Journal of Modern Physics, 2013, 4, 12-17
doi:10.4236/jmp.2013.45B003 Published Online May 2013 (
Laser Сonoscopiс Research Technique For Single
Crystals LiNbO3: Mg
O. Y. Pikoul1, N. V. Sidorov2, M. N. Palatnikov2, O. V. Makarova2
1Far Eastern State University of Transportation, Khabarovsk, Russia
2Tananayev Institute of Chemistry and Technology of Rare Elementsand, Mineral Raw Materials, Apatity, Russia
Received 2013
Optical homogeneity and subtle features of structural distortions in a series of single crystals of lithium niobate (LiNbO3)
of congruent composition doped Mg2+ [0.01 - 5.5 mol%] Investigated by laser conoscopic method using radiation
He-Ne laser (λ = 632.8 nm ) of less than 1 mW.
Keywords: Lithium Niobate; Conoscopic Patterns; Optical Homogeneity
1. Introduction
Initially, the conoscopic patterns obtained with a polar-
izing microscope were used in mineralogy in order to
identify minerals based on the data on crystal symmetry
and orientation [1]. Conoscopic pattern informativity
provides for the possibility to determine orientation and
nature of optical indicatrix, measure an angle between
the optical axes of a biaxial crystal, determine an optical
sign of the crystal, detect optical axes dispersion, identify
qualitative and quantitative changes in the optical indica-
trix in response to external action, etc. [1-11]. Cono-
scopic method is one way to analyze the properties of
optical crystals, which allows to determine their func-
tionality, and which has long and successfully used in
scientific research and a variety of optical devices.
In this article, it is proposed to obtain conoscopic pat-
terns using an optical system where diverging laser ra-
diation is let pass through an anisotropic crystal placed
between the polarizer and analyzer, rather than using a
polarizing microscope. The pattern on the screen is re-
corded by a digital camera and displayed on a computer.
Where the point symmetry group of the crystal is al-
ready known, the practical importance of such cono-
scopic studies lies in detection and analysis of various
distortions of optical elements of actual crystals [10,11].
Modern industrial technology for growing single crystals
of lithium niobate doped with different dopants influ-
encing the composition of the crystals and physical
properties, allowing them to adjust to a wide range. One
of the main criteria is the quality of produced crystals of
optical homogeneity. Use as a dopant cations Mg2+ pro-
vides lower interfering effect photorefraction in lithium
niobate single crystals, however, can complicate the
structure is strong enough crystal and, as a consequence,
lead to the optical inhomogeneity. The possibility of ob-
serving conoscopic patterns of large-scale appears when
you use the laser system in which divergent wide-beam
radiation is obtained through the diffuser placed in front
of the front face of the crystal [5].
The significant size of the image allows you to per-
form a detailed analysis of subtle features of the struc-
tural distortions in the crystal, as in the center of the field
of view, and in the peripheral region of the conoscopic
The development of laser conoscopic method also
relevant to studies of thin structural distortions, arising in
photorefractive crystals, for the detection and investiga-
tion of subtle features of structural distortions, as well as
micro-and nanostructures, inevitably present in doped
single-crystal materials [12].
In this paper, a laser conoscopic method investigated
the fine features of structural distortions in a series of
single crystals of lithium niobate (LiNbO3) congruent (R
= Li/Nb = 0.946), doped with Mg2+, characterized by low
effect photorefraction (optical damage), promising as
materials for electronics [12,13]. Used as a relatively
lightly doped crystal LiNbO3:Mg[0.01 - 1.5 mol%], аnd
crystals with a high concentration of Mg2+ (LiNbO3:Mg
[3.0 - 5.5 mol%]), рhotorefractive effect in which is
almost equal to zero [13].
2. Еxperimental Technique
The test samples were cut from a single crystal boule
grown in the direction of the Z (the polar axis of the
crystal). In order to evaluate the optical homogeneity of
single crystal boules grown samples were cut from dif-
Copyright © 2013 SciRes. JMP
ferent parts of the boule.
Тhe cylindrical portion of the boule was cut into
transverse disks from which the samples were cut into
parallelepipeds ~ 6×4.7 mm3 edges parallel crystal-
lophysical axes X, Y, Z, respectively. Faces of the paral-
lelepiped and plates carefully polished. Methods of crys-
tal growth and preparation of samples for research in
more detail in [12].
