<Emphasis Type="Italic">Ab initio</Emphasis> theory of the electronic structure and spectra of impurity <Emphasis Type="Italic">nl</Emphasis> ions

The fundamentals of the theory of the electronic structure of impurity clusters and the results of numerical calculations for the iron-, lanthanum-, and actinium-group ions in Me⁺ⁿ: [L]_k clusters are presented. The effects of the interionic distance and ligands in the Me⁺ⁿ: [L]_k clusters on the electronic structure of the nl ^N and nl^N?1n′l′ configurations of the 3d, 4f, and 5f ions are considered. The correspondence between the optical and x-ray spectra of different impurity crystals is also analyzed.     

Low-temperature heat capacity upon the transition from paramagnetic to ferromagnetic Heusler alloys Fe<Subscript>2</Subscript> <Emphasis Type="Italic">Me</Emphasis>Al (<Emphasis Type="Italic">Me</Emphasis> = Ti,V, Cr,Mn, Fe,Co, Ni)

The heat capacity of band magnets Fe₂MeAl (Me = Ti, V, Cr, Mn, Fe, Co, Ni) ordered in crystal structure L2₁ has been measured in the range 2 K ≤ T ≤ 50 K. The dependences of the Debye temperature Θ_D, the Sommerfeld coefficient γ, and the temperature-independent contribution to heat capacity C₀ on the number of valence electrons z in the alloys have been determined.     

Formation of a monolayer <Emphasis Type="Italic">h</Emphasis>-BN nanomesh on Rh (111) studied using <Emphasis Type="Italic">in-situ</Emphasis> STM

As a member of the 2D family of materials, h-BN is an intrinsic insulator and could be employed as a dielectric or insulating inter-layer in ultra-thin devices. Monolayer h-BN can be synthesized on Rh (111) surfaces using borazine as a precursor. Using in-situ variable-temperature scanning tunneling microscopy (STM), we directly observed the formation of h in real-time. By analyzing the deposition under variable substrate temperatures and the filling rate of the h-BN overlayer vacant hollows during growth, we studied the growth kinetics of how the borazine molecules construct the h-BN overlayer grown on the Rh surface.     

Microhardness and refractive index of titanium dioxide-based binary coatings

Glass composites covered by sol–gel TiO₂–m Me _x O _y (Me _x O _y = ZnO, CdO, SnO, CuO, and Fe₂O₃ and m = 2, or 10 wt %) binary oxide coatings have been studied. The microhardness of the composites and glass substrate has been measured, and the microhardness of the coatings has been determined from these measurements. A correlation between the microhardness of the coatings, their refractive index, and packing density of disperse sol phase particles in the coating has been established.     

Magneto-optical spectroscopy and optical detection of EPR spectra of Yb<Superscript>3+</Superscript> paramagnetic centers with cubic symmetry in single crystals <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Cd,Ca, Pb)

Cubic paramagnetic centers formed by Yb³⁺ impurity ions in fluorite-type crystals MeF₂ (Me = Cd, Ca, Pb) have been investigated using electron paramagnetic resonance, magnetic circular dichroism, magnetic circular polarization of luminescence, Zeeman splitting of optical absorption and luminescence lines, and optical detection of electron paramagnetic resonance. The g factors of the ²Γ₇ state in the excited multiplet ² F _5/2 of Yb³⁺ ions in Me F₂ crystals, the hyperfine interaction constant ¹⁷¹ A (¹⁷¹Yb) for the excited multiplet ² F _5/2 in the CaF₂ crystal, and the energies and symmetry properties of all energy levels of Yb³⁺ ions in MeF₂ crystals are determined. The crystal-field parameters for the crystals under investigation are calculated.     

Study of ferroelectric phase transitions in solid solutions based on bismuth vanadate

The structural, thermal, nonlinear optical, and conducting properties of Bi₄(V_{1 ? x} Me _x )₂O_{11 ? y} solid solutions (Me = Zr, Ga, Fe) have been investigated. The effect of domain wall pinning, accompanying the α ? β ferroelectric phase transition, is revealed. It is found that the conductivity in all studied series of solid solutions increases with an increase in x.     

Local structure of Tm<Superscript>2+</Superscript> and Yb<Superscript>3+</Superscript> impurity centers in <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Ca,Sr, Ba) fluoride crystals

The local structure of Tm²⁺ and Yb³⁺ cubic impurity centers in MeF₂: Tm²⁺ and MeF₂: Yb³⁺ (Me = Ca, Sr, Ba) fluoride crystals, as well as Yb³⁺ trigonal and tetragonal impurity centers in MeF₂: Yb ³⁺ crystals, is calculated within the shell model in the pair potential approximation.     

