Origin of morphotropic phase boundaries in ferroelectrics

Piezoelectrics made simple Application of mechanical force to a piezo­electric material generates a voltage; conversely, apply a voltage and you get a force. This combination of properties has many applications, primarily in the generation of ultrasound. The largest electromechanical responses tend to occur in highly complex materials, and the desired properties tend to be maximum when associated with a 'morphotropic' phase transition — an abrupt structural change usually linked to changes in composition. Muhtar Ahart et al . show that a similar phase transition can occur in a simple, pure compound, under high pressure. The compound is the prototypical ferroelectric, lead titanate, and it produces an electro-mechanical response larger than any known. It may be possible to chemically tune these effects to ambient pressures, which would potentially reduce the costs and enhance the utility of high-performance piezoelectric materials. This paper shows that even a pure compound, in this case lead titanate, can display a morphotropic phase boundary under pressure. The results are consistent with first principles theoretical predictions, but show a richer phase diagram than anticipated; moreover, the predicted electromechanical coupling at the transition is larger than any known. A piezoelectric material is one that generates a voltage in response to a mechanical strain (and vice versa). The most useful piezoelectric materials display a transition region in their composition phase diagrams, known as a morphotropic phase boundary 1 , 2 , where the crystal structure changes abruptly and the electromechanical properties are maximal. As a result, modern piezoelectric materials for technological applications are usually complex, engineered, solid solutions, which complicates their manufacture as well as introducing complexity in the study of the microscopic origins of their properties. Here we show that even a pure compound, in this case lead titanate, can display a morphotropic phase boundary under pressure. The results are consistent with first-principles theoretical predictions 3 , but show a richer phase diagram than anticipated; moreover, the predicted electromechanical coupling at the transition is larger than any known. Our results show that the high electromechanical coupling in solid solutions with lead titanate is due to tuning of the high-pressure morphotropic phase boundary in pure lead titanate to ambient pressure. We also find that complex microstructure