A Mössbauer study of octahedral site symmetries in systems Co x Mn3−x−y Fe y O4 and Ni x Mn3−x−y Fe y O4

A Mössbauer study of cation distribution in systems Co _x Mn_3?x?y Fe _y O₄ and Ni _x Mn_3?x?y Fe _y O₄ has been made. It has been found that in both systems all specimens withy<0.6 value exhibit quadrupole doublets corresponding to site symmetries Fe³⁺(I) and Fe³⁺(II) of octahedral site. As more and more cobalt or nickel is introduced into the matrix the intensity of the inner quadrupole doublet increases while on introducing iron that of the outer quadrupole doublet increases. After a certain concentration of iron the inner doublet starts becoming more intense. It is suggested that this arises possibly from the substitution of cations in the second co-ordination sphere of Fe³⁺(I) and Fe³⁺(II) sites. Fory>0.6 the Mössbauer spectra show relaxation effects.     

A Mössbauer study of hyperfine interaction in the systems Co x Mn3−x−y Fe y O4 and Ni x Mn3−x−y Fe y O4

A Mössbauer study of systems Co _x Mn_3?x?y Fe _y O₄ and Ni _x Mn_3?x?y Fe _y O₄ for values ofx=0·1, 0·5, 1·0 andy ranging from 0·1 to 2·0 in steps of 0·2 have been made. At room temperature samples fory values ranging in between 0·1 to 0·5 exhibit paramagnetic behaviour while all spectra for values ofy between 0·6 to 0·8 show relaxation effects. Well-defined hyperfine Zeeman spectra are observed for all the samples withy>0·8 and resolved in two sextets corresponding to octahedral and tetrahedral site symmetries and a central doublet probably due to the presence of super-paramagnetic particles in the system. The hyperfine field at⁵⁷Fe nucleus reduces with decreasing iron cobalt and nickel concentration. These observations have been explained in terms of site preference of cations and exchange interactions.     

Bonding in the Doubly Disordered Ba1−x Sn x Cl1+y F1−y Solid Solution

New materials were prepared in the SnF₂/BaCl₂ system by precipitation, and in the SnF₂/BaCl₂/BaF₂ system by direct reactions at high temperature in dry conditions. Stoichiometric BaSn₂Cl₂F₄ and BaSnClF₃?0.8H₂O, and a wide Ba_1?x Sn _x Cl_1+y F_1?y solid solution were prepared for the first time. Elemental analysis, X-ray diffraction and ¹¹⁹Sn Mössbauer spectroscopy were used for the characterization and study of bonding in the new materials. Mössbauer spectroscopy was shown to be an excellent method for probing both the type of bonding at tin(II) (ionic or covalent) and the bond strength at the tin sublattice. Tin(II) is covalently bonded in the stoichiometric phases and ionic (Sn²⁺ stannous ion) in the precipitated Ba_1?x Sn _x Cl_1+y F_1?y solid solution. The case of Ba_1?x Sn _x Cl_1+y F_1?y prepared in dry conditions is more complex. At negative y values (Cl: F <1) and more particularly at high x (solid solution rich in tin), a mixture of Sn²⁺ and covalent Sn(II) is observed, with a normal sublattice strength for Sn(II). At positive y values (Cl:F >1) and more particularly at low x (poor in tin), all the tin(II) is in the ionic form. Furthermore, at high x and high y, the tin(II) sublattice strength decreases so drastically that the tin recoil free fraction at ambient temperature is nearly zero. The bonding type and tin sublattice strength can be explained in terms of preference of covalent bonding with F and when tin clustering occurs, whereas an excess Cl around Sn(II) forms ionic bonding and tin rattling due to ionic size mismatch.     

