Growth and characterization of LuVO4 single crystals

Growth and characterization of LuVO4 single crystals

Accepted Manuscript Growth and characterization of LuVO4 single crystals D.Z. Dimitrov, P.M. Rafailov, Y.F. Chen, C.S. Lee, R. Todorov, J.Y. Juang PII...

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Accepted Manuscript Growth and characterization of LuVO4 single crystals D.Z. Dimitrov, P.M. Rafailov, Y.F. Chen, C.S. Lee, R. Todorov, J.Y. Juang PII: DOI: Reference:

S0022-0248(17)30379-2 CRYS 24189

To appear in:

Journal of Crystal Growth

Received Date: Revised Date: Accepted Date:

15 December 2016 27 April 2017 22 May 2017

Please cite this article as: D.Z. Dimitrov, P.M. Rafailov, Y.F. Chen, C.S. Lee, R. Todorov, J.Y. Juang, Growth and characterization of LuVO4 single crystals, Journal of Crystal Growth (2017), doi: j.jcrysgro.2017.05.023

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Growth and characterization of LuVO4 single crystals

D. Z. Dimitrov a, b, c, *, P. M. Rafailov a, Y. F. Chen c, C. S. Lee d, R. Todorov b, J. Y. Juang c a

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


Institute of Optical Materials and Technologies, Bulgarian Academy of Science, Acad. G. Bonchev Str., 109, Sofia, Bulgaria


Department of Electrophysics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, Taiwan


Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, Taiwan E-mail: [email protected]; [email protected]

Abstract Large LuVO4 single crystals have been successfully obtained by high-temperature solution method. The structure details of these crystals are determined by X-ray crystallographic analysis and Raman spectroscopy. It is observed that the crystal consists of LuVO4 phase with trace amount of imperfections possibly due to oxygen vacancies. The optical quality of the crystal is assessed by Spectroscopic Ellipsometry (SE). The crystal shows higher than +0.2 birefringence in a large interval of wavelengths. Keywords: A1.X-ray diffraction, A2.Growth from high temperature solutions, A2.Single crystal growth, B1.Rare earth compounds, B1.Vanadates, B2.Nonlinear optic materials

1. Introduction Rare earth orthovanadates (RVO4, where R = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) have been actively studied because of their interesting magnetic, optical and electronic properties. These compounds show applications in various fields, such as laser host crystals [1, 2], scintillators [3], sensors [4], phosphor materials [5] and catalysts [6]. In particular, due to their exceptional optical properties, namely wide-range optical transparency and large birefringence, RVO4 compounds are potential candidates for optical isolators, circulator’s beam displacers, and components for polarizing optics. Besides the technological importance, these 1

compounds have also generated significant theoretical interest due to emergent physical properties associated with the presence of 4f electrons [7]. Among RVO4 single crystals, LuVO4 is the one of particular interest for serving as a laser host material, because in comparison with those of other vanadate crystals, its absorption and emission cross sections are substantially larger in the vicinity of 800 nm and 1.064 µm, respectively [2]. These features have made LuVO4 the ideal candidate for fabricating diode pumped solid-state lasers with high pumping efficiency, low operation threshold, as well as high optical-to-optical efficiency. A wide variety of methods, including slow cooling from solution [8], the Czochralski process [9], top-seeded solution growth [10], the laser-heated pedestal growth method [11], the floating-zone (FZ) method [12] and the micro-FZ method [13], had been practiced to grow RVO4 single crystals. Although LuVO4 crystals obtained by these methods are of excellent quality, their size is often too small to conduct comprehensive characterizations. Moreover, from the practical applications point of view, it is essential to be able to grow large-sized single crystals with minimal amount of defects. Therefore, one of the primary goals of the our research on LuVO4 single crystals was to develop a method for preparing large, high quality single crystals and investigating their properties relevant to existing and future applications.

