133Cs nuclear magnetic resonance study of one-dimensional fluctuations in CsH2PO4 and its ferroelectric and antiferroelectric transitions at high pressure.

Abstract

Pressure and temperature effects on the one dimensional (1D) and higher-dimensionality correlations associated with the ferroelectric and antiferroelectric phase transitions in cesium dihydrogen phosphate were studied by means of the $^{133}\mathrm{Cs}$ nuclear magnetic resonance (NMR) spin-lattice relaxation time ${T}_{1}$. We measured ${T}_{1}$ at 6.5 MHz at temperatures down to the ferroelectric (FE) Curie point ${T}_{C}$ at 1 bar and at 1.5 and 3.0 kbar, down to the triple point ${T}_{t}$=124.6 K at 3.3 kbar, and down to the antiferroelectric (AFE) N\'eel point ${T}_{N}$ at 3.6 kbar. With decreasing temperature, ${T}_{1}$ first decreases exponentially due to 1D fluctuations associated with the ${J}_{b}$ interactions in disordered hydrogen-bonded chains running along b. As the temperature falls further, ${T}_{1}$ then decreases linearly as the ${J}_{c}$ interaction between these chains in hydrogen-bonded planes comes into play. From these results and the known pressure derivatives of ${T}_{C}$ and ${T}_{N}$, we calculated pressure dependences for ${J}_{b}$, ${J}_{c}$, and for the interplanar interaction ${J}_{a}$. At 3.3 kbar ${J}_{a}$ changes sign, so the plane stacking becomes AFE instead of FE. Above 8.9 kbar, where ${J}_{c}$ extrapolates to zero, a new AFE phase with a checkerboard arrangement of FE b chains is predicted.