Na–Ga–Si type-I clathrate single crystals grown via Na evaporation using Na–Ga and Na–Ga–Sn fluxes

Single crystals of a Na–Ga–Si clathrate, Na₈Ga_5.70Si_40.30, of size 2.9 mm were grown via the evaporation of Na from a Na–Ga–Si melt with the molar ratio of Na : Ga : Si = 4 : 1 : 2 at 773 K for 21 h under an Ar atmosphere. The crystal structure was analyzed using X-ray diffraction with the model of the type-I clathrate (cubic, a = 10.3266(2) Å, space group Pm$\bar{3}$n, no. 223). By adding Sn to a Na–Ga–Si melt (Na : Ga : Si : Sn = 6 : 1 : 2 : 1), single crystals of Na₈Ga_xSi_46−x (x = 4.94–5.52, a = 10.3020(2)–10.3210(3) Å), with the maximum size of 3.7 mm, were obtained via Na evaporation at 723–873 K. The electrical resistivities of Na₈Ga_5.70Si_40.30 and Na₈Ga_4.94Si_41.06 were 1.40 and 0.72 mΩ cm, respectively, at 300 K, and metallic temperature dependences of the resistivities were observed. In the Si L_2,3 soft X-ray emission spectrum of Na₈Ga_5.70Si_40.30, a weak peak originating from the lowest conduction band in the undoped Si₄₆ was observed at an emission energy of 98 eV.

Single crystals of a Na–Ga–Si clathrate, Na₈Ga_4.94Si_41.06, of size 3.7 mm were grown via the evaporation of Na from a Na–Ga–Si–Sn melt with the molar ratio of Na : Ga : Si : Sn = 6 : 1 : 2 : 1 at 873 K for 3 h under an Ar atmosphere.     

Effect of the itinerant electron density on the magnetization and Curie temperature of Sr2FeMoO6 ceramics

The itinerant electron density (n) near the Fermi level has a close correlation with the physical properties of Sr₂FeMoO₆. Two series of single-phase Sr_(2−y)Na_yFeMoO₆ (y = 0.1, 0.2, 0.3) and Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ (y = 2x; y = 0.1, 0.2, 0.3) ceramics were specially designed and the itinerant electron density (n) of them can be artificially controlled to be: n = 1 − y and n = 1 − y + 3x = 1 + 0.5y, respectively. The corresponding crystal structure, magnetization and the ferromagnetic Curie temperature (T_C) of two subjects were investigated systematically. The X-ray diffraction analysis indicates that Sr_(2−y)Na_yFeMoO₆ (y = 0.1, 0.2, 0.3) have comparable Fe/Mo anti-site defect (ASD) content in spite of decreased n. However, a drastically improved Fe/Mo ASD can be observed in Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ (y = 2x; y = 0.1, 0.2, 0.3) caused by the intrinsic wrong occupation of normal Fe sites with excess Mo. Magnetization–magnetic field (M–H) behavior confirms that it is the Fe/Mo ASD not n that dominantly determines the magnetization properties. Interestingly, approximately when n ≤ 0.9, T_C of Sr_(2−y)Na_yFeMoO₆ (y = 0.1, 0.2, 0.3) exhibits an overall increase with decreasing n, which is contrary to the T_C response in electron-doped SFMO. Such abnormal T_C is supposed to relate with the ratio variation of n(Mo)/n(Fe). Moreover, when n ≥ 1, T_C of Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ (y = 2x; y = 0.3) exhibits a considerable rise of about 75 K over that of Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ (y = 2x; y = 0.1), resulting from improved n caused by introducing excess Mo into Sr_(2−y)Na_yFeMoO₆. Maybe, our work can provide an effective strategy to artificially control n and ferromagnetic T_C accordingly, and provoke further investigation on the FeMo-baseddouble perovskites.

T _C of C6 exhibits a significant rise of 75 K over that of C2, resulting from introducing excess Mo in Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆.     