2. Experimental In the present study, the high temperature solution growth method was adopted to grow LuVO4 single crystals. Details of the growth processes were reported previously [14]. Briefly, stoichiometric amounts of 99.99%-pure Lu2O3 and V2O5 powders were mixed, compacted, and then calcinated in oxygen at 650 °C for 48 h to carry out the solid state reaction. The reacted product consists primarily the polycrystalline LuVO4, which was then ground and mixed with V2O5 flux with the ratio of V2O5:LuVO4 =12:1. In general, the flux growth technique is effective for preparing crystals at temperatures substantially lower than the melting point of the targeted crystal and, thus, can result in a lower concentration of thermally induced point defects. Specifically, researches have indicated that the vanadium enriched flux solutions are more favorable for growing stoichiometric RVO4 crystals. In the present case, the above-mentioned 12:1 mixture was further heated to 1100 °C and the melted solution was kept at the same


teperature for 48 h in a platinum crucible of 50 mm in diameter and 60 mm in depth, covered with a platinum lid. Subsequently, the solution was slowly cooled from 1100 to 700 °C at a cooling rate of 1 °C/h and then funance-cooled to room temperature. The residual flux was separated from the as-grown crystals by decanting with the crystals remaining at the bottom and on the walls of the crucible. The so obtained LuVO4 crystals were of tetragonal rectangular shape with a typical size of 8×10×2 mm3. The crystallographic analysis of the obtained LuVO4 single crystal was performed at room temperature on Bruker-AXS Smart Apex three-circle diffractometer equipped with a CCD detector [14]. The diffractometer was operated with Mo-Kα radiation (λ=0.71073 Å) and a graphite monochromatic crystal under a voltage and current of 50 kV and 30 mA, respectively. The total exposure time was 14.10 hours. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using a tetragonal unit cell yielded a total of 726 reflections to a maximum θ angle of 24.87° (0.84Å resolution), of which 82 were independent (average redundancy 8.854, completeness 100%, Rint = 4.86%, Rsig = 2.92%) and 79 of them (96.34%) were greater than 2σ(F2). The final cell constants are based upon the refinement of the XYZ-centroids of 664 reflections above 20 σ(I) with 8.746° < 2θ < 49.73°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.496. Spectroscopic ellipsometry was applied for the determination of the refractive index of LuVO4 crystal in the spectral range of 190-900 nm. The measurements were conducted by UV– Visible phase modulated spectroscopic ellipsometric platform UVISEL2 (HORIBA Jobin Yvon) at 70° incident angle. The instrument was operated in a rotating compensator configuration, with a white light source and a CCD detector providing fast data acquisition capabilities. For the Raman spectroscopic measurements LuVO4 single crystals elongated along the Z-axis with naturally grown {100} and {001} surfaces were selected, which additionally displayed the wellshaped {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 and the absolute accuracy was 0.5 cm-1.


3. Results and discussion As-grown LuVO4 single crystals have a pale yellow color. This coloration is believed to be due to oxygen vacancies resulted from crystal growth. Although LuVO4 melts congruently, some vanadium oxides vaporize incongruently, causing changes in the Lu/V ratio and oxygen stoichiometry. It is considered that the annealing process at high temperature (1100-1200 °C) in air or in the high oxygen concentration atmosphere is needed after the crystal growth in order to restore colorless appearance. The grown crystals were cut and polished into plate form. Fig.1 shows one polished crystal plate with dimensions ∼10x5 mm2 and thickness ∼1 mm.

