Realization of Negative Thermal Expansion in Lead-Free Bi0.5K0.5VO3 by the Suppression of Tetragonality.

ABSTRACT

Bi1/2K1/2VO3 is a lead-free PbTiO3-type compound with a tetragonality (c/a = 1.054) comparable to that of typical ferroelectric PbTiO3 (c/a = 1.064) with negative thermal expansion (NTE) during the tetragonal-to-cubic phase transition; therefore, Bi1/2K1/2VO3 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 Bi1/2K1/2VO3 was conducted through substituting K+ with La3+ to suppress the tetragonality and achieve NTE. La substitution successfully suppressed the tetragonality of Bi1/2K1/2VO3 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 Bi0.5K0.46La0.04VO3 during the tetragonal-to-cubic phase transition (ΔV = -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.