Effect of Bi2O3 content on the microstructure and electrical properties of SrBi2Nb2O9 piezoelectric ceramics

Lead-free ceramics, SrBi₂Nb₂O₉–xBi₂O₃ (SBN–xBi), with different Bi contents of which the molar ratio, n(Sr) : n(Bi) : n(Nb), is 1 : 2(1 + x/2) : 2 (x = −0.05, 0.0, 0.05, 0.10), were prepared by conventional solid-state reaction method. The effect of excess bismuth on the crystal structure, microstructure and electrical properties of the ceramics were investigated. A layered perovskite structure without any detectable secondary phase and plate-like morphologies of the grains were clearly observed in all samples. The value of the activation energy suggested that the defects in samples could be related to oxygen vacancies. Excellent electrical properties (e.g., d₃₃ = 18 pC N⁻¹, 2P_r = 17.8 μC cm⁻², ρ_rd = 96.4% and T_c = 420 °C) were simultaneously obtained in the ceramic where x = 0.05. Thermal annealing studies indicated the SBN–xBi ceramics system possessed stable piezoelectric properties, demonstrating that the samples could be promising candidates for high-temperature applications.

Lead-free ceramics, SrBi₂Nb₂O₉–xBi₂O₃ (SBN–xBi), with different Bi contents of which the molar ratio, n(Sr) : n(Bi) : n(Nb), is 1 : 2(1 + x/2) : 2 (x = −0.05, 0.0, 0.05, 0.10), were prepared by conventional solid-state reaction method.     

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.     

Probing the ionic structure of FLiNaK–ZrF4 salt mixtures by solid-state NMR

In this study, by applying ¹⁹F, ²³Na and ⁷Li high-resolution NMR methods, the evolution of the [Zr_xF_y]^4x−y local ionic structures in FLiNaK–ZrF₄ salt mixtures were elucidated. K₃ZrF₇, Na₃ZrF₇ and Na₇Zr₆F₃₁ crystal phases were identified when the melt salts were being solidified. The distribution of these [Zr_xF_y]^4x−y species was dependent on the content of ZrF₄ in FLiNaK eutectic salts. Moreover, K₃ZrF₇ phase transition from an orthorhombic lattice into a disordered cubic lattice was clarified, thereby causing dynamics of the coordinated F⁻ ions to be reduced and the well-ordered crystal lattices to be destroyed. These mentioned results provide a further insight into the Zr–F based ionic structure and the formation of the disordered Zr–F structure in ZrF₄-based eutectic salts.

The evolution of the [Zr_xF_y]^4x−y ionic structures in FLiNaK–ZrF₄ salt mixtures was elucidated through solid-state NMR techniques when the melt salts were being solidified.     

Promotional effect of silver nanoparticle embedded Ga–Zr-codoped TiO2 as an alternative anode for efficient blue,green and red PHOLEDs

Efficient blue, green and red phosphorescent OLEDs have been harvested from silver nanoparticles embedded at a glass:Ga–Zr-codoped TiO₂ interface. The embedded silver nanoparticles at the interface removed the non productive hole current and enhanced the efficiencies. The blue emitting device (456 nm) with emissive layer Ir(fni)₃ exhibits a maximum luminance (L) of 40 512 cd m⁻² (ITO – 37 623 cd m⁻²), current efficiency (η_c) of 41.3 cd A⁻¹ (ITO – 40.5 cd A⁻¹) and power efficiency (η_p) of 43.1 lm w⁻¹ (ITO – 39.8 lm w⁻¹) and external quantum efficiency (η_ex) of 19.4% (ITO – 6.9%). A newly fabricated green device based on emissive layer Ir(tfpdni)₂(pic) shows intensified emission at 514 nm, luminance of 46 435 cd m⁻² (ITO – 40 986 cd m⁻²), current efficiency of 49.7 cd A⁻¹ (ITO – 47.3 cd A⁻¹), power efficiency of 48.6 lm w⁻¹ (ITO – 41.4 lm w⁻¹) and external quantum efficiency of 17.5% (ITO – 14.9%). The red device (618 nm) with emissive layer Ir(bbt)₂(acac) shows luminance of 8936 cd m⁻² (ITO – 8043 cd m⁻²), current efficiency of 6.9 cd A⁻¹ (ITO – 4.6 cd A⁻¹), power efficiency of 5.7 lm w⁻¹ (ITO – 4.9 lm w⁻¹) and external quantum efficiency of 9.3% (ITO – 6.9%).

Efficient blue, green and red phosphorescent OLEDs have been harvested from silver nanoparticles embedded at a glass:Ga–Zr-codoped TiO₂ interface.     

