Reversible Thermosalient Effect of <i>N</i>′‑2-Propylidene-4-hydroxybenzohydrazide Accompanied by an Immense Negative Compressibility: Structural and Theoretical Arguments Aiming toward the Elucidation of Jumping Phenomenon

The temperature-induced reversible phase transition of <i>N</i>′-2-propylidene-4-hydroxy­benzo­hydrazide from the polymorphic Form II to Form III, and <i>vice versa</i>, is accompanied by the dramatic change of the macroscopic dimensions of the crystal which resulted in the pronounced mechanical motion (jumping) during the phase transition. Prior to the phase transition, the extremely large uniaxial negative thermal expansion along one crystal axis (<i>b</i> axis<i>)</i> was observed, together with the positive thermal expansions along the other two crystal axes. Form III of <i>N</i>′-2-propylidene-4-hydroxy­benzo­hydrazide exhibits the thermal expansion α_<i>c</i> = 360 × 10^–6 K^–1, which is the largest value ever noticed in any organic or metal–organic crystal. From the structural point of view, a thermosalient effect is escorted by the springlike behavior of the <i>zig-zag</i> molecular assemblies along the <i>c</i> axis. First-principles electronic structure calculations show that negative thermal expansion arises from the elastic properties of the crystal which show uniaxial negative compressibilities, NLC. Form III exhibits the negative compressibility along the 001 direction β₃ = −28 TPa^–1, which is 1 order of magnitude larger than that of any organic compound and, in fact, is comparable to compressibilities of molecular frameworks showing the most pronounced NLC behavior. Elastic properties are also the reason for the reversibility of Form II to Form III transition in contrast to the irreversible Form I to Form II transition. Low energy springlike phonons are easily thermally excited and can assist in the overcoming of the energy barrier between the two phases that precedes thermosalient transition

Reversible Thermosalient Effect of <i>N</i>′‑2-Propylidene-4-hydroxybenzohydrazide Accompanied by an Immense Negative Compressibility: Structural and Theoretical Arguments Aiming toward the Elucidation of Jumping Phenomenon

The temperature-induced reversible phase transition of <i>N</i>′-2-propylidene-4-hydroxy­benzo­hydrazide from the polymorphic Form II to Form III, and <i>vice versa</i>, is accompanied by the dramatic change of the macroscopic dimensions of the crystal which resulted in the pronounced mechanical motion (jumping) during the phase transition. Prior to the phase transition, the extremely large uniaxial negative thermal expansion along one crystal axis (<i>b</i> axis<i>)</i> was observed, together with the positive thermal expansions along the other two crystal axes. Form III of <i>N</i>′-2-propylidene-4-hydroxy­benzo­hydrazide exhibits the thermal expansion α_<i>c</i> = 360 × 10^–6 K^–1, which is the largest value ever noticed in any organic or metal–organic crystal. From the structural point of view, a thermosalient effect is escorted by the springlike behavior of the <i>zig-zag</i> molecular assemblies along the <i>c</i> axis. First-principles electronic structure calculations show that negative thermal expansion arises from the elastic properties of the crystal which show uniaxial negative compressibilities, NLC. Form III exhibits the negative compressibility along the 001 direction β₃ = −28 TPa^–1, which is 1 order of magnitude larger than that of any organic compound and, in fact, is comparable to compressibilities of molecular frameworks showing the most pronounced NLC behavior. Elastic properties are also the reason for the reversibility of Form II to Form III transition in contrast to the irreversible Form I to Form II transition. Low energy springlike phonons are easily thermally excited and can assist in the overcoming of the energy barrier between the two phases that precedes thermosalient transition

A 3D Oxalate-Based Network as a Precursor for the CoMn₂O₄ Spinel: Synthesis and Structural and Magnetic Studies

