Zero Thermal Expansion in a Flexible, Stable Framework: Tetramethylammonium Copper(I) Zinc(II) Cyanide

摘要

Materials that exhibit zero thermal expansion (ZTE) find usesnin applications where precise alignment must be maintained, andnthermal shock avoided, over a range of temperatures.1 AlthoughnZTE can be achieved by composites of ordinary compounds whichnexpand on heating and those which exhibit negative thermalnexpansion (NTE), this approach leads to strain within the ZTEnmaterial, and it is therefore preferable to engineer ZTE behavior atnthe atomic level within a single material. Compounds known tonexhibit near-ZTE over limited temperature ranges include Ag2Onand ReO3.2,3 Of the variety of compounds known for their NTEnbehavior, the family of metal cyanides is of particular interest innengineering new ZTE materials, since these compounds’ expansivitiesncan be readily tuned by varying the composition of thenframeworks4,5 and/or guest molecules within them.6–8 NTE in thesencompounds has been attributed to low-frequency vibrational modesninvolving transverse motion of the cyanide bridges, producing annet contraction of the lattice. Thermal expansion in these materialsncan be tuned by modifying either the framework composition, andnthus the flexibility afforded at each metal center,4 or the guestnoccupancy, and thus the steric hindrance to the transverse CNnmodes.7 The Cd(CN)2 · CCl4 clathrate, which contains CCl4 guestsnwithin a single cristobalite Cd(CN)2 framework, appears promisingnsince its coefficient of thermal expansion R ) da/adT varies substantiallynwith guest occupancy.8 Unfortunately, this compound isnimpractical for ZTE applications because the readiness with whichnthe CCl4 guests desorb makes it difficult to maintain a constant Rnvalue.