Structure and transport properties of triple-conducting BaxSr1−xTi1−yFeyO3−δ oxides

In this work, Ba_xSr_1−xTi_1−yFe_yO_3−δ perovskite-based mixed conducting ceramics (for x = 0, 0.2, 0.5 and y = 0.1, 0.8) were synthesized and studied. The structural analysis based on the X-ray diffraction results showed significant changes in the unit cell volume and Fe(Ti)–O distance as a function of Ba content. The morphology of the synthesized samples studied by means of scanning electron microscopy has shown different microstructures for different contents of barium and iron. Electrochemical impedance spectroscopy studies of transport properties in a wide temperature range in the dry- and wet air confirmed the influence of barium cations on charge transport in the studied samples. The total conductivity values were in the range of 10⁻³ to 10⁰ S cm⁻¹ at 600 °C. Depending on the barium and iron content, the observed change of conductivity either increases or decreases in humidified air. Thermogravimetric measurements have shown the existence of proton defects in some of the analysed materials. The highest observed molar proton concentration, equal to 5.0 × 10⁻² mol mol⁻¹ at 300 °C, was obtained for Ba_0.2Sr_0.8Ti_0.9Fe_0.1O_2.95. The relations between the structure, morphology and electrical conductivity were discussed.

Ba_xSr_1−xTi_1−yFe_yO_3−δ-based perovskite materials with different barium and iron contents are reported as triple conducting oxides (TCOs), which may conduct three charge carriers: oxygen ions, protons and electrons/holes.     

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.     

The cation–anion co-exchange in CsPb1−xFex(Br1−yCly)3 nanocrystals prepared using a hot injection method

All inorganic perovskite nanocrystals (NCs) have wide practical applications for their remarkable optoelectronic properties. To obtain blue-emitting perovskites with high photoluminescence quantum yield and room-temperature ferromagnetism, CsPb_1−xFe_x(Br_1−yCl_y)₃ NCs were synthesized using a hot injection method. The effects of the cation–anion co-exchange on the structural, luminescent and magnetic properties of CsPbBr₃ NCs were studied by X-ray diffraction spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, field emission scanning electron microscopy, and vibrating sample magnetometer. The results indicated that there was cation–anion co-exchange in CsPb_1−xFe_x(Br_1−yCl_y)₃ NCs, while the band-edge energies and PLQY were mainly affected by the anion exchange. The ferromagnetism of CsPb_1−xFe_x(Br_1−yCl_y)₃ NCs had been observed at room temperature, and there was an increase in saturation magnetization with increasing Fe concentration.

All inorganic perovskite nanocrystals (NCs) have wide practical applications for their remarkable optoelectronic properties.     

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.     

Characterization of NaxLi0.67+yNi0.33Mn0.67O2 as a positive electrode material for lithium-ion batteries

The relationship between the charge–discharge properties and crystal structure of Na_xLi_0.67+yNi_0.33Mn_0.67O₂ (0.010 ≤ x ≤ 0.013, 0.16 ≤ y ≤ 0.20) has been investigated. Li/Na_xLi_0.67+yNi_0.33Mn_0.67O₂ cells exhibit gradually sloping initial charge and discharge voltage–capacity curves. The initial charge capacity increased from 171 mA h g⁻¹ for thermally-treated Na_0.15Li_0.51Ni_0.33Mn_0.67O₂ to 226 mA h g⁻¹ for Na_0.010Li_0.83Ni_0.33Mn_0.67O₂ with an increase in the Li content. The initial maximum discharge capacity was 252 mA h g⁻¹ in the case of Na_0.010Li_0.83Ni_0.33Mn_0.67O₂ between 4.8 and 2.0 V at a fixed current density of 15 mA g⁻¹ (0.06C) at 25 °C. The predominance of the spinel phase leads to the high initial discharge capacity of Na_0.010Li_0.83Ni_0.33Mn_0.67O₂. This study shows that chemical lithiation using LiI is effective to improve the electrochemical properties.

The relationship between the charge–discharge properties and crystal structure of Na_xLi_0.67+yNi_0.33Mn_0.67O₂ (0.010 ≤ x ≤ 0.013, 0.16 ≤ y ≤ 0.20) has been investigated.     

