Strontium titanate toughens up

Materials scientists in Stuttgart have achieved significant plastic deformation in strontium titanate, an oxide ceramic material previously believed to be extremely fragile at room temperature.

Materials scientists at the Max-Planck-Institute for Metals Research have achieved significant plastic deformation in strontium titanate (SrTiO3), an oxide ceramic material previously believed to be extremely fragile and brittle at room temperature.

The results may change the way ceramic materials are treated as engineering materials.

Strontium titanate is a prominent representative of the group of ceramic oxides, which crystallise in the cubic perovskite structure. At ambient temperatures, perovskites behave as most of the other ceramic oxides in that they are brittle and shatter easily.

Scientists think this is due to the difficulty with which dislocations move through the crystalline structure of these materials.

Dislocations are defects of the regular crystal structure and serve as the elementary vehicle of permanent plastic deformation in most crystalline materials.

When a dislocation moves through the crystal, it shears the crystal along its plane of motion (slip) by a well-defined displacement vector.

The ductility of metals can be attributed to the ease of motion of these dislocations. In contrast, the ionic and covalent nature of the bonding in ceramic oxides normally makes this slipping process difficult and the dislocations are essentially immobile up to high temperatures of the order of 1000°C.

The lack of plastic deformation of strontium titanate was the feature that researchers at the Max-Planck Institute for Metals Research wanted to make use of when calibrating new mechanical testing equipment.

The researchers were surprised when this seemingly hard single crystal deformed plastically at flow stresses as low as 120 MPa (comparable to aluminium or copper alloys) and that it reached strains of up to 7% at room temperature.

The researchers launched an investigation into this behaviour and discovered that single strontium titanate crystals tested in compression not only display the usual transition from ductile behaviour at high temperatures (above 1000°C) to brittle behaviour but also a second transition back to ductile behaviour below 600°C.

Microscopical analysis revealed that the same type of dislocations carry the deformation in both the high-temperature and the low-temperature regime.

The researchers concluded that these dislocations in strontium titanate exist with two different inner (core) structures.

Since symmetry and crystal structure largely dictate the core structure of dislocations, this observation suggests that such different inner structures of the dislocations should also exist in other perovskites.

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