Über die Supraleitung von Hf-Rh-Legierungen

The transition temperaturesT _c of quenched Hf- Rh alloys are strongly increased in the hexagonal phase by Rh, similar to the homologous systems Ti-Rh and Zr-Rh. On the other hand a stabilization of the cubic high-temperature phase by Rh has not been possible. At higher amounts of Rh(>6 at%) the intermetallic compound Hf₂Rh with aT _c of 1.98 °K is formed. Moreover a transition temperature of 1.73 °K is found which is probably due to the intermetallic compound HfRh. The results above are interpreted with the help of X-ray diffraction and metallurgic techniques.     

Structure and thermoelastic martensitic transformations in ternary Ni–Ti–Hf alloys with a high-temperature shape memory effect

The effect of alloying by 12–20 at % Hf on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary alloys of the quasi-binary NiTi–NiHf section is studied by transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. The electrical resistivity is measured at various temperatures to determine the critical transformation temperatures. The data on phase composition are used to plot a full diagram for the high-temperature thermoelastic B2 ? B19’ martensitic transformations, which occur in the temperature range 320–600 K when the hafnium content increases from 12 to 20 at %. The lattice parameters of the B2 and B19’ phases are measured, and the microstructure of the B19’ martensite is analyzed.     

Magnetic properties of cobaltites doped with chromium,gallium, and iron ions

The magnetic and magnetotransport properties of cobaltites La_0.5Sr_0.5Co_1–xMe_xO₃ (Me = Cr, Ga, Fe) have been studied. The initial compound (x = 0) is a ferromagnet with T_C = 247 K and a saturation magnetization close to 2μ_B per formula unit. It has ben shown that chromium substitution (x = 0.2) decreases the spontaneous magnetization to 0.3μ_B, while the iron substitution (x = 0.2) does not change the magnetization. The obtained data have been interpreted in a model of positive superexchange interactions between cobalt and iron and negative superexchange interactions between cobalt and chromium.     

Ferrimagnetic nanoparticles for self-controlled magnetic hyperthermia

Based on the Heisenberg model including single-site uniaxial anisotropy and using aGreen’s function technique we studied the influence of size and composition effects on theCurie temperature T _C , saturationmagnetization M _S and coercivityH _C of spherical nanoparticles with astructural formulaM e _1?x Zn _x Fe₂O₄,Me = Ni, Cu, Co, Mn. It is shown that for x = 0.4–0.5and d = 10–20 nm these nanoparticles have aT _C  = 315 K and are suitable for aself-controlled magnetic hyperthermia.     

Effect of metal vacancies on the band structure of Nb,Zr, and Y diborides

The band structure of nonstoichiometric layered diborides Me_0.75B₂ (Me=Nb, Zr, or Y) is calculated for the first time in the framework of the self-consistent full-potential linearized muffin-tin orbital (FLMTO) method, and the energies of formation of Me vacancies are estimated numerically. It is established that metal vacancies affect the electronic properties and energy states of Group III–V metal diborides in radically different ways. The superconducting properties of these diborides are discussed with due regard for the results obtained.     

Effect of gallium alloying on the structure,the phase composition,and the thermoelastic martensitic transformations in ternary Ni–Mn–Ga alloys

The effect of gallium alloying on the structure, the phase composition, and the properties of quasibinary Ni₅₀Mn_50–zGa_z (0 ? z ? 25 at %) alloys is studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures is analyzed. A general phase diagram of the magnetic and structural transformations in the alloys is plotted. The temperature–concentration boundaries of the B2 and L2₁ superstructures in the austenite field, the tetragonal L1₀ (2M) martensite, and the 10M and 14M martensite phases with complex multilayer crystal lattices are found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to {011}_B2. Martensite crystals are twinned along one of the 24 \(24\left\{ {011} \right\}{\left\langle {01\bar 1} \right\rangle _{B2}}\) “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses.     

Phonon spectra and thermodynamic properties of rare-earth hexaborides

The phonon spectra g(ν) of rare-earth hexaborides MeB₃ are calculated both in the approximation MeB* + B (B* = B₆) without regard for bonding between metal and boron atoms and in the approximation making allowance for this bonding. The temperature dependences of the heat capacity are calculated from the dependences g(ν), and the results obtained are compared with the experimental data in the range 5–300 K. It is found that stretching vibrations in the metal and boron sublattices and between these sublattices variously affect the thermodynamic functions of hexaborides at low and high temperatures.     

Effect of copper on the structure–phase transformations and the properties of quasi-binary TiNi–TiCu alloys

The effect of copper alloying up to 25 at % on the structure–phase transformations and the physicomechanical properties of ternary alloys from the quasi-binary TiNi–TiCu section is studied by measuring the physicomechanical properties, transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction (XRD). The data of temperature measurements of the electrical resistivity and the magnetic susceptibility and XRD data are used to plot a general diagram for the thermoelastic B2 ? B19', B2 ? B19 ? B19', and B2 ? B19 martensitic transformations, which occur in the alloys upon cooling as the copper content increases in the ranges 0–8, 8–15, and 15–25 at % Cu, respectively. The experimental results are compared to the well-known data, including differential scanning calorimetry data, obtained for these alloys. The changes in the mechanical properties and the microstructure of the alloys in the state of B19 or B19' martensite are discussed.     

