We show that a synergy of the covalency effect associated with the $A$-site and $B$-site cations in ${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ (KBT) makes it unique among the Pb-free ferroelectrics by imparting a rare property of negative and zero thermal expansion (ZTE). We tuned this synergy in two different KBT alloys, ${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3-{\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ and ${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3-\mathrm{Bi}\left({\mathrm{Mg}}_{0.5}{\mathrm{Ti}}_{0.5}\right){\mathrm{O}}_3$, and identified a common critical tetragonality below which the ZTE behavior disappears completely in both the systems.

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• Ferroelectricity
• Ferroelectrics
• Perovskites
• Crystal structures
• Neutron diffraction
• Raman spectroscopy
• X-ray diffraction