Uniaxial Negative Thermal Expansion Research Articles

The Critical Role of Anharmonic Lattice Dynamics for Macroscopic Properties of the Visible Light Absorbing Nitride Semiconductor CuTaN2

AbstractTernary nitride semiconductors are rapidly emerging as a promising class of materials for energy conversion applications, offering an appealing combination of strong light absorption in the visible range, desirable charge transport characteristics, and good chemical stability. In this work, it is shown that finite‐temperature lattice dynamics in CuTaN2 – a prototypical ternary nitride displaying particularly strong visible light absorption – exhibit a pronounced anharmonic character that plays an essential role in defining its macroscopic optoelectronic and thermal properties. Low‐frequency vibrational modes that are Raman‐inactive from symmetry considerations of the average crystal structure and unstable in harmonic phonon calculations are found to appear as intensive Raman features near room temperature. The atomic contributions to the anharmonic vibrations are characterized by combining Raman measurements with molecular dynamics and density functional theory calculations. This analysis reveals that anharmonic lattice dynamics have large ramifications on the fundamental properties of this compound, resulting in uniaxial negative thermal expansion and the opening of its bandgap to a near‐optimal value for solar energy harvesting. The atomic‐level understanding of anharmonic lattice dynamics, as well as the finding that they strongly influence key properties of this semiconductor at room temperature, have important implications for design of new functional materials, especially within the emerging class of ternary nitride semiconductors.

Pressure‐Induced Volumetric Negative Thermal Expansion in CoZr2 Superconductor

AbstractThe study investigates the thermal expansion and superconducting properties of a CuAl2‐type (tetragonal) superconductor CoZr2 under high pressures. High‐pressure synchrotron X‐ray diffraction is performed in a pressure range of 2.9 GPa < P < 10.4 GPa, and it is discovered that CoZr2 exhibits volumetric negative thermal expansion (NTE) under high pressures. Although uniaxial positive thermal expansion (PTE) along the a‐axis is observed under ambient pressure, it is suppressed by pressure, whereas a large uniaxial NTE along the c‐axis is maintained under the pressure regime. Because of the combination of the suppressed uniaxial PTE along the a‐axis and uniaxial NTE along the c‐axis, volumetric NTE is achieved under high pressure in CoZr2. The volumetric NTE mechanism is based on the flexible crystal structure caused by the soft Co–Co bond, as observed in the isostructural compound FeZr2, which exhibits a uniaxial NTE along the c‐axis. High‐pressure electrical resistance measurements of CoZr2 are performed and confirm superconductivity at 0.03 GPa < P < 41.9 GPa. Because of the coexistence of the two phenomena, volumetric NTE and superconductivity, in CoZr2 under high pressure, coexistence can be achieved under ambient pressure by tuning the chemical composition after the present observation.

Anisotropic Thermal Expansion of Structurally Related Lanthanide-Mercury(II) Cyanide Coordination Polymers.

Three sets of related lanthanide-mercury(II) cyanide coordination polymers were synthesized by the reaction of LnCl3·xH2O (Ln = Ce, Nd, Eu, Gd, Tb, Dy, Ho, Tm, Yb, and Lu) with Hg(CN)2 and structurally characterized. [Ce(OH2)5][Hg(CN)2Cl]3·2H2O is a 3-D material with sheet-based architecture; its thermal expansion behavior shows uniaxial negative thermal expansion (-18.3(8), 39(2), and 68.3(16) ppm K-1 along the a, b, and c axes, respectively). This anisotropic thermal behavior is postulated to be driven elastically by weak Hg···Cl interactions: large area expansion of the sheets causes negative thermal expansion in the perpendicular direction. Using lanthanides heavier than Ce yielded 2-D sheet-based compounds with the formula [Ln(OH2)x]2[Hg(CN)2]5Cl6·2H2O (Ln = Nd and Eu, x = 7; Ln = Gd, Tb, Dy, Ho, Tm, Yb, and Lu, x = 6). Although there was also evidence for elastic behavior within these materials, both showed uniaxial zero thermal expansion (Ln = Nd: 27.9(17), 22.4(10), and 0.6(12) ppm K-1 along the I, II, and III principal axes, respectively; Ln = Tb: 39.6(12), 1.1(17), and 36.1(7) ppm K-1 along the a, b, and c axes, respectively). Despite their similar structural architecture, this zero thermal expansion was found to occur in different directions─within the plane of the 2-D sheets for [Nd(OH2)7]2[Hg(CN)2]5Cl6·2H2O but in the direction perpendicular to the 2-D sheets for [Tb(OH2)6]2[Hg(CN)2]5Cl6·2H2O. Overall, this system of compounds reveals the delicate relationship between coordination polymer structure and thermal expansion.

