• Characterization of high temperature piezoelectric crystals with an ordered langasite structure

Piezoelectric single crystals with the ordered langasite structure $A_3{BC}_3{D}_2{\text{O}}_{14}$, including ${\text{Sr}}_3{\text{TaGa}}_3{\text{Si}}_2{\text{O}}_{14}$, ${\text{Sr}}_3{\text{NbGa}}_3{\text{Si}}_2{\text{O}}_{14}$, ${\text{Ca}}_3{\text{TaGa}}_3{\text{Si}}_2{\text{O}}_{14}$, and ${\text{Ca}}_3{\text{TaAl}}_3{\text{Si}}_2{\text{O}}_{14}$ (CTAS), were studied as a function of temperature, up to $900°\text{C}$. The dielectric permittivity ${\epsilon }_{11}$ and piezoelectric coefficient $d_{11}$ of the ordered crystals were found to be on the orders of 12–16 and 4–5 pC/N, respectively, slightly lower than langasite $\left({\text{La}}_3{\text{Ga}}_5{\text{SiO}}_{14}\text{-LGS}\right)$ or langanite $\left({\text{La}}_3{\text{Ga}}_{5.5}{\text{Nb}}_{0.5}{\text{O}}_{14}\right)$ crystals which possess a disordered structure. The mechanical quality factor $Q$ and electrical resistivity $\rho$, however, were found to be greatly improved at elevated temperatures $\ge 500°\text{C}$, being one to two orders of magnitude higher, due to cation ordering. Of particular interest is the CTAS crystal, in which, the Ga cations are totally replaced by low cost Al cations. Together with its thermally stable piezoelectric properties and high electrical resistivity, CTAS crystals offer a competitive material for high temperature sensing applications.