Relative stability ofZrO2andHfO2structural phases

Abstract

The potentially hard oxides ZrO 2 and HfO 2 are investigated using ab initio electronic structure calculations and structural properties of the lowest phases compared. In general there are strong similarities between some phases apart from the softer monoclinic baddeyelite phase, which in HfO 2 is the lowest energy phase with a bulk modulus almost twice that of ZrO 2 . Other differences relate to the formation of the first orthorhombic phase, especially the inter-relation between Pbc21 and Pbca structures. The bulk moduli of the high-pressure, cotunnite-type phases are in good agreement with experimental results. The present calculations confirm that these phases are highly incompressible and are thus good candidates for hard materials.

Stability of the MgSiO3 analog NaMgF3 and its implication for mantle structure in super‐Earths

“…[13] According to the reported sequence of phase transitions in SiO 2 analogs, the SrI 2 type is most likely structure for phase X [Moriwaki et al, 2006;Vajeeston et al, 2006;Lowther et al, 1999]. However, no orthorhombic solutions satisfied the expected axial ratios for the SrI 2 -type: a:b:c ' 1:2:2.…”

“…Mixed phases may be experimentally inevitable if the phase stability fields are narrow in pressure. In addition, computer simulations of SiO 2 analogs usually find numerous phases within small energy differences of each other at 0 K [Lowther et al, 1999;Oganov et al, 2005;Moriwaki et al, 2006].…”

First-principles calculations of structural and electronic properties of monoclinic hafnia surfaces

“…At room temperature the equilibrium phase of HfO 2 is monoclinic which has the lowest free energy of formation and the largest volume. [26][27][28] At approximately 1300 K monoclinic HfO 2 transforms into the tetragonal structure and transforms to the CaF 2 cubic structure near 2700 K. 28 Here we fully relax both the cell lattice parameters as well as the ion positions. Table I reports our calculated structural parameters for the three phases of HfO 2 we consider along with the energy of the system per HfO 2 formula unit.…”

The origin of ferroelectricity in Hf1−xZrxO2: A computational investigation and a surface energy model

“…Kisi 5 found and structurally analyzed the polar orthorhombic Pca21 phase (f-phase) more than 2 decades ago in ZrO2 under asymmetric stress conditions. In this time theoretical calculations were performed 6 showing such a phase to be stable. Neither the dielectric properties were studied experimentally nor examined by calculations.…”