Dumbbell rattling in thermoelectric zinc antimony

Twofold Rattling Mode Induced Ultralow Thermal Conductivity in Vacancy-Ordered Double Perovskites Cs2SnI6

Here, we report a first-principles study of lattice vibrations and thermal transport in Cs2SnI6, the archetypal compound in the family of vacancy-ordered double perovskites. We show that twofold rattlers of...

A New Material with a Composite Crystal Structure Causing Ultralow Thermal Conductivity and Outstanding Thermoelectric Properties: Tl2Ag12Te7+δ

A new state-of-the-art thermoelectric material, Tl₂Ag₁₂Te_7+δ, which possesses an extremely low thermal conductivity of about 0.25 W m^-1 K^-1 and a high figure-of-merit of up to 1.1 at 525 K, was obtained using a conventional solid-state reaction approach. Its subcell is a variant of the Zr₂Fe₁₂P₇ type, but ultimately its structure was refined as a composite structure of a Tl₂Ag₁₂Te₆ framework and a linear Te atom chain running along the c axis. The super-space group of the framework was determined to be P6₃(00γ) s with a = b = 11.438(1) Å, c = 4.6256(5) Å, and that of the Te chain substructure has the same a and b axes, but c = 3.212(1) Å, space group P6(00γ) s. The modulation leads to the formation of Te₂ and Te₃ fragments in this chain and a refined formula of Tl₂Ag_11.5Te_7.4. The structure consists of a complex network of three-dimensionally connected AgTe₄ tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, Tl₂Ag₁₂Te₇, Tl₂Ag₁₂Te_7.33, and 2× Tl₂Ag₁₂Te_7.5, were computed using the WIEN2k package. Depending on the Te content within the chain, the models are either semiconducting or metallic. Physical property measurements revealed semiconducting properties, with an ultralow thermal conductivity, and excellent thermoelectric properties at elevated temperatures.

Ultralow thermal conductivity in all-inorganic halide perovskites

Controlling the flow of thermal energy is crucial to numerous applications ranging from microelectronic devices to energy storage and energy conversion devices. Here, we report ultralow lattice thermal conductivities of solution-synthesized, single-crystalline all-inorganic halide perovskite nanowires composed of CsPbI₃ (0.45 ± 0.05 W·m^-1·K^-1), CsPbBr₃ (0.42 ± 0.04 W·m^-1·K^-1), and CsSnI₃ (0.38 ± 0.04 W·m^-1·K^-1). We attribute this ultralow thermal conductivity to the cluster rattling mechanism, wherein strong optical-acoustic phonon scatterings are driven by a mixture of 0D/1D/2D collective motions. Remarkably, CsSnI₃ possesses a rare combination of ultralow thermal conductivity, high electrical conductivity (282 S·cm^-1), and high hole mobility (394 cm²·V^-1·s^-1). The unique thermal transport properties in all-inorganic halide perovskites hold promise for diverse applications such as phononic and thermoelectric devices. Furthermore, the insights obtained from this work suggest an opportunity to discover low thermal conductivity materials among unexplored inorganic crystals beyond caged and layered structures.

Structural stability and thermoelectric property optimization of Ca2Si

By using an ab initio evolutionary algorithm structure search, low enthalpy criterion as well as stability analysis, we have found that cubic Fm3̄m Ca₂Si can be achieved under a negative external pressure.

Organised chaos: entropy in hybrid inorganic-organic systems and other materials

Entropy is one of the fundamental quantities which links emerging research areas like flexibility and defect engineering in inorganic-organic hybrid materials. Additionally, a delicate balance between entropy and enthalpy can lead to intriguing temperature-driven transitions in such materials. Here, we briefly overview traditional material design principles, highlight the role of entropy in the past and discuss how computational methods can help us to understand and quantify entropic effects in inorganic-organic hybrid materials in the future.

