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Liquid immiscibility in a CTGS (Ca3TaGa3Si2O14) melt
Journal of Crystal Growth 454 (2016) 82–86
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Liquid immiscibility in a CTGS (Ca3TaGa3Si2O14) melt Jun Nozawa n, Hengyu Zhao, Chihiro Koyama, Kensaku Maeda, Kozo Fujiwara, Haruhiko Koizumi, Satoshi Uda Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 29 July 2016 Received in revised form 31 August 2016 Accepted 1 September 2016 Communicated by: T.F. Kuech Available online 3 September 2016
Although many studies have indicated that Ca3TaGa3Si2O14 (CTGS) grows congruently from a stoichiometric melt when using the Czochralski (Cz) technique, the occurrence of a secondary phase during growth when using the micro-pulling down (μ-PD) technique has been reported. We have examined the detailed growth mechanism of μ-PD grown CTGS as well as its congruency. Differential thermal analysis (DTA) at an elevated temperature up to 1650 °C shows no peaks associated with the presence of a secondary phase, whereas a secondary phase related peak was detected at an elevated temperature up to 1490 °C with the same heating rate. Back-scattered electron images (BEIs) revealed the occurrence of Ca3Ta2Ga4O14 (CTG) as a secondary phase. The secondary phase appears at the very early stage of growth, which is not possible to explain by a eutectic reaction. The experimental results suggest that liquid immiscibility was present in the melt at around 1490 °C during the growth of s-CTGS. Liquid immiscibility produces Si-rich and Si-poor melts, from which different phases with different compositions are solidified. The μ-PD technique poses a more static environment in the melt than that of Cz technique due to low melt convection and the lack of stirring, which enables liquid immiscibility to emerge. & 2016 Elsevier B.V. All rights reserved.
Keywords: A2. Growth from melt A2. Single crystal growth B1. Oxides B2. Piezoelectric materials
1. Introduction Pressure or temperature sensors that work in high temperature environments offer versatile applications in the energy, aerospace, and automotive industries [1–3]. In particular, sensitive and stable combustion pressure sensors used in cars [4] are in strong demand. Such sensors improve the consumption efficiency of engines and hence greatly mitigate their environmental load. A hightemperature pressure sensor requires high electrical resistivity at a high working temperature [5], and langasite-type crystals are regarded as promising materials for such applications. Quartz (α-SiO2) is now commonly employed in piezoelectric resonators and sensors because it possesses high thermal stability, a high Q value, and high electrical resistivity. However, the α-β phase transition of quartz at 573 °C is an obstacle for its use in high-temperature sensors. GaPO4 is a good candidate for a high temperature sensor due to the high temperature of its α-β transition (around 970 °C). However, growing large crystals using the hydrothermal method is difficult. In contrast, langasite-type crystals can be grown in large sizes by the Czochralski (Cz) method and Bridgeman technique [6], and no phase transitions occur up to the melting temperature. Moreover, each piezoelectric coefficient n
Corresponding author. E-mail address: [email protected] (J. Nozawa).
http://dx.doi.org/10.1016/j.jcrysgro.2016.09.005 0022-0248/& 2016 Elsevier B.V. All rights reserved.
