Growth and characterisation of RbTiOPO4. A new self-frequency doubling crystal.

Growth and characterisation of RbTiOPO

₄

:(Nb,Ln).

A new self-frequency doubling crystal

Joan Josep Carvajal Martí

Tarragona, 2003

Abstract (English)

Resumen (Español)

Resum (Català)

Table of contents

Papers

Growth and Characterisation of RbTiOPO

:(Nb,Ln). A New

Self-frequency Doubling Crystal.

Joan Josep Carvajal Martí

Física i Cristal·lografia de Materials

Departament de Química Física i Inorgánica

Universitat Rovira i Virgili

Resumen

Desde que Schawlow y Townes publicaron que el principio del M.A.S.E.R. podía ser extendido

a la parte óptica del espectro electromagnético, y un año y medio más tarde Mainman demostraba el

primer láser, la importancia de éste, para la ciencia y la sociedad, se ha ido incrementando. Hoy, los

láseres forman parte de nuestra vida cotidiana.

Actualmente, el desarrollo de nuevos láseres para almacenamiento de datos y otras aplicaciones

se ha incrementado gracias a la investigación en láseres de semiconductor de banda ancha, generación

de harmónicos por

phase-matching

en cristales de óptica no-lineal,

quasi-phase-matching

en cristales,

fibras ópticas y otras guías de onda, y láseres de

up-conversion

. Los láseres de estado sólido compactos

son útiles en una amplia gama de aplicaciones, como almacenamiento de datos en alta densidad,

proyección de imágenes en color, impresión láser, medicina, substitución de láseres de Ar,

biofluorescencia, comunicaciones submarinas, litografía estereográfica, terapia fotodinámica...

Estos láseres se pueden conseguir por procesos de

up-conversion

, que emiten a una longitud de

onda más corta que la de bombeo. Una alternativa es la conversión de frecuencia por procesos de óptica

no-lineal. Los cristales autodobladores de frecuencia son cristales de óptica no-lineal con posiciones

estructurales ocupadas por iones activos láser, preferiblemente Yb

, que pueden combinar la emisión

láser alrededor de 1

µ

m con la generación de segundo harmónico de la matriz produciendo emisión

verde por autodoblaje de frecuencia.

El objetivo principal de esta Tesis Doctoral ha sido la obtención de cristales de la familia del

KTiOPO

, con excelentes propiedades de óptica no-lineal, dopados con iones lantánidos (Ln).

al Ti en la red cristalina. Mientras que el Nb

sólo sustituye al Ti en la posición Ti(1), debido a un

efecto electrostático más que estérico, los iones lantánidos sustituyen al Ti en los octaedros Ti(1)O

y

Ti(2)O

con la misma probabilidad. Otro aspecto a destacar es la habilidad del material para

autocompensarse eléctricamente a la substitución pentavalente del Nb

por la creación de vacantes de

rubidio. La evolución de esta estructura con la temperatura muestra como la transición de fase de la

estructura ferroeléctrica a la paraeléctrica depende del contenido de Nb y que la sustitución de Ti

por

el par (Nb

-Rb

) estabiliza la fase de alta temperatura de estos materiales.

La caracterización óptica de la matriz ha mostrado que el incremento de la birrefringencia del

cristal posibilita aumentar el intervalo de doblaje de longitudes de onda que se puede efectuar con estos

cristales. La amplia ventana óptica y la eficiencia de conversión a segundo harmónico muestran que

ésta es una buena matriz para albergar iones lantánidos. Estas excelentes propiedades se mantienen

incluso cuando los iones lantánidos se hallan en el cristal. Finalmente, la caracterización

espectroscópica de los iones activos ha mostrado que se puede obtener una emisión eficiente por parte

de los dos iones, Er

y Yb

alrededor de 1.5 y 1.0

µ

m, respectivamente. En el caso del Yb

la banda

amplia de emisión y la elevada vida media obtenidas permiten utilizar estos cristales en aplicaciones

sintonizables y de subpicosegundos.

Todas estas observaciones nos llevan a concluir que con estos cristales se puede obtener un

nuevo material autodoblador de frecuencia.

Palabras Clave:

Óptica No-Lineal, Generación de Segundo Hamónico, Autodoblaje de Frecuencia,

Top- Seeded Solution Growth

, KTiOPO

, RbTiOPO

, Difracción de Rayos X, Difracción de neutrones

en polvo.

