Why is it that ABO3 perovskites generally do not exhibit negative thermal expansion(NTE)over a wide temperature range,whereas layered perovskites of the same chemical family often do?It is generally accepted that ther...Why is it that ABO3 perovskites generally do not exhibit negative thermal expansion(NTE)over a wide temperature range,whereas layered perovskites of the same chemical family often do?It is generally accepted that there are two key ingredients that determine the extent of NTE:the presence of soft phonon modes that drive contraction(have negative Grüneisen parameters);and anisotropic elastic compliance that predisposes the material to the deformations required for NTE along a specific axis.This difference in thermal expansion properties is surprising since both ABO3 and layered perovskites often possess these ingredients in equal measure in their high-symmetry phases.Using first principles calculations and symmetry analysis,we show that in layered perovskites there is a significant enhancement of elastic anisotropy due to symmetry breaking that results from the combined effect of layering and condensed rotations of oxygen octahedra.This feature,unique to layered perovskites of certain symmetry,is what allows uniaxial NTE to persist over a large temperature range.This fundamental insight means that symmetry and the elastic tensor can be used as descriptors in high-throughput screening and to direct materials design.展开更多
An understanding of the roles and representations of women in classical Chinese philosophy is here derived from central texts such as the Analects,the Lienii Zhuan,and the I Ching.We argue that the roles of women duri...An understanding of the roles and representations of women in classical Chinese philosophy is here derived from central texts such as the Analects,the Lienii Zhuan,and the I Ching.We argue that the roles of women during the classical period of Chinese philosophy tended to be as part of the "inner," working domestically as a housewife and mother.This will be shown from three passages from the Analects.Women were represented as submissive and passive,as with the qualities ascribed to yin energy,and therefore as rightfully subordinate to men.However,despite representations of women in philosophy being thus at this time,there were exceptions,specific women who could take a male "outer" political role.The story of Jing Jiang from the Lienii Zhuan suggests that although women being involved in "outer" affairs were looked down on,there were still women who would be and who would occasionally get praised for doing so.This shows that it was realized,explicitly or otherwise,that women were capable of taking those roles,but also that they were not allowed to take such roles at that time.展开更多
基金supported by an Imperial College Research Fellowship and a Royal Society Research Grantthe John Fell Fund,Oxford(DPD09750)for funding to support this project+2 种基金supported by a studentship in the Centre for Doctoral Training on Theory and Simulation of Materials at Imperial College London funded by the EPSRC(EP/L015579/1)supported by the Thomas Young Centre under grant TYC-101partially funded by EPSRC(EP/P020194/1).
文摘Why is it that ABO3 perovskites generally do not exhibit negative thermal expansion(NTE)over a wide temperature range,whereas layered perovskites of the same chemical family often do?It is generally accepted that there are two key ingredients that determine the extent of NTE:the presence of soft phonon modes that drive contraction(have negative Grüneisen parameters);and anisotropic elastic compliance that predisposes the material to the deformations required for NTE along a specific axis.This difference in thermal expansion properties is surprising since both ABO3 and layered perovskites often possess these ingredients in equal measure in their high-symmetry phases.Using first principles calculations and symmetry analysis,we show that in layered perovskites there is a significant enhancement of elastic anisotropy due to symmetry breaking that results from the combined effect of layering and condensed rotations of oxygen octahedra.This feature,unique to layered perovskites of certain symmetry,is what allows uniaxial NTE to persist over a large temperature range.This fundamental insight means that symmetry and the elastic tensor can be used as descriptors in high-throughput screening and to direct materials design.
文摘An understanding of the roles and representations of women in classical Chinese philosophy is here derived from central texts such as the Analects,the Lienii Zhuan,and the I Ching.We argue that the roles of women during the classical period of Chinese philosophy tended to be as part of the "inner," working domestically as a housewife and mother.This will be shown from three passages from the Analects.Women were represented as submissive and passive,as with the qualities ascribed to yin energy,and therefore as rightfully subordinate to men.However,despite representations of women in philosophy being thus at this time,there were exceptions,specific women who could take a male "outer" political role.The story of Jing Jiang from the Lienii Zhuan suggests that although women being involved in "outer" affairs were looked down on,there were still women who would be and who would occasionally get praised for doing so.This shows that it was realized,explicitly or otherwise,that women were capable of taking those roles,but also that they were not allowed to take such roles at that time.