A cobalt(II) hydrogen-bonded organic framework exhibiting uniaxial negative thermal expansion and field-induced slow magnetic relaxation

The magnetic and thermal expansion properties of a cobalt(II) hydrogen-bonded organic framework (CoHOF), [Co(H2O)6](NPS)2 (1, HNPS = naphthalene-2-sulphonic acid), constructed by hexaaquacobalt(II) and organsulfonate were reported. Single-crystal X-ray diffraction reveals a porous 2D hydrogen-bonded framework of 1 sustained by multiple and charge-assisted intermolecular hydrogen bonding interactions. Variable-temperature single-crystal combined with powder X-ray diffraction and thermal gravimetric analyses confirms the high thermal stability of the CoHOF. The dynamic changes of the crystal lattice indicate uniaxial negative thermal expansion (NTE) behavior along the c-axis direction through a possible slippage motion. Magnetic measurements indicate the Co2+ ions show uniaxial magnetic anisotropy with D = −92.5 cm−1. A.C. susceptibility measurements reveal field-driven slow magnetic relaxation of 1. These results provide a rare cobalt(II) hydrogen-bonded organic framework showing single-ion magnetism and negative thermal expansion properties.

Giant uniaxial negative thermal expansion in FeZr2 alloy over a wide temperature range

Negative thermal expansion (NTE) alloys possess great practical merit as thermal offsets for positive thermal expansion due to its metallic properties. However, achieving a large NTE with a wide temperature range remains a great challenge. Herein, a metallic framework-like material FeZr2 is found to exhibit a giant uniaxial (1D) NTE with a wide temperature range (93-1078 K, {bar{alpha }}_{l}=-34.01times {10}^{-6},{{{{{{rm{K}}}}}}}^{-1}). Such uniaxial NTE is the strongest in all metal-based NTE materials. The direct experimental evidence and DFT calculations reveal that the origin of giant NTE is the couple with phonons, flexible framework-like structure, and soft bonds. Interestingly, the present metallic FeZr2 excites giant 1D NTE mainly driven by high-frequency optical branches. It is unlike the NTE in traditional framework materials, which are generally dominated by low energy acoustic branches. In the present study, a giant uniaxial NTE alloy is reported, and the complex mechanism has been revealed. It is of great significance for understanding the nature of thermal expansion and guiding the regulation of thermal expansion.

Uniaxial Negative Thermal Expansion in a Tetragonal V3S-Type Superconductor IrZr3

Recently, uniaxial negative thermal expansion (NTE) has been observed in transition-metal zirconides including tetragonal (CuAl2-type) CoZr2 and orthorhombic (Re3B-type) CoZr3. Here, we report the observation of uniaxial NTE in tetragonal (V3S-type) IrZr3. With increasing temperature, the lattice parameter for the a-axis and volume of IrZr3 increase, but the lattice parameter for the c-axis decreases. IrZr3 is a superconductor with a transition temperature of 2.3 K. The present result suggests that uniaxial NTE and related anomalous axis thermal expansion occur in various transition-metal zirconides, which will be important for designing superconducting device robust to temperature cycles using zero-thermal-expansion superconductors.

In situ study of the crystallization in GeTe0.26 Se0.74 thick film by synchrotron X-ray diffraction

Crystallization in unencapsulated GeTe0.26Se0.74 amorphous 3 µm-thick films produced by thermal co-evaporation technique was studied by in situ temperature-dependent synchrotron powder X-ray diffraction from room temperature up to 418 °C and back to 30 °C. The experimental results obtained with a continuous heating rate of 2 °C/min under a protective nitrogen atmosphere showed that the crystallization process took place in several steps i) total crystallization of the amorphous phase to an intermediate crystallized phase, (ii) total transformation of the intermediate phase to a second crystallized phase. The intermediate phase corresponds to a solid solution with a metastable α-GeTe-like rhombohedral polar structure (R3m) presenting a uniaxial negative thermal expansion along the c-axis. The onset of crystallization to the polar structure is around 270 °C. The high-temperature phase has a stable hexagonal structure with a P63mc space group. When cooling, until back to room temperature, the hexagonal structure is conserved. The effect of a changing heating rate from 2° to 0.1°C/ min was also investigated. In this context, the crystallization scheme is modified.

