Kirill V. Yusenko

The scandium effect in multicomponent alloys

Despite its excellent elemental properties, lightweight nature and good alloying potential, scandium has received relatively little attention in the manufacturing community. The abundance of scandium in the Earths crust is quite high. It is more abundant than silver, cobalt, lead and tin. But, because scandium is so well dispersed in the lithosphere, it is notoriously difficult to extract in commercial quantities – hence low market availability and high cost. Scandium metallurgy is still a largely unexplored field – but progress is being made. This review aims to summarise advances in scandium metallurgical research over the last decade. The use of scandium as a conventional minor addition to alloys, largely in structural applications, is described. Also, more futuristic functional applications are discussed where details of crystal structures and peculiar symmetries are often of major importance. This review also includes data obtained from more obscure sources (especially Russian publications) which are much less accessible to the wider community. It is clear that more fundamental research is required to elevate the status of scandium from a laboratory-based curiosity to a mainstream alloying element. This is largely uncharted territory. There is much to be discovered.

Thermal decomposition of ammonium hexachloroosmate

Structural changes of (NH4)2[OsCl6] occurring during thermal decomposition in a reduction atmosphere have been studied in situ using combined energy-dispersive X-ray absorption spectroscopy (ED-XAFS) and powder X-ray diffraction (PXRD). According to PXRD, (NH4)2[OsCl6] transforms directly to metallic Os without the formation of any crystalline intermediates but through a plateau where no reactions occur. XANES and EXAFS data by means of Multivariate Curve Resolution (MCR) analysis show that thermal decomposition occurs with the formation of an amorphous intermediate {OsCl4}x with a possible polymeric structure. Being revealed for the first time the intermediate was subjected to determine the local atomic structure around osmium. The thermal decomposition of hexachloroosmate is much more complex and occurs within a minimum two-step process, which has never been observed before.

Structure and magnetic property of potassium intercalated pentacene: observation of superconducting phase in KxC22H14

We report the results from systematic investigations on the structure and magnetic properties of potassium intercalated pentacene as a function of potassium content, K x C22H14 (1  ⩽  x  ⩽  3). Synchrotron radiation powder x-ray diffraction technique revealed that there are two different stable phases can be obtained via potassium intercalation, namely, K1C22H14 phase and K3C22H14 phase. Structural phase transition was induced when the potassium content was increased to the nominal value x  =  3. This phase transition is accompanied by drastic change in their magnetic property, where those samples with compositions K1C22H14 shows ferromagnetic behavior and those with near K3C22H14 lead to observation of superconductivity with transition temperature, T c, of 4.5 K. It is first time that superconductivity was observed in linear oligoacenes. Both magnetization study and synchrotron radiation powder x-ray diffraction clearly indicates that the superconducting phase belong to K3C22H14 as a result of phase transition from triclinic to monoclinic structure induced by chemical doping.

Synthesis of Ir1-xRex (0.15 ≤ x ≤ 0.40) solid solutions under high-pressure and high-temperature

Abstract Ir1–xRex (0.15 ≤ x ≤ 0.40) phases prepared under high-pressure, high-temperature conditions from nanopowders of iridium and rhenium were characterized using powder X-ray diffraction and scanning electron microscopy. Structural characteristics of the phases obtained were identical with the corresponding parameters for the solid solutions prepared by means of melting and thermal decomposition of the precursors. The data obtained make it possible to improve solid-state solubility limits in the binary Ir–Re phase diagram. As revealed, the maximum solid solubility of Ir in Re is 68 at.%, and that of Re in Ir is 20 at.%.

Scandium-Based Hexagonal-Closed Packed Multi-Component Alloys

Since their early development, High-Entropy Alloys have fueled the investigation of exotic metal combinations. Here, we present a strategy for the rational design of a library for multi-component alloys based on six hcp-structured metals. Seven five- and six-component equimolar alloys based on Co, Gd, Y, Sc, Ti and Zr were prepared via induction melting and characterized by PXRD, SEM–EDX and Vickers hardness. They all present ternary hexagonal phases (ScTiZr or GdScY) co-existing with one or more cubic phases and intermetallic compounds. Both ScTiZr and GdScY appear promising as the starting point for new single-phase High-Entropy Alloys families.

Exothermal effects in the thermal decomposition of [IrCl6]2−-containing salts with [M(NH3)5Cl]2+ cations: [M(NH3)5Cl][IrCl6] (M = Co, Cr, Ru, Rh, Ir)

[M(NH3)5Cl][IrCl6], M = Co, Cr, Ru, Rh, and Ir, were proposed as single-source precursors for bimetallic alloys. Their thermal decomposition in inert and reductive atmospheres below 700 °C results in the formation of nanostructured porous Ir0.5M0.5 alloys. Salts decompose with a significant exothermal effect during the first stage of their thermal breakdown in an inert atmosphere above 200 °C. The exothermal effect gradually decreases in the series: [Co(NH3)5Cl][IrCl6] (1) > [Cr(NH3)5Cl][IrCl6] (2) > [Ru(NH3)5Cl][IrCl6] (3) > [Rh(NH3)5Cl][IrCl6] (4); [Ir(NH3)5Cl][IrCl6] (5) does not exhibit any thermal effects and decomposes at much higher temperatures. To shed light on their thermal decomposition and the nature of the exothermal effect, DSC–EGA, in situ and ex situ IR, Raman, XPS and XAFS studies were performed. A combination of complementary techniques suggests a simultaneous ligand exchange and a reduction of central atoms as key processes. In [Co(NH3)5Cl][IrCl6], Co(III) and Ir(IV) simultaneously oxidise coordinated ammonia, which can be detected due to a significant exothermal effect and the presence of Co(II) and Ir(III) in the intermediate product. The appearance of Ir–N frequencies demonstrates a ligand exchange between cations and the [IrCl6]2− anion. Salts with Cr(III), Ru(III), and Rh(III) show a much lower exothermal effect due to the stability of their oxidation states. Salts with Rh(III) and Ir(III) demonstrate a high thermal stability and a low tendency for ligand exchange as well as decomposition with exothermal effects.

