Abstract
Measurement of the dielectric constant along the polar axis for tetragonal and monoclinic Rb(H1-x D x )2PO4 crystals was carried out under the hydrostatic pressure using by an LCR mater. The values of the pressure derivative of the transition temperature, dT C/dp, hardly depend on the deuterium concentration for the monoclinic Rb(H1-x D x )2PO4 crystal. In the tetragonal crystals, the values of dT C/dp are −87 K/GPa for the tetragonal RbH2PO4 crystal and −38 K/GPa for the crystal with x = 0.74. The absolute value of dT C/dp for tetragonal RbH2PO4 decreases extremely on deuteration. The deuteration effect on dT C/dp in tetragonal RbH2PO4 crystals is large, though it is hardly observed in monoclinic RbH2PO4 crystals.