Cu3Ru6Sb8—a new ternary antimonide with a new structure type

Cu₃Ru₆Sb₈ is the first known ternary compound of the respective elements and crystallizes in its own structure type.

Origin of the highly anisotropic thermal expansion of the semiconducting ZnSb and relations with its thermoelectric applications

This article is devoted to the thermal expansion of ZnSb combining experiments (neutron and X-ray) and calculations based on density functional theory.

Unexpected high-temperature stability of β-Zn4Sb3 opens the door to enhanced thermoelectric performance

β-Zn4Sb3 has one of the highest ZT reported for binary compounds, but its practical applications have been hindered by a reported poor stability. Here we report the fabrication of nearly dense single-phase β-Zn4Sb3 and a study of its thermoelectric transport coefficients across a wide temperature range. Around 425 K we find an abrupt decrease of its thermal conductivity. Past this point, Zn atoms can migrate from crystalline sites to interstitial positions; β-Zn4Sb3 becomes metastable and gradually decomposes into Zn(hcp) and ZnSb. However, above 565 K it recovers its stability; in fact, the damage caused by decomposition can be repaired completely. This is key to its excellent thermoelectric performance at high temperature: the maximum ZT reaches 1.4. Molecular dynamics simulations are used to shed light on the microscopic behavior of the material.

Anomalous lattice dynamics near the ferroelectric instability in PbTe

A recent report of highly unusual ferroelectric fluctuations in PbTe by E. S. Božin et al. [Science 330, 1660 (2010)] raises fundamental questions about the nature of underlying lattice dynamics. We show by first-principles calculations that the reported results can be attributed to abnormally large-amplitude thermal vibrations that stem from a delicate competition of dual ionicity and covalency, which puts PbTe near ferroelectric instability. It produces anomalous properties such as partially localized low-frequency phonon modes, a soft transverse optical phonon mode, and a positive temperature coefficient for the band gap. These results account for experimental findings and resolve the underlying atomistic mechanisms, which have broad implications for materials near dynamic instabilities.

Giant anharmonic phonon scattering in PbTe

Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic mode and the longitudinal acoustic modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying longitudinal acoustic phonons. The longitudinal acoustic-transverse optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

A study of low-energy guest phonon modes in clathrate-II Na(x)Si136 (x = 3, 23, and 24)

Single-crystal x-ray diffraction from clathrate-II Na(x)Si(136) (x = 24) prepared by a new technique reveals the exceptionally large Na@Si(28) atomic displacement parameter (U(eq)) is strongly temperature dependent, and can be attributed to low-energy rattling modes associated with the Na guest. Inelastic neutron scattering (INS) spectra collected from Na(x)Si(136) powder specimens (x = 3, 23) confirm the presence of low-energy guest-derived phonon modes for Na@Si(28) and Na@Si(20). The lower energy Na@Si(28) rattler mode falls in the frequency range of the silicon host acoustic phonons, indicating the possibility for interaction with these phonons. The presence of these low-energy modes combined with the ability to controllably vary the guest content presents a unique opportunity for exploring the influence of guest-framework interactions on the lattice dynamics in intermetallic clathrates.

Off-center rattling and anisotropic expansion of type-I clathrates studied by Raman scattering

Dynamical motions of the guest ions in type-I clathrates Sr8Ga16Si(30-x)Gex and Ba8Ga16Si(30-x)Gex have been studied by Raman scattering spectroscopy, to clarify the role of guest vibration modes in these systems with unusual thermal transport behaviors. An anomalous decrease of the guest energies with decreasing temperature is observed for both systems. The Ge-doping expands the cage surrounding the 6d site anisotropically for Sr8Ga16Si(30-x)Gex, but isotropically for Ba8Ga16Si(30-x)Gex. Especially for Sr8Ga16Si(30-x)Gex, off-center rattling arises simultaneously with the anisotropic expansion, and it is confirmed that these anomalies play a crucial role to suppress lattice thermal conductivity in these systems.