Low-temperature Specific Heat Research Articles

Two-Dimensional Cobalt(II) Benzoquinone Frameworks for Putative Kitaev Quantum Spin Liquid Candidates.

The realization and discovery of quantum spin liquid (QSL) candidate materials are crucial for exploring exotic quantum phenomena and applications associated with QSLs. Most existing metal-organic two-dimensional (2D) quantum spin liquid candidates have structures with spins arranged on the triangular or kagome lattices, whereas honeycomb-structured metal-organic compounds with QSL characteristics are rare. Here, we report the use of 2,5-dihydroxy-1,4-benzoquinone (X2dhbq, X = Cl, Br, H) as the linkers to construct cobalt(II) honeycomb lattices (NEt4)2[Co2(X2dhbq)3] as promising Kitaev-type QSL candidate materials. The high-spin d7 Co2+ has pseudospin-1/2 ground-state doublets, and benzoquinone-based linkers not only provide two separate superexchange pathways that create bond-dependent frustrated interactions but also allow for chemical tunability to mediate magnetic coupling. Our magnetization data show antiferromagnetic interactions between neighboring metal centers with Weiss constants from -5.1 to -8.5 K depending on the X functional group in X2dhbq linkers (X = Cl, Br, H). No magnetic transition or spin freezing could be observed down to 2 K. Low-temperature susceptibility (down to 0.3 K) and specific heat (down to 0.055 K) of (NEt4)2[Co2(H2dhbq)3] were further analyzed. Heat capacity measurements confirmed no long-range order down to 0.055 K, evidenced by the broad peak instead of the λ-like anomaly. Our results indicate that these 2D cobalt benzoquinone frameworks are promising Kitaev QSL candidates with chemical tunability through ligands that can vary the magnetic coupling and frustration.

Magnetic properties, phase diagram and low-temperature specific heat of Ni50Mn50-xSbx alloys

This study examines the magnetic properties and temperature-composition phase diagram of the Ni50Mn50-xSbx alloys. The addition of Sb into the antiferromagnetic Ni50Mn50 alloy introduces ferromagnetic interaction, resulting in a sudden decrease in the Néel temperature. The characteristic regions of ferromagnetic (FM) and antiferromagnetic (AFM) interactions were determined on temperature-composition phase diagram, as well as compositional dependence of Néel temperature was computed. Increase of Sb content gives rise to an interplay between AFM and FM interactions, leading to the emergence of a spin glass-like state. We identified six distinct magnetic phases in the Ni50Mn50-xSbx system, encompassing AFM, paramagnetic, and FM martensite, as well as spin glass, and paramagnetic and FM austenite. Importantly, this behavior may extend to other Ni50Mn50-xZx (Z = In, Sn) alloys, where addition of Z elements leads to the introduction of FM interaction. Moreover, a significant impact of magnetic ordering on low-temperature specific heat is demonstrated. Neglecting the magnetic contribution in ferromagnetic phase leads to the overestimation of electronic specific heat coefficient by factor of 2.

Signature of elementary excitations in Se-substituted layered triangular-lattice antiferromagnet CuCrS2

Geometrical spin frustration in a layered triangular-lattice system prohibits the long-range magnetic order and could give rise to unusual spin-disordered states of matter in which spins are strongly correlated and highly entangled with low-energy excitations. Here we investigate the low-temperature magnetic and specific heat properties of the layered triangular-lattice system CuCr(S1-xSex)2 within the entire Se-for-S substitution range. In magnetization measurements, the x = 0 phase exhibits a sharp cusp-like antiferromagnetic transition at 38 K, while for all the Se-substituted samples a broad maximum along with a spin-glass-like freezing at low temperatures is seen. The Curie-Weiss temperature systematically increases from −122 K for x = 0 to + 15 K for x = 1. These observations suggest that the magnetic ground state is determined by the competition between antiferromagnetic direct exchange interactions and ferromagnetic super-exchange interactions. For x = 0, the magnetic contribution to heat capacity exhibits a sharp jump at the magnetic transition temperature indicative of 3D long-range magnetic order, while for the x > 0 samples it increases smoothly across the magnetic transition range following a power-law (∼T2.1) behavior. All these results are in line with an existence of unusual gapless spin-liquid like excitations originating from the spin frustrations and competing nearest-neighbor interactions in CuCr(S,Se)2.