Search for the stannous ion in a chloride/fluoride matrix: cases of Ba1−x Sn x Cl1+y F1−y and of Ba2SnCl6

With the exception of anhydrous SnCl₂, in divalent tin fluorides and chlorides, tin(II) is always covalent bonded, i.e. its valence orbitals are hybridized and the tin lone pair is located in one of the hybrid orbitals. This lone pair is highly stereoactive and generates a large efg, resulting in a large quadrupole splitting. A doubly disordered Ba_1?x Sn _x Cl_1+y F_1?y solid solution has been prepared and found to contain either ionic tin(II) (Sn²⁺ ions) or a mixture of ionic and covalent tin(II), depending on x, y and the method of preparation. The ionic tin(II) spectrum in Ba_1?x Sn _x Cl_1+y F_1?y gives a Mössbauer single line that is broadened by the lattice efg, like in SnCl₂. Now, Sn²⁺ has been found to be present in an undistorted octahedral coordination in a newly isolated compound, Ba₂SnCl₆. It should be the first example of Sn²⁺ that is fully ionic and has a perfectly spherical lone pair.     

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La_{1 ? x} Ag _y MnO₃ (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρ_FM(T) = ρ₀ + AT ² + BT ^4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.     

Mössbauer studies of ɛ-Fe3−x Ni x N and γ′-Fe4−y Ni y N nanoparticles

Nanocrystalline ?-Fe_3?x Ni _x N (0.0?≤?×?≤?0.8) particles are synthesized by precursor technique and nitridation of decomposed products in NH₃ (g) in the temperature range 673 K-823 K. For x?=?0.1–0.4 compositions, single phase ?-Fe_3?x Ni _x N hexagonal structure with space group P6₃/mmc is formed, while for x?=?0.5–0.8, fcc γ′-Fe_4?y Ni _y N phase is also precipitated. The room temperature Mössbauer spectrum for all the compositions shows the presence of superparamagnetic doublet, which is attributed to ?-Fe_3?x Ni _x N phase. For x?=?0.5–0.8 compositions, two additional sextets are observed corresponding to two different iron sites, the corner position (Fe^c) and the fcc position (Fe^f), in γ′-Fe_4?y Ni _y N. The added Ni atoms preferentially substitute the corner Fe^c positions. The isomer shift, quadrupole splitting and hyperfine field values are found to change with the Ni content.     

Mössbauer study of super and semi-conducting samples of57Fe-doped (La2−y Sr y ) CuO4−δ

From Mössbauer and resistivity measurements on (La_2?y Sr _y ) CuO_4?δ samples doped with 5/1000⁵⁷Fe, three successive phases are observed depending on they value: a semiconducting antiferromagnetic phase fory<0.02; a semiconducting phase with two-dimensional spin-glass Cu magnetic order for 0.02<y<0.07; and a superconducting phase fory>0.07. The Mössbauer data show that a Cu spin-glass order survives in the superconducting phase up toy～0.15. Fory>0.15, a different type of magnetic ordering is observed on⁵⁷Fe, which is clearly governed by the iron impurities interacting through the Cu sublattice.     

No Zero Energy States for the Supersymmetric x 2 y 2 Potential

We show that the positive supersymmetric matrix-valued differential operator H = p _x ² + p _y ² + x ² y ² + x₃ + y₁ has no zero modes, i.e., H = 0 implies = 0. The result depends on a virial type argument for the corresponding supercharge. The model may be regarded as a simple relative of dimensional reductions of supersymmetric Yang–Mills theories.     

Obtaining YBa2Cu3O y crystals via YO1.5-BaO-CuO x melt electrolysis

Mass transfer during the melt electrolysis of Y_0.02Ba_0.30Cu_0.70O _y and Y_0.02Ba_0.25Cu_0.75O _y samples is investigated at a temperature of 950°C (0.5 h) and a current of 5–10⁵⁰ A in a cell. Crystal deposits of YBa₂CVu₃O_{6 + δ} tetragonal oxide (123) are grown, and their cation composition and structure are investigated by means of X-ray phase analysis, electron diffraction, elemental analysis, and high resolution on a transmission electron microscope. Deviation of the cation composition of oxide (123) from the stoichiometric ratio and its nanostructured state at nanocrystallite sizes of 2–5 nm are observed. The temperature dependence of magnetic susceptibility after oxygen annealing (450°C, 1 h) has four curve bends, indicating there are four superconducting phases with T _s = 45, 52, 75, and 86 K.     

