Mechanochemical Synthesis and Magnetic Characterization of Nanosized Cubic Spinel FeCr₂S₄ Particles

Nanosized samples of the cubic thiospinel FeCr2S4 were synthesized by ball milling of FeS and Cr2S3 precursors followed by a distinct temperature treatment between 500 and 800 °C. Depending on the applied temperature, volume weighted mean (Lvol) particle sizes of 56 nm (500 °C), 86 nm (600 °C), and 123 nm (800 °C) were obtained. All samples show a transition into the ferrimagnetic state at a Curie temperature TC of ∼ 167 K only slightly depending on the annealing temperature. Above TC, ferromagnetic spin clusters survive and Curie–Weiss behavior is observed only at T ≫ TC, with T depending on the heat treatments and the external magnetic field applied. Zero-field-cooled and field-cooled magnetic susceptibilities diverge significantly below TC in contrast to what is observed for conventionally solid-state-prepared polycrystalline samples. In the low-temperature region, all samples show a transition into the orbital ordered state at about 9 K, which is more pronounced for the samples heated to higher temperatures. This observation is a clear indication that the cation disorder is very low because a pronounced disorder would suppress this magnetic transition. The unusual magnetic properties of the samples at low temperatures and different external magnetic fields can be clearly related to different factors like structural microstrain and magnetocrystalline anisotropy

Comparative high-pressure investigations of Ag2ZnSnS4 and Ag2CdSnS4 compounds

Quaternary kesterite-type (KS) compounds have attracted worldwide attention from the scientific community as promising materials for solar cells. On the route to optimizing their performance, the effect of stress and strain constitutes a critical factor when it comes to thin film applications. Following a recent theoretical study, we report here joint experimental and computational high-pressure investigations on the KS Ag2ZnSnS4 and wurtz–kesterite (WZ–KS)-type Ag2CdSnS4 compounds. Our results reveal that both materials undergo successive transformations, first into a GeSb-type and then toward a CrN-type modification at ambient temperature. Our theoretical calculations predict a metallic character for all Ag2ZnSnS4 and Ag2CdSnS4 high-pressure phases. In addition, structural disorder is observed in KS Ag2ZnSnS4 upon moderate compression, prior to its KS → GeSb-type transition. Decompression leads to the recovery of a disordered zinc blende-type structure in the latter, whereas Ag2CdSnS4 retains the disordered GeSb-type modification. The similarities and deviations from the archetypical KS Cu2ZnSnS4 are discussed

Na2MgSnS4 – a new member of the A(2)(I)B(II)C(IV)X(4) family of compounds

A new member of the A2IBIICIVX4 compound family, Na2MgSnS4, has been synthesized by ball milling of the binary sulfides SnS, Na2S, and MgS with elemental sulfur in a high-energy planetary mill, followed by annealing in an atmosphere of H2S (T = 600 °C/3 h). Na2MgSnS4 adopts the NaCrS2-type structure (rhombohedral, space group R¯3m) with a = 3.7496(11) and c = 19.9130(6) Å. The Na atoms occupy Wyckoff position 3b, whereas the Mg and Sn atoms are statistically distributed on the cation sites 3a; all cations are surrounded by six sulfur atoms

High-Pressure Behavior and Disorder for Ag2ZnSnS4 and Ag2CdSnS4

We carried out first-principles calculations to simulate Ag2ZnSnS4 and Ag2CdSnS4 and calculated enthalpies of different plausible structural models (kesterite-type, stannite-type, wurtzkesterite-type, wurtzstannite-type, and GeSb-type) to identify low- and high-pressure phases. For Ag2ZnSnS4, we predict the following transition: kesterite-type→[8.2GPa]→ GeSb-type. At the transition pressure, the electronic structure changes from semiconducting to metallic. For Ag2CdSnS4, we cannot decide which of the experimentally observed structures (kesterite-type or wurtzkesterite-type) is the ground-state structure because their energy difference is too small. At 4.7 GPa, however, we predict a transition to the GeSb-type structure with metallic character for both structures. Regarding the sensitivity of the material to disorder, a major drawback for solar cell applications, Ag2CdSnS4 behaves similar to Cu2ZnSnS4, both showing a high tendency to cationic disorder. In contrast, the disordered structures in Ag2ZnSnS4 are much higher in energy, and therefore, the material is less affected by disorder

New compounds of the Li2MSn3S8 type

The substitution of Cu/Ag by lithium in complex thiospinels with the general formula AI2BIICIV3XVI8 was achieved by ball milling and a subsequent annealing step in an atmosphere of H2S. Four hitherto unknown compounds Li2MSn3S8 with M = Mg, Mn, Fe, Ni were obtained without side phases and have been structurally investigated. From X-ray powder diffraction experiments, space group Fd¯3m and a spinel-type structure are suggested. In these so-called normal spinels, lithium occupies one eighth of the tetrahedral voids (Wyckoff position 8a) of the cubic closest packing of the sulfide ions whereas M and Sn can be found on one half of the octahedral voids (Wyckoff position 16d)

Crystal structure of mechanochemically synthesized Ag2CdSnS4

Ag2CdSnS4 was synthesized by a two step mechanochemical synthesis route. From a detailed analysis of the observed reflections in the X-ray powder diffraction pattern, the crystal structure proposed in the literature (space group Cmc21 [E. Parthé, K. Yvon, R. H. Deitch, Acta Crystallogr.1969, B25, 1164–1174; O. V. Parasyuk, I. D. Olekseyuk, L. V. Piskach, S. V. Volkov, V. I. Pekhnyo, J. Alloys Compd.2005, 399, 173–177]) is questionable. Our structural investigations presented in this contribution point to the fact that Ag2CdSnS4 crystallizes in the monoclinic wurtzkesterite-type structure (space group Pn). At around T = 200°C, a phase transition to the orthorhombic wurtzstannite-type structure (space group Pmn21) is observed

