First-order Structural Transformation Research Articles

Martensitic transformation and magnetocaloric effect of Cr-doped Mn–Ni–Fe–Ti all-d-metal Heusler alloys near room temperature

Since the discovery of all-d-metal Heusler alloys, Ni–Co–Mn–Ti quaternary alloys have been widely studied and reported, but there are few studies on Mn-rich Mn–Ni–Ti all-d-metal Heusler alloys. In the present work, the structure, martensitic transformation, and magnetocaloric properties of the Mn50-xCrxNi36Fe4Ti10 (x = 1, 2, and 3) alloys were investigated. The behavior of the alloys during martensitic transformation accompanied by thermal hysteresis indicates the nature of first-order structural transformation, and the doping of Cr results in the transition temperature closer to the room temperature. Martensitic transformation temperature decreases from 309 K to 288 K with increasing Cr content. For this system, the inverse of the magnetic susceptibility follows the Curie-Weiss law, and the Curie paramagnetic temperatures (TCW) obtained by fitting in the high-temperature paramagnetic region are 170 K (x = 1), 119 K (x = 2), and 116 K (x = 3), respectively. The contribution of the magnetic entropy change to the total entropy is not large because of the paramagnetic nature of the alloys. The magnetic entropy changes ΔSm of the alloys are 0.58 Jkg−1K−1 (x = 1), 0.72 Jkg−1K−1 (x = 2), and 0.29 Jkg−1K−1 (x = 3) at room temperature under 5 T magnetic field.

Structural ordering, magnetic and electrical transport properties in Ni60-xFe13+xGa27 Heusler alloys

The evolutions of phase transformation, magnetic and electrical transport properties with the change of Fe concentration have been systematically investigated in a series of Ni60-xFe13+xGa27 (x = 0–10) Heusler alloys. The structure and heat flow measurements reveal every sample undergoes a disordered-ordered transition from a disordered B2 structure to a L21 ordered structure, and a first-order structural transformation from the L21 cubic structure to a L10 tetragonal structure is corresponding to the martensitic transformation (MT). With increasing Fe concentration, the disordered-ordered transition and ferromagnetic transition shift to higher temperature, while the MT shifts to lower temperature. According to phase diagram established in the present investigation, it is also found that the increase of Fe content can convert the first-order MT from a paramagnetic type to a ferromagnetic one, which is accompanying by an obvious increase of both effective and spontaneous magnetic moments. Furthermore, the calculated magnetocrystalline anisotropy constant (K1) of the studied alloys monotonically increases as Fe increases. It is evident that the magnetic degrees of freedom in these two MTs remain almost unchanged so that the electron-spin scattering doesn't change, which results in the resistivity difference between the austenitic and martensitic states is feeble. Such an intrinsic nature leads to a weak magnetoresistance (MR) and baroresistance (BR) effects in the studied samples because of only contribution of electron-lattice scattering change on the resistivity. This demonstrates from a negative perspective that the electron-spin scattering change originated from the variation in the magnetic degrees of freedom acts as a dominant role in improvement of the MR and BR effects for the Heusler system possessed the first-order MT.

Superconductivity in efficient thermoelectric material Cu3Sb0.98Al0.02Se4

Inspired by the rich physical properties of thermoelectric materials, we have investigated the electrical transport, vibrational, and structural properties of efficient thermoelectric material Cu3Sb0.98Al0.02Se4 under pressure up to 40.1 GPa. At a critical pressure of 8.5 GPa, a superconducting phase sets in and persists in the pressure range studied. A superconducting dome in the temperature-pressure phase diagram of Cu3Sb0.98Al0.02Se4 was observed. Meanwhile, pressure-induced structural transition from its initial phase to disorder body-centered cubic structure in Cu3Sb0.98Al0.02Se4 has been verified by the X-ray diffraction and Raman scattering measurements. Interestingly, the onset of superconductivity coincides with the structural transition, accompanied with the reduction of volume. The superconducting phase was observed in the ultimate formation of the Cu–Sb–Al–Se alloy and was determined to be Im-3m. Based on Hall coefficient measurements, we evaluated the carrier concentration of Cu3Sb0.98Al0.02Se4, which shows a dramatic increase in the superconducting state. These results suggest that the novel superconductor is realized through the charge transfers and the first-order structural transformation.

Sequential Spin State Transition and Intermetallic Charge Transfer in PbCoO3.

