Significant Enhancement of Negative Thermal Expansion Under Low Pressure in Cu2P2O7.

Much effort is made to achieve the negative thermal expansion (NTE) control, but rare methods reached the improvement of intrinsic NTE. In the present work, a significantly enhanced NTE is realized in Cu2P2O7 by applying low pressure. Especially, the volumetric coefficient of thermal expansion (CTE) of Cu2P2O7 reached to -50.0 × 10-6 K-1 (150-325K) under 0.25 GPa, which is increased by 47.5% compared to its NTE in a similar temperature range under atmosphere pressure. This character enables a more effective manifestation of the thermal compensation role of Cu2P2O7 in composites. The enhanced NTE mechanisms are analyzed by high pressure synchrotron X-ray diffraction, neutron diffraction at variable temperature and pressure, as well as density functional theory (DFT) calculations. The results show that applied pressure accelerates the contraction of the distance between adjacent CuO layers and CuO columns. Meanwhile, the low-frequency phonon contribution to NTE in α-Cu2P2O7 is improved. This work is meaningful for the exploration of methods to enhance NTE and the practical application of NTE materials.

High-density, spontaneous magnetic biskyrmions induced by negative thermal expansion in ferrimagnets.

Magnetic skyrmions are topologically protected quasiparticles that are promising for applications in spintronics. However, the low stability of most magnetic skyrmions leads to either a narrow temperature range in which they can exist, a low density of skyrmions, or the need for an external magnetic field, which greatly limits their wide application. In this study, high-density, spontaneous magnetic biskyrmions existing within a wide temperature range and without the need for a magnetic field were formed in ferrimagnets owing to the existence of a negative thermal expansion of the lattice. Moreover, a strong connection between the atomic-scale ferrimagnetic structure and nanoscale magnetic domains in Ho(Co,Fe)3 was revealed via in situ neutron powder diffraction and Lorentz transmission electron microscopy measurements. The critical role of the negative thermal expansion in generating biskyrmions in HoCo3 based on the magnetoelastic coupling effect is further demonstrated by comparing the behavior of HoCo2.8Fe0.2 with a positive thermal expansion.

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, [Formula: see text]). 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.

Negative and zero thermal expansion in α-(Cu2-xZnx)V2O7 solid solutions.

Negative or zero thermal expansion (NTE or ZTE) of materials is intriguing for controllable thermal expansion. We report a series of orthorhombic α-Cu2-xZnxV2O7 (x = 0, 0.1, 0.2), in which the volumetric coefficients of thermal expansion are successfully tuned from -10.19 × 10-6 K-1 to -1.58 × 10-6 K-1 in the temperature range of 100-475 K by increasing the content of Zn2+. It has been revealed that the transverse vibrations of oxygen bonded with vanadium are dominant in the contraction of the bc plane, leading to the overall volume NTE in α-Cu2V2O7. The introduction of Zn2+ densifies the crystal structure, which is presumed to suppress the space of transverse vibrations and results in the ZTE in α-Cu1.8Zn0.2V2O7. This work presents an effective method to realize ZTE in anisotropic framework systems.

Strong Negative Thermal Expansion in a Low-Cost and Facile Oxide of Cu2P2O7.

Negative thermal expansion (NTE) behaviors have been observed in various types of compounds. The achievement in the merits of promising low-cost and facile NTE oxides remains challenging. In the present work, a simple and low-cost Cu2P2O7 has been found to exhibit the strongest NTE among the oxides (αV ∼ -27.69 × 10-6 K-1, 5-375 K). The complex NTE mechanism has been investigated by the combined methods of high-resolution synchrotron X-ray diffraction, neutron powder diffraction, X-ray pair distribution function, extended X-ray absorption fine structure spectroscopy, and density functional theory calculations. Interesting, the direct experimental evidence reveals that the coupling twist and rotation of PO4 and CuO5 polyhedra are the inherent factors for the NTE nature of Cu2P2O7, which is triggered by the transverse vibrations of oxygen atoms. The present new NTE material of Cu2P2O7 also has been verified for the practical application.

Negative thermal expansion in cubic FeFe(CN)6 Prussian blue analogues.

Negative thermal expansion (NTE) behavior is an interesting physical phenomenon, but the number of NTE materials is limited. In this study, a new NTE compound has been found, FeFe(CN)6 Prussian blue analogue, where the average linear coefficient of thermal expansion (αl) is -4.260 × 10-6 K-1 between 100 and 450 K. The NTE properties and local vibration dynamics have been investigated by joint experiments of synchrotron X-ray diffraction, X-ray pair distribution function, and extended X-ray absorption fine structure spectroscopy. It has been observed that the Fe-C/Fe-N bonds expand with increasing temperature, while the unit cell shrinks in FeFe(CN)6. The vibration directions of both Fe-C and Fe-N prefer to be perpendicular to the linkage of Fe-C[triple bond, length as m-dash]N-Fe rather than being parallel. More pieces of evidence indicate that the transverse vibrations of N atoms dominate the NTE behavior of FeFe(CN)6. The present results prove directly that the transverse thermal vibrations of C and N atoms are crucial for the occurrence of the NTE of Prussian blue analogues.

Tunable Thermal Expansion from Negative, Zero, to Positive in Cubic Prussian Blue Analogues of GaFe(CN)6.

The achievement of controlling thermal expansion is important for open-framework structures. The present work proposes a feasible way to adjust the coefficient of thermal expansion continuously from negative to positive via inserting guest Na+ ions or H2O molecules into a GaFe(CN)6 framework. The guest ions or molecules have an intense dampening effect on the transverse vibrations of CN atoms in the -Ga-N≡C-Fe- linkage, especially for the N atoms. This study demonstrates that electrochemical or redox intercalation of guest ions will be an effective way to tune thermal expansion in those negative thermal expansion open-framework materials induced by low-frequency phonons.

Low-Frequency Phonon Driven Negative Thermal Expansion in Cubic GaFe(CN)6 Prussian Blue Analogues.

The understanding of the negative thermal expansion (NTE) mechanism is vital not only for the development of new NTE compounds but also for effectively controlling thermal expansion. Here, we report an interesting isotropic NTE property in cubic GaFe(CN)6 Prussian blue analogues (α l = -3.95 × 10-6 K-1, 100-475 K), which is a new example to understand the complex NTE mechanism. A combined study of synchrotron X-ray diffraction, X-ray total scattering, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations shows that the NTE of GaFe(CN)6 originates from the low-frequency phonons (< ∼100 cm-1), which are directly related to the transverse vibrations of the atomic -Ga-N≡C-Fe- chains. Both the Ga-N and Fe-C chemical bonds are much softer to bend than to stretch. The direct evidence that transverse vibrational contribution to the NTE of GaFe(CN)6 is dominated by N, instead of C atoms, is illustrated. It is interesting to find that the polyhedra of GaFe(CN)6 are not rigid, which is a starting assumption in some models describing the NTE properties of other systems. The NTE mechanism can be vividly described by the "guitar-string" effect, which would be the common feature for the NTE property of many open-framework functional materials, such as Prussian blue analogues, oxides, cyanides, metal-organic frameworks, and zeolites.