Large negative linear compressibility of Ag ₃ [Co(CN) ₆ ]

Abstract

Silver(I) hexacyanocobaltate(III), Ag3[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.negative linear compression ͉ negative thermal expansion ͉ high-pressure ͉ framework materials ͉ flexibility N egative linear compressibility (NLC), whereby a material expands along a specific direction on increasing hydrostatic pressure, is a very unusual effect. Indeed in a study of elastic constant data from 500 noncubic crystal phases, only 13 displayed NLC, and of those, 11 structures were of monoclinic or lower symmetry (1). Despite its rarity, NLC is a highly attractive mechanical property, with a key application being the development of effectively incompressible optical materials (1, 2). Such materials could be used in high-pressure environments, such as in optical telecommunications devices that must function at deep-sea pressures Ͼ1,000 atm. NLC also offers a means of producing ultrasensitive pressure detectors, such as interferometric optical sensors for sonar and aircraft altitude measurements. The effect is also often coupled to so-called ''auxetic'' behavior, which is itself being used to improve shock resistance in, e.g., body armor (3).Of the few known examples among inorganic materials, the most pronounced NLC effects have been reported for LaNbO 4 (4), elemental Se (5), the BXO 4 (X ϭ P, As) family (6), and the spin-Peierls compound ␣Ј-NaV 2 O 5 (7). In some other cases, transient NLC behavior may occur only at pressures just above a strain-induced phase transition to a structure of lower symmetry (e.g., refs. 8 and 9), or as a result of an uptake of additional interstitial molecules (10, 11).One fundamental barrier to the practical application of NLC is that its magnitude is generally much smaller than the ''normal'' (positive) compressibilities of standard materials.* By convention, linear compressibility is defined as the relative rate at which a given dimension ᐉ decreases with pressure (at constant temperature): K ᐉ ϭ Ϫ(Ѩlnᐉ/Ѩp) T . Typical values for crystalline materials lie in the range K ᐉ Ӎ 5-20 TPa Ϫ1 (12) (i.e., their linear dimensions decrease by Ϸ1% for each GPa increase in pressure). In cont...

Uniaxial negative thermal expansion and metallophilicity in Cu3[Co(CN)6]

“…This is probably reasonable given that both terms will be governed by the chemistry of the [Co(CN) ] [34] for A = H, from Ref. [17] for A = Ag, and from our present study for A = Cu.…”

“…Compounds with large thermal expansion coefficients often show extreme and counterintuitive responses to pressure [16,17], and may harbour various other anomalous elastic properties, such as negative Poisson's ratios [18] or thermosalient effects [19,20]. So it is perhaps unsurprising that some of the most extreme ('colossal') thermal expansion known has been observed in framework materials whose lattice dimensions are a function of weak metallophilic interactions [14,17,21].…”

“…1. The same mechanism operates under application of hydrostatic pressure, such that volume compression actually results in linear expansion for a particular set of directions [17]-so-called negative linear compressibility (NLC) [25][26][27]. NTE and NLC are valuable material properties, exploitable in the design of a thermal composites used in optical devices and next-generation pressure sensors.…”

“…Compounds with large thermal expansion coefficients often show extreme and counterintuitive responses to pressure [16,17], and may harbour various other anomalous elastic properties, such as negative Poisson's ratios [18] or thermosalient effects [19,20]. So it is perhaps unsurprising that some of the most extreme ('colossal') thermal expansion known has been observed in framework materials whose lattice dimensions are a function of weak metallophilic interactions [14,17,21]. The canonical system of this type is Ag 3 [Co(CN) 6 ], which adopts a lattice structure [22] that can flex in such a way as to vary argentophilic Ag + …Ag + separations without affecting covalent interactions within the lattice itself [23,24].…”

Negative linear compressibility in common materials

“…The recent discovery of materials and structures [2][3][4][5][6][7][8] displaying NLC suggest that this property is not as rare as previously considered and may lead to interesting developments. The main aim of this study is to re-examine previously published elastic properties and to identify NLC in common materials.…”