Identifying the forces that drive a temperature‐induced phase transition is always challenging in the prospect of the first‐principles methods. Herein, we perform a first‐principles study of the ...temperature effects on structural, energetic, electronic, and vibrational properties of four BaTiO3 polymorphs using quasi‐harmonic approximations. Study of the stability between these four phases, which we break into contributions arising from the vibration of the lattice, electronic structure, and volume expansion/contraction, is helpful to confirm the sequence of phase transitions as cubic → tetragonal → orthorhombic → rhombohedral, as well as its transition temperatures. A general mechanism was proposed based on the combination between structural distortions at TiO6 clusters, vibrational characteristics, and electronic structure. These findings confirm the power of quasi‐harmonic approximations to disclose the main fingerprints associated with both thermic and mechanical phase transitions, serving as a guide for further theoretical studies.
Phase transitions in BaTiO3 single crystals are very interesting due to the structural distortions, which enable the connection between the polymorphs. First‐principles calculations are appropriate to tackle such problem, but they are commonly restricted to studying the electronic structure at 0 K. In this context, the quasi‐harmonic approximation provides an interesting and alternative tool to compute quasi‐harmonic thermal properties of solids beyond the harmonic approximation.
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BFBNIB, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
•Mn0.5Zn0.5Cr2O4 nanoparticles were synthesized by solution combustion method.•Structural, electronic and magnetic properties are discussed using DFT.•Magnetic parameters are estimated confirming the ...dominant antiferromagnetic character.•Mn0.5Zn0.5Cr2O4 nanoparticles are good candidates for superior electronic properties.
In the present investigation, we report the structural, vibrational, electronic and magnetic properties of Mn0.5Zn0.5Cr2O4 nanoparticles fabricated by the solution combustion method and complemented by Density Functional theory (DFT) calculations. X-ray diffraction (XRD), Neutron diffraction, X-ray photoelectron spectroscopy and Raman analysis confirms the formation of single-phase with spinel cubic structure. The average crystallite size was found to be 8 nm. The theoretical calculations suggest that Zn-doping on the MnCr2O4 matrix induces a unit cell contraction associated with structural distortions along both AO4 (A = Mn, Zn) and CrO6 clusters, in agreement with the experimental evidence. These structural distortions contribute to narrowing the band-gap of Mn0.5Zn0.5Cr2O4 from disturbed energy levels in the vicinity of Fermi level. Field dependent magnetization confirms that the samples exhibit paramagnetic nature at 300 K and antiferromagnetic nature at 3 K. In the theoretical context, the exchange coupling constant for pure and Zn2+ substituted MnCr2O4 materials were calculated confirming the dominant antiferromagnetic character of Cr-Cr interactions. The temperature dependent susceptibility reveals that the magnetic transition from paramagnetic phase to antiferromagnetic phase occurs at 19 K (TN). The spin frustration factor of Mn0.5Zn0.5Cr2O4 is found to be 22 K. Hence, our experimental and theoretical result suggests that synthesized materials are useful for low and high frequency applications.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The antibacterial activity of a calixarene derivative, p-tert-butylcalix6arene (Calix6), was assessed and was shown not to inhibit the growth of E. coli, S. aureus and B. subtilis bacteria. With the ...aim of gaining more insights into the absence of antibacterial activity of Calix6, the interaction of this derivative with DPPG, a bacterial cell membrane lipid, was studied. Langmuir monolayers were used as the model membrane. Pure DPPG and pure Calix6 monolayers, as well as binary DPPG:Calix6 mixtures were studied using surface pressure measurements, compressional modulus, Brewster angle and fluorescence microscopies, ellipsometry, polarization-modulation infrared reflection absorption spectroscopy and molecular dynamics simulations. Thermodynamic properties of the mixed monolayers were additionally calculated using thermodynamic parameters. The analysis of isotherms showed that Calix6 significantly affects the DPPG monolayers, modifying the isotherm profile and increasing the molecular area, in agreement with the molecular dynamics simulations. The presence of Calix6 in the mixed monolayers decreased the interfacial elasticity, indicating that calixarene disrupts the strong intermolecular interactions of DPPG hindering its organization into a compact arrangement. At low molar ratios of Calix6, the DPPG:Calix6 interactions are preferentially attractive, due to the interactions between the hydrophobic tails of DPPG and the tert-butyl groups of Calix6. Increasing the proportion of calixarene generates repulsive interactions. Calix6 significantly affects the hydrophobic tail organization, which was confirmed by PM-IRRAS measurements. Calix6 appears to be expelled from the mixed films at a biologically relevant surface pressure, π = 30 mN m-1, indicating a low interaction with the cell membrane model related to the absence of antibacterial activity.
