The unusual property of negative thermal expansion is of fundamental interest and may be used to fabricate composites with zero or other controlled thermal expansion values. Here we report that colossal negative thermal expansion (defined as linear expansion <−10−4 K−1 over a temperature range ~100 K) is accessible in perovskite oxides showing charge-transfer transitions. BiNiO3 shows a 2.6% volume reduction under pressure due to a Bi/Ni charge transfer that is shifted to ambient pressure through lanthanum substitution for Bi. Changing proportions of coexisting low- and high-temperature phases leads to smooth volume shrinkage on heating. The crystallographic linear expansion coefficient for Bi0.95La0.05NiO3 is −137×10−6 K−1 and a value of −82×10−6 K−1 is observed between 320 and 380 K from a dilatometric measurement on a ceramic pellet. Colossal negative thermal expansion materials operating at ambient conditions may also be accessible through metal-insulator transitions driven by other phenomena such as ferroelectric orders.

No abstract

No abstract

No abstract

No abstract

A TiS 2 crystal with a layered structure was found to have a large thermoelectric power factor. The in-plane power factor S 2 /ρ at 300 K is 37.1 µW/K 2 cm with resistivity (ρ) of 1.7 mΩcm and thermopower (S) of -251 µV/K, and this value is comparable to that of the best thermoelectric material, Bi 2 Te 3 alloy.The electrical resistivity shows both metallic and highly anisotropic behaviors, suggesting that the electronic structure of this TiS 2 crystal has a quasi-twodimensional nature. The large thermoelectric response can be ascribed to the large density of state just above the Fermi energy and inter-valley scattering.In spite of the large power factor, the figure of merit, ZT of TiS 2 is 0.16 at 300 K, because of relatively large thermal conductivity, 68 mW/Kcm. However, most of this value comes from reducible lattice contribution. Thus, ZT can be improved by reducing lattice thermal conductivity, e.g., by introducing a rattling unit into the inter-layer sites.