Recently synthesized (Ti1−xMox)2AlC (0 ≤ x ≤ 0.20) solid solutions: deciphering the structural,electronic, mechanical and thermodynamic properties via ab initio simulations

The structural, electronic, mechanical and thermodynamic properties of (Ti_1−xMo_x)₂AlC (0 ≤ x ≤ 0.20) were explored using density functional theory. The obtained lattice constants agree well with the experimental values. The electronic band structure confirms the metallic nature. Strengthening of covalent bonds due to Mo substitution is confirmed from the study of band structure, electronic density of states and charge density mapping. The elastic constants satisfy the mechanical stability criteria. Strengthening of covalent bonds leads to enhanced mechanical properties. (Ti_1−xMo_x)₂AlC compounds are found to exhibit brittle behavior. The anisotropic nature of (Ti_1−xMo_x)₂AlC is revealed from the direction dependent Young''s modulus, compressibility, shear modulus and Poisson''s ratio as well as the shear anisotropic constants and the universal anisotropic factor. The Debye temperature, minimum thermal conductivity, Grüneisen parameter and melting temperature of (Ti_1−xMo_x)₂AlC have been calculated for different Mo contents. Our calculated values are compared with reported values, where available.

The structural, electronic, mechanical and thermodynamic properties of (Ti_1−xMo_x)₂AlC (0 ≤ x ≤ 0.20) were explored using density functional theory.     

The photoelectron-imaging spectroscopic study and chemical bonding analysis of VO2−, NbO2− and TaO2−

The transition-metal di-oxides, namely VO₂⁻, NbO₂⁻ and TaO₂⁻ have been studied using photoelectron velocity map imaging (PE-VMI) in combination with theoretical calculations. The adiabatic electron affinities of VO₂⁻, NbO₂⁻ and TaO₂⁻ are confirmed to be 2.029(8), 1.901(10) and 2.415(8) eV, respectively. By combining Franck–Condon (FC) simulation with theoretical calculations, the vibrational feature related to Nb–O and Ta–O stretching modes for the ground state has been unveiled. The photoelectron angular distribution (PAD) for VO₂⁻, NbO₂⁻ and TaO₂⁻ is correlated to the photo-detachment of the highest occupied molecular orbitals (HOMOs), which primarily gets involved in s- and d-orbitals of the V, Nb and Ta atoms. A variety of theoretical calculations have been used to analyze the chemical bonding features of VO₂^−1/0, NbO₂^−1/0 and TaO₂^−1/0, which show that the strong M–O (M = V, Nb and Ta) bond is mainly characterized as ionicity.

The transition-metal di-oxides, namely VO₂⁻, NbO₂⁻ and TaO₂⁻ have been studied using photoelectron velocity map imaging (PE-VMI) in combination with theoretical calculations.     

Random anion distribution in MSxSe2−x (M = Mo,W) crystals and nanosheets

The group VIb dichalcogenides (MX₂, M = Mo, W; X= S, Se) have a layered molybdenite structure in which M atoms are coordinated by a trigonal prism of X atoms. Ternary solid solutions of MS_xSe_2−x were synthesized, microcrystals were grown by chemical vapor transport, and their morphologies and structures were characterized by using synchrotron X-ray diffraction, Rietveld refinement, DIFFaX simulation of structural disorder, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Double aberration corrected scanning transmission electron microscopy was used to determine the anion distributions in single-layer nanosheets exfoliated from the microcrystals. These experiments indicate that the size difference between S and Se atoms does not result in phase separation, consistent with earlier studies of MX₂ monolayer sheets grown by chemical vapor deposition. However, stacking faults occur in microcrystals along the layering axis, particularly in sulfur-rich compositions of MS_xSe_2−x solid solutions.

Nanosheets exfoliated from single crystals of the group VIb sulfoselenides (MS_xSe_2−x, M = Mo, W) are solid solutions at the atomic level.     

