A. R. López García’s research while affiliated with National Scientific and Technical Research Council and other places

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Publications (29)


Perturbed angular correlation study of 181Ta-doped PbTi1−xHfxO3 compounds
  • Article

July 2012

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18 Reads

Physica B Condensed Matter

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A.R. López García

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M.A. de la Rubia

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In this work, the hyperfine quadrupole interaction at Ta-doped PbTi1−xHfxO3 polycrystalline samples is studied for the first time. Powders with x=0.25, 0.50 and 0.75 were prepared and characterized by X-ray diffraction analysis. Perturbed Angular Correlation (PAC) analyses were done as a function of temperature, using low concentration 181Ta nuclei as probes. In the ferroelectric and paraelectric phases of these compounds two sites were occupied by the probes. For each site the quadrupole frequency, asymmetry and relative distribution width parameters were obtained as a function of temperature above and below the Curie temperature (TC). One of these sites was assigned to the regular Ti–Hf site, while the other one was assigned to some kind of defect. The behavior of the hyperfine parameters as a function of temperature was analyzed in terms of a recent published phase diagram and the presence of disorder below and above TC. For the three compositions measured, the obtained hyperfine parameters present discontinuities which correspond to the ferroelectric–paraelectric phase transition. In both phases it was found broad frequency distributed interactions. The disorder in the electronic distribution would be responsible for the broad line width of the hyperfine interaction.


Phase stability study in SrZrO3 vs sintering temperature

November 2007

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35 Reads

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2 Citations

Boletin de la Sociedad Espanola de Ceramica y Vidrio

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M.A. De la Rubia

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[...]

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A.R. López García

Recent studies of the phase transitions in SrZrO3 using X-ray and Neutron Diffraction, together with Perturbed Angular Correlation spectroscopies reveal that the phase diagram of this compound is not simple. The temperatures of the transitions appear to be highly dependent of the thermal history. In this work we present for the first time a study of the phase stability as a function of the annealing using the Impedance Spectroscopy technique. This work will focus on the Pnma ® Imma (second order) and the Imma ® 14/mcm (first order) phase transitions, previously reported at 750° C and 840° C respectively.


PAC investigation on the Zr-rich region of the PZT phase diagram

February 2007

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13 Reads

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1 Citation

Physica B Condensed Matter

Previous studies using perturbed angular correlation (PAC) spectroscopy in the PbZr1-xTixO3 (PZT) family of compounds show that whereas PbZrO3 is characterized by a single probes site with a very well-defined hyperfine interaction, in PZT with x >= 0.1 the probes occupy two distributed sites. In this work, we investigate the Zr-rich region of the PZT phase diagram from x = 0.02 up to x = 0.08 using PAC in order to correlate the hyperfine parameters with the antiferroelectric-ferroelectric phase transition reported to occur at x = 0.05. Two static electric quadrupole interactions, one of them fairly distributed, were detected over the whole Ti concentration range, and an abrupt change in the hyperfine parameters for 0.02 < x < 0.04 is observed. The behaviour of these parameters and the existence of two probe sites is discussed. (c) 2006 Elsevier B.V. All rights reserved.


PAC study on the thermal evolution of the 60ZrF 4 –40BaF 2 glass

September 1999

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12 Reads

Hyperfine Interactions

By using 181Ta probes, the PAC technique has been used to investigates the behaviour of the hyperfine interaction in 60ZrF4–40BaF2 glass from room temperature (RT) up to the glass temperature Tg (562 K). A first serie of measurements was obtained while heating the as-prepared sample from RT to Tg. A second serie was performed on the as-treated sample after cooling it down to RT and again heated to Tg. In both series the same three components of the quadrupole interaction were observed. Dynamical effects were detected in the main component and activation energies of 0.181 eV and 0.213 eV were obtained. The results are interpreted as dynamical effects associated to the movement in the fluorine ions during polyhedra rearrangements.