Calculation for High Pressure Behaviour of Potential Solar Cell Materials Cu2FeSnS4 and Cu2MnSnS4

Exploring alternatives to the Cu2ZnSnS4 kesterite solar cell absorber, we have calculated first principle enthalpies of different plausible structural models (kesterite, stannite, P4¯ and GeSb type) for Cu2FeSnS4 and Cu2MnSnS4 to identify low and high pressure phases. Due to the magnetic nature of Fe and Mn atoms we included a ferromagnetic (FM) and anti-ferromagnetic (AM) phase for each structural model. For Cu2FeSnS4 we predict the following transitions: P4¯ (AM) −→−−−−16.3GPa GeSb type (AM) −→−−−−23.0GPa GeSb type (FM). At the first transition the electronic structure changes from semi-conducting to metallic and remains metallic throughout the second transition. For Cu2MnSnS4, we predict a direct AM (kesterite) to FM (GeSb-type) transitions at somewhat lower pressure (12.1 GPa). The GeSb-type structure also shows metallic behaviour

Crystal structure of mechanochemically prepared Ag2FeGeS4

Ag2FeGeS4 was synthesized as a phase-pure and highly crystalline product by mechanochemical milling from the binary sulfides and iron metal, followed by annealing in H2S atmosphere. The structure evaluation was carried out using X-ray powder diffraction with subsequent Rietveld refinements. As Fe and Ge atoms are not distinguishable using conventional X-ray methods, the chalcopyrite-type structure (space group I¯42d ), exhibiting a statistical distribution of Fe and Ge on Wyckoff position 4b, was considered. However, quantum-chemical calculations at hybrid density-functional level indicate that mechanochemically prepared Ag2FeGeS4 crystallizes in the kesterite-type structure (space group I¯4 ) where the cations are arranged in an ordered way. Ag2FeGeS4 is a further example of a mechanochemically prepared compound differing structurally from the commonly known polymorph exhibiting the stannite type (solid-state route)

Mechanochemical Synthesis and Magnetic Characterization of Nanosized Cubic Spinel FeCr2S4 Particles

Nanosized samples of the cubic thiospinel FeCr2S4 were synthesized by ball milling of FeS and Cr2S3 precursors followed by a distinct temperature treatment between 500 and 800 °C. Depending on the applied temperature, volume weighted mean (Lvol) particle sizes of 56 nm (500 °C), 86 nm (600 °C), and 123 nm (800 °C) were obtained. All samples show a transition into the ferrimagnetic state at a Curie temperature TC of ∼ 167 K only slightly depending on the annealing temperature. Above TC, ferromagnetic spin clusters survive and Curie–Weiss behavior is observed only at T ≫ TC, with T depending on the heat treatments and the external magnetic field applied. Zero-field-cooled and field-cooled magnetic susceptibilities diverge significantly below TC in contrast to what is observed for conventionally solid-state-prepared polycrystalline samples. In the low-temperature region, all samples show a transition into the orbital ordered state at about 9 K, which is more pronounced for the samples heated to higher temperatures. This observation is a clear indication that the cation disorder is very low because a pronounced disorder would suppress this magnetic transition. The unusual magnetic properties of the samples at low temperatures and different external magnetic fields can be clearly related to different factors like structural microstrain and magnetocrystalline anisotropy

Time-retrenched synthesis of BaTaO2N by localizing an NH3 delivery system for visible-light-driven photoelectrochemical water oxidation at neutral pH: Solid-state reaction or flux method?

Among 600 nm class transition-metal oxynitrides, BaTaO2N with a cubic Pm3̅m perovskite-type structure is promising for solar water oxidation due to its absorption of visible light up to 660 nm, narrower band gap (Eg = 1.9 eV), appropriate valence band edge position for oxygen evolution, good stability in concentrated alkaline solutions, and nontoxicity. However, high defect density stemmed from long high-temperature ammonolysis limits the separation and transfer efficiency of photogenerated charge carriers in BaTaO2N. Here, a NH3 delivery system is specifically localized just above the synthesis mixture to reduce the synthesis time and defect density of BaTaO2N by a fresh supply of more active nitriding species and minimizing the generation of N2 and H2. Particularly, the effects of synthesis temperature (700–950 °C), synthesis time (1–8 h), and gas composition are systematically investigated to gain insights into the formation of single-phase BaTaO2N by solid-state reaction and flux method. Time-dependent experiments conducted at 950 °C show that single-phase BaTaO2N can be synthesized ≥6 and ≥4 h by solid-state reaction and flux method, respectively, revealing the advantage of the flux method over solid-state reaction in a localized NH3 delivery system. Subsequently, the separation and transfer efficiency and kinetics of photogenerated charge carriers are studied in BaTaO2N samples. Photoelectrochemical studies made it possible to resolve trends during visible-light-induced water oxidation, evidencing the inverse relationship between recombination and charge transfer phenomena. Transient absorption spectroscopy reveals that the dynamics of the photogenerated charge carriers in both types of BaTaO2N samples are different: (i) BaTaO2N synthesized by flux method has a greater number of holes despite the similar number of deeply trapped charge carriers and (ii) solid-state reaction led to the formation of a higher number of free electrons in BaTaO2N. The findings demonstrate the advantage of reducing the transfer distance of active nitriding species to the surface of the synthesis mixture for enhancing the photoelectrochemical water oxidation of BaTaO2N at neutral pH.EC/H2020/793882/EU/Carbon-Oxynitride Coupled Artificial Photosynthesis System For Solar Water Splitting Beyond 600 nm/H2O-SPLI