NMR Proton Second Moments in KH2PO4-type Crystals

Abstract
Calculated second moments of the NMR proton lines for different cases of dynamic proton modes and static ordering in KH2PO4-type crystals show differences that are too small to be experimentally observable. It is shown, however, that a previously proposed disordered rotation of the H2PO4 group cannot account for the proton linewidth narrowing at the high-temperature (180°C) transition in KH2PO4. An incidental result of this investigation is the 14N nuclear quadrupole coupling constant in NH4H2AsO4, of 30.7 kHz.