Article pubs.acs.org/crystal
Top Seeded Solution Growth and Structural Characterizations of α‑Quartz-like Structure GeO2 Single Crystal Adrien Lignie,† Bertrand Ménaert,‡ Pascale Armand,*,† Alexandra Peña,‡ Jérôme Debray,‡ and Philippe Papet† †
Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UM2-ENSCM-UM1, C2M, UM2, CC 1504, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France ‡ Institut Néel, Dpt. Matière Condensée, Matériaux et Fonctions, 25 avenue des Martyrs, Bâtiment F, BP 166, 38042 Grenoble Cedex 9, France ABSTRACT: A high-transparency and large-size single crystal, up to 0.5 cm3, of the piezoelectric phase of GeO2 was grown by the top seeded solution growth method from a high-temperature solution using K2Mo4O13 as solvent. The obtained volume makes this flux-grown GeO2 single crystal, with the metastable α-quartz like structure, the largest reported in the literature to our knowledge. Several oriented plates, X-, Y-, and Z-cut according to the dielectric frame, were obtained from the grown crystal, which exhibits a typical hexagonal morphology. The presence of hydroxyl groups as chemical impurities, known to damage the piezoelectric property, was not detected by infrared spectroscopy in transmission mode or Raman spectroscopy on the resulting oriented plates of α-GeO2. The effect of a prolonged annealing (up to five months) at high-temperature (800−900 °C) was followed by Raman spectroscopy: no structural evolution as well as no macroscopic modification of the transparency or the morphology of the α-GeO2 single crystal were observed. These results were consistent with a high optical quality crystal as checked by UV−vis−NIR spectroscopy.
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INTRODUCTION A high electromechanical coupling coefficient1 value predicted around 20% for GeO2 single crystal with the α-quartz-like structure (α-GeO2) makes this oxide a promising material for piezoelectric devices.2,3 However, advanced in situ investigations of the outstanding properties exhibited by α-GeO2 as potential piezoelectric material4,5 or nonlinear optical material6 are hindered by the difficulty to obtain very large-sized single crystals. Previous studies7,8 claimed that the main complication of the crystal growth of α-GeO2 lies in its narrow thermal stability range, from 1033 °C to its melting point, 1116 °C. Below 1033 °C, the rutile polymorph r-GeO2 is the stable phase. The transition α-GeO2/r-GeO2 is complex, and the temperature of the corresponding phase transition depends on the presence of impurities9,10 in the α-quartz structure. Furthermore, the inclusion of hydroxyl groups (OH) or the presence of catalysts as the nonpiezoelectric phase of GeO2 fasten the transition kinetics to the rutile polymorph. Thus, hydrothermally grown crystals of α-GeO2, made in aqueous solution at low-temperature and exhibiting significant incorporation of hydroxyl groups in their structure,11 rapidly transform into the rutile structure once heated (phase transition around 180 °C). For high-purity commercial powder of α-GeO2, transition temperatures of 830 °C or 720−730 °C are reported4,10−12 depending on the percentage of the rutile polymorph as secondary phase in the commercial product. However, if the purity of α-GeO2 material is high enough (no © XXXX American Chemical Society
trace of the rutile phase or OH-groups), this phase transition could remain unobserved.10,13 In the 1970s, a colorless hexagonal α-GeO2 single crystal with a maximum final diameter of 5 mm was grown by the top seeded solution growth method in a Li2O−WO3 flux.14 However, the experimental procedure was quite unclear, and some important parameters, such as seed diameter and applied cooling program, were missing. Moreover, no information concerning the crystal quality was given. Recently, interesting results were obtained on spontaneously nucleated single crystal of α-GeO2 and SiO2-substituted αGeO2 in inorganic fluxes.13,15 For example, no phase transition was observed in the 20−1000 °C temperature range linked to their very low level of defects.16 Unfortunately, the polynucleation occurring during the spontaneous nucleation stage limits the dimensions of the crystals achieved by this method (cubic millimeter scale). To confirm the interesting piezoelectric properties of the αquartz structure of GeO2, large oriented plates with high crystalline quality must be produced to conceive reliable resonators. Based on our previous experiments on the flux growth of α-GeO2 single crystals by spontaneous nucleation in high temperature solution, we decided to overcome the Received: January 10, 2013 Revised: August 15, 2013
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dx.doi.org/10.1021/cg4000523 | Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
