AL-27 NMR OF THE INCOMMENSURATE PHASE IN ALPO4

The temperature dependence of the quadrupole perturbed Al-27 NMR spect ra has been measured in alpha, beta, and incommensurate phases of AlPO 4. The observed Al-27 NMR line shapes show that the incommensurate pha se is not a superposition of differently oriented single-k domains but rather corresponds to a planar triply-k modulated structure. The thre e modulation waves are correlated and the relative phase shifts are Ps i(1)+Psi(2)+Psi(3)=0. The amplitude of the AlO4 twist angle in the I p hase is estimated as 1.5 degrees-2 degrees.