2H nuclear quadrupole resonance study of the Ubbelohde effect in K(H1-xDx),PO4

Temperature dependence of the average H-2 nuclear quadrupole resonance frequency (v) over bar,(v) over bar= (v(+)+v(-))/2, has been measured in K(H1-xDx)(2)PO4 (x = 0.20, 0.35, and 0.53) by a double-resonance technique. The H-2 NQR data indirectly show that in K(H1-xDx)(2)PO4 the length of the O-D ... O hydrogen bonds and presumably also the length of the O-H ... O hydrogen bonds increases linearly with increasing deuterium concentration. These data together with the linear dependence of T-c on deuterium concentration suggest that the length of the hydrogen bonds plays a major role in determining the phase-transition temperature in KH2PO4-type systems.