Metal$ndash$insulator and magnetic transitions in heavily irradiated NaCl$ndash$KBF4

Abstract

We have performed electron spin resonance (ESR), nuclear magnetic resonance (NMR) and static magnetic susceptibility measurements on heavily irradiated NaCl-KBF 4 single crystals in the temperature range 4.2 < T < 350 K. In these samples, up to about 10% of the NaCl molecules are transformed into extremely small metallic Na particles and Cl 2 precipitates. At high temperatures a oneline ESR signal, i.e. common mode due to strong exchange interaction between conduction electrons and F-aggregate centres, is observed. We propose that the smooth decrease of the ESR spin susceptibility with decreasing temperature, which can be as large as 50%, is due to a metal-insulator transition, taking place at about 40 K. In the same temperature range, the linewidth increases by 18±2 G with decreasing temperature. This anomalous broadening is explained by a reduction of the exchange narrowing at low temperatures. NMR spinlattice relaxation on 23 Na shows a Korringa-type behaviour down to 10 K, which suggests that the conducting phase in heavily irradiated NaCl-KBF 4 behaves as a three-dimensional metal. SQUID experiments have revealed antiferromagnetic ordering at 40 K and a ferromagnetic phase below 20 K. The nature of the observed effects is discussed.

Characterization of the size and orientation of Na and Cl2nanocrystals in electron irradiated NaCl crystals by means of synchrotron radiation

“…Extension of the experiments to low temperatures (between 4 and 300 K) leads to a far more delicate picture. It appears that the system of conduction electrons undergo a metal-insulator phase transition [27]. At high temperature the precipitates behave more or less as reasonably large metallic objects, consisting of many individual nano-particles with sizes of only a few nanometres in diameter, because the conduction electrons are delocalized.…”

Characterization of ultra‐heavily damaged NaCl

“…The results for Na-precipitates suggest that the defect structure consists of a dense network of quasi-1D coupled nano-particles, which in some cases shows a Metal-Insulator (M.I.) transition at low temperature [1][2][3][4]. Raman scattering experiments have provided evidence for the existence of narrow quasi-1D geometrical structures of sodium nano-particles with a width of a few nm [5].…”

Radiolysis of NaCl at high and low temperatures: development of size distribution of bubbles and colloids

“…Another effect related to the chemical nature of point defects in NaCl is the formation of Na colloid nano-structures with one-dimensional (1D) magnetic behaviour [13][14][15][16][17]. Direct AFM measurements have revealed ring-and chain-like clusters (or agglomerates) of Na colloid particles which are situated very close to each other [18].…”