Jorge A. Martínez

• Ph.D.
• National University of La Plata

The mechanochemical activation process has proved to be an effective technique to enhance a solid-state reaction at relatively low temperatures. In such a process, the mechanical effects of milling, such as reduction of particle size and mixture homogenization, are accompanied by chemical effects. The actual chemical phases during the mechanical tr...

As HfO 2 appears as a good candidate to replace SiO 2 in Si complementary metal-oxide-semiconductor devices, a refined knowledge of the possible solid-state reactions between Si and HfO 2 is valuable. Being the Perturbed Angular Correlations technique a very sensitive method to detect small changes in solid state, the goal of this work is to follow...

The short range technique of the Perturbed Angular Correlation (PAC) and x-ray diffraction (Rietveld) methods have been employed to determine the phase content in commercial mullite–zirconia and zircon raw materials that are ordinarily used to produce ceramic materials. The PAC technique, which probes zirconium-containing compounds at nanoscopic le...

In the field of refractory materials, ceramics containing mullite–zirconia are the basis of those most used in the industry of glass and steel. It is known that the addition of zircon improves the behavior of the refractory used in service. Knowing that some mullite–zirconia composites properties as fracture strength and the elastic modulus E are a...

It has been already published that, at nanoscopic level, zircon exhibits wide regions of aperiodic material not detected by the ordinary techniques used to analyse the obtained product in the production of ceramic materials. In this paper it is reported how the Perturbed Angular Correlations (PAC) technique has assisted the interpretation of a mism...

The Perturbed Angular Correlations technique was used to determine the configurations around Zirconium ions and their thermal behavior in non-aggregated sub-micron zirconia spherical particles. Three residues containing- Zr surroundings were determined for the non-crystalline starting particles, which were identified under the assumption of a certa...

Solid solutions of ZrO2 and HfO2 are potential electrolyte materials for intermediate-temperature SOFC because both are oxygen-ion conductors. The main challenge for these compounds is to reduce the relatively high value of the activation energies vacancies diffusion, which is influenced by several factors. In this work the thermal evolution of CaO...

This paper deals with the phase stability of an aqueous suspension of tetragonal Zr(0.9)Pr(0.1)O(2) (20 wt%/vol%) at room temperature as a function of the aging time. The suspension is investigated in situ using the highly localized Perturbed Angular Correlations technique. The results indicate that an almost fully reversible degradation process to...

The hyperfine quadrupole interaction at Hf sites in films and powders of 14 mol% CaO–HfO2 and 20 mol% CaO–HfO2 has been determined as a function of temperature. Results indicate the formation of a cubic solid solution and other microstructures assigned to the ϕ1 (CaHf4O9) and ϕ2 (Ca6Hf19O44) phases. Dynamical effects on the electric field gradient...

A perturbed angular correlation (PAC) study as a function of temperature has been carried out on spray pyrolysis-derived powders and compacts of 2.5 mol% Y2O3–ZrO2 and 2 mol% Gd2O3–ZrO2 nanostructured tetragonal zirconias. The powders undergo the ordinary thermal transformation between the two known defective t′- and regular t-tetragonal forms and...

The Aurivillius type oxide Bi1.9Te0.1SrNb1.9Hf0.1O9 has been studied by Perturbed Angular Correlations spectroscopy using Ta-181 probes. The spin precession curves were measured from room temperature up to 873 K. Two sites are occupied by probes and the temperature dependence of both indicates a continuous phase transition at about 625 K. One site...

A Zr(10 mol % Pr)O2 powder obtained by high-energy ball milling has been investigated at nanoscopic scale using primarily the Perturbed Angular Correlations technique. The aim has been to determine the nanoconfigurations around Zr4+ cations present in the solid solution and their thermal evolution with the intention of providing knowledge on the st...

This paper deals with a perturbed angular correlation investigation of the microstructure and subsequent thermal evolution of a commercial ZrO2-2. 8 mol% Y2O3 ceramic powder and of a pellet obtained upon sintering the powder at 1450°C. The perturbed angular correlation results, complemented by those of X-ray diffraction and Raman spectrometry, indi...

