Geoscience Reference
In-Depth Information
conditions. However, the synthesis of phosphates was started neither to understand
the peculiarities in nature nor to use them in technology. The first work on the syn-
thesis of phosphates began in the nineteenth century by Berstelius in 1816
[225]
.He
synthesized sodium pyrophosphate artificially for the first time. In 1889, Johnson
obtained LaP
5
O
14
—the first artificial rare earth phosphate
[226]
. However, the
works on the synthesis of inorganic phosphates or their growth as monocrystals
attracted less attention. Even after the beginning of the synthesis of various types of
rare earth phosphates from the early 1940s
[227,228]
up to early 1970s [229,230],
the interest was confined only to the studies concerning the X-ray and morphological
analyses. The first technological phosphate material was Potassium Dihydrogen
Phosphate (KDP) which was first obtained during the 1940s for telecommunication
purpose. The interest on this material continues to progress. Except this, virtually
phosphates did not carry any technological significance for a very long time.
However, a systematic study of rare earth phosphates began in the 1970s with the
developments in the field of communication and the problems associated with the
miniature sources of light, particularly lasers based on dielectrics. Until recently,
these materials contained a low concentration of active ions, which was a major
obstacle in the attempts to use them in optoelectronics.
In 1973, laser beams were obtained from NaP
5
O
14
crystal containing 3.86
10
12
ions of neodymium atoms/cm
3
[231,232]
. Such a high concentration of active ions
led to the further decrease in size of the laser crystal to tens of micromillimeters. An
analogous effect was found in the monocrystals of MNd(PO
3
)
4
(where M
5
Li, Na,
K)
[233,234]
,Na
3
Nd(PO
4
)
2
[235]
,K
3
Nd(PO
4
)
2
[236]
, etc. Prior to the development
of these rare earth phosphates, laser beams were obtained in crystals mostly YAG:
Nd containing only 2
3
3% of active ions mostly as dopants, but not as the stoichio-
metric components. Today it has become a flourishing field in science and technol-
ogy, particularly in the field of fiber-optic communication. In 1976, the authors
[237]
reported a three-dimensional high ionic
conductivity in NASICON
(Na
1
1
x
Zr
2
Si
x
P
3
2
x
O
12
;0
3) type of phosphate which is isostructural with
Na
3
Sc
2
P
3
O
12
[238]
. Later on ionic conductivity was reported in some other rare
earth phosphates like RbNdP
4
O
12
[239]
and CsNdP
4
O
2
[225,240]
. Since NASICON
has some problems like nonstoichiometry, nonavailability of single crystals, and
highly complex structures, there is a search for new materials with simple structures.
The rare earth phosphates containing other elements like sodium, transitional ele-
ments, and some alkaline earth elements are found to be the ideal substitutes for
NASICON
[241]
. In all these materials, the ionic conductivity, the optical proper-
ties, and so on depend mainly upon the type of cation and the crystalline structure.
However, the structures of many new solid electrolytes are yet to be studied because
of the nonavailability of these materials in the form of single crystals. It is well
known that the rare earth phosphates exhibit island, ribbon, layered, ring, and frame-
work types of structures, which are highly distorted with a very wide variation in
P
x
#
#
a
O bond lengths. The inter-isolation of the rare earth polyhedra in most of the rare
earth phosphates and the distortion of the overall structures are mainly responsible
for the presence of luminescence and ionic conductivity in these materials. The acti-
vation energy and ionic conductivity depend upon the radii of the rare earth cation
