Structural Characterization of a High-Temperature, Ionic Conducting Ceramic using Perturbed Angular Correlation Spectroscopy

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

Thermal expansion of compounds of zircon structure

The thermal expansion behavior of 13 members of ABO<SUB>4</SUB> compounds of the zircon family is examined in terms of crystal chemical (size, charge, and mass of cations) and crystallographic (a and c) parameters. The systematic trend in the thermal expansion coefficients, α<SUB>a</SUB> and α<SUB>c</SUB>, with the ionic radii, r<SUB>A</SUB> and r<SUB>B</SUB>, can be explained in terms of the unique arrangement of M-O polyhedra along a and c directions of this lattice. In the zircon structure, edge-sharing ZrO<SUB>8</SUB> dodecahedra form a chain along the a direction while the chain along the c direction consists of alternate edgesharing SiO<SUB>4</SUB> tetrahedra and ZrO<SUB>8</SUB> triangular dodecahedra. Substitution in the A sites affects a and α<SUB>a</SUB> more than c and α<SUB>c</SUB> and the reverse is true for replacements in the B sites. Unequal valencies on the A and B sites affect thermal expansion coefficients, particularly α<SUB>c</SUB>

Perturbed-angular-correlation Spectroscopy

Perturbed-angular-correlation (PAC) spectroscopy measurements were used to characterize the structure of an ionic conducting ceramic Sr(Zr3.95Hf0.05)P6O24 (SZP). Hafnium was substituted primarily into the Zr sites to provide the PAC probe nuclei, 181Hf/181Ta, and the measurements were made over a range of temperatures from 77 to 1320 K. The results of the analysis were compared to the results of a previously reported analysis of measurements on the \u27\u27isostructural\u27\u27 ceramic, Ca(Zr3.95Hf0.05)P6O24 (CZP). The major difference is that the SZP data show two distinct static electric quadrupole interactions that correspond to two distinct, approximately equally numerous Zr sites, whereas the CZP measurements show one well-defined static interaction at approximately 80% of the sites. Another difference is that no time-varying interactions were observed in the SZP measurements—unlike the CZP results which do show them above 960 K. Using the measurements made below 800 K, the SZP sites were characterized in terms of the presence of or the absence of Sr2+ ions in the nearest-neighbor environments of the Zr sites. These assignments were based (1) on point-charge-model calculations, which were used to estimate the electric field gradient (EFG) magnitudes at low temperatures, and (2) on the effective Debye-Waller factors, which were determined from the EFG temperature dependence. These factors also were used to estimate a Debye temperature ratio for the sites. Analysis of the measurements made above 800 K did not provide unique PAC parameters. This problem has limited the information content of the high-temperature data