Zirconia powders, prepared by a sol-gel method using zirconium n-propoxide (ZNP) and different H2O/ZNP and alcohol/ZNP ratios, were investigated by the perturbed-angular-correlation method, along with X-ray diffraction (XRD), Raman spectroscopy, differential thermal analysis (DTA), and thermogravimetric analysis (TGA) techniques. The hyperfine inte...

By using Perturbed Angular Correlation Spectroscopy, a suitable technique to explore internal fields at nanoscopic scale, the electric field gradients at Zr4+ sites in the Pr2Zr2O7 compound have been determined as a function of temperature. Three nonequivalent nanoconfigurations are present, which have been interpreted with the aid of point charge...

A nanocrystalline ZrO2–10 mol% Y2O3 powder synthesized by a nitrate–citrate gel-combustion process and stabilized in the metastable t′′-form of the tetragonal phase has been characterized using mainly the Perturbed Angular Correlations technique. The determined hyperfine interaction allows a clear distinction between the zirconium surroundings in t...

The PrxZr1-xO2 ceramic system obtained by coprecipitation has been investigated at zirconium sites as a function of Pr content (1, 10, and 15 mol %) and of annealing temperature by means of the perturbed angular correlations (PAC) technique. Praseodimium enters zirconia forming a tetragonal solid solution whose extent increases with Pr content. In...

The thermal behavior of the hyperfine interaction at Zr sites on two sol-gel derived 6 mole% CaO-ZrO2 powders, obtained from the particulate and non-particulate regimes, has been studied by Perturbed Angular Correlations and complementary techniques. The aim was to get experimental support on the thermal stability of the obtained material in order...

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 samp...

The hyperfine quadrupole interaction of CoHfF6.6H2O and ZnHfF6.6H2O was determined at room temperature for the first time. The thermal behaviour of the electric field gradient at hafnium sites in the compounds AHfF6.6H2O (where A is Co, Ni or Zn) determined using the time differential perturbed angular correlations (TDPAC) technique, is presented i...

The hyperfine quadrupole interaction of was determined in the room-temperature to range. The study of the thermal behaviour of the hyperfine parameters identified an extended phase transition from to and the existence of a distorted low-temperature -phase.

The evolution of the phase content in zirconia powders that have been prepared by sol-gel acid hydrolysis has been investigated using the perturbed-angular-correlation (PAC) technique and X-ray diffractometry. As a consequence of performing annealing treatments at increasing temperatures between room temperature and 1,000 C, the amorphous starting...

By means of a facile chemical modification of the bovine serum albumin molecule, it was possible to measure its hydrodynamic radius with high accuracy (approximately 3%) using the TDPAC technique. The new approach presented here allows a wide use of the TDPAC technique to perform high precision studies of backbone dynamics of almost any protein.

The rehydration process at room temperature, after the dehydration of ZnTiF6· 6H2O to ZnTiF6, was studied by means of the Time Differential Perturbed Angular Correlations technique.

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 exposur...

Yttria-stabilized cubic zirconia powders and coatings produced by the sol-gel method have been investigated by Perturbed Angular Correlation Spectroscopy (PAC). Results indicate that the metastable cubic phase is retained during heating and cooling cycles. The hyperfine interaction that describes this cubic phase, once crystallized, exhibits two co...

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...

Using the time differential perturbed angular correlation technique, the hyperfine interactions as titanium sites in ZnTiF6·6H2O and its decomposition products were determined between room temperature (300 K) and 1273 K. The different steps in the decomposition pattern were identified and a simple model is given for it.

Approximate analytical expressions for the attenuation factor of the time-dependent magnetic hyperfine interaction have been calculated assuming the hypothesis of the random phase approximation. The Fourier transforms of those expressions were also computed. Different relaxation regimes were considered rangingλ/ω L over 1/4, 1/2, 3/4 and 1/1 ratios...

By use of the time differential perturbed angular correlations technique, the hyperfine interactions at zirconium sites in Cu2(ZrF7)2·16H2O and its decomposition products were determined between 300 and 700 K. A simple model to give account of the decomposition process is presented in agreement with angular correlation and gravimetric results.

For the first time the time differential perturbed angular correlation Hf-181-TDPAC technique is used to investigate a recrystallization process from solution. The time evolution of the electric field gradient during the recrystallization of CoHfF6 center dot 6H2O from aqueous solution was determined and suitable explanations for the behavior of th...

