Correlation between Polymorphism and Optical Bandwidths in
AgNd(WO
4
)
2
C. Colo ´n,
†
A. Alonso-Medina,
†
F. Ferna ´ndez,
‡
R. Sa ´ez-Puche,
§
V. Volkov,
|
C. Cascales,*
,|
and C. Zaldo*
,|
Departamento de Fı ´sica Aplicada and Departamento de Quı ´mica Industrial y Polı ´meros. E.U.I.T.
Industrial, UniVersidad Polite ´ cnica de Madrid, Ronda de Valencia 3, 28012 Madrid, Spain,
Departamento de Quı ´mica Inorga ´ nica, Facultad de Ciencias Quı ´micas, UniVersidad Complutense de
Madrid, AVenida Complutense s/n. 28040 Madrid, Spain, and Instituto de Ciencia de Materiales de
Madrid, Consejo Superior de InVestigaciones Cientı ´ficas, c/ Sor Juana Ine ´ s de la Cruz,
3. Cantoblanco, 28049 Madrid, Spain
ReceiVed August 10, 2005. ReVised Manuscript ReceiVed October 17, 2005
The preparation and characterization of two polymorphic phases of AgNd(WO
4
)
2
are described. The
high-temperature phase of AgNd(WO
4
)
2
is prepared as a polycrystalline powder and as a single crystal.
X-ray diffraction analysis indicates that the crystal has at 300 K the tetragonal symmetry of the space
group (SG) I4 h (No. 82), with two independent crystal sites, 2b and 2d, for Nd
3+
cations and structural
disorder around them. The 5 K ground state optical absorption of this tetragonal crystal clearly differs
from that corresponding to the monoclinic SG C2/m (No. 12) ordered phase found in polycrystalline
samples prepared below 800 °C. Four times larger bandwidths and a weaker crystal field (CF), that is,
lesser CF splitting for all Nd
3+ 2S+1
L
J
manifolds, are observed for the tetragonal phase. Well-defined S
4
polarization rules have been determined in the tetragonal phase, and then the observed 99 Nd
3+
energy
levels were labeled with the appropriate Γ
7,8
or Γ
5,6
irreducible representations. A detailed Hamiltonian
of 26 free ion and CF parameters have been used in the simulation of the phenomenological energy
levels and associated wave functions of the 4f
3
configuration of Nd
3+
in the tetragonal AgNd(WO
4
)
2
single crystal, with final σ ) 12.6 cm
-1
. The validity of the above set of CF parameters and wave functions
has been established through the good reproduction of the thermal variation of the measured anisotropic
paramagnetic susceptibility . As a result of this simulation it is shown that the larger bandwidths of the
tetragonal phase contain nonresolved contributions from the two Nd
3+
sites. A method to control
overheating events is proposed on the basis of the nonreversibility of the tetragonal phase into the
monoclinic one.
I. Introduction
Polymorphism is the early, best-known structural charac-
teristic of double tungstate (DT) compounds with stoichi-
ometry XT(WO
4
)
2
[X ) alkaline and Ag monovalent cations;
T ) In, Bi, and rare-earth (RE) trivalent cations]. In fact,
depending on the X-T ionic radii relationship and on the
temperature of the synthesis (or crystal growth), different
phases have been found, including the tetragonal scheelite-
type CaWO
4
, orthorhombic, monoclinic or even triclinic
symmetries.
1
The strong anisotropy (leading to very large optical cross
sections) and large lanthanide impurity acceptance of the
monoclinic [space group (SG) C2/c] KY(WO
4
)
2
and KGd-
(WO
4
)
2
single crystals with “ordered” cation environments
have promoted their use as efficient laser hosts mainly for
Nd
3+ 2,3
and Yb
3+ 4
ions. Moreover, these crystals are also
known as efficient Raman shifters.
5
These applications have
promoted the study of their properties. The DT single crystals
with tetragonal crystalline structure (“disordered” materials
with regard to their actual cationic distribution) are also
known as laser Raman shifters,
6-8
and they received initial
attention as laser crystals.
9,10
These early laser experiments
evidenced larger bandwidths and lower peak optical cross
sections than those observed in the ordered phases prepared
* To whom correspondence should be addressed. E-mail: ccascales@
icmm.csic.es (C.Cascales); cezaldo@icmm.csic.es (C.Zaldo).
†
Departamento de Fı ´sica Aplicada, Universidad Polite ´cnica de Madrid.
‡
Departamento de Quı ´mica Industrial y Polı ´meros, Universidad Polite ´cnica
de Madrid.
§
Universidad Complutense de Madrid.
|
Instituto de Ciencia de Materiales de Madrid.
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6635 Chem. Mater. 2005, 17, 6635-6643
10.1021/cm051789l CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/23/2005