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Initial Page

Table of Contents

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Chapter 7

Preface

The Ph.D. work contained in this Thesis was carried out at the group of

Física i Cristal·lografia

de Materials

(FiCMA), in the Department of Physical and Inorganic Chemistry of the Rovira i Virgili

University in Tarragona (Spain) between January 1998 and May 2003, directed by Professor Francesc

Díaz González and Doctor Rosa Mª Solé Cartañá.

Within the framework of this Thesis, we have collaborated actively with the following research

groups: Dr. V.Nikolov at the Bulgarian Academy of Sciences (Sofia, Bulgaria); Dr. C. Zaldo at the

Instituto de Ciencia de Materiales de Madrid

(Cantoblanco, Spain); Prof. X. Solans at the University of

Barcelona (Barcelona, Spain); Dr. J.L. García-Muñoz at the

Institut de Ciència de Materials de

Barcelona

(Bellaterra, Spain) and the Institute Laue-Langevin (Grenoble, France); Prof. J. Fernández at

the Basc Country University (Bilbao, Spain); Prof. B. Boulanger at the Joseph Fourier University

(Grenoble, France); Prof. G. Boulon at the Claude Bernard University (Lyon, France); and Prof. C.F.

Woensdregt at the University of Utrecht (Utrecht, The Netherlands).

This project was possible thanks to generous grants from the

Departament d’Universitats,

Recerca i Societats de la Informació

of the

Generalitat de Catalunya

(2000FI 00633 URV APTIND,

2001BV 00195, 2002BV 00013, 2003BV 00073) and the Rovira i Virgili University (1999PSR-13,

2000M3C-36, 2001M3C-83, 2002M3C-22) and the research projects of the CICyT (TIC96-1039,

MAT99-1077-C02, MAT2002-04603-C05-03, 2FD97-0912-C02, 2001-477,

FIT-070000-2002-461) and CIRIT (1999SGR 00183, 2001SGR 317)

4

Joan J. Carvajal, Rosa M. Solé, Josefina Gavaldà, Jaume Massons, Patricia Segonds, Benoît

Boulanger, Alain Brenier, Georges Boulon, J. Zaccaro, Magdalena Aguiló and Francesc Díaz*

Interest has increased in developing compact green laser sources for high-density

optical data storage and display applications, such as colour projection, laser printing, medicine,

biofluorescence, underwater communications, stereo lithography and photodynamic therapy[1] since the promising investigations into wide-gap diode lasers,[2,3] harmonic generation by phase matching in nonlinear crystals,[4] quasi-phasematching in bulk,[5] optical fibres and other waveguides,[6] and upconversion lasers in crystals[7] and fibres.[8] Using non linear optical processes such as frequency doubling in laser materials is very promising for the realization of

these compact all-solid-state laser sources, but so far only a few non linear crystals doped with

Nd3+ have been used to do this:[1] LiNbO3 (LNB), LaBGeO5, Ba2NaNb5O15, β-Gd2(MoO4)3, YAl3(BO3)4 (YAB), CaY4O(BO3)3 (YCOB) and CaGd4O(BO3)3.

More recently, Yb3+ also appeared as a promising ion in the same range of emission wavelength and several highly efficient Yb-lasing in self-frequency doubling materials have

been reported in YAB, LNB and YCOB. [1] This is due to a very simple energy level scheme of Yb3+ which is made up of only two levels: the 2F7/2 ground state and the 2F5/2 excited state. Moreover, there is no excited state absorption to reduce the effective laser cross-section, no

up-conversion, no concentration quenching and no absorption in the green. Finally, the intense

Yb3+ absorption lines are well suited for laser diode pumping near 980 nm from high-power

* _{Mr. J. J. Carvajal, Dr. R. M. Solé, Dr. Jna. Gavaldà, Dr. J. Massons, Dr. M. Aguiló, Dr. F. Díaz,}

Física i Cristal·lografia de Materials, Universitat Rovira i Virgili, Pl. Imperial Tarraco, 1, 43005, Tarragona (Spain) E-mail : diaz@quimica.urv.es

Dr. P. Segonds, Dr. B. Boulanger

Laboratoire de Spectrométrie Physique, Université Joseph Fourier, Sant Martin d'Hères (France) Dr. A. Brenier, Dr. G. Boulon

Laboratoire de Physico-Chimie de Matériaux Luminescents, Université Claude Bernard, Villeurbane (France) Dr. J. Zaccaro

Laboratoire de Cristallographie, CNRS, Grenoble (France)

** _{This research is supported by the CIRIT (2001SGR317) and CICyT (MAT2002-04603-C05-03 and}

FIT-070000-2002-461). J.J. Carvajal gratefully acknowledges the grant from the Catalan Government (2000FI 00633)

thermal loading of the material during the laser operation.