Controlled Assembly and Anomalous Thermal Expansion of Ultrathin Cesium Lead Bromide Nanoplatelets

Quantum confined lead halide perovskite nanoplatelets are anisotropic materials displaying strongly bound excitons with spectrally pure photoluminescence. We report the controlled assembly of CsPbBr3 nanoplatelets through varying the evaporation rate of the dispersion solvent. We confirm the assembly of superlattices in the face-down and edge-up configurations by electron microscopy, as well as X-ray scattering and diffraction. Polarization-resolved spectroscopy shows that superlattices in the edge-up configuration display significantly polarized emission compared to face-down counterparts. Variable-temperature X-ray diffraction of both face-down and edge-up superlattices uncovers a uniaxial negative thermal expansion in ultrathin nanoplatelets, which reconciles the anomalous temperature dependence of the emission energy. Additional structural aspects are investigated by multilayer diffraction fitting, revealing a significant decrease in superlattice order with decreasing temperature, with a concomitant expansion of the organic sublattice and increase of lead halide octahedral tilt.

Multifunctional Triggering by Solid-Phase Molecular Motion: Relaxor Ferroelectricity, Modulation of Magnetic Exchange Interactions, and Enhancement of Negative Thermal Expansion

Although the development of artificial molecular machines has garnered considerable attention in recent years, the construction of multifunctional solid-state molecular machines still faces several challenges. Herein, we report a supramolecular approach as an efficient strategy for building multifunctional trigger systems. In crystals composed of [Ni(dmit)2]− with a spin of S = 1/2 and supramolecular structures consisting of 4-aminopyridinium+ and benzo[18]crown-6, supramolecular cations with dynamic degrees of freedom affect the magnetic and dielectric properties and induce negative thermal expansion (NTE). The supramolecular cations in the crystals form one-dimensional columns. Two adjacent columns form a supramolecular ladder structure via π···π interactions between the phenylene groups of benzo[18]crown-6 and are arranged within a two-dimensional layer. A disorder between the two sites of the phenylene ring was observed in one of the crystallographically independent benzo[18]crown-6. Disordered benzo[18]crown-6 formed polar domains within the crystal, resulting in relaxor ferroelectricity. With increasing temperature, the supramolecular ladders elongated and the translational motion of benzo[18]crown-6 caused the molecular ladder to move closer to each other. Consequently, the crystals shrunk in the direction perpendicular to the ladder, exhibiting uniaxial NTE, and the magnetic exchange interaction between the [Ni(dmit)2]− crystals was disrupted.

Axis thermal expansion switching in transition-metal zirconides TrZr2 by tuning the c/a ratio

This study examines the temperature-dependent evolution of the lattice constants for various CuAl2-type compounds, including NiZr2, (Co,Rh,Ir)Zr2, (Fe,Co,Rh,Ir)Zr2, and (Co,Ni,Cu,Rh,Ir)Zr2, in the pursuit of negative or zero thermal expansion. Results reveal that NiZr2 has a positive thermal expansion, while the other compounds exhibit uniaxial negative thermal expansion along the c-axis contraction. The study suggests that the c-axis thermal expansion can be controlled by manipulating the c/a ratio through Tr-site substitution, providing a design principle for achieving negative thermal expansion of the c-axis and potentially zero thermal expansion in a single compound in TrZr2 compounds.

Uniaxial Negative Thermal Expansion in an Orthorhombic Superconductor CoZr3

We investigated the temperature evolution of crystal structure of orthorhombic CoZr3, which is a superconductor with a transition temperature of 4.3 K, by synchrotron and laboratory (CuK{\alpha}) X-ray diffraction. Uniaxial negative thermal expansion along the c-axis, which is similar to that observed in tetragonal CoZr2, has been observed at a wide temperature range of T = 90-800 K in CoZr3, while a-and b-axis exhibit positive thermal expansion.

Anomalous thermal expansion of strontium squarate trihydrate induced by hydrogen-bond weakening

A strontium squarate framework featuring diverse chemical bonds was found to exhibit uniaxial negative thermal expansion.