A novel heterometallic oxalate-based compound of the formula {[Co­(bpy)₃]­[Mn₂(C₂O₄)₃]·H₂O}_<i>n</i> (<b>1</b>; bpy = 2,2′-bipyridine) was synthesized and characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction (XRD), and magnetization measurement. The molecular structure of <b>1</b> is made of a three-dimensional (3D) anionic network, [Mn₂(C₂O₄)₃]_<i>n</i>^2<i>n</i>–, and tris-chelated cations [Co­(bpy)₃]²⁺ occupying the vacancies of the framework. Splitting between the zero-field-cooled (ZFC) and field-cooled (FC) branches of susceptibility below the small peak at 13 K indicates magnetic ordering. Compound <b>1</b> was used as a single-source precursor for the formation of the mixed-metal oxide CoMn₂O₄. This conversion via thermal decomposition was explored by thermal analysis (TGA and DTA), IR spectroscopy, powder XRD, and magnetic susceptibility measurement. From refined structural parameters, it could be seen that the spinel obtained by the thermal treatment of <b>1</b> at 800 °C is characterized by the inversion parameter δ = 21%, and therefore the structural formula at room temperature can be written as ^tet[Co_0.79Mn_0.21]^oct[Co_0.105Mn_0.895]₂O₄. The temperature dependence of magnetization for CoMn₂O₄ points to at least three magnetic phases: the ferrimagnetic state is observed below 83 K, and up to 180 K blocking of the magnetic moments of nanocrystallites of 31 nm appears, transforming to paramagnetic-like behavior above 180 K. Microstructural characterization of the CoMn₂O₄ sample was carried out by means of XRD line-broadening analysis

Single-Step Preparation of the Mixed Ba^II–Nb^V Oxides from a Heteropolynuclear Oxalate Complex

A novel oxalate-based complex of the formula {Ba₂(H₂O)₅[NbO­(C₂O₄)₃]­HC₂O₄}·H₂O (<b>1</b>) was prepared from an aqueous solution containing the [NbO­(C₂O₄)₃]^3– and Ba²⁺ entities in the molar ratio 1:2, and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. The crystal packing of <b>1</b> reveals a complex three-dimensional (3D) network: the Nb polyhedron is connected to eight neighboring Ba polyhedra through the oxalate ligands and the oxo-oxygen group, whereas the Ba polyhedra share edges and vertices. The ability of compound <b>1</b> to act as a single-source precursor for the formation of bimetallic oxides was investigated by the thermal analysis (TGA and DSC) and X-ray powder diffraction. Thermal processing of <b>1</b> resulted in the formation of mixed-metal oxide phases, Ba₄Nb₂O₉ and Ba₅Nb₄O₁₅. Three stable polymorphs of Ba₄Nb₂O₉ were isolated: the known, hexagonal <i>α-</i> and orthorhombic γ-Ba₄Nb₂O₉, and another one, not previously reported, hexagonal δ-Ba₄Nb₂O₉ polymorph. The new, δ-Ba₄Nb₂O₉ polymorph has the 6H-perovskite structure (space group <i>P</i>6₃/<i>m</i>), in which the Nb₂O₉^8– face-sharing octahedral dimers are interconnected via corners to the regular BaO₆^10– octahedra. Formation of the mixed-metal oxides takes place at different temperatures: the Ba₅Nb₄O₁₅ oxide occurred at ∼700 °C, as the major crystalline oxide phase; by heating the sample up to 1135 °C, the α-Ba₄Nb₂O₉ form was obtained, whereas the heating at 1175 °C caused the crystallization of two polymorphs, γ-Ba₄Nb₂O₉ and δ-Ba₄Nb₂O₉. Special focus was set on the electrical properties of the prepared mixed Ba^II–Nb^V oxides obtained by this molecular pathway in a single-step preparation

Single-Step Preparation of the Mixed Ba^II–Nb^V Oxides from a Heteropolynuclear Oxalate Complex

