Thermal and mechanical properties of LaNbO4-based ceramics

Abstract Thermal and mechanical properties of polycrystalline La 1− x A x NbO 4 ( 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 LaNbO 4 (∼3 GPa) due to a significantly smaller average grain size. The fracture toughness of La 0.98 Sr 0.02 NbO 4 , measured by single edge V-notched beam method, was 1.7 ± 0.2 MPa m 1/2 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 LaNbO 4 -based materials are discussed with focus on ferroelasticity, microcracking due to crystallographic anisotropy and pinning of ferroelastic domain boundaries.