CaCu5-type Structure Research Articles

Hydrogen storage and electrochemical properties of annealed low-Co AB5 type intermetallic compounds

The structure, hydrogen storage and electrochemical properties of annealed low-Co AB5-type intermetallic compounds have been investigated. La-alloy, Nd-alloy and Cr-alloy are used to represent La0.8Ce0.2Ni4Co0.4Mn0.3Al0.3, La0.6Ce0.2Nd0.2Ni4Co0.4Mn0.3Al0.3 and La0.6Ce0.2Nd0.2Ni3.8Co0.4Mn0.3Al0.3Cr0.2, respectively. The XRD results indicated that annealed samples are all single-phase alloys with CaCu5 type structure. The maximum of both hydrogen content and discharge capacity is obtained for La-alloy 1.23 wt%H2 and 321.1 mA h/g, respectively. All the investigated alloys are quiet stable with ΔH of hydrogen desorption about 36–38 kJ/mol H2. Cycle life of alloy electrode has been improved by partial substitution of La for Nd and Ni for Cr. The highest capacity retention of 92.2% after 100 charge/discharge cycles at 1C has been observed for Nd-alloy. The hydrogen diffusion coefficient measured by PITT is higher at the start of charging process and dramatically reduces by 2–3 order of magnitude with saturation of β-hydride. The highest value 6.9 × 10−13 cm2/s is observed for La alloy at 100% SOC. Partial substitution La for Nd and Cr for Ni in low-Co AB5 metal hydride alloys slightly reduces maximum discharge capacity, HRD performance and hydrogen diffusion kinetics. Low-Co alloys show good overall electrochemical properties compared to high-Co alloys and might be perspective materials for various electrochemical applications.

Investigation of La0.8Ce0.2Ni4Co0.4Mn0.3Al0.3 for possible use as a metal hydride fuel cell anode

The low-Co AB5-type La0.8Ce0.2Ni4Co0.4Mn0.3Al0.3 alloy was investigated as a potential material for a metal hydride fuel cell anode. The intermetallic compound was prepared by arc melting in argon atmosphere followed by annealing at 1223 K and hardening in cold water. According to XRD analysis, the alloy has the hexagonal CaCu5 type structure without inclusions of secondary impurities. Electrodes were prepared by cold pressing of alloy powder (20%) with carbonyl nickel (80%). Activation at 100 mA/g was almost done in 3 charge/discharge cycles. Full capacity was 340 mAh/g at current density 300 mA/g. The metal hydride electrodes demonstrate stable high rate dischargeability dropping to 317 mAh/g (-7%) at 1000 mA/g. The electrodes have shown high values of the hydrogen diffusion coefficient, with a maximum of 9×10–11 cm2/s at 10% SOC.

Long-term hydrogen absorption/desorption properties of an AB5-type LaNi4.75Mn0.25 alloy

AB5-type hydrogen storage alloys have been extensively studied due to their wide applications, but their degradation process is still controversial, hindering the improvement in their cycling performance. In this work, the degradation mechanism of an AB5-type LaNi4.75Mn0.25 alloy with single CaCu5-type phase structure has been investigated based on its long-term hydrogen storage properties and the structural changes during cycling. It is found that within 1000 hydrogen absorption/desorption cycles in the temperature range of 343–383 K, the P-C-T curves remain a single plateau, but the plateau slope becomes steeper and the hydrogen storage capacity decreases. These degradation phenomena are caused by the structural evolution of the alloy including particle pulverization and crystal lattice damage. The internal driving force of the structural changes is found to be the microstrains generated during hydrogen absorption/desorption. But with cycling, the isotropic parameter c/a increases, decreasing the hysteresis and slowing down the degradation process.

