Direct Observation of a Transverse Vibrational Mechanism for Negative Thermal Expansion in Zn(CN)_2: An Atomic Pair Distribution Function Analysis

摘要

The instantaneous structure of the cyanide-bridged negative thermal expansion (NTE) material Zn(CN)_2 has been probed using atomic pair distribution function (PDF) analysis of high energy X-ray scattering data (100-400 K).The temperature dependence of the atomic separations extracted from the PDFs indicates an increase of the average transverse displacement of the cyanide bridge from the line connecting the Zn~(II) centers with increasing temperature.This allows the contraction of non-nearest-neighbor Zn...Zn' and Zn...C/N distances despite the observed expansion of the individual direct Zn-C/N and C-N bonds.Thus,this analysis provides definitive structural confirmation that an increase in the average displacement of bridging atoms is the origin of the NTE behavior.The lattice parameters reveal a slight reduction in the NTE behavior at high temperature from a minimum coefficient of thermal expansion (alpha=dl/ldT) of -19.8 x 10~(-6) K~(-1) below 180 K,which is attributed to interaction between the doubly interpenetrated frameworks that comprise the structure.