First-principles study of thermoelectric properties of Mg2Si–Mg2Pb semiconductor materials

Mg₂X^IV (X^IV = Si, Ge, Sn) compounds are semiconductors and their solid solutions are believed to be promising mid-temperature thermoelectric materials. By contrast, Mg₂Pb is a metal and few studies have been conducted to investigate the thermoelectric properties of Mg₂Si–Mg₂Pb solid solutions. Here, we present a theoretical study exploring whether Mg₂Pb–Mg₂Si solid solutions can be used as thermoelectric materials or not. We firstly constructed several Mg₂Si_1−xPb_x (0 ≤ x ≤ 1) structures and calculated their electronic structures. It is suggested that Mg₂Si_1−xPb_x are potential thermoelectric semiconductors in the range of 0 ≤ x ≤ 0.25. We then explicitly computed the electron relaxation time and both the electronic and lattice thermal conductivities of Mg₂Si_1−xPb_x (0 ≤ x ≤ 0.25) and studied the effect of Pb concentration on the Seebeck coefficient, electrical conductivity, thermal conductivity, and thermoelectric figure of merit (ZT). At low Pb concentration (x = 1/16), the ZT of the Mg₂Si_1−xPb_x solid solutions (up to 0.67 at 900 K) reaches a maximum and is much higher than that of Mg₂Si.

Thermoelectric figure of merit of Mg₂Si_1−xPb_x solid solutions as a function of temperature.     

Enhanced thermoelectric properties in N-type Mg2Si0.4−xSn0.6Sbx synthesized by alkaline earth metal reduction

Mg₂Si_1−xSn_x-based compounds have been recognized as promising thermoelectric materials owing to their high figure-of-merit ZTs, abundance of raw constituent elements and nontoxicity. However, further improvement in the thermoelectric performance in this type of material is still constrained by the high thermal conductivity. In this work, we prepared a series of representative Mg₂Si_0.4−xSn_0.6Sb_x (x = 0, 0.0075, 0.008, 0.009, 0.01, 0.011) samples via the alkaline earth metal reduction method through a combination of ball milling and spark plasma sintering (SPS) processes. The samples featured many dislocations at the grain boundaries and plenty of nanoscale-coherent Mg₂Si–Mg₂Sn spinodal phases; both of which can effectively scatter heat-carrying phonons and have nearly no impact on the carrier transport. Meanwhile, Sb-doping can efficiently optimize the carrier concentration and significantly suppress the bipolar effects. As a result, a maximal ZT of 1.42 at 723 K and engineering (ZT)eng of 0.7 are achieved at the optimal Sb-doping level of x = 0.01. This result indicates that the alkaline earth metal reduction method could be an effective route to engineer phonon transport and improve the thermoelectric performance in Mg₂Si_1−xSn_x-based materials.

Sb doped Mg₂Si_0.4Sn_0.6 materials feature with lots of dislocations at grain boundaries and plenty of nanoscale Mg₂Si–Mg₂Sn spinodal phases, both of which can scatter heat carrying phonons and suppress the bipolar effects, with a optimal ZT of 1.42.     

In situ electrochemical investigation of the reaction progress between Zr and a CuCl–SnCl2 mixture in a LiCl–KCl molten salt

The electrochemical behaviors of CuCl, SnCl₂ and a CuCl–SnCl₂ mixture were investigated by cyclic voltammetry (CV) and square wave voltammetry (SWV). The reduction potentials of Cu(i) and Sn(ii) on CV curves are −0.49 and −0.36 V, respectively, while the reduction potentials of Cu(i)–Sn(ii) in the CuCl–SnCl₂ mixture almost overlap. The co-chlorination reaction progress between CuCl–SnCl₂ and Zr was also studied by monitoring the concentration changes of Cu(i), Sn(ii) and Zr(iv) ions in situ by CV, SWV and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analyses. The results indicate that during the reaction, the concentration of Zr(iv) ions increases gradually, while those of Cu(i) and Sn(ii) decrease rapidly until they disappear. When the molar ratios of Cu(i) to Sn(ii) are 1 : 1 and 1 : 0.5, the reaction between Cu(i) and Zr is faster but cannot exceed twice that of Sn(ii) and Zr in a short time. When the theoretical product of ZrCl₄ is a constant, and with the proportion of CuCl to SnCl₂ decreasing from 1 : 0 to 0 : 1, the chlorination reaction time periods increase from 40 to 170 min. Chloride products such as Cu_xSn_y, Sn_xZr_y, and Cu_xZr_y, are formed with different molar ratios. The coupling effect caused by the formation of alloys will promote the chlorination reaction when the ratios of CuCl to SnCl₂ are 0.66 : 0.17 and 0.5 : 0.25. The results provide a theoretical basis for the electrolytic refinement of zirconium.

