Stability of monoclinic phase in pure and Gd-doped HfO2: a hyperfine interaction study

The present Time Differential Perturbed gamma-gamma Angular Correlation (TDPAC) measurements in bulk HfO2 report, for the first time, the presence of three regular sites at ambient condition, apart from the stable monoclinic phase (m-HfO2). Also, the effects of Gd impurity (5 at%) on HfO2 crystal structure was investigated by PAC. In pure HfO2, apart from the monoclinic component (similar to 76%), four other minor frequency components have been found at room temperature. To further investigate the origin of these minor frequency components, PAC measurement at 673 K was performed. At 673 K, population of two of the four minor components was enhanced at the expense of the m-HfO2 as the population of m-HfO2 reduces to similar to 58%. On doping with 5 at% Gd, the population of monoclinic site drastically reduced to similar to 10% while one of the minor components got enhanced to a large extent (68%). The enhancements of the three minor components, both by temperature or by Gd impurity, indicate that these are the regular phases of HfO2 and could therefore be attributed to three orthorhombic phases of HfO2. The fourth minor component (5-10%) with values of omega(Q) similar to 7 Mrad/s, eta = 0 can be assigned to crystalline defect site.