Fig.1. Polished LuVO4 single crystal plate According to X-ray powder diffraction analysis [14], the grown crystals are single-phased with all diffraction peaks being corresponding to the tetragonal zircon-type structure of LuVO4 phase without any discernible second phase peaks. It has been conceived that the symmetry space group of the tetragonal zircon-type LuVO4 can be denoted as I41/amd (Nr. 141). Namely, the Ln3+ ions are situated in the dodecahedral cages formed by the eight-fold coordinated oxygen and have the D2d point symmetry. Whereas the V5+ ions are locating at the tetrahedral sites coordinated by oxygen atoms [15-18]. Detailed crystal structure of the present LuVO4 was solved and refined using the Bruker SHELXTL Software Package, based on the space group I41/amd, with Z = 4 for the formula unit, LuVO4. The obtained crystal structure analysis data, structural refinements, and some physical parameters are summarized in Tables 1 and 2. Selected bond lengths and bond angles are listed in Table 3. The obtained cell constants of a=b=7.0236(3) and c=6.2293(4) Å are comparable to those reported in previous references: 7.0263(2), 6.2329(2) [19]; 7.0254(1), 6.2347(1) [16]; 7.0266(3), 6.2329(4) [20] and 7.0230(1), 6.2305(1) [21]. Based on the detailed structural information described above, it can be briefly summarized that LuVO4 crystallizes as xenotime structure being made up of chains formed by 4

alternating LuO8 polyhedra (bisdisphenoid) and VO4 (tetrahedra) along the c-axis. These chains are joined together by sharing other edges of the LuO8 polyhedra along the a- and b-directions to form a three-dimensional lattice. The connection of LuO8 units as well as LuO8 and VO4 units in the xenotime-type structure are as follows [22]: there are four neighboring Lu and four neighboring V atoms to each Lu atom at the same distance. Along the c-axis, there are two additional V atoms with shorter Lu-V distances than the other four V atoms. The LuO8 polyhedra made up of two sets of four identical Lu-O bonds can be described as two interpenetrating tetrahedra. The set of four longer bonds (Lu-Oc) forms a tetrahedron elongated along the c axis. Similarly, the set of four shorter bonds (Lu-Oa) is compressed along the c-axis and thus oriented more toward the (a) or (b) directions. Table 1 Sample and crystal data of LuVO4 crystal Chemical formula LuVO4 Formula weight 289.91 Temperature 296(2) K Wavelength 0.71073 Å Crystal system tetragonal Space group I 41/amd Unit cell dimensions a=b =7.0236(3) Å; c=6.2293(4) Å α=90°, β=90°, γ=90° Volume 307.30(3) Å3 Z 4 Density (calculated) 6.266 g/cm3 Absorption coefficient 34.794 mm-1 F(000) 504 Table 2 Data collection and structure refinement θ range for data 4.37 to 24.87° collection Index ranges -8<= h <=6, -8<= k <=5, -7<= l <=7 Reflections 726 collected Independent 82[R(int) = 0.0486] reflections Absorption Multi-scan correction

Refinement program

SHELXL-97 [18]

Function minimized

Σ w(Fo2-Fc2)2

Data/restraints/parameters 82/0/12 Goodness of fit on F2


Final R indices

79 data: I >2σ(I); R1 = 0.0211, wR2 = 5

Structure solution technique Structure solution program Refinement method

Direct method

Extinction coefficient

0.0465 all data; R1 = 0.0217, wR2 = 0.0465 0.0108(14)

Largest diff. peak and 1.233 and -1.496 eÅ-3 hole least- RMS deviation from 0.326 eÅ-3 mean

SHELXS-97 [18] Full-matrix squares on F2

Table 3 Bond lengths (Å) and selected bond angles (°) O-V 1.710 (4) Lu- O (#5) O-Lu (#6) 2.412 (4) Lu- O (#11) Lu-O (#9) 2.245 (4) Lu- O (#8) V-Lu (#6) 3.1146(2) Lu-O (#12) Lu-O (#7) 2.412 (4) Lu-O (#2) Lu-Lu (#4) 3.84161(14) V-O (#8) O-V-O (x2) 100.50(3) O-Lu-O (x8) O-V-O (x4) 114.12(2) O-Lu-O (x4)

2.245 (4) 2.412 (4) 2.412 (4) 2.245 (4) 2.412 (4) 1.710(4) 80.02 (2) 92.45 (4)

Lu-V Lu-Lu (#10) V-O (#1) Lu-O (#3) Lu-V (#7) V-O (#11) O-Lu-O (x2) O-Lu-O (x4)

