Temperature-dependent X-ray Diffraction Research Articles

Tailoring the structural and magnetic properties of epitaxially growth Ni–Mn–Ga magnetic films by Fe addition

ABSTRACTNi–Mn–Ga is one of the major magnetic materials in Heusler family due to its interesting magnetic and structural properties. In this study, the epitaxial 200 nm thin films are prepared by magnetron co-sputtering from a NiMnGa alloy target and an elemental Fe target on MgO substrates. The epitaxial growth mechanism is pursued and controlled by ex-situ X-ray refractometry (XRR) and X-ray diffraction (XRD) measurements. On account of analysing the physical parameters, temperature-dependent X-ray diffraction (XRD), transmission electron (TEM), scanning electron microscopy (SEM), temperature dependence of resistivity (ρ(T)), atomic force microscopy (AFM) and magnetisation (MOKE) measurements are performed systematically for different Fe contents. Structural and magnetic measurements show that the NiMnGa parent thin film transitions could be adjusted from martensitic to austenitic phases by adding Fe element from high temperature to the vicinity of room temperature. Measurements are carried out to get a further structural characterisation of 14M martensite which was found to consist of nano-twinned structures. The epitaxial growth and the stable coexisting austenite and martensitic phases for the thin film of the NiMnGa chemical composition are shown near room temperature. Due to this feature, magnetic thin films could increase the likelihood of possible room temperature using thin film applications.

Strong Electron-Phonon Coupling in the Excitonic Insulator Ta2NiSe5.

An excitonic insulating (EI) state is a fantastic correlated electron phase in condensed matter physics, driven by screened electron-hole interaction. Ta2NiSe5 is an excitonic insulator with a critical temperature (TC) of 328 K. In the current study, temperature-dependent Raman spectroscopy is used to investigate the phonon vibrations in Ta2NiSe5. The following observations were made: (1) an abnormal blue shift around TC is observed, which originates from the monoclinic to orthorhombic structural phase transition; (2) the splitting of a mode and two new Raman modes at 147 and 235 cm-1 have been observed with the formation of an EI state. With the help of first-principles calculations and temperature-dependent X-ray diffraction (XRD) experiments, it is found that the TaSe6 octahedra are "frozen" and the NiSe4 tetrahedra are greatly distorted below TC. Thus, it seems that the distortion of NiSe4 tetrahedra plays an important role in the strong electron-phonon coupling (EPC) in Ta2NiSe5, while the strong EPC, coupled with electron-hole interaction, opens the energy gap to form the EI state in Ta2NiSe5.

Local Structure and Influence of Sb Substitution on the Structure-Transport Properties in AgBiSe2.

Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I-V-VI2 family, and especially AgBiSe2, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe2, to evaluate possible stereochemical activity of the 6s2 lone pair of Bi3+, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBi1-xSbxSe2 (0 ⩽ x ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe2. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure-transport correlations of the thermoelectric AgBiSe2.

Characterization of structural transitions and lattice dynamics of hybrid organic–inorganic perovskite CH3NH3PbI3**Project supported by the National Natural Science Foundation of China (Grant No. 11774419) and the Ministry of Science and Technology of China (Grant Nos. 2016YFA0300504 and 2017YFA0302904).

By combining temperature-dependent x-ray diffraction (XRD) with temperature-dependent Raman scattering, we have characterized the structural transitions and lattice dynamics of the hybrid organic–inorganic perovskite CH3NH3PbI3. The XRD measurements cover distinct phases between 15 K and 370 K and demonstrate a general positive thermal expansion. Clear anomalies are found around the transition temperatures. The temperature evolution of the lattice constants reveals that the transition at 160 K/330 K is of the first-/second-order type. Raman measurements uncover three strong low-frequency modes, which can be ascribed to the vibration of the Pb/I atoms. The temperature evolution of the modes clearly catches these transitions at 160 K and 330 K, and confirms the transition types, which are exactly consistent with the XRD results. The present study may set an experimental basis to understand the high conversion efficiency in methylammonium lead iodide.

Tunable spin-state bistability in a spin crossover molecular complex

The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{H2B(pz)2}2(bipy)] polycrystalline films on Al2O3 substrates have been studied, where pz = pyrazol-1-yl and bipy = 2,2′-bipyridine. For [Fe{H2B(pz)2}2(bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{H2B(pz)2}2(bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{H2B(pz)2}2(bipy)]/Al2O3 interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperature-dependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{H2B(pz)2}2(bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition.