When conducting an experiment to observe cono-
scopic patterns of optical crystals with optical system
(Figure 1), consisting of a source of radiation, polarizer,
diffuser, crystal, analyzer and the screen, which allows
you to receive conoscopic pattern of considerable size
(0.5 meters or more).
Investigated crystal plate is located on the two-coor-
dinate optical mobile stand that allows you to scan the
entire plane with a laser beam entrance face and get a
series of conoscopic patterns. In the experiments, radia-
tion He-Ne laser (λ = 632.8 nm) power not exceeding 1
mW in order to minimize the possible impact of the
photorefractive effect on conoscopic pattern.
To investigate the defect micro and macrostructure
single-crystal LiNbO3:Mg was applied high-performance
and flexible image analyzer Thixomet ®, based on mod-
ern hardware (microscope of Carl Zeiss - Axio Observer)
and software [14].
3. Experimental Results and Discussion
Conoscopic picture perfect uniaxial crystals obtained
with linearly polarized radiation is well known, ex-
plained and described in the literature [1,6,10,11].
This picture of the propagation of a diverging beam of
light along the optical axis is composed of concentric
rings centered at the output of the optical axis. Rings
superimposed on the characteristic intensity distribution -
black "maltese cross" In this case, each ring is the same
line of the phase shift and the cone of rays with the same
angle of incidence at the coincidence of the axis of the
conical radiation beam with the optical axis of the crys-
Izohrom form depends on the orientation of the optical
axis with respect to the input face of the crystal. With
some of the angle between the optical axis and the nor-
mal to the entrance face of the ring transformed into el-
lipses. With significant corners form izohrom approach-
ing hyperbole.
Branch of the "maltese cross", consisting of two
isogyre minimum intensity, intersect in the center of the
visual field, perpendicular to each other and coincide
with the axes of transmission of the polarizer and the
The characteristic feature of arising anomalous optical
biaxiality in which there is a deformation of the optical
indicatrix of the crystal is the rupture of black “maltese
cross” in two parts with the enlightenment in the center
of the visual field.
In our experiments, for samples LiNbO3:Mg[0.01 - 1.5
mol%] were observed conoscopic pattern of the stan-
dard form, in which the black “maltese cross”preserves
the integrity of the center of the field of view, and
isochromes have the form of concentric circles.
For samples with the same thickness in the direction of
the optical axis, but with a different concentration of
dopant Mg, for example, LiNbO3:Mg [0.5 mol%] and
the LiNbO3:Mg [1.0 mol%] general view of conoscopic
patterns has coincided Figure 2 with preservation of di-
ameter ring-izohrom.
Conoscopic pattern crystal LiNbO3:Mg [0.01 - 1.5
mol%] and LiNbO3:Mg [3.0 - 5.5 mol%] are very dif-
Figure 1. Diagram of the optical sign identifying system:
1–He-Ne laser; 2–polarizer; 3–diffuser; 4–investigated crys-
tal plate; 5–analyzer crossed with polarizer; 6–screen;
Figure 2. Conoscopic pattern of single crystals of LiNbO3:
Mg:(a)–[0.5 mol%]; (b)–[1.0 mol%].
Copyright © 2013 SciRes. JMP
When scanning the plane entrance face with a rather
high concentration of the impurities LiNbO3:Mg [3.0
mol %], in addition to standard patterns and similar in
appearance (Figure 3(a)) were observed and the dis-
torted conoscopic pattern (Figures 3(b)-(e)).
On conoscopic pattern (Figure 3(b)) black“maltese
cross” cut in half with the enlightenment in the center
field of view. The azimuthal direction of displacement on
parts of “maltese cross” amounts to the angle of ~ 1-
1clockwise from the vertical. Isochromen keep integ-
rity, but some extend in the direction of displacement of
the fragments of the cross and take the form of ellipses
with the attitude of the minor and major axes of ~ 0.9:1.