Hyperfine characterization of β-zirconium tetrafluoride

October 1997

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11 Reads

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1 Citation

Hyperfine Interactions

The evolution of the hyperfine quadrupole interaction in \beta-ZrF4 at Zr sites, is measured between 273 and 740 K via the Perturbed Angular Correlations technique. Two different quadrupole interactions, in a ratio 1:2 of relative fractions, are determined. It is observed that the compound remains stable all over the thermal range. Moisture exposure at room temperature seems not to affect the hyperfine interaction suggesting that \beta-ZrF4 is less sensitive to air water than other varieties of zirconium fluoride.


Characterization of metastable tetragonal forms in ZrO{sub 2}-2.8 mol% Y{sub 2}O{sub 3} ceramics

April 1996

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8 Reads

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2 Citations

Journal of the American Ceramic Society

This paper deals with a perturbed angular correlation investigation of the microstructure and subsequent thermal evolution of a commercial ZrO{sub 2}-2.8 mol% Y{sub 2}O{sub 3} ceramic powder and of a pellet obtained upon sintering the powder at 1,450 C. The perturbed angular correlation results, complemented by those of X-ray diffraction and Raman spectrometry, indicate that the metastable tetragonal phase exhibits two hyperfine interaction forms instead of the interaction associated with the conventional t-ZrO{sub 2} phase, both having different oxygen vacancy configurations around zirconium sites. While the powder sample exhibits mainly a very defective tetragonal form, the pellet exhibits a slightly distorted one as the major form. As temperature is increased, a reversible transformation between both forms occurs involving a redistribution of the oxygen vacancy. Once the transition is completed, the oxygen vacancy movement is described by a fast relaxation regime with an activation energy E{sub act} = 0.50 {+-} 0.04 eV.


On the occurrence of a metastable tetragonal t′-phase in a ZrO2-13.6 mole% MgO ceramic and its microscopic thermal evolution

March 1993

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22 Reads

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20 Citations

Journal of Materials Research

The time-differential perturbed angular correlation technique has been used to investigate the thermal behavior of a ZrO2−13.6 mole % MgO ceramic between room temperature and 1423 K. Two different quadrupole hyperfine interactions corresponding to a tetragonal structure have been found to result on cooling the ceramic from the single-phase cubic field. One of them agrees with that depicting the pure t-ZrO2 tetragonal phase and the other one has been interpreted as describing a high-MgO-content nontransformable t'–ZrO2 phase. As temperature increases, the latter gives rise to a similar but fluctuating interaction related to the oxygen vacancies mobility and which shows a thermal behavior analogous to that already reported for the stabilized cubic ZrO2. Above 1100 K these dynamic t'-sites transform into pure tetragonal ones which behave ordinarily, suffering the t → m phase transition when cooling to room temperature. Differences found between TDPAC results and information drawn from other techniques are discussed.


Gammaradiation detection limits for electrochemically induced deuterium cold-fusion rates

July 1992

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3 Reads

Nuovo Cimento Della Societa Italiana Di Fisica A nuclei Particles and Fields 

Summary An experiment was performed in order to detect cold fusion in an electrochemical set-up similar to that described by Fleischmannet al. The detection of gamma-rays from deuteron formation was made with a NaI(Tl) scintillator. Upper bounds for the neutron production rate of 0.53 c/s and 0.23 c/s for the stationary and the transient processes, respectively, were determined.


Temperature dependence of the hyperfine interaction in the cubic phase of BaHfO_ {3}
  • Article
  • Full-text available

November 1991

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58 Reads

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27 Citations

Physical Review B

The temperature dependence of the hyperfine interaction in the paraelectric phase of BaHfO3 in the temperature range from liquid-nitrogen temperature to 1350 K has been studied. The experimental results show an Abragam-Pound mechanism for the attenuation of the angular correlation. In this cubic structure the attenuation would be consistent with a relaxation mechanism that involves O-ion hopping between vacant sites in the lattice. Using the point-charge model for the estimation of the electric field gradient produced by an O vacancy in the coordination sphere of the probe ion, the process is characterized by a correlation time τc=12±1 ps at RT.

