Pressure-Induced Phase Transformations of Quasi-2D Sr3Hf2O7.

We present an abinitio study of the quasi-2D layered perovskite Sr3Hf2O7 compound, performed within the framework of the density functional theory and lattice dynamics analysis. At high temperatures, this compound takes a I4/mmm centrosymmetric structure (S.G. n. 139); as the temperature is lowered, the symmetry is broken into other intermediate polymorphs before reaching the ground-state structure, which is the Cmc21 ferroelectric phase (S.G. n. 36). One of these intermediate polymorphs is the Ccce structural phase (S.G. n. 68). Additionally, we have probed the C2/c system (S.G n. 15), which was obtained by following the atomic displacements corresponding to the eigenvectors of the imaginary frequency mode localized at the Γ-point of the Ccce phase. By observing the enthalpies at low pressures, we found that the Cmc21 phase is thermodynamically the most stable. Our results show that the I4/mmm and C2/c phases never stabilize in the 0-20 GPa range of pressure values. On the other hand, the Ccce phase becomes energetically more stable at around 17 GPa, surpassing the Cmc21 structure. By considering the effect of entropy and the constant-volume free energies, we observe that the Cmc21 polymorph is energetically the most stable phase at low temperature; however, at 350 K, the Ccce system becomes the most stable. By probing the volume-dependent free energies at 19 GPa, we see that Ccce is always the most stable phase between the two structures and also throughout the studied temperature range. When analyzing the phonon dispersion frequencies, we conclude that the Ccce system becomes dynamically stable only around 19-20 GPa and that the Cmc21 phase is metastable up to 30 GPa.

Noncollinear magnetism of Mn nanowires on Fe(1 1 0).

Magnetic properties of Mn linear nanochains on a bcc Fe(1 1 0) surface have been studied using the first-principles real space-linear muffin-tin orbital atomic sphere approximation (RS-LMTO-ASA) method. We have considered up to nine Mn atoms deposited on bcc Fe(1 1 0). Our ab initio calculations reveal the competition between the antiferromagnetic Mn-Mn and Mn-Fe couplings, presenting a behavior which is very different from Mn nanowires on Fe(0 0 1), as shown in a previous publication. Due to this competition and non-negligible Dzyaloshinskii-Moriya interaction, noncollinear magnetic structures are stabilized as ground states for the Mn nanochains on Fe(1 1 0).

Magnetic properties of Fe(x)Co(1-x) nanochains on Pt(1 1 1) surfaces.

The magnetic properties of FexCo1-x nanochains on Pt(1 1 1) were studied using the first-principles real-space linear muffin-tin orbital-atomic sphere approximation (RS-LMTO-ASA) method within the density functional theory. The relative amounts of Fe and Co atoms in a chosen nanochain were varied and several possible arrangements of the atomic species were taken into account. The results of the exchange interaction demonstrates ferromagnetic coupling for the nanowires. Our calculations of Fe and Co average magnetic moments reveal a large enhancement of both spin and orbital moments compared to Fe-Co films deposited on a Pt(1 1 1) surface. The trend for the orbital moments with respect to stoichiometry differs from all previous higher-dimensional Fe-Co alloys on Pt(1 1 1) studies.

Electric field gradients in (111)In-doped (Hf/Zr)3Al2 and (Hf/Zr)4Al3 mixed compounds: ab initio calculations, perturbed angular correlation measurements and site preference.

The quadrupolar hyperfine interactions of in-diffused (111)In --> (111)Cd probes in polycrystalline isostructural Zr(4)Al(3) and Hf(4)Al(3) samples containing small admixtures of the phases (Zr/Hf)(3)Al(2) were investigated. A strong preference of (111)In solutes for the contaminant (Zr/Hf)(3)Al(2) minority phases was observed. Detailed calculations of the electric field gradient (EFG) at the Cd nucleus using the full-potential augmented plane wave + local orbital formalism allowed us to assign the observed EFG fractions to the various lattice sites in the (Zr/Hf)(3)Al(2) compounds and to understand the preferential site occupation of the minority phases by the (111)In atoms. The effects of the size of the supercell and relaxation around the oversized In and Cd probe atoms were investigated in detail.

Hyperfine and magnetic properties of Fe-Cu clusters and Fe precipitates embedded in a Cu matrix.

Using the first-principles real-space linear muffin-tin orbital method within the atomic sphere approximation (RS-LMTO-ASA) we study hyperfine and local magnetic properties of substituted pure Fe and Fe-Cu clusters in an fcc Cu matrix. Spin and orbital contributions to magnetic moments, hyperfine fields and the Mössbauer isomer shifts at the Fe sites in Fe precipitates and Fe-Cu alloy clusters of sizes up to 60 Fe atoms embedded in the Cu matrix are calculated and the influence of the local environment on these properties is discussed.

Hyperfine interaction studies with (181)Ta and (111)Cd probes in the compound Ti(2)Ag.

By using the time-differential perturbed angular correlation technique, the electric field gradients (EFG) at (181)Hf/(181)Ta and (111)In/(111)Cd probe sites in the MoSi(2)-type compound Ti(2)Ag have been measured as a function of temperature in the range from 24 to 1073 K. Ab initio EFG calculations have been performed within the framework of density functional theory using the full-potential augmented plane wave+local orbitals method as implemented in the WIEN2k package. These calculations allowed assignments of the probe lattice sites. For Ta, a single well-defined EFG with very weak temperature dependence was established and attributed to the [4(e)4mm] Ti site. For (111)Cd probes, two of the three measured EFGs are well defined and correlated with substitutional lattice sites, i.e. both the [4(e)4mm] Ti site and the [2(a)4/mmm] Ag site.