Uniaxial Negative Thermal Expansion, Negative Linear Compressibility, and Negative Poisson’s Ratio Induced by Specific Topology in Zn[Au(CN)2]2

Giant Negative Thermal Expansion in Ultralight NaB(CN)4

Negative thermal expansion (NTE) is crucial for controlling the thermomechanical properties of functional materials, albeit being relatively rare. This study reports a giant NTE (αV ∼-9.2 ⋅ 10-5  K-1 , 100-200 K; αV ∼-3.7 ⋅ 10-5  K-1 , 200-650 K) observed in NaB(CN)4 , showcasing interesting ultralight properties. A comprehensive investigation involving synchrotron X-ray diffraction, Raman spectroscopy, and first-principles calculations has been conducted to explore the thermal expansion mechanism. The findings indicate that the low-frequency phonon modes play a primary role in NTE, and non-rigid vibration modes with most negative Grüneisen parameters are the key contributing factor to the giant NTE observed in NaB(CN)4 . This work presents a new material with giant NTE and ultralight mass density, providing insights for the understanding and design of novel NTE materials.

The uniaxial zero thermal expansion and zero linear compressibility in distorted Prussian blue analogue RbCuCo(CN)6

The uniaxial zero thermal expansion (ZTE) in distorted Prussian blue analogue (PBA) RbCuCo(CN)6 is reproduced by employing first-principles calculations, which agrees well with the experimental data. Also, the zero linear compressibility (ZLC) behavior in RbCuCo(CN)6 can be found. The special Jahn-Teller distortion introduced by Cu2+ in RbCuCo(CN)6 is noticed by investigating the change of the local structure with temperature and hydrostatic pressure. The lattice thermal conductivity (LTC) and phonon group velocity of RbCuCo(CN)6 are studied, where the LTC and phonon group velocity are significantly anisotropic. Especially, RbCuCo(CN)6 exhibits a quite low LTC, and its c-axis shows a characteristic of glasslike LTC at low temperatures. Our work facilitates a deep understanding of the coexistence mechanisms of uniaxial ZTE and ZLC properties in RbCuCo(CN)6.

Penta-graphene and phagraphene: thermal expansion, linear compressibility, and Poisson's ratio

Nonplanar penta-graphene and planar phagraphene, which are connected by carbon pentagons and penta-hexa-hepta carbon rings, respectively, are two allotropes of graphene. Graphene as a star material in two-dimensional materials has been widely studied. However, the studies around penta-graphene and phagraphene are still insufficient. We are interested in both materials' response to temperature, hydrostatic pressure, and stress. In this work, the thermal expansion, linear compressibility, and Poisson's ratio of penta-graphene and phagraphene have been investigated systematically. It is found that both materials can exhibit abnormal negative thermal expansion behavior, while their linear compressibility behavior is normal. The negative Poisson's ratio behavior only occurs in penta-graphene, which is consistent with other work. Through an analysis of the lattice vibrations and associated mode Grüneisen parameters, it is found that there are anomalies in the phonon spectra of both penta-graphene and phagraphene. It is noted that acoustic phonons contribute most to their respective anomalies, especially the transverse acoustic mode. The librational motion of the lowest-frequency optical phonon of both materials is identified and also associated with their novel properties. In general, the unique topological arrangement of carbon atoms can play a decisive role in determining the performances of penta-graphene and phagraphene.

Anharmonic Interaction in Negative Thermal Expansion Material CaTiF6

Although the quasi-harmonic approximation (QHA) method applies to many materials, it is necessary to study the anharmonic interaction for extremely anharmonic materials. In this work, the unusual negative thermal expansion (NTE) property of CaTiF₆ is studied by combing QHA and anharmonic interaction. The improved self-consistent phonon approximation (ISCPA), which treats anharmonic effects in solids nonperturbatively, is employed. The agreement of NTE behavior between the calculation and the experiment can be further promoted from qualitative consistency by QHA to quantitative consistency by the ISCPA. From mode Grüneisen parameters, it is found that the low-frequency phonons, especially acoustic phonons, contribute greatly to the NTE behavior of CaTiF₆. The rigid unit modes (RUMs) of low-frequency optical phonons can be identified. The phonon lifetime of CaTiF₆ is calculated from three-phonon interactions; thereby, the NTE mechanism can be further explored by phonon lifetimes of phonons with different frequencies on heating. The anomalous lattice thermal conductivity (LTC) is predicted using the Boltzmann transport equation within the relaxation time approximation. The glasslike LTC can occur in crystal CaTiF₆.

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.

Thermally boosted upconversion and downshifting luminescence in Sc2(MoO4)3:Yb/Er with two-dimensional negative thermal expansion

Rare earth (RE³⁺)-doped phosphors generally suffer from thermal quenching, in which their photoluminescence (PL) intensities decrease at high temperatures. Herein, we report a class of unique two-dimensional negative-thermal-expansion phosphor of Sc₂(MoO₄)₃:Yb/Er. By virtue of the reduced distances between sensitizers and emitters as well as confined energy migration with increasing the temperature, a 45-fold enhancement of green upconversion (UC) luminescence and a 450-fold enhancement of near-infrared downshifting (DS) luminescence of Er³⁺ are achieved upon raising the temperature from 298 to 773 K. The thermally boosted UC and DS luminescence mechanism is systematically investigated through in situ temperature-dependent Raman spectroscopy, synchrotron X-ray diffraction and PL dynamics. Moreover, the luminescence lifetime of ⁴I_13/2 of Er³⁺ in Sc₂(MoO₄)₃:Yb/Er displays a strong temperature dependence, enabling luminescence thermometry with the highest relative sensitivity of 12.3%/K at 298 K and low temperature uncertainty of 0.11 K at 623 K. These findings may gain a vital insight into the design of negative-thermal-expansion RE³⁺-doped phosphors for versatile applications.

