Elastic anomalies due to structural phase transitions in mechanoluminescent SrAl₂O₄:Eu

Elastic and anelastic properties of a ceramic sample of elasticoluminescent SrAl₂O₄:Eu have been characterized as a function of temperature by resonant ultrasound spectroscopy. Both the bulk (K) and shear (G) moduli show changes attributable to the influence of the P6₃22→ P6₃ (√3A) →P2₁ sequence of structural phase transitions. Softening of K and stiffening of G at the P6₃22→ P6₃ (√3A) transition (T_c ≈1135 K) is consistent with weak strain/order parameter coupling and tricritical character. In marked contrast, the first order P6₃(√3A) →P2₁ transition near 930 K is accompanied by stiffening such that G is ∼50% larger at room temperature than the value for a P6₃22 structure extrapolated directly from high temperatures and K is ∼15% larger. Softening of G at high temperatures is consistent with the existence of a soft acoustic mode. Both the P6₃(√3A) and P6₃22 phases show an increasing acoustic dissipation with increasing temperature and there is an additional peak in the dissipation behavior below ∼650 K. Landau theory has been used to analyze the overall strain and elastic behavior of SrAl ₂O₄:Eu. The stability of the P2₁ structure is considered in terms of coupling between order parameters with F₆ and M₂ symmetry, though it could also be stabilized by pairwise coupling of order parameters corresponding to irreps F₄ + M₂, F₄ + M₃, F₆ + M₃, and M₂ + M₃. Twin walls in this material should have interesting properties, which are quite distinct from those of the bulk material and could contribute differently to the elasticoluminescent properties.