27 Al NMR of the incommensurate phase in AlPO 4 s

The temperature dependence of the quadrupole perturbed $^{27}$ Al NMR spectra has been measured in \ensuremath{\alpha}, \ensuremath{\mathrm{B}}, and incommensurate phases of ${\mathrm{AlPO}}_{4}$ . The observed $^{27}$ Al NMR line shapes show that the incommensurate phase is not a superposition of differently oriented single-k domains but rather corresponds to a planar triply-k modulated structure. The three modulation waves are correlated and the relative phase shifts are ${\mathrm{\ensuremath{\Psi}}}_{1}$ +${\mathrm{\ensuremath{\Psi}}}_{2}$ +${\mathrm{\ensuremath{\Psi}}}_{3}$ =0. The amplitude of the ${\mathrm{AlO}}_{4}$ twist angle in the I phase is estimated as 1.5\ifmmode^\circ\else\textdegree\fi{}\char21{}2\ifmmode^\circ\else\textdegree\fi{}.