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Résumé |
The unusual property of negative thermal expansion (NTE) is of fundamental interest and may be used to fabricate composites with controlled thermal expansion values. Systems, such as ZrW2O8, show NTE over a wide temperature range. NTE can result from transitions between different electronic or magnetic states coupled to the lattice, as in the case of (Mn0.96Fe0.04)3(Zn0.5Ge0.5)N. BiNiO3 is an antiferromagnetic insulator with a unique charge distribution of Bi3+0.5Bi5+0.5Ni2+O3. It shows a 2.6% volume reduction under pressure due to a Bi/Ni charge transfer accompanied by an insulator to metal transition. The charge transfer transition is shifted to ambient pressure through La substitution for Bi. We found that the relative proportion of the low- and high-T phases changes as a function of temperature, thus leading to a smooth volume decrease upon heating. By means of dilatometric measurements, the linear expansion coefficient of a Bi0.95La0.05NiO3 pellet was measured to be -13710-6K-1 at RT and -82x10-6K-1 in the 320-380 K range.
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