No abstract

No abstract

A neutron-diffraction experiment (X =0.704 A) has been performed on liquid arsenic above the triple point. The coordination number (Z -3) does not change thru the semimetal-to-semiconductor transition on melting. This is in sharp contrast to the elemental semiconductors (Si,Ge) that undergo a semiconductor-to-metal transition upon melting, accompanied by an increase (4-• 6.4) of the coordination number. The nonmetallic behavior of As, which has a half-filled p band, can be understood as a Peierls distortion in the p-bonding mechanism (three p-p bonds are reinforced; the others are considerably weakened). PACS numbers: 61.25.-f, 61.12.GzThe elemental group-IV semiconductors (S^Ge) 1 and their isoelectronic III-V compounds (InSb, 2 GaAs 3 ) drastically change their structural and electronic properties upon melting: Their coordination number increases from four to six and they undergo a semiconductor-tometal transition associated with a high value of the entropy of melting. In this paper, we show that arsenic, a group-V semimetal, has a completely different behavior as it keeps its coordination number Z = 3 in the liquid state. Moreover liquid arsenic is semiconducting (E g =0.5 eV).The valence three of group-V elements (P, As, Sb, and Bi) in their crystalline structures can be simply explained as the result of a Peierls distortion of /^-bonded atoms. Indeed, an atom of group-V element has an electronic configuration s 2 p 3 .The s level lying far below ( -8.4 eV) the p level, the fully occupied s band in the crystal does not contribute to cohesion which is dominated by the half-filled p shell. The six lobes of the p orbitals pointing at right angles lead to a simple cubic (or related) structure. However, such a structure is unstable against doubling of the periodicity for a half-filled band. Doubling of the periodicity in the three principal directions of the simple cubic structure can be achieved by the alternation of short (covalent) bonds and long (weakly covalent) van der Waals bonds: It leads to the Al rhombohedral structures for As, Sb, and Bi or to the orthorhombic structure of P and As; both structures consist of double layers of tricoordinated atoms. 4 In addition to a gain in the electronic energy, doubling of the periodicity opens a gap at the Fermi level. (Group-V elements are semiconductors or semimetals.) In the crystal, the opening of a gap results from the orthogonality of the dispersion relations at the Brillouin-zone boundaries. The classical explanation of the Peierls distortion mechanism requires the periodicity. In this paper, we demonstrate for the first time that a Peierls-type distortion may still exist in a liquid. Indeed, we show that the coordination number of liquid arsenic is exactly three as in the crystal; the absence of a Peierls-type distortion would have given a sixfold-coordinated structure like that of polonium at room temperature.The difficulty in performing an experiment on liquid As is due to the high pressure of its triple point which prevents it from melting at atmospheric p...

The reaction between nanometric Ni films and Ge is analyzed using isothermal x-ray diffraction measurements and transmission electron microscopy. It is found that NiGe is formed during deposition at room temperature. The metal rich phase that grows during heat treatment has been clearly identified to be Ni5Ge3. The simultaneous growths of Ni5Ge3 and NiGe have been observed on amorphous and polycrystalline germanium. This is in contrast with the usual sequential growth reported in thin films.