollen grains of these two varieties excelsa and ovata present polymorphism. The leaf of that species has structure of a higrophyllous plants. The stem is rich in mucilaginous cells; vascular bundles are bicollateral; the leafknot is bilacunar, and the trace is formed by two vascular bundles

Elusive β‑Zn₈Sb₇: A New Zinc Antimonide Thermoelectric

Zn₈Sb₇ was theoretically predicted to exhibit superior thermoelectric properties; however a crystalline phase with a similar composition was only stabilized in the form of nanoparticles. We report a new metastable compound, β-Zn₈Sb₇, which was synthesized in the form of bulk polycrystalline powder via high-temperature solid-state annealing followed by quenching. Single crystal X-ray diffraction indicates that β-Zn₈Sb₇ crystallizes in a new structure type (noncentrosymmetric orthorhombic space group <i>Pmn</i>2₁ (no. 31) with unit cell parameters of <i>a</i> = 15.029(1) Å, <i>b</i> = 7.7310(5) Å, <i>c</i> = 12.7431(9) Å, which is different from the nanoparticulate phase. According to differential scanning calorimetry, the β-Zn₈Sb₇ phase melts incongruently at 825(5) K. β-Zn₈Sb₇ is a <i>p</i>-type semiconductor with high Seebeck thermopower and low thermal conductivity stemming from the complex crystal structure. β-Zn₈Sb₇ exhibits a promising thermoelectric figure-of-merit, <i>zT</i>, of 0.33 at 400 K, which is comparable to the state-of-the-art thermoelectric materials based on binary zinc antimonides

Elusive β‑Zn₈Sb₇: A New Zinc Antimonide Thermoelectric

Zn₈Sb₇ was theoretically predicted to exhibit superior thermoelectric properties; however a crystalline phase with a similar composition was only stabilized in the form of nanoparticles. We report a new metastable compound, β-Zn₈Sb₇, which was synthesized in the form of bulk polycrystalline powder via high-temperature solid-state annealing followed by quenching. Single crystal X-ray diffraction indicates that β-Zn₈Sb₇ crystallizes in a new structure type (noncentrosymmetric orthorhombic space group <i>Pmn</i>2₁ (no. 31) with unit cell parameters of <i>a</i> = 15.029(1) Å, <i>b</i> = 7.7310(5) Å, <i>c</i> = 12.7431(9) Å, which is different from the nanoparticulate phase. According to differential scanning calorimetry, the β-Zn₈Sb₇ phase melts incongruently at 825(5) K. β-Zn₈Sb₇ is a <i>p</i>-type semiconductor with high Seebeck thermopower and low thermal conductivity stemming from the complex crystal structure. β-Zn₈Sb₇ exhibits a promising thermoelectric figure-of-merit, <i>zT</i>, of 0.33 at 400 K, which is comparable to the state-of-the-art thermoelectric materials based on binary zinc antimonides

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

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 <i>Pnma</i> (No. 62, <i>Z</i> = 4), Pearson symbol <i>oP</i>48. 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

Synthesis, Crystal Structure, and Properties of Three La–Zn–P Compounds with Different Dimensionalities of the Zn–P Framework

Two novel ternary compounds in a La–Zn–P system, La₂Zn₁₁P₉ and La₇Zn₂P₁₁, were synthesized via high-temperature transport reactions. The crystal structures for both compounds were established by means of single crystal X-ray diffraction. The complex three-dimensional (3D) crystal structure of metal-rich La₃Zn₂P₄ is composed of a Zn–P framework with large channels accommodating four atomic columns of La atoms. The isolated columns of La atoms alternating with Zn–P tetrahedral chains and disordered P₃ chains, resembling polyacene fragments, build up the crystal structure of phosphorus-rich La₇Zn₂P_11. The previously reported La₃Zn_2–<i>x</i>P₄ compound with intermediate phosphorus content has a two-dimensional (2D) structural motif composed of Zn₂P₂ and La₃P₂ layers. A structural dimensionality reduction from 3D La₂Zn₁₁P₉ to 2D La₃Zn_2‑<i>x</i>P₄ to 1D La₇Zn₂P₁₁ is due to both the flexibility of the Zn–P framework with ZnP₄ tetrahedra and ZnP₃ planar building units and the ability of phosphorus to form homonuclear bonds and polyatomic phosphorus chains. A polycrystalline sample La₃Zn_1.75P₄ was purified by a high-temperature solid-state method. The electron counting rules and computations predict the <i>n</i>-type metallic nature of La₃Zn_1.75P₄. The transport properties tests performed on a sintered pellet of La₃Zn_1.75P₄ confirm its metallic behavior with negative thermopower indicating that the major carriers are electrons. La₃Zn_1.75P₄ exhibits moderate thermal conductivity, 4.5 W m^–1 K^–1 at 300 K, where lattice thermal conductivity has the dominating contributions

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

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