Effects of 4d transition metal Pd doping on the magnetic and superconducting properties of 112-type iron pnictide EuFeAs2

The recently discovered 112-type EuFeAs2 compound that shows complex Eu-spin magnetism provides a new platform to study the interplay between superconductivity (SC) and magnetism in iron pnictide superconductors. In this paper, by substituting Fe with the 4d transition metal Pd, we have successfully synthesized a series of EuFe1-xPdxAs2 (0 ≤ x ≤ 0.08) samples and studied the doping effect on SC and magnetism by means of electrical transport and magnetization measurements. The systematic evolution of the lattice parameters indicates that the Pd atoms have been successfully substituted into the Fe sites. With Pd doping, the Fe-related spin density wave (SDW) transition at TFe m in the parent phase is rapidly suppressed, and SC simultaneously emerges, exhibiting a domelike shape with a maximum onset transition temperature Tonset c of around 18.5 K at x = 0.04. On the other hand, the Eu-related AFM order at TEu m is suppressed very slowly and persists up to x = 0.08, covering the whole SC dome region. Also, the reentrant spin-glass and spin-reorientation transitions below TEu m remain unchanged with Pd doping. All these results indicate that the Eu spins have little effect on SC in EuFe1-xPdxAs2. Finally, based on the resistivity and magnetization data, the T-x phase diagram of EuFe1-xPdxAs2 is constructed and compared with those of 3d transition metals Co/Ni and La doped ones.

Influence of Batch Mass on Formation of NiTi Shape Memory Alloy Produced by High-Energy Ball Milling

A high-energy ball milling technique was used for production of the equiatomic NiTi alloy. The grinding batch was prepared in two quantities of 10 and 20 g. The alloy was produced using various grinding times. Scanning electron microscopy, X-ray diffraction, hardness measurement and differential scanning calorimetry were used for materials characterization at various milling stages. The produced alloy was studied by means of microstructure, chemical and phase composition, average grain and crystallite size, crystal lattice parameters and microstrains. Increasing the batch mass to 20 g and extending the grinding time to 140 h caused the increase in the average size of the agglomerates to 700 µm while the average crystallites size was reduced to a few nanometers. Microstrains were also reduced following elongation of milling time. Moreover, when the grinding time is extended, the amount of the monoclinic phase increases at the expense of the body-centered cubic one—precursors of crystalline, the B2 parent phase and the B19′ martensite. Crystallization takes place as a multistage process, however, at temperatures below 600 °C. After crystallization, the reversible martensitic transformation occurred with the highest enthalpy value—4 or 5 J/g after 120 and 140 h milling, respectively.

Microstructural Characteristic and Solid Particle Erosion Behavior of Plasma-Sprayed Ni60/Ni-WC Composite Coating

A Ni60/Ni-WC composite coating was fabricated by the plasma spraying technology and microstructure of the coating was analyzed. Moreover, erosion resistance of the coating under different erosion angles was tested. Results demonstrated that the coating has lamellar structures and contain some pores. WC particles distribute evenly in the coating and bond well with the parent phase. When the erosion angle increase, the weight loss of the erosion-induced coating increases firstly and then decreases, showing plastic-brittle composite erosion characteristics.

The wavevector star channel and symmetry group

The concepts of `wavevector star channel' and `wavevector star channel group' are newly defined, which allow the effective study of phase transitions considering directly the translational symmetry breaking in crystals. A method is suggested by which the wavevector star channels can be found using the image of the representation of the translational group. According to this method, the wavevector star channels are found for the 80 Lifschitz stars in the reciprocal lattice. The wavevector star channel group is defined as the set of symmetry elements of the parent phase which leave the star channel invariant, and the wavevector star channel groups with one, two, three and four arms are calculated. It is shown that the complicated symmetry changes in the pyroelectric crystal Pb1−x Ca x TiO3 (PCT) can be described using the new five-component reducible order parameter transformed according to the representation of the wavevector star channel group, rather than the nine-component one based on the theory of the full irreducible representation of the space group.

Alkali sulfates with aphthitalite-like structures from fumaroles of the Tolbachik volcano, Kamchatka, Russia. III. Solid solutions and exsolutions

ABSTRACT Six different exsolution types are found in crystals of aphthitalite-group alkali sulfates from exhalations of the active Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. The coexisting minerals in these exsolutions are metathénardite, ideally Na2SO4 (P63/mmc), and vanthoffite, Na6Mg(SO4)4 (P21/c) (Type I); metathénardite and belomarinaite, KNaSO4 (P3m1) (Type II); thénardite, Na2SO4 (Fddd), and aphthitalite, K3Na(SO4)2 (Pm1) (Type III); aphthitalite and arcanite, K2SO4 (Pnma) (Type IV); metathénardite and natroaphthitalite, KNa3(SO4)2 (Pm1) (Type V); and two chemical varieties of metathénardite (Type VI). The exsolution processes occur in crystals belonging to the high-temperature, hexagonal Na2SO4(I) (= metathénardite, P63/mmc) structure type with different K:Na ratios formed at temperatures higher than 500 °C. The similarity and hexagonal close-packed nature of the crystal structures of the coexisting phases, all representatives of aphthitalite-like structure types, cause the coherent conjugation of domains during diffusion and cation ordering in the parent phase. The breakdown of solid solution can be facilitated by the mosaic character of crystals of a parent phase (incoherent grain boundaries) and the presence of coherent twin boundaries. The heating of samples with exsolution Types II and V up to 700 °C over 24 h shows that diffusion of K and Na through the domain borders does not result in the complete disorder of these cations and the extinction of domains with different crystal structures.