toward the crystal during the crystal growth process. The saturation temperature (Ts) value of the growth solution was determined accurately by controlling the growth or dissolution of a test seed placed 2 mm under the surface of the solution. The obtained value was Ts = 945.5 ± 0.3 °C for the molar ratio GeO2/K2Mo4O13 = 0.23:0.77. The hexagon-shaped seed (8 mm in length), mounted onto a Pt holder, was partially dipped (2 mm) into the growth solution. Its Zaxis (dielectric axes frame) was perpendicular to the surface of the solution. In the dielectric system, also called orthogonal system, the Zaxis coincides with the crystallographic c-axis; the X-axis matches the crystallographic a-axis, and the Y-axis is normal to the X and Z axes (baxis being in the XY plane at 120° from the a-axis). The growth solution was kept 1 °C above the saturation temperature, that is, at 946.5 °C, to begin the growth with a slight dissolution process of the seed, in order to prevent a capping zone between the seed and the grown crystal. Then, a slow cooling rate (0.05 °C/h) was applied to begin the growth process and a continuous mass measurement of the seed was initiated. During the crystal growth process, the crystal was rotated alternatively (one turn clockwise, one turn counterclockwise) with a constant angular rotation of 30 rpm. The cooling rate was progressively increased during the TSSG process in order to obtain an efficient growth rate, that is, a constant mass deposition rate. At the end of the growth process, the obtained crystal was removed from the solution and allowed to cool, during 2 days, to room temperature inside the furnace at some millimeters above the surface of the solution. Characterization. Unpolarized Raman spectra were recorded on a LabRam Aramis (Horiba Jobin-Yvon) spectrometer with a blue diode laser (λ = 473 nm, laser spot of approximately 1 μm in diameter). This device was equipped with a motorized stage and a CCD detector cooled by Peltier effect. Sample positioning was made under a microscope (Olympus) with a long focal length objective (magnification ×50). At each use of the spectrometer, a previous calibration was made by the measure of pure silicon standard and by a systematic record of the Raman spectrum of α-GeO2 commercial powder to compensate for the experimental shift due to the equipment. Multipoint measurements were done on each sample surface to validate the collected Raman spectra. The atomic composition of the single crystal was analyzed by energy-dispersive X-ray spectroscopy (EDX). The scanning electron microscope (SEM) used to perform those measurements was a Quanta 200 FEG (FEI) equipped with an SDD diode as detector (Oxford INCA). A previous calibration was made with known elemental standards. Analyses were made at 15 kV and under medium vacuum (around 10−3 Pa). Sample composition and homogeneity were estimated by the analysis of several points on the sample (at least three different areas checked). Oriented plates were cut in the bulk single crystal following orientations determined by Laue diffractometer (I = 35 mA, V = 40 kV). Laue patterns were recorded on image plates (23 × 25 cm2) in a backscattering geometry and an exposure time of 20 min. The resulting orientation was deduced from the recorded diffraction spots with OrientExpress software, version 3.3. Fourier Transform infrared reflection spectroscopy (FTIR) was performed in air at ambient temperature on an IFS 66v spectrometer (Bruker) supplied with a MCT (HgCdTe) detector. Numerical correction due to the absorption of atmospheric water and carbon dioxide was made by subtraction of a blank spectrum, previously recorded. A Hyperion microscope was used for the sample positioning with a magnification ×15. Optical transmittance spectra were recorded on a VARIAN Cary 50 UV−vis−NIR spectrometer. Measurements were done through a rectangular mask (2 × 3 mm in length) in the 200−1100 nm range. Baseline correction of the sample transmittance was made in air.
uncontrolled polynucleation and thus to increase crystal dimensions by using the top seeded solution growth (TSSG) method. In this paper, we report on the growth of large single crystals of the piezoelectric polymorph of GeO2 (space group P3121 or P3221) using K2Mo4O13 as solvent. The developed morphology of the as-grown crystal, compared with the well-known macroscopic morphologies of quartz material, permits a visual identification of the sample habit. Structure, crystalline quality, and thermal stability of α-GeO2 plates with simple X-, Y-, and Z-orientations, extracted from this TSSG experiment, are analyzed by Laue diffraction, infrared spectroscopy in transmission mode, UV−vis−NIR spectroscopy, and Raman spectroscopy.
