Structural and optical properties of LuVO4 single crystals

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Structural and optical properties of LuVO4 single crystals

View the table of contents for this issue, or go to the journal homepage for more 2017 J. Phys.: Conf. Ser. 794 012029

(http://iopscience.iop.org/1742-6596/794/1/012029)

Structural and optical properties of LuVO

4

single crystals

D Dimitrov1, 2, 4*_{, P Rafailov}1_{, V Marinova}2_{, T Babeva}2_{, E Goovaerts}3_{, Y F Chen}4_,

C S Lee5_{, and J Y Juang}4

1_{Institute of Solid State physics, Bulgarian Academy of Science, 72 Tzarigradsko} Chaussee Blvd., Sofia, Bulgaria

2_{Intstitute of Optical Materials and Technologies, Bulgarian Academy of Science,} Acad. G. Bonchev Str., 109, Sofia, Bulgaria

3

Experimental Condensed Matter Physics, Department of Physics, University of Antwerpen, Universiteitsplein, B-2610, Antwerpen, Belgium

4_{Department of Electrophysics, National Chiao Tung University, 1001 Ta Hsueh} Road, Hsinchu, Taiwan

5_{Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh} Road, Hsinchu, Taiwan

E-mail: *[email protected]; [email protected]

Abstract. The synthesis of large single crystals with good optical quality which is a preliminary

condition for the practical applications of these materials frequently is complicated. It is found that large LuVO4 single crystals with high optical quality are possible to be prepared using high temperature solution growth method. It is obtained by X-ray crystallographic analysis that the grown crystals possess centrosymmetric tetragonal structure with the point group symmetry D4h and space group I41/amd (zircon-type structure). The unit cell parameters of a = 7.0236 Å, b = 7.0236 Å, c = 6.2293 Å, volume = 307.30(3) Å3_{are measured. The crystals composition as well} as vanadium oxidation state were measured in order to confirm that the crystal phase is mainly LuVO4. Optical transmission and Raman Spectroscopy are further performed on LuVO4 single crystal to reveal the optical quality and structure details

1. Introduction

Rare-earth orthovanadates (RVO4, where R = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) compounds demonstrate diverse applications in various fields. Due to their exceptional optical properties, as wide optical transparency and large birefringence, RVO4 are potential candidates for optical isolators, circulators, beam displacers, and components for polarizing optics [1].

Up to now, crystal growth process of RVO4 has been attempted by several methods. Slow cooling from solution [2], the Czochralski process [3], top-seeded solution growth [4], the laser-heated pedestal growth method [5], the floating-zone (FZ) method [6] and the micro-FZ method [7] have been already reported. All those methods offer single crystals with excellent quality, however with relatively small sizes. The reason is that RVO4 melts congruently and although the melting temperatures are as high as 1400 °C and above, and consequently large crystals were not obtained.

Among RVO4 single crystals, LuVO4 is one with a particular interest as a laser host material due to the larger absorption cross section near 800 nm and larger emission cross section at 1.064 ȝm in comparison with other vanadate crystals [8]. These features are very desirable for diode pumped solid-state lasers, as they make it feasible to achieve highly efficient pumping and to realize low threshold

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laser operation with high optical-to-optical efficiency. The synthesis of large single crystals with good optical quality which is a prerequisite for the practical applications however is quite complicated.This paper reports the crystal growth process of large, high quality LuVO4 crystals and study of their structural and optical properties.

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2. Experimental details and discussion

2.1. Crystal growth, structural analysis and composition

Single crystals of LuVO4 were grown by high temperature solution growth method. As a first step, polycrystalline LuVO4 was synthesized by the solid state reaction. Stoichiometric amounts of Lu2O3 and V2O5 with minimum purity of 99.99% were mixed, compacted, and then calcinated in oxygen at 650 °C for 48 h. The reacted product was ground and mixed with V2O5 flux in V2O5: LuVO4 =12:1 ratio. The mixture was melted and heated to 1100 °C for 48 h in a platinum crucible of 50 mm diameter and 60 mm depth, covered with a platinum lid. Single crystals were obtained by cooling the solution from 1100 to 700 °C at a cooling rate of 1 °C/h. The residual flux was separated from the as-grown crystals by decanting. The obtained crystals remained on the bottom and the walls of the crucible. The LuVO4 single crystals with high optical quality have tetragonal rectangular shape and typical size of 6 x 8 x 0.5 mm.

Figure 1: Powder XRD of LuVO4 crystal (inset: main peaks assignment)

X-ray Powder Diffraction, using a D8 Bruker powder diffractometer equipped with a Cu anticathode, an incident slit of 0.51, an anti-scatter slit of 0.51, a detector slit of 1mm, and a graphite monochromator in the diffracted beam was performed on LuVO4 powder prepared from single crystal sample. The powder diffraction pattern and main peaks assignments (inset) are shown in figure1.

A single crystal LuVO4 specimen was used for the X-ray crystallographic analysis The X-ray intensity data were measured at room temperature using Bruker-AXS Smart Apex three-circle diffractometer equipped with a CCD detector which verified that the grown crystals possess centrosymmetric tetragonal structure with the point group symmetry D4h and space group I41/amd (zircon-type structure).