is higher than that of quartz [7]. The general chemical formula of a langasite-family crystal is A3BC3D2O14. Langasite-type crystals are subdivided into ordered and disordered structures. Langasite (LGS, La3Ga5SiO14) and langatate (LTG, La3Ta0.5Ga5.5O14) have disordered structures, and are well-known piezoelectric materials that can be applied to pressure sensors [8,9] and in surface acoustic wave (SAW) devices [10]. The constituent element in an ordered structure occupies only one site in the crystal, which is why the ordered structure is expected to have good piezoelectric properties since no vacancies are formed due to anti-site defects. CTGS and CNGS are typical ordered langasite-family crystals [11,12]. Langasite-family crystals with ordered structures such as CNGS, CTGS, and SNGS have been well investigated [13–20] because they pose higher electrical resistivity than LTG, LNG, and LGS at high temperature. Here, the congruency is crucial for the growth of single-phase crystals. Langasite-type crystals with three elements, such as langasite (LGS) and langatate (LTG), have been shown to be incongruent [21,22], which causes fatal polycrystallization defects during growth. For the growth of large-scale crystals intended for use as high temperature pressure sensors, a congruent langasite-type crystal with an ordered structure is desired. In order to grow high quality single crystals, congruent growth is required. However, controversial results have been obtained regarding the congruency of CTGS. Many groups have reported
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that CTGS crystals grown using the Czochralski (Cz) technique do not show secondary phases [23–25]. In contrast, Yokota et al. [26] observed a secondary phase in the peripheral portion of a CTGS crystal grown from a stoichiometric melt using the micro-pulling down (μ-PD) technique. In the present study, the growth mechanism of CTGS in association with the presence of secondary phases is investigated for a crystal grown by the μ-PD technique. The formation mechanism of the secondary phase of the CTGS crystals is discussed based on the results of DTA carried out at different maximum temperatures and backscattered electron images (BEI) of secondary phases.
2. Experimental To study the congruency of CTGS, DTA measurements were carried out using a Linseis DTA/DSC PT1600 under an air atmosphere with heating and cooling rates of 20 °C/min. Pt crucibles were used for the sample and the reference. Sintered powder of stoichiometric CTGS (s-CTGS) (50 mg) was placed in the sample crucible, while the reference crucible remained empty. For temperature calibration, the melting point of Au (1064.4 °C) was used and confirmed before and after the measurement of CTGS. Thus, the accuracy of the DTA data for CTGS was assured. A CTGS bulk crystal was grown using the micro-pulling-down (μ-PD) technique. A pipe-shaped capillary with a length of 1 mm and a diameter of 2 mm was attached to the bottom of the container portion of the crucible. The container portion of the crucible was Pt–Rh (10% Rh), while the capillary was pure Pt. Electric power (5 V, 10 A) was applied directly to the Pt–Rh crucible for selfheating. The temperature of the melt was controlled by varying the current supplied to the crucible. The after-heater in the furnace was placed 2 cm below the capillary and was set at 800 °C. Assuming that the temperature of the liquid–solid interface was approximately 1430 °C, the temperature gradient at the interface was in the range of 500–700 °C/cm. A langatate crystal with a c-axis orientation was used as a seed to grow the CTGS crystal. Langatate has the same langasite crystal structure as CTGS, which results in only a slight mismatch of the crystal lattice. Thus, a low degree of distortion and a high degree of coherency was achieved in the lattice of the obtained CTGS crystal. In addition, the melting point of langatate is approximately 1450 °C, which is close to that of CTGS (1430 °C). The langatate seed crystal was moved upward until it contacted the CTGS melt at the tip of the capillary. The CTGS crystal was then grown by pulling down the seed at a rate of 2 mm per hour. The interface between the melt and the crystal was observed during crystal growth (Fig. 1).
Fig. 1. Growing CTGS with the micro-pulling-down (μ-PD) technique.
Fig. 2. Two cycles of DTA measurements for melting and solidification of sintered stoichiometric CTGS; (a) heated to 1650 °C, (b) heated to 1490 °C.
3. Results and discussion 3.1. Congruency of CTGS Fig. 2 shows two cycles of DTA plots for the melting and solidification of sintered powder of CTGS (s-CTGS) of stoichiometric composition. Fig. 2(a) presents the curves for heating the sample up to 1650 °C, while Fig. 2(b) shows the curves for heating up to 1490 °C. In Fig. 2(a), endothermic peaks near 1430 °C in both cycles represent the melting of CTGS. During the cooling process for both cycles, the minor peaks near 1285 °C indicate the solidification of CTGS from the melt. These peaks have weak intensities because most of the melt turned into glass instead of crystal. The exothermic peak observed during heating in the 2nd cycle indicates the transition of CTGS from the glass to the crystalline state around 1050 °C. Notably, both cycles exhibit a pair of peaks, an endothermic peak for melting and an exothermic peak for crystallization, demonstrating that CTGS is congruent. When the sample was heated to 1490 °C [Fig. 2(b)], melting peaks near 1430 °C were observed in both cycles. They were also observed when the sample was heated to 1650 °C. However, during the cooling process, an exothermic peak appeared at 1280 °C followed by a smaller exothermic peak at 1160 °C. This smaller peak may represent the crystallization of the secondary phase, which was not detected when the sample was heated to 1650 °C. The difference in the maximum heating temperature led to this difference, and its cause will be discussed later.