The temperature dependence above room temperature of the hyperfine quadrupole interactions measured by the time differential perturbed angular correlations technique at hafnium sites in AHfF6nH2O, with A=Co and Zn andn=6, 4 and 0, is presented. The different steps of the dehydration process are reflected by modifications on the hyperfine quadrupole...

The thermal evolution of the quadrupole parameters determined using the time differential perturbed angular correlation technique at titanium sites, in NiTiF66H2O is presented. The study of the thermal behavior of the hyperfine quadrupole interaction allows one to observe the occurrence of a structural phase transition around 140 K. The thermal evo...

Time-differential perturbed-angular-correlation measurements on NiHfF6· 6H2O were performed as a function of temperature in the 77–521 K range. This compound was observed to dehydrate to NiHfF6 · 4H2O and NiHfF6 at 421 and 476 K, respectively. At higher temperatures the last substance undergoes a phase transition from the hexagonal to the cubic str...

The hyperfine interaction in Ni2HfF812H2O has been determined between 77 K and 1100 K by means of the time-differential perturbed angular correlation technique. From 200 K on, the one-site phase existing at lower temperatures undergoes a gradual phase transition until, at room temperature, the populations of both phases attain a 2:1 ratio. While th...

Using time-differential perturbed-angular correlation, two thermal reversible phase transitions have been observed in both isostructural compounds K3HfF7 and K3ZrF7: the orthorhombic-to-tetragonal transition, occurring in K3HfF7 around 275 K, and the tetragonal-to-disordered cubic transition, which developed from room temperature (RT) over a wide t...

The quadrupole hyperfine interaction of the tetragonal phase of the zirconium oxide has been measured at 1523 K using the time-differential perturbed angular correlation technique. The electric field gradient at zirconium sites was determined to beV zz =(17.50.4)1017 Vcm−2 and axially symmetric.

The quadrupole hyperfine interaction of ZrF4.3H2O is reported. This compound is observed to dehydrate to ZrF4 at 325 K. Spectra taken on cooling were observed to be time dependent, and the rehydration process involved was investigated at 293, 298 and 302 K. Contrary to what had previously been determined in HfF4.3H2O, no intermediate compound was o...

The thermal behavior of the quadrupole hyperfine interaction in Rb2HfF6 and Rb2ZrF6 has been studied between 293 and 800 K with high-resolution time-differential perturbed-angular-correlation equipment. Both compounds exhibit a very similar temperature dependence, undergoing a phase transition near 740 K. An embryonic state of the high-temperature...

The thermal behavior of the hyperfine interaction in K2HfF6 was investigated by means of the time-differential perturbed-angular-correlation technique between 293 and 750 K. The quadrupole parameters associated with the monoclinic, orthorhombic, cubic, and trigonal phases have been determined. An activation energy Ea=86±16 kJ/mol could be calculate...

The quadrupole hyperfine interaction in Li2ZrF6 and Li2HfF6 has been studied as a function of temperature. Both lattices are characterized by very similar low and symmetric electric field gradients at probe sites and no phase transitions were observed up to 770 K. On heating, between 700 and 830 K, the Li2ZrF6 undergoes chemical reactions which giv...

It has been determined that the thermally activated alpha-->beta transition occurring in (NH4)2HfF6 at about 370 K is not always reversible. The beta-phase hysteresis was investigated between 14 and 400 K. It was possible to explain previously published results without considering the Goldanskii-Karyagin effect.

A TDPAC investigation has been accomplished on (NH4)2 HfF6 between 14 and 620 K. A phase transition was observed below 390 K. The low temperature phase is characterized by two non-equivalent sites for hafnium atoms in a 1∶1 ratio. The high temperature phase, on the other hand, is depicted by a unique quadrupole interaction. Above 400 K, the compoun...

The thermal evolution of the hyperfine interaction in α-ZrF4 is investigated between 293 K and 788 K. The first change, observed ay 534 K, is attributed to the appearance of the non-stoichiometric compound ZrO1.3F1.4. During the transformation, the relative fraction of the latter shows an Arrhenius behaviour with an activation energyE a=(22.6±5.5)...

The hyperfine quadrupole interaction of HfF4.3H2O at Hf sites is investigated from 14 to 820 K. No transitions have been found. After the complete dehydration of this compound at 393 K, chemical reactions take place which give rise to hafnium oxifluorides and metastable forms of hafnium oxide. Heating treatments at increasing temperatures make HfO2...