Rare-earth doped phosphors with negative thermal expansion (NTE) may display thermally-enhanced emission, but their performance is generally limited. Here the authors report thermally-boosted green upconversion luminescence and near-infrared downshifting luminescence in Sc₂(MoO₄)₃:Yb/Er phosphors with two-dimensional NTE, and their application in temperature sensing.

Crystal engineering of aurophilic supramolecular architectures and coordination polymers based on butterfly-like Copper-dicyanoaurate complexes: vapochromism, P-T behaviour and multi-metallic cocrystal formation

Using the equilibrium properties of CuII in the presence of the chelating ligand and the characteristics of the dicyanoaurate anion, we were able to obtain a family of 10 bimetallic...

Biaxial negative thermal expansion in Zn[N(CN)2]2

A 2D-layered network Zn[N(CN)2]2, is reported in which the transverse vibrations of C atoms and the rotation of ZnN4 tetrahedra dominate its biaxial NTE behavior.

Adjustable uniaxial zero thermal expansion and zero linear compressibility in unique hybrid semiconductors: the role of the organic chain

Zero thermal expansion (ZTE) and zero linear compressibility (ZLC) are unique and rare properties. Materials combining ZTE and ZLC will have promising prospects. A novel route is proposed in this work to design the coexistence of uniaxial-ZTE and ZLC based on layered hybrid semiconductors [ZnTe(L)0.5] [L = N2H4, ethylenediamine (en), propyldiamine (pda)]. In the framework of [ZnTe(L)0.5], the organic chain contains the attractive and repulsive interactions that arise from the different organic components. It is demonstrated that changing the length of the organic chain can effectively regulate the interaction between different organic components and then achieve the uniaxial-ZTE and ZLC or the desired thermal expansion and compression behaviors. The origin of the coexistence of abnormal axial responses has been traced from thermodynamic formalisms, model Grüneisen parameters and specific vibration modes. It is found that low-energy phonons play an important internal role in realizing the multi-peculiar properties.

A new polar perovskite coordination network with azaspiroundecane as A-site cation

We report a new polar ABX₃ perovskite coordination network based on azaspiroundecane as A-site cation and dicyanamide as X-site anion.

Black phosphorene exhibiting negative thermal expansion and negative linear compressibility

Positive expansion on heating, and positive contraction on hydrostatic compression are our general understanding. Actually, the abnormal negative thermal expansion (NTE) or negative linear compressibility (NLC) behavior is permissible, but exists in very few materials. Interestingly, we find that the NTE and NLC behaviors can coexist in the low-dimensional black phosphorene using first-principles calculations. In the temperature-field, the volume-NTE of black phosphorene can be exhibited below about 200 K, while the axial-NTE exists in the whole studied temperature range. In the hydrostatic pressure-field, the NLC behavior can occur along a zigzag direction or armchair direction in different pressure ranges. The phonon abnormality in black phosphorene is detected. It is found that acoustic phonons, especially the lowest-frequency TA _z mode, play a very important role. The re-entrant honeycomb network in black phosphorene, as the specific topology, will facilitate the occurrence of NTE and NLC behaviors. This work can provide some foundations and clues for exploring the abnormal properties in other 2D materials.

Anomalous Lattice Dynamics in AgC4N3: Insights From Inelastic Neutron Scattering and Density Functional Calculations

We have performed temperature dependent inelastic neutron scattering measurements to study the anharmonicity of phonon spectra of AgC4N3. The analysis and interpretation of the experimental spectra is done using ab-initio lattice dynamics calculations. The calculated phonon spectrum over the entire Brillouin zone is used to derive linear thermal expansion coefficients. The effect of van der Waals interaction on structure stability has been investigated using advanced density functional methods. The calculated isothermal equation of states implies a negative linear compressibility along the c-axis of the crystal, which also leads to a negative thermal expansion along this direction. The role of elastic properties inducing the observed anomalous lattice behavior is discussed.

Control of Uniaxial Negative Thermal Expansion in Layered Perovskites by Tuning Layer Thickness

Uniaxial negative thermal expansion (NTE) is known to occur in low n members of the A n+1B n O3n+1 Ruddlesden-Popper (RP) layered perovskite series with a frozen rotation of BO6 octahedra about the layering axis. Previous work has shown that this NTE arises due to the combined effects of a close proximity to a transition to a competing phase, so called "symmetry trapping", and highly anisotropic elastic compliance specific to the symmetry of the NTE phase. We extend this analysis to the broader RP family (n = 1, 2, 3, 4, …, ∞), demonstrating that by changing the fraction of layer interface in the structure (i.e., the value of 1/n) one may control the anisotropic compliance that is necessary for the pronounced uniaxial NTE observed in these systems. More detailed analysis of how the components of the compliance matrix develop with 1/n allows us to identify different regimes, linking enhancements in compliance between these regimes to the crystallographic degrees of freedom in the structure. We further discuss how the perovskite layer thickness affects the frequencies of soft zone boundary modes with large negative Grüneisen parameters, associated with the aforementioned phase transition, that constitute the thermodynamic driving force for NTE. This new insight complements our previous work-showing that chemical control may be used to switch from positive to negative thermal expansion in these systems-since it makes the layer thickness, n, an additional design parameter that may be used to engineer layered perovskites with tuneable thermal expansion. In these respects, we predict that, with appropriate chemical substitution, the n = 1 phase will be the system in which the most pronounced NTE could be achieved.

Negative/zero thermal expansion in black phosphorus nanotubes

The thermal expansion of BPNTs has been given a comprehensive study. It is found that the abnormal thermal expansion can be tunable by changing diameter and bonding in the unique configurations of armchair and zigzag BPNTs.