Indirect-to-direct band gap transition and optical properties of metal alloys of Cs2Te1−xTixI6: a theoretical study

In recent years, double perovskites have attracted considerable attention as potential candidates for photovoltaic applications. However, most double perovskites are not suitable for single-junction solar cells due to their large band gaps (over 2.0 eV). In the present study, we have investigated the structural, mechanical, electronic and optical properties of the Cs₂Te_1−xTi_xI₆ solid solutions using first-principles calculations based on density functional theory. These compounds exhibit good structural stability compared to CH₃NH₃PbI₃. The results suggest that Cs₂TeI₆ is an indirect band gap semiconductor, and it can become a direct band gap semiconductor with the value of 1.09 eV when the doping concentration of Ti⁴⁺ is 0.50. Moreover, an ideal direct band gap of 1.31 eV is obtained for Cs₂Te_0.75Ti_0.25I₆. The calculated results indicate that all the structures are ductile materials except for Cs₂Te_0.50Ti_0.50I₆. Our results also show that these materials possess large absorption coefficients in the visible light region. Our work can provide a route to explore stable, environmentally friendly and high-efficiency light absorbers for use in optoelectronic applications.

In recent years, double perovskites have attracted considerable attention as potential candidates for photovoltaic applications.     

Comparison of electrochemical performance of LiNi1−xCoxO2 cathode materials synthesized from coated (1−x)Ni(OH)2@xCo(OH)2 and doped Ni1−xCox(OH)2 precursors

LiNi_1−xCo_xO₂ cathode materials were successfully synthesized from coated (1−x)Ni(OH)₂@xCo(OH)₂ and doped Ni_1−xCo_x(OH)₂ precursors, and the effects of the Co site and content in the precursor and final cathode material on the structure, morphology, and electrochemical performance of the cathodes were investigated using X-ray diffraction, scanning electron microscopy, and charge–discharge tests. The electrochemical performance of the materials prepared from the coated precursor was generally better than that of the materials prepared from the doped precursor. However, with increasing Co content, the performance difference gradually decreased. Among the as-prepared samples, the sample coated with 12 mol% Co delivered an excellent reversible capacity of 213.8 mA h g⁻¹ at 0.1C and the highest capacity retention of 88.5% after 100 cycles at 0.2C in the voltage range of 2.75–4.3 V. High-performance LiNi_1−xCo_xO₂ materials were successfully synthesized, and our findings clearly reveal the differences in the electrochemical properties of the materials prepared from the two different precursors with increasing Co content, thereby providing a valuable reference for the synthesis of high-performance Ni-rich layered cathode materials for Li-ion batteries.

The effects of Co site and content on electrochemical performance of LiNi_1−xCo_xO₂ cathodes materials were investigated.     

Colossal dielectric permittivity,reduced loss tangent and the microstructure of Ca1−xCdxCu3Ti4O12−2yF2y ceramics

Ca_1−xCd_xCu₃Ti₄O_12−2yF_2y (x = y = 0, 0.10, and 0.15) ceramics were successfully prepared via a conventional solid-state reaction (SSR) method. A single-phase CaCu₃Ti₄O₁₂ with a unit cell ∼7.393 Å was detected in all of the studied ceramic samples. The grain sizes of sintered Ca_1−xCd_xCu₃Ti₄O_12−2yF_2y ceramics were significantly enlarged with increasing dopant levels. Liquid-phase sintering mechanisms could be well matched to explain the enlarged grain size in the doped ceramics. Interestingly, preserved high dielectric permittivities, ∼36 279–38 947, and significantly reduced loss tangents, ∼0.024–0.033, were achieved in CdF₂ codoped CCTO ceramics. Density functional theory results disclosed that the Cu site is the most preferable location for the Cd dopant. Moreover, F atoms preferentially remained close to the Cd atoms in this structure. An enhanced grain boundary response might be a primary cause of the improved dielectric properties in Ca_1−xCd_xCu₃Ti₄O_12−2yF_2y ceramics. The internal barrier layer capacitor model could well describe the colossal dielectric response of all studied sintered ceramics.

CdF₂ defect clusters result in enhancement of dielectric properties of the Ca_1−xCd_xCu₃Ti₄O_12−2yF_2y ceramics.     