The structural, electronic, and vibrational properties of two leading representatives of the Zn‐based spinel oxides class, normal ZnX2O4 (X = Al, Ga, In) and inverse Zn2MO4 (M = Si, Ge, Sn) crystals, ...were investigated. In particular, density functional theory (DFT) was combined with different exchange‐correlation functionals: B3LYP, HSE06, PBE0, and PBESol. Our calculations showed good agreement with the available experimental data, showing a mean percentage error close to 3% for structural parameters. For the electronic structure, the obtained HSE06 band‐gap values overcome previous theoretical results, exhibiting a mean percentage error smaller than 10.0%. In particular, the vibrational properties identify the significant differences between normal and inverse spinel configurations, offering compelling evidence of a structure‐property relationship for the investigated materials. Therefore, the combined results confirm that the range‐separated HSE06 hybrid functional performs the best in spinel oxides. Despite some points that cannot be directly compared to experimental results, we expect that future experimental work can confirm our predictions, thus opening a new avenue for understanding the structural, electronic, and vibrational properties in spinel oxides.
The predictive power of the density functional theory (DFT) method is dependent on the relative precision of the exchange‐correlation functional. In this study, a performance test was carried out in order to analyze the description of structural, electronic, and vibrational properties of normal and inverted spinel. Theoretical results indicate that HSE06 performs the best in reproducing experimental findings for such a class of compounds, being the most suitable choice for the in silico prediction of spinel oxide properties.
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In this paper, we present a combined experimental and theoretical study to disclose, for the first time, the structural, electronic, and optical properties of Ca
V
O
crystals. The microwave-assisted ...hydrothermal (MAH) method has been employed to synthesize these crystals with different morphologies, within a short reaction time at 120 °C. First-principle quantum mechanical calculations have been performed at the density functional theory level to obtain the geometry and electronic properties of Ca
V
O
crystal in the fundamental and excited electronic states (singlet and triplet). These results, combined with the measurements of X-ray diffraction (XRD) and Rietveld refinements, confirm that the building blocks lattice of the Ca
V
O
crystals consist of three types of distorted 6-fold coordination CaO
clusters: octahedral, prism and pentagonal pyramidal, and distorted tetrahedral VO
clusters. Theoretical and experimental results on the structure and vibrational frequencies are in agreement. Thus, it was possible to assign the Raman modes for the Ca
V
O
superstructure, which will allow us to show the structure of the unit cell of the material, as well as the coordination of the Ca and V atoms. This also allowed us to understand the charge transfer process that happens in the singlet state (s) and the excited states, singlet (s*) and triplet (t*), generating the photoluminescence emissions of the Ca
V
O
crystals.
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•We have prepared CoCr2-yScyO4 nanoparticles with shape-selective control of nanomagnetism.•Singular magneto-structural behavior was observed Sc doped CoCr2O4.•Shape-oriented magnetic ...properties indicate the anisotropic spin density distribution and surface exposure;•Two magnetic transitions at Tc and Ts were detected, depending upon the Sc content;•DFT calculations revealed the role of surface exposure on magnetic properties;
In this study CoCr2-yScyO4 (y = 0, 0.01, 0.02, and 0.03) nanoparticles (NPs) have been synthesized following the solution combustion method and characterized by combining experimental techniques with high-throughput Density Functional Theory calculations. The structural analysis confirmed the crystalline nature with spinel cubic structure exhibiting an average particle size between 7 and 10 nm. The morphological analysis confirmed the exposure of the (220), (311), (400), and (422) planes in agreement with theoretical results based on Wulff Construction. Magnetic analysis indicated the existence of paramagnetic to ferrimagnetic phase transition at the critical temperature (Tc) and a conical spiral spin phase was found at the spiral transition temperature (Ts). In both the cases, the transition temperature was seen to decrease with increase in the Sc content in agreement with the exchange-coupling constants calculated by DFT, where a singular magneto-structural behavior was observed. Theoretical results for shape-oriented magnetic properties indicate the anisotropic spin density distribution and surface exposure, providing a general picture of shape-selective control of nanomagnetism.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this paper, the aggregate formation of para-tert-butylcalix6arene molecules (Calix6) in dimeric structures was investigated at the water/air interface using experimental and theoretical studies. A ...specific orientation for such Calix6 molecules was observed with an average area of 133 Å(2), which corresponds to a flat-on orientation with the OH groups parallel to the interface. By varying the pressure on the Calix6 monolayer, the molecules tend to organize at the water/air interface and subsequently, at higher pressures, aggregates were formed atop the monolayer as cluster structures. Morphological characterization by the Brewster Angle Microscopy technique showed the formation of larger domains at lower pressures. Based on such experimental evidence, molecular dynamics (MD) simulations were performed to investigate possible dimeric structures for aggregated Calix6 molecules, which are localized at the water/air interface, where one molecule remains in the water phase and the other remains in the air phase. By increasing surface pressure, experimental and theoretical results corroborate the intermolecular interactions among Calix6 molecules. These results are relevant because a dimeric structure has a molecular cavity, which is a candidate for host-guest chemistry, an ion receptor or a drug-delivery system.
