We predict the occurrence of large ferroelectric polarization and piezoelectricity in the hypothetical perovskite-structure oxides, bismuth aluminate (BiAlO 3 ) and bismuth gallate (BiGaO 3 ), using density functional theory within the local density approximation. We show that BiGaO 3 will have a similar structure to PbTiO 3 , although with much stronger tetragonal distortion and therefore improved ferroelectric properties. Likewise, BiAlO 3 shares structural characteristics with antiferrodistortive PbZrO 3 , but it is also a ferroelectric with large polarization. Therefore, we propose the Bi(Al,Ga)O 3 system as a replacement for the widely used piezoelectric material, Pb(Zr,Ti)O 3 (PZT), that will avoid the environmental toxicity problems of lead-based compounds. Finally, we show that, in both BiAlO 3 and BiGaO 3 , the large distortions from the prototypical cubic structure are driven by the stereochemical activity of the Bi lone pair.

We present results of an ab initio density-functional theory study of three bismuth-based multiferroics, BiFeO 3 , Bi 2 FeCrO 6 , and BiCrO 3 . We disuss differences in the crystal and electronic structure of the three systems and show that the application of the LDA+ U method is essential to obtain realistic structural parameters for Bi 2 FeCrO 6 . We calculate the magnetic nearest-neighbor coupling constants for all three systems and show how Anderson's theory of superexchange can be applied to explain the signs and relative magnitudes of these coupling constants. From the coupling constants we then obtain a mean-field approximation for the magnetic ordering temperatures. Guided by our comparison of these three systems, we discuss the possibilities for designing a multiferroic material with large magnetization above room temperature.

We describe the design of a new magnetic ferroelectric with large spontaneous magnetization and polarization using first-principles density functional theory. The usual difficulties associated with the production of robustly-insulating ferromagnets are circumvented by incorporating the magnetism through ferri-magnetic behavior. We show that the the ordered perovskite Bi2FeCrO6 will have a polarization of ∼80 µC/cm 2 , a piezoelectric coefficient of 283 µC/cm 2 , and a magnetization of ∼160 emu/cm 3 (2 µB per formula unit), far exceeding the properties of any known multiferroic.

The structure and properties of the possible multiferroic, BiMnO3, are calculated using the LDA+U method of density functional theory. The symmetry is found to be centrosymmetric C2/c with zero ferroelectric polarization. The stereochemically active Bi lone pairs form local dipole moments which order in an anti-polar arrangement.

Computations on NaH(n), n=6-12, show that NaH(9) is stable by P=25  GPa. Cmc2(1)-NaH(9) containing both H(2) and H(-) units is metallic at P>250  GPa. Other phases with only H(2) units metallize at lower pressures as a result of the partial filling of the H(2) σ(u)* bands by the Na 3s electrons. Pressure induced overlap of the Na 2p cores forestalls closure of the band gap in the odd phases with H(-) atoms, but the even phases remain good metals up to 300 GPa. The lower the IP of the metal, the lower the pressure at which MH(n) with n>1 become stable. The larger the radius of M, the greater the optimal value of n.

Spin-orbit coupling has been introduced into our newly developed ligand field density functional theory (LFDFT), using the zero-order regular approximation as , is used to manifest further the effect of bonding changes on the sign and magnitude of the spin-orbit constant. Ligand field and spin-orbit coupling matrices are found to be correlated, with the higher erxtent of antibonding being accompanied by lower values of the spin-orbit coupling constant. In cases of little or no symmetry, this leads to situations in which ligand field and spin-orbit coupling cannot be neatly separated in the mathematical description. Using these results, the electronic energy levels of this series of compounds are predicted to be in good agreement with available spectral and magnetic data from literature.

The new DFT based ligand field (LF) model is proposed to calculate the g-and A-tensors of [Co(acacen)] that is known to be a difficult case. The results obtained are compared with the ZORA approach implemented in ADF as well as with the experimental values. The calculations are in good agreement with the experimental data and demonstrate the ability of the method to reproduce the large anisotropy typical for this type of complexes. The ligand field -density functional theory method is therefore not simply a method to calculate multiplet structure, ligand field splittings and UV-Vis transitions, but is also appropriate to compute magnetic properties.

The heavier alkali metal hydrides MH (M = K, Rb, Cs) undergo a series of pressure induced structural phase transitions: B1 (NaCl) → B2 (CsCl) → CrB. Experiments reveal that the latter occurs at 85 and 17.5 GPa for RbH and CsH, but it has not yet been observed for KH. Herein, evolutionary algorithms coupled with density functional theory calculations are employed to explore the potential energy surface of the aforementioned hydrides up to pressures of 300 GPa. The computations support previous theoretical work which predicts that KH will adopt the CrB structure when compressed. In addition, for KH and RbH we find configurations with Pnma and I41/amd symmetry that are thermodynamically competitive with the CrB structure at 300 GPa. Between 100–150 GPa, a Pnma structure which is analogous to a high-pressure form of CsI is found to be the most stable phase for the heaviest alkali hydride considered. At higher pressures a hitherto unknown CsH–P63/mmc arrangement becomes thermodynamically preferred up to at least 400 GPa. A detailed analysis of the geometric and electronic structures of the various phases is provided.