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Fig. 3. (a) The s-CTGS crystal grown with the μ-PD technique. (b) BEI image of thin section perpendicular to growth direction, close to the interface between seed as langatate (LTG, La3Ta0.5Ga5.5O14) and grown crystal. (c) Enlarged view of (b).
3.2. Occurrence of secondary phase in
μ-PD grown crystal
A colorless and transparent CTGS single crystal with a mass of 120 mg, a length of 7 mm, and a diameter of 2 mm was obtained via the μ-PD technique [Fig. 3(a)] using a langatate single crystal as a seed. To evaluate the degree of homogeneity of the crystal and to determine whether any secondary phases were present in the CTGS crystal, a back-scattered electron image (BEI) of a cross section of the obtained crystal was obtained. The specimen was cut from the top portion of the obtained CTGS crystal, which is the initial stage of the growth, and mirror polished. BEI analysis revealed secondary phases in the matrix as white spheres [Fig. 3(b)]. The bright region enclosed by the white dashed line is the langatate (LTG, La3Ta0.5Ga5.5O14) seed crystal. Fig. 3 (c) shows a more highly magnified BEI image of the area where CTGS contacts the langatate seed crystal. The compositions of the matrix and secondary phases were determined via electron-probe microanalysis (EPMA), and the secondary phase was identified as a Ca–Ta–Ga oxide (CTG:Ca3Ta2Ga4O14). The compositions of the matrix CTGS and secondary CTG phases are presented in Table 1. The atomic ratios were normalized in such a way that their values were the same as the chemical formula of CTGS when it has stoichiometric composition. The matrix phase of CTGS was rich in Si and poor in Ga and Ta compared to the s-CTGS (Ca:Ta:Ga: Si ¼3:1:3:2). It should be noted that CTG appeared in the very early stage of CTGS crystallization. Many groups have reported that CTGS crystals grown using the Czochralski (Cz) technique do not appear to have secondary phases [23–25]. In addition, Yokota et al. [26] observed a secondary phase in the peripheral portion of a CTGS crystal grown from a stoichiometric melt using the μ-PD technique, which is similar to the result obtained in the present study.
mechanism. Incongruency of a material yields a primary phase different from the host phase during the early stage of the cooling process. However, as shown in Fig. 2, a pair of endothermic and exothermic peaks for melting and solidification of CTGS without any other reaction peaks indicates that CTGS is congruent. The eutectic reaction yields a secondary phase when the melt reaches the eutectic point. As reported by Zhao et al. [27], the stoichiometric composition of CTGS was found to be outside the solid-solution region. The schematic phase diagram for the CTGS– CTG binary system is shown in Fig. 4. During CTGS crystallization from the stoichiometric melt, the composition of the melt travels along the liquidus line, and the secondary CTG phase appears when the composition of the melt reaches the eutectic point (e). However, such a secondary phase induced by the eutectic reaction only forms during the late stage of crystal growth. In the present study, the CTG phase was observed during the beginning stage of crystal growth [Fig. 3(b), (c)], which indicates that it was not formed due to the eutectic reaction. If the phase diagram shown in Fig. 4 is plausible, CTG would appear at the final solidification stage by the Cz technique. However, no report of the secondary phase was found in references [23–25]. We think that this is due in part to the low solidification fraction of the Cz technique (in general 1/3 to 1/2). Thus, the formation of the secondary phase of CTG can be explained by liquid immiscibility. Liquid immiscibility is the coexistence of two liquids with different compositions in equilibrium with each other. It has been studied as an important
3.3. Liquid immiscibility in CTGS melts Three possible mechanisms for the growth of CTGS from the stoichiometric melt with secondary phase were considered: incongruency, eutectic reaction, and liquid immiscibility. Based on the experimental results, we examined the validity of each Table 1 Composition of the host CTGS and the secondary CTG phases in CTGS crystal grown by the μ-PD technique. Values indicate atomic ratios. Elements
Matrix phase:CTGS
Secondary phase:CTG
Ca Ta Ga Si O
2.94 0.99 2.53 2.54 15.18
3.53 2.12 3.19 0.16 15.44
Fig. 4. Schematic phase diagram for the pseudo-binary CTGS-CTG system assuming a eutectic reaction.