Effects on structure and magnetic properties of SmCo5 based intermetallic compounds by increasing configuration entropy from binary to quaternary equiatomic rare earths at Sm site

The rare earth configuration entropy dependence of the crystal structure, microstructure and magnetic properties for SmCo5 based intermetallic compounds has been systematically investigated by using power X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy and magnetic measurement. The studies show that the single phase with CaCu5 type structure forms in (Sm1/2Nd1/2)Co5, (Sm1/3Nd1/3Y1/3)Co5 and (Sm1/4Nd1/4Y1/4Tb1/4)Co5. The formation of single phase is derived from Miedema's model due to the mixing enthalpy of 0 kJ·mol−1 at rare earth site. The high entropy (Sm1/4Nd1/4Y1/4Tb1/4)Co5 intermetallic compound is obtained due to its configuration entropy of 1.39R and rare earth equiatomic ratio at Sm site. The morphologic images show that the grains are growing and agglomerating with doping equiatomic rare earth Y and Tb. The magnetization of sample cannot reach saturation within the maximum field of 30 kOe at room temperature, the magnetizing curve follows the Langevin model for SmCo5 and (Sm1/2Nd1/2)Co5, and the modified Langevin model for (Sm1/3Nd1/3Y1/3)Co5 and (Sm1/4Nd1/4Y1/4Tb1/4)Co5, respectively. The fitted magnetization is going down with substitution of nonmagnetic Y and heavy rare earth Tb due to the diluting magnetic effect of nonmagnetic Y substitution for Sm and the weakened effects on moment of Tb moment arrangement antiparallel to that of Co. The coercivity tends to decrease due to the agglomeration and growth of grains. It implies that the configuration entropy seems to have little effect on the room temperature magnetizing process.

Crystal structure, spin reorientation, and rotating magnetocaloric properties of NdCo5-xSix compounds

The structure, spin-reorientation transitions, and the rotating magnetocaloric effect of field-oriented polycrystalline NdCo5-xSix (x = 0.0, 0.2, and 0.4) compounds were investigated. These compounds crystallize in the CaCu5-type structure with the space group P6/mmm, and the doping element Si prefers to occupy the 2c site. All these compounds show two spin-reorientation temperatures (TSR1 and TSR2). When Si content x reaching up to 0.4, TSR1 of the compounds monotonously increases from 260 K to 285 K and TSR2 increases from 285 K to 317 K, while the rotating entropy change value slightly decreases from 3.23 J/kg K to 2.60 J/kg K under a magnetic field of 2 T. These materials will be suitable refrigerants for application in room-temperature rotating magnetic refrigeration technology.

Effects of Ce/Y on the cycle stability and anti-plateau splitting of La5-xCexNi4Co (x = 0.4, 0.5) and La5-yYyNi4Co (y = 0.1, 0.2) hydrogen storage alloys

Rare-earth based AB5-type alloys are one of the most popular and potential hydrogen storage materials. However, their overall hydrogen storage performance, especially hydrogen absorption/desorption durability needs to be further optimized. In this study, the microstructure and long-term hydrogen absorption/desorption properties of La5-xCexNi4Co (x = 0.4, 0.5) and La5-yYyNi4Co (y = 0.1, 0.2) alloys are investigated. All the alloys are composed of CaCu5-type hexagonal single-phase structure. The addition of either Ce or Y increases the anisotropic c/a factor of cell lattices, thus enhancing the alloys' pulverization resistance and structural stability. Ce and Y elements also help to eliminate the second plateau in the alloys’ P–C-T curves and increase their application efficiency. In particular, La4.5Ce0.5Ni4Co alloy shows a single and long P–C-T plateau which reaches 95.5% of the full capacity compared with 67%–77% of the other three alloys. Furthermore, it experiences low microstrains and possesses high pulverization resistance during hydrogen absorption/desorption, exhibiting an excellent cycling retention of 96.0% after 1000 cycles. In addition, this alloy boosts the feeding hydrogen pressure from 1.5 MPa at room temperature to above 20 MPa at 175 °C with small hysteresis and flat and long plateau, showing a potential in the application of hydrogen compressor.

