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Anomalous thermal behaviour of β-(NH4)2HfF6
Abstract
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.
The thermal, optical, and dielectric properties of fluoride Rb2TaF7 were investigated. It was observed that the variation in chemical pressure in fluorides A2⁺TaF7 caused by the cation substitution of rubidium for ammonium does not affect the ferroelastic nature of structural distortions, but leads to stabilization of the high- and low-temperature phases and enhancement of birefringence. The entropy of the phase transition P4/nmm ↔ Cmma is typical of the shift transformations, which is consistent with a model of the initial and distorted phase structures. The anisotropy of chemical pressure causes the change of signs of the anomalous strain and baric coefficient dT/dp of Rb2TaF7 as compared with the values for its ammonium analog.
This paper gives an elementary introduction to time differential perturbed angular correlation of γ-rays emitted from excited nuclei with particular reference to nuclear quadrupole interactions. We start with a comparison to other techniques such as nuclear quadrupole resonance and the Mössbauer effect, quote all necessary formulae, describe a typical spectrometer, list suitable isotopes, and finally give some examples for modern applications.
Perturbed angular correlation (PAC) spectroscopy has been used to characterize several structural aspects of a high-temperature, ionic conducting ceramic, CaZr3.95Hf0.05P6O24. Hafnium was introduced into the material to provide the PAC probe nuclei, 181Hf/181Ta, which were located primarily at Zr sites. PAC measurements were made over a range of temperatures from 77 to 1180 K, and they have been analyzed and interpreted using several simple models. The distorted octahedral crystal field at the Zr site produced a (low-frequency) static electric quadrupole interaction which can be accurately described by the point-charge model. But, the temperature dependence of the associated electric field gradient (EFG) cannot be described accurately by purely static considerations via the point-charge model and high-temperature x-ray diffraction data. Although a high-frequency static interaction was also observed, the measurements were not sufficiently accurate to identify its origin unambiguously. Some of the high-temperature measurements show evidence of a time-varying interaction, which may result from Ca2+-ion jumping. But, jump frequencies derived classically from high-temperature electrical dc conductivity measurements are too low to agree with those indicated by the PAC data. However, the dc conductivity measurements support a simple model of thermally activated Ca2+-ion transport. The temperature dependence of the EFG (corresponding to the low-frequency interaction) was used to determine an effective Debye-Waller factor. As a result of using this approach to analyze this type of PAC data, this factor was shown also to agree qualitatively with the predictions of the Debye crystal model, although significant theoretical limitations were encountered. These particular results suggest that the PAC technique may provide new insights into understanding advanced ceramic materials.
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)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.
Time-differential angular correlation studies of the electric quadrupole interaction of 181Ta in various Hf compounds are presented. Methods for the analysis of the time spectra are discussed. The experiments yield information on the strength of the quadrupole interaction, and on the axial asymmetry and distribution of the field gradient. Electric field gradients Vzz are derived for 181Ta in Hf metal, HfO_2, and (NH_4)_2HfF_6.
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 764 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.
The angular correlation of the 133 keV-482 keV-yy-cascade in the decay of Hf181 is strongly attenuated if solid sources of hafniumammoniumhexafluoride are used. The unperturbed correlation was observed however when a single crystal of hafniumammoniumhexafluoride was used whose main axis pointed into the direction of one of the two detectors. This proves that the perturbation is static and that the maximum component of the electric field gradient at the position of the hafnium nucleus coincides with the direction of the main axis of the crystal.