Order-disorder mixed phase in Tl H2 P O4

An intermediate phase of partial order, associated with antiferroelectricity, is elucidated in hydrogen-bonded Tl H2 P O4 undergoing separated antiferroelectric and ferroelastic phase transitions. Our H1 and P31 nuclear magnetic resonance linewidth measurements revealed marked changes at an intermediate phase boundary, involving an intermediate phase of mixed order of coexistence of paraelectricity and antiferroelectricity, which was identified by means of impedance spectroscopy. A coupling of the proton and the P O4 tetrahedral motions to the proton ordering in the hydrogen bonds is also manifested.