New red-emitting phosphor RbxK3−xSiF7:Mn4+ (x = 0, 1, 2, 3): DFT predictions and synthesis

Finding new phosphors through an efficient method is important in terms of saving time and cost related to the development of phosphor materials. The ability to identify new phosphors through preliminary simulations by calculations prior to the actual synthesis of the materials can maximize the efficiency of novel phosphor development. In this paper, we demonstrate the use of density functional theory (DFT) calculations to guide the development of a new red phosphor. We performed first-principles calculations based on DFT for pristine and Mn-doped Rb_xK_3−xSiF₇ (x = 0, 1, 2, 3) and predicted their stability, electronic structure, and luminescence properties. On the basis of the results, we then synthesized the stable Rb₂KSiF₇:Mn⁴⁺ red conversion phosphor and investigated its luminescence, structure, and stability. As a result, we confirmed that Rb₂KSiF₇:Mn⁴⁺ emitted red light with a longer wavelength than that emitted by K₃SiF₇:Mn⁴⁺ and a wavelength similar to that of K₂SiF₆:Mn⁴⁺. These results show that DFT calculations can provide rational insights into the design of a phosphor material before it is synthesized, thereby reducing the time and cost required to develop new red conversion phosphors.

We performed first-principles calculations for pristine and Mn-doped Rb_xK_3−xSiF₇ and predicted their various properties. In addition, we synthesized Rb₂KSiF₇:Mn⁴⁺ as a stable red phosphor and verified its PL properties, structure, and stability.     

Co1−xBaxFe2O4 (x = 0, 0.25, 0.5, 0.75 and 1) nanoferrites as gas sensor towards NO2 and NH3 gases

Co_1−xBa_xFe₂O₄ (x = 0, 0.25, 0.5, 0.75 and 1) nanoferrites were synthesized using a controlled chemical co-precipitation technique. Their structural, optical, dielectric and gas sensing properties were characterized by X-ray diffractometry, UV-Vis spectroscopy and an LCR meter with a gas sensing unit. The crystalline sizes were estimated using the Scherrer formula and were found to be 7.8 nm, 14.4 nm, 21.8 nm, 16.5 nm and 30.3 nm for x = 0, 0.25, 0.5, 0.75 and 1, respectively. The fundamental optical band gaps were calculated by extrapolating the linear part of (αhυ)²vs. hυ of the synthesized nanoferrites. The SEM and EDX spectra also confirmed the formation of nanoferrites. Dramatic behavior was observed in the dielectric constant and dissipation factor with varying temperature, which provides a substantial amount of information about electric polarization. The synthesized nanoferrites were tested towards NO₂ and NH₃ gases. The order of sensitivity (%) towards NH₃ was analyzed as x = 0.75 > x = 0.5 > x = 0.25 > x = 0 > x = 1, while the order was x = 0 > 0.75 > 1 > 0.5 > 0.25 for NO₂ gas.

XRD pattern and sensitivity (%) as a function of flow rate (ppm) of Co_1−xBa_xFe₂O₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) nanoferrites towards NO₂ and NH₃ gases.     

Electronic structure of iron dinitrogen complex [(TPB)FeN2]2−/1−/0: correlation to Mössbauer parameters

Low-valent species of iron are key intermediates in many important biological processes such as the nitrogenase enzymatic catalytic reaction. These species play a major role in activating highly stable N₂ molecules. Thus, there is a clear need to establish the factors which are responsible for the reactivity of the metal–dinitrogen moiety. In this regard, we have investigated the electronic structure of low-valent iron (2−/1−/0) in a [(TPB)FeN₂]^2−/1−/0 complex using density functional theory (DFT). The variation in the oxidation states of iron in the nitrogenase enzyme cycle is associated with the flexibility of Fe→B bonding. Therefore, the flexibility of Fe→B bonding acts as an electron source that sustains the formation of various oxidation states, which is necessary for the key species in dinitrogen activation. AIM calculations are also performed to understand the strength of Fe→B and Fe–N₂ bonds. A detailed interpretation of the contributions to the isomer shift (IS) and quadrupole splitting (ΔE_Q) are discussed. The major contribution to IS comes mainly from the 3s-contribution, which differs depending on the d orbital population due to different shielding. The valence shell contribution also comes from the 4s-orbital. The Fe–N₂ bond distance has a great influence on the Mössbauer parameters, which are associated with the radial distribution, i.e. the shape of the 4s-orbital and the charge density at the nucleus. A linear relationship between IS with Fe–N₂ and ΔE_Q with Fe–N₂ is observed.

We use density functional theory studies to explore the electronic structure, bonding and spectroscopic analysis of a low-valent iron (2−/1−/0) complex [(TPB)FeN₂]^2−/1−/0 and reveled the factor which affects the reactivity of the metal–dinitrogen moiety.     

