Perturbed-angular-correlation spectroscopy: Structural anomalies in (Sr,Ca)(Zr3.95Hf0.05)P6O24 ceramics

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, Hf181/181Ta, 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 isostructural 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 measurementsunlike 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.