Abstract: High-temperature piezoelectric sensors are critically needed in extreme-environment applications such as aerospace， nuclear energy， and industrial process monitoring. Traditional piezoelectric ceramics and polymers often exhibit significant performance degradation under high-temperature and low-oxygen conditions. Among emerging high-temperature piezoelectric single crystals， langasite-family crystals， particularly the structurally ordered Ca₃TaGa₃Si₂O₁₄ （CTGS） have attracted considerable attention due to their high resistivity， absence of phase transitions up to their melting point （>1 400 ℃）， and suitability for large-size growth via the Czochralski method. However， further enhancement of their electromechanical properties and thermal stability is essential to meet the demands of advanced sensor applications. This study aims to optimize the high-temperature electromechanical properties of CTGS crystals through lattice substitutions. Two distinct substitution strategies were employed： Sr substitution at the A-site （Ca site） to form （Sr _x Ca_1-x ）₃TaGa₃Si₂O₁₄ （SCTGS）， and Al substitution at the C-site （Ga site） to form Ca₃Ta（Ga _x Al_1-x ）₃Si₂O₁₄ （CTGAS）. Crystals with varying substitution ratios （x=0.25， 0.50 for Sr； x=0.30， 0.50 for Al） were grown by the Czochralski method under N₂ atmosphere using iridium crucibles. The crystalline quality was evaluated by high-resolution X-ray diffraction rocking curves， showing full-width half-maximum values between 33.41″ and 58.73″， which confirmed good crystallinity. Resistivity was measured from 350 ℃ to 800 ℃ along the crystallographic Y- and Z-directions. Results indicate that the Al substitution significantly increases the high-temperature resistivity， with 50%CTGAS reaching approximately 1×10⁷ Ω·cm at 800 ℃ along the Z-direction. In contrast， Sr substitution reduces resistivity， with 50%SCTGS exhibiting ~1×10⁵ Ω·cm under the same conditions. Dielectric and piezoelectric properties were systematically characterized from room temperature to 800 ℃ using specifically designed crystal cuts （X-cut， Z-cut， XYt/0°， and YZt/45°）. Results indicate that the Sr substitution notably enhances the room-temperature piezoelectric coefficients： d₁₁ and d₁₄ reaches 4.84 and -20.17 pC/N for 50%SCTGS， representing increases of 16.1% and 81.2%， respectively， over pure CTGS. The relative dielectric permittivity {\epsilon }_{\mathrm{11}}^T/{\epsilon }_{\mathrm{0}} also increases with the increase of Sr content. In contrast， Al-substituted crystals retaines piezoelectric coefficients similar to pure CTGS while markedly improving thermal stability. The variation in piezoelectric coefficient d₁₁ is less than 10.5% across 25~800 ℃， and dielectric loss remained below 0.2 up to 600 ℃. To elucidate the mechanisms， bond-valence-based calculations of polyhedral dipole moments were performed. Results indicate that the large dipole moment of the ［SrO₈］ polyhedron （12.204 5 D） in SCTGS accounts for its enhanced piezoelectric response， whereas the reduces dipole moment in ［CaO₈］ and improves structural order upon Al substitution explain the superior resistivity and thermal stability of CTGAS. In conclusion， Sr substitution could effectively enhance the piezoelectric activity of CTGS crystals， while Al substitution could significantly improve high-temperature resistivity and electromechanical stability. This work demonstrates that site-specific substituting is a powerful strategy for tailoring CTGS-based materials toward specific high-temperature sensor requirements.

Key words: langasite; substitution; Czochralski method; resistivity; piezoelectric property; high-temperature