Structural growth pattern of neutral and negatively charged yttrium-doped silicon clusters YSin0/− (n=6–20): from linked to encapsulated structures

A global search for the low energy of neutral and anionic doped Si clusters YSi_n^0/− (n = 6–20) was performed using the ABCluster global search technique coupled with a hybrid density functional method (mPW2PLYP). In light of the calculated energies and the measured photoelectron spectroscopy values, the true minima of the most stable structures were confirmed. It is shown that the structural growth pattern of YSi_n⁻ (n = 6–20) is from Y-linked two subcluster structure to a Y-encapsulated structure in Si cages, while that of YSi_n (n = 6–20) is from substitutional to linked structures, and as the number of Si atoms increases, it evolves toward the encapsulated structure. Superatom YSi₂₀⁻ with a high-symmetry endohedral I_h structure has an ideal thermodynamic stability and chemical reactivity, making it the most suitable building block for novel optical, optoelectronic photosensitive or catalytic nanomaterials.

Superatom YSi₂₀⁻, with an ideal thermodynamic stability and chemical reactivity, is the most suitable building block for novel optical, optoelectronic photosensitive or catalytic nanomaterials.     

Anomalous behavior above the Curie temperature in (Nd1−xGdx)0.55Sr0.45MnO3 (x = 0, 0.1, 0.3 and 0.5)

Magnetic properties were studied just above the ferromagnetic–paramagnetic (FM–PM) phase transition of (Nd_1−xGd_x)_0.55Sr_0.45MnO₃ with x = 0, 0.1, 0.3 and 0.5. The low-field inverse susceptibility (χ⁻¹) of Nd_0.55Sr_0.45MnO₃ exhibits a Curie–Weiss-PM behavior. For x ≥ 0.1, we observe a deviation in χ⁻¹(T) behavior from the Curie–Weiss law. The anomalous behavior of the χ⁻¹(T) was qualified as Griffiths phase (GP)-like. The study of the evolution of the GP through a susceptibility exponent, the GP temperature and the temperature range of the GP reveals that the origin of the GP is primary due to the accommodated strain. Likewise, the magnetic data reveal distinct features visible only for x = 0.5 at a low magnetic field that can be qualitatively understood as the result of ferromagnetic polarons, entailed by the strong effect of chemical/structural disorder, whose concentration increases upon cooling towards the Curie temperature. We explained the magnetic properties at a high temperature for the heavily Gd-doped sample (x = 0.5) within the phase-separation scenario as an assembly of ferromagnetic nanodomains, antiferromagnetically coupled by correlated Jahn–Teller polarons.

Magnetic properties were studied just above the ferromagnetic–paramagnetic (FM–PM) phase transition of (Nd_1−xGd_x)_0.55Sr_0.45MnO₃ with x = 0, 0.1, 0.3 and 0.5.     

Synthesis and characterisation of Ba(Zn1−xCox)2Si2O7 (0 ≤ x ≤ 0.50) for blue-violet inorganic pigments

Ba(Zn_1−xCo_x)₂Si₂O₇ (0 ≤ x ≤ 0.50) solid solutions were synthesized as novel blue-violet inorganic pigments by a conventional solid-state reaction method. The crystal structure, optical properties, and colour of the pigments were characterized. All the pigments were obtained in a single-phase form. The pigments strongly absorbed visible light at wavelengths from 550 to 650 nm, corresponding to the range of green to orange light. This optical absorption was caused by the d–d transition of the tetrahedrally coordinated Co²⁺ (⁴A₂(F) → ⁴T₁(P)), which was the origin of the blue-violet colour of the pigments. The most intense colour was obtained for Ba(Zn_0.85Co_0.15)₂Si₂O₇, where a* = +52.2 and b* = −65.5 in the CIE (Commission Internationale de l''Éclairage) L*a*b* system. These absolute values were significantly larger than those of commercial violet pigments such as Co₃(PO₄)₂ (a* = +33 and b* = −32) and NH₄MnP₂O₇ (a* = +39 and b* = −21). Therefore, the Ba(Zn_0.85Co_0.15)₂Si₂O₇ pigment could be a novel blue-violet inorganic pigment.

Ba(Zn_1−xCo_x)₂Si₂O₇ (0 ≤ x ≤ 0.50) solid solutions were synthesized as blue-violet inorganic pigments and the colour gradually changed from pale blue-violet to deep blue-violet with increasing the Co²⁺ concentration.     

