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 degrees C and over a broad composition range, 10(-4) less than or equal to x less than or equal to 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 (Delta B-PP = 0.2 mT) stay constant up to x similar to 0.05. Above this composition a clear increase of Delta B-PP is observed indicating the NM-M transition. The spin susceptibility kappa(s) 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 similar to 0.05 kappa(s) rises steeply with x. The overall variation of kappa(s) 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 less than or equal to 0.05, and a simple band model describing the strong increase of kappa(s) approaching the NM-M transition. Both approaches yield a satisfactory description of the observed variation of kappa(s). Further support of this interpretation is found in the spin dynamics which are qualitatively discussed. (C) 1996 American Institute of Physics.