Study of the23Na EFG tensor as determined from first order NMR satellite lines near the ferroelectric phase transition of AgNa(NO2)2

The²³Na NMR first order quadrupolar satellite lines are detected and studied in AgNa(NO₂)₂ single crystals near the paraelectric-ferroelectric phase transition. Although the intensities of the satellites are small the components of the electric field gradient tensor (EFG) at the²³Na sites could be deduced from the angular dependence of the first order quadrupolar line splitting with respect to rotations around the three crystallographic axes by applying the Volkoff method. In the paraelectric phase the principal axes system of the EFG coincides with the crystallographic axes system whereas in the ferroelectric phase there is a strongly temperature dependent small non diagonal element _xz (T). The EFG principal components _xx and _yy are strongly influenced whereas the EFG principal component _zz is nearly not affected by the ferroelectric phase transition. The observed temperature dependences of the EFG components are related to the temperature variation of the normalized spontaneous polarizationS(T) by assuming a coupling term which is quadratic inS(T). Finally the problem of the intensities of the satellite lines is discussed.