First-princples Study On Piezoelectric Property Of Lead-free Piezoelectric Materials

Piezoelectric material is an important functional material in the scientific research and industrial fields,which can convert mechanical energy to electricity.In macroscopic scale,piezoelectric material has been used as underwater sound transducer,B-ultrasound medical probe,ultrasonic motor,oil logging detector,nondestructive testing equipment and acceleration sensor.In nanoscale,micro and nano electromechanical system has potentially valuable application in biomedical and national defense fields.Developing new lead-free bulk and nano piezoelectric materials that provides power for nano devices is two important research directions in piezoelectric fields.Pb?Zr,Ti?O_3- based bulk piezoelectric materials exhibit high piezoelectric,electromechanical and dielectric properties,so they have been used to make sensors and actuators.Due to Pb's toxicity,their preparation,use and reuse would be a great threat for human health and environment.Therefore,it is an urgent and important task to develop new leadfree piezoelectric materials.It is worth noting that?K,Na?NbO₃ and BaTiO₃ based bulk piezoelectric materials are the potential candidates for lead-free piezoelectric materials.At micro-scale,lead-based bulk materials in nano level have lower performance,and even do not show piezoelectric effect.These weaknesses result in that these materials are not used in nano-electromechanical system.To provide power for nano devices,the researchers began to explore the piezoelectric performance of low-dimensional materials.Thus,the theoretical study on piezoelectricity in?K,Na?Nb O₃,BaTiO₃ based and two-dimensional materials is very important,which can provide some useful information for their application.The polymorphic phase boundary and piezoelectricity of K_1-xNa_xNbO₃ are studied.The total energy,crystal structure,electronic structure,elastic modulus,compliance constant and piezoelectric coefficient are investigated by firstprinciples.The polymorphic phase boundary in K_1-xNa_xNbO₃ is predicted to occur at x=0.521,which is in good agreement with experimental data.The high piezoelectric response in K_1-xNa_xNbO₃ is attributed to the coexistence of two phases at the polymorphic phase boundary.The rotated maximum d₃₃ of orthorhombic K_0.5Na_0.5NbO₃ is close to [001] direction.By analyzing the energy barrier on polarization rotation paths from [011] to [001] directions,it is found that the polarization rotation of K_0.5Na_0.5NbO₃ becomes easier compared with KNbO₃.The polymorphic phase boundary and piezoelectricity of BaTi_1-xZr_xO₃ are investigated.The total energy,structure parameter,piezoelectric coefficient,dielectric constant,elastic property and Born effective charge are calculated by first-principles.The polymorphic phase boundary of BaTi_1-xZr_xO₃ occurs in range of x=0.077-0.088,which is coincident with experimental data.The high piezoelectric activity at the polymorphic phase boundary is due to the coexistence of two phases.The high d₃₃ in tetragonal and orthorhombic BaTi_1-xZr_xO₃ occur at x=0.080,which confirms the position of polymorphic phase boundary.Moreover,the enhancement of both structural instability and Ti-O covalent bonding is responsible for high piezoelectric performance in BaTi_1-xZr_xO₃.Using the first-principles method,we predict a two-dimensional honeycomb structure of BC,and study its bonding,optical and piezoelectric properties.Twodimensional BC consists of the sp2 hybridized boron and carbon on?001?plane and the pz-pz???bonding carbon along [001] direction.The structure is thermodynamically stable and possesses lower formation energy than other candidates.The calculations within GW approximation reveal that BC exhibits an indirect band gap of 2.40 e V and large exciton binding energy of 1.35 e V.In optical absorption spectra,a Frenkel-class bound exciton has been discovered at 2.98 e V,which is desirable for light emitting applications.The relationship between piezoelectric and structure for BC is investigated.The results indicate that piezoelectricity of BC is derived from its honeycomb structure.The angle of piezoelectric coefficients of BC is about 120°.The piezoelectric coefficient(e₂₂)of-2.38×10^-10 C/m is predicted for BC,which indicates that it is a potential candidate for piezoelectric applications in micro-nano-electromechanical system.Using first-principles method and evolutionary algorithm,we predict two honeycomb structures of BSi.Their structure,bonding,optical and piezoelectric properties are studied.A weak covalent bonding between B atoms along [001] direction plays a very important role in structural stability of BSi.Phonon spectra calculations show that both structures are thermodynamically stable.P1-type BSi is semi-metallic,while C2-type exhibits a semiconductor band structure with a gap of 1.07 e V.The relationship between piezoelectric and structure for C2-type BSi is investigated.The results indicate that piezoelectricity of C2-type BSi is also derived from its honeycomb structure similar to that of BC.The piezoelectric coefficient(e₂₂)of 1.65×10^-10 C/m is predicted for C2-type BSi.Due to obvious difference between C2 type BSi and BC,they have a huge difference in piezoelectric anisotropy.When making nanometer piezoelectric devices by them,the differences should be considered.The above results can provide some useful information in designing and developing new lead-free piezoelectric materials.