Abstract
The relation between optical second harmonic generation (SHG) and ferroelectric order is discussed in the case of NaNO2. The nonlinear susceptibility χNL determining SHG in the ferroelectric state shows the decrease of long range order of the NO 2 -sublattice in approaching the Curie point. In the disordered paraelectric state the third-rank tensor χNL vanishes and SHG has to be described by a tensor χN of fourth rank, χN is interpreted as a measure of short range order. Its temperature dependence reveals an anomaly at 207°C, also shown by the thermal expansion coefficients.

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