Temperature-Induced Single-Crystal-to-Single-Crystal Transformations with Consequential Changes in the Magnetic Properties of Fe(III) Complexes.

The present article deals with an one-to-one structure–property correspondence of a dinuclear iron complex, [Dipic(H2O)FeOH]2·H2O (1) (Dipic = pyridine-2,6-dicarboxylic acid). Variable-temperature X-ray single-crystal structural analysis confirms a phase transition of complex 1 to complex 2 ([Dipic(H2O)FeOH]2) at 120 °C. Further, single-crystal-to-single-crystal (SCSC) transformation was monitored by temperature-dependent single crystal X-ray diffraction, powder X-ray diffraction, time-dependent Fourier-transform infrared spectroscopy, and differential scanning calorimetry. SCSC transformation brings the change in space group of single crystal. Complex 1 crystallizes in the C2/c space group, whereas complex 2 crystallizes in the Pi̅ space group. SCSC transformation brings the change in packing diagram as well. Complex 1 shows two-dimensional network through H-bonding, whereas the packing diagram of complex 2 shows a zigzag-like arrangement. Phase transformation not only fetches structural changes but also in the magnetic properties. Difference in Fe–O–Fe bond angles of two complexes creates notable variation in their antiferromagnetic interactions with adjacent metal centers.

Mechanosynthesis of the Whole Y1-xBixMn1-xFexO3 Perovskite System: Structural Characterization and Study of Phase Transitions.

Perovskite BiFeO3 and YMnO3 are both multiferroic materials with distinctive magnetoelectric coupling phenomena. Owing to this, the Y1−xBix Mn1−xFexO3 solid solution seems to be a promising system, though poorly studied. This is due to the metastable nature of the orthorhombic perovskite phase of YMnO3 at ambient pressure, and to the complexity of obtaining pure rhombohedral phases for BiFeO3-rich compositions. In this work, nanocrystalline powders across the whole perovskite system were prepared for the first time by mechanosynthesis in a high-energy planetary mill, avoiding high pressure and temperature routes. Thermal decomposition temperatures were determined, and structural characterization was carried out by X-ray powder diffraction and Raman spectroscopy on thermally treated samples of enhanced crystallinity. Two polymorphic phases with orthorhombic Pnma and rhombohedral R3c h symmetries, and their coexistence over a wide compositional range were found. A gradual evolution of the lattice parameters with the composition was revealed for both phases, which suggests the existence of two continuous solid solutions. Following bibliographic data for BiFeO3, first order ferroic phase transitions were located by differential thermal analysis in compositions with x ≥ 0.9. Furthermore, an orthorhombic-rhombohedral structural evolution across the ferroelectric transition was characterized with temperature-dependent X-ray diffraction.

Temperature Dependent X-Ray Diffraction in K2FeP2O7 Pyrophosphate: Polymorphism and Preliminary Electrochemistry Towards Li, Na, and K Ion Intercalation

