P. Shiv Halasyamani , University of Houston
Piezoelectricity phenomena are observed as both direct and converse. With the direct piezoelectric effect, the change in polarization, (delta)P, is caused by an external stress, and is is formulated as (delta)P = d(delta)S where d, a third rank tensor, dijk, is the piezoelectric coefficient (in C/N) of the material. With the converse piezoelectric effect, an applied field, (delta)E, results in a macroscopic strain on the material. The converse effect may be formulated as (delta)S = d(delta)E, where the piezoelectric coefficient d is in units of m/V. It should be noted that 1 C/N = 1 m/V. Both direct and converse piezoelectric effects are used in a variety of electromechanical energy conversion applications. The direct effect results in generator action – the piezoelectric material converts mechanical energy to electrical energy. This generator action is used in solid-state batteries, sensing devices, and fuel lighting applications. The converse effect results in motor action – the piezoelectric material converts electrical energy to mechanical energy. This motor action is used in ultrasonic and acoustic applications, micromotor devices, and electromechanical transducers. Materials that exhibit piezoelectricity must be crystallographically non-centrosymmetric (NCS), that is the material cannot contain a center of symmetry. So the fundamental issue is – how to design and synthesize new materials that are NCS and will therefore exhibit piezoelectric phenomena? We have developed a design methodology whereby we synthesize oxides that contain metal cations that are susceptible to second-order Jahn-Teller (SOJT) distortions. These cations are octahedrally coordinated d0 transition metals (Ti4+, Nb5+, W6+, etc.) and cations with a lone-pair (Se4+, Te4+, I5+, etc.). Cations that undergo SOJT distortions, are usually in asymmetric coordination environments. When these asymmetric environments constructively add in the crystal structure, a macroscopic NCS material is created. By using this methodology, we have increased the incidence of NCS in any new material to nearly 50%. Another issue with piezoelectrics is the inclusion of Pb2+ - a lone-pair cation that is also toxic. Currently, commercially used piezoelectrics contain Pb2+. Thus there is a need for non-Pb-based piezoelectric materials.
With our ACS-PRF funded research, we have synthesized a host of new oxide materials that exhibit piezoelectric phenomena. These include A2Ti(IO3)6 (A = Li, Na, K, Rb, Cs, or Tl), KNbW2O9, RbNbW2O9, and KTaW2O9, RbSe2V3O12, TlSe2V3O12, Rb2(MoO3)(SeO3), Tl2(MoO3)3(SeO3), VOSe2O5, and VO(SeO2OH)2. All of these materials are NCS – a requirement for piezoelectric behavior – and include cations that are susceptible to second-order Jahn-Teller distortions. These materials were synthesized as single crystals, as well as phase-pure polycrystalline powders – at least 1g of each. Converse piezoelectric measurements done in our laboratory revealed d33 coefficients between 10 – 100 pm/V. In addition to the synthesis, crystal structure, and piezoelectric properties, the materials were characterized by thermogravimetric analysis, differential scanning calorimetry, UV-Vis spectroscopy, second-harmonic generation, and in cases where the materials are polar, by ferroelectric and pyroelectric measurements. By investigating in detail the bonding interactions, we were able to develop a host of structure-property relationships. These relationships enabled us to explain why the materials had specific physical property phenomena. Also, through synthesizing and characterizing these materials we are able to suggest additional stoichiometries that will have piezoelectric phenomena. These experiments are in progress.