M. Witschas et H. Eckert, P-31 and Na-23 solid-state NMR studies of cation dynamics in HT-sodium orthophosphate and the solid solutions (Na2SO4)(x)-(Na3PO4)(1-x), J PHYS CH A, 103(50), 1999, pp. 10764-10775
The high-temperature phases of sodium orthophosphate, HT-Na3PO4, and of the
solid solutions (Na2SO4)(x)-(Na3PO4)(1-x) are characterized by their plast
ic crystalline state with dynamically disordered PO43- and SO42- anions and
a remarkably high cation conductivity. Since HT-Na3PO4 possesses a fully o
ccupied cation sublattice (no vacancies), it has been proposed that cation
transport and anion reorientations are dynamically coupled ("paddle-wheel m
echanism"). However, no direct evidence for this coupling has been reported
. In the present study, the validity of this mechanism is investigated on t
he basis of Na-23 and P-31 nuclear magnetic resonance (NMR) experiments. Te
mperature-dependent measurements of the static P-31 linewidth indicate that
in the solid solutions with 0.04 less than or equal to x less than or equa
l to 0.25 the acceleration of sodium ionic mobility is closely correlated w
ith the acceleration of phosphate rotational motion, associated with a seco
nd-order phase transition near 400 K. Temperature-dependent measurements of
the 23Na longitudinal and transverse relaxation times have been analyzed u
sing the theory of quadrupolar relaxation under nonextreme narrowing condit
ions. Consistent with theoretical predictions sizeable dynamic frequency sh
ifts are detected. All of the data are consistently analyzed quantitatively
in terms of two distinct motional processes. A low-temperature process, wh
ose relaxation strength is independent of sample composition, is clearly ac
celerated by the onset of fast anion rotation occurring at the second-order
phase transition temperature. In addition, a high-temperature process, whi
ch is almost absent in HT-Na3PO4 but whose importance increases with increa
sing sulfate content, signifies vacancy hopping. This dependence on composi
tion is easily understood because the substitution of PO43- by SO42- genera
tes cation vacancies. The activation energies of both processes are near 0.
45 eV, and the corresponding timescales grow increasingly similar with incr
easing sodium sulfate content. Altogether, the results give strong evidence
for a dynamic coupling between anionic reorientation and cation diffusion,
supporting the concept of a paddle-wheel mechanism.