Nuclear-quadrupole double-resonance study of RbH2PO4 in the supercooled high-temperature monoclinic phase.

Abstract

Rubidium nuclear-quadrupole-resonance (NQR) spectra have been measured in the high-temperature monoclinic phase of ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ above ${\mathit{T}}_{\mathit{p}}$=86 \ifmmode^\circ\else\textdegree\fi{}C and also in the supercooled high-temperature phase below ${\mathit{T}}_{\mathit{p}}$. At the transition into the high-temperature phase, the rubidium quadrupole-coupling constants drop by a factor of 5. The high-temperature phase is metastable below ${\mathit{T}}_{\mathit{p}}$. The kinetics of the transformation of the metastable phase into the stable tetragonal phase in a polycrystalline sample is determined at room temperature from the time dependence of the rubidium NQR spectra. A comparison of the present rubidium NQR data in the high-temperature monoclinic ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ with the rubidium NQR data in monoclinic ${\mathrm{RbD}}_{2}$${\mathrm{PO}}_{4}$ suggests that at low temperatures, supercooled monoclinic ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ may be isostructural with the phase III of monoclinic ${\mathrm{RbD}}_{2}$${\mathrm{PO}}_{4}$ whereas above ${\mathit{T}}_{\mathit{p}}$ ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ may be isostructural with phase II of ${\mathrm{RbD}}_{2}$${\mathrm{PO}}_{4}$.