Distinct Bond Lengths Research Articles

Evidence for charge localization in the ferromagnetic phase ofLa1−xCaxMnO3from high real-space-resolution x-ray diffraction

High real-space-resolution atomic pair distribution functions of La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using high-energy x-ray powder diffraction to study the size and shape of the MnO_6 octahedron as a function of temperature and doping. In the paramagnetic insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95 and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and T=20K, we find a single Mn-O bond-length; however, as the metal-insulator transition is approached either by increasing T or decreasing x, intensity progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong evidence that charge localized and delocalized phases coexist close to the metal-insulator transition in the FM phase.

Distorted icosahedral cage structure ofSi60clusters

Using full-potential linear-muffin-tin-orbital molecular-dynamics method, we have performed calculations on the structure and the binding energy of cluster ${\mathrm{Si}}_{60}.$ It is found that the stable structure for ${\mathrm{Si}}_{60}$ cluster is a distorted truncated icosahedron, with ${T}_{h}$ symmetry. Its fullerene cage structure remains, but it looks like a puckered ball. The lower symmetry and four distinct Si-Si bond lengths make the structure different from that with ${I}_{h}$ symmetry and two C-C bond lengths in ${\mathrm{C}}_{60}.$

A full-potential linear-muffin-tin-orbital molecular-dynamics study on the distorted cage structures of Si60 and Ge60 clusters

Using a full-potential linear-muffin-tin-orbital molecular-dynamics (FP-LMTO-MD) method, we have performed calculations on the structures and the binding energies of Si60 and Ge60 clusters. The results calculated suggest that the stable structures for Si60 and Ge60 clusters are both distorted truncated icosahedra, with Th symmetry. Their fullerene cage structures remain, but they look like puckered balls. Four distinct Si-Si bond lengths in Si60, three distinct Ge-Ge bond lengths in Ge60, and lower Th symmetry make the two structures different from that with Ih symmetry and two C-C bond lengths in C60.

Atomic Structure of the GaAs(001)-β2(2 × 4) Surface

We present detailed results of ab initio pseudopotential calculations for equilibrium atomic geometry and chemical bonding on the arsenic-terminated GaAs(001)-β2(2 × 4) surface. We find that this surface is characterized by the presence of two distinct Ga-As bond lengths between the first and second surface layers. This feature is due to the presence of both threefold- and fourfold-coordinated Ga atoms in the second layer.

Atomic geometry and bonding on the GaAs(001)- beta 2(2 x 4) surface from ab initio pseudopotential calculations.

We present detailed results of ab initio pseudopotential calculations for equilibrium atomic geometry and chemical bonding on the arsenic terminated GaAs(001)-\ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4) surface. Of particular note is our finding that there are two distinct Ga-As bond lengths between the first and second layers. This feature is due to the presence of both threefold and fourfold coordinated Ga atoms in the second layer. Our results add significantly to the information available from recent first-principles calculations, and from scanning tunneling microscopy, reflection high-energy electron diffraction, and low-energy electron diffraction analyses. \textcopyright{} 1996 The American Physical Society.