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Reducing the size of a material, from a bulk solid to a nanomaterial, may lead to drastic changes of various properties including reactivity and optical properties. Chemical reactivity is often inc...

Thin film samples of Ge9SnSb2Te9Se4, Ge4.5SnSb2Te4.5Se4, Ge2.5SnSb2Te2.5Se4, and GeSnSb2TeSe4 were prepared via co-sputtering of GeTe and SnSb2Se4 and compared to the well-investigated phase change material GeTe. All samples were obtained in an amorphous state. Temperature-dependent in situ X-ray diffraction experiments reveal a crystallization temperature that increases with an increasing SnSb2Se4 content, leading to a higher stability of the amorphous phase. The electrical contrast between the amorphous and metastable crystalline state investigated via the van der Pauw method is as large as 4 orders of magnitude up to a 4.5:1 GeTe:SnSb2Se4 ratio. Increasing the SnSb2Se4 content leads to a decrease in the electrical contrast. Investigations of the samples by applying Fourier transform infrared spectroscopy and variable incident angle spectroscopic ellipsometry show that the optical properties of the amorphous phase are not affected by changes in stoichiometry. In striking contrast, the impact of SnSb2Se4...

The need to identify lithium battery anodes consisting of new materials exhibiting high energy density and good cycling stability has intensified the research on reversible so-called conversion reactions between lithium and oxidic spinels such as, e.g., CoFe2O4. Operando nondestructive synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were applied to directly determine the complex reaction mechanisms occurring during discharge and charge processes. Distinct reaction mechanism steps related to different electrochemical features of nanosized CoFe2O4 as the anode material are elucidated. The different discharge steps include uptake of a small fraction of Li into an amorphous layer covering the particles, the transformation of the spinel structure into a NaCl-like structure, and the formation of nanosized metallic Co and Fe embedded in a Li2O matrix. The metals are oxidized to form Co2+ and Fe3+ during charging. The reaction Fe3+ ↔ Fe0 is observed for many discharge/charge processes,...

The quality and temperature stability of 1 nm Bi2Te3/5 nm Sb2Te3 superlattices prepared by molecular beam epitaxy for the first time was investigated by in situ and ex situ X-ray diffraction and transmission electron microscopy. Upon heating, the superlattice structures are not stable against interdiffusion of the components, with micro- and nanostructural changes occurring at temperatures as low as 200 °C. The interdiffusion preferably starts next to superlattice defects. At 300 °C the Bi2Te3 and Sb2Te3 layers were mostly interdiffused, forming the thermodynamically stable Sb1.66Bi0.33Te3 alloy. The data suggests that structural integrity of Bi2Te3/Sb2Te3 superlattices will not be stable for extended times above 200 °C, thus inhibiting application of such superlattices over a wide temperature range.

The opto-electronic properties of the Blu-ray phase-change material Ge8Sb2Te11 were investigated and compared to partially substituted compounds. Investigations were performed on Ge8Sb2Te6Se5 (I) and SnGe7Sb2Te7Se4 (II). To monitor the influence of substitution on both cation and anion sites onto the properties, in Ge8Sb2Te6Se5, only Te was substituted by Se, while in SnGe7Sb2Te7Se4, Ge also was partially replaced by Sn, yielding an equivalent degree of substitution. The crystallization of the amorphous compound to a rhombohedral phase was observed. The first transition temperature (Tc1) is 210 °C for I and 185 °C for II, while the second transition temperature (Tc2) cannot be clearly determined, because of the coexistence of both phases. Thin-film properties were examined and an increase of density and roughness was observed for both materials upon crystallization. In situ electrical resistance measurements show a huge electrical contrast between the amorphous samples and the crystalline samples, which a...

The ternary compounds GeSb2Te4 and Ge2Sb2Te4 are promising phase change materials for application in the fields of data storage and recording. In the present study, thin films of the materials were deposited on inert substrates using the magnetron sputter technique. The microstructures of the amorphous, metastable, and stable crystalline states were investigated by X-ray powder diffractometry and high resolution transmission electron microscopy (HRTEM). In the as-deposited state, the materials are amorphous. Upon heating, they crystallize first into a metastable phase with a NaCl-type structure, followed by a transformation into a layered structure (stable phase) at higher temperatures. HRTEM studies reveal that nanosized particles with a granular morphology are present in all materials for the crystalline metastable state. In the case of GeSb2Te4, these nanocrystals do not grow significantly when the sample transforms to the stable phase, which contains layers of structural vacancy. Ge2Sb2Te4 behaves dif...

