68 research outputs found
Interacting Dipoles in Type-I Clathrates: Why Glass-like though Crystal?
Almost identical thermal properties of type-I clathrate compounds to those of
glasses follow naturally from the consideration that off-centered guest ions
possess electric dipole moments. Local fields from neighbor dipoles create many
potential minima in the configuration space. A theoretical analysis based on
two-level tunneling states demonstrates that interacting dipoles are a key to
quantitatively explain the glass-like behaviors of low-temperature thermal
properties of type-I clathrate compounds with off-centered guest ions.From this
analysis, we predict the existence of a glass transition
Glasslike vs. crystalline thermal conductivity in carrier-tuned Ba8Ga16X30 clathrates (X = Ge, Sn)
The present controversy over the origin of glasslike thermal conductivity
observed in certain crystalline materials is addressed by studies on
single-crystal x-ray diffraction, thermal conductivity k(T) and specific heat
Cp(T) of carrier-tuned Ba8Ga16X30 (X = Ge, Sn) clathrates. These crystals show
radically different low-temperature k(T) behaviors depending on whether their
charge carriers are electrons or holes, displaying the usual crystalline peak
in the former case and an anomalous glasslike plateau in the latter. In
contrast, Cp(T) above 4 K and the general structural properties are essentially
insensitive to carrier tuning. We analyze these combined results within the
framework of a Tunneling/Resonant/Rayleigh scatterings model, and conclude that
the evolution from crystalline to glasslike k(T) is accompanied by an increase
both in the effective density of tunnelling states and in the resonant
scattering level, while neither one of these contributions can solely account
for the observed changes in the full temperature range. This suggests that the
most relevant factor which determines crystalline or glasslike behavior is the
coupling strength between the guest vibrational modes and the frameworks with
different charge carriers.Comment: 8 pages, 4 figures, 4 tables, submitted to Phys. Rev.
Cage-size control of guest vibration and thermal conductivity in Sr8Ga16Si30-xGex
We present a systematic study of thermal conductivity, specific heat,
electrical resistivity, thermopower and x-ray diffraction measurements
performed on single-crystalline samples of the pseudoquaternary type-I
clathrate system Sr8Ga16Si30-xGex, in the full range of 0 < x < 30. All the
samples show metallic behavior with n-type majority carriers. However, the
thermal conductivity and specific heat strongly depend on x. Upon increasing x
from 0 to 30, the lattice parameter increases by 3%, from 10.446 to 10.726 A,
and the localized vibrational energies of the Sr guest ions in the
tetrakaidekahedron (dodecahedron) cages decrease from 59 (120) K to 35 (90) K.
Furthermore, the lattice thermal conductivity at low temperatures is largely
suppressed. In fact, a crystalline peak found at 15 K for x = 0 gradually
decreases and disappears for x > 20, evolving into the anomalous glass-like
behavior observed for x = 30. It is found that the increase of the free space
for the Sr guest motion directly correlates with a continuous transition from
on-center harmonic vibration to off-center anharmonic vibration, with
consequent increase in the coupling strength between the guest's low-energy
modes and the cage's acoustic phonon modes.Comment: 7 pages, 7 figures, submitted to PR
Significance of Off-Center Rattling for Emerging Low-lying THz Modes in type-I Clathrates
We show that the distinct differences of low-lying THz-frequency dynamics
between type-I clathrates with on-center and off-center guest ions naturally
follow from a theoretical model taking into account essential features of the
dynamics of rattling guest ions. Our model analysis demonstrates the drastic
change from the conventional dynamics shown by on-center systems to the
peculiar dynamics of off-center systems in a unified manner. We claim that
glass-like plateau thermal conductivities observed for off-center systems stem
from the flattening of acoustic phonon dispersion in the regime |k|<|G|/4. The
mechanism is applicable to other systems such as glasses or relaxers
Optical conductivity of rattling phonons in type-I clathrate BaGaGe
A series of infrared-active optical phonons have been detected in type-I
clathrate BaGaGe 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 with 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.Comment: 4 pages, 4 figure
Anomalous infrared spectra of hybridized phonons in type-I clathrate BaGaGe
The optical conductivity spectra of the rattling phonons in the clathrate
BaGaGe are investigated in detail by use of the terahertz
time-domain spectroscopy. The experiment has revealed that the lowest-lying
vibrational mode of a Ba(2) ion consists of a sharp Lorentzian peak at
1.2 THz superimposed on a broad tail weighted in the lower frequency regime
around 1.0 THz. With decreasing temperature, an unexpected linewidth broadening
of the phonon peak is observed, together with monotonic softening of the phonon
peak and the enhancement of the tail structure. These observed anomalies are
discussed in terms of impurity scattering effects on the hybridized phonon
system of rattling and acoustic phonons.Comment: Submitted to JPS
Lattice instability and elastic dispersion due to the rattling motion in the type-I clathrate Ba_8Ga_<16>Sn_<30>
To investigate the off-center rattling motion and its charge-carrier dependence in type-I clathrate compounds, we carried out ultrasonic measurements on type-I Ba8Ga16Sn30 and a reference compound, K8Ga8Sn38. We found elastic softening of C44 originating from a lattice instability due to the off-center rattling motion of Ba atom in Ba8Ga16Sn30. Elastic softening below 1 K suggests that the lattice instability remains at very low temperatures. We also found ultrasonic dispersion which has no mode selectivity. No-mode-selective ultrasonic dispersion in Ba8Ga16Sn30 would be caused by a strong electron-phonon coupling. No charge-carrier dependence is observed between n-type and p-type Ba8Ga16Sn30. The significant softening on the bulk modulus in Ba8Ga16Sn30 contrasts to the continuous hardening in K8Ga8Sn38, indicating the central role of the rattling motion in the softening
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