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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. 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)
5. Piezoelectricity across 2D Phase Boundaries

   https://doi.org/10.1002/adma.202206425  

   Puthirath, Anand B.; Zhang, Xiang; Krishnamoorthy, Aravind; Xu, Rui; Samghabadi, Farnaz Safi; Moore, David C.; Lai, Jiawei; Zhang, Tianyi; Sanchez, David E.; Zhang, Fu; et al (August 2022, Advanced Materials)

   Abstract Piezoelectricity in low‐dimensional materials and metal–semiconductor junctions has attracted recent attention. Herein, a 2D in‐plane metal–semiconductor junction made of multilayer 2H and 1T′ phases of molybdenum(IV) telluride (MoTe₂) is investigated. Strong piezoelectric response is observed using piezoresponse force microscopy at the 2H–1T′ junction, despite that the multilayers of each individual phase are weakly piezoelectric. The experimental results and density functional theory calculations suggest that the amplified piezoelectric response observed at the junction is due to the charge transfer across the semiconducting and metallic junctions resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials. 

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