Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Adjusting Local Molecular Environment for Giant Ambient Thermal Contraction

A low-energy triggered switch that can generate mechanoresponse has great technological potential. A submolecular moiety, S-dibenzocyclooctadiene (DBCOD) that is composed of a flexible eight-membered ring connecting to a phenyl ring at each end, undergoes a conformational change from twist-boat to chair under a low-energy stimulus such as near infrared irradiation, resulting in thermal contraction of DBCOD-based polymer. Experimental evidence corroborated by theoretical calculations indicates that introducing molecular asymmetry can reduce crystallinity significantly and consequently facilitate the kinetics of the conformational change. It has been demonstrated that the negative thermal expansion (NTE) coefficient of a DBCOD-based polymer system can be adjusted in a range from -1140 to -2350 ppm K^-1 . -2350 ppm K^-1 is ≈10 times better than the value reported by the second best NTE system.

Stabilities and defect-mediated lithium-ion conduction in a ground state cubic Li3N structure

A stable ground state structure with cubic symmetry of Li3N (c-Li3N) is found by an ab initio initially symmetric random-generated crystal structure search method. Gibbs free energy, calculated within quasi-harmonic approximation, shows that c-Li3N is the ground state structure for a wide range of temperatures. The c-Li3N structure has a negative thermal expansion coefficient at temperatures lower than room temperature, mainly due to two transverse acoustic phonon modes. This c-Li3N phase is a semiconductor with an indirect band gap of 1.90 eV within hybrid density functional calculations. We also investigate the migration and energetics of native point defects in c-Li3N, including lithium and nitrogen vacancies, interstitials, and anti-site defects. Lithium interstitials are found to have a very low migration barrier (∼ 0.12 eV) and the lowest formation energy among all possible defects. The ionic conduction in c-Li3N is thus expected to occur via an interstitial mechanism, in contrast to that in the well-known α-Li3N phase which occurs via a vacancy mechanism.

Phase transition and negative thermal expansion in orthorhombic Dy2W3O12

Orthorhombic Dy₂W₃O₁₂ shows NTE (−2.6 × 10⁻⁵ °C⁻¹) in the temperature range of 150–500 °C. So far, this value is the largest coefficient of negative thermal expansion in the A₂W₃O₁₂ family (A = rare earth element).

Relationship between negative thermal expansion and lattice dynamics in a tetragonal PbTiO3–Bi(Mg1/2Ti1/2)O3 perovskite single crystal

The negative thermal expansion of a tetragonal PbTiO₃–Bi(Mg_1/2Ti_1/2)O₃ perovskite single crystal is correlated to its lattice dynamics and spontaneous polarization.

Zero thermal expansion with high Curie temperature in Ho2Fe16Cr alloy

Ho₂Fe₁₆Cr with a high T_C and moderate coercivity behaves like a zero thermal expansion material in the temperature range 13–330 K.

A novel material of HfScMo2VO12 with negative thermal expansion and intense white-light emission

A novel material of HfScMo₂VO₁₂ with negative thermal expansion (NTE) and intense white-light emission is presented.

Structure and control of negative thermal expansion of Nd/Sm substituted 0.5PbTiO3–0.5BiFeO3 ferroelectrics

Introducing Nd/Sm substitution in 0.5PbTiO₃–0.5BiFeO₃ is an efficient way to control negative thermal expansion (NTE), which meets the mechanism of spontaneous volume ferroelectrostriction.

Topochemical molten salt synthesis for functional perovskite compounds

A topochemical molten salt synthesis (TMSS) method, which is rapid and a modification of the molten salt synthesis method (MSS), to facilitate crystal growth with improved phase-purity and particle homogeneity, is one of the strategic approaches aimed at a controllable morphology synthesis. The TMSS method is an environmentally friendly and mild way to prepare pure and controllable perovskites, which are famous functional materials, used as piezoelectrics, catalysts, ferroelectrics, multiferroics, and negative thermal expansion compounds, at a moderate reaction temperature in a short soaking time. This report reviews various TMSS reactions and their applications in fulfilling the demand for the tunable morphology of perovskite materials, such as one dimensional, two dimensional and three dimensional perovskites in molten salts, which mainly include: piezoelectrics, photocatalysts, negative thermal expansion matters and other functional perovskites. The double and layered perovskites obtained by TMSS methods are also discussed. We believe that a comprehensive understanding of the TMSS method for functional perovskites will definitely promote the development of a clean, efficient and tunable production process for advanced functional materials.

Cation deficiency effect on negative thermal expansion of ferroelectric PbTiO3

This article describes a strong correlation between the spontaneous polarization and NTE features in 8% Pb²⁺ deficient PT (P₉₂T), 2% Ti⁴⁺ deficient PT (PT₉₈) and pure PbTiO₃(PT) samples, which meets the mechanism of spontaneous volume ferroelectrostriction.

Unidirectional thermal expansion in KZnB3O6: role of alkali metals

The size and mass of alkali metals have profound effects on the area zero thermal expansion behavior of edge-sharing KZnB₃O₆.