Large negative linear compressibility of Ag3[Co(CN)6]

Abstract

Silver(I) hexacyanocobaltate(III), Ag(3)[Co(CN)(6)], shows a large negative linear compressibility (NLC, linear expansion under hydrostatic pressure) at ambient temperature at all pressures up to our experimental limit of 7.65(2) GPa. This behavior is qualitatively unaffected by a transition at 0.19 GPa to a new phase Ag(3)[Co(CN)(6)]-II, whose structure is reported here. The high-pressure phase also shows anisotropic thermal expansion with large uniaxial negative thermal expansion (NTE, expansion on cooling). In both phases, the NLC/NTE effect arises as the rapid compression/contraction of layers of silver atoms--weakly bound via argentophilic interactions--is translated via flexing of the covalent network lattice into an expansion along a perpendicular direction. It is proposed that framework materials that contract along a specific direction on heating while expanding macroscopically will, in general, also expand along the same direction under hydrostatic pressure while contracting macroscopically.

Fig. 1.

Compression mechanism in Ag₃[Co(CN)₆], exaggerated for illustrative purposes: In order for the framework to reduce its volume (either in response to a decrease in temperature or an increase in pressure), it must expand along the trigonal axis (c).

Fig. 2.

Pressure dependence of various structural parameters: lattice parameters (A), linear compressibilities extracted from parameterized fits to the lattice parameter data (B), and volume per formula unit V/Z (C). The Inset in C shows linear (Phase I) and third-order Birch–Murnaghan (Phase II) fits to the V/Z data near the phase transition.

Fig. 3.

Representations of the pressure-induced structural transition in Ag₃[Co(CN)₆]. (A) In both the ambient trigonal phase I (Left; shown with the outline of its monoclinic supercell for ease of comparison) and high-pressure monoclinic phase II (Center and Right), [Co(CN)₆] octahedra (shown in polyhedral representation) are connected via Ag atoms (large spheres). (B) The framework topology differs between phases because the shear parallel to a_I/a_II is coupled to a shift of 2/3 of the Ag atoms by 1/4 of a unit cell in the same direction. This transforms the argentophilic Kagome layers of phase I into a reentrant honeycomb (“bow-tie”) arrangement in phase II. (C) Subsequent compression increases the extent of “puckering” in this 2-dimensional lattice, approaching a triangular network.

Fig. 4.

High pressure thermal expansion and Grüneisen isotropy in Ag₃[Co(CN)₆]. (A) Temperature dependence of phase II lattice parameters measured at p = 0.395(10) GPa. (B) Uniaxial compression in layered materials is strongly direction dependent (Upper), whereas that in connected frameworks produces similar internal strains (here, compression of the same framework “struts”) for very different applied directions (Lower).