Spin state transitions and intermetallic charge transfers can essentially change material structural and physical properties while excluding external chemical doping. However, these two effects have rarely been found to occur sequentially in a specific material. In this article, we show the realization of these two phenomena in a perovskite oxide PbCoO3 with a simple ABO3 composition under high pressure. PbCoO3 possesses a peculiar A- and B-site ordered charge distribution Pb2+Pb4+3Co2+2Co3+2O12 with insulating behavior at ambient conditions. The high spin Co2+ gradually changes to low spin with increasing pressure up to about 15 GPa, leading to an anomalous increase of resistance magnitude. Between 15 and 30 GPa, the intermetallic charge transfer occurs between Pb4+ and Co2+ cations. The accumulated charge-transfer effect triggers a metal-insulator transition as well as a first-order structural phase transition toward a Tetra.-I phase at the onset of ∼20 GPa near room temperature. On further compression over 30 GPa, the charge transfer completes, giving rise to another first-order structural transformation toward a Tetra.-II phase and the reentrant electrical insulating behavior.

Statistical model for the martensitic transformation simulation in Heusler alloys

In this work, the one-dimensional statistical model based on the phenomenological approach for the study of the magnetic and magnetocaloric properties in Heusler alloys is considered. The proposed model is approved for Ni-Co-Mn-In exhibiting both the first-order structural transformation and second-order magnetic phase transition. Temperature dependencies of magnetization and magnetic entropy change under applied magnetic field and external pressure are studied. The results of the simulations are in good agreement with the experimental data.

Equilibrium and metastable phase transitions in silicon nitride at high pressure: A first-principles and experimental study

We have combined first-principles calculations and high-pressure experiments to study pressure-induced phase transitions in silicon nitride (Si${}_{3}$N${}_{4}$). Within the quasi-harmonic approximation, we predict that the $\ensuremath{\alpha}$ phase is always metastable relative to the $\ensuremath{\beta}$ phase over a wide pressure-temperature range. Our lattice vibration calculations indicate that there are two significant and competing phonon-softening mechanisms in the $\ensuremath{\beta}$-Si${}_{3}$N${}_{4}$, while phonon softening in the $\ensuremath{\alpha}$-Si${}_{3}$N${}_{4}$ is rather moderate. When the previously observed equilibrium high-pressure and high-temperature $\ensuremath{\beta}$ $\ensuremath{\rightarrow}$ $\ensuremath{\gamma}$ transition is bypassed at room temperature (RT) due to kinetic reasons, the $\ensuremath{\beta}$ phase is predicted to undergo a first-order structural transformation to a denser $P\overline{6}$ phase above 39 GPa. The estimated enthalpy barrier height is less than 70 meV$/$atom, which suggests that the transition is kinetically possible around RT. This predicted new high-pressure metastable phase should be classified as a ``postphenacite'' phase. Our high-pressure x-ray diffraction experiment confirms this predicted RT phase transition around 34 GPa. No similar RT phase transition is predicted for $\ensuremath{\alpha}$-Si${}_{3}$N${}_{4}$. Furthermore, we discuss the differences in the pressure dependencies of phonon modes among the $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$ phases and the consequences on their thermal properties. We attribute the phonon modes with negative Gr\"uneisen ratios in the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases as the cause of the predicted negative thermal expansion coefficients (TECs) at low temperatures in these two phases, and predict no negative TECs in the $\ensuremath{\gamma}$ phase.

Ferroelectric and proton conducting behavior of a new elpasolite-related vanadium oxyfluoride (NH4,K)3VO2F4

A new elpasolite-type (NH4,K)3VO2F4 compound was prepared and characterized by x-ray diffraction, differential scanning calorimeter (DSC), impedance analysis, and electrical polarization measurements. It crystallizes in an orthorhombic lattice with unit-cell parameters: a = 8.9584(4), b = 18.6910(14), c = 6.2174(4) Å, V = 1041.04(11) Å3, Z = 6. NH4+, and K+ ions are distributed statistically over crystallographically four equivalent sites. There are two distinguishable vanadium atoms forming cis- and trans-VO2F4 octahedra present in the unit cell. High-temperature studies by DSC and in situ x-ray diffraction revealed a first-order structural transformation from orthorhombic to cubic lattice around 343 K. Impedance measurements show two different kinds of conductivity behaviors for the two phases. In orthorhombic phase a significant conductivity resulting from involvement of protonic species is observed. In the orthorhombic phase, a clear ferroelectric hysteresis loop is observed.

First order structural transformation and inverse magnetocaloric effect in melt-spun Ni–Mn–Sn ribbons

First order structural transformation (FOST) and inverse magnetocaloric effect (IMCE) were investigated in melt-spun ribbons of Ni49Mn37.4Sn13.6 and Ni50Mn34.5Sn15.5 Heusler alloys. Thermal and magnetic characterization of the ribbons revealed the existence of FOST. Interestingly, in the Ni49Mn37.4Sn13.6 ribbon, the structural transformation occurs prior to magnetic transition incongruous to that observed in the Ni50Mn34.5Sn15.5 ribbons. In addition to FOST, the ribbons also exhibit a large IMCE at low magnetic field. A maximum positive magnetic entropy change ΔSM ∼ 6.0 J kg−1 K−1 and ∼1.6 J kg−1 K−1 for a field change of 2 T was observed in the Ni49Mn37.4Sn13.6 and Ni50Mn34.5Sn15.5 ribbons, respectively.

