Ferroelectric order versus metallicity in Sr1-xCaxTiO3-δ (x=0.009)

We report on a thermal-expansion study of the ferroelectric phase transition in insulating Sr1-xCaxTiO3 (x=0.009) and its evolution upon increasing charge-carrier concentration up to n≃60×1019cm-3. Although electric polarization is screened by mobile charge carriers, we find clear signatures of the ferroelectric phase transition in the thermal-expansion coefficient α of the weakly doped metallic samples. Upon increasing n, the transition temperature TC(n) and the magnitude of the anomalies in α rapidly decrease up to a threshold carrier density n above which broadened anomalies remain present. There is no indication for a sign change of α as is expected for a pressure-dependent quantum phase transition with n as the control parameter. Thus, the ferroelectriclike transition is either continuously fading away or it transforms to another low-temperature phase above n, but this change hardly affects the temperature-dependent α(T) data.