Negative thermal expansion (NTE), whereby lattices contract upon heating, is of considerable interest for its wide applications in many fields. Molecular materials have been widely investigated as catalysts, sensors, etc., which usually endure temperature vibration. NTE can become a substantial means for controlling the coefficients of thermal expansion. Molecular materials possess plentiful structures and can be easily decorated, making them ideal platforms for thermal expansion modification. In this feature article, we provide an overview of the recent developments in utilizing NTE in molecular materials and summarize some mechanisms leading to NTE. The discussion of NTE in molecular materials concerns many factors, including transverse vibration, geometric flexibility, host-guest interactions, spin crossover, molecular packing rearrangement and molecular conformational changes.

Thermal expansion properties of solids are of fundamental interest and control of thermal expansion is important for practical applications but can be difficult to achieve. Many framework-type materials show negative thermal expansion when internal cages are empty but positive thermal expansion when additional atoms or molecules fill internal voids present. Here we show that redox intercalation offers an effective method to control thermal expansion from positive to zero to negative by insertion of Li ions into the simple negative thermal expansion framework material ScF3, doped with 10% Fe to enable reduction. The small concentration of intercalated Li ions has a strong influence through steric hindrance of transverse fluoride ion vibrations, which directly controls the thermal expansion. Redox intercalation of guest ions is thus likely to be a general and effective method for controlling thermal expansion in the many known framework materials with phonon-driven negative thermal expansion.

Chloride ion batteries (CIBs) are regarded as promising energy storage systems due to their large theoretical volumetric energy density, high abundance, and low cost of chloride resources. Herein, the synthesis of CoFe layered double hydroxide in the chloride form (CoFe-Cl LDH), for use as a new cathode material for CIBs, is demonstrated for the first time. The CoFe-Cl LDH exhibits a maximum capacity of 239.3 mAh g −1 and a high reversible capacity of ≈160 mAh g −1 over 100 cycles. The superb Cl − ion storage of CoFe-Cl LDH is attributed to its unique topochemical transformation property during the charge/discharge process: a reversible intercalation/deintercalation of Cl − ions in cathode with slight expansion/contraction of basal spacing, accompanied by chemical state changes in Co 2+ /Co 3+ and Fe 2+ /Fe 3+ couples. First-principles calculations reveal that CoFe-Cl LDH is an excellent Cl − ion conductor, with extremely low energy barriers (0.12−0.25 eV) for Cl − diffusion. This work opens a new avenue for LDH materials as promising cathodes for anion-type rechargeable batteries, which are regarded as formidable competitors to traditional metal ion-shuttling batteries.

Due to the advantage of invariable length with temperatures, zero thermal expansion (ZTE) materials are intriguing but very rare especially for the metals based compounds. Here, we report a ZTE in the magnetic intermetallic compounds of Tb(Co,Fe) over a wide temperature range (123-307 K). A negligible coefficient of thermal expansion (α = 0.48 × 10 K) has been found in Tb(CoFe). Tb(Co,Fe) exhibits ferrimagnetic structure, in which the moments of Tb and Co/Fe are antiparallel alignment along the c axis. The intriguing ZTE property of Tb(Co,Fe) is formed due to the balance between the negative contribution from the Tb magnetic moment induced spontaneous magnetostriction and the positive role from the normal lattice expansion. The present ZTE intermetallic compounds are also featured by the advantages of wide temperature range, high electrical conductivity, and relatively high thermal conductivity.

The structure and thermal expansion behavior of the tetragonal tungsten bronze oxide Pb2KNb5O15 were investigated by neutron powder diffraction and high-temperature X-ray diffraction. Below the Curie temperature, T(C) (orthorhombic phase, T(C) ≈ 460 °C), the cell parameters a and c increase with temperature, while b decreases. The thermal expansion coefficients are α(a) = 1.29 × 10(-5) °C(-1), α(b) = -1.56 × 10(-5) °C(-1), and α(c) = 1.62 × 10(-5) °C(-1). Temperature-dependent second harmonic generation (SHG), dielectric, and polarization-electrical field (P-E) hysteresis loop measurements were performed to study the symmetry and electric properties. We show that the distortion and cooperative rotation of NbO6 octahedrons are directly responsible for the negative thermal expansion coefficient along the polar b axis. It is suggested that Pb-O covalency, especially in the large and asymmetric pentagonal prisms, may be related to orthorhombic distortion and abnormal spontaneous polarization along the b axis. This study shows that tungsten bronze families are possible candidates for exploring negative thermal expansion materials.