Unified theory for light-induced halide segregation in mixed halide perovskites

Mixed halide perovskites that are thermodynamically stable in the dark demix under illumination. This is problematic for their application in solar cells. We present a unified thermodynamic theory for this light-induced halide segregation that is based on a free energy lowering of photocarriers funnelling to a nucleated phase with different halide composition and lower band gap than the parent phase. We apply the theory to a sequence of mixed iodine-bromine perovskites. The spinodals separating metastable and unstable regions in the composition-temperature phase diagrams only slightly change under illumination, while light-induced binodals separating stable and metastable regions appear signalling the nucleation of a low-band gap iodine-rich phase. We find that the threshold photocarrier density for halide segregation is governed by the band gap difference of the parent and iodine-rich phase. Partial replacement of organic cations by cesium reduces this difference and therefore has a stabilizing effect.

Automated reconstruction of parent austenite phase based on the optimum orientation relationship

Characterization of the austenite phase at high temperatures is important for understanding the microstructural evolution during steel processing. The austenite phase structure can be reconstructed from the room-temperature microstructure employing the crystallographic orientation relationship between the parent and product phases. The actual orientation relationships in steels are often calculated on the basis of well known relations (e.g. Kurdjumov–Sachs), which may differ from the experimentally observed orientation relationships. This work introduces a new approach to improve the current state of the art in prior phase reconstruction. The proposed approach consists of two new algorithms that are sequentially applied on an electron backscatter diffraction (EBSD) measured data set of the product phase microstructure: (i) an automated identification of the optimum orientation relationship using the observed misorientation distribution of the entire EBSD scan and (ii) reconstruction of the parent phase microstructure using a random walk clustering technique. The latter identifies groups of closely related grains according to their angular deviation from the proposed orientation relationship. The results were validated by near in situ experimental observations of phase transformation in an Fe–Ni alloy whereby the experimentally measured parent phase structure could be compared point by point with the reconstructed counterpart.

Thermodynamic model for lattice point defect-mediated semi-coherent precipitation in alloys

The formation of precipitates with an atomic volume different from their parent phase eventually leads to a loss of the lattice continuity at the matrix–precipitate interface. Here, we show the creation or removal of lattice sites mediated by lattice point defects is an accommodation mechanism of the coherency loss and even a precipitation driving force. We introduce a thermodynamic approach that rationalizes the selection of phases resulting from chemical and crystallographic constraints in relation to point defect properties. The resulting semi-coherent phase diagram and the precipitation kinetic model depend on the equilibrium phase diagram, the eigenstrain of the precipitating phase, and the chemical potential of point defects. From a joint experimental and modeling study, we uncover the prominent role of excess point defects in unforeseen phase transformations of the Fe–Ni metallic system under irradiation. By addressing the fundamental role of lattice point defects in the accommodation mechanisms of precipitation, we provide a step torwards the understanding of semi-coherent phase transformations occurring in solid materials upon synthesis and in use.

Abnormal Effect of Phase Transition on Thermal Transport in Soft Porous Crystals

<p></p><p>Soft porous crystals (SPCs) can undergo large-amplitude phase transitions under external stimulus such as mechanical pressure, gas adsorption, and temperature while retaining their structural integrity. During the gas adsorption process, the generated latent heat is needed to be effectively removed. Thus, understanding the effect of phase transition on the thermal transport in SPCs becomes extremely important for their applications in storage and separation applications. </p> <p>In this paper, taking isorecticular DUT series as an example, the evolution of the thermal transport in SPCs during the phase transition from the large pore (lp) phase to the narrow pore (np) phase is comprehensively investigated by molecular dynamics (MD) simulations together with the Green-Kubo method. After the phase transition, an abnormal thermal transport property is found in the np phase of DUT materials. We find that although the transformed np phase of DUT-48 has a density much larger than its parent phase, the thermal conductivity of its np phase is smaller than its lp phase. This result is in contrast to the previous finding that SPCs with larger density possess a larger thermal conductivity. However, as for other DUT crystals including DUT-47, DUT-49, DUT-50, and DUT-151, the np phase is found to have a higher thermal conductivity than their lp phase counterpart, which is in accordance with the previous finding. This complicated effect of phase transition on thermal transport in SPCs can be explained by the porosity-dominated competition mechanism between the increased volumetric heat capacity and the aggravated phonon scattering during the phase transition process. Overall, the finding extracted from the present study can greatly expand current knowledge about the thermal conductivity of metal-organic frameworks that is previously found to grow usually with increasing porosity.</p><br><p></p>