Studies on (NaCl)
x
(KBr)
y-x
(KI)
1-y
solid solutions: 1. Lattice
and thermal parameters
G. Selvarajan Æ C. K. Mahadevan
Received: 26 January 2004 / Accepted: 23 September 2005 / Published online: 7 November 2006
Ó
Springer Science+Business Media, LLC 2006
Abstract Solid solutions of NaCl, KBr and KI were
prepared by the melt method for the first time.
Densities and refractive indices of all the prepared
solid solutions were determined and also used for the
estimation of the bulk composition in the crystal.
Lattice parameters and thermal parameters like
Debye–Waller factor, mean square amplitude of vibra-
tion, Debye temperature and Debye frequency were
determined from the X-ray powder diffraction data.
The observed lattice parameters showed the existence
of three phases in solid solutions each nearly corre-
sponds to NaCl, KBr and KI. The thermal parameters
show a highly non-linear bulk composition depen-
dence.
Introduction
A solid solution or mixed crystal is obtained by
crystallizing together two isomorphous crystals like
KCl and KBr with comparable lattice constants. For
ionic crystals like alkali halides complete miscibility is
possible only above a temperature T (K) given by
T = 4.5 S
2
, S being the percentage difference in lattice
constants [1]. S takes a value of 8% for alkali halides at
room temperature. Sixteen pairs of alkali halides are
completely miscible at room temperature and several
got limited miscibility [2].
Like the alkali halides their mixed crystals are also
equally important. Alkali halide mixed crystals are of
the completely disordered substitutional type. Harib-
abu and Subbarao [3] have reviewed the aspects of the
growth and characterization of alkali halide mixed
crystals. Sirdeshmukh and Srinivas [2] have reviewed
the physical properties.
A study of literature has shown that there are
broad miscibility gaps in several binary systems of
alkali halides. NaCl–KCl [4] is an example. Barrett
and Wallace [5] determined the lattice parameters
of Na
x
K
1-x
Cl crystals and found that this system
does not form a continuous series. Nair and Walker
[6] observed that for the extreme concentra-
tion ranges x <0.3andx >0.7theKBr
1-x
I
x
crystals
couldbecharacterizedbyasinglef.c.c.lattice
parameter, while in the intermediate region
three f.c.c. phases characterized by three lattice
parameters.
Alkali halide crystals are widely used as neutron
monochromaters, infrared prisms, infrared transmit-
ters, laser window materials, etc. However, the uses are
limited by their mechanical properties and hence there
exists the need to strengthen these. The mixed and
impurity added (doped) crystals of alkali halides are
found to be harder than the end members and so they
are more useful in these applications. Also, it is a
known fact that alloys are more useful than the pure
simple metals in device fabrications. In addition, mixed
alkali halides find their applications in optical, opto-
electronic and electronic devices. In view of this, it
becomes necessary and useful to prepare binary
and ternary solid solutions regardless of miscibility
G. Selvarajan Á C. K.Mahadevan (&)
Physics Research Centre, S.T. Hindu College, Nagercoil
629 002, India
e-mail: mahad@sancharnet.in
G. Selvarajan
Department of Physics, Sivanthi Aditanar College,
Pillayarpuram, Nagercoil 629 501, India
J Mater Sci (2006) 41:8211–8217
DOI 10.1007/s10853-006-0999-2
123