EXAFS study of orientational glasses

The extended X-ray absorption fine structure (EXAFS) study of K1-x(NH4)(x)I and KBr1-x(NO2)(x) mixed crystals was performed to elucidate the local str ucture of their glassy state associated with the reorientational freezing o f molecular ions without any long-range order. The magnitude of EXAFS spect ra of K1-x(NH4)(x)I was reduced with increasing ammonium ion concentration, while that of KBr1-x(NO2)(x) exhibits no anomalous changes with increasing nitrite ion concentration even below the grass transition temperature T-g. The results indicate that the local distortion which breaks the symmetry o f fee lattice is induced by the short-range antiferroelectric order of ammo nium ions in K1-x(NH4)(x) I and the local structure is retained by the shor t-range ferroelectric order of nitrite ions in KBr1-x(NO2)(x). The temperat ure dependence of mean-square displacement of bromine-potassium pair-distri bution in KBr1-x(NO2)x obeys the Debye approximation even below T-g, which conflicts the result of previous X-ray diffraction studies that the mean-sq uare displacement deviates largely from the Debye approximation.