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Fig. 5. Schematic phase diagram for the pseudo-binary CTGS-CTG system showing the liquid immiscibility gap in the CTGS melt; (a) cooled down from 1490 °C and (b) cooled down from 1650 °C.
differentiation mechanism in silicate melts [28–30]. The spherical shape of the secondary phase [Fig. 3(b)] is typical of the shape of a pre-solidified melt embedded in a host melt for liquid immiscibility. Although liquid immiscibility has been reported in langasite-type crystals with three elements [6], it is not known for those containing four elements. When the melt with a stoichiometric CTGS composition was cooled down from 1490 °C, liquid–liquid phase separation possibly occurred prior to initiation of the solidification process, leading to differentiation into Si-poor and Si-rich melts [Fig. 5(a)]. CTGS subsequently crystallized from the Si-rich melt, while the CTG secondary phase precipitated from the Si-poor melt. However, the melt cooled from 1650 °C did not experience such a phase separation but directly reached the CTGS liquidus line [Fig. 5(b)]. This would be due to the difference of thermal history of DTA measurements for heating up to 1490 and 1650 °C. The measurement heating up to 1490 °C stays longer time in the liquid immiscibility region than that of the 1650 °C one. Since the time is required to reach equilibrium for the liquid immiscibility state, we infer that this difference is attributed to the fact that the liquid immiscibility was observed only for the 1490 °C measurement. The immiscibility of the liquid is metastable and easily suppressed by stirring the melt, which is conducted during Cz growth. In contrast, there is no stirring of the melt in the μ-PD technique and the melt flow is not significant [31,32], thus, a static state of the melt is obtained. Liquid immiscibility can occur under such a situation. In addition, the lower solidification rate of Cz causes the secondary phases to be hidden in the melt that remains separate from the growing crystal. However, with the μ-PD method, all the melt is consumed, so secondary phases cannot stay hidden. For the growth of single CTGS crystals, stability against liquid immiscibility in the melt is found to be important. The occurrence of liquid immiscibility of CTGS growth indicates that it could happen in other ordered four-element langasite crystals. 4. Conclusion The congruency and liquid immiscibility of CTGS grown from
the stoichiometric melt were investigated. The formation mechanism of the CTG secondary phase was also clarified. CTGS was shown to be a congruently melting material, with only a pair of melting and solidification peaks observed in the DTA curves. Considering that langasite-type crystals with three elements are known to be incongruently melting materials, CTGS, which is a langasite-type crystal with four elements, is advantageous for crystal growth. CTGS crystal grown by the μ-PD technique contains a secondary phase that was determined to be CTG poor in Si. Using EPMA, the composition of the host CTGS phase was determined to be rich in Si and poor in Ga and Ta. Considering the early crystallization of the CTG phase, liquid immiscibility might have occurred in the s-CTGS melt near 1490 °C, and Si-rich and Si-poor melts were formed. The host CTGS crystal solidified from the Si-rich melt that belonged to the solid-solution region, whereas the CTG secondary phase precipitated from the Si-poor melt. Even a crystal that grows congruently produces secondary phases when liquid immiscibility occurs in a melt.
Acknowledgements This work was supported in part by JSPS KAKENHI Grant No. 24360002.
