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Hyperfine interactions in Li 2 ZrF 6 and Li 2 HfF 6
Abstract
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 give rise to ZrO2 and then to Li3ZrF7 (high temperature α-phase) and Li2ZrO3. On cooling, below 750 K, the α→β transition in Li3 ZrF7 already reported is found to take place. An analogous behaviour is determined for Li2HfF6. Accordingly, the same decomposition steps and exstence of high and low temperature phases for Li3HfF7 can be inferred.
In this note we report results of studies on the temperature dependence of the electric field gradient (EFG) in Li2HfO3. The time differential perturbed angular correlation (TDPAC) method has been used. The compound has been prepared by the conventional ceramic powder procedure. X-ray diffraction analysis has been used for its identification.
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.