A novel oxalate-based complex of the formula {Ba₂(H₂O)₅[NbO­(C₂O₄)₃]­HC₂O₄}·H₂O (<b>1</b>) was prepared from an aqueous solution containing the [NbO­(C₂O₄)₃]^3– and Ba²⁺ entities in the molar ratio 1:2, and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. The crystal packing of <b>1</b> reveals a complex three-dimensional (3D) network: the Nb polyhedron is connected to eight neighboring Ba polyhedra through the oxalate ligands and the oxo-oxygen group, whereas the Ba polyhedra share edges and vertices. The ability of compound <b>1</b> to act as a single-source precursor for the formation of bimetallic oxides was investigated by the thermal analysis (TGA and DSC) and X-ray powder diffraction. Thermal processing of <b>1</b> resulted in the formation of mixed-metal oxide phases, Ba₄Nb₂O₉ and Ba₅Nb₄O₁₅. Three stable polymorphs of Ba₄Nb₂O₉ were isolated: the known, hexagonal <i>α-</i> and orthorhombic γ-Ba₄Nb₂O₉, and another one, not previously reported, hexagonal δ-Ba₄Nb₂O₉ polymorph. The new, δ-Ba₄Nb₂O₉ polymorph has the 6H-perovskite structure (space group <i>P</i>6₃/<i>m</i>), in which the Nb₂O₉^8– face-sharing octahedral dimers are interconnected via corners to the regular BaO₆^10– octahedra. Formation of the mixed-metal oxides takes place at different temperatures: the Ba₅Nb₄O₁₅ oxide occurred at ∼700 °C, as the major crystalline oxide phase; by heating the sample up to 1135 °C, the α-Ba₄Nb₂O₉ form was obtained, whereas the heating at 1175 °C caused the crystallization of two polymorphs, γ-Ba₄Nb₂O₉ and δ-Ba₄Nb₂O₉. Special focus was set on the electrical properties of the prepared mixed Ba^II–Nb^V oxides obtained by this molecular pathway in a single-step preparation

Photoinduced Segregation Behavior in 2D Mixed Halide Perovskite: Effects of Light and Heat

Photoinduced halide segregation (PHS) is a process of critical importance for the performance of perovskite solar cells with mixed halide absorber layers. However, PHS is still not well understood, especially in the case of layered mixed halide perovskites (MHPs), which are less commonly studied compared to their 3D counterparts. Here, we investigated temperature- and light-induced PHS in 2D MHPs with a phenylpropylammonium (PPA) spacer. We found that 2D PPA-based MHPs exhibited complex segregation behavior dependence on temperature and illumination intensity with the suppression of segregation observed at high temperature (attributed to the highly exothermic nature of the process) as well as moderate illumination intensities, illustrating the importance of additional processes present in this particular material, which exhibits distinctly different behavior compared to 2D MHPs with other aromatic cations

Ba₄Ta₂O₉ Oxide Prepared from an Oxalate-Based Molecular PrecursorCharacterization and Properties

A novel heterometallic oxalate-based compound, {Ba₂(H₂O)₅[TaO­(C₂O₄)₃]­HC₂O₄}·H₂O (<b>1</b>), was obtained by using an (oxalato)­tantalate­(V) aqueous solution as a source of the complex anion and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. Compound <b>1</b> is a three-dimensional (3D) coordination polymer with the Ta atom connected to eight neighboring Ba atoms through the oxalate ligands and the oxo oxygen group. Thermal treatment of <b>1</b> up to 1200 °C leads to molecular precursor-to-material conversion that yields the mixed-metal γ-Ba₄Ta₂O₉ phase. This high-temperature γ-Ba₄Ta₂O₉ polymorph has the 6<i>H</i>-perovskite structure (space group <i>P</i>6₃/<i>m</i>), in which the Ta₂O₉ face-sharing octahedral dimers are interconnected via corners to the regular BaO₆ octahedra. To date, γ-Ba₄Ta₂O₉ has never been obtained at room temperature, because of the reduction of symmetry (<i>P</i>6₃/<i>m</i> → <i>P</i>2₁/<i>c</i>) that usually occurs during the cooling. Spectroscopic, optical, photocatalytic, and electrical properties of the obtained γ-Ba₄Ta₂O₉ phase were investigated. In addition to the experimental data, an absorption spectrum and band structure of the γ-Ba₄Ta₂O₉ polymorph were calculated using density functional theory

