Hydrogen adsorption on low-index surfaces of B2 titanium alloys

Physics of the Solid State - Systematic first-principles calculations of hydrogen adsorption on two surfaces (001) and (110) of B2 titanium alloys with inclusion of complete relaxation of the...     

Order-to-order phase transition in alloys with face-centered superstructures

The possibility of a temperature phase transition from one superstructure in a binary alloy to a superstructure of another type is theoretically considered. A possible phase diagram for these transitions is constructed.Translated from Izvestiya Vysshikh Uchebnykh. Zavedenii, Fizika, No. 8, pp. 26–30, August, 1982.     

Low temperature phase transition in ZnSe doped with nickel

Neutron diffraction and ultrasonic experiments as well as measurements of heat conductivity in ZnSe and Zn_1−xNi_xSe (x=0.0025) semiconductors have been carried out. As a result, a structural transition induced by Ni impurity has been found at      

New Heusler alloys with a metamagnetostructural phase transition

Investigations of new ferromagnetic shape-memory Ni-Mn-Z Heusler alloys (Z = In, Sn, Sb) are reviewed. Experimental data are described and explained on the assumption that these alloys undergo a phase transition from the ferromagnetic to the antiferromagnetic state (metamagnetic transition). The results of theoretical studies of the phase diagrams of these alloys are considered with regard to the possible change in the character of magnetic ordering (from ferromagnetic to antiferromagnetic) and interaction of the structural martensitic transformation with the metamagnetic transition.     

First order magnetic transition in doped CeFe2 alloys: phase coexistence and metastability

First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped CeFe2 alloys is studied with the micro-Hall probe technique. Clear visual evidence of magnetic phase coexistence on micrometer scales and the evolution of this phase coexistence as a function of temperature, magnetic field, and time across the first order FM-AFM transition is presented. Such phase coexistence and metastability arise as a natural consequence of an intrinsic disorder-influenced first order transition. The generality of these phenomena involving other classes of materials is discussed.     

Planar-homeotropic transition observed for B2 phase of banana-shaped thioester

1,3-Phenylene bis{4-[(4-dodecyloxybenzoyl) sulfanyl] benzoate} (12OSOR) possessing banana-like achiral molecules has been studied by complementary methods to investigate linear dielectric as well as antiferroelectric properties of its B₂ phase. To this end, the following experimental methods were used: differential scanning calorimetry, polarizing microscopy, dielectric spectroscopy and reversal current method. As found, 12OSOR compound with bent-core molecules exhibits only B₂ phase that happens to be antiferroelectric. Dielectric spectra have been measured using the dielectric spectrometer based on Agilent 4294A impedance analyzer. The measurements have been done using HG 5 µm cells (giving homogeneous orientation) with gold electrodes. Thermal, dielectric, and electro-optic properties of 12OSOR are discussed.     

Martensitic phase transition due to soft phonon mode in β′ phase alloys

The soft phonon modes relating to the martensitic phase transition in the β′ alloys are analysed by group- theoretical methods. The characteristic of the transition in the O⁵_h type β′ alloy are different from that of the O¹_h type one.     

Investigation of a magnetic phase transition in fcc iron-nickel alloys

A magnetic phase transition in carbon-doped (0.1 and 0.7 at. %) Fe₇₀Ni₃₀ Invar alloys was investigated by the method of depolarization of a transmitted neutron beam and by small-angle scattering of polarized neutrons. It is shown that for both alloys, two characteristic length scales of magnetic correlations coexist above T _c. Small-angle scattering by critical correlations with radius R _c is described well by the Ornstein-Zernike (OZ) expression. The longer-scale (second) correlations, whose size can be estimated from depolarization data, are not described by the OZ expression, and hypothetically can be modeled by a squared OZ expression, which in coordinate space corresponds to the relation 〈M(r)M(0)〉∝exp(−r/R _d), where R _d is the correlation length of the second scale. The temperature dependence of the correlation radius R _c was obtained: R _c ∝ ((T−T _c)/Tc)^−ν , where ν≈2/3 is the critical exponent for ferromagnets, over a wide temperature range up to T _c ^exp , at which the correlation radius becomes constant and equals its maximum value R _c(T _c)=R _c ^max . The maximum correlation radius established (R _c ^max =140 Å and 230 Å for the first and second alloys, respectively) characterizes the length-scale of the fluctuation for which the appearance of critical correlations first results in the formation of a ferromagnetic phase, and the phenomenon itself exhibits a “disruption” of the second-order phase transition at T=T _c ^exp , as a result of which a first-order transition arises. Temperature hysteresis was also detected in the measured polarization of the transmitted beam and intensity of small-angle neutron scattering in the alloy above T _c, confirming the character of this magnetic transition as a first-order transition close to a second-order transition. Zh. éksp. Teor. Fiz. 112, 2134–2155 (December 1997)     

Reversible structural phase transition in Ni-Mn-Ga alloys in a magnetic field

In Ni_2+x Mn_1?x Ga shape-memory ferromagnetic alloys with coincident magnetic and structural phase transitions, a reversible structural field-induced phase transition was observed at constant temperature and pressure in magnetic fields of about 10 T. Computational results are in qualitative agreement with experiment.     

