Systematic study of electron localization in saltrich M-x(MX)(1-x) melts (M=Na, K, Cs; X=Cl, I) by optical absorption spectroscopy

We report optical absorption spectra of K-x(KI)(1-x), Cs-x(CsI)(1-x), and C s-x(CsCl)(1-x) solutions at temperatures around 800 degrees C and in the sa ltrich concentration range up to metal mole fractions of x(M)less than or e qual to 0.04 for energies 0.5 less than or equal to (h) over bar omega less than or equal to 5 eV. Employing a high-temperature electrochemical cell, we were able to change and determine the alkalimetal activity and the alkal imetal mole fraction in situ simultaneously with the optical absorption spe ctra. The high quality of the absorption spectra allows to distinguish spec tral contributions to localized electronic states and mobile electrons. Tog ether with previously measured spectra of Na-x(NaI)(1-x) melts a systematic investigation of the alkali iodide melts reveals differences in the nature of the strongly localized electronic states with varying cation. Interpret ation of the spectroscopic results with the aid of a chemical defect model shows that the formation of localized dimeric electron states (bipolarons) is pronounced in melts of smaller cations. The analysis of the optical cont ribution due to mobile electrons has been performed with a simple Drude mod el for nearly free electrons and the results are in good agreement with ind ependent measurements of electronic transport properties. (C) 1999 American Institute of Physics. [S0021-9606(99)70405-8].