Realization of Negative Thermal Expansion in Lead-Free BiKVO by the Suppression of Tetragonality.

BiKVO is a lead-free PbTiO-type compound with a tetragonality (/ = 1.054) comparable to that of typical ferroelectric PbTiO (/ = 1.064) with negative thermal expansion (NTE) during the tetragonal-to-cubic phase transition; therefore, BiKVO is a potential lead-free NTE material if its metastable perovskite structure can be maintained at high temperatures. In the present experiment, electron doping in BiKVO was conducted through substituting K with La to suppress the tetragonality and achieve NTE. La substitution successfully suppressed the tetragonality of BiKVO and also improved its thermal stability. Moreover, both composition- and temperature-induced tetragonal-to-cubic phase transitions occurred. In particular, a large volume shrinkage with a large negative thermal coefficient of expansion (CTE) was obtained for BiKLaVO during the tetragonal-to-cubic phase transition (Δ = -0.66%). Hence, this study extends the NTE family and also sheds light on the exploration of lead-free piezoelectric materials with controllable thermal expansion.