Chemical structure stabilities of a SixFy (x ≤ 6, y ≤ 12) series

In this paper, we construct a Si_xF_y (x ≤ 6, y ≤ 12) series optimised at the B3LYP/6-31G(d,p) level. At the same level, we perform frontline molecular orbital (FMO), Mayer bond order (MBO), molecular surface electrostatic potential (MS-EPS) and natural population analysis (NPA) calculations to study the chemical structure stabilities of these Si_xF_y molecules. The FMO and MBO results demonstrate that the chemical structure stabilities of the Si_xF_y (x ≤ 6, y ≤ 12) series are ranked (from strong to weak) as SiF₄ > Si₂F₆ > Si₃F₈ > Si₄F₁₀ > SiF₂ > Si₅F₁₂ > Si₃F₆ (ring) > Si₅F₁₀ (ring) > Si₆F₁₂ (ring) > Si₄F₈ (ring). Furthermore, the chemical structure stabilities of the chains are stronger than those of the rings, while the number of silicon atoms is the same. In addition, infrared spectroscopy analysis shows that SiF₄ is the most stable among the Si_xF_y (x ≤ 6, y ≤ 12) series, followed by Si₂F₆, and SiF₂ is unstable. The experimental results are consistent with theoretical calculations. Finally, the MS-EPS and NPA results indicate that compounds in the Si_xF_y (x ≤ 6, y ≤ 12) series tend to be attacked by nucleophiles rather than by electrophiles; also, they show poor chemical structure stability when encountering nucleophiles.

The chemical structure stabilities of a set of Si_xF_y (x ≤ 6, y ≤ 12) compounds were explored using theoretical and experimental methods.     

Structural exploration of AuxM− (M = Si,Ge, Sn; x = 9–12) clusters with a revised genetic algorithm

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters. The most interesting finding in the structural study of Au_xSi⁻ (x = 9–12) is the 3D (Au₉Si⁻ and Au₁₀Si⁻) → quasi-planar 2D (Au₁₁Si⁻ and Au₁₂Si⁻) structural evolution of the Si-doped clusters, which reflects the competition of Au–Au interactions (forming a 2D structure) and Au–Si interactions (forming a 3D structure). The Au_xM⁻ (x = 9–12; M = Ge, Sn) clusters have quasi-planar structures, which suggests a lower tendency of sp³ hybridization and a similarity of electronic structure for the Ge or Sn atom. Au₉Si⁻ and Au₁₀Si⁻ have a 3D structure, which can be viewed as being built from Au₈Si⁻ and Au₉Si⁻ with an extra Au atom bonded to a terminal gold atom, respectively. In contrast, the quasi-planar structures of Au_xM⁻ (x = 9–12; M = Ge, Sn) reflect the domination of the Au–Au interactions. Including the spin–orbit (SO) effects is very important to calculate the simulated spectrum (structural fingerprint information) in order to obtain quantitative agreement between theoretical and future experimental PES spectra.

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters.     

Electronic and optical properties of perovskite compounds MA1−αFAαPbI3−βXβ (X = Cl,Br) explored for photovoltaic applications

As outstanding light harvesters, solution-processable organic–inorganic hybrid perovskites (OIHPs) have been drawing considerable attention thanks to their higher power conversion efficiency (PCE) and cost-effective synthesis relative to other photovoltaic materials. Nevertheless, their further development is severely hindered by the drawbacks of poor stability and rapid degradation in particular. First-principles calculations based on density functional theory (DFT) are hence performed towards the perovskite compounds MA_1−αFA_αPbI_3−βX_β (X = Cl, Br), with the aim of exploring more efficient and stable OIHPs. In addition to that, a hybrid density functional is adopted for exact electronic properties, and their band structures indicate that the doped series are all direct band-gap semiconductors. Moreover, the defect formation energies indicate that the stability of perovskite compounds can be significantly enhanced via ion doping. Meanwhile, it is unveiled that the optical performance of the doped perovskite series is also effectively improved through ion doping. Therefore, the investigated perovskite compounds MA_1−αFA_αPbI_3−βX_β (X = Cl, Br) are promising candidates for enhancing solar-energy conversion efficiency. Our results pave a way in deeper understanding of the inherent characteristics of OIHPs, which is useful for designing new-type perovskite-based photovoltaic devices.

The absorption performance of perovskite CH₃NH₃PbI₃ can be significantly improved via mono-, or co-doping of organic cations and halide ions.     