Spontaneous and induced magnetic phase transitions in Tb0.9Er0.1Ni5

Abstract Magnetic and neutron powder diffraction measurements have been carried out on Tb0.9Er0.1Ni5 intermetallic compound. The intermetallide crystallizes in the hexagonal CaCu5-type structure and possesses a long-range magnetic order at temperatures below 22 K. A fan-like magnetic structure is described by two propagation vectors: k1 = 0 and k2 = 2π/c(0, 0, 0.036), at 20 K. The total Tb-ion magnetic moment has the ferromagnetic and modulated components. The latter is a transverse spin wave. When the sample is cooled at a temperature below 8 K, the module of the k2 vector does not change and is equal to k2 = 2π/c(0, 0, 0.027). An “incommensurate – lock-in” magnetic transition takes place at 8 K. The k2 vector exhibits a temperature hysteresis of about (5–6) K, whereas the intensities of Bragg reflections and satellites do not show up appreciable changes. When an external magnetic field is applied to the sample, the satellites and the module of the k2 vector decrease, while the Bragg intensities increase. The sample becomes a ferromagnetic at a field of ∼2 kOe, and the Tb-ion magnetic moment is equal to 8.3 μB. A general notion of the Tb0.9Er0.1Ni5 magnetic state evolution with an external field is given using the field dependence of the background intensity in diffraction patterns. First principle calculations for TbNi5 and Tb0.9Er0.1Ni5 are performed including the 4f states into the orbital basis and accounting for strong electronic correlations and spin-orbital coupling. This allowed obtaining both spin and orbital moments of the effective Tb-ion moment and estimating also the value of Tb-Tb exchange interaction.

Complex Crystal Chemistry of Yb6(CuGa)50 and Yb6(CuGa)51 Grown at Different Synthetic Conditions

Two new compounds in the Yb–Cu–Ga system were discovered. Yb6(CuGa)50 was obtained as single crystals grown from gallium metal flux, and Yb6(CuGa)51 was synthesized by high-frequency induction heating technique. Structure analysis reveals new structure type for Yb6(CuGa)50 (C2/m space group), while Yb6(CuGa)51 crystallizes in rhombohedral space group R3̅m adopting the Th2Zn17 structure type. Both structures could be derived from the simplest layered packing, through CaCu5-type structure, as a common parent. Physical properties of the Yb6(CuGa)51 sample obtained bulk quantity were explored. The magnetic susceptibility study of Yb6(CuGa)51 in the temperature range of 2–300 K suggests a valence fluctuation at lower temperatures. X-ray absorption near edge spectroscopy suggests that Yb exists in mixed-valent state in Yb6(CuGa)51. Electrical resistivity measurement reveals that Yb6(CuGa)51 is metallic in nature and shows Fermi-liquid behavior in the temperature range from 3.5 to 25 K.

Preparation and characterization of mechanically alloyed AB3-type based material LaMg2Ni5Al4 and its solid-gaz hydrogen storage reaction

We developed in the present paper the synthesis of a new AB3-type compound LaMg2Ni5Al4 by mechanical alloying (MA) process. ​​X-ray diffraction analysis (XRD) was used to determine the structural properties and the phase evolution of the powder mixtures. Two different synthesis pathways have been investigated. The first starting from elemental metals and the second from a mixture of two binary compounds LaNi5 (CaCu5-type structure, P6/mmm space group) and Al(Mg) solid solution (cubic Fm-3 m space group). The results show multiphase alloys which contain LaMg2Ni5Al4 main phase with hexagonal PuNi3-type structure (R-3 m space group). Rietveld analysis shows that using a planetary ball mill, we obtain a good yield of LaMg2Ni5Al4 compound after 5 h of mechanical alloying for both synthesis pathways. TEM analysis confirmed XRD results. SEM-EDX analysis of the final product was in agreement with the nominal chemical formula. A setup of possible solid-gaz hydrogenation reaction will be described so far at the end of this work. Electrochemical results demonstrate evidence on hydrogen absorption in the AB3 material and the discharge capacity was equal to 5.9 H/f.u.

