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

Effect of Octahedral Connectivity on the Negative Thermal Expansion of SrZrS3

Sulfide perovskites (ABX₃) are under increasing investigation for use in photovoltaic, optoelectronic, dielectric, and thermoelectric devices due to their favorable band gaps, dynamical properties, environmental stability, and structural diversity. A key material parameter to optimize in such devices is the constituent materials' coefficient of thermal expansion (CTE) in order to minimize thermomechanical stress during fabrication and operation. This can be done by avoiding materials that have large CTE mismatch or by compensating positive thermal expansion by including materials with negative thermal expansion (NTE). Here, we evaluate the CTE of (edge-connected) α- and (corner-connected) β-SrZrS₃ with density functional theory and the self-consistent quasiharmonic approximation. We find that both materials exhibit positive thermal expansion at 0 GPa and host pressure-induced negative thermal expansion. The β phase has a smaller CTE (37 × 10^-6 K^-1) at room temperature and ambient pressure and also has a larger NTE response under pressure due to its more flexible corner-connected framework structure. We use our findings to suggest that corner-shared motifs should be prioritized over edge- or face-shared octahedral networks to maximize NTE arising from vibrational (phononic) mechanisms.

Sign change in c-axis thermal expansion constant and lattice collapse by Ni substitution in transition-metal zirconide superconductor Co1-xNixZr2

Recently, c-axis negative thermal expansion (NTE) was observed in a CoZr₂ superconductor and related transition-metal zirconides. Here, we investigated the structural, electronic, and superconducting properties of Co_1-xNi_xZr₂ to achieve systematic control of c-axis NTE and switching from NTE to positive thermal expansion (PTE) by Ni substitution. At x ≤ 0.3, c-axis NTE was observed, and the thermal expansion constant α_c approached zero with increasing x. At x = 0.4-0.6, c-axis thermal expansion close to zero thermal expansion (ZTE) was observed, and PTE appeared for x ≥ 0.7. On the superconducting properties, we observed bulk superconductivity for x ≤ 0.6, and bulk nature of superconductivity is suppressed by Ni heavy doping (x ≥ 0.7). For x ≤ 0.6, the evolution of the electronic density of states well explains the change in the superconducting transition temperature (T_c), which suggests conventional phonon-mediated superconductivity in the system. By analyzing the c/a ratio, we observed a possible collapsed transition in the tetragonal lattice at around x = 0.6-0.8. The lattice collapse would be the cause of the suppression of superconductivity in Ni-rich Co_1-xNi_xZr₂ and the switching from NTE to PTE.

Negative Thermal Expansion Properties of Sm0.85Sr0.15MnO3-δ

A novel negative thermal expansion (NTE) material composed of Sm0.85Sr0.15MnO3-δ was synthesized using the solid-state method. By allowing Sr2+ to partially replace Sm3+ in SmMnO3, the ceramic material Sm0.85Sr0.15MnO3-δ exhibits NTE properties between 360K and 873K, and its average negative thermal expansion coefficient was −10.08 × 10−6/K. The structure of Sm0.85Sr0.15MnO3-δ is orthogonal, the space group is pbnm, the morphology is regular, and the grain size is uniform. The results of X-ray diffraction and XPS (X-ray photoelectron spectroscopy) suggest that the NTE phenomenon is related to the electron transfer of Mn ions. With the increase in temperature, Mn4+ is rapidly transformed into Mn3+, accompanied by Mn4+O6 octahedron distortion and oxygen defects. It was found that the sample volume continually decreased at the same time.

Bi0.33Zr2(PO4)3, a negative thermal expansion material with a Nasicon-type structure

A new negative thermal expansion ceramic material, Bi0.33Zr2(PO4)3, with a sodium zirconium phosphate structure, has been investigated and the results are presented.