Theoretical design of dielectric enhancement in Tm-doped LaCOB crystals†

LaCOB crystals are promising for ultra-high-temperature piezoelectric sensing application due to their high melting point, high electrical resistivity, superior piezoelectric properties, and low temperature drift under extreme temperatures. However, the high symmetry of the local octahedral structure in LaCOB crystals limits the dielectric response, reducing the potentials of mechanical-to-electrical energy conversion. Using density functional perturbation theory, the microstructural origins influencing the dielectric properties for LaCOB crystals have been elucidated. Thereby we propose the La and Ca site doping with Tm ions, leveraging the smaller ionic radius and larger mass of Tm, to significantly enhance the Jahn–Teller distortion, breaking the high symmetry of La–O₆ and Ca–O₆ octahedra. This modification markedly boosts the low-frequency phonon vibrations and increases the static dipole moment, effectively enhancing the dielectric response of the crystal. Our findings reveal that the rotational vibrations of La/Tm/Ca–O₆ octahedra, driven by low-frequency optical phonons, are key to the enhanced polarization response. Additionally, oxygen-deficient or boron-rich atmosphere treatments could facilitate Tm doping at Ca sites, further improving the Tm doping concentrations and enhancing the dielectric properties. These findings provide valuable insights for designing new high-temperature piezoelectric crystals with improved performances.