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EXPERIMENTAL SECTION
Syntheses and Crystal Growth. K2Mo4O13 was prepared as polycrystalline powder by heat treatment of the starting materials (MoO3, 99.95%; K2CO3, 99.95% (Alfa Aesar)) mixed with a 4:1 molar ratio at 500 °C (solid state reaction) during 2 weeks. As a source of GeO2 powder material, we used a commercial product (Metaleurop 99.999%). To prevent any presence of GeO2 in its rutile structure, GeO2 solute was synthesized in the glassy form by quenching the commercial powder in an iced bath after melting at 1200 °C during 2 h. This later process avoids the presence of any undissolved r-GeO2 material. Thus, no parasitic growth of the nonpiezoelectric phase r-GeO2 should occur in the growth medium during the TSSG experiment. Millimeter-sized as-grown α-Ge1−xSixO2 single crystals (starting from a glassy solute with x = 0.03) obtained by spontaneous nucleation in a high-temperature solution of K2Mo4O1313 were used as seeds. Structural16 and chemical13 characterizations undertaken on these seeds have pointed out their high crystalline quality. In order to enhance the crystal dimension, the TSSG technique was developed. The growth solution was prepared by mixing an appropriate ratio of GeO2/K2Mo4O13 in a platinum crucible and homogenizing at high temperature (1000 °C) for 2 days. Then, the platinum crucible containing the growth solution was placed in the center of a homemade tubular three-zone resistively heated furnace. The axial temperature gradient of the growth solution was measured with a Pt/Pt−Rh thermocouple. Because the crucible position inside the growth furnace was fixed, an axial temperature gradient was assigned to a value of 0.1 °C/mm (Figure 1) by changing the control temperature of the three resistive zones of the furnace. The obtained value would lead to a homogeneous diffusion of the dissolved nutrient
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RESULTS AND DISCUSSION In a published work by Goodrum14 in the 1970s on TSSG experiments using Li2O−WO3 flux, a colorless hexagonal αGeO2 single crystal with a maximum final diameter of 5 mm
Figure 1. Axial temperature gradient inside the growth solution used to obtain the bulk α-GeO2 crystal by top seeded solution growth (TSSG) method. B
dx.doi.org/10.1021/cg4000523 | Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 4. (a) X-plate (surface (S) ≈ 13 mm2; thickness: t = 390 μm), (b) Y-plate (S ≈ 18 mm2; t = 230 μm), and (c) Z-plate (S ≈ 13 mm2; t = 330 μm) cuts from the α-GeO2 single crystal grown by the TSSG technique.
Figure 2. Pictures of the α-Ge0.97Si0.03O2 growth seed (a) and the αGeO2 single crystal obtained by the TSSG technique (b).
was obtained. The as-grown crystal, which did not present hexagonal morphology, was not sharply faceted or homogeneously transparent (white inclusions). Moreover, the experimental procedure followed by Goodrum was quite unclear, and some important parameters were missing (size of the seed, precise thermal program, etc.). Concerning the chemical and structural quality of this TSSG crystal, little information was available. Therefore, based on Goodrum’s paper,14 some unseeded growth experiments using X2O−WO3 (X = Li, K) fluxes were run in our laboratory.13 The results were very poor as the majority of the as-grown α-GeO2 crystals contained yellow flux inclusions. Thus, other inorganic solvents were investigated such as K2Mo4O13, and new growth parameters were determined in order to decrease the starting growth temperature from 105014 to 950 °C. The K2Mo4O13 flux gives GeO2based crystals with no flux inclusions and presents a quite low volatility at high temperature (loss of mass
