J. A. Martínez

Time‐differential perturbed angular correlations investigation of the (NH4)2ZrF6 thermal decomposition

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→485u2009KNH4ZrF5→560u2009KNH4Zr2F9 →620u2009KZrF4→760u2009KZrO2. Assuming that the first transformation takes place through the simple chemical reaction (NH4)2ZrF6→NH4ZrF5+NH4F, the corresponding activation energy has been determined to be Ea=(82±6) kJ/mol. The hyperfine parameters of the NH4Zr2F9 decomposition product have been determined between 14 and 560 K. A complete analogy with the isomorphous compound NH4Hf2F9 was observed.

TDPAC investigation on thermally related HfF4.3H2O and HfO2

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 turn into its monoclinic phase.

On the kinetics of chemical reactions in hafnium tetrafluorides using time differential-perturbed angular correlations

Abstract Quadrupole hyperfine interactions in HfF 4 and HfF 4 ·3H 2 O determined by time differential-perturbed angular correlations technique are reported. Changes in the hyperfine interactions of HfF 4 ·3H 2 O 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 hydration obeys a first-order rate law, a rate constant k = 0.49 ± 0.10 d −1 is obtained.

Temperature dependence of the electric field gradient at Ta nuclei in hafnium pyrovanadate

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 110°C.

High-resolution TDPAC measurement in K2ZrF6

A time resolution of 2τ=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.

TDPAC study on the thermal stability of zirconium tetrafluoride

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 energyEa=(22.6±5.5) kJ/mol. At approximately 700 K, ZrO1.3F1.4 gives rise to the monoclinic phase of ZrO2 and to β-ZrF4.

TDPAC investigation on NH4Hf2F9

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 inequivalence between the two sites occupied by the hafnium atoms in the molecule.

The thermal behaviour of the quadrupole hyperfine interaction in (NH4)2HfF6

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 compound decomposes successively to NH4 HfF5, NH4 Hf2F9 and HfF4. An enthalpy of 76±4 kJ/mol could be determined for the (NH4)2 HfF6→NH4 HfF5+NH4F chemical reaction. The hyperfine interaction and thermal evolution of (NH4)2 HfF6 was found to be quite similar to that of (NH4)2 ZrF6.

Temperature defendence of the hyperfine interaction in (NH4)2ZrF6

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 interaction frequencies. On the other hand, the high-temperature phase corresponds to an unique axially symmetric and more intense electric field gradient.

Temperature dependence of the hyperfine quadrupolar interaction in K2ZrF6

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 in the range 270 °C–335 °C. The results at temperatures above 437 °C can be understood assuming a thermal decomposition of the compound.