3.1146(2) 3.84161(14) 1.710(4) 2.245 (4) 3.1146(2) 1.710(4) 66.10 (1) 68.88 (2)

Symmetry transformations used to generate equivalent atoms: #1 –x+0,-y+3/2, z+0; #2 –x+0,-y+3/2, z+1; #3 x+5/4, -y-1/4, z+3/4; #4 –x,-y+1, -z; #5 x-1/2,y, -z-1/2; #6 x, y, z-1; #7 x, y, z+1; #8 x+3/4, -y+3/4, -z-3/4; #9 –x+1/2, -y+3/2, z-1/2; #10 –x+1/2, -y+3/2, -z+1/2; #11 –x-5/4, y+5/4, -z-3/4; #12 –x-3/4, y+5/4, z+3/4

For analyzing the optical results obtained from the present LuVO4 crystals, the DeltaPsi2 software was used for ellipsometric modeling. We used a layer stack model, consisting of the semi-infinite anisotropic substrate and surface layer for the determination of the refractive index from the recorded spectra of Is and Ic. Where Is and Ic are related to ellipsometric angles ψ and ∆ as follows: Is = sin2Ψsin∆ and Ic = sin2ψcos∆. The anisotropic substrate was described as uniaxial medium with different refractive indices (nx, ny, nz) parallel to the main axes, where nx = ny = no and nz = ne. The Euler angles were used to find the orientation of (nx, ny, nz) in (N, S, P) laboratory coordinates. The direction of N is parallel to the normal vector of the sample surface, while S and P determine the plane parallel to the sample surface. The surface layer is assumed to consist of 50 % of void in order to describe the surface roughness of the sample. The refractive indices (nx, ny, nz) were approximated by a Lorentz dispersion model for the dielectric function εi = ni2 (where i = x, y, z):


̂  = , +  ,,  ,


+ ∑  

 , .,







Wherein, ε∞,i is the high frequency dielectric constant, εs,i gives the value of static dielectric function at zero frequency, ωt,i is the resonant frequency of the oscillator, Γ0,i is the damping factor, fj,i represents the oscillator strength in the multiple Lorentz oscillator part, ω0j,i is resonant energy of an oscillator, and Γj,i is a broadening parameter corresponding to the peak energy of each oscillator, respectively. The validity of the model is confirmed by the calculation (Eq. (2)) of a common mean square error function (χ2), which accounts for the discrepancies between the measured and simulated spectra for Ic and Is: 

  = !" ∑! #$%&'()' ℎ+ − %&-.(/ ℎ+0 + $%1'()' ℎ+ − %1-.(/ ℎ+0 2


where K and L are the total number of data points and the number of fitted parameters, respectively. Fig. 2 shows the typical wavelength-dependent refractive indices and extinction coefficients along the extraordinary (n1, k1) and ordinary (n2, k2) axes obtained from the present LuVO4 single crystals. Fig. 3, displays the corresponding birefringence dispersion derived from the refractive index data.

Refractive index







2.0 1.8


1.4 200



1.6 400



Extinction coefficient

2.0 2.6


Wavelength [nm] Fig. 2. Wavelength dependence of the refractive indices and extinction coefficients for the present tetragonal LuVO4 single crystals


0.8 0.7

∆n = n1 -n2

0.6 0.5


0.4 0.3 0.2 0.1 0.0 200








Wavelength [nm] Fig. 3. Birefringence dispersion of the tetragonal LuVO4 single crystal

