Raúl Gómez - Academia.edu (original) (raw)

Papers by Raúl Gómez

$Research paper thumbnail of El departamento de Física de la Facultad de Ciencias de la Universidad Nacional Autónoma de México:\ un estudio comparativo$

Revista Mexicana De Fisica, 1986

We study the similitudes and differences between YBa2Cu3- x Fe x Odelta and PrBa2Cu3- x Fe x Odel... more We study the similitudes and differences between YBa2Cu3- x Fe x Odelta and PrBa2Cu3- x Fe x Odelta systems, using Mössbauer spectroscopy. Both systems, with z=0.04, were studied at several temperatures. The spectra of PrBa2Cu3- x Fe x Odelta showed four asymmetric quadrupole doublets with apparently different line widths, which were fitted with four symmetric superimposed quadrupole doublets with the same line width, but assuming a small (˜4 6 kOe) residual magnetic field in the Fe sites, which are mainly the Cu(1) sites of the 1 2 3 structure. On the other hand, the PrBa2Cu3- x Fe x Odelta spectra showed only three asymmetric quadrupole doublets which can be fitted with three superimposed symmetric doublets and a residual magnetic field. In this case, the Fe atoms also occupy the Cu(1) sites of the 1 2 3 structure. The temperature variations of the Mössbauer spectra and their parameters seem to indicate that, in the Pr-system, a structural change takes place between 12 K and 40 K.

Physica B: Condensed Matter, 2020

Polycrystalline samples of the W 1-x Fe x B 3 (x ¼ 0.00, 0.15) were synthesized by the first time... more Polycrystalline samples of the W 1-x Fe x B 3 (x ¼ 0.00, 0.15) were synthesized by the first time by arc melting. The effect of the chemical pressure due to the substitution of tungsten by iron atoms on the crystal structure and electronic properties were studied by X-ray diffraction, electron microscopy, M€ ossbauer spectroscopy, X-ray photoelectron spectroscopy, and Density Functional Theory calculations. Rietveld refinement analysis indicate the presence of transition metal vacancies in the structure. The M€ ossbauer spectroscopy shows the presence of two non-equivalent crystallographic sites with different chemical environments, which are symptomatic of a charge density redistribution for one of the two iron sites and suggesting a vacancies formation mechanism. X-ray photoelectron spectroscopy and Density Functional Theory calculations show a similar density charge behavior for the tungsten atoms. Finally, an increment of electronic states at the Fermi level was predicted due to the partial iron substitution in the highest boride of tungsten.

Journal of Molecular Structure, 2016

The structural, elastic, vibrational, thermodynamic and electronic properties of the Mo 2 B inter... more The structural, elastic, vibrational, thermodynamic and electronic properties of the Mo 2 B intermetallic under pressure are assessed using firstprinciples calculations based on the generalized gradient approximation (GGA) proposed by Perdew-Wang (PW91). Our results show that the calculated structural parameters at a pressure of zero GPa are in good agreement with the available experimental data. The effect of high pressures on the lattice constants shows that the compression along the c-axis and along the a-axis are similar. The elastic constants were calculated using the static finite strain technique, and the bulk shear moduli are derived from the ideal polycrystalline aggregate. We find that the elastic constants, elastic modulus and hardness monotonically increase as a function of pressure; consequently, the structure is dynamically stable and tends from brittle to ductile behavior under pressure. The Debye temperature θ D increases and the so-called

Physica B: Condensed Matter, 2017

A sample of Bi 25 FeO 39 was synthesized by the molten salts method in 2 hours at 750ºC with KCl-... more A sample of Bi 25 FeO 39 was synthesized by the molten salts method in 2 hours at 750ºC with KCl-KBr as reaction media. The sample was characterized with X-ray powder diffraction and Mössbauer spectroscopy. The Rietveld refinement of the X-ray diffraction confirms that a sillenite type structure with a cell parameter a =10.190(4) Å was achieved, with only small traces of Bi 2 O 3. In order to get a Mössbauer spectrum with a good signal to noise ratio, the synthesis was made using 95% enriched α-57 Fe 2 O 3. To our knowledge, this is the first time that a Mössbauer spectrum for the Bi 25 FeO 39 has been recorded. The nonappearance of a magnetic sextet confirms its paramagnetic behavior. The measured Mössbauer parameters (IS=0.20±0.004 mm/s and QS=0.2±0.07 mm/s) correspond to Fe +3 in very symmetric surroundings, and rule out some interpretations given to the doublet observed in some Mössbauer spectra of BiFeO 3 .