Giant magnetocaloric effects by tailoring the phase transitions

The MnCoGe alloy can crystallize in either the hexagonal Ni2In- or the orthorhombic TiNiSi-type of structure. In both phases MnCoGe behaves like a typical ferromagnet with a second-order magnetic phase transition. For MnCoGeBx with B on interstitial positions, we discover a giant magnetocaloric effect associated with a single first-order magnetostructural phase transition, which can be achieved by tuning the magnetic and structural transitions to coincide. The results obtained on the MnCoGe-type alloys may be extensible to other types of magnetic materials undergoing a first-order structural transformation and can open up some possibilities for searching magnetic refrigerants for room-temperature applications.

First-principles comparative study of multiferroic compound PbVO 3

We report a comprehensive first-principles investigation of the structural, electronic, magnetic and phase transition properties in multiferroic compound PbVO 3 with systematic comparisons of various exchange-correlation (XC) functionals. The antiferromagnetic (AFM) insulating ground state of tetragonal phase has been obtained in the framework of the band theory, which is characterized by C-type two-dimensional AFM magnetic ordering in the ab plane. A first-order structural transformation from tetragonal phase to idea cubic perovskite structure takes place at 1.75 GPa, corresponding to the ferroelectric to paraelectric phase transition. Electronic structure calculations suggest that the ground state of the cubic paraelectric phase is a nonmagnetic orbital-disorder metal.

Magnetic and crystal structures ofEr5(SixGe1−x)4

The magnetic and crystallographic structures in theEr5(SixGe1−x)4 system have beeninvestigated for x = 1,0.875 and 0 using macroscopic magnetic measurements and neutron powder diffraction. Theexistence of a first-order structural transformation from a high-temperature orthorhombicO (I) structureto a low-temperature monoclinic M polymorph in the temperature range 160–220 K has been confirmed forEr5Si4. In contrast,compounds with x = 0.875 and 0 retain their room-temperature crystallographic phases—M andO (II), respectively—on cooling down to 2 K. Both Si-containing alloys adopt cantedferromagnetic structures below K with an easy magnetization direction along theb-axis and a weak antiferromagnetic coupling in the (010) plane. TheEr5Ge4 compound ordersantiferromagnetically at TN = 14 K in a complex incommensurate structure.

Molecular metals with widely tunable band filling. Structure/stoichiometry/counterion relationships in the electrochemistry of a cofacially joined polymeric phthalocyanine metal

The oxidative electrochemistry of the cofacially joined phthalocyanine polymer (Si(Pc)O){sub n} to yield molecular metals/conductive polymers of the type ((Si(Pc)O)X{sub y}){sub n} is studied by a combination of X-ray diffractometric and spectroscopic techniques. Electrochemical methodology includes controlled-potential coulometry and electrochemical potential spectroscopy (ECPS) applied to rapidly stirred slurries or to microcompactions of the solid polymer. For X{sup {minus}} = BF{sub 4}{sup {minus}} in acetonitrile, oxidation (doping) of as-polymerized orthorhombic (Si(Pc)O){sub n} to yield tetragonal ((Si(Pc)O)(BF{sub 4}){sub n}) (y {approx} 0.50) is accompanied by a significant overpotential, minimal tunability in y, and involves a first-order structural phase transformation. Electrochemical undoping occurs smoothly and over a broader potential range (0.90 V) to afford tetragonal (Si(Pc)O){sub n}, which is also accessible by thermally undoping ((Si(Pc)O)I{sub 1.1}){sub n}. In contrast to these results, ECPS studies of solid Ni(Pc) (monoclinic slipped-stack {beta} phase) reveal a first-order structural transformation to yield tetragonal Ni(Pc)(BF{sub 4}){sub y} (y {approx} 0.48) upon oxidative doping, and a subsequent first-order transformation to another slipped-stack Ni(Pc) structure (monoclinic slipped-stack {gamma} phase) upon undoping. Doping/undoping occurs over a relatively narrow potential range; consequently there is far less tunability in y than in the ((Si(Pc)O)X{sub y}){sub n} materials, and large overpotentials are observed. ECPSmore » studies of (Ge(Pc)O){sub n} reveal irreversible oxidative processes, and polymer decomposition via Ge-O bond cleavage is implicated.« less