RESUMO
We present a first-principles investigation of structural and elastic properties of experimentally observed phases of bulk SrRuO3 - namely orthorhombic, tetragonal, and cubic - by applying density functional theory (DFT) approximations. First, we focus our attention on the accuracy of calculated lattice constants in order to find out DFT approaches that best represent the crystalline structure of SrRuO3, since many important physical quantities crucially depend on change in volume. Next, we evaluate single-crystal elastic constants, macroscopic elastic parameters, and mechanical stability trying to at least partially compensate for the existing lack of information about these fundamental features of SrRuO3. Finally, we analyze the anomalous behavior of low-temperature orthorhombic phase under C44 related shear deformation. It turns out that at critical strain values the system exhibits a distinct deviation from the initial behavior which results in an isosymmetric phase transition. Moreover, under C44 related shear deformation tetragonal SrRuO3 becomes mechanically unstable raising an open question of what makes it experimentally observable at high temperatures.
RESUMO
The charge state distributions of Fe, Na, and F are determined in a photoionized laboratory plasma using high resolution x-ray spectroscopy. Independent measurements of the density and radiation flux indicate unprecedented values for the ionization parameter xi=20-25 erg cm s(-1) under near steady-state conditions. Line opacities are well fitted by a curve-of-growth analysis which includes the effects of velocity gradients in a one-dimensional expanding plasma. First comparisons of the measured charge state distributions with x-ray photoionization models show reasonable agreement.