Applied Clay Science, 2016

The sludge generated in the treatment of surface water for human consumption represents an enviro... more The sludge generated in the treatment of surface water for human consumption represents an environmental hazard which can be mitigated by the valorisation of this solid waste. This paper presents an analysis of the process parameters and physicochemical mechanisms involved in the elaboration of a glass-ceramic foam using clay and the sludge generated in a drinking water plant. Optimisation of the process conditions was performed by a 3 3 experimental design. For structural purposes, compression strength is the optimising parameter; if used as a catalytic support or filtering medium, porosity and permeability have to be maximised. An open cellular structure can be obtained when the materials are fired in a mould which allows the coexistence of solid, liquid and gas phases. The latter are responsible for foaming and were identified as CO 2 , SO 2 and H 2 O. Thermogravimetric analysis coupled to mass spectrometry and Mössbauer spectroscopy show that CO 2 was generated mainly from pyrolysis, SO 2 was generated from aluminium sulphate used as a coagulant in the drinking water treatment but H 2 O resulting from the decomposition of lepidocrocite at high temperature is likely to be the main agent for the foaming effect.

$Research paper thumbnail of M\"{o} ssbauer study of corrosion and abrasion products in oil transporting pipes$

It is known that one of the main technological problems in carbon steel oleoducts is the corrosio... more It is known that one of the main technological problems in carbon steel oleoducts is the corrosion produced by different substances, such as water, carbon dioxide, sulfur, and microorganisms. In addition, if in such mixture there is sand, aggressive sludge can be form that abrasions material from the oleoduct. A room temperature Mössbauer study of corroded material taken from different sites of oleoducts is presented. Most of the Mössbauer spectra reveal the presence of nanoparticles, indicating that in these pipes the abrasion problem is severe. A preliminary identification of the oxidized samples suggests the presence of magnetite, and some Iron hydroxides. Further studies are in course in order to identify unambiguously the products present in the corroded materials.

MRS Proceedings, 1995

ABSTRACTA series of coordination compounds -macrocycle tetraaza derivatives with extended electro... more ABSTRACTA series of coordination compounds -macrocycle tetraaza derivatives with extended electronic conjugations- as molecular solids have been synthesized and analyzed by Raman spectroscopy. Their resistivity vs. temperature characteristics, measured in thin films, show three types of behavior: metallic, semiconductor and, in some cases, semiconductor with a transition to metallic below certain temperature. Room temperature Raman spectra were obtained in a FT-IR spectrometer and some stretching modes show different Raman shifts depending on the reactants. We relate these shifts with the degree of charge transfer between donor and acceptor radicals.

Physica C: Superconductivity and its Applications, 1989

Hyperfine Interactions, 1991

The Jahn-Teller distortion mechanism in the Nd2_xCexCuO 4 is analyzed. This analysis is based on ... more The Jahn-Teller distortion mechanism in the Nd2_xCexCuO 4 is analyzed. This analysis is based on a quasi-molecular model for the Cut 4 cluster having D4h symmetry. In order to compare with experimental MiSssbauer measurements in the doped Ndl.85CeoAsCuo.99Feo.01 04_ 8 superconductor, the FeO 4 cluster is also analyzed for the case of Fe 2+ in the high spin state. The results show not evidence of a Jahn-Teller effect in these superconductors.

Crystal Research and Technology, 1997

... A. SALMERON-VALVERDE*, J. G. ROBLES-MART~NEZ*, J. GARC~A-SERRANO*, A. ZEHE**, R. GOMEZ, R. RI... more ... A. SALMERON-VALVERDE*, J. G. ROBLES-MART~NEZ*, J. GARC~A-SERRANO*, A. ZEHE**, R. GOMEZ, R. RIDAURA, M . QUINTANA ... 1, applying the stretching band C=N (bu) of each complex (IV, V, VI) with respect to its corresponding acceptors (DDFO, DTFO, DT4NFo). ...

$Research paper thumbnail of M\"ossbauer and X-ray Photoelectron Spectroscopy Studies of Fe2BiMO7 (M = Sb, Ta) pyrochlore compounds synthesized by molten salts method$

Cornell University - arXiv, Nov 21, 2018

Polycrystalline samples of Fe2BiMO7 (M = Sb, Ta) compounds with pyrochlore-type structure were sy... more Polycrystalline samples of Fe2BiMO7 (M = Sb, Ta) compounds with pyrochlore-type structure were synthesized for the first time by the molten salts method. The compounds were obtained in one hour at 950ºC. The structures were determined by Rietveld refinement. Through Mössbauer spectroscopy and X-ray photoelectron spectroscopy, the site occupancies and ionic states of the cations in the pyrochlore structure were investigated.

DURANTE LA SEMANA DE LA SUPERCONDUCTIVIDAD, LA MAESTRA EN CIENCIAS E INVESTIGADORA DE LA FACULTAD... more DURANTE LA SEMANA DE LA SUPERCONDUCTIVIDAD, LA MAESTRA EN CIENCIAS E INVESTIGADORA DE LA FACULTAD DE CIENCIAS VIVIANNE MARQUINA, SENALO QUE DURANTE LA CONFERENCIA INTERNACIONAL SOBRE MATERIALES Y MECANISMOS DE SUPERCONDUCTIVIDAD SE REPORTARON NUEVOS MATERIALES SUPERCONDUCTORES PERO NO TEMPERATURAS MAS ALTAS DE TRANSICION. POR SU PARTE, EL MAESTRO EN CIENCIAS RAUL GOMEZ, DEL INSTITUTO DE FISICA, COMENTO QUE LA APORTACION ECONOMICA DE PEMEX AL PROGRAMA UNIVERSITARIO DE SUPERCONDUCTORES DE ALTA TEMPERATURA DE TRANSICION (PUSCATT) PERMITIRA QUE LOS DIFERENTES GRUPOS UNIVERSITARIOS COLABOREN HACIA UNA FINALIDAD ESPECIFICA.

