Oxide pyrochlores — A review

The transition metal oxides ZrO2 and HfO2 as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO2 thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO2–ZrO2 mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc21, whose noncentrosymmetric nature is deemed responsible for the spontaneous...

Ferroelectricity in Simple Binary ZrO2 and HfO2

Within the past 20 years or so, there has occurred an explosion of interest in the magnetic behavior of pyrochlore oxides of the type $A_{2}^{3+}$$B_{2}^{4+}$O$_{7}$ where $A$ is a rare-earth ion and $B$ is usually a transition metal. Both the $A$ and $B$ sites form a network of corner-sharing tetrahedra which is the quintessential framework for a geometrically frustrated magnet. In these systems the natural tendency to form long range ordered ground states in accord with the Third Law is frustrated, resulting in some novel short range ordered alternatives such as spin glasses, spin ices and spin liquids and much new physics. This article attempts to review the myriad of properties found in pyrochlore oxides, mainly from a materials perspective, but with an appropriate theoretical context.

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Magnetic Pyrochlore Oxides

Systematic trends of the 57Fe Mössbauer isomer shifts in (FeOn) and (FeFn) polyhedra. Evidence of a new correlation between the isomer shift and the inductive effect of the competing bond T-X (→ Fe) (where X is O or F and T any element with a formal positive charge)

The structural, thermal, and dielectric properties of the ferroelectric phase of HfO2, ZrO2, and Hf0.5Zr0.5O2 (HZO) are investigated with carefully validated density functional computations. We find that the free bulk energy of the ferroelectric orthorhombic Pca21 phase is unfavorable compared to the monoclinic P21/c and the orthorhombic Pbca phase for all investigated stoichiometries in the Hf1−xZrxO2 system. To explain the existence of the ferroelectric phase in nanoscale thin films, we explore the Gibbs/Helmholtz free energies as a function of stress and film strain and find them unlikely to become minimal in HZO films for technological relevant conditions. To assess the contribution of surface energy to the phase stability, we parameterize a model, interpolating between existing data, and find the Helmholtz free energy of ferroelectric grains minimal for a range of size and stoichiometry. From the model, we predict undoped HfO2 to be ferroelectric for a grain size of about 4 nm and epitaxial HZO below 5 nm. Furthermore, we calculate the strength of an applied electric field necessary to cause the antiferroelectric phase transformation in ZrO2 from the P42/nmc phase as 1 MV/cm in agreement with experimental data, explaining the mechanism of field induced phase transformation.

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

Synthesis of a new perovskite CaFeO3

A time-of-flight neutron diffraction experiment was undertaken on a powder sample of orthorhombic ZrO2 synthesized at 600°C and 6GPa for 30min. The Rietveld analysis for the diffraction data shows that the space group of this phase is Pbca. The refined lattice parameters are a0=1.00861, b0=0.52615, and c0=0.50910nm.

Structural Analysis of Orthorhombic ZrO2 by High Resolution Neutron Powder Diffraction

The stability fields of two high-pressure polymorphs of ZrO2 (ortho I and ortho II) were determined by both calorimetry and phase equilibrium experiments. Enthalpies of transition were measured by transposed temperature drop calorimetry. The entropies of transition and slopes of phase boundaries were calculated using the measured enthalpies and free energies calculated from the results of phase equilibrium experiments. From the thermodynamic measurements, it is seen that the entropy increases and the volume decreases during the monoclinic–ortho I transition, whereas both the entropy and the volume decrease during the ortho I–ortho II transition. Accordingly, the gradient of phase boundaries, dP/dT, is negative in the former and positive in the latter. These trends are consistent with those of phase equilibrium experiments.

Stability of Monoclinic and Orthorhombic Zirconia: Studies by High-Pressure Phase Equilibria and Calorimetry

The crystal structure of orthorhombic HfO2 synthesized at 600°C and 6 GPa was investigated by a time-of-flight neutron powder diffraction experiment. Rietveld analysis of these data revealed that the space group of this phase is Pbca. The refined lattice parameters are a0= 1.00177, b0= 0.52276, and c0= 0.50599 nm. The structure is derived from a distorted fluorite (CaF2) structure by a b-glide parallel to the a-axis. The hafnium atom is in seven-fold coordination. The structure of orthorhombic HfO2 is found to be the same as that of an orthorhombic ZrO2, which occurs under high pressure above 3 GPa.

Shoichi Kume

Papers

Synthesis of a new perovskite CaFeO3

Structural Analysis of Orthorhombic ZrO2 by High Resolution Neutron Powder Diffraction

Stability of Monoclinic and Orthorhombic Zirconia: Studies by High-Pressure Phase Equilibria and Calorimetry

Structural analysis of orthorhombic hafnia by neutron powder diffraction

Synthesis of SrNiO3 and related compound, Sr2Ni2O5