Orthorhombic Phases In Bulk Pure Hfo2: Experimental Observation From Perturbed Angular Correlation Spectroscopy

In pure HfO2, only P21/c monoclinic phase was known to exist from previous investigations and there is no report of orthorhombic phase in pure bulk HfO2 at ambient temperature and pressure. Present atomic scale measurements by perturbed angular correlation (PAC) spectroscopy, report two orthorhombic phases in pure bulk HfO2 at room temperature along with the most commonly observed monoclinic phase (similar to 80%). By comparing with the calculated results of density functional theory using full potential (linearized) augmented plane wave method (FP-LAPW), these two orthorhombic phases have been attributed to structures with space group Pbca and Pca2(1). The structures with space group Pca2(1) and Pmn2(1) are non-centrosymmetric and were found to be responsible for ferroelectricity in doped thin film HfO2. At room temperature, the site percentages for the Pca2(1) and Pbca have been found to be similar to 6% and similar to 10%, respectively. At high temperatures of 973 and 1073 K, the Pca2(1) phase was not found but, the Pbca orthorhombic phase was found to be present in the whole temperature range from 298 to 1073 K. The third orthorhombic phase Pmn2(1) was not observed in bulk HfO2. Interestingly, In Gd doped (similar to 5 at%) HfO2, these two orthorhombic phases enhance more than the monoclinic phase. In Gd doped oxide, the P21/c monoclinic phase reduces to similar to 24% while the site percentages for the two orthorhombic phases have been found to be similar to 54% (Pbca) and similar to 17% (Pca2(1)).