Mössbauer studies in amorphous Fe80−x−y Ni x Mn y B12Si8 alloys

Room temperature Mössbauer studies have been carried out on amorphous Fe_80?x?y Ni _x Mn _y B₁₂Si₈ (x=35, 40, 45, 50 and 0≤y≤1). Thex=40 and 45 samples, showed an increase in the average hyperfine field, wheny increased from 0.0 to 0.2 at% in the former and 0.0 to 0.5 at% in the latter. The hyperfine field decreased fory=1.0. This is similar to the results of the magnetic moment on these samples, published earlier.     

Investigation of La1−x Ca x FeO3−y (0≦x≦0.50) by means of X—ray diffraction and Mössbauer spectroscopy

Compounds La_1?x Ca _x FeO_3?y (0≤x≤0.50) were prepared and characterized by X-ray diffraction and Mössbauer measurements. The diffraction patterns were defined to be orthorhombic. The lattice constants of orthorhombic perovskite La_1?x Ca _x FeO_3?y decrease linearly with increasingx. The Mössbauer spectra at room temperature indicate that the Néel temperature drops and the isomer shift decreases with increasingx. The spectrum of La_0.50 Ca_0.50 FeO_3?y at 80 K shows two hyperfine splitting patterns which may be related to the Fe³⁺ and Fe⁵⁺ ions.     

Magnetic behaviour of Eu x (Rh1−y Fe y )3B2;x=0.6, 0.75, 1.04 andy=0.02

⁵⁷Fe ME spectra taken for Eu_x(Rh_1–yFe_y)₃B₂ compounds (x=0.6, 0.75, 1.04 and y=0.02) show quadrupole doublet and a magnetically split pattern for T < T_c ^*. Above T_c ^* the spectra merge into a single quadrupole doublet. It is seen that the addition of Fe shifts the T_c corresponding to undoped compound to T_c ^*, T_c ^*>300 K for all the three samples.     

Structural and electronic properties of zincblende phase of Tl x Ga1−x As y P1−y quaternary alloys: First-principles study

Using the first-principles band-structure method, we have calculated the structural and electronic properties of zincblende TlAs, TlP, GaAs and GaP compounds and their new semiconductor Tl _x Ga_1?x As _y P_1?y quaternary alloys. Structural properties of these semiconductors are obtained with the Perdew and Wang local-density approximation. The lattice constants of Tl _x Ga_1?x As, Tl _x Ga_1?x P ternary and Tl _x Ga_1?x As _y P_1?y quaternary alloys were composed by Vegard’s law. Our investigation on the effect of the doping (Thallium and Arsenic) on lattice constants and band gap shows a non-linear dependence for Tl _x Ga_1?x As _y P_1?y quaternary alloys. The band gap of Tl _x Ga_1?x As _y P_1?y , E _g (x, y) concerned by the compositions x and y. To our awareness, there is no theoretical survey on Tl _x Ga_1?x As _y P_1?y quaternary alloys and needs experimental verification.     

Photoluminescence properties of heavily doped heterostructures based on (Al x Ga1 − x As)1 − y Si y solid solutions

It has been established that the photoluminescence spectra of heavily doped heterostructures based on Al _x Ga_{1 ? x} As)_{1 ? y} Si _y solid solutions exhibit quenching of the main exciton bands of Al _x Ga_{1 ? x} As ternary solid solutions and appearance of other maxima. The quenching of the main exciton bands can be associated both with the DX-center formation and with the change in the character of the band structure of (Al _x Ga_{1 ? x} As)_{1 ? y} Si _y quaternary solid solutions.     

Preparation,stoichiometry and magnetic properties of Cu y Cr2Se4−z Br x spinels with 0.5≲x≲2

The chemical composition of Cu _y Cr₂Se_4?z Br _x spinels depends strongly on the preparation parameters. Spinels with 0.8?y?1.2, 0.5?x?2, and 0?z?x?0.2 have been prepared. Whereas the lattice constanta _o of these spinels differs only by less than approximately 0.6%, their Curie temperatureT _c depends sensitively on the spinel composition. For Cu_1.1Cr₂Se_3.4Br_o.46,a _o=1.0410 nm andT _c=310 K were found to compared witha ₀=1.0447 nm andT _c=84 K of CuCr₂Se₂Br₂.     