First-principles investigation of the electronics, optical, mechanical, thermodynamics and thermoelectric properties of Na based Quaternary Heusler alloys (QHAs) NaHfXGe (X = Co, Rh, Ir)

In this study, we explored the electronic and thermoelectric (TE) properties of the Na-based Quaternary Heusler Alloys (QHAs) NaHfXGe (X = Co, Rh, Ir) using density functional theory (DFT). We performed the spin-polarized DFT calculations at the general gradient approximation (GGA) level and confirmed the ground state non-magnetic configuration of NaHfXGe. The mechanical and thermodynamical stabilities are analyzed and discussed to validate the stability by calculating the elastic constant and phonon dispersion curve. A thorough investigation on the electronic properties are carried out by performing the GGA, GGA+U, and GGA+SOC formalism where we report the semi-conducting characteristic of NaHfCoGe and NaHfRhGe QHAs. However, NaHfIrGe is predicted to be a non-magnetic metal. From the calculated optical properties we found that the most active optical absorption occurs within the vis–UV region with 105 cm−1, therefore the studied QHAs are proposed to be a promising optoelectronic materials. The results of the thermodynamic properties have shown that NaHfXGe follows Debye’s low-temperature specific heat law and the classical thermodynamics of the Dulong-Petit law at high temperatures. The calculated TE efficiency using GGA+SOC formalism at T = 1200 K are ZT∼1.22 and 0.57 for NaHfCoGe and NaHfRhGe, suggesting that these materials are potential TE materials to operate at high temperature.

Thermodynamic properties of the phosphate glass Dy(PO3)3 – Potential influence of boson peak on spin relaxation

An experimental study was conducted to investigate the magnetic properties of dysprosium-doped phosphate glasses. Low-temperature (LT) measurements of the specific heat of Dy(PO3)3 were performed in the range of 0.38–300 K in the magnetic fields up to 9 T. The LT specific heat of amorphous materials is characterized by the presence of a broad maximum named boson peak (BP). The LT specific heat of the Dy-doped sample is dominated by the magnetic contribution, which overlaps the BP. Due to that reason, the specific heat of Y(PO3)3 nonmagnetic glasses was also measured at the same temperature range, revealing the BP at 12–14 K.The magnetic susceptibility was measured from 1.8 K up to room temperature, yielding the effective magnetic moment of 10.65 μB, which is close to the theoretical prediction for Dy3+. Magnetization curves were measured up to B = 5 T with temperatures ranging from 2 to 50 K.X-band electron-paramagnetic resonance spectra were measured from 0 to 1 T, revealing a maximum at 100 mT. The line achieves maximal intensity at temperatures around 12–14 K, which coincides with the appearance of BP in specific heat. The coincidence suggests the presence of the magnetoelastic coupling between the ground quasi-doublet and boson peak.

Non-magnetic regenerator material of silver oxide for 4 K cryocoolers

In regenerative cryocoolers, magnetic specific heat is often utilized for heat regeneration at cryogenic temperature, such as below 20 K. Magnetic regenerator materials of rare earth compound which have sufficient specific heat enabled cryocooler to reach below 4 K. However, the magnetic noise emitted from the magnetic materials during the refrigeration cycle affects the performance of noise-sensitive devices. In this study, we propose a non-magnetic regenerator material having “optical phonon degree of freedom” even at cryogenic temperatures, in which silver oxide (Ag2O) is selected. Ag2O decomposes at temperature above 410 K in ambient atmosphere, whereas we succeeded in fabricating a sintered Ag2O bulk and small particles of the size of 0.5 mm with high-density more than 90%. Specific heat measurement down to 100 mK was performed for the sintered bulk sample of Ag2O. Below 10 K, a non-Debye behavior was observed, suggesting the contribution from optical phonon modes. Calculation of the phonon density of state (phDOS) performed by the Evolution Strategy algorithm in addition to the first-principles calculation reveals that phDOS has several distinct peaks at low energies, and the low-energy optical phonons contribute to the non-Debye behavior. The low temperature specific heat of Ag2O enhanced by the optical phonons is larger than conventional non-magnetic regenerator materials when compared in terms of specific heat per volume. Moreover, the calculation of cooling performance of cryocooler using Ag2O particles is found to be reached below 4.2 K. This study shows that non-magnetic material with low temperature specific heat enhanced by optical phonon mode can be used as a regenerator material for regenerative cryocoolers.

Low-Temperature Specific Heat Capacity of Water-Ammonia Mixtures Down to the Eutectic.