On conoscopic pattern (Figures 3(c), (d), (e)) the
black “maltese cross” in the center of the field of view,
on the contrary, is an integer, and retain the form
isochromen rings. However, in the periphery of the field
of view at a considerable angular distance from the cen-
ter of the picture, starting with a 5 - 6th isochromen, in
only one branch of the “maltese cross” is observed by
imposing additional interference structure. While the re-
maining three branches “maltese cross” retain their usual
All observed by scanning the plane entrance face
conoscopic pattern LiNbO3:Mg [5,0 mol.%] Characteris-
tic of uniaxial crystals, as indicated by the black “maltese
cross” on the background of the rings-izohrom (Figures
4(a)-(e)). However, on some conoscopic patterns on a
small angular distance from the center of one of the four
branches of the “maltese cross” there is the imposition of
additional distinct interference fringes (Figures 4(b)-(e)).
(a) (b)
(c) (d)
(e) (f)
Figure 3. Conoscopic pattern of single crystal of LiNbO3:
Mg:[3.0 mol%]
Conoscopic pattern samples with the highest concen-
tration of the dopant LiNbO3:Mg[5.5 mol%]. Charac-
teristic of uniaxial crystals (Figures 5(a)-(e)), but at
some point the input face light up with some pictures of
conoscopic anomalies. One type of anomaly is a superpo-
sition of additional interference pattern at an angular dis-
tance from the center, corresponding to a 3 - 4th isochromen,
inone branch of the “maltese cross” (Figures 5(b)-(c)).
(a) (b)
(c) (d)
(e) (f)
Figure 4. Conoscopic pattern of single crystals of LiNbO3:
Mg[5.0 mol%].
(a) (b)
(c) (d)
(e) (f)
Figure 5. Conoscopic pattern of single crystals of LiNbO3:
Mg [5.5 mol%].
Copyright © 2013 SciRes. JMP
Another kind of anomaly is manifested as additional
interference pattern, but in the center of the field of view
of a conoscopic pattern on the background of black
crossing branches “maltese cross” It should be noted that
the conoscopic patterns of each of the three samples
LiNbO3:Mg[3.0 - 5.5 mol%] with circular polarizer and
the analyzer, which allows you to remove beclouding
“maltese cross” have a standard form of rings and show
no noticeable distortion (Figures 3(f), 4(f) and 5(f)).
Results conoscopic method study of crystals LiNbO3,
doped Mg cations to varying concentrations of interest,
grown under different conditions, show that lightly doped
lithium niobate samples containing Mg [0.003 - 1.0
mol %] have a high optical homogeneity.
Analysis of the effect of the dopant Mg on the form
the conoscopic pattern LiNbO3:Mg showed that when the
concentration of Mg dopant in the samples with the same
geometric parameters of the scale of the conoscopic pat-
tern, intensity distribution, shape and size of the “maltese
cross” and izohrom saved.
Conoscopic technique to study samples of lithium nio-
bate with the content of Mg [3.0 - 5.5 mol%] suggests
that a stronger doping Mg cations while maintaining
overall uniaxial crystal leads to the appearance of local
birefringent inclusions, which are recorded in the form of
additional interference pattern on the background main
conoscopic pattern in the center of the field of view, and
in its peripheral region.
Small anomalous biaxiality in a bounded domain is
registered for a sample LiNbO3:Mg[3.0 mol%], which
is confirmed by the break and enlightenment “maltese
cross” in the center of the conoscopic pattern of the crys-
The differences in conoscopic patterns of single crys-
tals of LiNbO3: Mg [0.01 – 1.5 mol%] аnd LiNbO3:Mg
[3.0 - 55 mol%] сan be explained as follows.
Feature of lithium niobate single crystals doped with
cations Mg2+ at relatively high ( 3 mol%) dopant con-
centration is uneven of impurity [13, 14] and, therefore,
the appearance of growth bands associated with gradients
of dopant concentration, as in plane perpendicular to and
in the plane parallel to the growth axis (Figures 6-7).
Banding is accompanied by the growth of microde-
fects in the form of dislocations, microdomains of do-
main walls and block structure, especially in the high
impurity concentration gradients at the boundaries of
growth bands (Figure 8).
Growth bands, the gradient of the impurity concentra-
tion, the concentration of microdefects leads to a local
change of the elastic characteristics of the crystal and
appearance of mechanical stress [14], locally distorting
the optical indicatrix of optically uniaxial crystal.
