Slow ferroelectric cluster dynamics in KH2aso4 and KH2po4 from Cr5+ paramagnetic resonance

We succeeded in substituting the paramagnetic Cr5+ (S=12) ion with the same size as As5+ in KH2AsO4, for P5+ in KH2PO4, and in their deuterated isomorphs. In the crystals the high-temperature resonance spectrum showing a local tetragonal symmetry is broken on the time scale of the EPR experiment below T*>TC, so that orthorhombic spectra appear in the paraelectric tetragonal phase. T* shifts proportionally to the Curie temperature TC upon deuteration. We deduce the existence of ferroelectric clusters with lifetimes τ longer than 1.5×10-8 sec below T*. © 1976 The American Physical Society.