Chemical nature of giant strain in Mn-doped 0.94(Na0.5Bi0.5)(3)

Figure3shows the X-ray photoelectron spectra for Na1s,Bi4f and Ti2p of Mn:NBBT6single crystals. The shifts of core-level spectra due to the charging effect were corrected by calibration with the C1s peak set at 285eV.The spectra were deconvoluted using Lorentz-ian–Gaussian functions.The Na1s XPS spectra werefit-ted by two components at binding energies(BEs)of 1072and1073.5eV,respectively.The intensity of the peak located at the lower BE decreases for OS.How-ever,it increases for VS.For Bi4f7/2,two components werefitted at BEs of164and164.8eV.For Bi4f5/2, the BEs werefitted to be158.8and159.3eV,respec-tively.No obvious change was observed in the Bi4f spec-tra after annealing.The two peaks in the Ti2p spectra correspond to the angular momentum of the titanium

electrons.With the shape of the Ti2p3/2and Ti2p1/2 peaks and thefitting results,the conclusion can be drawn that only Ti4+exists in the crystal lattice.There-

fore,the positive charge of V

O is not compensated by

the reduction of Ti4+to Ti3+.

According to the charge potential model suggested by Siegbahn et al.[31],the binding energy mainly depends on the average electron density of the element and the neighboring environment.The difference in the BEs of Na+and Bi3+is the consequence of the chemical disor-der,which results in different effective electron densities of elements.Similar behavior has also been observed in PMN,where the presence of anomalous Pb4+was as-signed to the the coexistence of two unequivalent Pb–O bond lengths[32,33].The different changes in the Na1s and Bi7f line intensities upon annealing reveal the difference in the chemical binding natures of Bi and Na.According to Jones and Thomas[34],Bi3+is underbonded in NBT,with a32%deficiency from its ideal values–much higher than the deficiency(8%)ob-served for Na+.The Bi3+ions displace and interact with oxygen,resulting in a much“harder”Bi–O bond than the Na–O bond.Therefore,the binding energy of Na+ ions is more sensitive to the change in the coordinate environments.

Figure4shows the X-band EPR spectra for Mn:NBBT6single crystals measured at ambient temper-ature.The AS samples exhibit a hyperfine structure, with six prominent lines of typically isolated Mn2+, where Mn2+substitutes Ti4+ions at the B-site and acts as an acceptor[35,36].Oxygen vacancies(V

O

)will be created simultaneously for charge compensation:

Mn MnþTiÂ

Ti

!Mn00

Ti

þTiþV

O

A superimposed signal originating from Mn2+and Mn4+was observed in the EPR spectrum of OS,indicat-ing that Mn2+ions were partly oxidized to Mn4+:

Mn00

Ti

þ2h !MnÂ

Ti

According to impedance spectra in Figure2,this pro-cess is accompanied by absorption of oxygen into the oxygen vacancy sites as a way of compensating electrical neutrality:

V

O

þ

1

2

O2!OÂ

O

þ2h

The EPR spectrum of VS exhibits axial symmetry, indicating the presence of nearest-neighbor oxygen vacancies.This is because the long-time annealing in a vacuum favors the mobility of oxygen vacancies,which are preferentially“trapped”on these isolated Mn00

Ti

defect centers and form½Mn00

Ti

ÀV

O

defect dipoles.

Based on these results,the giantfield-induced strain

plex impedance spectra of Mn:NBBT6single crystals as

function of temperature from40to1MHz:(a)AS;(b)OS;(c)VS;

(d)Arrhenius plot of r dc conductivity.

Figure3.XPS profiles of Mn:NBBT6single crystals:(a)Na1s;(b)Bi4

(c)Ti2p.X-band EPR spectra for Mn:NBBT6single crystals.

Materialia75(2014)50–53