(192ar) Leveraging Heterostructural Alloying to Design Metastable Nitrides with Improved Piezoelectric Properties

The tailoring of materials properties by alloying is routinely utilized to design materials for targeted technological applications. Despite the great successes of alloying in isostructural systems, heterostructural alloying remains a fundamentally unexplored area. In heterostructural alloys, the crossover between different crystal structures enables the control of the atomic structure by variation of the composition. The deliberate manipulation of local atomic coordination symmetry introduces an additional materials design parameter for tuning the piezoelectric response of a material. The piezoelectric effect underlies a wide range of industrial applications including quartz resonators in wristwatches, powerful hydrophones for naval sonar, medical ultrasound, and the microelectromechanical (MEMs) resonator-based bandpass filters in wireless handsets. However, many of the mature alloys used (eg. PZT) have been designed around large strain and electromechanical coupling coefficients for use in high power, low operating frequency applications while high frequency resonant, high temperature, and/or low power integrated devices typically use simple unary compounds such as quartz or aluminum nitride despite their relatively low piezoelectric response. In this talk, we present a complementary theoretical and experimental investigation of piezoelectrically active nitride alloys to develop design principles and approaches for utilizing heterostructural alloying as a materials design strategy. We use ab initio methods to predict the structural and electronic properties of Al_1-xSc_xN heterostructural alloys for the energetically competitive polymorphs and compute their corresponding equilibrium phase diagrams and materials properties. Combinatorial sputtering is employed as a non-equilibrium growth technique to overcome thermodynamic solubility limits and produce metastable thin-film samples spanning the alloy composition range. The experimentally validated predictions, theory guided combinatorial synthesis, and characterization of piezoelectric heterostructural alloys exemplify how our integrated research strategy is used to design and realize functional metastable materials which can be utilized in a variety of industrial applications. We apply these design principles to screen and computationally prototype new metastable nitride alloys with improved piezoelectric properties. Our approach establishes a new route for the control of structure-property and composition-structure relationships by accessing non-equilibrium phase space to develop new piezoelectric materials with uniquely tailored properties for specific applications.