Microscopic origin of polarity in quasiamorphous BaTiO
3
A. I. Frenkel,
1
Y. Feldman,
2
V. Lyahovitskaya,
2
E. Wachtel,
2
and I. Lubomirsky
2
1
Physics Department, Yeshiva University, 245 Lexington Avenue, New York, New York 10016, USA
2
Weizmann Institute of Science, Rehovot 76100, Israel
Received 10 September 2004; published 28 January 2005; corrected 8 February 2005
The recent observation of pyroelectricity in quasiamorphous thin films of BaTiO
3
introduced a previously
unreported type of polar ionic solid where the appearance of a macroscopic dipole moment is not accompanied
by long-range crystal-like order. This poses a question regarding the mechanism of polarity in noncrystalline
ionic systems and the nature of their local dipoles. By combining x-ray diffraction and x-ray-absorption
fine-structure spectroscopy techniques we have identified the local dipoles as stable but distorted TiO
6
octa-
hedra. The magnitude of the off-center displacement of the Ti ion and the concomitant dipole moment in both
quasiamorphous polar and amorphous nonpolar BaTiO
3
were found to be nearly twice as large as those in
bulk BaTiO
3
. We propose that the mechanism of macroscopic polarity in quasiamorphous BaTiO
3
is in a weak
orientational ordering of the TiO
6
bonding units. In this view, one may expect that other amorphous ionic
oxides containing stable local bonding units, for example NbO
6
, TiO
6
, or VO
6
, may also form noncrystalline
polar phases.
DOI: 10.1103/PhysRevB.71.024116 PACS numbers: 61.10.Ht, 61.43.Er, 64.70.Nd
It has been reported recently
1
that amorphous BaTiO
3
thin
films on Si 100 prepared by radio frequency RF magne-
tron sputtering do not crystallize if pulled through a tempera-
ture gradient. According to x-ray and electron diffraction
these films remain noncrystalline below 700 °C, which is the
temperature that normally leads to rapid crystallization of
BaTiO
3
. These films nevertheless exhibit significant 5–15 %
of bulk BaTiO
3
pyroelectric and piezoelectric effects, indi-
cating that the phase is polar. This noncrystalline but polar
phase was named quasiamorphous, in contrast to the as-
deposited amorphous phase that is neither pyroelectric nor
piezoelectric and therefore nonpolar.
1,2
This paper investi-
gates the nature of this surprising effect in disordered films.
The origin of polarity in ionic solids is generally associ-
ated with crystallinity.
3,4
Nevertheless, quasiamorphous films
with no crystallites at all are pyroelectric, whereas partially
crystallized films amorphous matrix containing nanocrystal-
lites are not. This agrees with the fact that nanocrystalline
BaTiO
3
with grains smaller than 30 nm is usually not
ferroelectric
5–10
with the exception of some special cases.
11,12
This also implies that the quasiamorphous phase is not a
transient state between the amorphous and the crystalline
phases and that the origin of polarity in quasiamorphous
films remains to be determined. Polarity in quasiamorphous
films is particularly surprising because amorphous BaTiO
3
is
not expected to form a stable network of covalent bonds it is
85% ionic
13
and must be viewed as a dense random pack-
ing of hard spheres,
2
which is kinetically stabilized. It is thus
expected to be neither polar nor thermally stable. It has been
suggested that the steep temperature gradient employed dur-
ing formation of the quasiamorphous films generates a gra-
dient of mechanical strain. The latter causes orientational
ordering of hypothetical crystal motifs the regions with
short-range order on the scale of one of two unit cells, i.e.,
below the detection limit by TEM and, consequently, the
macroscopic polarity of the quasiamorphous phase.
1
How-
ever, the nature of these hypothetical crystal motifs that ef-
fectively function as dipoles, as well as the degree of their
ordering remain uncertain.
Elucidating the microscopic origin of polarity in qua-
siamorphous BaTiO
3
requires direct measurement of the lo-
cal bonding geometry of the Ti atom. Therefore x-ray-
absorption fine-structure XAFS spectroscopy was chosen
as the most appropriate technique for this purpose. The
XAFS technique is sensitive to short-range order only and is
thus particularly valuable for structural determination when
long-range periodicity is absent. For BaTiO
3
, XAFS mea-
surements directly confirmed previous experimental
14,15
and
theoretical
16
results that Ti atoms are displaced in the 111
direction of the TiO
6
octahedral bonding units within a very
wide range of temperatures
17
and grain sizes.
18
Similar be-
havior of the Nb 111 displacement in KNbO
3
was demon-
strated by XAFS as a function of pressure.
19
For the current studies, 100–180-nm-thick quasiamor-
phous BaTiO
3
films were prepared by pulling as-deposited
amorphous RF sputtered films through a temperature gradi-
ent with peak temperature of 600 °C. A compressive stress of
= 2.0–2.2 GPa developed in the films which were passed
through the temperature gradient. The details of the experi-
mental procedure are given in Ref. 1. For comparison, three
other types of samples were measured as well: as-deposited
amorphous films; polycrystalline films prepared by annealing
as-deposited films under isothermal conditions at 600 °C;
and partially crystallized films amorphous matrix with
clearly detectable nanocrystallites. The presence of the
nanocrystallites in the partially crystallized samples was de-
tected by electron diffraction performed in the transmission
electron microscope TEM Phillips CM-120. No indication
of crystallites as small as three to five unit cells was found in
the quasiamorphous films either by electron diffraction Fig.
1a or by using different x-ray-diffraction XRD scanning
protocols including 2 scans at fixed , and pole figure mea-
surements. The XRD patterns -2 scans, Rigaku D-Max/B
diffractometer of the quasiamorphous films and the as-
deposited films were indistinguishable Fig. 1b. Scanning
PHYSICAL REVIEW B 71, 024116 2005
1098-0121/2005/712/0241165/$23.00 ©2005 The American Physical Society 024116-1