Thermal and mechanical properties of LaNbO4-based ceramics

Thermal and mechanical properties of polycrystalline La1−xAxNbO4 (x = 0, 0.005, 0.02 and A = Ca, Sr and Ba) are reported. The materials possess a ferroelastic to paraelastic phase transition close to 500 °C, and the linear thermal expansion is significantly lower (∼8.6 ± 0.5 × 10−6 °C−1) for the paraelastic phase compared to the ferroelastic phase (∼15 ± 3 × 10−6 °C−1). The hardness was significantly higher for acceptor doped materials (∼6 GPa) compared to pure LaNbO4 (∼3 GPa) due to a significantly smaller average grain size. The fracture toughness of La0.98Sr0.02NbO4, measured by single edge V-notched beam method, was 1.7 ± 0.2 MPa m1/2 independent of temperature up to 600 °C. The ferroelastic properties of the materials were confirmed by non-linear relationships between stress and strain during compression/decompression, a remnant strain after decompression and the presence of ferroelastic domains. The mechanical properties of LaNbO4-based materials are discussed with focus on ferroelasticity, microcracking due to crystallographic anisotropy and pinning of ferroelastic domain boundaries.