Ba₄Ta₂O₉ Oxide Prepared from an Oxalate-Based Molecular PrecursorCharacterization and Properties

A novel heterometallic oxalate-based compound, {Ba₂(H₂O)₅[TaO­(C₂O₄)₃]­HC₂O₄}·H₂O (<b>1</b>), was obtained by using an (oxalato)­tantalate­(V) aqueous solution as a source of the complex anion and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. Compound <b>1</b> is a three-dimensional (3D) coordination polymer with the Ta atom connected to eight neighboring Ba atoms through the oxalate ligands and the oxo oxygen group. Thermal treatment of <b>1</b> up to 1200 °C leads to molecular precursor-to-material conversion that yields the mixed-metal γ-Ba₄Ta₂O₉ phase. This high-temperature γ-Ba₄Ta₂O₉ polymorph has the 6<i>H</i>-perovskite structure (space group <i>P</i>6₃/<i>m</i>), in which the Ta₂O₉ face-sharing octahedral dimers are interconnected via corners to the regular BaO₆ octahedra. To date, γ-Ba₄Ta₂O₉ has never been obtained at room temperature, because of the reduction of symmetry (<i>P</i>6₃/<i>m</i> → <i>P</i>2₁/<i>c</i>) that usually occurs during the cooling. Spectroscopic, optical, photocatalytic, and electrical properties of the obtained γ-Ba₄Ta₂O₉ phase were investigated. In addition to the experimental data, an absorption spectrum and band structure of the γ-Ba₄Ta₂O₉ polymorph were calculated using density functional theory

Aqueous Sol–Gel Route toward Selected Quaternary Metal Oxides with Single and Double Perovskite-Type Structure Containing Tellurium

Highly crystalline SrFe_2/3Te_1/3O₃, Ba₃Fe₂TeO₉, and Ba₂NiTeO₆ have been synthesized by using a specially developed sol–gel route methodology, reducing the time needed employing solid-state routes and resulting in high reaction yield up to 75%. These materials have been studied by X-ray powder diffraction (XRPD), scanning and transmission electron microscopy, Raman spectroscopy, and dielectric and magnetic measurements. At room temperature, the crystal structure of SrFe_2/3Te_1/3O₃ is cubic, space group <i>Pm</i>3̅<i>m</i>, with <i>a</i> = 3.9373(2) Å, whereas Ba₃Fe₂TeO₉ crystallizes in the hexagonal crystal system, space group <i>P</i>6₃/<i>mmc</i>, <i>a</i> = 5.7691(4) Å, and <i>c</i> = 14.208(1) Å. The third studied perovskite Ba₂NiTeO₆ crystallizes in the trigonal <i>R</i>3̅<i>m</i> space group with <i>a</i> = 5.7974(4) Å and <i>c</i> = 28.599(2) Å. Based on structural characterization results, the obtained single and double perovskite crystallites are nearly in nanometer regime, ranging from 45 to 164 nm, building micrometer-sized particles with visible well-faceted hexagonal morphology. Magnetic measurements show the onset of ferrimagnetic ordering at relatively high temperature of 667 K for the SrFe_2/3Te_1/3O₃, whereas Ba₃Fe₂TeO₉ and Ba₂NiTeO₆ show antiferromagnetic ordering below 80 and 8.6 K, respectively. The measured room temperature dielectric constants are in the range between 15 and 77

Synthesis of Lead-Free Perovskite Films by Combinatorial Evaporation: Fast Processes for Screening Different Precursor Combinations

We demonstrate an evaporation-based combinatorial approach for fast screening of precursor combinations for the synthesis of novel perovskite materials. Nine material combinations can be explored simultaneously, which enabled us to synthesize nine different lead-free perovskite compounds. The structural properties (morphology, crystal structure) and optical properties (UV–vis absorption spectra, photoluminescence) of the prepared materials were investigated. Among these materials, several Sn-based and Pd-based perovskites exhibit strong absorption in the visible spectral range and thus may be of interest for photovoltaic applications. In addition, butyl ammonium tin iodide exhibits bright red emission, and it is of interest for potential light emitting applications