Optical conductivity of rattling phonons in type-I clathrateBa8Ga16Ge30

Abstract

A series of infrared-active optical phonons have been detected in type-I clathrate Ba8Ga16Ge30 by terahertz time-domain spectroscopy. The conductivity spectra with the lowest-lying peaks at 1.15 and 1.80 THz are identified with so-called rattling phonons, i.e., optical modes of the guest ion Ba 2+ (2) with T1u symmetry in the oversized tetrakaidecahedral cage. The temperature dependence of the spectra from these modes are totally consistent with calculations based on a one-dimensional anharmonic potential model that, with decreasing temperature, the shape becomes asymmetrically sharp associated with a softening for the weight to shift to lower frequency. These temperature dependences are determined, without any interaction effects, by the Bose-factor for optical excitations of anharmonic phonons with the nonequally spaced energy levels.PACS numbers: 63.20. Ry, During the past decade, thermoelectric materials such as clathrates and filled-skutterudites have renewed an interest in phonons [1]. These conductors are formed by polyhedral building blocks, where each polyhedral cage accommodates a guest ion like an alkali-earth or rareearth element. If the cage is oversized, the guest ions can vibrate with large amplitude around an on-center or off-center site in the cage potential. These Einsteinlike local modes have been called rattling phonons. The renewed interest above has been paid to interactions of these rattling phonons with acoustic phonons propagating through the cage network and carrying heat entropy, and, equally or more interestingly, with conduction electrons. However, these issues are still far from being well understood that even the charge dynamics of a single ion in rattling vibration have been little known.This paper reports on the optical conductivity of rattling phonons detected in a type-I clathrate Ba 8 Ga 16 Ge 30 (hereafter abbreviated as BGG), featuring the anharmonicity effects on the conductivity spectra in comparison with model calculations.This compound belongs to a family of ternary type-I clathrates having cubic crystal structure with space group P m3n [2]. The unit cell of the host framework consists of 6 tetrakaidecahedra and 2 dodecahedra. The latter smaller cages around 2a sites occupy the bodycentered-cubic sites, while the former oversized cages around 6d sites line up along the principal axes by sharing both the pentagonal faces with the dodecahedral cages and the hexagonal faces with neighboring tetrakaidecahedra. Every cage encapsulates one Ba 2+ ion, and the guest ions in the smaller (2a) and oversized (6d) cages are defined as Ba 2+ (1) and (2), respectively. These guest ions satisfy the Zintl rule for charge compensations that the more electro-positive guest atom donates electrons to the more electro-negative cage; 16 electrons are transferred from 8 Ba atoms to 16 Ga atoms of the cage. With finely tuned Ga/Ge concentration, therefore, the system can be a heavily doped semiconductor, in general, having both the charge-carrier's sign and density controlled.So far the electric...

Optical conductivity from local anharmonic phonons

“…Recently, time-domain terahertz spectroscopy 28 has been successfully applied for the first time to observe the rattling phonons around 1THz in a type-I clathrate Ba 8 Ga 16 Ge 30 (BGG) 29 . In this paper, we investigate systematically the optical conductivity spectra from the rattling phonons in an on-centered and off-centered potential, and apply the obtained theoretical classifica-tion of the spectra to analyse the experimental result of BGG.…”

“…In this paper, we investigate systematically the optical conductivity spectra from the rattling phonons in an on-centered and off-centered potential, and apply the obtained theoretical classifica-tion of the spectra to analyse the experimental result of BGG. The detail of the experiment will be presented elsewhere 29 . We express the optical conductivity by level transitions among states in an anharmonic potential 30 .…”

Optical conductivity spectra of rattling phonons and charge carriers in the type-VIII clathrate Ba8Ga16Sn30Iwamoto1, Mori2, Kajitani3 et al. 2013Phys. Rev. BSelf Cite7050View full textAdd to dashboardCite

“…14,15,17,23 The measured time evolution of the transmitting electric fields through vacuum (Vac. ), substrate only (Sub.…”

“…Such a behavior is also observed in other clathrate materials. 14,15,17,20 . Thus, we write the fitting expression forσ fit (ω) aŝ…”

NMR relaxation and rattling phonons in the type-I Ba8Ga16Sn30clathrate

“…Understanding the guest atom vibration modes, often called "rattling", has been considered to be one of the most important ways to reveal the essence of these phenomena. Many methods including Raman scattering 10,11 , inelastic neutron scattering 12 , optical conductivity 13,14 , NMR relaxation 15 and theoretical calculations 16,17 have been reported to analyze the rattling atoms in clathrates using different models.…”