Since KTiOPO4 (KTP) has been established as unique inorganic non linear crystal for frequency conversion applications[9-12] and as the material of choice for frequency doubling Nd-lasers at 1 µm radiation,[9,13-15] literature reports doping of KTP with lanthanide elements using different techniques. [16] However, the concentration achieved in bulk crystals is limited to 5 × 1017 - 6 × 1018 ions·cm-3, which is weak. Higher concentrations of lanthanide ions have been obtained by thermal diffusion of Ln2O3 layers deposited by laser ablation on KTP surfaces,[17] or implantation and ion beam mixing,[18,19] but damage caused to the sample, the degradation of its surface and the deposition of LnPO4 phases restrict the use of such techniques.

On the other hand, substituting K+ with Rb+ in the KTP structure lead to similar non linear properties for RbTiOPO4 (RTP). Moreover, using Nb5+ as codopant in RTP seems to be a good way to obtain a high enough concentration of Ln3+ to produce an efficient laser emission. [20,21]

In this communication, we report for the first time the crystal growth of RbTiOPO4 doped with (Nb,Yb). We have performed the spectroscopic characterization: we show

absorption and emission spectra and second harmonic generation measurements. The results

predict a favourable disposition for the use of Yb-doped RTP crystals as self-frequency

doubling laser materials.

We obtained single crystals of high optical quality with a concentration of Yb3+ of 1.96

× 1020 ions·cm-3, emitting at around 1µm. Like KTP and isomorphs, this material crystallizes in the orthorhombic system, with the space group of symmetry Pna21. Figure 1 shows a single

crystal of RbTi0.935Nb0.043Yb0.022OPO4 (RTP:(Nb,Yb)) grown by the top-seeded solution growth (TSSG) and a schematic view of its morphology.

Electron probe microanalysis (EPMA) reports that the concentration of Yb3+ is the highest ever obtained in a bulk crystal of the KTP family. Figure 2 shows the homogeneity of

distribution of Nb and Yb along two of the three different crystallographic directions. The

concentration of doping ions decreases the closer we are to the crystal seed, because their

distribution coefficients are less than one. Then, during the growth process, the solution

becomes richer in these ions and their incorporation into the crystal increases. However, there

Er in an octahedrical environment, which is closer to that of Ti , reinforces this

assumption.

The absorption and emission spectra around 1 µm of the RTP:(Nb,Yb) crystal are shown in Figures 3 and 4. The absorption spectra were recorded with the electrical field of the

incident light parallel to the a_, b _and c_{-axis, and were compared with Raman spectra to}

distinguish the vibronic and electronic contributions. The three expected peaks, corresponding

to the splitting of the 2F5/2 multiplet, were found around 903, 955 and 972 nm. The absorption peaks at 903 and 972 nm were the strongest. The absorption spectra show a large splitting of the 2

F5/2 sublevels of about 70 nm, which is one of the largest crystal field, comparable with Yb-doped fluoroapathites.[26] This large splitting indicates that a broadband fluorescence can be obtained, which is interesting for tunability and subpicosecond-pulse generation applications in

the visible and in the infrared domains. The value of the absorption cross section (σa) depended strongly on the polarization of the incident light as it is shown in Figure 3. This means that the

number and positions of the absorption peaks are independent of the polarization but that the

intensity of a gien peal can depend on the polarization. We found that the two external peaks

invert their intensity in the spectra parallel to the b_{- and} c_{-axes and that the spectrum collected}

parallel to the a_{-axis is less intense than the other ones. The emission spectra shown in Figure 4}

were obtained by exciting the sample at 972 nm, and were recorded with the electrical field of

the emitted light parallel to the a_-, b_{- and} c_{-axes. These spectra show the typical emission of}

Yb3+ at around 1 µm wavelength and confirm the broadband fluorescence expected from the large splitting of the absorption spectra, which is very important for population inversion in the

laser emission process. The peaks in the measured spectrum correspond to the emission from

the excited 2F5/2 manifold to the four sublevels of the ground state manifold 2F7/2 located at 972, 1025, 1051 and 1071 nm. The intensities of the spectra again depended on the polarization of

the emitted signal in this order: c _> b _> a_.