A LiCl–KCl–ZrCl₄ melt was prepared via a co-chlorination reaction between a binary mixture of CuCl–SnCl₂ and Zr, and the reaction progress was electrochemically monitored.     

Significant enhancement of dielectric permittivity and percolation behaviour of La2−xSrxNiO4/poly(vinylidene fluoride) composites with different Sr doping concentrations

The percolation behaviour and dielectric properties of La_2−xSr_xNiO₄ (LSNO)/poly(vinylidene fluoride) (PVDF) composites with different Sr doping concentrations were investigated. The semiconducting LSNO filler particles with x = 0.2 (LSNO-1) and x = 0.4 (LSNO-2) were prepared using a chemical combustion method. The microstructures, thermal properties, and phase compositions of the polymer composites and filler particles were systematically investigated. The conductivity of the LSNO fillers increased with the Sr content and had an important impact on the dielectric properties of the LSNO/PVDF composites. The percolation threshold of the LSNO-2/PVDF composite was lower than that of the LSNO-1/PVDF composite. An ultra-high dielectric permittivity (ε′) of 3384.7 (at 1 kHz and room temperature), which was approximately 340 times higher than that of pure PVDF, was obtained for the LSNO-2/PVDF composite with a filler volume fraction of 25 vol%. The enhanced dielectric properties were attributed to interfacial polarisation at the semiconductor–insulator interface, a micro-capacitor model, and the intrinsically remarkable dielectric properties of the LSNO ceramic.

The percolation behaviour and dielectric properties of La_2−xSr_xNiO₄ (LSNO)/poly(vinylidene fluoride) (PVDF) composites with different Sr doping concentrations were investigated.     

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.     

Enhanced osteogenic activity of Ti alloy implants by modulating strontium configuration in their surface oxide layers

To guarantee the long-term stability of an orthopaedic implant, non-degradable surface coatings with the ability to selectively release bioactive drugs or ions are especially desirable. In this study, SrO–TiO₂ composite coatings were deposited on the surface of Ti alloys, whose release behavior of bioactive Sr ions was modulated by the Sr configurations, either interstitial atoms in solid solution (Ti_ySr_2−2yO₂) or strontium titanate (SrTiO₃). A perfect linear relationship between the amount of the released Sr ions and the Sr content in the coating was observed. Among the SrO-doped TiO₂ coatings, the 20% SrO–TiO₂ coating where Sr existed in both forms of Ti_ySr_2−2yO₂ and SrTiO₃ not only promoted proliferation of bone cells but also enhanced their osteogenic differentiation, which was proved to be related to its Sr release behavior. However, overdosing with 30% SrO only resulted in one single Sr configuration (SrTiO₃) and an inferior osteogenic function. This study suggests that Sr configurations of both interstitial atoms of the solid solution and SrTiO₃ can realize the selective release of Sr, but they possibly have different effects on the biological functions and other properties including corrosion resistance.

Strontium configurations can modulate its release in the SrO–TiO₂ coating system, thus being able to control the interfacial osteogenesis.     

Preparation,characterization and application in cobalt ion adsorption using nanoparticle films of hybrid copper–nickel hexacyanoferrate

Different mole ratios (n_Cu : n_Ni = x : y) of hybrid copper–nickel metal hexacyanoferrates (Cu_xNi_yHCFs) were prepared to explore their morphologies, structure, electrochemical properties and the feasibility of electrochemical adsorption of cobalt ions. Cyclic voltammetry (CV), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) indicated that the x : y ratio of Cu_xNi_yHCF nanoparticles can be easily controlled as designed using a wet chemical coprecipitation method. The crystallite size and formal potential of Cu_xNi_yHCF films showed an insignificant change when 0 ≤ x : y < 0.3. Given the shape of the CV curves, this might be due to Cu²⁺ ions being inserted into the NiHCF framework as countercations to maintain the electrical neutrality of the structure. On the other hand, crystallite size depended linearly on the x : y ratio when x : y > 0.3. This is because Cu tended to replace Ni sites in the lattice structure at higher molar ratios of x : y. Cu_xNi_yHCF films inherited good electrochemical reversibility from the CuHCFs, in view of the cyclic voltammograms; in particular, Cu₁Ni₂HCF exhibited long-term cycling stability and high surface coverage. The adsorption of Co²⁺ fitted the Langmuir isotherm model well, and the kinetic data can be well described by a pseudo-second order model, which may imply that Co²⁺ adsorption is controlled by chemical adsorption. The diffusion process was dominated by both intraparticle diffusion and surface diffusion.