Robust thermodynamic data are essential for the development of geodynamic and geochemical models of ocean worlds. The water-ammonia system is of interest in the study of ocean worlds due to its purported abundance in the outer solar system, geological implications, and potential importance for origins of life. In support of developing new equations of state, we conducted 1 bar specific heat capacity measurements (Cp) using a differential scanning calorimeter (DSC) at low temperatures (184-314 K) and low mass fractions of ammonia (5.2-26.9 wt %) to provide novel data in the parameter space most relevant for planetary studies. This is the first known set of data with sufficient fidelity to investigate the trend of specific heat capacity with respect to temperature. The obtained Cp in the liquid phase domain above the liquidus generally increases with temperature. Deviations of our data from the currently adopted equation of state by Tillner-Roth and Friend[Tillner-Roth R.; Friend D. G.J. Phys. Chem. Ref. Data1998, 27, 63-96]. are generally negative (ranging from +1 to -10%) and larger at lower temperatures. This result suggests that suppression of the critical behavior of supercooled water (rapid increase in specific heat with decreasing temperature) by ammonia starts at a smaller concentration than that set by Tillner-Roth and Friend.[Tillner-Roth R.; Friend D. G.J. Phys. Chem. Ref. Data1998, 27, 63-96]. Cp measurements of the liquid were also obtained in the partial melting domain between the eutectic and liquidus. This novel data set will be useful in future investigations of conditions where such partial melt may exist, such as the ice shell-ocean boundary or the interiors of ocean worlds that may contain relatively large proportions of dissolved ammonia.

Griffiths phase, re-entrant spin-glass behaviour and schottky anomaly in anti-site disordered double perovskite Pr2MnNiO6

In the present study, the effect of anti-site disorder on the magnetic properties of Pr2MnNiO6 is explored. Due to anti-site disorder, a reduced T C preceded by a Griffith phase has been observed. At low temperatures, we also report the development of the spin glass phase in co-existence with the cluster-like ferromagnetic order. The signature of the re-entrant spin glass phase, where the material is ferromagnetic below T c, and undergoes a transition to the spin glass state, is revealed by the frequency-dependent ac-susceptibility measurements. The spin glass behavior is also supported by the slow decay of thermo-remanent magnetization. The low temperature specific heat shows a small field dependent behaviour which may be attributed to Schottky effects.

Charge density wave and superconductivity in 6R-TaS[formula omitted

The layered transition metal dichalcogenide compounds 1T-TaS2 and 4H-TaS2 are well known for their exotic properties, which include charge density wave, superconductivity, Mott transition, etc., and lately quantum spin liquid. Here, we report the magnetic, transport and transmission electron microscopy study of the charge density wave and superconductivity in 6R-TaS2 which is a relatively less studied polymorph of this dichalcogenide TaS2. Our high temperature electron microscopy reveals multiple charge density wave transitions between room temperature and 660K. Magnetization, and the electrical resistivity measurements in the temperature range of 2-400 K reveal that 6R-TaS2 undergoes a charge density wave transition around 305 K and is followed by a transition to a superconducting state around 3.5 K. The low temperature specific heat measurement exhibits anomaly associated with the superconducting transition around 2.4 K. The estimated Ginzburg Landau parameter suggests that this compound lies at the extreme limit of type-II superconductivity.

Strong acoustic-optical phonon coupling in high-performance thermoelectric material Bi0.5Sb1.5Te3

Understanding the origin of intrinsic lattice thermal conductivity in crystalline solids is essential for the optimization of thermoelectric properties. Here, we investigate the phonon dynamics of Bi0.5Sb1.5Te3 by means of Raman scattering, specific heat measurement, and first-principles calculations. All the Raman modes display the three-phonon scattering processes below 490 K. The Boson peak of the low-temperature specific heat and the avoided-crossing of the phonon dispersion together reveal the existence of strong coupling between low-lying optical branches and acoustic branches. The vibration of Bi atoms is responsible for this acoustic-optical phonon coupling. Specifically, the lower group velocity, larger scattering space, larger scattering rate, and larger Grüneisen parameter near the avoided-crossing frequency lead to the lower lattice thermal conductivity of Bi0.5Sb1.5Te3. This work provides new insights for understanding the thermal properties of Bi2Te3-based compounds.