Phase transition in Cu2+xSnS3+y (0 ≤ x ≤ 2; 0 ≤ y ≤ 1) ternary systems synthesized from complexes of coumarin derived thiocarbamate motifs: optical and morphological properties

Tetragonal Cu₂SnS₃ and orthorhombic Cu₄SnS₄ nanocubes were synthesized by a heat up procedure with oleylamine (OLA) and dodecanethiol (DT) acting as both solvent and capping ligands. Both mohite–anorthic and monoclinic phases were obtained from the same variant of precursors mixture, by hot injection synthesis, at 200 and 250 °C. Changing the reaction conditions also leads to the formation of different morphologies. When OLA was used as a solvent, nanosheets or nanocubes were obtained, while the reaction with DT resulted in the formation of particles in the form of nanohexagons. The growth process of copper tin sulphide starts with the formation of Cu⁺ seeds, followed by the oxidation of Sn²⁺ to Sn⁴⁺. Dodecanethiol was an additional source of sulphur. The overall reaction leads to the formation of either phase pure Cu₂SnS₃ or Cu₄SnS₄, depending on the reaction conditions, with band-gap energies of 1.05–1.45 eV, which are in the optimum range for photovoltaic applications.

Tetragonal Cu₂SnS₃ and orthorhombic Cu₄SnS₄ nanocubes were synthesized by a heat up procedure with oleylamine (OLA) and dodecanethiol (DT) acting as both solvent and capping ligands.     

Bifunctional thiosquaramide catalyzed asymmetric reduction of dihydro-β-carbolines and enantioselective synthesis of (−)-coerulescine and (−)-horsfiline by oxidative rearrangement

Tetrahydro-β-carboline (THBC) is a tricyclic ring system that can be found in a large number of bioactive alkaloids. Herein, we report a simple and efficient method for the synthesis of enantiopure THBCs through a chiral thiosquaramide (11b) catalyzed imine reduction of dihydro-β-carbolines (17a–f). The in situ generated Pd–H employed as hydride source in the reaction of differently substituted chiral THBCs (18a–f) afforded high selectivities (R isomers, up to 96% ee) and good isolated yields (up to 88%). Moreover, the chiral thiosquaramide used also afforded exceptional catalyst activity in the syntheses of (−)-coerulescine (5) and (−)-horsfiline (6) with excellent enantioselectivities up to 98% and 93% ee, respectively, via an enantioselective oxidative rearrangement approach.

A simple and efficient asymmetric synthesis of THBCs through a chiral thiosquaramide 11b catalyzed imine reduction of dihydro-β-carbolines (17a−f) and syntheses of (−)-coerulescine and (–)-horsfiline via enantioselective oxidative rearrangement.     

Color tuning of Bi3+-doped double-perovskite Ba2(Gd1−x,Lux)NbO6 (0 ≤ x ≤ 0.6) solid solution compounds via crystal field modulation for white LEDs

Nowadays, rare-earth-free color-tunable solid solutions are receiving great attention. Here, we synthesize a type of double-perovskite Ba₂(Gd_1−x,Lu_x)NbO₆:Bi³⁺ (0 ≤ x ≤ 0.6) solid solution compound using high temperature solid state reaction. The structural phase purity and photoluminescence (PL) properties of the samples are characterized by powder X-ray diffraction (XRD), density functional theory (DFT) calculations, UV-visible diffuse reflectance spectroscopy and PL spectroscopy. Structural analysis shows that all the samples are crystallized in the double-perovskite structure with a cubic space group of Fm$\bar{3}$m. Moreover, with the increase of Lu³⁺ content, the XRD positions are gradually shifted toward higher diffraction angles, indicating the shrinkage of the crystal lattice. Our PL results show that the Ba₂(Gd_1−x,Lu_x)NbO₆:Bi³⁺ solid solutions can show Bi³⁺ tunable emissions from 462 nm to 493 nm and excitation peaks from 363 nm to 390 nm with the increase of Lu³⁺ content from 0 to 0.6, owing to the crystal field modulation around the Bi³⁺ ion. In addition, the blue-shift of the host excitation peaks is also observed, which is ascribed to the increase of bandgap energies (i.e., from 3.01 eV to 4.14 eV based on the DFT calculations). Besides, due to the closer structural rigidity induced by the replacement of larger Gd³⁺ ions with smaller Lu³⁺ ions, the Ba₂(Gd_1−x,Lu_x)NbO₆:Bi³⁺ solid solutions show an increase of Bi³⁺ emission intensity and QE values followed by a subsequent decrease. As a result, the highest QE value, which corresponds to Ba₂Gd_0.8Lu_0.2NbO₆:Bi³⁺, is 49%. Finally, by coating this optimal blue phosphor and the red CsPb(Br_0.4I_0.6)₃ phosphor on a commercial UV LED chip, a white LED device with a color temperature (CT) of 3633 K, CIE value at (0.381, 0.379), color rendering index (CRI) of 78.4, and luminous efficiency of 48 lm W⁻¹ is achieved.