Phase transition of the doubly disordered Ba1−x Sn x Cl1+y F1−y solid solution at high temperature

The Ba_1−x Sn _x Cl_1+y F_1−y solid solution has the BaClF structure, with full disorder of the cations and partial disorder of the anions. It can be prepared either by the wet method or the dry method. Most samples prepared by the aqueous route give a tin(II) single line at ca. 4.1 mm/s characteristic of the Sn²⁺ stannous ion, and therefore of ionic bonding. In some rare cases, a very weak quadrupole doublet indicates the presence of a small amount of covalently bonded tin(II). The solid solution prepared by the dry method has different unit-cell parameters, and it contains a larger amount of covalently bonded tin, except at very low x values or highly positive y values, and the Sn²⁺ recoil-free fraction is much smaller. In the present work, a study of the phase transition that takes place when precipitated Ba_1−x Sn _x Cl_1+y F_1−y is heated has been undertaken. A dramatic increase of the amount of covalently bonded tin(II) occurred on heating.     

Neutral current cross-section measurement at low Q 2 and high y with the ZEUS detector at HERA

The measurements of the reduced cross-sections for e ^?+? p deep inelastic scattering at high inelasticities y for three different centre-of-mass energies, 318, 251 and 225 GeV have been extended to lower momentum transferred squared, Q ². The analysis of satellite vertex events allows one to extend the cross-section measurement at high y down to Q ² = 4.5 GeV², substantially lower that the previously published cross-section measurement from which the longitudinal structure function, F _L, was extracted.     

57Fe Mössbauer study of Li x Fe1-y Co y PO4 (y = 0, 0.1, 0.2) as cathode material for Li-ion batteries

Lithium iron phosphates LiFe_1-y Co _y PO₄ (y = 0, 0.1, 0.2) exposed to a charging process were studied by ⁵⁷Fe Mössbauer spectroscopy taking into account XRD and SEM data. Hyperfine parameters of the spectra were determined above and below the magnetic ordering temperature for all the samples. It was shown that the presence of Co impurity atoms in lithium phosphates gives no effect on the hyperfine interaction of ⁵⁷Fe²⁺ cations. However, Co atoms in the nearest cation environment of Fe atoms lead to a significant change of the hyperfine interactions of ⁵⁷Fe³⁺ cations. The Co impurity atoms distribution over the positions of the iron atoms in the structure is found not to be statistical,but correlated.     

57Fe,119Sn and151Eu Mössbauer study of EuBa2(Cu1−x−y 119Sn x 57Fe y )3O7−δ superconductor

¹⁵¹Eu,¹¹⁹Sn and⁵⁷Fe Mössbauer spectroscopy was used to study a EuBa₂(Cu_1?x?y Sn _x Fe _v )₃O_7?δ superconductor in which sites were replaced by all three Mössbauer isotopes in the same sample in order to get information about the site preferences and structural changes. We have found changes in the Mössbauer parameters of the⁵⁷Fe,¹¹⁹Sn and¹⁵¹Eu spectra compared to those recorded in the separately substituted EuBa₂Cu₃O_7?δ, EuBa₂(Cu_1?x Sn _x )₃O_7?δ, YBaCu_3?x Fe _x O_7?δ YBaCu_3?x Sn _x O_7?δ materials. These results can be interpreted as a consequence of the site preferences and the changes in the structure due to the presence of other additional substitutional elements.     

Phase diagrams of (d x 2−y 2+id xy) superconductors on a square lattice

A model of strongly coupled electrons on a square lattice with attraction of the electrons to nearest-neighbor and next-nearest neighbor lattice sites is studied. For this model, the phase diagrams containing d _x ²−y ², d _xy, and (d _x ²−y ²+id _xy) states are constructed in the variables temperature versus chemical potential for different ratios of the corresponding potentials. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 356–360 (10 March 1998)