Investigation of the structure, phase composition, and electrochemical characteristics of hydrogen-sorbing intermetallics of the La–Mg–Ni system obtained by the sintering method

The crystal structure, phase composition, and electrochemical properties of the materials obtained by sintering of (LaNi3+Mg+Ni) powder mixture in the temperature range 640–1020°С have been investigated. Experimental results show that, at temperatures of ≤850°С, the sintered multiphase material, whose major phases are phases with PuNi3-type structure (LaMg2Ni9 and LaNi3) and CaCu5-type structure (LaNi5), is formed. With increase in temperature, the number of phases in the sintered material decreases to a single major phase LaNi5, and the content of the LaMg2Ni9 phase practically does not change. It has been established that increase in sintering temperature deteriorates the activation of the electrode materials and slows down the hydrogen absorption process. At the same time, the maximum discharge capacity and cyclic stability of electrodes increase.

The magnetic properties of (La,Ce)Co5 ((La,Ce)=La0.35Ce0.65, La-Ce mischmetal) nanoflakes prepared by surfactant-assisted ball milling

The hard magnetic (La,Ce)Co5 nanoflakes with high coercivity and narrow thickness distribution have been successfully obtained by surfactant-assisted ball milling (SABM). The magnetic properties, morphology and interaction of (La,Ce)Co5 nanoflakes are studied in this work. The coercivity and remanence ratio of (La,Ce)Co5 nanoflakes are 5.48 kOe and 0.71, respectively. The X-ray powder diffraction (XRD) patterns indicate that the (La,Ce)Co5 nanoflakes are CaCu5-type hexagonal crystal structure. The average thickness and aspect ratio are 47 nm and 40, respectively. The intergrain interaction of the (La,Ce)Co5 nanoflakes is studied using the δm(H)-curves technique which shows the magnetostatic-dominated particle interaction. The high coercivity and narrow thickness distribution of (La,Ce)Co5 nanoflakes could be promising for the future development of the high performance soft/hard exchange spring magnets.

Creating the hydrogen absorption capability of CeNi5 through the addition of Al

The hydrogenation capacities of CeNi5−xAlx (x = 0, 0.75, 1.0, 1.25, 1.5 and 1.75) alloys were assessed. In contrast to LaNi5-based alloys, the addition of Al to CeNi5 enables hydrogen absorption by creating larger interstitial sites as the result of an expanded lattice. Structure analyses indicate that each of these alloys maintained a CeNi5 (CaCu5-type) structure in which Al atoms exclusively occupied Ni 3g sites. Among these specimens, CeNi4Al absorbed the greatest proportion of hydrogen atoms, decreasing its c/a ratio from 0.826 to 0.802 upon the formation of CeNi4AlH4.3. This drastic decrease in the c/a ratio is attributed to an anisotropic lattice expansion along the a-axis, as verified by in situ X-ray diffraction under H2. The enhanced hydrogen absorption of these alloys is thought to be associated with the preferential situation of hydrogen atoms at interstitial sites located at the centers of octagons as well as a charge transition of Ce ions upon hydrogenation. It is verified that CeNi4Al after hydrogenation shows high catalytic activity toward propyne hydrogenation: 100% conversion of propyne even at room temperature.

On the phase stability of CaCu5-type compounds

We present a hybrid method to inspect the phase stability of compounds having a CaCu5-type crystal structure. This is done using 2D stability plots using the Miedema parameters that are based on the work function and electron density of the constituent elements. Stable compounds are separated from unstable binary compounds, with a probability of 94%. For stable compounds, a linear relation is found, showing a constant ratio of charge transfer and electron density mismatch. DFT calculations show the same trend. Elements from the s,d,f-block are all reliably represented, elements from the p-block are still challenging.

