H-1-AS-75 NUCLEAR-QUADRUPOLE DOUBLE-RESONANCE STUDY OF THE HIGH-TEMPERATURE PHASE-TRANSITION IN TLH2ASO4

The As-75 nuclear-quadrupole-resonance frequency has been measured around the high-temperature phase transition in TlH2AsO4 with a proton-arsenic cross-relaxation technique. The experimental results are analyzed within an order-disorder model where above the high-temperature phase transition each AsO4 group forms four dynamically disordered symmetric double-minimum-type O-H ... O hydrogen bonds, whereas below the high-temperature phase transition two of the four hydrogen bonds become asymmetric and ordered. The temperature dependence of the order parameter associated with the high-temperature phase transition is calculated from the nuclear-quadrupole-resonance data.