Tilt strain glass in Sr and Nb co-doped LaAlO3 ceramics

Strain glass, a glassy state of lattice strain, has been found in Ti_50-xNi_50+x alloys and later in many metallic ferroelastic/martensitic systems, where shear or shuffle serves as a primary order parameter (POP). Another class of non-metallic ferroelastic ceramics are also known to widely exist and commonly possess a polyhedral tilt as the POP. So far, it is unclear whether a "tilt" strain glass exists. Here, we report a finding of a tilt strain glass in La_1-xSr_xAl_0.95Nb_0.05O_3-δ ceramics. With increasing Sr²⁺ dopants, the ferroelastic transition from cubic to rhombohedral phases is gradually suppressed. At a critical concentration (x_c ∼10%), a strain glass transition emerges, characterized by five sets of evidence: (I) an invariance of average structure; (II) frequency dependence of elastic moduli at a strain glass transition temperature T_g; (III) non-ergodicity; (IV) formation of rhombohedral nanodomains; (V) a gradual increase of tilt angle upon cooling. Surprisingly, the established phase diagram shows an increase of T_g with increasing dopants (a positive correlation), which is different from previous strain glass phase diagrams. The positive and negative correlations can be explained as a balance between two factors of strain glass transition: a global transition factor producing a negative contribution competes with a local field one producing a positive contribution. Our discovery of strain glass in ceramics may also bring novel properties as in metals.