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Liquid immiscibility in a CTGS (Ca3TaGa3Si2O14) melt Jun Nozawa n, Hengyu Zhao, Chihiro Koyama, Kensaku Maeda, Kozo Fujiwara, Haruhiko Koizumi, Satoshi Uda Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 29 July 2016 Received in revised form 31 August 2016 Accepted 1 September 2016 Communicated by: T.F. Kuech Available online 3 September 2016
Although many studies have indicated that Ca3TaGa3Si2O14 (CTGS) grows congruently from a stoichiometric melt when using the Czochralski (Cz) technique, the occurrence of a secondary phase during growth when using the micro-pulling down (μ-PD) technique has been reported. We have examined the detailed growth mechanism of μ-PD grown CTGS as well as its congruency. Differential thermal analysis (DTA) at an elevated temperature up to 1650 °C shows no peaks associated with the presence of a secondary phase, whereas a secondary phase related peak was detected at an elevated temperature up to 1490 °C with the same heating rate. Back-scattered electron images (BEIs) revealed the occurrence of Ca3Ta2Ga4O14 (CTG) as a secondary phase. The secondary phase appears at the very early stage of growth, which is not possible to explain by a eutectic reaction. The experimental results suggest that liquid immiscibility was present in the melt at around 1490 °C during the growth of s-CTGS. Liquid immiscibility produces Si-rich and Si-poor melts, from which different phases with different compositions are solidified. The μ-PD technique poses a more static environment in the melt than that of Cz technique due to low melt convection and the lack of stirring, which enables liquid immiscibility to emerge. & 2016 Elsevier B.V. All rights reserved.
Keywords: A2. Growth from melt A2. Single crystal growth B1. Oxides B2. Piezoelectric materials
1. Introduction Pressure or temperature sensors that work in high temperature environments offer versatile applications in the energy, aerospace, and automotive industries [1–3]. In particular, sensitive and stable combustion pressure sensors used in cars [4] are in strong demand. Such sensors improve the consumption efficiency of engines and hence greatly mitigate their environmental load. A hightemperature pressure sensor requires high electrical resistivity at a high working temperature [5], and langasite-type crystals are regarded as promising materials for such applications. Quartz (α-SiO2) is now commonly employed in piezoelectric resonators and sensors because it possesses high thermal stability, a high Q value, and high electrical resistivity. However, the α-β phase transition of quartz at 573 °C is an obstacle for its use in high-temperature sensors. GaPO4 is a good candidate for a high temperature sensor due to the high temperature of its α-β transition (around 970 °C). However, growing large crystals using the hydrothermal method is difficult. In contrast, langasite-type crystals can be grown in large sizes by the Czochralski (Cz) method and Bridgeman technique [6], and no phase transitions occur up to the melting temperature. Moreover, each piezoelectric coefficient n
Corresponding author. E-mail address: [email protected] (J. Nozawa).
http://dx.doi.org/10.1016/j.jcrysgro.2016.09.005 0022-0248/& 2016 Elsevier B.V. All rights reserved.
is higher than that of quartz [7]. The general chemical formula of a langasite-family crystal is A3BC3D2O14. Langasite-type crystals are subdivided into ordered and disordered structures. Langasite (LGS, La3Ga5SiO14) and langatate (LTG, La3Ta0.5Ga5.5O14) have disordered structures, and are well-known piezoelectric materials that can be applied to pressure sensors [8,9] and in surface acoustic wave (SAW) devices [10]. The constituent element in an ordered structure occupies only one site in the crystal, which is why the ordered structure is expected to have good piezoelectric properties since no vacancies are formed due to anti-site defects. CTGS and CNGS are typical ordered langasite-family crystals [11,12]. Langasite-family crystals with ordered structures such as CNGS, CTGS, and SNGS have been well investigated [13–20] because they pose higher electrical resistivity than LTG, LNG, and LGS at high temperature. Here, the congruency is crucial for the growth of single-phase crystals. Langasite-type crystals with three elements, such as langasite (LGS) and langatate (LTG), have been shown to be incongruent [21,22], which causes fatal polycrystallization defects during growth. For the growth of large-scale crystals intended for use as high temperature pressure sensors, a congruent langasite-type crystal with an ordered structure is desired. In order to grow high quality single crystals, congruent growth is required. However, controversial results have been obtained regarding the congruency of CTGS. Many groups have reported
J. Nozawa et al. / Journal of Crystal Growth 454 (2016) 82–86
83
that CTGS crystals grown using the Czochralski (Cz) technique do not show secondary phases [23–25]. In contrast, Yokota et al. [26] observed a secondary phase in the peripheral portion of a CTGS crystal grown from a stoichiometric melt using the micro-pulling down (μ-PD) technique. In the present study, the growth mechanism of CTGS in association with the presence of secondary phases is investigated for a crystal grown by the μ-PD technique. The formation mechanism of the secondary phase of the CTGS crystals is discussed based on the results of DTA carried out at different maximum temperatures and backscattered electron images (BEI) of secondary phases.
