Growth of GdCa4O(BO3)3 by the Czochralski Method and Some Structure Properties A. Pajaczkowska,†,* A. Klos,† B. Hilczer,‡ N. Menguy,§ and A. Novosselov† Institute of Electronic Materials Technology, 01-919 Warszawa, Poland, Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznan, Poland, and Laboratoire L2MP CNRS, Universite´ Aix-Marseille III, France
CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 5 363-365
Received July 18, 2001
ABSTRACT: Self-frequency doubling calcium oxoborate compounds GdCa4O(BO3)3 were prepared as a single crystal by the Czochralski method. To reveal the nature and the distribution of defects, crystals were investigated by chemical etching, X-ray methods, high-resolution electron microscopy (HREM), and Raman spectroscopy. XPS measurements were performed to reveal the impurity of the single crystal. GCOB crystal showed high ordering in the lattice up to room temperature. Introduction Recently, GdCa4O(BO3)3 (GCOB) single crystals have been reported as an excellent candidate for application in nonlinear optics.1 The compound melts congruently at 1753 K2, and the crystal has high hardness (6.5), is nonhydroscopic, chemically stable, and easy to polish. GCOB belongs to a new class of calcium oxoborate compounds with rare earth elements, R ) La, Nd, Sm, Gd, Y, obtained by Norrestam et al.3 by solid-state reaction, and Iliukhin et al.4 obtained some crystals by the flux method with R ) Gd, Tb, Lu ions and measured the crystal structure of GCOB by X-ray diffraction methods, which is related to the structure of fluoroborate and fluoroapatite. The unit cell parameters are a ) 0.8095(7), b ) 1.6018(6), c ) 0.3558(8) nm, and β ) 101.260°.4 The space group is monoclinic noncentrosymmetric Cm. There are two types of Ca2+ ions that occupy distorted octahedral sites. All octahedra share corners with BO3 triangles to form a three-dimensional network. There are two kinds of boron sites, B(1) and B(2), with 3-fold coordination. Three planar borate units lie approximately parallel to the (001) plane.4 The Gd3+ ions are located in the crystallographic mirror plane. The environment of Gd3+ is a distorted octahedron with Cs site symmetry. Four oxygen ions are shared with the BO3 groups. The existence of a probable disorder between calcium and gadolinium atoms in the two octahedral positions is expected.5 Crystal Growth The GCOB compound was prepared by solid-state reaction, with Gd2O3 and CaCO3 of 4N purity and B2O3 of 5N purity. B2O3 was prepared in the special way to III-V technology and contains water no higher than 70 ppm. The mixture was heated at 950 °C, cooled and ground, and then heated again at 1150 °C, for 20 h, respectively. Crystals were grown by the Czochralski method. The synthesized charge was melted in a iridium crucible, * To whom correspondence should be addressed.
[email protected]. † Institute of Electronic Materials Technology. ‡ Institute of Molecular Physics. § Universite ´ Aix-Marseille III.
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Figure 1. Single crystal of GdCa4O(BO3)3.
and the seed of orientation (010) was introduced into the crucible, at the top, and kept in contact with the melt. The growth processes were computer monitored by a weight-and-diameter system. A nitrogen atmosphere was provided during growth. The typical growth rate was about 1 mm/h, and the crystal was rotated at 20-35 rpm. The crystals obtained in this way are colorless, with a good optical quality. The grown crystals had a nonuniform outer part of crystal surface. The size of obtained crystals without macroscopic defects and without crackings was 25 mm in diameter and 50 mm in length, using a crucible 50 mm diam. The as-grown crystal is shown in Figure 1. Defect Investigations To reveal the nature and the distribution of defects such as inclusions and dislocations, crystals were investigated by chemical etching and high-resolution electron microscope (HREM). Chemical methods were developed on planes of parallelepiped cut along the three monoclinic axes and on plates of the orientation (010) perpendicular to the growth direction from three regions of crystal (top, central, and bottom). The etched pits on (010) and (100) planes of the cube were detected (Figure 2). The (010) plates showed a similar amount of dislocations at the outer part of plates of these three parts of the crystal. The density of etched pits was 1.8 × 104 cm-2.
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Figure 2. Etching pits.
Figure 4. The model of stacking faults perpendicular to the b-monoclinic axis. Figure 3. HREM image of a defected part of GCOB crystal (850 000 magn.) with the corresponding diffraction pattern in the inset. The direction of observation is [104].
It means this result is related to the outer part of the crystal; the central part was dislocation free. No etched pits were observed on (001) planes. This is probably because this is a rough type surface. HREM experiments were performed at room temperature with a JEOL 2010F transmission electron microscope operating at 200 kV. Single crystals were crushed in an agate mortar, and the particles were deposited on a carbon film. In general, the crystal is characterized by a regular ordering; however, in some (but rather very few) particles stacking faults perpendicular to the monoclinic b-axis were observed. Figure 3 shows an HREM image of a defected part of an as-grown GCOB single crystal with the corresponding diffraction pattern insert. The picture was obtained in the [104] direction at the magnification 850 000. The diffused streaks in the diffraction pattern correspond to more or less regularly spaced stacking faults visible in the center of the picture. The stacking faults are perpendicular to the b-monoclinic axis, and the model proposed is shown in Figure 4. The minimum distance between the stacking fault layers shown was ∼0.8 nm (b/2).