Defect model of the solid-to-smectic B phase transition

The going out of register of adjacent two-dimensional lattices is attributed to the formation of infinitely extended lines of maladjustment. Continuum theory is used to derive line energy and width. An elastic modulus of reasonable order of magnitude is estimated from the transition temperature.     

The magnetic phase transition in titanium oxide induced by proton irradiation

We have studied the magnetic properties of ⁵⁷Fe-doped TiO₂ compounds irradiated by proton with 0, 5 and 10 pC/μm². We have observed the enhancement of the magnetic moment, measured by superconducting quantum interference device magnetometer, with increasing proton irradiation ranging from 0 to 10 pC/μm². Mössbauer spectra of proton irradiated Ti_0.99⁵⁷Fe_0.01O₂ samples were taken at room temperature. Two sites of the wing (sextet) and the central (doublet) are shown in the spectra, which suggest the magnetically ordered phase and the paramagnetic phase, respectively. With increasing proton irradiation, the part of Fe³⁺ ions was converted to Fe²⁺ ions by compensation charge. This clearly suggests that the enhancement of magnetic moment after proton irradiation is contributed to the moment by the spin-orbit coupling of Fe²⁺ ions.     

Magnetic phase transition in MnTe2

The magnetic phase transition in MnTe₂ has been investigated by neutron powder diffraction. MnTe₂ orders at T_N=86.6 K with a second-order transition. Strong critical scattering has been observed both below and above T_N and the critical scattering seems to be centered at a slightly higher angle than the magnetic 100 Bragg peak.     

f-State occupancy at the gamma-alpha phase transition of Ce-Th and Ce-Sc alloys

Resonant inelastic x-ray scattering spectra were measured for a series of Ce solid solutions (Ce-Th and Ce-Sc) across the gamma-alpha phase transition. They reveal a well-defined feature associated with the 4f2 configuration when the incident energy is tuned to the Ce L3 preedge region. This component is normally hidden in x-ray absorption spectra because of lifetime broadening. The f1/f2 ratio estimated by resonant inelastic x-ray scattering presents a sharp drop across the gamma-alpha transition and hysteresis as a function of temperature that closely resemble the magnetization loop. These measurements confirm recent dynamical mean-field theory calculations that unexpectedly predict significant double occupancy of f orbitals in the ground state.     

Internal energy and parameters of the order-disorder phase transition in titanium monoxide TiO y

Quantum-mechanical ab initio calculations are used to simulate the free energy functions for titanium monoxide TiO _y . The effect of the long-range order of the Ti₅O₅ type superstructure on the internal energy of the compound is studied by the supercell method. The dependences of the configuration entropy and free energy on the long-range order parameter are determined. It is found that the order-disorder phase transition in titanium monoxide must occur in accordance with the mechanism of the first-order phase transition with a critical value of the long-range order parameter of 0.971. The calculated parameters of the phase transition are compared with the experimental data and the results obtained using the model of point charges and by calculating the Madelung energy. It is concluded that the short-range order and the phonon entropy must be taken into account in calculating the equilibrium phase diagrams for strongly nonstoichiometric compounds.     

Study of the phase transitions b2-a2 and B32-A2 in alloys using the modified Kirkwood method

The modified Kirkwood method gives an improved treatment of the problem because both intraphase and heterophase fluctuations in the alloy can be taken into account. In other methods of the theory of ordering each superstructure must be treated as a separate case, whereas the advantage of the modified method is that the resulting expression for the free energy of the alloy describes the ordering in both the B2 and B32 phases, as well as the decomposition of the alloy. The reduced B2-A2 transition temperature obtained here agrees more closely with the asymptotic estimates from series expansions than the results of other approximations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 11–16, August, 1989.     

Effect of alloying on the structure and phase transformations in Ni-Mn alloys alloyed by titanium

The crystal structure of NiMn alloy alloyed by titanium in a wide range of temperatures and compositions has been investigated using resistivity measurements, transmission electron microscopy, electron diffraction, and X ray diffraction. It is found that alloying by titanium not only decreases the martensitic transformation temperature but also changes the martensite crystal structure. The martensitic transformation temperatures are determined and the diagram of martensitic transformations for Ni₅₀Mn_{50 ? x} Ti _x alloys is constructed.     

Influence of chemical and phase composition on the emergence of the electroplastic effect in titanium alloys

The nature of differently directed stress jumps observed in stress-strain diagrams with tension under a pulsed current is analyzed. It is shown that the amplitude and direction of stress jumps are determined by competition between the electroplastic effect (EPE) and phase transformation, while the EPE is a structurally sensitive property. The EPE diminishes when the structure is refined and even disappears in the nanocrystalline state.