Chemical nature of giant strain in Mn-doped 0.94(Na0.5Bi0.5)(2)

deviation in the Ba/Ti ratio will result in a valencefluc-tuation between Ti3+and Ti4+in BaTiO3,the end mem-ber of NBBT[24].It has been found that Mn doping is an effective way to reduce leakage current and enhance the piezoelectric constant[14,25].However,the underly-ing mechanism is still not clear,including its site occupa-tion and valence state.Thus,clarifying the crystal’s chemical nature,i.e.the elements’oxidation states, chemical defects and ion binding energies,is crucial for revealing the microscopic origin of NBBT’s excellent properties.

In present work,the electrical properties and defect chemistry of Mn-doped0.94Na0.5Bi0.5TiO3–0.06BaTiO3 (Mn:NBBT6)single crystal have been studied in detail. Field-induced bipolar and unipolar strains have been measured.The temperature-dependent conductivity was studied by impedance spectra.The oxidation states and binding energy of Na,Bi,and Ti were studied by X-ray photoelectron spectra(XPS).The site occupation and valencefluctuation Mn ions were investigated by paramagnetic resonance spectra(EPR)spectra.The de-fect structures and their relation to the improved electri-cal properties have also been discussed.

Mn:NBBT6single crystals were grown by a carefully controlled top-seeded solution growth method.Induc-tive coupled plasma atomic emission spectrometry mea-surements show that the concentrations of Ba2+and Mn2+ions are6and0.1at.%,respectively.The as-grown single crystals were cut along the pseudocubic (001)/(100)/(010)planes with dimensions of 5Â5Â0.5mm3.The samples were then annealed under different conditions:(i)as-grown,labeled as“AS”;(ii) 20h at600°C in O2atmosphere,labeled as“OS”;and (c)20h at600°C in a vacuum,labeled as“VS”.To characterize the electrical properties,gold electrodes were sputtered on both of the main faces.The bipolar strain curves(S–E)and unipolar strain curves(e–E) were measured using a ferroelectric test system (aixACCT TFanalyzer1000).Temperature dependence impedance spectra were measured using an Agilent 4294impedance analyzer.The X-band(9.8GHz) EPR measurements were performed on a Bruker EMX spectrometer(Karlsruhe,Germany)at ambient temperature.

Figure1a shows bipolar strain curves(S–E)for Mn:NBBT6single crystals after different annealing pro-respectively.The obvious increases in the coercivefield for OS and VS reveal that the potential energy of field-induced phase transformation from pesudocubic to tetragonal increases after annealing[9,26].The increase in the remanent strain(S rem)and the develop-ment of negative strain(S neg)indicate that the stability of thefield-induced ferroelectric order is also enhanced [27].Similar behavior was also observed in unipolar strain curves(e–E),as shown in Figure1b.The values of normalized strain e max for OS and VS samples were 1086and1361pm V–1,respectively,much larger than that of the AS samples.The electrical properties of Mn:NBBT6single crystals were compared with previ-ously reported data and are summarized in Table1.It is clear that controlling chemical defects is an effective way to tailor the electromechanical properties.

Figure2a–c shows complex impedance spectra of Mn:NBBT6single crystals from40to1MHz at different temperatures.A single semicircle was observed for all samples,indicating that a single localized relaxation mechanism dominates the impedance in the measured temperature range.Upon heating,the semicircles gradu-ally contract and shift to the real axis(Z0).This behavior can be assigned to the activation of the weakly trapped charge carriers,which results in the increase in conduc-tivity(r).The impedance was simulated as an equiva-lent circuit consisting of parallel connection of resistance(R)and capacitance(C).By extrapolating the low-frequency intercept of the simulated semicircles with real axis,the values of R were obtained.The relation between the activation energy(E a)and r can be denoted by the Arrhenius equation:

r¼r0expðÀE a=kTÞð1Þwhere r0is a constant,k is the Boltzmann constant and T is the absolute temperature.The logarithms of r vs. 1000/T are plotted in Figure2d.The solid lines are the best least-squaresfitting of Eq.(1).The values of activa-tion energy for AS,OS and VS are evaluated to be1.3, 1.38and1.2eV,respectively.In ABO3perovskite mate-rials,the values of E a for A-and B-site cations are around4and12eV,respectively[29].For oxygen vacancies,it varies from0.5to2eV,depending on their concentration[30].Therefore,it is reasonable to suggest that oxygen vacancies dominant the conductivity in this temperature range.The enhanced E a reveals the reduc-tion in oxygen vacancies in OS,while the suppressed E a in VS indicates it has experienced an increase in the concentration of oxygen vacancies.

Table1.Electrical properties of Mn:NBBT6single crystals as com-pared to NBT–BT–KNN and PZT ceramics.

Material P r

(l C cm–2)

E c

(V mm–1)

S max

(%)

e max/

E max

(pm V–1)

Reference

AS30.7512500.23372This work OS11.8520000.721086This work VS25.8018320.521361This work NBT–BT-5KNN33.00–0.30–[1]

NBT–BT-2KNN––0.41560[2]

Hard PZT,PZT8–––150[28]

Soft PZT,PZT5H––400–590[28]

Figure1.(a)Bipolar strain curves and(b)unipolar strain curves of

[001]-oriented Mn:NBBT6single crystals measured under5kV mm–1

H.Zhang et al./Scripta Materialia75(2014)50–5351