Phosphate-based polyanionic materials have changed the era of commercial secondary batteries after the discovery of triphylite LiFePO4 in 1997. Such materials are encouraged due to its open and robust framework, chemical/ thermal stability, tunable redox potential and desirable electrochemical performance. Several phosphate-based electrode materials have been reported for lithium (Li+) and sodium (Na+) insertion to develop lithium-ion batteries (LIB) and sodium-ion batteries (SIB). The fact that Na+ can't be intercalated into graphite (which is widely used negative electrode for battery application) has diverted researcher’s attention toward potassium-ion batteries (KIB).[1] Also regarding redox center, polyanion chemistry has only created a basis for iron-based compounds to be used as cathode in LIB. The elemental abundance and non-toxic nature has encouraged battery community to explore Fe-based battery insertion materials. The progressive studies on pyrophosphate polyanionic cathodes has developed them as potential compounds for next generation batteries. Promising electrochemical properties of Li2FeP2O7 and Na2FeP2O7 has motivated us to study potassium-based analogues K2FeP2O7. [2] K2FeP2O7 was synthesized using solid state and combustion method. In particular, at first this phospho-anionic compound was prepared by solution combustion synthesis using low cost Fe(III) based precursors [Fe(NO3)3.9H2O]. Fe(NO3)3 was dissolved in distilled water followed by the addition of ascorbic acid to convert all Fe (III) into Fe(II). Then stoichiometric amount of KH2PO4 and combustion fuel like citric acid or urea were added. These solutions were heated at 110 ºC to trigger an exothermic reaction developing an intermediate complex. Upon annealing this complex at high temperature (500, 600 and 700 ºC), similar unknown phase members at all three temperature were obtained inferred from X-ray diffraction technique (XRD). To validate the unknown phase, stoichiometric amounts of FeC2O4.2H2O and KH2PO4 were added and ball milled. Similarly, the ball milled intermediate were annealed at high temperature (500, 600 and 700 ºC). The synthesized phases were analyzed using XRD, 500 and 600 ºC compounds are same as obtained by solution combustion synthesis. Interestingly, 700 ºC synthesized phase was found to crystalize into the structure type of Ca2MgSi2O7 (previously reported by Keates et al. K2FeP2O7).[3] Such simple observation of change in phase observed from XRD profile gave us a clue of polymorphism phenomenon which was further cross-validated by temperature dependent XRD in K2FeP2O7 obtained at 500 ºC. High temperature XRD results clearly shows the existence of two stable phases (Figure 1. in the middle), low temperature (500 ºC) one can be assigned as β-K2FeP2O7 and high temperature (700 º C) can be assigned as α-K2FeP2O7. Structure determination of β-K2FeP2O7 is in its almost final stage with monoclinic (P21/c) crystal structure. α-K2FeP2O7 crystalizes into tetragonal crystal system of P-421 m space group. Pyrophosphate compounds offer a robust three-dimensional P2O7 4--framework with multiple sites for alkali ions and existence of polymorphism in such compounds is clear in the present case. Polymorphism arises due to the different angular orientation between the two phosphate units. Here, we report the electrochemistry of the β-K2FeP2O7 and α-K2FeP2O7 with respect to Li+, Na+ and K+. Galvanostatic charge-discharge profile of β-K2FeP2O7 and α-K2FeP2O7 is compared and shown in the left and right side of the figure respectively. Both materials have low insertion voltages which is lesser than 3 V. This low working voltage is possibly coming due the existence of FeO4 tetrahedral environment in the crystal structure which leads to the weak inductive effects as compared to FeO6 octahedral environment during Fe2+/Fe3+ redox mechanism.[4] Seeing these unpromising intercalation chemistry both polymorphs were subjected to study the ac impedance conductivity by varying the temperature from room temperature to 500 ºC in neutral atmosphere. β-K2FeP2O7 polymorph was found to have conductivity ~10-6 S cm-1 at 300 ºC and 10-3 S cm-1 at 400 ºC. However, α-K2FeP2O7 was found to have very poor conductivity experimentally which is in sync with BVSE energy barrier of 1.08 eV. Thus the current work will summarize recent findings in ‘pyrophosphate’ class of battery materials capable of reversible insertion of Li+ into β-K2FeP2O7 along with high temperature conductivity measurements using AC-impedance analyzer.

Nonmagnetic Substitution in Pyrochlore Iridate Y2(Ir1–xTix)2O7: Structure, Magnetism, and Electronic Properties

Tuning of spin-orbit coupling and electron correlation effects in pyrochlore iridates is considered for many interesting phenomena. We have investigated the temperature evolution of structural, magnetic and electronic properties in doped Y$_2$(Ir$_{1-x}$Ti$_{x}$)$_2$O$_7$ ($x$ = 0.0, 0.02, 0.05, 0.10 and 0.15) where the substitution of nonmagnetic Ti$^{4+}$ (3$d^0$) for Ir$^{4+}$ (5$d^5$) amounts to dilution of magnetic network and tuning of these parameters in opposite way. The system retains its original structural symmetry but local structural parameters show an evolution with Ti content. While the magnetic transition temperature is not largely influenced, both magnetic moment and magnetic frustration decreases with Ti doping. Magnetic relaxation measurement shows the parent compound Y$_2$Ir$_2$O$_7$ as well as its Ti doped analogues are in nonequilibrium magnetic state where the magnetic relaxation rate increases with Ti. Temperature dependent Raman measurements indicate no changes in structural symmetry, however, across the magnetic transition temperature an anomaly in A$_{1g}$ Raman mode is observed. Temperature dependent x-ray diffraction data also support the Raman spectroscopy data, however, an evolution of lattice parameters with temperature is observed. The electrical resistivity data of Y$_2$(Ir$_{1-x}$Ti$_{x}$)$_2$O$_7$ series exhibits insulating behavior throughout the temperature range, however, the resistivity decreases with Ti doping. The nature of charge conduction is found to follow power-law behavior in whole series but the validity of this model varies with temperature. A negative magnetoresistance has been observed at low temperature in present series which is explained with weak localized mechanism. Similar to other Ir based oxides, a crossover from negative to positive MR has been observed in present system.

