Lattice dynamics and high-pressure Raman scattering studies of ferroelectric K
2
MgWO
2
(PO
4
)
2
M. Maczka,
1
W. Paraguassu,
2
A. G. Souza Filho,
3
P. T. C. Freire,
3
A. Majchrowski,
4
J. Mendes Filho,
3
and J. Hanuza
5
1
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw 2, Poland
2
Departamento de Fisica, Universidade Federal do Maranhão, São Luis-MA, 65085-580, Brazil
3
Departamento de Fisica, Universidade Federal do Ceara, P.O. Box 6030, Fortaleza-CE, 60455-970, Brazil
4
Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Street, 00-908 Warszawa, Poland
5
Department of Bioorganic Chemistry, University of Economics, 53—345 Wroclaw, Poland
Received 11 April 2008; published 26 August 2008
K
2
MgWO
2
PO
4
2
ferroelectric single crystal, derived from KTiOPO
4
KTP by replacement of two Ti
4+
ions with Mg
2+
and W
6+
cations, was investigated at ambient pressure by micro-Raman scattering and infrared
spectroscopies with a focus on polarization properties of vibrational modes. These results were analyzed based
on classical lattice dynamics calculations which allowed to propose the normal-mode symmetries and assign-
ments. In addition to the ambient pressure studies, high-pressure Raman scattering studies were performed.
These studies showed the onset of a reversible first-order phase transition near 2.0 GPa which is associated
with marked softening of the low wave number mode corresponding to motions of the K
+
ions. The structural
changes at the phase transition are relatively weak and either the crystal exhibits transformation from the
ambient pressure P1 to a high-pressure monoclinic structure or there is no symmetry change at the phase
transition, i.e., this transition is isosymmetric.
DOI: 10.1103/PhysRevB.78.064116 PACS numbers: 77.80.Bh, 78.30.Hv, 77.84.Dy
I. INTRODUCTION
One of the most outstanding and widely used nonlinear-
optical phosphates is KTiOPO
4
KTP.
1–3
KTP undergoes a
second-order displacive-type structural phase transition at
1206 K from the high-temperature paraelectric Pnan phase to
a low-temperature ferroelectric Pna2
1
phase.
4
It was reported
that the change of space group is mainly determined by the
behavior of the K
+
cation but the TiO
6
octahedron also plays
a role in this phase transition.
5
A soft optical mode was ob-
served but its temperature dependence was complicated due
to a coupling to a relaxation mode.
5,6
Pressure-dependent
Raman and x-ray studies of KTP revealed a first-order phase
transition at 5.5–5.8 GPa.
6–8
It was shown that this transition
is purely displacive and it does not lead to any symmetry
change, i.e., this transition is an isosymmetric phase
transition.
8
It was also shown that KTP may exhibit a second
phase transition near 10 GPa but nature of this transition was
not established.
7
The former studies showed that properties of KTP-type
family of compounds can be greatly modified by replace-
ment of K
+
Ti
4+
or PO
4
3-
by Rb
+
, Cs
+
, Tl
+
Zr
4+
, Sn
4+
,
Ge
4+
or AsO
4
3-
, GeO
4
4-
, SiO
4
4-
.
1,5,9
In particular, such re-
placement has a significant impact on lattice instabilities and
mechanism of the phase transition. For instance, replacement
of K
+
by Tl
+
ions led to the appearance of a very clear soft
mode typical for a displacive transition,
5
whereas the second-
order phase transition in germanate analogs of KTP was
shown to be both displacive and order-disorder in nature.
9
Although the discussed above replacement of ions leads
to significant changes in properties, the parent orthorhombic
structure Pnan is preserved. When, however, Ti
4+
ions are
replaced with two ions of different valence state such as W
6+
and Mg
2+
or Ni
2+
, Co
2+
, Fe
2+
, Mn
2+
, Cd
2+
, the crystal struc-
ture is also modified.
10–12
For instance, the high-temperature
structure of K
2
MgWO
2
PO
4
2
KMWP, which is derived
from KTP by replacement of two Ti
4+
ions with Mg
2+
and
W
6+
cations, is tetragonal, with the space group P4
1
2
1
2.
10–12
The polymorphism of this crystal is also much richer than
that of KTP. The previous calorimetric, ionic conductivity,
and dielectric studies showed that KMWP undergoes succes-
sive structural transitions at T
3
= 537, T
2
= 535, and
T
1
=436 K from the P4
1
2
1
2 tetragonal phase into the
P2
1
2
1
2
1
orthorhombic, P2
1
monoclinic and P1 triclinic
phases.
10–12
Below 537 K, KMWP displays ferroelastic prop-
erties, and below 535 K, in addition, ferroelectric
properties.
10–12
The previous studies showed also that
KMWP undergoes two additional phase transitions at
T
4
=637 and T
5
= 782 K. It was suggested that these transi-
tions occur without any alteration of the tetragonal
symmetry.
10–12
Recently, temperature-dependent studies of
acoustic properties of KMWP were carried out using Bril-
louin technique.
13,14
These studies revealed that the phase
transition at T
4
has an order-disorder nature whereas the
phase transitions at T
2
and T
1
have both order-disorder and
displacive nature.
13,14
They also showed that the phase tran-
sition at T
2
is induced by instability of the soft optic mode.
14
The presented examples show that in order to understand
phonon and structural properties, and obtain insight into the
origin of lattice instabilities in the family of compounds re-
lated to KTP, it is necessary to perform studies of these ma-
terials as a function of composition, pressure, and tempera-
ture. In contrast to KTP and its analogs crystallizing in the
orthorhombic structure, the phonon properties of the crystals
derived from KTP by replacement of two Ti
4+
ions with two
ions of different valence states are unknown and the under-
standing of the nature of lattice instabilities is still very far
from being satisfactory. We report therefore in this paper
detailed ambient pressure polarized IR and Raman studies,
results of lattice dynamics calculations, and pressure-
dependent Raman studies of a representative member of this
family of compounds, KMWP. The obtained results indicate
that KMWP exhibits a first-order structural transformation at
PHYSICAL REVIEW B 78, 064116 2008
1098-0121/2008/786/06411611 ©2008 The American Physical Society 064116-1