Specific features of the electrical resistivity of half-metallic ferromagnets Co<Subscript>2</Subscript> <Emphasis Type="Italic">Me</Emphasis>Al (<Emphasis Type="Italic">Me</Emphasis> = Ti,V, Cr,Mn, Fe)

The transport properties of half-metallic ferromagnetic Heusler alloys Co₂MeAl (Me = Ti, V, Cr, Mn, Fe are transition 3d metals) have been measured in the temperature range of 4–900 K. The specific features of the behavior of the resistivity have been considered in the framework of the two-current model of conductivity that takes into account the existence of the energy gap in the electronic spectra of the alloys near the Fermi level of one of electron subbands that differs in the spin direction.     

Magnetic phase diagram of the Bi<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

The magnetic and elastic properties of the Bi_1-xCa_xMnO₃ manganites are studied. The phase transformations revealed are ferromagnet-spin glass (x≥0.15) and spin glass-charge-ordered antiferromagnet (x≥0.25). The ferromagnetic state is characterized by ordering of the Mn³⁺d _x ²-y orbitals. It is suggested that thespin glass state originates from local static Jahn-Teller distortions. The antiferromagnetic charge-ordered and the spin-glass disordered phases coexist in samples with 0.25<x<0.32, which may be due to the charge order-disorder phase transformation being martensitic in character. The magnetic phase diagram is constructed.     

Line shape of <Superscript>57</Superscript>Co sources exhibiting self absorption

The effect of selfabsorption in Mössbauer sources is studied in detail. Spectra were measured using an old ⁵⁷ C o/R h source of 74M B q activity with an original activity of ca. 3.7G B q and a 0.15G B q ⁵⁷ C o/α ? F e source magnetized by an in-plane magnetic field of 0.2 T. The ⁵⁷ C o/α ? F e source of a thickness of 25 μ was used both from the active and the inactive side giving cause to very different selfabsorption effects. The absorber was a single crystal of ferrous ammonium sulphate hexahydrate (FAS). Its absorption properties were taken over from a detailed study (Bull et al., Hyperfine Interact. 94(1–3), 1; Spiering et al. 2). FAS (space group P21/c) crystallizes as flat plates containing the (\(\overline {2}\)01) plane. The γ-direction was orthogonal to the crystal plate. The ⁵⁷ C o atoms of the ⁵⁷ C o/R h source were assumed to be homogeneously distributed over a 6μ thick Rh foil and to follow a one dimensional diffusion profile in the 25 μ Fe-foil. The diffusion length was fitted to 10 μ. The theory follows the Blume-Kistner equations for forward scattering (Blume and Kistner, Phys. Rev. 171, 417, 3) by integrating over the source sampled up to 128 layers.     

Untersuchung der Zustandsdichte und der charakteristischen Energieverluste von Ir,Rh, Pt und Pd mit Isochromatenmessungen

X-ray isochromats of the above metals measured with high resolution are presented. Assuming a rigid band model for the four metals they give — with some usual approximations — the qualitative form of the density of states. It turns out that the densities of statesZ(ε) at the Fermi limits decrease in the succession Ir, Rh, Pt, Pd. This is in contradiction to low temperature measurements of the electronic specific heatc _e which for the same succession give an appreciable raising inZ(ε). It is assumed that this discrepancy is caused by a special coupling of the electrons to the transverse phonons. In consequence of this,Z(ε) of the renormalized electron system measured byc _e can be very different from that of the bare electrons measured by the isochromats. Assuming that the transition temperature of elemental superconductors is related according to BCS-theory to the latter density of states the absence of superconductivity of Rh, Pt and Pd can be understood. In addition the isochromats give values for the characteristic energy losses of the four metals.     

Superconductivity and metal-insulator transition in Cu(Ir<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Rh<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>S<Subscript>4</Subscript>

A new thiospinel CuIr₂S₄ exhibits a metal-insulator (M-I) transition at 226 K, while CuRh₂S₄ shows a superconducting transition at 4.70 K. We present a systematic study of electrical and magnetic properties of Cu(Ir_1?x Rh _x )₂S₄. TheM-I transition of CuIr₂S₄ is accompanied by a structural phase transition from tetragonal symmetry in insulating phase to cubic symmetry in high temperature metallic phase. With increasing Rh contentx, the sharpM-I transition shifts to lower temperature forx≦0.10. The samples show semiconductive behavior for 0.10≦0.30 between 4.2 and 300 K, and recover the metallic state forx≧0.50. The superconducting transition may occur for very close tox=1.00. Magnetic susceptibility shows the jump at theM-I transition temperature and the variation ofx leads to a systematic change of the magnetic susceptibilities, which is consistent with the electrical characteristic feature.