Enhancement of martensite transition temperature and inverse magnetocaloric effect in Ni43Mn47Sn11 alloy with B doping

Ni43Mn47Sn11 and Ni43Mn47Sn10.5B0.5 polycrystalline alloys have been prepared by using arc-melting and characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), magnetization measurements and microhardness tests. Temperature dependent XRD measurements shows the evolution of monoclinic martensite phase (P12/m1) from the cubic austenite L21 (Fm-3m) phase. An enhancement (73 K) in the martensite transition temperature from 210 K for Ni43Mn47Sn11 to 283 K for Ni43Mn47Sn10.5B0.5 was observed by DSC measurements, which is attributed to the increase in cell volume on the account of B doping. Microhardness measurements showed that the hardness reduces with B doping in Ni43Mn47Sn11 alloy. The magnetic entropy change evaluated from the magnetization measurements was found to be 14 J/kg-K and 17 J/kg-K for Ni43Mn47Sn11 and Ni43Mn47Sn10.5B0.5 alloys respectively. The effective magnetic refrigeration capacity at a maximum field of 5 T was found to be 94 J/kg for Ni43Mn47Sn11 and 55 J/kg for Ni43Mn47Sn10.5B0.5, suggesting that these alloys have potential for magnetocaloric applications (magnetic refrigeration).

Temperature-dependent studies of exciton binding energy and phase-transition suppression in (Cs,FA,MA)Pb(I,Br)3 perovskites

Multiple-cation mixed-halide (Cs,FA,MA)Pb(I,Br)3 perovskites containing cesium, formamidinium (FA), and methylammonium (MA) possess excellent properties for a wide range of optoelectronic applications such as thin-film photovoltaics or lasers. We investigate the role of excitons and the exciton binding energy EB, relevant for the effectiveness of charge separation in solar cells, as well as the temperature-dependent bandgap energy Eg which is used as an indicator for crystal phase transitions. Generalized Elliott fits of absorption spectra offer the possibility to determine both EB and Eg. However, since excitonic effects are non-negligible even at room temperature, a careful and detailed analysis of the spectra is crucial for a correct interpretation. Therefore, an additional evaluation based on a so-called f-sum rule is applied to achieve an improved reliability of the results at higher temperatures. The obtained EB values of 20–24 meV for Cs-containing mixed perovskite compounds are below the ones of 24–32 meV and 36–41 meV for pure methylammonium lead iodide (MAPbI3) and bromide (MAPbBr3), respectively, and, thus, facilitate charge-carrier separation in photovoltaic applications. Furthermore, temperature-dependent (T = 5–300 K) studies of Eg in (Cs,FA,MA)Pb(I,Br)3 indicate a suppressed crystal phase transition by the absence of any phase-transition related signatures such as the well-known jump of about 100 meV in MAPbI3. We verify these results using temperature-dependent electroreflectance spectroscopy, which is a very reliable technique for the direct and non-destructive determination of optical resonances of the absorber layer in complete solar cells. Additionally, we confirm the suppression of the phase transition in Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 by temperature-dependent X-ray diffraction.

