crystals for laser and luminescence applications

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1 Journal of Crystal Growth 198/199 (1999) Search for impurity phases of Nd :YV crystals for laser and luminescence applications L. Sangaletti, B. Allieri, L.E. Depero *, M. Bettinelli, K. Lebbou, R. Moncorgé Istituto Nazionale per la Fisica della Materia and Dipartimento di Chimica e Fisica per l+ingegneria e per i Materiali, Universita% di Brescia, Via Valotti, 9, Brescia, Italy Istituto Policattedra, Facolta% di Scienze, Universita% di Verona, Ca+ Vignal, Strada Le Grazie, Verona, Italy Physico-Chimie Minerale II, LPCML, CNRS 5620, Université Lyon I, Lyon, France Laboratoire de Spectroscopie Atomique, ISMRA-UPRESA 6084 CNRS, Univerisite& de Caen, Caen, France Abstract Structural investigation of the Y system was undertaken with the aim of characterizing possible segregated phases appearing during the synthesis of pure and doped yttrium vanadate. Nd : YV single crystals obtained by LHPG and flux growth techniques were studied by high spatial resolution micro-raman probe (up to 1 μm in the X ½ plane) spectroscopy. The confocality of microscope coupled to the Raman microprobe allowed us to select the in-depth resolution of the probe. By mapping several single crystals grown with different methods, a segregated phase was identified in the flux grown crystal, about 70 μm below the surface. Micro-Raman maps of powder samples prepared by solid state reaction starting from 4Y and mixtures gave evidence of this phase. The spectral features are very similar to the precipitate found in YV :Nd single crystal Elsevier Science B.V. All rights reserved. PACS: 78.30; S; H Keywords: YV ; Microprobe Raman spectroscopy; Laser; Impurity phases 1. Introduction Optical applications of YV laser crystals can be limited by the presence of precipitates inside the crystals, which generate unwanted scattering of light [1]. * Corresponding author. depero@bsing.ing.unibs.it Microprobe Raman spectroscopy (MPRS) has demonstrated to be a powerful tool for the investigation of impurities and segregations in crystals, which are suitable for different technological applications. In fact, the microprobe techniques offer a characteristic vibrational Raman spectrum, which may be obtained routinely from μm-sized regions of crystalline or amorphous solids, even when these structures are in the form of inclusions /99/$ see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S ( 9 8 )

2 L. Sangaletti et al. / Journal of Crystal Growth 198/199 (1999) within a host material (for a review on Raman microprobe see Ref. [2]). In this case the MPRS has already been employed to investigate micro-inhomogeneities in YV single crystals grown by different methods [3]. The authors have ascribed the observed anomalies of the Raman spectra to oxygen deficiency which causes distortions in the V tetrahedra and makes some bands weaker or produces frequency shifts. However, the structural characterisation of precipitates is far from being completed. Based on the possibility of detecting and identifying on a microscopic scale undesired phases, the present study is aimed to characterise YV single crystals doped with Nd grown by the flux and LHPG method [4] using MPRS technique. In case of the Nd -doped crystals grown from Pb flux an internal precipitate was clearly detected and its vibrational Raman spectrum was also carefully mapped. In order to make an assessment of the structural features of this precipitate, several polycrystalline samples were prepared in the Y system and phases detected in this systems were compared to that found in the single crystal. The comparison of different specimens allowed us to discuss the origin and the structural properties of possible precipitates in yttrium orthovanadate. 2. Experimental procedure Micro-Raman spectra were collected by a Dilor Labram spectrograph. The exciting source was a HeNe laser (632.8 nm) with a power of less than 10 mw at the sample. The microscope was coupled confocally to the spectrograph. A 100 objective with a numerical aperture NA"0.9 and a confocal hole opened at 200 μm were used. Suppression of the exciting line was obtained with a holographic notch filter. All the spectra were measured at room temperature. The diffraction experiments were carried out with a Philips MPD 1830 automated powder diffractometer with graphite-monochromated CuK α radiation in the Bragg Brentano parafocusing geometry. The crystals investigated were grown as described in Ref. [4]. In particular, the crystal where impurities were found was grown by the flux method. The procedure involves the dissolution of the rare earth oxide Y in molten Pb at high temperature with spontaneous nucleation and crystal growth of YV achieved by slow cooling of the solution. The dopant was added as Nd. The concentration of Nd was nominally 3 at % weighed in YV. The crystals grew in the form of blue grey rods elongated in the direction of the crystallographic c-axis. Their size was approximately 4 2 2mm. Powder samples were prepared from 4Y and mixtures. After mixing and crushing in an agate mortar, powders were pressed into pellets under 20 t and fired at different temperatures ranging between 1200 C and 1450 C under one bar of oxygen. The heat treatment was repeated several times in order to obtain a good homogeneity. An excess of was used to compensate vanadium oxide losses at high temperatures. The pellets were fired in alumina crucibles. 3. Results and discussion Fig. 1a shows the optical image collected from the YV single crystal. The focus of the image was set in order to evidence the grey spot at the centre of the image. By moving the microscope sample holder 68 μm lower, the crystal surface was put into the objective focus, as shown in Fig. 1b. In this way a small crystal fragment laying on the sample surface is clearly detectable. These images prove that the opaque spot lies inside the crystal, about 70 μm below the surface. A micro-raman mapping of a μm area including the opaque spot appearing in Fig. 1 was carried out. The results can be seen in Fig. 2, where two representative spectra collected outside (Fig. 2a) and inside (Fig. 2b) the opaque spot are shown. The spectrum of Fig. 2a shows the typical Raman bands of YV [5] (Fig. 2c,d), while that of Fig. 2b shows, in addition to the YV bands, two broad features. The first, peaked at about 320 cm, ranges from 300 to 400 cm, while the second is found in the cm range. Fig. 2f shows a mapping of the intensity of the 320 cm band throughout the region sampled out. It is evident that the band

