ENDOR and pulsed ESR study of proton glass behavior in the mixed crystal (betaine phosphate)(0.15)(betaine phosphite)(0.85)

The proton ENDOR spectra of BP0.15BPI0.85 taken between 90 and 290 K allow a direct determination of the local polarization distribution W(p) of the protons in the hydrogen bonds linking the phosphate and phosphite groups to chains. The temperature dependence of the Edwards-Anderson glass order parameter can be described by a random-field dominated proton glass behavior in BP0.15BPI0.85 with a glass temperature of T-G=30 K and a random-field induced freezing temperature of T-f=(95+/-5) K. The experimental results do not permit one to discriminate between three-dimensional (3D) or quasi-one-dimensional interactions of the hydrogen bonds. The experimentally determined polarization distribution function W(p) can be described to a certain extent within the 3D random-bond random-field model if a long-range order of the protons is introduced by a non-zero mean (J) over tilde 0/k(B)=160 K of the random-bond interaction. Additionally, the local field variance <(Delta)over tilde> has to be taken as a temperature-dependent parameter with a minimal value at the resulting phase transition at 144 K. However, even at lowest temperatures the maxima of the order parameter distribution function W(p) appear at values of the local proton polarization distinctly smaller than one. This is not describable within the above models. We conclude that proton tunneling is of importance in betaine phosphite-phosphate proton glasses.