From the obtained results, it is clear that LuVO4 is a positive uniaxial crystal (ne > no) with optical properties very well suited for crystal polarizers especially at the infrared spectral interval. At the Na D line the ordinary and extraordinary indices of LuVO4 were 2.1107 and 2.3134 respectively, giving a birefringence of +0.2027, which is substantially larger than that of the typical calcite's value of -0.1720. Moreover, LuVO4 possesses high transmittance and has no absorption in the wavelength range between 2.5 and 15 µm [23]. The analysis of the Raman active phonons of LuVO4 was made on the basis of the point group symmetry D4h of the zircon-type structure with 4 formula units in the unit cell (2 formula units per primitive cell). The structure consists of Lu3+ ions and VO43- tetrahedrons 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 Lu3+ ions [24]. 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) as well as X’ (110) and Y’(-110) for the main crystal axes were used. It was obvious, from the measured spectra [14] that the 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 the allowed and forbidden intensity between different Raman modes, which can be minimized by using focusing/collecting optics with small numerical 8

aperture (N.A.) and by keeping the beam path short within the sample [25]. These requirements were successfully met by using a 20× microscope objective with N.A. = 0.4. The spectra also contain features from impurities or substitutional defects that do not obey the Raman selection rules and obviously do not pertain to the vibrational spectrum of LuVO4 crystal. However, in most cases, their frequencies are far from that of the LuVO4 signal and, with one exception, thus do not pose any obstacle to the assignment of the LuVO4 modes. Based on the polarization behavior of the different spectral lines, most of them could be unambiguously assigned to zonecenter Raman active phonons of the LuVO4 crystal, which amounts to a total of 12 modes, namely 2A1g + 4B1g + B2g + 5Eg [24]. The mode assignment is given in Table 4 along with the related vibrational species of the isolated VO43- tetrahedron [26]. Totally 11 out of all 12 Raman active modes of LuVO4 are identified and assigned in the Raman spectra obtained from the present samples compared to only 9 Raman active modes observed previously [27, 28].

Table 4 Frequencies (cm-1) of the Raman lines of LuVO4 and assignment to zone-center phonon modes Raman Symmetry Assignment Symmetry species -1 shift/ cm species (D4h) isol. VO43- (Td) 103 Eg Lu3+ / VO43 translation 113 B1g Lu3+ / VO43- translation (in phase) 157.5 Eg Lu3+ / VO43- translation pure translation 252 Eg VO43- libration pure rotation 261.5 B2g E(v2) ν2, O–V–O symmetric bending 269 B1g Lu3+ / VO43- translation pure translation (out-of-phase) 381 A1g E(v2) ν2, O–V–O symmetric bending missing Eg F2(v4) ν4, VO4 asymmetric bending/stretching 493 B1g F2(v4) ν4, VO4 asymmetric bending 826 B1g F2(v3) ν3, VO4 asymmetric stretching 846 Eg F2(v3) ν3, VO4 asymmetric stretching 900 A1g A1(v1) ν1, VO4 symmetric stretching 9

In order to confirm the assignment of B1g(2) and Eg(4) modes, which are known to possess exceptionally weak scattering intensity [29], we measured Raman spectra as a function of the rotation angle φ by rotating the sample around the exciting beam direction in the YZ plane. For each spectrum, φ is defined as the angle between the Y-axis and the actual polarization direction of the exciting laser beam. In these cases the intensity of a B1g mode is expected to vary as cos4φ for parallel polarization and as sin22φ for perpendicular polarization. As can be seen from Fig. 4 (a) and (b), these functional dependencies are only fulfilled by the faint line at 269 cm-1, confirming its assignment to the B1g(2) mode. This assignment is further corroborated by a comparison to the corresponding Raman modes of YVO4 which are found at nearly the same frequency and are ordered in the same sequence [29] (Eg(3) mode at 260 cm-1, B2g mode at 260 cm-1, and B1g(2) mode at 269 cm-1). The Eg(1) mode, which is very faint too, could also be unambiguously assigned in a similar way. Due to their low intensity and the complications from birefringence effects, these modes are frequently missing [28] or wrongly assigned [30,31] in a number of recent Raman studies on orthovanadates.

Fig. 4. Rotation-angle dependent Raman spectra of the LuVO4 crystal in parallel (a) and perpendicular (b) polarization. Assignment of the B1g (2) mode is made on the basis of the characteristic rotation-angle dependence of its intensity. Features not inherently related to the first-order Raman spectrum of LuVO4 are marked with asterisks.