ture changed from 80 to 300 K and was stabilized to within 0.1 K. Room temperature IR and optical Raman studies of polycrystalline and single crystals Ca4GdO(BO3)3 were investigated earlier.6,7 The Raman spectra reported in the papers were performed at room temperature. We observed well-resolved lattice modes of Gd, Ca, and BO3 translations and BO3 librations in the wavenumber range from 90 to 510 cm-1. Internal modes of two different BO3 (3-) anions appeared in the range 6001350 cm -1, and the number of the modes observed corresponds to that obtained from factor group analysis. Figure 5 shows an example of temperature variation from 100 to 400 K NIR Raman spectra in polarized of GCOB (Y (Z, Z) -Y) geometry) single crystal. The bands of the lattice modes are sharp and well separated. Temperature changes of spectral profiles of both the lattice and internal modes are rather small, i.e., the increase in the line width on heating from 80 to 400 K amounts to 12-21%. The results show that the crystal exhibits low dynamics of molecular and lattice vibrations at room temperature, which seems to be a characteristic property of the crystal. Figure 6 shows the temperature variation of the half-width of libratory lattice mode T(BO3) at 100 K. X-ray Photoelectron Spectroscopy (XPS)
Raman Investigations Polarized NIR Raman spectra were studied for b-cut sample in backscattering geometry using an FT BRUKER IFS66 FRA 106 spectrometer. The samples were placed in LINKAM THM 600 heating-cooling stage controlled with a TSM 91 LINKAM unit; the tempera-
The XPS spectra of GCOB were measured with monochromitized Al KR radiation (1486.6 eV) at room temperature using PHI 5700/660 physical electronics spectrometer. The spectra of the photoelectrons in function of their kinetic energy were analyzed by a hemispherical mirror
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Figure 7. XPS spectra of GCOB crystal before and after annealing in oxygen.
in O2 might improve the crystal structure of GCOB. Carbon impurity was only found in as-grown CGBO crystal and was removed after annealing in oxygen. Figure 5. Temperature variation of Raman spectra (Y (Z, Z) -Y) geometry.
Figure 6. Temperature variation of the half bandwidth of T′ (BO3) mode at 181 cm-1.
analyzer with an energy resolution about 0.3 eV. To eliminate the surface contamination, all samples were broken in a vacuum condition of less than 10-9 Torr before the analysis. Thus, it was possible to measure XPS spectra of fresh surface without exposing the surface to air. A broad survey with a scan range from 0 to 1400 eV was run to identify all elements present in the samples. XPS depth profiling studies have revealed the existence of Ca, Gd, B, O, and C atoms. It should be noted that the presence of carbon atoms can be due to the expected adsorption of adventitious carbon, primary hydrocarbons, C-O-C, C-OH, and CdO species6 and, in addition, incompetence of the decomposition of CaCO3 used for the preparation of the starting melts of GCOB by solid-state reaction. No other elements such as nitrogen or iridium were detected. The XPS spectra of GCOB crystal before and after annealing in oxygen at 1050 °C during 36 h are shown in Figure 7. Differences between spectra of as-grown samples and the samples annealed in O2 may be explained by inelastic scattering of photoelectrons producing higher background signals on as-grown samples. One can conclude that annealing
Conclusions The investigated crystals were partially defected showing etched pits and stacking faults along the crystal growth direction [010]. The etched pits were revealed mainly at the outer part of crystal along the [010] direction, and maximum content was 1.8 × 104 cm2. The observed stacking faults were perpendicular to the monoclinic b-axis, and the minimum distance between the stacking faults layers was ∼0.8 nm (b/2). The number of internal Raman modes observed in spectra corresponds to that obtained from factor group analysis. The crystal structure was stable at a temperature range of 100-400 K because the differences in the bandwidth of Raman spectra were rather small. The significant improvement of GCOB crystal structure after annealing in oxygen was shown by XPS investigations. Acknowledgment. The Polish Committee supported this work for Scientific Research under Grant No. 8T11B00716. References (1) Aka G.; Bloch L.; Godard J.; Kahn-Harari A.; Salin F.; Vivien D. International patent No. WO 96/26464, 29.08., 1996. (2) Vivien D.; Mougel F.; Aka G.; Kahn-Harari A.; Pelenc D. Laser Phys. 1998, 8, 759-763. (3) Norrestam R.; Nygren M.; Bovin J. O. Chem. Mater. 1992, 4, 737-743. (4) Aka G.; Kahn-Harari A.; Mougel F.; Vivien D.; Salin F.; Coquelin P.; Colin P.; Pelenc D.; Damelet J. P. J. Opt. Soc. Am. B, 1997, 14, 2238-2247. (5) Iliukhin A. B.; Dzhurinskii B. F. Russ. J. Inorg. Chem. 1993, 38, 917-920. (6) Dominiak-Dzik G.; Ryba-Romanowski W.; Goła¸ b S.; Macalik L.; Hanuza J.; Pajaczkowska A. J. Mol. Struct. 2000, 555, 213-225. (7) Lorriaux-Rubbens A.; Aka G.; Antic-Fidancev E.; Keszler D. A.; Wallart F. J. Raman Spectrosc. 2000, 31, 535-538. (8) Pawlak D. A.; Wozniak K.; Frukacz Z.; Barr T. L.; Fiorentino D.; Seal S. J. Phys. Chem. B 1999, 103, 1454-1461.
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