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1. Piezoelectricity in hafnia

   https://doi.org/10.1038/s41467-021-27480-5  

   Dutta, Sangita; Buragohain, Pratyush; Glinsek, Sebastjan; Richter, Claudia; Aramberri, Hugo; Lu, Haidong; Schroeder, Uwe; Defay, Emmanuel; Gruverman, Alexei; Íñiguez, Jorge (December 2021, Nature Communications)

   Abstract Because of its compatibility with semiconductor-based technologies, hafnia (HfO 2 ) is today’s most promising ferroelectric material for applications in electronics. Yet, knowledge on the ferroic and electromechanical response properties of this all-important compound is still lacking. Interestingly, HfO 2 has recently been predicted to display a negative longitudinal piezoelectric effect, which sets it apart from classic ferroelectrics (e.g., perovskite oxides like PbTiO 3 ) and is reminiscent of the behavior of some organic compounds. The present work corroborates this behavior, by first-principles calculations and an experimental investigation of HfO 2 thin films using piezoresponse force microscopy. Further, the simulations show how the chemical coordination of the active oxygen atoms is responsible for the negative longitudinal piezoelectric effect. Building on these insights, it is predicted that, by controlling the environment of such active oxygens (e.g., by means of an epitaxial strain), it is possible to change the sign of the piezoelectric response of the material. 

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2. Piezoelectricity in chalcogenide perovskites

   https://doi.org/10.1038/s41467-024-50130-5  

   Abir, Sk_Shamim_Hasan; Sharma, Shyam; Sharma, Prince; Karla, Surya; Balasubramanian, Ganesh; Samuel, Johnson; Koratkar, Nikhil (July 2024, Nature Communications)

   Abstract Piezoelectric materials show potential to harvest the ubiquitous, abundant, and renewable energy associated with mechanical vibrations. However, the best performing piezoelectric materials typically contain lead which is a carcinogen. Such lead-containing materials are hazardous and are being increasingly curtailed by environmental regulations. In this study, we report that the lead-free chalcogenide perovskite family of materials exhibits piezoelectricity. First-principles calculations indicate that even though these materials are centrosymmetric, they are readily polarizable when deformed. The reason for this is shown to be a loosely packed unit cell, containing a significant volume of vacant space. This allows for an extended displacement of the ions, enabling symmetry reduction, and resulting in an enhanced displacement-mediated dipole moment. Piezoresponse force microscopy performed on BaZrS₃confirmed that the material is piezoelectric. Composites of BaZrS₃particles dispersed in polycaprolactone were developed to harvest energy from human body motion for the purposes of powering electrochemical and electronic devices. 

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3. Negative Longitudinal Piezoelectricity Coexisting with both Negative and Positive Transverse Piezoelectricity in a Hybrid Formate Perovskite

   https://doi.org/10.1021/acsami.2c09828  

   Ghosh, Partha Sarathi; Lisenkov, Sergey; Ponomareva, Inna (October 2022, ACS Applied Materials & Interfaces)
4. Mechanisms of direct and converse piezoelectricity in ferroelectric polymers

   https://doi.org/10.1016/j.polymer.2025.128290  

   Zhu, Zhiwen; Rui, Guanchun; Allahyarov, Elshad; Zhang, Honghu; Li, Ruipeng; Taylor, Philip L; Zhu, Lei (April 2025, Polymer)

   Within the linear regime of mechanical and electrical responses, it is commonly accepted that direct and converse piezoelectric coefficients should be the same. However, we observed a consistently higher converse d31 (∼54 pm/V) than the direct d31 (∼42 pC/N) for a quenched, stretched, annealed, and electrically poled poly(vinylidene fluoride-co-trifluorethylene) [P(VDF-TrFE)] 52/48 mol.% sample (abbreviated as coP-52/48QSAP). On the contrary, the direct and converse d31 values were the same for coP-65/35QSAP and coP-55/45QSAP. Small-angle X-ray scattering results showed that coP-52/48QSAP had a higher amount of relaxor-like secondary crystals (SCs) in the oriented amorphous fraction (OAF) (SCOAF) than coP-55/45QSAP and coP-65/35QSAP. To explain the experimental observation, we performed molecular dynamics (MD) simulation of the pure PVDF (without TrFE) to estimate direct and converse piezoelectricity for the PVDF OAF. Based on the MD simulation, the direct d31 had a plateau value around 350 pC/N for the transverse (i.e., along the chain direction) strain up to 1 %, whereas the simulated converse d31 could be lower (for electric field E < 0.8 MV/m), equal (for E = 0.8 MV/m), or higher (for E > 0.8 MV/m) than the direct d31, depending on the poling electric field. From the MD simulation, both mechano-electrostriction and electrostatic interaction were identified in the OAF as the driving force for enhanced piezoelectricity in ferroelectric PVDF. When ferroelectric domains were formed in the OAF by electric poling, the simulated converse d31 became higher than the direct d31. Combining both experimental and MD simulation results, the higher converse d31 than direct d31 for coP-52/48QSAP was understood qualitatively. 

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5. Piezoelectricity in nominally centrosymmetric phases

   https://doi.org/10.1103/PhysRevResearch.3.043221  

   Aktas, Oktay; Kangama, Moussa; Linyu, Gan; Catalan, Gustau; Ding, Xiangdong; Zunger, Alex; Salje, Ekhard K. (December 2021, Physical Review Research)