Uniaxial negative thermal expansion induced by molecular rotation in a one-dimensional supramolecular assembly with associated peculiar magnetic behavior

An alternative approach for achieving negative thermal expansion (NTE) in a molecular crystal of [Ni(dmit)2]− salt with supramolecular cation composed of pyridazinium and dibenzo[24]crown-8. Deformation of supramolecular cation induces NTE and peculiar magnetic responses.

Preparation, uniaxial negative thermal expansion and Raman study in semiconductor oxide Nb2WO8 ceramic

Negative thermal expansion materials exhibit important promising to tailor thermal expansion due to their abnormal thermal shrinkage characteristics. Here, the Nb2WO8 ceramic has been synthesized, which exhibits axial negative thermal expansion (αa = −2.05 × 10−6 K−1) from 300 to 1000 K without water absorption or phase transformation. Along a axis, the one O atom is located between two identical Nb atoms and the Nb–O–Nb atomic linkages are formed, in which the Nb2–O2–Nb2 bond angle decreases upon heating, resulting in the negative thermal expansion of a axis. Although the b axis and c axis show positive thermal expansion, the low thermal expansion (αl = 2.30 × 10−6 K−1, 173–1073 K) occurs in unit cell volume due to the negative thermal expansion of a axis. The variable pressure Raman shows that the translational and rotational modes of the polyhedron play a key role in axial negative thermal expansion. This work shows potential application prospects of Nb2WO8 as a low thermal expansion semiconductor ceramic.

Designing Geometric Degrees of Freedom in ReO3‐Type Coordination Polymers

AbstractEngineering the interplay of structural degrees of freedom that couple to external stimuli such as temperature and pressure is a powerful approach for material design. New structural degrees of freedom expand the potential of the concept, and coordination polymers as a chemically versatile material platform offer fascinating possibilities to address this challenge. Here, we report a new class of perovskite‐like AB2X6 coordination polymers based on a [BX3] − ReO3‐type host network ([Mn(C2N3)3] −), in which the spatial orientation of divalent A2+ cations ([R3N(CH2)nNR3]2+) with separated charge centers that bridge adjacent ReO3‐cavities is introduced as a new geometric degree of freedom. Herringbone and head‐to‐tail order pattern of [R3N(CH2)nNR3]2+ cations are obtained by varying the separator length n and, together with distortions of the pseudocubic [BX3] − network, they determine the materials’ stimuli‐responsive behavior such as counterintuitive large negative compressibility and uniaxial negative thermal expansion. This new family of coordination polymers highlights the chemists’ capabilities of designing matter on a molecular level to address macroscopic material functionality and underpins the opportunities of the design of structural degrees of freedom as a conceptual framework for rational material synthesis in the future.

Anomalous and colossal thermal expansion, photoluminescence, and dielectric properties in lead halide-layered perovskites with cyclohexylammonium and cyclopentylammonium cations

SummaryA detailed study of lead halide-layered perovskites with general formula A2PbX4 (where A is cyclohexylammonium (CHA) or cyclopentylammonium (CPA) cation and X is Cl− or Br− anion) is presented. Using variable temperature synchrotron X-ray powder diffraction, we observe that these compounds exhibit diverse crystal structures above room temperature. Very interestingly, we report some unconventional thermomechanical responses such as uniaxial negative thermal expansion and colossal positive thermal expansion in a perpendicular direction. For the polymorphs of (CHA)2PbBr4, the volumetric thermal expansion coefficient is among the highest reported for any extended inorganic crystalline solid, reaching 480 MK−1. The phase transitions are confirmed by calorimetry and dielectric measurements, where the dielectric versus temperature curves show anomalies related with the order-disorder phase transitions. In addition, these compounds exhibit a broad photoluminescence (PL) emission with a large Stokes shift, which is related with an exciton PL emission.

Tunable uniaxial, area, and volume negative thermal expansion in quartz-like and diamond-like metal-organic frameworks.

