Colossal negative thermal expansion in BiNiO 3 induced by intermetallic charge transfer

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^-4K^-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 Bi _0.95La_0.05NiO₃ is - 137×10 ^-6K^-1 and a value of - 82×10^-6K ^-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.