Materials Sciences and Applicatio ns, 2011, 2, 634-637
doi:10.4236/msa.2011.26086 Published Online June 2011 (http://www.SciRP.org/journal/msa)
Study on Formation Mechanism of Defects in
Monoclinic KLu(WO4)2 Crystals
Kunpeng Wang, Jinzhi Fang
National Center for Materials Service Safety, University of Science and Technology Beijing, Beijing, China.
Received December 23rd, 2010; revised January 20th, 2011; accepted May 17th, 2011.
ABSTRACT
The laser host crystals of KLu(WO4)2(KLuW) with large sizes up to 43 × 35 × 16 mm3 have been grown along [111],
[110], b, and c crystallographic directions, respectively, by the top-seeded solution growth (TSSG) slow-cooling method.
The macro defects are observed using optical microscopy. The main defects found were cracks, inclusions, growth
striations, sector boundaries and twin boundaries. The formation mechanism and approaches to reduce or eliminate the
defects have been analyzed.
Keywords: Growth Defects, KLu(WO4)2 Crystals, Top-Seeded Solution Growth
1. Introduction
Double tungstates containing potassium and rare-earth
ions have been extensively investigated because of their
applicability in diode-pumped solid-state lasers. Crystals
such as Nd:KGd(WO4)2 [1,2], Yb:KY(WO4)2 [3], and
KYb(WO4)2 [4] have been recently used as laser materials
or Raman shifters. KLu(WO4)2, which has KGd(WO4)2
related structure, is also an attractive laser host material
and has found application in a wide range of optoelec-
tronic devices [5,6]. More recently KLuW crystals with
large sizes and good optical quality have been grown by
TSSG method in our group [7]. The lattice constants
were determined by using a Four-Circle Diffractometer
to be a = 10.591(5) Å, b = 10.244(6) Å, c = 7.500(3) Å
and β = 130.73(2)˚, which are slightly smaller than those
of KGW [8]. The experimentally grown crystal shapes
developed along different crystalline directions were
compared to the predicted habits, and the influences of
seed orientations on the crystal growth habits, qu ality and
utilization ratio are analyzed by BFDH law and PBC’
theory [7]. The anisotropy thermal properties were care-
fully studied [9].
In the present work, the crystal defects are observed
and analyzed using optical microscopy. Their formation
mechanism and the approach to abate and even eliminate
these defects are presented.
2. Experiments
Single crystals of KLuW with different seed orientations
have been grown by TSSG method and as an example the
as-grown KLuW crystal with seed orientations along b
directions is shown in Figure 1. Three wafers parallel to
the (110), (200), and (020) facets, respectively, are cut
from the crystal and the natural grown planes are cleaned
by absolute alcohol. The Opton polarizing microscope is
used to observe the morphologies of (110), (200), and
(020) facets and all pictures were taken by a video cam-
era.
3. Results and Discussions
3.1. Cracking and Inclusions
Cracks have been observed in crystals with seed orienta-
tions along [111], b, and [110] directions, respectively.
Whereas crystals grown along c directions have good
optical quality and no clear cracks could be observed. As
shown in the Figure 1, the directions of the cracks are
mostly parallel to b and (a + c) directions. We suggest
that when the cooling rate is very fast the anisotropic
thermal expansion coefficients of KLuW crystals results
in cracks along b and (a + c) directions. In addition, the
local crystal lattices are distorted greatly by the solution,
impurity or gaseous solutions and therefore mechanical
stresses are induced. At the same time, the thermal ex-
pansion coefficients of the inclusions are also different
from those of the crystal, which also results in cracks. So
understanding the forming mechanisms of inclusions is
also an important issue for crystal researchers. Various
kinds of inclusions have been observed in the KLuW

Study on Formation Mechanism of Defects in Monoclinic KLu(WO) Crystals635
4 2
Figure 1. The as-grown KLuW crystal with seed orienta-
tions along b directions and severe cracks along a and (a + c)
directions.
