Sr_1-xCa_xTiO_3: An XY Quantum Ferroelectric with Transition to Randomness

A series of twenty homogeneous Sr 1- x Ca x TiO 3 mixed crystals has been measured dielectrically between 4.2 and 300 K. In the tetragonal phase; the dielectric constant perpendicular to the c axis becomes peaked above x c =0.0018; the quantum mechanical onset for displacive ferroelectricity. The polarization ⥠c can be switched between the two equivalent a axes; i.e.; the system is an XY ; n =2; quantum ferroelectric. Above x r =0.016±0.002; the ε( T ) peaks round in a distinct manner which we attribute to the onset of a random-field-induced domain state.