Department of Chemistry, University of Oxford , Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, U.K.
Diamond Light Source, Harwell Campus, Oxfordshire, OX11 0DE, U.K.
J Am Chem Soc. 2016 May 4;138(17):5479-82. doi: 10.1021/jacs.5b13192. Epub 2016 Mar 7.
The layered perovskite Ca3-xSrxMn2O7 is shown to exhibit a switching from a material exhibiting uniaxial negative to positive thermal expansion as a function of x. The switching is shown to be related to two closely competing phases with different symmetries. The negative thermal expansion (NTE) effect is maximized when the solid solution is tuned closest to this region of phase space but is switched off suddenly on passing though the transition. Our results show for the first time that, by understanding the symmetry of the competing phases alone, one may achieve unprecedented chemical control of this unusual property.
层状钙钛矿 Ca3-xSrxMn2O7 表现出随 x 变化的从单轴负热膨胀材料到正热膨胀材料的转变。这种转变与两种具有不同对称性的密切竞争相有关。当固溶体最接近该相空间区域时，负热膨胀（NTE）效应最大化，但在通过转变时突然关闭。我们的结果首次表明，仅通过了解竞争相的对称性，就可以实现对这种不寻常性质的前所未有的化学控制。