3 456 L. Sangaletti et al. / Journal of Crystal Growth 198/199 (1999) Fig. 1. Optical images of the YV :Nd single crystal collected with the microscope coupled to the Raman spectrometer. (a) The focus of the objective lens is set onto the opaque inclusion inside the crystal, at an estimated depth of about 70 μm. (b) Focus on the crystal surface. The opaque inclusion is out of focus, whereas a small crystal fragment lying on the surface is clearly visible on the bottom of the image.

4 L. Sangaletti et al. / Journal of Crystal Growth 198/199 (1999) Fig. 2. Raman spectra of pure and Nd-doped YV single crystals. (a) Raman spectrum collected in a point of the Nd : YV single crystal far from the opaque inclusion. (b) Raman spectrum collected by focusing the laser beam onto the opaque inclusion found inside the Nd : YV single crystal. The band ascribed to the phase of the opaque inclusion is marked by a symbol ( ). Additional features ascribed to the impurity phase are found in the background below the three sharp peaks above 800 cm. (c, d) Raman spectra collected from a needle-like pure YV single crystal. Two polarization of the laser beam were used, with the electrical field E perpendicular (c)or parallel (d) to the major axis of the crystal. (e) Raman spectrum collected from an impurity crystallite of the - polycrystalline sample. (f) Micro-Raman mapping of the band at 320 cm collected over a μm area including the opaque inclusion. The intensity of the impurity Raman band is shown by the bar on the right side of the plot. intensity is higher in correspondence to the opaque spot. Information about the opaque inclusion can be obtained by investigating the Y system. To this purpose, powder samples were prepared by solid state reaction starting from 4Y and mixtures. After annealing at 1450 C for 100 h both samples resulted to be composed of an impurity phase (hereafter denoted as I-phase) and YV, the yttrium vanadate content being higher in the 4Y sample. The X-ray diffraction patterns obtained from the sintered samples are shown in Fig. 3a (4Y ) and Fig. 3b ( ), along with a reference pattern for YV (Fig. 3c; structural parameters are taken from Ref. [6]). The most important difference between the patterns of the sintered samples and the reference pattern of YV is represented by the group of reflections in the θ range. Other groups of reflections are found in the and θ ranges. The XRD data have been analysed by performing a Rietveld refinement based onto two phases, the YV phase and the I-phase. This phase has been modelled on the basis of the Zr type of structure [7]. The Rietveld program of the CERIUS package was used to refine the cell