Design and Synthesis of a Metallic Polar Aurivillius Oxide Bi2ReO6

The number of reported metallic compounds with polar crystal structures is limited due to the incompatibility of the acentricity and metallicity. Particularly, metallic oxides with polar crystal structures are rare. In this work, we designed a single-layer Aurivillius oxide Bi2ReO6 to explore metallic polar oxides. Single crystals and polycrystalline samples of Bi2ReO6 are synthesized by high-pressure methods. Bi2ReO6 crystallizes in the tetragonal structure with the polar space group P42cm (No. 101). Bi2ReO6 consists of alternative stacking of (Bi2O2)2+ layers and perovskite-like [ReO4]2– layers. Theoretical calculations further support that Bi2ReO6 with the polar space group is more stable and the tilts of ReO6 octahedra stabilize the system and break the inversion symmetry. Our magnetic and electrical data and low-temperature specific heat data indicate that Bi2ReO6 is a Pauli metal. The calculated electronic structures for Bi2ReO6 support the metallic nature, and the density of states at the Fermi level is contributed mainly from Re 5d orbitals hybridized with O 2p orbitals.

Magnetic phase diagram in the three-dimensional triangular-lattice antiferromagnet Sr3CoTa2O9 with small easy-axis anisotropy

We report the results of low-temperature magnetization and specific heat measurements of ${\mathrm{Sr}}_{3}{\mathrm{CoTa}}_{2}{\mathrm{O}}_{9}$ powder, in which ${\mathrm{Co}}^{2+}$ ions with effective spin-1/2 form a uniform triangular lattice in the $ab$ plane. It was found that ${\mathrm{Sr}}_{3}{\mathrm{CoTa}}_{2}{\mathrm{O}}_{9}$ undergoes successive antiferromagnetic transitions at ${T}_{\mathrm{N}1}=0.97\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=0.79\phantom{\rule{4pt}{0ex}}\mathrm{K}$ at zero magnetic field. As the magnetic field increases, both ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$ decrease monotonically. The obtained magnetic field vs temperature phase diagram together with a sharp magnetization anomaly at a saturation field of ${\ensuremath{\mu}}_{0}{H}_{\mathrm{s}}=2.3$ T indicates that ${\mathrm{Sr}}_{3}{\mathrm{CoTa}}_{2}{\mathrm{O}}_{9}$ is described as a spin-1/2 three-dimensional triangular-lattice antiferromagnet with a weak easy-axis anisotropy. We discuss the characteristics of the magnetic phase diagram, which approximates the phase diagram for the magnetic field perpendicular to the $c$ axis.

Layered Monophosphate Tungsten Bronzes [Ba(PO4 )2 ]Wm O3m-3 : 2D Metals with Locked Charge-Density-Wave Instabilities.

Phosphate tungsten and molybenum bronzes represent an outstanding class of materials displaying textbook examples of charge-density-wave (CDW) physics among other fundamental properties. Here we report on the existence of a novel structural branch with the general formula [Ba(PO4 )2 ][Wm O3m-3 ] (m=3, 4 and 5) denominated 'layered monophosphate tungsten bronzes' (L-MPTB). It results from thick [Ba(PO4 )2 ]4- spacer layers disrupting the cationic metal-oxide 2D units and enforcing an overall trigonal structure. Their symmetries are preserved down to 1.8 K and the compounds show metallic behaviour with no clear anomaly as a function of temperature. However, their electronic structure displays the characteristic Fermi surface of previous bronzes derived from 5d W states with hidden nesting properties. By analogy with previous bronzes, such a Fermi surface should result into CDW order. Evidence of CDW order was only indirectly observed in the low-temperature specific heat, giving an exotic context at the crossover between stable 2D metals and CDW order.

A Thorough Investigation of Electronic, Optical, Mechanical, and Thermodynamic Properties of Stable Glasslike Sodium Germanate under Compressive Hydrostatic Pressure: Ab Initio Study.

In this paper, we have tried to elucidate the variation of structural, electronic, and thermodynamic properties of glasslike Na2GeO3 under compressive isotropic pressure within a framework of density functional theory (DFT). The result shows stable structural (orthorhombic → tetragonal) and electronic (indirect → direct) phase transitions at P ∼ 20 GPa. The electronic band gap transition plays a key role in the enhancement of optical properties. The results of the thermodynamic properties have shown that Na2GeO3 follows Debye's low-temperature specific heat law and the classical thermodynamic of the Dulong-Petit law at high temperature. The pressure sensitivity of the electronic properties led us to compute the piezoelectric tensor (both in relaxed and clamped ions). We have observed significant electric responses in the form of a piezoelectric coefficient under applied pressure. This property suggested that Na2GeO3 could be a potential material for energy harvest in future energy-efficient devices. As expected, Na2GeO3 becomes harder and harder under compressive pressure up to the phase transition pressure (∼20 GPa) which can be read from Pugh's ratio (kH) > 1.75, however, at pressures above 20 GPa kH < 1.75, which may be due to the formation of fractures at high pressure.

