Low-temperature phase transition in Ba₂TiGe₂O₈

Abstract

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (∼ 223 K on cooling, ∼ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.

Key Words:

• Ba₂TiGe₂O₈
• ferroelastic crystal
• incommensurate crystal
• lock-in phase transition