5 458 L. Sangaletti et al. / Journal of Crystal Growth 198/199 (1999) Fig. 3. (a) X-ray diffraction pattern of the 4Y polycrystalline sample polycrystalline sample (dots) and result of the Rietveld analysis (solid line). (b) X-ray diffraction pattern of the polycrystalline sample polycrystalline sample (dots) and result of the Rietveld analysis (solid line). (c) Calculated powder XRD pattern of the tetragonal YV cell. (d) Calculated XRD pattern of the I-phase. parameters and calculate the ratio between the two phases in the experimental pattern. No restriction was imposed on the unit cell symmetry and six coefficients of the polynomial background were refined. As for the cell parameters the following values have been obtained: a"0.734 nm, b" nm, c"0.750 nm, α"90.2, β"120.4, γ"90.7. The relationship between this unit cell

6 L. Sangaletti et al. / Journal of Crystal Growth 198/199 (1999) and the cubic cell of zirconia is given by the following matrix (¹) transformation: ¹" 1! !1 0 1 In the Rietveld refinement the atomic fractional coordinates have been fixed to those of the ideal zirconia. The ratio between the weight of the tetragonal YV and the I-phases was 0.07 in the sample and 0.21 in the 4Y sample. Therefore, the solid state synthesis of the and 4Y samples yielded in both cases a mixture of two phases, the I-phase with a nearly trigonal unit cell derived from the Zr type-of structure and the tetragonal YV. The relative content of YV is larger in the 4Y sample. The simulated XRD pattern of the I-phase is shown in Fig. 3d, while the calculated patterns resulting from the the Rietveld analysis are shown in Fig. 3a and Fig. 3b below the corresponding experimental XRD pattern. Also the micro-raman maps collected from the powder samples gave evidence, in addition to YV, of formation of an impurity phase with spectral features very similar to those observed in the opaque spot found in the single crystal. Fig. 2e shows the Raman spectrum collected from the sample. As in the case of Fig. 2b, two broad bands appear in the cm and in the cm regions. These bands are ascribed to the I-phase. Sharp peaks are detected superimposed to these bands and ascribed to crystalline YV. Similar spectra were observed in the case of 4Y, but the intensity of the bands ascribed to the YV phase was much higher, in agreement with X-ray diffraction analysis. The width of the Raman bands of the I-phase can be ascribed to disorder effects such as cation lattice distortions, disorder in oxygen sublattice, or distortions of the oxygen coordination polyhedra. In conclusion, a structural investigation of the Y system was carried out on possible segregated phases appearing during the synthesis of pure and doped yttrium vanadate. An opaque segregated phase was identified inside a flux grown crystal, a few microns below the surface. Micro- Raman maps collected from this inclusion showed that the spectral features are quite similar to those found in powder samples prepared by solid state reaction starting from either 4Y or mixtures and ascribed to a nearly trigonal phase derived from the Zr type of structure. In view of the close relationship between the Zr and Y type of structures the segregated I-phase can be related to that of Y by introducing into the close-packed yttrium sublattice distortions due to the presence of the small V cations. The present results confirm the suggestion made by Erdei et al. [8] that Y -rich inclusions can indeed form in the crystal growth of YV. The I-phase has an XRD pattern not much different from those reported by Levin [9] and Yamaguchi et al. [10] for the 4Y and phases. However, we believe that, due to possible distortions introduced by the V cation into the Y sublattice and also in the oxygen coordination polyhedra, the relative intensities of the reflections may change depending on the sintering procedure, which may account for the observed differences with respect to other patterns [9,10] as well as for a different indexing [10]. References [1] S. Erdei, J. Crystal Growth 134 (1993) 1, and references therein. [2] G. Turrel, J. Corset, Raman Microscopy: Developments and Applications, Academic Press, London, [3] B. Jin, S. Erdei, A.S. Bhalla, F.W. Ainger, Mater. Lett. 22 (1995) 281. [4] C. Goutadier, F.S. Ermeneux, M.T. Cohen-Adad, R. Moncorgé, M. Bettinelli, E. Cavalli, Mater. Res. Bull. 33 (1998) [5] S.E. Miller, H.H. Caspers, H.E. Rast, Phys. Rev. 168 (1968) 964. [6] G. Lohmueller, G. Schmidt, B. Deppish, V. Gramlich, C. Scheringer, Acta Crystallogr B 29 (1973) 141. [7] L.E. Depero, unpublished data. [8] S. Erdei, G.G. Johnson Jr., F.W. Ainger, Cryst. Res. Technol. 29 (1994) 815. [9] E.M. Levin, J. Am. Ceram. Soc. 50 (1967) 381. [10] O. Yamaguchi et al., J. Electrochem. Soc. 136 (1989) 1557.