Formation of nanostructured hexagonal alkali tungstates (HTBs) from ammonium metatungstate (AMT) and alkali chlorides has been investigated by a combined strategy encompassing in situ spectroscopic techniques and hydrothermal synthetic studies. They demonstrate how the morphology of the HTBs can be tuned by the straightforward hydrothermal reaction of ammonium metatungstate with the choice of appropriate alkali chlorides. The resulting particle shapes vary from highly anisotropic fibers to multilevel assemblies of hierarchically grown nanorods. Microwave-hydrothermal techniques have further expanded this morphological spectrum. Next, the growth process of alkali HTBs was explored for selected cases by combination of in situ EXAFS and EDXRD investigations. They both show that nucleation starts after a certain induction time and is strongly dependent on the concentration. Though the individual kinetics of alkali HTB formation differ slightly from each other, all alkali cations have a nucleation-controlled g...

The intercalation reaction of Li into the complex ternary selenide Cr4TiSe8 was investigated as function of time and temperature. The treatment of the host compound with a solution of butyl lithium in n-hexane (BuLi) leads to Li insertion up to the composition Li2.8Cr4TiSe8. The intercalation reaction is accompanied by a structural phase transition of the host material. The monoclinic symmetry of pristine Cr4TiSe8 is changed to trigonal when x in LixCr4TiSe8 exceeds approximately 0.4. With increasing Li content the axes of the new trigonal unit cell are significantly enlarged. The structural phase transition was characterized by Rietveld refinement of X-ray (XRD) powder patterns for different values of x. The phase transition is electronically driven rather than being due to geometric reasons. Immediately after removal of the excess BuLi of partially intercalated phases, the product is not in equilibrium and undergoes a structural relaxation. When the lithiated products are treated with water, the interca...

The crystal growth and kinetics of the formation of [Co(C6H18N4)][Sb2S4] were investigated under solvothermal conditions with combined in situ EDXRD (energy dispersive X-ray diffraction) and in situ EXAFS (extended X-ray absorption fine structure) at different temperatures. In the overwhelming number of reactions all product reflections occur simultaneously in the EDXRD spectra. A detailed analysis of the extent of reaction α vs time clearly shows that the mechanism changes during the reaction. Such a change occurs at different temperatures after different reaction times. At the early stages the reactions are controlled by diffusion, and later the reaction exponents m suggest a more complex behavior. At the end of the formation of [Co(C6H18N4)][Sb2S4] a second phase is formed which does not contain Co. The results of the in situ experiments suggest that [Co(C6H18N4)][Sb2S4] is not only a metastable phase but rather that the second product crystallizes because the Co2+ concentration in solution is too low ...

The five quaternary compounds Ba3Gd2[P4S16] (1), Ba3Tb2[P4S16] (2), Ba3Dy2[P4S16] (3), Ba3Ho2[P4S16] (4), and Ba3Er2[P4S16] (5) were synthesized at elevated temperatures in carbon-coated quartz tubes. The isostructural compounds crystallize in the orthorhombic space group Pbcn (No. 60), Z = 4, with a = 19.127(3) A, b = 10.2795(5) A, and c = 12.648(2) A for 1; a = 19.113(2) A, b = 10.261(2) A, and c = 12.616(2) A for 2; a = 19.112(2) A, b = 10.2646(8) A, and c = 12.602(2) A for 3; a = 19.088(3) A, b = 10.248(2) A, and c = 12.565(2) A for 4; and a = 19.078(3) A, b = 10.241(2) A, and c = 12.568(2) A for 5.The structures are composed of one-dimensional infinite helical anionic [Ln2(PS4)4]n6n- chains and charge-compensating Ba2+ ions. The Ln atoms are surrounded by four [PS4]3- tetrahedra, leading to a strong distorted LnS8 rectangular antiprism. Three of the anions act as tetradentate ligands, and one is bidentate. Three Ln3+ centers are joined via one tetradentate [PS4]3- ligand, and this connection mode yie...

The solvothermal synthesis of the layered compound Mn2Sb2S5·DAP (DAP = 1,3-diaminopropane) has been studied using in situ energy-dispersive X-ray diffraction. The results clearly demonstrate that the induction time strongly depends on the reaction temperature. At lower temperatures two crystalline intermediates could be detected. The decay of the second intermediate and the product growth show a strong correlation. A detailed analysis of the extent of reaction α suggests that a small fraction of the intermediate is either dissolved or converted into an amorphous state. The experimentally determined extent of reaction α versus time was compared with various theoretical models. The reaction exponents indicate similar mechanisms for temperatures between 105 and 125 °C. The best agreement is obtained with a first-order reaction and/or phase-boundary-controlled mechanisms. A rigorous analysis reveals that it is highly likely that with increasing α the mechanism changes, suggesting consecutive and/or parallel k...

The new compounds K4Nb2Se11O and Cs4Nb2Se11O have been synthesized from a K2Sen and Cs2Sen flux, respectively. K4Nb2Se11O crsytallizes in the orthorhombic space group Pbca with lattice parameters a...