One-step synthesis of Sc2W3O12:Eu3+ phosphors with tunable luminescence for WLED

Sc2W3O12 is an important host matrix for rare-earth doped luminescence. However, the conventional method to prepare the material is solid-state reaction, which results into coarse and irregular morphologies. In this work, Eu3+ doped Sc2W3O12 phosphors with high crystallinity and pure phase were successfully synthesized via one-step hydrothermal method. It was found that the crystalline phase changed from Sc2W3O12 phase to Na4Sc2(WO4)5 phase when the molar ratio between Sc(NO3)3 and Na2WO4 decreased. The temperature-dependent X-ray diffraction analysis was performed to prove the negative thermal expansion property of Sc2W3O12. A systematic study on the effect of reaction time, temperature and Eu3+ doping concentration was explored. It was also found that the as-prepared samples displayed tunable emission colors, ranging from blueish white to orange red. Particularly, the white light emission with the chromaticity coordinate of (0.3395, 0.3289) can be realized in Sc2W3O12: 5% Eu3+. What's more, the photoluminescence properties of the samples were investigated under different ambient temperatures between 97 and 280 K. The result clearly showed energy transfer between Eu3+ and WO42−. The above results suggested that Sc2W3O12:Eu3+ can be excellent candidate for solid-state lasing, panel display and WLEDs.

Multiple Magnetic Phase Transitions in Mn-deficient Co0.4Ni0.6Mn1−xGe Compounds

The structural and magnetic transition in Mn-deficient Co0.4Ni0.6Mn1−xGe was investigated by DC magnetization and temperature-dependent X-ray diffraction measurements. By introducing Mn vacancies in the stoichiometric Co0.4Ni0.6MnGe compound, multiple magnetic phase transitions and tunable martensitic transitions were observed. A paramagnetic martensite (Ni2In-type structure) to ferromagnetic austenite (TiNiSi-type structure) transition was realized in Co0.4Ni0.6Mn0.98Ge around 275 K. A large volume expansion of 2.6% accompanied this transition. In addition, a martensitic transition induced by the magnetic field was also observed near room temperature. Furthermore, a significant magnetic entropy variation of − 21.3 J/kg K at 263 K (∆B = 5 T) was obtained in the Co0.4Ni0.6Mn0.98Ge compound.

Influence of Cr Alloying (1.5 to 5 at.%) on Martensitic Phase Transformation Temperatures in Co-Ni-Ga-Cr Thin Films

A Co-Ni-Ga-Cr thin film materials library with a Cr concentration gradient ranging from 1.5 at.% to 5 at.% was magnetron-sputtered to determine the influence of Cr additions on the martensite transformation temperature of non-stoichiometric Co2NiGa-based Heusler alloys. An increase of the phase transformation temperature from 30 °C for 5 at.% Cr up to 100 °C for 1.5 at.% Cr was determined by a peak fitting analysis of temperature-dependent X-ray diffraction data.

Connection between Unusual Lattice Thermal Expansion and Cooperative Jahn-Teller Effect in Double Perovskites LaPbMSbO6 (M = Mn, Co, Ni).

Lattice thermal expansion (LTE) has been investigated in double perovskites LaPbMSbO6 (M = Mn, Co, Ni). Ordinary LTE behavior with good thermal stability is identified for the Mn sample, whereas unusual LTE with a preferably expanded interplanar distance of (040) is revealed for Co and Ni samples. Temperature-dependent X-ray diffraction patterns ( T-XRD), Raman spectra ( T-Raman), and specific heat capacities ( T- Cp) consistently indicate that a rare isostructural displacive phase transition (IDPT) with a second-order phase transition nature is predominant near the critical temperature. Refinements of neutron powder diffraction (NPD) and in situ T-XRD data present temperature-sensitive bond parameters which are relevant to planar oxygen O1. X-ray photoelectron spectra (XPS) further confirm the Jahn-Teller (J-T) activated Co2+ (HS) or Ni3+ (HS/LS) cations at the B-site sublattice. This unusual LTE behavior could be understood by the cooperative J-T effect contributed by a Pb2+ ion and Co2+/Ni3+ ion from A- and B-site sublattices, respectively. The importance of 6s(Pb)-2p(O)-3d(Co/Ni) extended orbital hybridization on affecting thermal expansion behavior is highlighted on the basis of temperature-induced phonon mode softening. This study presents a microscopic description of connection between anisotropic thermal expansion and a cooperative J-T effect, which inspired exploration of thermal-mechanical coupled functional materials based on LaPbMSbO6 double perovskites.