This leads to a distortion of the conoscopic patterns
(Figures 3-5). Moreover, the maximum distortion is ob-
served for the conoscopic patterns on the borders growth
bands, where the concentration of structural defects and
the dopant concentration gradients are maximized.
In the series of crystals investigated by us striation of
samples, in general, decreases with increasing impurity
concentration from 3.0 to 5.5 mol% (Figures 6-7). In
the same row is somewhat reduced degree of distortion
conoscopic patterns (Figures 4-5).
Thus, the deficiency of the crystal associated with the
inhomogeneity of admixture disposition, passes through
a maximum at a certain concentration of ~3 mol%
Mg2+. The latter may be due to a change in the mecha-
nism of admixture disposition when changing dopant
concentration [13, 15].
In particular, the research methods of microanalysis
found a reduction ratio R=Li/Nb ( 0,94) at a concentra-
tion in the crystal Mg2+ ~ 3% [16]. With such a concen-
tration of Mg2+ defects NbLi (cation Nb5+, are in the posi-
tions of lithium of the ideal structure of the stoichiomet-
ric composition) completely forced out in the cation
sublattice [13].
(a) (b) (c)
Figure 6. Bands of crystal growth LiNbO3: Mg in the plane
perpendicular to the growth: (а)–[3,0 mol.%]; b–[5,0
mol.%]; c–[5. mol%].
(a) (b) (c)
Figure 7. Bands of crystal growth LiNbO3: Mg in the plane
parallel to the growth:(а)–[3.0 mol%];(b)–[5.0 mol%];(c)
– [5.5 mol%].
(a) (b) (c)
Figure 8. Microdefects at the boundaries of crystal growth
bands LiNbO3: Mg in the plane perpendicular to the axis of
growth:(a)-3.0 mol%];(b) -[5.0 mol%];(c)-5.5 mol%].
Copyright © 2013 SciRes. JMP
At a concentration of Mg2+ > 3% are replaced by base
cations Li+, accompanied by an increase of defects VLi
(vacant oxygen octahedrons, which in an ideal structure
stoichiometric composition should be located the cations
Li+) [13,16].
When approaching the value R = Li/Nb to the value of
0.84 [Mg2+ 8%], corresponding to the stability bound-
ary LiNbO3 phase in the phase diagram [13], the cations
Mg2+ are included in both (lithium and niobium) position
of the ideal structure of stoichiometric composition with
a simultaneous decrease concentration of compensating
defects VLi [13,16].
4. Conclusions
Results conoscopic method study of lithium niobate
crystals doped with cations Mg2 +, show that samples
containing Mg2+ (0.01 - 1.5 mol%), even cut from dif-
ferent crystals, grown under different conditions, have
high optical homogeneity.
And the scale of the conoscopic pattern, intensity dis-
tribution, shape and size of the “maltese cross”and
izohrom are fully maintained for samples with the same
geometric parameters.
Stronger doping cations Mg2+ (3.0 - 5.5 mol%),
while maintaining overall uniaxial crystal leads to the
appearance of local birefringent inclusions, which are
recorded as additional interference pattern on the back-
ground main conoscopic pattern in the center of the field
of view and in its peripheral region.
A small anomalous biaxiality in a limited region is reg-
istered for a sample containing Mg2+ 3.0 mol%, which
confirmed the break and enlightenment “maltese cross”in
the center of the conoscopic pattern of the crystal.
When the concentration of the dopant appears uneven
of impurity, accompanied by the appearance of growth
bands and microdefects in the form of dislocations, mi-
crodomains of domain walls and block structure, espe-
cially in the high impurity concentration gradients at the
boundaries of growth bands.
Defect structure of the crystals, associated with ir-
regular occurrence impurities leads to a local change of
the elastic characteristics of the crystal and the appear-
ance of mechanical stresses that cause distortion of the
conoscopic patterns. And the imperfection of the crystal
samples and the degree of distortion conoscopic patterns,
generally decrease with increasing impurity concentra-
tion from 3.0 to 5.5 mol%.
Thus, laser conoscopic research technique on the pro-
posed scheme in this paper to identify subtle changes in
the optical properties of the crystal at its doping. Cono-
scopic same study samples under a polarizing micro-
scope, in which the light source is used as a incandescent
lamp, make it possible to observe a picture of the crystal
as a whole, without detailing the fine features of the
structure in the form of local distortions.
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