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First principles calculations were used to study the effects of Si, Ti, Zr, and Ta (+N) substitutional impurities on the structure and dielectric properties of crystalline HfO2. The dielectric constant of monoclinic HfO2 can be enhanced by substituting more polarizable ions for Hf, but the band gap is decreased. Enhancing the permittivity without decreasing the band gap requires forming the tetragonal or cubic phase of HfO2. Among the ions studied, Si alone is found to stabilize a nonmonoclinic phase of HfO2 relative to the monoclinic phase, but only at an atomic concentration above about 20%. Various experiments have reported the formation of nonmonoclinic phases of HfO2 with increased permittivity when other ions are substituted for Hf. It is concluded that these structures are, in general, either metastable or are stabilized by extrinsic factors or by a layered arrangement of the substitutional cations. [ABSTRACT FROM AUTHOR]

The first-principles calculations are used to compute the dielectric permittivities of hypothetical [[La,Sc].sub.2][O.sub.3] solid solutions in the cubic (bixbyite) and hexagonal [La.sub.2][O.sub.3] phases. The calculated data for a model amorphous [La.sub.2][O.sub.3] structure showed little change in permittivity when a small amount of Sc are substituted for La.

First-principles calculations were used to compute the dielectric permittivities of hypothetical [La,Sc]2O3 solid solutions in the cubic (bixbyite) and hexagonal La2O3 phases. Dielectric enhancement is predicted at small Sc concentrations due to the rattling ion effect. Similar calculations for a model amorphous La2O3 structure show little change in permittivity when a small amount of Sc is substituted for La. In this case, the local environment around the Sc changes in a way that compensates for the rattling ion effect. [ABSTRACT FROM AUTHOR]

First-principles density functional theory calculations have been used to study the reasons for the contrasting dielectric properties of CaTiO3 (CT) and CaAl1/2Nb1/2O3 (CAN). It was found that, in CAN, the low-frequency polar phonons have higher frequency and qualitatively different phonon eigenvectors than in CT.

CaTiO[sub 3] (CT) and CaAl[sub 1/2]Nb[sub 1/2]O[sub 3] (CAN) have similar structures but very different dielectric properties. For CT at room temperature, the relative permittivity κapprox. 170 and the temperature coefficient of resonance frequency τ[sub f]approx. 900×10[sup -6] K[sup -1], but for CAN, κapprox. 27 and τ[sub f]approx. -88×10[sup -6] K[sup -1]. I use first-principles density functional theory calculations to investigate the origin of the contrasting behaviors. I find important differences between the two systems in both the frequencies and the eigenvectors of the low-frequency polar phonons. In CT, the frequencies are lower and the perovskite B site cations move against the surrounding oxygen octahedra; whereas in CAN, the frequencies are higher and the B site cations move with the oxygen octahedra. These two factors are equally important in explaining the differences in κ. I introduce and solve a decoupled quantum oscillator model for the temperature-dependent permittivity. This model predicts a large positive τ[sub f] for CT and a small τ[sub f] for CAN, in qualitative agreement with experiment. I relate the different dielectric behaviors to differences in the electronic structures. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]