μSR studies of hydrogen-bonded ferroelectrics and antiferroelectrics

We have performed measurements on crystals of KH2PO4 (KDP, ferroelectric (FE) transition at TFE=123K), NH4H2PO4 (ADP, antiferroelectric (AFE) transition at TAFE=148K) and deuterated ADP (DADP, ND4D2PO4, antiferroelectric transition at TAFE=242K). Compounds of this family are covalently bonded molecular PO4 units linked by a network of hydrogen O-H-O bonds. At any temperature between 2 and 310 K, a substantial fraction of the muons implanted into the crystal appears as muonium. At low temperatures (below ∼140K), this fraction reaches about one half. The onset of the ferroelectric or antiferroelectic phase transition seems to differently affect the muon charge state. Longitudinal field decoupling measurements show only weak dynamics in the diamagnetic fraction with decoupling already at fields of about 1.5 mT. In the antiferroelectric ADP, the temperature dependence of the static Kubo-Toyabe width obtained from LF global fits exhibits a pronounced increase at the transition temperature.