Stoichiometry modulates the optoelectronic functionality of zinc phosphide (Zn3−xP2+x)

Altres ajuts: ICN2 is funded by the CERCA Programme/Generalitat de Catalunya.Predictive synthesis-structure-property relationships are at the core of materials design for novel applications. In this regard, correlations between the compositional stoichiometry variations and functional properties are essential for enhancing the performance of devices based on these materials. In this work, we investigate the effect of stoichiometry variations and defects on the structural and optoelectronic properties of monocrystalline zinc phosphide (ZnP), a promising compound for photovoltaic applications. We use experimental methods, such as electron and X-ray diffraction and Raman spectroscopy, along with density functional theory calculations, to showcase the favorable creation of P interstitial defects over Zn vacancies in P-rich and Zn-poor compositional regions. Photoluminescence and absorption measurements show that these defects create additional energy levels at about 180 meV above the valence band. Furthermore, they lead to the narrowing of the bandgap, due to the creation of band tails in the region of around 10-20 meV above the valence and below the conduction band. The ability of zinc phosphide to form off-stoichiometric compounds provides a new promising opportunity for tunable functionality that benefits applications. In that regard, this study is crucial for the further development of zinc phosphide and its application in optoelectronic and photovoltaic devices, and should pave the way for defect engineering in this kind of material

Phonons in Copper Diphosphide (CuP₂):Raman Spectroscopy and Lattice Dynamics Calculations

Copper diphosphide (CuP2) is an emerging binary semiconductor with promising properties for energy conversion and storage applications. While functionality and possible applications of CuP2 have been studied, there is a curious gap in the investigation of its vibrational properties. In this work, we provide a reference Raman spectrum of CuP2, with a complete analysis of all Raman active modes from both experimental and theoretical perspectives. Raman measurements have been performed on polycrystalline CuP2 thin films with close to stoichiometric composition. Detailed deconvolution of the Raman spectrum with Lorentzian curves has allowed identification of all theoretically predicted Raman active modes (9Ag and 9Bg), including their positions and symmetry assignment. Furthermore, calculations of the phonon density of states (PDOS), as well as the phonon dispersions, provide a microscopic understanding of the experimentally observed phonon lines, in addition to the assignment to the specific lattice eigenmodes. We further provide the theoretically predicted positions of the infrared (IR) active modes, along with the simulated IR spectrum from density functional theory (DFT). Overall good agreement is found between the experimental and DFT-calculated Raman spectra of CuP2, providing a reference platform for future investigations on this material

AAg₂(M′_1/3M_2/3)[VO₄]₂: Synthesis, Magnetic Properties, and Lattice Dynamics of Honeycomb-Type Lattices

The magnetic honeycomb lattice series of compounds, AAg₂<i>(</i>M′_1/3M_2/3)­[VO₄]₂ with A = Ba²⁺, Sr²⁺, M′ = Mg²⁺, Zn²⁺, and M = Mn²⁺, Co²⁺, and Ni²⁺, have been synthesized and their physical properties are reported. This series of compounds contains the M′ and M cations in a 1:2 ratio on a single crystallographic site. In an ordered arrangement, this could generate a magnetic honeycomb-type lattice. Presented X-ray diffraction data, spectroscopic measurements of lattice dynamics, along with ab initio calculations, magnetic, and specific heat data for these compounds clearly point toward the formation of magnetic honeycomb-type lattices

Narrow Gap Semiconducting Germanium Allotrope from the Oxidation of a Layered Zintl Phase in Ionic Liquids

A metastable germanium allotrope, Ge­(oP32), was synthesized as polycrystalline powders and single crystals from the mild-oxidation/delithiation of Li₇Ge₁₂ in ionic liquids. Its crystal structure, from single crystal X-ray diffraction (<i>Pbcm</i>, <i>a</i> = 8.1527(4) Å, <i>b</i> = 11.7572(5) Å, <i>c</i> = 7.7617(4) Å), features a complex covalent network of 4-bonded Ge, resulting from a well-ordered topotactic oxidative condensation of [Ge₁₂]^7– layers. It is a diamagnetic semiconductor (<i>E</i>_g = 0.33 eV), and transforms exothermically and irreversibly to α-Ge at 363 °C. This demonstrates the potential of ionic liquids as reactive media in the mild oxidation of Zintl phases to new highly crystallized modifications of elements and simple compounds

Narrow Gap Semiconducting Germanium Allotrope from the Oxidation of a Layered Zintl Phase in Ionic Liquids