Wie mit Hilfe der Titelmethode und komplementären RÖntgen-Beugungsanalysen gezeigt wird, läuft die thermische Zers. der Titelverbindung im Temp.-Bereich 400-800 Kgemäß Reaktionsfolge A ab.

Via the TDPAC technique and complementary x‐ray diffraction analysis, the thermal decomposition of (NH4)2ZrF6 has been observed to occur in the range 400–800 K according to the thermolysis scheme (NH4)2ZrF6→485 KNH4ZrF5→560 KNH4Zr2F9 →620 KZrF4→760 KZrO2. Assuming that the first transformation takes place through the simple chemical reaction (NH4)2...

Quadrupole hyperfine interactions in HfF4 and HfF4·3H2O determined by time differential-perturbed angular correlations technique are reported. Changes in the hyperfine interactions of HfF4·3H2O with temperature are interpreted in terms of a dehydration process. The reversal process is studied at room temperature as a function of time. Assuming that...

The thermal dependence of the hyperfine quadrupolar interaction of181Ta at Zr sites in K2ZrF6 has been studied by TDPAC between RT and 511 °C. Four phases of ever increasing symmetry have been found in the range RT-437 °C. The possible existence of a healing mechanism with an activation energy of 0.187±0.040 eV is discussed for the lattice recovery...

The thermal evolution of the hyperfine quadrupole interaction in (NH4)2ZrF6 has been investigated via TDPAC from 14 to 400 K. A phase transition is detected near 370 K. The low-temperature phase exhibits two sites equally populated for181Ta probes. The associated electric field gradients are described by both high asymmetry parameters and high inte...

Time-differential perturbed angular correlation (TDPAC) measurements in NH4Hf2F9 were performed between 15 and 580 K. The compound was found to be stable in the whole temperature range and no phase transitions were observed. Experimental results could be explained assuming two equally populated quadrupole interactions which suggest a neat inequival...

The attenuation factor of gamma-gamma perturbed angular correlation in the isotropic random-phase approximation is deduced for an axially symmetric quadrupole interaction and spins 1, 3/2, 2 and 5/2. It is shown that modifications to the usual expression are significant. Useful expressions for fitting procedures are obtained.

Time-differential perturbed-angular-correlation measurements on ${\mathrm{K}}_{2}$Hf${\mathrm{F}}_{6}$ have been performed as a function of temperature in the range 20-500\ifmmode^\circ\else\textdegree\fi{}C. Three phase transitions are observed at temperatures around 165, 310, and 440\ifmmode^\circ\else\textdegree\fi{}C. The last two phases exhibi...

A time resolution of 2tau=0.75 ns, achieved with a conventional TDPAC setup with FCs detectors, allowed a determination of the very high electric field gradient at Hf impurity sites in K2ZrF6. Simple theoretical calculations seem to indicate that nearest neighbours are responsible for the interaction observed.

The time differential gamma—gamma perturbed angular correlation technique has been applied to the system phosphoric acid plus water in its phases of glass, superviscous liquid and normal liquid. For the superviscous case the extension of an existing model for the relaxation process has been used to obtain the temperature dependence of the rotationa...

The general problem of anomalous diffusion is discussed in connection with the way in which the TDPAC (time differential perturbed angular correlation) technique may help determine the relative fraction of substitutional or interstitial positions of the diffusing atoms. The TDPAC technique referred to the Ag-In system is outlined. The present measu...

The hyperfine interaction of140Ce in nickel has been investigated by the time-differential perturbed-angular-correlation technique (TDPAC). The probe was produced by isotope separator implantation of the fission product140Xe, the β- decay chain of which finally populates excited states of140Ce. Different spin rotation spectra were observed before a...

The electric field gradient EFG in hafnium pyrovanadate has been measured at Hf sites at different temperatures with the time-differential perturbed angular correlation method. The results obtained show the existence of a phase transition around 110C.

The half-life of the 379.3 keV state of ¹⁶⁹ Tm has been reinvestigated by the delayed coincidence technique. The result, T 1/2 = 49.8 ± 1.5 ns is about 40% larger than the result by Berlovich et al. which had been generally accepted in the literature.