4. Conclusions


Large single crystals of lutetium vanadate (LuVO4) have been successfully grown by the hightemperature solution method using excessive V2O5 as flux medium. The single crystals are transparent with high quality crystalline structure as confirmed by X-ray diffraction and Raman spectroscopy measurements. Facilitated by the sufficiently large size of the obtained crystals, the birefringence dispersion of LuVO4 was determined in a broad spectral interval ranging from the UV to the near IR region and the birefringence properties of LuVO4 were shown to be superior to those of calcite. Investigation of polarized rotational-angle dependent Raman spectra of LuVO4 revealed the correct assignment of 11 of its 12 first-order Raman active phonons.

Acknowledgements: The financial supports by the Ministry of Science and Technology, Taiwan under contracts MOST 103-22221-E-009-079, MOST 105-2811-M-009-044, MOST 103-2112M009-015-MY3 and Bulgarian Science Fund under the project ДH-08/9 are gratefully acknowledged. JYJ was also supported in part by MOE-ATU program operated at NCTU. References [1] C. Maunier, J. L. Douglas, R. Moncorge, A. Speghini, M. Bettinelli, E. Cavalli, J. Opt. Soc. Am. B 19 (2002) 1794 [2] H. Zhang, J. Liu, J. Wang, X. Xu, M. Jiang, Appl. Opt. 44 (2005) 7439 [3] J. - F. Liu, Y.- D. Li, J. Mater. Chem. 17 (2007) 1797 [4] E. V. Tsipis, M. V. Patrakeev, V. V, Kharton, N. P. Vyshatko, J. R. Frade, J. Mater. Chem. 12 (2002) 3738 [5] M. Yu, J. Lin, S. B. Wang, Appl. Phys. A: Mater. Sci. Proc. 80 (2005) 353 [6] C. T. Au, W. D. Zhang, J. Chem. Soc. Faraday Trans. 93 (1997) 1195 [7] V. Panchal, D. Errandonea, A. Segura, P. Rodríguez-Hernandez, A. Muñoz, S. Lopez-Moreno, M. Bettinelli, J. Appl. Phys. 110 (2011) 043723 [8] S. H. Smith, B. M. Wanklyn, J. Crystal Growth 21 (1974) 23 [9] K. Chow, H. G. McKnight, Mater. Res. Bull. 8 (1973) 1343. [10] J. Loriers, M. Vichr, J. Crystal Growth 13/14 (1972) 593 [11] S. Eedei, F. W. Ainger, J. Crystal Growth 128 (1993) 1025. [12] K. Muto, K. Awazu, Jpn. J. Appl. Phys. 8 (1969) 1361 [13] K. Oka, H. Unoki, H. Shibata, H. Eisaki, Journal of Crystal Growth 286 (2006) 288 [14] D. Dimitrov, P. Rafailov, V. Marinova, T. Babeva, E. Goovaerts, Y.- F. Chen, C.- S. Lee, J. - Y. Juang, Journal of Physics: Conference Series 794 (1) (2017) 012029M [15] S. Mahapatra, A. Rahmanan, J. Alloys Compounds 395 (2005) 149 [16] B. C. Chakoumakos, M. M. Abraham, L. A. Boatner, J. Solid State Chem. 109 (1994) 197 [17] X. Wang, I. Loa, K. Syassen, M. Hanfland, B. Ferrand, Phys. Rev. B 70 (2004) 064109 [18] G. M. Sheldrick, Acta Cryst. A 64 (2008) 112 [19] A. T. Aldred, Acta Cryst. B 40 (1984) 569 [20] R. Mittal, A. B. Garg, V. Vijayakumar, S. N. Achary, A. K. Tyagi, B. K. Godwal, E. Busetto, A. Lausi, S. L. Chaplot, J. Phys.: Condens. Matter 20 (2008) 075223 11

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Highlights 1. Large size high- quality LuVO4 single crystals 2. Optical transparency and large birefringence 3. Correct assignment of the first-order Raman active phonons