Electron localization and metal–nonmetal transition in fluid KxKCl1−x: An electron spin resonance study of the magnetic properties with in situ variation of x (10−4⩽x⩽10−1)

We have measured the electron spin resonance (ESR) spectra of saltrich KxKCl1−x melts at temperatures near 800 °C and over a broad composition range, 10−4≤x≤10−1, approaching the metal–nonmetal (M–NM) transition in these solutions. Emphasis has been given to a precise in situ variation of composition which has been achieved for the first time in a high temperature ESR experiment by Coulometric titration. The spectra are characterized by a motional linewidth narrowing with a Lorentzian line shape. The ESR characteristics as determined from the Lorentz fits of the spectra exhibit the following features as a function of x: In the nonmetallic regime the g factor (g=1.9938±0.0002) and the peak-to-peak halfwidth (ΔBPP=0.2 mT) stay constant up to x∼0.05. Above this composition a clear increase of ΔBPP is observed indicating the NM–M transition. The spin susceptibility ϰs has been determined from the imaginary part of the Lorentz fits and has been calibrated against a sapphire signal measured simultaneously with the KxKCl1−x liquid samples. The spin susceptibility strongly deviates from simple Curie behavior even at low x which gives evidence for spin paired states in the nonmetallic solutions. Above x∼0.05 ϰs rises steeply with x. The overall variation of ϰs with x is interpreted by two limiting models: a thermodynamic defect model focusing on the particle character of localized paramagnetic and diamagnetic states for x≤0.05, and a simple band model describing the strong increase of ϰs approaching the NM–M transition. Both approaches yield a satisfactory description of the observed variation of ϰs. Further support of this interpretation is found in the spin dynamics which are qualitatively discussed.