In this paper, we present a combined experimental and theoretical study to disclose, for the first time, the structural, electronic, and optical properties of Ca10V6O25 crystals. The ...microwave-assisted hydrothermal (MAH) method has been employed to synthesize these crystals with different morphologies, within a short reaction time at 120 °C. First-principle quantum mechanical calculations have been performed at the density functional theory level to obtain the geometry and electronic properties of Ca10V6O25 crystal in the fundamental and excited electronic states (singlet and triplet). These results, combined with the measurements of X-ray diffraction (XRD) and Rietveld refinements, confirm that the building blocks lattice of the Ca10V6O25 crystals consist of three types of distorted 6-fold coordination CaO6 clusters: octahedral, prism and pentagonal pyramidal, and distorted tetrahedral VO4 clusters. Theoretical and experimental results on the structure and vibrational frequencies are in agreement. Thus, it was possible to assign the Raman modes for the Ca10V6O25 superstructure, which will allow us to show the structure of the unit cell of the material, as well as the coordination of the Ca and V atoms. This also allowed us to understand the charge transfer process that happens in the singlet state (s) and the excited states, singlet (s*) and triplet (t*), generating the photoluminescence emissions of the Ca10V6O25 crystals.
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ZnO is a semiconductor material largely employed in the development of several electronic and optical devices due to its unique electronic, optical, piezo-, ferroelectric and structural properties. ...This study evaluates the properties of Ba-doped wurtzite-ZnO using quantum mechanical simulations based on the Density Functional Theory (DFT) allied to hybrid functional B3LYP. The Ba-doping caused increase in lattice parameters and slight distortions at the unit cell angle in a wurtzite structure. In addition, the doping process presented decrease in the band-gap (Eg) at low percentages suggesting band-gap engineering. For low doping amounts, the wavelength characteristic was observed in the visible range; whereas, for middle and high doping amounts, the wavelength belongs to the Ultraviolet range. The Ba atoms also influence the ferroelectric property, which is improved linearly with the doping amount, except for doping at 100% or wurtzite-BaO. The ferroelectric results indicate the ZnO:Ba is an strong option to replace perovskite materials in ferroelectric and flash-type memory devices.
In this study, we present a combined experimental and theoretical study of the geometry, electronic structure, morphology, and photoluminescence properties of CaZrO3:Eu3+ materials. The polymeric ...precursor method was employed to synthesize CaZrO3:Eu3+ crystals, while density functional theory calculations were performed to determine the geometrical and electronic properties of CaZrO3:Eu3+ in its ground and excited electronic states (singlet and triplet). The results were combined with X-ray diffraction (XRD) measurements to elucidate the local structural changes induced by the introduction of Eu3+ in the crystal lattice. This process results in the formation of intermediate levels in the band-gap (Egap) region, narrowing its width. The PL emissions were rationalized by characterizing the electronic structure of the excited singlet and triplet electronic states, which provided deep insight into the main structural and electronic fingerprints associated with CaO8, EuO8, and ZrO6 clusters. In addition, the Wulff construction, obtained from the first-principles calculations, was used to clarify the experimental morphologies. These results extend our fundamental understanding of the atomic processes that underpin the Eu doping of CaZrO3.
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