“Unconventional covalent” KAgF₃ is metallic above 50 K

Abstract

. 30.+h, 72.80.Ga, 74.10.+v Using microwave cavity perturbation and magnetic susceptibility measurements we demonstrate for the first time that for KAgF 3 the antiferromagnet-paramagnet transition leads to metallic state above 50 K.Are fluorides always prototypical ionic insulators? Is it possible that electrons hop between metal centers via deep-lying F(2p) orbitals [1], thus leading to metallic conductivity, in analogy to many conducting oxides? Could the strongly-bound d electrons of Ag(II), spatially localized around nuclei, be set free to move at a moderate temperature? Or, in yet another words, how deep in the absolute energy scale could the Fermi level of a metallic substance could lye? In order to answer these fundamental questions we are currently exploring the challenging chemistry of high oxidation states in multinary silver fluorides, at the very limits of the positive redox potentials.Over sixty fluorides of Ag(II) (fluoroargentates) are known, among which only seven, representing the MAgF 3 and M 2 AgF 4 families (M = Cs, Rb, K) and the binary AgF 2 , have extended pseudo-2D structures (see Fig. 1) [2].The electrical properties of these substances are of particular interest because fluoroargentates are isoelectronic and sometimes also isostructural (K 2 AgF 4 ≈ La 2 CuO 4 ) with the oxides of Cu(II); the latter become high temperature superconductors under proper doping. DFT computations [3] and ultra-high resolution XPS spectroscopical results [4] indicate strong mixing of the Ag(4d) and F(2p) levels, which may lead to metallic [5], and possibly even superconducting behaviour. However, measuring the electrical conductivity of strongly oxidizing and highly air-and moisture-sensitive microcrystalline powders, such as fluoroargentates, presents a formidable experimental challenge. And the scarce attempts of the classical contact dc resistivity measurements have failed for these compounds [6]. Prompted by similar problems, we have applied here the nonintrusive microwave cavity perturbation (MWCP) technique to measure the ac electrical conductivity of powder samples without the need for sample contacts [7]. At high frequencies (here 3.2 GHz), the effects of powder intergrain contacts are negligible, and the microwave method thus probes directly the intragrain conductivity. The details of the equipment used and of the quantitative analysis of the electric conductivity are described elsewhere [7].The variation of the resonance microwave frequency, ν, and of the resonance band width, ∆ν ½ , [8] for powder samples of KAgF 3 and AgF 2 (referenced to an empty glass tube), are shown in Fig. 2.

Origin of ferromagnetism inCs2AgF4: The importance ofAg−Fcovalency

“…For example, KAgF 3 shows a metal insulator transition coincident with an antiferromagnetic ordering. 4 This is much more natural in an itinerant system than in a strongly correlated local moment system, which would tend to be insulating on both sides of the ordering temperature at half integer band fillings. The structure of that compound shows compressed octahedra and Ag-F-Ag chains along the c-axis direction with short bond lengths.…”

“…While transport measurements have not been reported for these compounds, it is known that the related distorted perovskite compound KAgF 3 is metallic at high temperatures, and then has a metal insulator transition coincident with an antiferromagnetic ordering temperature. 4 In the doped high-T c cuprates, superconductivity develops from a paramagnetic metallic phase, with Fermi surfaces coming from hybridized Cu d -O p bands. These are formally antibonding bands of d x 2 −y 2p σ character.…”

How do electrons travel in unusual metallic fluorides of Ag2+?

“…Any metallic electrode (including those made of pure gold), becomes fluorinated on the surface attached to fluoroargentates, thus preventing reliable ac conductivity measurements. However, it has recently been shown by the (non-contact) microwave cavity resonance technique that KAgF 3 (which is an antiferromagnetic insulator below 70 K) becomes metallic above its magnetic ordering (Néel) temperature [3]. Above this temperature it is also a Pauli paramagnet, its free electron character being confirmed by temperature-independent magnetic susceptibility [3,4].…”

Plasticity of the coordination sphere of Ag2+

“…Naturally, the idea arises to attempt to generate metallic conductivity or even superconductivity in strongly correlated fluoroargentates [1]. Indeed, metallic conductivity of KAgF 3 [3] and possible Meissner-Ochsenfeld anomalies have been reported for the poorly defined minority phases in the Be-Ag-F system [4].…”