Transient Nuclear-Magnetic-Resonance Study of Phase Transitions in Metallic Sodium Tungsten Bronzes

Measurements of line shape and spin-lattice relaxation of ${\mathrm{Na}}^{23}$ in ${\mathrm{Na}}_x\mathrm{W}{\mathrm{O}}_3$ have been performed by pulse techniques in the temperature range 100-700 °K for three samples with $x=0.517, 0.72, 0.855$. It has been observed that below a certain temperature the sodium nuclei experience a nonzero electric field gradient which cannot be ascribed to the presence of sodium vacancies. The temperature dependence of the ${\mathrm{Na}}^{23}$ quadrupole coupling constant, obtained from an elaboration of the free-precession data, shows that a phase transition from the hight-emperature ideal perovskite structure to a distorted structure occurs. The observation of an anomalous rise in the spin-lattice relaxation rate indicates that the phase transition is driven by microscopic critical dynamics. The transition temperatures $T_c$ depend upon the sodium content and were found to be ≃ 390 °K for $x=0.517, 400$ °K for $x=0.72, \mathrm{and} 425$°K for $x=0.855\mathrm{}$. The transition occurs in a continous way: From the experimental results it cannot be decided whether it is of second order, with a departure from the Landau behavior close to $T_c$, or of first order with a transition temperature close to the critical temperature.