We used the Früchbauer-Ladenburg method to calculate the Yb3+ emission cross sections σa,b,c in polarization parallel to a-, b- and c- axes in the RbTi0.935Nb0.043Yb0.022OPO4 crystal, leading to the following expression:

( )

( )

∫

τ π

λ λ =

λ σ

d I I I c n

I

c b a f

c , b , a c

, b , a

3

8 2

4

Eq.1

where λ is the wavelength, Ia,b,c is the -, -, -polarized emission spectrum, n is the refractive

index, c is the vacuum speed of the light, and τfis the spontaneous fluorescence time.

It is important to accurately measure the excited-state lifetime as a further step to

characterising a laser material. It is well known that radiation trapping and total internal

reflection can strongly affect the measured lifetimes, related with the reabsorption of the initial

emission by ions in the ground state, followed by reemission and lengthening of the measured

lifetime. However, these effects can be neglected in our crystals because the concentration of

Yb3+ is not extremely high. We determined the fluorescence lifetime τf by collecting the decay of the luminescence intensity of the strongest peak of the luminescence spectra. The

fluorescence decay is characterized by single exponential behaviour and has a quite long value

of 2.2 ms. Such a long lifetime is favourable to get a high inversion of population under CW

pumping.

We measured the fundamental wavelength of type II angular non critical

phase-matching (NCPM) for second harmonic generation (SHG), λNCPM, of RTP samples doped in different concentration with Nb and/or Yb. According to the crystallographic orientation of

these crystals we were able to study only the x-principal axis which corresponds to the a

-crystallographic one. The relation between the involved principal refractive indices is then:

(

)

(

)

2 2

NCPM z NCPM y NCPM y

n n

n = λ + λ

     λ

Eq. 2

The values of λNCPM are summarized in Table 1. It is not reasonable to establish a precise rule of λNCPM and of the principal refractive indices ny and nz with the concentration

from these measurements. Nevertheless, it appears clearly that the increasing of Yb3+ concentration leads to an increasing of λNCPM, so that RbTi0.993Yb0.007OPO4 has exactly the same NCPM wavelength than KTiOAsO4 (KTA). [27] We also verified that all the samples studied exhibit a conversion efficiency of the order of that of RTP.

In conclusion, we have successfully grown for the first time high optical quality

crystals of RbTiOPO4 doped with Yb3+. These crystals show a broadband fluorescence around 1µm, which is interesting for tunability and sub-picosecond-pulse generation applications in the infrared. The long radiation lifetime of Yb3+ in these crystals is also favourable for achieving efficient power laser emission in this region. Their good non linear optical properties are close

to those of RTP, KTP, and KTA. Thus these crystals are good candidates for

self-frequency doubling applications. We are working on a future research line which can match

with the possibility of co-dope RTP with Er3+ and Yb3+: it is of great importance in optical amplifiers for telecommunication Er3+ is pumped indirectly via Yb3+.

Experimental

Crystal growth: Briefly, single crystals of RTP:(Nb,Yb) were grown by the

TSSG-slow-cooling technique using a vertical tubular furnace controlled by a Eurotherm 902

controller/programmer connected to a thyristor.[21] Solutions of about 160 g were prepared by mixing suitable amounts of Rb2CO3, NH4H2PO4, TiO2, Nb2O5 and Yb2O3, used as initial reagents, in a Pt crucible of 125 cm3. These reagents were decomposed by heating them until the bubbling of NH3, H2O and CO2 was complete and maintaining the solution at a temperature of about 50-100 K above the expected saturation temperature for 3-5 h to homogenise them.

The crystals were grown on RTP:(Nb,Yb) seeds of 5.0 × 1.5 × 5.0 mm along a × b × c

crystallographic axes respectively. The c_{-axis was oriented perpendicular to the surface of the}

solution, displaced 4-5 mm from the rotation axis, and fixed in a growth device that included a

platinum turbine to also stir the solution.[20] The a_{-axis of the two seeds used in every growth}

experiment was always placed in the radial direction of the rotation movement. We determined

the saturation temperature from the growth or dissolution of the seeds in contact with the

surface of the solution. The rotation was kept constant at 65 rpm in all experiments. When the

growth process was complete, we slowly removed the crystals from the solution and decreased

the temperature of the furnace to room temperature to minimise any thermal stress.