Journal of molecular modeling, Jan 4, 2017

First principles calculations have been performed for C60@C180 carbon double-layer endofullerenes... more First principles calculations have been performed for C60@C180 carbon double-layer endofullerenes with up to: three diatomic radioiodine molecules (131I2), two potassium radio-iodide (K131I), and three sodium radio-iodide (Na131I) inside. The plane-wave pseudopotential (PP) method within the general gradient approximation (GGA) in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) was used to perform geometric optimizations (GOs) and molecular dynamics (MD) at 310 K and atmospheric pressure. We found that the double-layer carbon nanocapsules formed by two concentric fullerenes (C180 surrounding C60) are very stable and may contain a radiodosis, without altering their configuration; that is, the 3(131I2)@C60@C180, 2(K131I)@C60@C180, and 3(Na131I)@C60@C180 systems constitute stable nanocapsules. We analyzed the interaction of double-layer endofullerene with radioactive content with some calcium, phosphorus, and strontium atoms, [n(X131I)@C60@C180 + mY...

Journal of Molecular Modeling, 2016

This paper inquires the C60 capabilities to contain radio-iodide ((131)I2) molecules. The encapsu... more This paper inquires the C60 capabilities to contain radio-iodide ((131)I2) molecules. The encapsulation conditions are investigated applying first principles method to simulate with geometric optimizations and molecular dynamics at 310 K and atmospheric pressure. We find that the n(131)I2@C60 system, where n = 1, 2, 3…, is stable if the content does not exceed three molecules of radio-iodide. The application of density functional theory allows us to determine that, the nanocapsules content limit is related with the amount of charge that is transferred from the iodine (131)I2 molecules to the carbon atoms in the fullerene surface. The Mulliken population analysis reveals that the excess of charge increases the repulsive forces between atoms and the bond length average in the C60 structure. The weakened bonds easily break and will critically damage the encapsulation properties. Additionally, we test the interaction nanocapsules with different amounts of radioactive iodine diatomic molecules content with calcium atoms, and find that only the fullerene containing one radioactive iodine diatomic molecule was able to interact with up to nine atoms of calcium without disrupting or cracking. Other fullerenes with two and three radio iodine diatomic molecules cannot resist the interaction with a single calcium atom without cracking or being broken. Graphical Abstract Instability of 3(131)I2@C60 Ca.

Boletin de la Sociedad Espanola de Ceramica y Vidrio

MFe 2 O 4 (M = Ni and Zn) nanoparticles were prepared by the hydrothermal method. The obtained sa... more MFe 2 O 4 (M = Ni and Zn) nanoparticles were prepared by the hydrothermal method. The obtained samples were characterized by X-ray and electron diffraction, Scanning and Transmission Electron Microscopy and Mössbauer spectroscopy. The transmission images show homogeneous shape and particle size ranging from 10 to 40 nm, depending on the nature of the M cation. Mössbauer spectroscopy yields to a ratio of occupancy between the A and B sites of 0.7 in the case of NiFe 2 O 4 oxide. DC magnetization (2-300 K) measurements reveal a superparamagnetic behaviour for the ZnFe 2 O 4 sample with a blocking temperature of 20 K. By contrast, in the case of the NiFe 2 O 4 ferrite the blocking temperature appears to be above 300 K and at lower temperature, it shows a ferrimagnetic behaviour arising from the superexchange interactions that take place in this inverse spinel. Mössbauer spectroscopy results confirm the bulk magnetic measurements. Nanopartículas de ferrita MFe 2 O 4 (M = Ni y Zn): sínte...

MRS Proceedings, 1994

The evolution of the R-phase transformation temperature with iron concentration is studied by Mös... more The evolution of the R-phase transformation temperature with iron concentration is studied by Mössbauer spectroscopy (MS), magnetic susceptibility and calorimetric measurements in a series of three Ti50Ni50-xFex, samples, with x = 1, 2 and 3. The changes in the Mössbauer spectra, from a single line at room temperature to a quadrupole doublet when the temperature is lowered below 280 K, are related to structural transformations occurring in the system. The temperature changes in the magnetic susceptibility are interpreted in terms of a gap opening characteristic of charge density waves (CDW) formation when the R-phase transformation takes place. The calorimetric measurements show specific heat fluctuations near room temperature in two of the samples (x = 1 and 3) whose peaks shift during the cooling and heating cycles.