A metastable germanium allotrope, Ge­(oP32), was synthesized as polycrystalline powders and single crystals from the mild-oxidation/delithiation of Li₇Ge₁₂ in ionic liquids. Its crystal structure, from single crystal X-ray diffraction (<i>Pbcm</i>, <i>a</i> = 8.1527(4) Å, <i>b</i> = 11.7572(5) Å, <i>c</i> = 7.7617(4) Å), features a complex covalent network of 4-bonded Ge, resulting from a well-ordered topotactic oxidative condensation of [Ge₁₂]^7– layers. It is a diamagnetic semiconductor (<i>E</i>_g = 0.33 eV), and transforms exothermically and irreversibly to α-Ge at 363 °C. This demonstrates the potential of ionic liquids as reactive media in the mild oxidation of Zintl phases to new highly crystallized modifications of elements and simple compounds

BaMn₉[VO₄]₆(OH)₂: A Unique Canted Antiferromagnet with a Chiral “Paddle-Wheel” Structural Feature

BaMn₉[VO₄]₆(OH)₂ was synthesized by hydrothermal methods. We evaluated the crystal structure based on the two possible space groups <i>P</i>2₁3 and <i>Pa</i>3̅ [<i>a</i> = 12.8417(2) Å] using single-crystal and powder X-ray diffraction techniques. The structure contains three-dimensionally linked Mn₉ units of a chiral “paddle-wheel” type. Experimental IR and Raman spectra were analyzed in terms of fundamental vanadate and hydroxide vibrational modes. The magnetic properties were investigated, and the specific heat in applied fields was studied. The dominant magnetic interactions (Mn²⁺, <i>S</i> = ⁵/₂) are of antiferromagnetic origin, as indicated by a Curie–Weiss fit above 175 K with Θ ≈ −200 K. Canting of the spins on the geometrically frustrated triangle segment of the structural feature is considered to account for the ferrimagnetic type of long-range order at <i>T</i>_C ≈ 18 K. We propose a model for the spin structure in the ordered regime. Dielectric constants were measured and indicate a magnetodielectric effect at <i>T</i>_C, which is assigned to spin–lattice coupling

Structural Polymorphism in "Kesterite" Cu2ZnSnS4: Raman Spectroscopy and First-Principles Calculations Analysis

This work presents a comprehensive analysis of the structural and vibrational properties of the kesterite Cu2ZnSnS4 (CZTS, I4\u305 space group) as well as its polymorphs with the space groups P4\u3052c and P4\u3052m, from both experimental and theoretical point of views. Multiwavelength Raman scattering measurements performed on bulk CZTS polycrystalline samples were utilized to experimentally determine properties of the most intense Raman modes expected in these crystalline structures according to group theory analysis. The experimental results compare well with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Vibrational patterns of the most intense fully symmetric modes corresponding to the P4\u3052c structure were compared with the corresponding modes in the I4\u305 CZTS structure. The results point to the need to look beyond the standard phases (kesterite and stannite) of CZTS while exploring and explaining the electronic and vibrational properties of these materials, as well as the possibility of using Raman spectroscopy as an effective technique for detecting the presence of different crystallographic modifications within the same material

Structure, electrochemical impedance and Raman spectroscopy of lithium-niobium-titanium-oxide ceramics for LTCC technology

Resultsof Electrochemical Impedance Spectroscopy (EIS) are reported for lithium-niobium-titanium-oxide (LNTO) ceramics synthesized by a solid-state reaction method with two functional additives (MoO3 or ZnO) in the temperature range 323 K - 573 K and frequencies between 10−1 Hz and 107 Hz. Scanning electron microscopy (SEM) reveals a textured morphology of rod and plate-like particles that are typical for M-phase LNTO materials, while X-ray diffraction (XRD) analysis confirms the formation of an M-phase member compound with an approximate structure of Li7Nb3Ti5O21. Complex impedance analysis indicates that its overall electrical resistivity behavior depends mostly on the grain boundary processes. EIS analysis shows a negative temperature coefficient of resistance behavior (NTCR) in a defined temperature range in two LNTOs and thermal activation of the conduction mechanisms. The low dielectric constants of 5.5 and 12.1 at 1 MHz were found for the first and second LNTOs, respectively. Complimentary Raman spectroscopic measurements, despite very large crystallographic unit cell of LNTO, reveal only a small number of lines, which is the consequence of a “molecular” nature of materials