Measuring techniques: We analysed the concentration of dopants by EPMA operating

in wavelength dispersive mode in a CAMECA SX-50 electron microprobe. The

crystallographic orientation was realized by an X-ray automatic diffractometer. Optical

absorption was performed with a Perkin-Elmer Lamda 900 Spectrophotometer. A

Glan-Thompson quartz polarizer located before the sample was used in order to eliminate the

possible polarization induced by the optical components of the spectrophotometer and to ensure

that we were working only with a polarized component of the incident light. Luminescence

spectra were recorded by exciting the samples at 972 nm with a Laser Analytical Systems dye

laser that delivered excitation pulses of 10 ns in duration and at a repetition rate of 10 Hz. This

luminescence was analysed with an HRS2 Jovin-Yvon monochromator, with a focal length of

70 cm and equipped with 1 µm blazed grating, and detected by a Hamamatsu R1767 photomultiplier. The radiative lifetime was measured with a LeCroy 9400 digital oscilloscope.

The type II angular non critical phase-matching (NCPM) fundamental wavelengths for second

harmonic generation (SHG) were measured by illuminating the samples with a focused tunable

beam. The laser source was a Continuum Panther OPO pumped by a Continuum SLI-10

YAG:Nd laser which is 10 Hz-repetition rate and 4 ns-FWHM pulse duration. This emitted

between 410 and 2550 nm. An achromatic half-wave plate was used to adjust the polarization of

the incident beam to ensure type II phase-matching. The wavelength of the OPO was controlled

by a CHROMEX 250 SM scanning monochromator.

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Sciences, 1999, 24, 103

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Fig. 1. Single crystal of RbTi0.93Nb0.05Yb0.02OPO4 grown by the TSSG technique and schematic view of its morphology.

Fig. 2. Variation of the concentration of Yb3+ (upper value) and Nb5+ (lower value) in atom % along the b _and c _{crystallographic directions in an internal surface of a}

RbTi0.93Nb0.05Yb0.02OPO4 crystal grown with a seed oriented with the c crystallographic direction normal to the surface of the solution of growth.

Fig. 3. Absorption spectra of Yb3+ in the RbTi0.93Nb0.05Yb0.02OPO4 crystal, corresponding to the 2

F7/2→2F5/2 transitions.

Fig. 4. Emission spectra of Yb3+ in the RbTi0.93Nb0.05Yb0.02OPO4 crystal, corresponding to the 2

F5/2→2F7/2 transitions.

Table 1. Type II angular non critical phase-matching (NCPM) SHG fundamental wavelength,

λNCPM, as a function of chemical composition.

Sample λNCPM± 1 nm

RbTiOPO4 1144

RbTi0.942Nb0.058OPO4 1101

RbTi0.993Yb0.007OPO4 1148

RbTi0.935Nb0.043Yb0.022OPO4 1119

Fig. 1

9

900

950

1000

1050

1100

0

2x10

4x10

6x10

8x10

Absorption cross-section (cm

2

)

Wavelength (nm)

pol // a

pol // b

pol // c

Fig. 3

950

1000

1050

1100

0

2x10

4x10

6x10

8x10

pol a

pol b

pol c

E

m

ission cross-section (cm

2

)

Wavelength (nm)

Fig. 4

10

Phase Transitions in RbTiOPO

Doped with Niobium

J. J. Carvajal, R. Sole´, Jna. Gavalda`, J. Massons, F. Dı´az, and M. Aguilo´*

43005 Tarragona, Spain

Received February 24, 2003. Revised Manuscript Received April 10, 2003

We used X-ray powder diffraction and differential thermal analysis (DTA) to study the effects of temperature on the cell parameters and phase transitions in RbTi1-xNbxOPO4(x )0-0.9) crystals. The linear thermal expansion coefficients were calculated from the results of X-ray diffraction. We determined their Curie temperature (Tc) by the change in the slope of the evolution of thecparameter of the crystals with the temperature, which is a function of the Nb content in the crystals. The transition temperature from RTP orthorhombic to RbTiPO5cubic phase and the decomposition temperature of this cubic phase, which are also functions of the Nb concentration in the crystals, were determined by DTA and X-ray diffraction. Finally, we also studied how the cell parameters of the RTP phase changed as the concentration of Nb in the crystals was increased at room temperature.

