Origin of complex crystal structures of elements at pressure

We present a unifying theory for the observed complex structures of the sp-bonded elements under pressure based on nearly free electron picture (NFE). In the intermediate pressure regime the dominant contribution to crystal structure arises from Fermi-surface Brillouin zone (FSBZ) interactions - structures which allow this are favoured. This simple theory explains the observed crystal structures, transport properties, the evolution of internal and unit cell parameters with pressure. We illustrate it with experimental data for these elements and ab initio calculation for Li.Comment: 4 pages 5 figure

STRUCTURE AND THERMODYNAMICS OF SIXGE1-X ALLOYS FROM ABINITIO MONTE-CARLO SIMULATIONS

Si(x)Ge1-x alloys are studied with a new method based on density-functional theory and Monte Carlo sampling. Using perturbation theory with respect to the virtual crystal, we are able to map the alloy onto a lattice gas with long-range interactions, which are determined from first principles. Monte Carlo simulations show that Si(x)Ge1-x is a model random alloy with a miscibility gap below almost-equal-to 170 K. The bond-length distribution displays three well-defined peaks whose positions depend on composition, but not on temperature. The resulting lattice parameter follows Vegard's law very closely

PHONON DISPERSIONS IN GAXAL1-XAS ALLOYS

The vibrational properties of GaxAl1−xAs alloys have been studied using large supercells to simulate the disorder and ab initio interatomic force constants. In agreement with recent experimental evidence, our results indicate that well defined GaAs-like and AlAs-like phonon dispersions exist for any concentration. Besides broadening phonon states with definite wave vector, alloying narrows the optic branches and lowers the longitudinal modes more than the transverse ones, thus reducing the LO-TO splitting. The acoustic bands are instead rather insensitive to the composition

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We report far-infrared reflectivity measurements on (GaAs)m/(AlAs)n superlattices (SL's) with systematically varied layer thicknesses in the range 1 less-than-or-equal-to m,n less-than-or-equal-to 7. Taking advantage of an appropriate choice of the total SL thickness D less-than-or-equal-to 0.3-mu-m, we measure the frequencies of AlAs-like TO1 confined phonons from the peak of the reststrahlen band. The GaAs-like TO1 frequencies are obtained by fitting reflectivity spectra to the SL dielectric-response-theory model. Microscopic calculations of confined TO frequencies are performed within an ab initio scheme and successfully compared with the experimental data