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Stabilization and Hyperfine Characterization of Metastable Tetragonal ZrO2

September 1990

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14 Reads

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10 Citations

Journal of Materials Research

Hydrolyzed ZrCl4 and ZrO2 · nH2O have been used as starting compounds in a time-differential perturbed-angular correlation (TDPAC) investigation on the stabilization and thermal evolution of the metastable tetragonal form of ZrO2. This phase, of quadrupole parameters very similar to those reported for the high temperature tetragonal form, emerges at moderate temperatures previous to the monoclinic phase, when starting from hydrolyzed ZrCl4 and from ZrO2 · 2H2O treated previously at 673 K. Though in all cases zirconia appears initially as an amorphous compound characterized by unique hyperfine parameters, two different precursors have been observed to exist immediately previous to the occurrence of either the monoclinic or the metastable tetragonal crystal phases. Each of them exhibits a quadrupole frequency identical with and an asymmetry parameter higher than the ones characterizing the forthcoming corresponding crystal phases. A crystallization enthalpy of (33 ± 5) kJ/mol has been determined for the formation of the metastable tetragonal phase out of its precursor.


Citations (9)


... However, there is no report of the thermal decomposition of (NH 4 ) 3 ZrF 7 in air. The thermal behavior of (NH 4 ) 2 ZrF 6 and NH 4 Zr 2 F 9 in air was analyzed but a great difference is observed [22][23][24][25], as listed in Table 1. For example, Yamada et al. [22] and Rodrfguez et al. [23] indicated that NH 4 ZrF 9 lost NH 4 F to form ZrF 4 at 347°C and then converted to ZrO 2 at 400 and 487°C, respectively. ...

Reference:

Thermal Decomposition of Ammonium Heptafluorozirconate in Air
Time Differential Perturbed Angular Correlations Investigation of the (NH4)2ZrF6 Thermal Decomposition
  • Citing Article
  • February 1985

... This observation is contrary to what has been reported for structural-charge-compensating vacancies that are inherent to doped stabilized zirconias. 12,13,18 The DTA/TGA and XRD results can be correlated to some extent with the evolution of the structure of the Zr environments that are revealed in detail by the PAC experiments. This correlation can be performed despite the dissimilar thermal treatments of the experiments that have been conducted in this work (10°C/min in DTA/TGA, at least 2 d at each temperature in in situ PAC measurements and after annealing at increasing temperatures in XRD). ...

Characterization of metastable tetragonal forms in ZrO{sub 2}-2.8 mol% Y{sub 2}O{sub 3} ceramics
  • Citing Article
  • April 1996

Journal of the American Ceramic Society

... It is interesting to note that for HfF 4 ·3H 2 O we observe about 2/3 of a discrete signal and about 1/3 of the anisotropy is lost rapidly [8]. A previous study [13] of this compound did not quote A eff 22 , but the spectra looked like ours and started with about −15% anisotropy which, compared to assumed A eff 22 = −23.0(1)% as in [4], would also give about 2/3 of the total anisotropy for the discrete component. ...

On the kinetics of chemical reactions in hafnium tetrafluorides using time differential-perturbed angular correlations
  • Citing Article
  • November 1983

Chemical Physics Letters

... Gaudreau [2] observed that HfF 4 ·3H 2 O dehydrates to HfF 4 ·H 2 O at 353 K in air. On the other hand, Martínez et al. [3,4], from time-differential perturbed angular correlation (TDPAC) measurements showed that final dehydrated product of HfF 4 ·3H 2 O was HfF 4 and it was produced at the still lower temperature of 330 K. These authors [3,4] found that in the transformation from HfF 4 ·3H 2 O to HfF 4 , monohydrate HfF 4 ·H 2 O was formed as an intermediate product. ...