Introduction

Rubidium titanyl phosphate, RbTiOPO4 (RTP), is isostructural with KTiOPO4 (KTP). This family of compounds is attractive because of their excellent nonlinear optical properties.1,2_{RTP crystallizes, below} its Curie temperature and at atmospheric pressure, in the noncentrosymmetric Pna21 space group with the lattice parametersa)12.974(2),b)6.494(3), andc)

10.564(6) Å.3

At high temperature, the KTP and RTP structures show a reversible ferroelectric-to-paraelectric phase transition, with a Curie temperature (Tc) in the case of RTP ranging from 1058 to 1102 K.4-7_{After this} second-order displacive phase transition, the crystal structure belongs to the centrosymmetric space groupPnan.8

A phase transition to a cubic phase (called decomposi-tion in the literature) is observed for RTP at 1374 K.9 This cubic phase, with the stoichiometry RbTiPO5, crystallizes in the space group of symmetry Fd3hm, similarly to CsTiPO5.10It seems that this cubic phase is stabilized by valence compensation when Nb5+ sub-stitutes for the pair Ti4+_/Cs+ _{in CsTiAsO}

5.11 In the

literature, however, 1343 K was reported as the tem-perature of incongruent melting of RTP.5,7 _{There is} therefore an incongruity in the results in the literature regarding the decomposition of RTP at high tempera-ture.

We have already studied rare earth (RE3+_{) doping of} KTP in previous papers.12,13 _{The RE}3+ _distribution coefficients were very low, although codoping with Nb5+ enhanced the distribution coefficients of RE3+_.11-₁₃ However, Nb5+ _{is incorporated into the crystals and} affects both the morphology and the physical properties of the materials.12-15

Interest in these crystals as laser materials makes it important to determine their linear thermal expansion coefficients and, thus, their thermal behavior. As is well-known, an important part of the pump power can be converted into heat inside the laser material during operation, so studying thermal effects on the solid-state laser materials is crucial in the design of solid-state laser devices.

In this paper, we determined the linear thermal expansion coefficients of RTP and RTP/Nb crystals and discuss their phase transitions with temperature. We show the different phase transitions of these crystals with temperature, and to make progress in the charac-terization of this host for RE3+_{doping, we clarify the} problems we found in the literature regarding the decomposition of RTP.

Experimental Procedures

Crystal Growth.RbTi1-xNbxOPO4crystals withx)0-0.09 were obtained from high-temperature solutions as described * Corresponding author. E-mail: aguilo@quimica.urv.es.

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(8) Harrison, W. T. A.; Gier, T. E.; Stucky, G. D.; Schultz, A. J. Mater. Res. Bull.1995,30, 1341.

(9) Cheng, L. K.; McCarron, E. M.; Calabrese, J.; Bierlein, J. D.; Ballman, A. A.J. Cryst. Growth1993,132, 280.

(10) Kunz, M.; Dinnebier, R.; Cheng, L. K.; McCarron, E. M.; Cox, D. E.; Parise, J. B.; Gehrke, M.; Calabrese, J.; Stephens, P. W.; Vogt, T.; Papoular, R.J. Solid State Chem.1995,120, 299.

(11) Sole´, R.; Nikolov, V.; Koseva, I.; Peshev, P.; Ruiz, X.; Zaldo, C.; Martı´n, M. J.; Aguilo´, M.; Dı´az, F.Chem. Mater.1997,9, 2745.

(12) Carvajal, J. J.; Nikolov, V.; Sole´, R.; Gavalda`, Jna.; Massons, J.; Rico, M.; Zaldo, C.; Aguilo´, M.; Dı´az, F.Chem. Mater.2000,12, 3171.

(13) Carvajal, J. J.; Nikolov, V.; Sole´, R.; Gavalda`, Jna.; Massons, J.; Aguilo´, M.; Dı´az, F.Chem. Mater.2002,14, 3136.

(14) Carvajal, J. J.; Sole´, R.; Gavalda`, Jna.; Massons, J.; Rico, M.; Zaldo, C.; Aguilo´, M.; Dı´az, F.J. Alloys Compd.2001,323-<sub>324</sub>, 231. (15) Carvajal, J. J.; Sole´, R.; Gavalda`, Jna.; Massons, J.; Aguilo´, M.; Dı´az, F.Cryst. Growth Des.2001,1, 479.

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