Experimental Investigation of the Isothermal Section at 800 °C of the Nd-Fe-Co Ternary System

The phase equilibrium of the ternary Nd-Fe-Co system at 800 °C was investigated by means of powder x-ray diffraction and scanning electron microscopy–energy dispersive x-ray spectroscopy. Seven binary compounds, i.e., Nd2Co17, NdCo5, Nd5Co19, Nd2Co7, NdCo3, NdCo2, Nd2Fe17 were identified to exist at this isothermal section. This isothermal section consists of ten single-phase, ten two-phase and six three-phase regions. All measured compositions and unit-cell refinements were performed at room temperature from quenched samples annealed at 800 °C for one week. The maximum solubility at 800 °C of Fe in NdCo2−xFex (MgCu2-type structure, Fd-3 m), NdCo3−xFex (PuNi3-type structure, R-3 m space group), Nd2Co7−xFex (Ce2Ni7-type structure, R-3 m), Nd5Co19−xFex (CeCo19-type structure, R-3 m space group), NdCo5−xFex (CaCu5-type structure, P6/mmm), Nd2Co17−xFex (Th2Zn17 type structure, R-3 m) and Nd2Fe17−xCox (Th2Zn17 type structure, R-3 m) are about 31.6 at.% Fe, 47.9 at.% Fe, 13.3 at.% Fe, 8.6 at.% Fe, 10.37 at.%, 36.35 at.% Fe, and 58.23 at.% Fe respectively. The solid solubility range of Co in Nd2Fe17 form discontinuous series of 2 ranges is about 0-30.14 at.% Co, and 51.9-100 at.% Co and the solid solubility range of Fe in Nd2Co17 is about 0-48.1 at.% Fe, and 69.86-100 at.% Fe.

The magnetic properties of MMCo5 (MM=Mischmetal) nanoflakes prepared by multistep (three steps) surfactant-assisted ball milling

The hard magnetic MMCo5 nanoflakes with high coercivity and narrow size distribution have been successfully obtained by three steps surfactant-assisted ball milling (SABM). The magnetic properties, phase structure and morphology of these MMCo5 nanoflakes were studied in this work. The coercivity and the remanence ratio of MMCo5 nanoflakes reached to 5.89 kOe and 0.75, respectively. The X-ray powder diffraction (XRD) patterns indicated that the MMCo5 nanoflakes were CaCu5-type hexagonal crystal structure. The average thickness, in-plane size and aspect ratio reached to 20 nm, 0.9 μm and 60, respectively. The low cost and great properties of MMCo5 nanoflakes with a centralized thickness distribution could be the building blocks for the future high-performance nanocomposite permanent magnets with an enhanced energy product.

Powder diffraction data of the AlCeCo2Ni2 quaternary compound

A new quaternary compound AlCeCo2Ni2 was synthesized and studied by means of X-ray powder diffraction technique. The powder pattern of AlCeCo2Ni2 was indexed and refined, giving a hexagonal structure, space group P6/mmm (No. 191) with the CaCu5 structure type. a = 4.9242(2) Å, c = 4.0524(1) Å, V = 85.1 Å3, Z = 1, ρx = 7.85 g cm−3, F30 = 130.2(0.010, 30), and RIR = 0.71(2).

Spin glass and ferromagnetic properties of Ce(Cu1-xNix)4Mn alloys: Multicritical points in the magnetic phase diagram

We report on the CeNi4Mn (ferromagnet FM) - CeCu4Mn (spin-glass SG) transformation leading to a complex magnetic phase diagram (MPD). It is verified that all the Ce(Cu1-xNix)4Mn alloys are isostructural and the transformation is governed only by the Cu-Ni substitution. MPD is built based on the magnetic dc/ac susceptibility measurements and reveals SG formation as well as the region of the coexistence of the FM and SG state in the middle range of the Ni concentration. The complex MPD is explained by clusters formation and a competition of interactions between various crystallographic sites of the hexagonal CaCu5-type structure, mainly the 3g-3g and 3g-2c interactions. The predominance of the SG state is confirmed by the analysis of the frequency dependence of the ac magnetic susceptibility components and the relaxation of the remanent magnetization. Additionally, the presence of two multicritical points is observed.