Nowadays, rare-earth-free color-tunable solid solutions are receiving great attention.     

Mixing thermodynamics and electronic structure of the Pt1−xNix (0 ≤ x ≤ 1) bimetallic alloy

The development of affordable bifunctional platinum alloys as electrode materials for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) remains one of the biggest challenges for the transition towards renewable energy sources. Yet, there is very little information on the optimal ratio between platinum and the transition metal used in the alloy and its impact on the electronic properties. Here, we have employed spin-polarised density functional simulations with long-range dispersion corrections [DFT–D3–(BJ)], to investigate the thermodynamics of mixing, as well as the electronic and magnetic properties of the Pt_1−xNi_x solid solution. The Ni incorporation is an exothermic process and the alloy composition Pt_0.5Ni_0.5 is the most thermodynamically stable. The Pt_0.5Ni_0.5 solid solution is highly ordered as it is composed mainly of two symmetrically inequivalent configurations of homogeneously distributed atoms. We have obtained the atomic projections of the electronic density of states and band structure, showing that the Pt_0.5Ni_0.5 alloy has metallic character. The suitable electronic properties of the thermodynamically stable Pt_0.5Ni_0.5 solid solution shows promise as a sustainable catalyst for future regenerative fuel cells.

Density functional theory simulations complemented by force-field based calculations show that the bimetallic Pt_0.5Ni_0.5 equilibrium composition is highly ordered at room conditions.     

The effects of microstructure,Nb content and secondary Ruddlesden–Popper phase on thermoelectric properties in perovskite CaMn1−xNbxO3 (x = 0–0.10) thin films

CaMn_1−xNb_xO₃ (x = 0, 0.5, 0.6, 0.7 and 0.10) thin films have been grown by a two-step sputtering/annealing method. First, rock-salt-structured (Ca,Mn_1−x,Nb_x)O thin films were deposited on 1$\bar{1}$00 sapphire using reactive RF magnetron co-sputtering from elemental targets of Ca, Mn and Nb. The CaMn_1−xNb_xO₃ films were then obtained by thermally induced phase transformation from rock-salt-structured (Ca,Mn_1−xNb_x)O to orthorhombic during post-deposition annealing at 700 °C for 3 h in oxygen flow. The X-ray diffraction patterns of pure CaMnO₃ showed mixed orientation, while Nb-containing films were epitaxially grown in [101] out of-plane-direction. Scanning transmission electron microscopy showed a Ruddlesden–Popper (R–P) secondary phase in the films, which results in reduction of the electrical and thermal conductivity of CaMn_1−xNb_xO₃. The electrical resistivity and Seebeck coefficient of the pure CaMnO₃ film were measured to 2.7 Ω cm and −270 μV K⁻¹ at room temperature, respectively. The electrical resistivity and Seebeck coefficient were reduced by alloying with Nb and was measured to 0.09 Ω cm and −145 μV K⁻¹ for x = 0.05. Yielding a power factor of 21.5 μW K⁻² m⁻¹ near room temperature, nearly eight times higher than for pure CaMnO₃ (2.8 μW K⁻² m⁻¹). The power factors for alloyed samples are low compared to other studies on phase-pure material. This is due to high electrical resistivity originating from the secondary R–P phase. The thermal conductivity of the CaMn_1−xNb_xO₃ films is low for all samples and is the lowest for x = 0.07 and 0.10, determined to 1.6 W m⁻¹ K⁻¹. The low thermal conductivity is attributed to grain boundary scattering and the secondary R–P phase.

Reduction of thermal conductivity of sputtered CaMn_1−xNb_xO₃ thin films by secondary Ruddlesden–Popper phase and grain size optimization.