This paper proposes that it will be an effective way to discover and explore organic negative thermal expansion (NTE) materials based on the specific topologies in inorganic NTE materials. Various NTE behaviors from the uniaxial, area, and volume-NTE can be achieved by adjusting the topology, for instance, quartz-like and diamond-like. Zn(ISN)2 and InH(BDC) metal–organic frameworks (MOFs) with quartz-like topology have been studied by first principles calculations. The calculated area-NTE of Zn(ISN)2 and uniaxial-NTE of InH(BDC) within quasi-harmonic approximation (QHA) agree well with the experimental data. Through the calculation of Grüneisen parameters, it is shown that low-frequency optical phonons appear dominant resulting in their NTE, but the coupling to high-frequency phonons is of greater ultimate importance. The lattice vibrational modes of great contribution to area-NTE of Zn(ISN)2 and uniaxial-NTE of InH(BDC) are analyzed in detail. Also, four MOFs with diamond-like topology are predicted to exhibit volume-NTE behavior. Moreover, it is found that there is a bulk modulus anomaly in some studied MOFs with the quartz-like and diamond-like framework, where the temperature dependence of bulk modulus does not follow the inverse dependence on that of volume. These specific topologies provide key geometric frameworks for various NTE behaviors of MOFs, and meanwhile, the local structure and bond environment in MOFs can lead to abnormal interatomic force, i.e., bulk modulus anomaly. This abnormal elastic property also deserves more attention.

Ferroelectricity and Uniaxial Negative Thermal Expansion in a Purely Organic Multifunctional Material

Ferroelectricity and Uniaxial Negative Thermal Expansion in a Purely Organic Multifunctional Material

Theoretical Study of Abnormal Thermal Expansion of CuSCN and Effect on Electronic Structure

CuSCN, as a new type of inorganic hole-transporting semiconductor with a wide bandgap (>3.4 eV), is attracting much attention in the fabrication of perovskite solar cells. In this article, by using first-principles density functional theory (DFT) and the quasi-harmonic approximation (QHA) approach, we have studied lattice dynamics and abnormal thermal expansion of the system, including α- and β-CuSCN phases. The influence of the abnormal thermal expansion of the lattice on the electronic structure, especially on the bandgap of the system, was explored and discussed. We found that due to the shearing modes and the three acoustic modes along the direction of the c-axis, the α- and β-CuSCN show a negative thermal expansion (NTE) in the direction of the c-axis. The torsion modes of the Cu–N–C–S atomic chains in the α-CuSCN may lead to an NTE in the directions of the a, b-axes of the α-phase. As a result, our theoretical results demonstrated that the α-CuSCN exhibits an anisotropic bulk NTE. While the β-CuSCN displays a strong uniaxial negative thermal expansion in the direction of the c-axis, in the directions of the a, b-axes, it exhibits positive thermal expansion. Our DFT calculations also predicted that the α-CuSCN has a direct bandgap, which increases slightly with increasing temperature. However, the β-CuSCN has an indirect bandgap at low temperature, which converts to a direct bandgap near the temperature of 375 K due to the strong positive expansion in the ab plane of the phase. Our work revealed the mechanisms of the abnormal thermal expansion of the two phases and a strong coupling between the anisotropic thermal expansion and the electronic structures of the system.

A survey of thermal expansion coefficients for organic molecular crystals in the Cambridge Structural Database.

Typical ranges of thermal expansion coefficients are established for organic molecular crystals in the Cambridge Structural Database. The CSD Python API is used to extract 6201 crystal structures determined close to room temperature and at least one lower temperature down to 90 K. The data set is dominated by structure families with only two temperature points and is subject to various sources of error, including incorrect temperature reporting and missing flags for variable-pressure studies. For structure families comprising four or more temperature points in the range 90-300 K, a linear relationship between unit-cell volume and temperature is shown to be a reasonable approximation. For a selected subset of 210 structures showing an optimal linear fit, the volumetric expansion coefficient at 298 K has mean 173 p.p.m. K-1 and standard deviation 47 p.p.m. K-1. The full set of 6201 structures shows a similar distribution, which is fitted by a normal distribution with mean 161 p.p.m. K-1 and standard deviation 51 p.p.m. K-1, with excess population in the tails mainly comprising unreliable entries. The distribution of principal expansion coefficients, extracted under the assumption of a linear relationship between length and temperature, shows a positive skew and can be approximated by two half normal distributions centred on 33 p.p.m. K-1 with standard deviations 40 p.p.m. K-1 (lower side) and 56 p.p.m. K-1 (upper side). The distribution for the full structure set is comparable to that of the test subset, and the overall frequency of biaxial and uniaxial negative thermal expansion is estimated to be < 5% and ∼30%, respectively. A measure of the expansion anisotropy shows a positively skewed distribution, similar to the principal expansion coefficients themselves, and ranges based on suggested half normal distributions are shown to highlight literature cases of exceptional thermal expansion.