Effect of Annealing Temperature on Hydrogen Storage Properties of LaNi3.8Al1.0Mn0.2 Alloy

Abstract The LaNi3.8Al1.0Mn0.2 hydrogen storage alloy annealed at different temperatures (T=1173, 1223, 1273, 1323 K) was investigated to examine the effect of annealing temperature on the crystal structure, the activation and the hydrogen absorption/desorption performance. It is found that the non-CaCu5 phases becomes tinier and more dispersive after annealing. All the alloys consist of the same matrix phase with CaCu5 type structure. Although it is more difficult to activate the annealed alloy than the as-cast alloy, the activation becomes easier with the increase of annealing temperature for the annealed alloy. The plateau pressure decreases at first with the increase of annealing temperature, when T≤1223 K and then increases. Changes in both the lattice strain and the pressure plateau slope factor of Sf in the alloys show the same trends with that of the annealing temperature, exhibiting a maximum at T=1223 K. It is also found that the absolute value of the enthalpy increases with the increase of annealing temperature, when T≤1223 K and then decreases, showing the same change trend as that of the cell volume. Moreover, the pulverization resistance and kinetics of the alloy are improved by annealing. The pulverization resistance is weakened at first with the increase of annealing temperature when T≤1223 K and then increases, but the kinetics of the alloy is almost not influenced by the annealing temperature.

Structural, atomic Hirschfeld surface, magnetic and magnetocaloric properties of SmNi5 compound

The SmNi5 intermetallic compound has been investigated by arc-melting. Powder X-ray diffraction analysis and Rietveld refinement revealed that the sample crystallized in the hexagonal CaCu5-type structure P6/mmm space group with the following lattice parameters: a = 4.9203 (1) Å, c = 3.9662 (1) Å. These lattice parameters for the compound are in good agreement with previous theoretical result and experimental data. The EDX analysis has been performed to confirm the composition of this compound. The chemical bonding in SmNi5 was analyzed using atomic Hirshfeld surfaces, and this analysis supports the presence of the structural elements and the coordination of Sm (1a), Ni (2c) and Ni (3 g). This study indicates the different types of interatomic interactions between the Sm and Ni atoms and the weak interactions between Sm–Sm atoms were also observed along the c axis. The magnetic properties and magnetocaloric effect (MCE) have been established by the magnetization and isothermal magnetization of different temperature measurements. The magnetization curve as a function of temperature shows a magnetic transition from ferromagnetic to paramagnetic state at the Curie temperature TC = 29 K. We have studied the MCE phenomena in the vicinity of magnetic phase transitions in terms of magnetic entropy change. The temperature dependence of the magnetization, the magnetic entropy changeΔSM, as well as the relative cooling power around the second-order magnetic transition and the Arrott plots for the alloys are reported.

Structural and Hydriding Properties of the LaZr2Mn4Ni5-AB3 Type Based Alloy Prepared by Mechanical Alloying from the LaNi5 and ZrMn2 Binary Compounds

In this study, we report on the synthesis of a new AB3-type compound LaZr2Mn4Ni5 (with a content of 40 wt %) at room temperature during 5h of mechanical alloying. This compound was synthesized from the two binary compounds LaNi5 (CaCu5-type structure, P6/mmm space group) and ZrMn2-Laves phase (MgZn2-type structure, P63/mmc space group) in order to take advantage of the hydrogen absorption properties of these two types of intermetallic compounds. Structural properties were investigated using X-ray diffraction (XRD). The surface morphology of the cycled electrode was observed by a scanning electron microscope (SEM). The electrochemical properties of the LaZr2Mn4Ni5–based alloy were determined using the chrono-potentiometry method. The experimental results indicate that the discharge capacity reaches a maximum value of 300 mAh/g. Solid-gaz reaction shows that this compound is able to form the LaZr2Mn4Ni5H13 hydride at room temperature at an absorption plateau pressure of about 7 bar.