crystal. Figure 2(a) shows the melt inclusions observed
at the edge of the (110) face and we can see that they are
large size up to hundreds of microns in length. They may
be caused by the rapid cooling-rate during the last stage
of crystal growth, the segregation phases formed in the
system, or the multi-steps’ meeting in the developing
process. In addition, we suggest that the negative crystal,
which is formed by the negative growth, has been formed
in the melt inclusion shown in Figure 2(b) due to its
regular morphology as the same of the crystal. On the
other hand, the inclusions can also be caused by the im-
purities and insoluble particles. Figure 2(c) shows the
inclusions containing insoluble particles with micrometer
scale observed on the (200) faces. They often result in
the step bunching and so high-purity materials should be
selected to avoid impurities and insoluble particles. The
inclusions described above are often symbiotic in the cry-
stal during crystal growth. However, when the temperature
(a) (b)
(c) (d)
Figure 2. (a) The melt inclusions observed at the edge of the (110) face, (b) the negative crystal formed in the melt inclusion, (c)
the inclusions containing insoluble particles with micrometer scale observed on the (200) faces, (d) cloud-like defects caused
by large quantity of inclusions.

Study on Formation Mechanism of Defects in Monoclinic KLu(WO) Crystals 636 4 2
(a) (b)
Figure 3. (a) The growth striations, sector boundaries and insoluble solid inclusions that are observed on the (110) face, (b)
the twin boundaries formed on the (110) face.
fluctuates greatly, it will increase the inclusion density
and cause cloud-like defects shown in Figure 2(d). The
amount of cloud-like defects is related to the temperature
perturbation and the cloud-like defects can usually be
alleviated by annealing the as-grown crystals at proper
temperature. As described in our former literature [7], the
crystals grown along [111], b, and [110] directions are
often poor due to the macro defects appear and however
no macroscopic defects such as cracks, clouds and scat-
tering particles were detected under the irradiation of a 5
mW green laser beam when grown along c directions.
3.2. Growth Striations and Sector Boundaries
As analyzed in our former literature [7], the (110), (020),
(111) faces are parallel to two or more shortest PBC’s
and so they are well developed. (200) and (202) are the
main growth faces due to their higher attached energy.
The difference of growth rates between these two faces
results in structure mismatches and lattice distortion at
the interface and forms sector boundaries. Impurities can
be easily captured on the sector boundaries, which results
in the formation of inclusions. The growth kinetics fac-
tors and the undulation of the growth conditions result in
growth striations. Figure 3(a) shows the growth stria-
tions, sector boundaries and insoluble solid inclusions
that are observed on the (110) face.
3.3. Twin Boundary
Twins might form as “accidents” during the growth of a
crystal and can also develop after a crystal has grown
during polymorphic transformation or during deforma-
tion. According the Hurl model [5,6], twins always gen-
erate on the edge facets which are anchored to the
three-phase boundary (TPB) at the solid-liquid interface
during the crystal growth. The nucleation free energy is
low and the step’s absorption energy is large at the TPB.
So the twins will energetically favor nucleation at the
TPB and impurities are easy to be absorbed at the TPB.
When the steps absorb impurities, the position and direc-
tion of all future growth will be altered from the normal
orientation. Figure 3(b) shows the formed twins.
3.4. Methods of Reducing Defects
1) Optical quality KLuW crystals can be easier obtained
with seed orientations along c directions than along other
directions.
2) Raw materials should be prepared and purified in
contamination-free environment and then fully mixed.
3) High-quality seeds must be selected to avoid the
extension of the dislocations to the crystal.
4) Keeping suitable growth rate and reducing tem-
perature fluctuation help in decreasing the crystal thermal
stress.
4. Conclusions
KLuW crystals have been successfully grown by TSSG
method. Macro defects have been observed using optical
microscopy. The main defects found were cracks, inclu-
sions, growth striations, sector boundaries and twin
boundaries. Methods of reducing and eliminating the
defects have been discussed.
5. Acknowledgements
Supported by Program for New Century Excellent Tal-
ents in University (NCET-08-0722).
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