Cu_xNi_yHCF films with appropriate Cu/Ni ratios are expected to be prepared as designed for the recovery of Co²⁺ from spent LIBs.     

Off-stoichiometric Na3−3xV2+x(PO4)3/C nanocomposites as cathode materials for high-performance sodium-ion batteries prepared by high-energy ball milling

Na₃V₂(PO₄)₃ (NVP) is regarded as a promising cathode material for sustainable energy storage applications. Here we present an efficient method to synthesize off-stoichiometric Na_3−3xV_2+x(PO₄)₃/C (x = 0–0.10) nanocomposites with excellent high-rate and long-life performance for sodium-ion batteries by high-energy ball milling. It is found that Na_3−3xV_2+x(PO₄)₃/C nanocomposites with x = 0.05 (NVP-0.05) exhibit the most excellent performance. When cycled at a rate of 1C in the range of 2.3–3.9 V, the initial discharge capacity of NVP-0.05 is 112.4 mA h g⁻¹, which is about 96% of its theoretical value (117.6 mA h g⁻¹). Even at 20C, it still delivers a discharge capacity of 92.3 mA h g⁻¹ (79% of the theoretical capacity). The specific capacity of NVP-0.05 is as high as 100.7 mA h g⁻¹ after 500 cycles at 5C, which maintains 95% of its initial value (106 mA h g⁻¹). The significantly improved electrochemical performance of NVP-0.05 is attributed to the decrease of internal resistance and increase of the Na⁺ ion diffusion coefficient.

Na₃V₂(PO₄)₃ (NVP) is regarded as a promising cathode material for sustainable energy storage applications.     

Thermodynamics of the double sulfates Na2M2+(SO4)2·nH2O (M = Mg,Mn, Co,Ni, Cu,Zn, n = 2 or 4) of the blödite–kröhnkite family

The double sulfates with the general formula Na₂M²⁺(SO₄)₂·nH₂O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 2 or 4) are being considered as materials for electrodes in sodium-based batteries or as precursors for such materials. These sulfates belong structurally to the blödite (n = 4) and kröhnkite (n = 2) family and the M cations considered in this work were Mg, Mn, Co, Ni, Cu, Zn. Using a combination of calorimetric methods, we have measured enthalpies of formation and entropies of these phases, calculated their Gibbs free energies (Δ_fG°) of formation and evaluated their stability with respect to Na₂SO₄, simple sulfates MSO₄·xH₂O, and liquid water, if appropriate. The Δ_fG° values (all data in kJ mol⁻¹) are: Na₂Ni(SO₄)₂·4H₂O: −3032.4 ± 1.9, Na₂Mg(SO₄)₂·4H₂O: −3432.3 ± 1.7, Na₂Co(SO₄)₂·4H₂O: −3034.4 ± 1.9, Na₂Zn(SO₄)₂·4H₂O: −3132.6 ± 1.9, Na₂Mn(SO₄)₂·2H₂O: −2727.3 ± 1.8. The data allow the stability of these phases to be assessed with respect to Na₂SO₄, MSO₄·mH₂O and H₂O(l). Na₂Ni(SO₄)₂·4H₂O is stable with respect to Na₂SO₄, NiSO₄ and H₂O(l) by a significant amount of ≈50 kJ mol⁻¹ whereas Na₂Mn(SO₄)₂·2H₂O is stable with respect to Na₂SO₄, MnSO₄ and H₂O(l) only by ≈25 kJ mol⁻¹. The values for the other blödite–kröhnkite phases lie in between. When considering the stability with respect to higher hydrates, the stability margin decreases; for example, Na₂Ni(SO₄)₂·4H₂O is still stable with respect to Na₂SO₄, NiSO₄·4H₂O and H₂O(l), but only by ≈20 kJ mol⁻¹. Among the phases studied and chemical reactions considered, the Na–Ni phase is the most stable one, and the Na–Mn, Na–Co, and Na–Cu phases show lower stability.

The double sulfates with the general formula Na₂M²⁺(SO₄)₂·nH₂O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 2 or 4) are being considered as materials for electrodes in sodium-based batteries or as precursors for such materials.