TDPAC investigation on thermally related HfF 4 .3H 2 O and HfO 2
  • Citing Article
  • September 1985

Hyperfine Interactions

... Different strategies for phase quantification have been developed. Like Raman spectroscopy [17][18][19][20], neutron diffraction [21], hyperfine interacions [22], X ray absortion [23][24][25]. Particularly the X ray diffraction (XRD) is one of the common techniques for crystalline phase identification. ...

Hyperfine Interaction of ZrO2-Tetragonal Phase
  • Citing Article
  • June 1988

Hyperfine Interactions

... At higher temperatures the two solids recombine to form a new structure, K 2 ZrF 6 , containing a sextuply-coordinated zirconium. Many other parameters have been investigated as a function of temperature for pure K 2 ZrF 6 : 19 F NMR, impedance spectroscopy, DTA, XRD patterns, TDPAC, and some other spectroscopies [5][6][7][8][9][10]. It was established that the β-K 2 ZrF 6 phase is characterized by high ionic (superionic) conductivity above 540 K (σ~3.2 × 10 −4 S/cm, T = 560 K). ...

Temperature dependence of the hyperfine quadrupolar interaction in K 2 ZrF 6
  • Citing Article
  • September 1983

Hyperfine Interactions

... As indicated in the Supplemental Material [28], CaHfO 3 , SrZrO 3 , and SrHfO 3 qualitatively behave as CZO when under pressure since: (1) their adopted state at low pressures crystallizes in Pv-Pnma as consistent with experiments [8,9]; (2) they then undergo a phase transition towards the ppPv-Pnma(I) state; (3) finally experience the isostructural ppPv-Pnma(I)-to-ppPv-Pnma(II) transition at higher pressures; and (4) possess similar piezochromic effects in their ppPv-Pnma(II) phase. Figure S3 of the Supplemental Material [28] also indicates that BaHfO 3 qualitatively behaves as BZO since its ground state is a perovskite at low pressures (it is Pv-P m3m as consistent with measurements [47,48]) then transforms into the Pv-I 4/mcm phase at about 10 GPa and into ppPv-Pnma(III) at about 90 GPa and remains in this post-post-perovskite state up to 200 GPa. Note that the difference in post-post-perovskite types of phase as well as the two different slopes of the volume collapse-versuscritical pressure, shown in Fig. S8 of the Supplemental Material [28], definitely indicate that the two classes of materials should be distinguished for these high-melting-point oxides: one that gathers CaZrO 3 , CaHfO 3 , SrZrO 3 , and SrHfO 3 and another one formed by the Ba-based BaZrO 3 and BaHfO 3 compounds. ...

Temperature dependence of the hyperfine interaction in the cubic phase of BaHfO_ {3}

Physical Review B

... This observation is contrary to what has been reported for structural-charge-compensating vacancies that are inherent to doped stabilized zirconias. 12,13,18 The DTA/TGA and XRD results can be correlated to some extent with the evolution of the structure of the Zr environments that are revealed in detail by the PAC experiments. This correlation can be performed despite the dissimilar thermal treatments of the experiments that have been conducted in this work (10°C/min in DTA/TGA, at least 2 d at each temperature in in situ PAC measurements and after annealing at increasing temperatures in XRD). ...

On the occurrence of a metastable tetragonal t′-phase in a ZrO2-13.6 mole% MgO ceramic and its microscopic thermal evolution
  • Citing Article
  • March 1993

Journal of Materials Research

... Another report was also published with TDPAC measurements obtained using the 181 Hf( 181 Ta) probe, but at room temperature with different ratios of x = 0.02, x = 0.04, x = 0.06, and x = 0.08 [27]. The authors referred to the observation of an antiferroelectric-ferroelectric phase transition. ...

PAC investigation on the Zr-rich region of the PZT phase diagram
  • Citing Article
  • February 2007

Physica B Condensed Matter