A new approach towards the study of thermal decomposition and formation processes of nanoalloys: the double complex salt [Pd(NH3)4][PtCl6]

A new approach based on a combination of synchrotron radiation techniques, such as X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), hard X-ray photoelectron spectroscopy (HAXPES), and powder X-ray diffraction (PXRD), has been applied to in situ study the processes of thermal decomposition of inorganic compounds and the formation of bimetallic nanoalloys. As an example, a double complex salt, [Pd(NH3)4][PtCl6], was selected because of (i) its sufficiently complicated structure and previous studies conducted via thermal analysis, ex situ PXRD and XPS, and (ii) its use as a prospective single-source precursor for the preparation of bimetallic (PdPt) nanoparticles or nanoalloys. The differences between the mechanisms based on ex situ and in situ data have been discussed for the first time. It has been found that the first step of thermal decomposition is related to the formation of crystalline [Pd(NH3)2Cl2][Pt(NH3)2Cl4]. Further decomposition results in the formation of {PdCl2}, {PtCl2}, and (NH4)2[PtCl6] in the second step. In the final step, the intermediates are completely reduced, and a bimetallic nanoalloy is formed. The different means of Pd and Pt reduction on the surface and in the bulk result in the formation of a disordered nanoalloy with possible monometallic inclusions. Further heating orders the nanoalloy that is accompanied by a decrease in the positive charge on Pt.

The Effect of Scandium Ternary Intergrain Precipitates in Al-Containing High-Entropy Alloys

We investigate the effect of alloying with scandium on microstructure, high-temperature phase stability, electron transport, and mechanical properties of the Al2CoCrFeNi, Al0.5CoCrCuFeNi, and AlCoCrCu0.5FeNi high-entropy alloys. Out of the three model alloys, Al2CoCrFeNi adopts a disordered CsCl structure type. Both of the six-component alloys contain a mixture of body-centered cubic (bcc) and face centered cubic (fcc) phases. The comparison between in situ high-temperature powder diffraction data and ex situ data from heat-treated samples highlights the presence of a reversible bcc to fcc transition. The precipitation of a MgZn2-type intermetallic phase along grain boundaries following scandium addition affects all systems differently, but especially enhances the properties of Al2CoCrFeNi. It causes grain refinement; hardness and electrical conductivity increases (up to 20% and 14% respectively) and affects the CsCl-type → fcc equilibrium by moving the transformation to sensibly higher temperatures. The maximum dimensionless thermoelectric figure of merit (ZT) of 0.014 is reached for Al2CoCrFeNi alloyed with 0.3 wt.% Sc at 650 °C.

An Alternative Route to Pentavalent Postperovskite

Two different high-pressure and -temperature synthetic routes have been used to produce only the second-known pentavalent CaIrO3-type oxide. Postperovskite NaOsO3 has been prepared from GdFeO3-type perovskite NaOsO3 at 16 GPa and 1135 K. Furthermore, it has also been synthesized at the considerably lower pressure of 6 GPa and 1100 K from a precursor of hexavalent Na2OsO4 and nominally pentavalent KSbO3-like phases. The latter synthetic pathway offers a new lower-pressure route to the postperovskite form, one that completely foregoes any perovskite precursor or intermediate. This work suggests that postperovskite can be obtained in other compounds and chemistries where generalized rules based on the perovskite structure may not apply or where no perovskite is known. One more obvious consequence of our second route is that perovskite formation may even mask and hinder other less extreme chemical pathways to postperovskite phases.

Structural modulations in multiferroic tetragonal tungsten bronze KxMnxFe1+xF3

Multiferroic materials showing coupling of the different order parameters (ferroelectric, ferromagnetic, ferroelastic) are interesting not only from a fundamental perspective, but also from a technological point of view, e.g. for to the development of new storage technologies. However, the coexistence of (ferro)magnetism and ferroelectricity is considered a rare phenomenon. Whilst this may be true for perovskite oxides, where empty d shells favor the off-centering of ions but counteract magnetism, this intrinsic limitation can be avoided by moving to different structure types, and/or away from oxides. An example of non-perovskite, non-oxide multiferroic systems are the tetragonal tungsten bronze (TTB) fluorides KxM>2+xM3+1−xF3 (x = 0.4 – 0.6), which show coexistence of electric and magnetic ordering 1. Here we present a detailed structural study on a series of TTB fluorides, KxMnxFe1−xF3 (x = 0.4 – 0.55). KMnFeF6 has been previously described as tetragonal P42bc and orders ferrimagnetically below T = 148 K 2. Additional satellite reflections were found in transmission electron microscopy experiments and attributed to ferroelastic domains arising from tilting of MF6 octahedra, but the reported bulk powder XRD measurements indicated only tetragonal symmetry 3. We used high-resolution powder diffraction techniques to reinvestigate the crystal structure as a function of temperature in comparison with DSC data. Our results reveal a structural distortion to orthorhombic symmetry (Ccc2) at room temperature, which diminished when moving to the end members of the series (x → 0.4 and x → 0.6). Although structurally subtle, this distortion may indicate a ferroelectric state, similar to KxFeF3, where ferroelectricity is observed only in the orthorhombic phase. On heating, an anomaly in the c-axis lattice parameter accompanies a phase transition to centrosymmetric P42/mbc around 320 – 350 K, marking the transition from ferroelectric – paraelectric state.