Temperature-dependent structure and transport of ionic liquids with short-and intermediate-chain length pyrrolidinium cations

We report on an observed connection between the structural evolution and dynamics in a series of pyrrolidium-based ionic liquids. Temperature-dependent X-ray diffraction experiments were compared to viscosity and conductivity measurements for 1-alkyl-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([CnPyrr][Tf2N], n = 2, 3, 4, 6, 8). We find that the temperature evolution of the charge alternation peak in the structure factor, S(q), is consistent with intuitive order decoherence for shorter chain lengths, but anomalous for longer chain lengths. Structure data show that [C2Pyrr][Tf2N] behaves fundamentally differently than [C4Pyrr][Tf2N], [C6Pyrr][Tf2N] and [C8Pyrr][Tf2N] over the temperature range investigated with [C3Pyrr][Tf2N] behaving as an intermediate case. Both the conductivity and viscosity mirror these trends providing insight into the fundamental connection between structural ordering and dynamics in these liquids.

Unusual temperature-sensitive excimer fluorescence from discrete π-π dimer stacking of anthracene in a crystal.

Herein, an unusual blue shift in excimer fluorescence with increasing temperature was observed from a crystal with a discrete π-π anthracene dimer, originating from the more temperature-sensitive intermolecular geometry of the π-π dimer (e.g., π-π distance) compared to the intramolecular geometry of the monomer, which is confirmed by temperature-dependent single-crystal X-ray diffraction (XRD) measurements.

Unusual adsorption behaviours and responsive structural dynamics via selective gate effects of an hourglass porous metal-organic framework.

An hourglass porous metal–organic framework, LIFM-12, constructed on a T-shaped flexible ligand with Cu2+ paddle-wheel clusters, shows temperature and gas adsorption responsive structural dynamics upon reversible molecular guest binding. Temperature-dependent single crystal and powder X-ray diffraction experiments show that the open gate status of the framework with adaptive behaviours facilitates kinetic diffusion of gas molecules resulting in the sequential filling of pores of different sizes, thus creating a breathing behaviour reminiscent of the observation of several steps in adsorption isotherms. In addition, adsorption studies revealed that LIFM-12 performs exceptional adsorption selectivity of 10–25 for CO2versus light gases N2, CH4, and CO and up to 200 for C3H6versus CH4.

Reversible adiabatic temperature change in the shape memory Heusler alloyNi2.2Mn0.8Ga: An effect of structural compatibility

The large magnetocaloric effect (MCE) observed in Ni-Mn based shape-memory Heusler alloys put them forward to use in magnetic refrigeration technology. It is associated with a first-order magnetostructural (martensitic) phase transition. We conducted a comprehensive study of the MCE for the off-stoichiometric Heusler alloy Ni$_{2.2}$Mn$_{0.8}$Ga in the vicinity of its first-order magnetostructural phase transition. We found a reversible MCE under repeated magnetic field cycles. The reversible behavior can be attributed to the small thermal hysteresis of the martensitic phase transition. Based on the analysis of our detailed temperature dependent X-ray diffraction data, we demonstrate the geometric compatibility of the cubic austenite and tetragonal martensite phases. This finding directly relates the reversible MCE behavior to an improved geometric compatibility condition between cubic austenite and tetragonal martensite phases. The approach will help to design shape-memory Heusler alloys with a large reversible MCE taking advantage of the first-order martensitic phase transition.

Kinetic and Electrochemical Reaction Mechanism Investigations of Rodlike CoMoO4 Anode Material for Sodium-Ion Batteries.

Sodium-ion batteries are considered the most promising power source for electrical energy storage systems because of the abundance of sodium and their significant cost advantages. However, high-performance electrode materials are required for their successful application. Herein, we report a monoclinic-type CoMoO4 material which is synthesized by a simple solution method. An optimized calcination temperature with a high crystallinity and a rodlike morphology of the material are selected after analyzing the as-synthesized powder by temperature-dependent time-resolved X-ray diffraction. The CoMoO4 rods exhibit initial discharge and charge capacities of 537 and 410 mA h g-1, respectively, when used as an anode for sodium-ion batteries. The sodium diffusion coefficient in the bimetallic CoMoO4 anode is measured using the galvanostatic intermittent titration technique and calculated in the range of 1.565 × 10-15 to 4.447 × 10-18 cm2 s-1 during the initial cycle. Further, the reaction mechanism is investigated using ex situ X-ray diffraction and X-ray absorption spectroscopy, and the obtained results suggest an amorphous-like structure and reduction/oxidation of Co and Mo during the sodium insertion/extraction process. Ex situ transmission electron microscopy and energy-dispersive spectroscopy images of the CoMoO4 anode in fully discharged and recharged state reveal the rodlike morphology with homogenous element distribution.