Jianjun Dong

Theoretical study of two expanded phases of crystalline germanium : clathrate-I and clathrate-II

We study two expanded phases of Ge crystals, type-I (Ge46) and type-II (Ge136) clathrates, using an ab initio density functional planewave pseudopotential method. The equation of state, electronic structure and vibrational modes have been determined. These expanded Ge phases are shown to be only slightly higher in energy than the diamond phase (0.05 eV/atom), and considerably lower in energy than the compressed -tin phase by about 0.14 eV/atom. The electronic band-gaps of Ge clathrates are about 1.21 eV (type-I) and 0.75 eV (type-II) wider that band-gaps in the diamond phase. We have determined lattice vibrational modes and the frequencies of the Raman and infrared modes. The results of Ge clathrates are compared and contrasted with those of Si clathrates, where experiments are more numerous.

Chemical trends of the rattling phonon modes in alloyed germanium clathrates

Alloys based on Ge clathrates are promising thermoelectric materials because of their expected “rattling” properties. We have incorporated the elements of columns I and II into the cages of (Ge, Ga)46 type-I clathrates as cation guests and have theoretically examined their “rattling” behavior using density functional theory. The potential energy curves of guest atoms in the cages are evaluated to understand the nature of the weak guest–framework interaction. Some atoms are unstable at the center of the cages, while others appear to be bonded by weak restoring forces. We calculate the phonon modes and the Raman spectra and find that heavy alkali-earth elements, such as Sr and Ba, induce low-frequency “rattling” phonon modes as predicted by Slack’s model, while heavy alkali metal atoms (K, Rb, and Cs) are less “rattler-like” since they interact less with the acoustic modes of the Ge-based clathrate framework.

The Raman spectra of the hexagonal and cubic (spinel) forms of Ge3N4: an experimental and theoretical study

The Raman spectra of the hexagonal (β) and cubic spinel (γ) forms of Ge3N4, newly obtained by high-pressure synthesis, have been measured and are compared with zone-center phonon frequencies calculated using first principles methods in a plane wave pseudopotential local density approximation. The observed and calculated phonon frequencies are in excellent agreement and we use the calculated values to perform mode assignment for both the phases. Both phases show all the expected Raman active modes.

Comment on “Assignment of the Raman active vibration modes of β-Si3N4 using micro-Raman scattering” [J. Appl. Phys. 85, 7380 (1999)]

In this comment to a recent experimental paper by Honda et al. [J. Appl. Phys. 85, 7380 (1999)], we report first-principles calculations of phonon modes of β-Si3N4, which theoretically confirm the experimental micro-Raman assignments. In addition, the theory is able to locate the “missing” Ag mode, and we show that this mode is a bond-stretching mode. Hence, it is not in the low-frequency band as previously believed, but rather at intermediate frequency. We discuss the possible reasons this mode is “missing” in the spectra, and show that the application of high pressure will separate this mode from others.

Theoretical study of the structural, electronic and vibrational properties of CdIn2Te4

Abstract The structural, electronic and vibrational properties of the ordered-vacancy compound CdIn 2 Te 4 have been studied theoretically in three phases. They are the tetragonal I4 and I42m phases, and a potential high-pressure Fd3m phase. We found that: (i) the I4 and I42m phases were virtually identical in total energy; (ii) a high pressure phase transition from either tetragonal phase to the Fd3m phase was unlikely; and (iii) the tetragonal phases were semiconducting with a direct band gap, while the Fd3m phase was metallic. We calculated the Raman and IR modes for the I42m phase and compared these with experimental values.

Theoretical calculation of the vibrational modes in Ge 46 clathrate and related M x Ga y Ge 46-y type clathrates

We have calculated theoretically the vibrational modes of the type-I Ge 46 clathrate and some related Zintl phase structures M x Ga y Ge 46- y using ab initio density functional theory. The vibrational modes of pure Ge 46 (without guest species) are compared to the M x Ga y Ge 46- y structures, and it is found that small metal atoms (e.g. Na) have vanishing restoring force in the large cage, while the larger alkali-earth metal Ba has a restoring force constant less than 10% of that of the a framework atom. This makes them ideal “rattler” systems.