Conventional s-wave superconductivity in the non-centrosymmetric compounds Re3W, Re3W0.5Nb0.5, and Re3W0.5Ta0.5

We carried out an investigation of the structural and superconducting properties of Re3W1−xMx alloys, which can crystallize in both a centrosymmetric (CS) hexagonal, and a non-centrosymmetric (NCS) cubic α-Mn phase. The superconducting transition temperatures (Tc) of the CS and NCS phases in pure Re3W are ∼ 9.1 and ∼ 7.7 K, respectively. While boules fast cooled from the melt have a mixture of CS phases, the NCS phase can be stabilized upon annealing at very high temperatures (∼ 1700 °C), or upon the partial substitution of the transition metals M = Ta, or Nb for W. X-ray diffraction and wavelength dispersive X-ray spectroscopy (WDS) data suggest that the optimal composition for the formation of the α-Mn phase directly from the melt in Re3W1−xMx is x ≈ 0.5. A detailed study of the superconducting properties of the Re3W1−xMx solid solutions was carried out by means of measurement of magnetization, ac magnetic susceptibility, electrical transport, and low temperature specific heat. As the M content is increased, the superconducting critical temperature Tc progressively decreases, though the overall superconducting properties remain similar to those of the parent NCS Re3W compound. The partial substitution at the W site does not seem to affect the pairing mechanism. Given the exponentially vanishing character of the electronic specific heat Ce(T) at low temperatures, the superconducting behavior of the NCS alloys is consistent with a nodeless, s-wave superconducting gap symmetry in all NCS samples. These findings add the pseudo-binary Re-M-W systems to the small roll of NCS materials where the pairing symmetry of the superconducting state can be explored.

Unusually strong electronic correlation and field-induced ordered phase in YbCo2

We report the first study of electrical resistivity, magnetization, and specific heat on YbCo2. The measurements on a single-phased sample of YbCo2 bring no evidence of magnetic ordering down to 0.3 K in a zero magnetic field. The manifestations of low Kondo temperature are observed. The specific heat value divided by temperature, C/T, keeps increasing logarithmically beyond 7 J/mol K2 with decreasing temperature down to 0.3 K without no sign of magnetic ordering, suggesting a very large electronic specific heat. Analysis of the magnetic specific heat indicates that the large portion of the low-temperature specific heat is not explained simply by the low Kondo temperature but is due to the strong intersite magnetic correlation in both the 3d and 4f electrons. Temperature-dependent measurements under static magnetic fields up to 7 T are carried out, which show the evolution of field-induced transition above 2 T. The transition temperature increases with increasing field, pointing to a ferromagnetic character. The extrapolation of the transition temperature to zero field suggests that YbCo2 is in the very proximity of the quantum critical point. These results indicate that in the unique case of YbCo2, the itinerant electron magnetism of Co 3d-electrons and the Kondo effect within the vicinity of quantum criticality of Yb 4f-local moments can both play a role.

Islands of chiral solitons in integer-spin Kitaev chains

An intriguing chiral soliton phase has recently been identified in the $S=\frac{1}{2}$ Kitaev spin chain. Here we show that for $S$ = 1, 2, 3, 4, 5 an analogous phase can be identified, but contrary to the $S=\frac{1}{2}$ case the chiral soliton phases appear as islands within the sea of the polarized phase. In fact, a small field applied in a general direction will adiabatically connect the integer spin Kitaev chain to the polarized phase. Only at sizable intermediate fields along symmetry directions does the soliton phase appear centered around the special point ${h}_{x}^{★}={h}_{y}^{★}=S$ where two exact product ground states can be identified. The large-$S$ limit can be understood from a semiclassical analysis, and variational calculations provide a detailed picture of the $S=1$ soliton phase. Under open boundary conditions, the chain has a single soliton in the ground state, which can be excited, leading to a proliferation of in-gap states. In contrast, even length periodic chains exhibit a gap above a twice-degenerate ground state. The presence of solitons leaves a distinct imprint on the low-temperature specific heat.