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Table 1: Single crystal data

Chemical formula LuVO4 Unit cell dimensions a=7.0236(3) Å

Formula weight 289.91 b=7.0236(3) Å

Temperature 296(2) K c=6.2293(4) Å

Wavelength 0.71073 Å α=90°,β=90°,γ=90°

Crystal system tetragonal Volume 307.30(3) Å3

Space group I 41/amd Z 4

F(000) 504 Density(calculated) 6.266 g/cm3

Absorption coefficient 34.794 mm-1

In this structure, the vanadium atom is in tetrahedral coordination, while the trivalent R cation is coordinated by eight oxygen atoms. The unit cell parameters of a = 7.0236(3) Å, b = 7.0236 Å, c = 6.2293 Å, volume = 307.30 Å3 _{are measured. The structure was solved and refined using the Bruker} SHELXTL Software Package, using the space group I41/amd, with Z = 4 for the formula unit, LuVO4. A selection of the data obtained is shown in table 1.

The single crystals composition and the valence state of vanadium were further verified with EDAX Energy Dispersive Spectrometry and X-ray Absorption Spectroscopy (XAS) respectively. EDAX data (shown on figure 2) revealed some hafnium impurities in the LuVO4 single crystals prepared by our method.

Figure 2: EDS spectra and composition data of LuVO4 single crystal plate

For LuVO4, the oxidation state of vanadium ion is very close to V5+, since its V-L2,3 X-ray Absorption Spectroscopy (XAS) spectra (figure 3) is very similar to the spectra of reference V2O5, V2p and O1s spectra shown on figure 3 top (adapted from [9]) as well as Ni3V2O8 [11], Co3V2O8 [10], and YVO4 [11]. Moreover, if there were V4+, an enhancement of the structures below the main peaks will be observed, as observed for example in beta-Sr0.17V2O5 [12]. However, this effect is absent in our single crystal LuVO4. Therefore, we could conclude that V in LuVO4 is in 5+ state.

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Figure 3: XAS spectra of LuVO4 single crystal

Figure 4: Optical Transmission of LuVO4 single crystal 200-3000 nm

2.2. Optical transmittance and Raman spectroscopy analysis

The LuVO4 single crystals are moreover analysed by Optical spectroscopy and Raman spectroscopy methods.

Optical transmission spectra were measured on crystal polished plate using Cary 5E UV-VIS spectrophotometer. As it seen from figure 4, the obtained crystal shows high transparency in a wide spectral range from 500 to 3000 nm. The absorption bands in the spectral interval 500-750 nm could be due to the hafnium impurities as shown in EDAX/EDS data.

For the Raman spectroscopic measurements single crystals with elongated shapes along the Z axis with naturally grown {100} and {001} surfaces were selected which additionally displayed well shaped {001} surfaces. The Raman spectra were measured in the range of (80 - 1200) cm-1 on a HORIBA Jobin Yvon LabRAM HR visible spectrometer equipped with a Peltier-cooled CCD detector. The 1.95 eV line of a He-Ne laser was used for excitation, the absolute accuracy being 0.5 cm-1_.

Lutetium vanadate LuVO4 crystallizes in a centrosymmetric tetragonal structure with the point group symmetry D4h and space group I41/amd (zircon-type structure). The structure consists of Lu3+_{ions and} VO43-_{tetrahedra that can be approximately regarded as separate units due to the strong internal bonds} within each tetrahedron. Therefore, the lowest-frequency vibrations of LuVO4 comprise translations and librations of the VO43- _{tetrahedrons as rigid units against the Lu}3+_{ions. The higher-frequency phonons,} on the other hand, are almost entirely due to internal vibrations of the VO43-_{tetrahedral complexes.} For the polarized Raman measurements, the notations X (100), Y (010) and Z (001) for the main crystal axes as well as X’ (110) and Y’ (-110) were used. The measured spectra are presented in figure 5 with scattering configurations given in Porto notations. Raman selection rules are clearly discernible despite the considerable depolarisation effects caused by the extremely strong birefringence of the LuVO4 crystal. These effects leading to partial mixing of allowed and forbidden intensity between different Raman modes [Porto] can be minimized by using focusing/collecting optics with small numerical aperture (N.A.) and keeping short the beam path within the simple. These two requirements were successfully met by using a 20× microscope objective with NA = 0.4. The spectra also contain features from impurities or second-phase inclusions that do not obey the Raman selection rules and obviously do not pertain to the vibrational spectrum of LuVO4 crystal. Their frequencies lie in most

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cases far from LuVO4 signal and with one exception they posed no obstacle to the assignment of the LuVO4 modes.

Figure 5: Raman spectra with mode assignment of LuVO4 single crystal in different polarization configurations given in Porto notations. Features

stemming from impurities or

second-phase inclusions are marked with asterisks

3. Conclusion

Large, optically homogenous single crystals of LuVO4 were prepared by using high temperature solution growth method. The single crystal structure quality details as well as composition and oxidation state of vanadium for LuVO4 crystal phase were confirmed. The samples possess high optical transparency in the spectral interval 500-3000 nm.

Acknowledgements

Financial support by the EU 7th_{FP REGPOT project INERA (GA No 316309), Ministry of Science and} Technology, Taiwan under contracts MOST 103-22221-E-009-079 and MOST 105-2811-M-009-044, Bulgarian Science Fund under the project FNI T-02/26 and bilateral collaboration project supported by the Fund for Scientific Research Flanders (FWO), Belgium and the Bulgarian Academy of Sciences.

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