Ordered and disordered variants of the triangular lattice antiferromagnet Ca3NiNb2O9 : Crystal growth and magnetic properties

Single crystals of the triangular lattice antiferromagnet (TLAF) ${\mathrm{Ca}}_{3}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{9}$ and its nonmagnetic analog ${\mathrm{Ca}}_{3}{\mathrm{MgNb}}_{2}{\mathrm{O}}_{9}$ are grown using the four-mirror optical floating-zone furnace. By varying the growth conditions, two types of mm-sized crystals are obtained: monoclinic crystals of the form ${\mathrm{Ca}}_{3}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{9}$ where ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Nb}}^{5+}$ exhibits 1:2 ordering and Ni ions (spin 1) are arranged on a triangular lattice, and orthorhombic crystals of the form ${\mathrm{CaBO}}_{3}$ where B site is occupied randomly by 1/3 Ni and 2/3 Nb. Low-temperature specific heat and low-field magnetization on these crystal modifications revealed contrasting ground states. In B-site ordered ${\mathrm{Ca}}_{3}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{9}$, the Ni spins undergo two successive phase transitions at ${\mathrm{T}}_{{\mathrm{N}}_{1}}=4.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${\mathrm{T}}_{{\mathrm{N}}_{2}}=4.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The magnetic propagation vector below ${\mathrm{T}}_{{\mathrm{N}}_{2}}$ is found to be $\stackrel{P\vec}{k}=(0,1/3,0)$ confirming the ${120}^{\ensuremath{\circ}}$ spin structure. The high-field magnetization reveals the presence of plateaus characteristic of TLAFs. In contrast, the orthorhombic ${\mathrm{CaBO}}_{3}$ crystals exhibit a spin-glass-like ground state.

Strong spin frustration and magnetism in kagomé antiferromagnets LnCu3(OH)6Br3 (Ln = Nd, Sm, and Eu)

To study the effects of lanthanide ions on the geometrically frustrated antiferromagnets and their magnetic properties, we grew high-quality single crystals of LnCu3(OH)6Br3 (Ln = Nd, Sm, and Eu) by hydrothermal method and studied their crystal structures and magnetic properties. The refinements of the crystal structure referred to the powder x-ray diffraction data show that LnCu3(OH)6Br3 adopt a Kapellasite-type layer structure, which is isostructural to their chlorine analogue. Magnetic susceptibilities demonstrate that LnCu3(OH)6Br3 have strong antiferromagnetic coupling and a pronounced magnetic frustration effect. Magnetization measurements indicate canted antiferromagnetic ordering of Cu2+ ions around 16 K within the kagomé plane and weak ferromagnetic coupling. Moreover, shoulder-like anomalies in specific heat around 16 K could be a signature of emergent of magnetic ordering. The low-temperature negative magnetization and specific heat of LnCu3(OH)6Br3 (Ln = Nd, Sm, and Eu) indicate that Ln 3+ ions induce more exotic magnetic ground state properties.

Strong Antibonding I (p)-Cu (d) States Lead to Intrinsically Low Thermal Conductivity in CuBiI4.

Chemical bonding present in crystalline solids has a significant impact on how heat moves through a lattice, and with the right chemical tuning, one can achieve extremely low thermal conductivity. The desire for intrinsically low lattice thermal conductivity (κlat) has gained widespread attention in thermoelectrics, in refractories, and nowadays in photovoltaics and optoelectronics. Here we have synthesized a high-quality crystalline ingot of cubic metal halide CuBiI4 and explored its chemical bonding and thermal transport properties. It exhibits an intrinsically ultralow κlat of ∼0.34-0.28 W m-1 K-1 in the temperature range 4-423 K with an Umklapp crystalline peak of 1.82 W m-1 K-1 at 20 K, which is surprisingly lower than other copper-based halide or chalcogenide materials. The crystal orbital Hamilton population analysis shows that antibonding states generated just below the Fermi level (Ef), which arise from robust copper 3d and iodine 5p interactions, cause copper-iodide bond weakening, which leads to reduction of the elastic moduli and softens the lattice, finally to produce extremely low κlat in CuBiI4. The chemical bonding hierarchy with mixed covalent and ionic interactions present in the complex crystal structure generates significant lattice anharmonicity and a low participation ratio in low-lying optical phonon modes originating mostly from localized copper-iodide bond vibrations. We have obtained experimental evidence of these low-lying modes by low-temperature specific heat capacity measurement as well as Raman spectroscopy. The presence of strong p-d antibonding interactions between copper and iodine leads to anharmonic soft crystal lattice which gives rise to low-energy localized optical phonon bands, suppressing the heat-carrying acoustic phonons to steer intrinsically ultralow κlat in CuBiI4.