2. Experimental To study the congruency of CTGS, DTA measurements were carried out using a Linseis DTA/DSC PT1600 under an air atmosphere with heating and cooling rates of 20 °C/min. Pt crucibles were used for the sample and the reference. Sintered powder of stoichiometric CTGS (s-CTGS) (50 mg) was placed in the sample crucible, while the reference crucible remained empty. For temperature calibration, the melting point of Au (1064.4 °C) was used and confirmed before and after the measurement of CTGS. Thus, the accuracy of the DTA data for CTGS was assured. A CTGS bulk crystal was grown using the micro-pulling-down (μ-PD) technique. A pipe-shaped capillary with a length of 1 mm and a diameter of 2 mm was attached to the bottom of the container portion of the crucible. The container portion of the crucible was Pt–Rh (10% Rh), while the capillary was pure Pt. Electric power (5 V, 10 A) was applied directly to the Pt–Rh crucible for selfheating. The temperature of the melt was controlled by varying the current supplied to the crucible. The after-heater in the furnace was placed 2 cm below the capillary and was set at 800 °C. Assuming that the temperature of the liquid–solid interface was approximately 1430 °C, the temperature gradient at the interface was in the range of 500–700 °C/cm. A langatate crystal with a c-axis orientation was used as a seed to grow the CTGS crystal. Langatate has the same langasite crystal structure as CTGS, which results in only a slight mismatch of the crystal lattice. Thus, a low degree of distortion and a high degree of coherency was achieved in the lattice of the obtained CTGS crystal. In addition, the melting point of langatate is approximately 1450 °C, which is close to that of CTGS (1430 °C). The langatate seed crystal was moved upward until it contacted the CTGS melt at the tip of the capillary. The CTGS crystal was then grown by pulling down the seed at a rate of 2 mm per hour. The interface between the melt and the crystal was observed during crystal growth (Fig. 1).
Fig. 1. Growing CTGS with the micro-pulling-down (μ-PD) technique.
Fig. 2. Two cycles of DTA measurements for melting and solidification of sintered stoichiometric CTGS; (a) heated to 1650 °C, (b) heated to 1490 °C.
3. Results and discussion 3.1. Congruency of CTGS Fig. 2 shows two cycles of DTA plots for the melting and solidification of sintered powder of CTGS (s-CTGS) of stoichiometric composition. Fig. 2(a) presents the curves for heating the sample up to 1650 °C, while Fig. 2(b) shows the curves for heating up to 1490 °C. In Fig. 2(a), endothermic peaks near 1430 °C in both cycles represent the melting of CTGS. During the cooling process for both cycles, the minor peaks near 1285 °C indicate the solidification of CTGS from the melt. These peaks have weak intensities because most of the melt turned into glass instead of crystal. The exothermic peak observed during heating in the 2nd cycle indicates the transition of CTGS from the glass to the crystalline state around 1050 °C. Notably, both cycles exhibit a pair of peaks, an endothermic peak for melting and an exothermic peak for crystallization, demonstrating that CTGS is congruent. When the sample was heated to 1490 °C [Fig. 2(b)], melting peaks near 1430 °C were observed in both cycles. They were also observed when the sample was heated to 1650 °C. However, during the cooling process, an exothermic peak appeared at 1280 °C followed by a smaller exothermic peak at 1160 °C. This smaller peak may represent the crystallization of the secondary phase, which was not detected when the sample was heated to 1650 °C. The difference in the maximum heating temperature led to this difference, and its cause will be discussed later.
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Fig. 3. (a) The s-CTGS crystal grown with the μ-PD technique. (b) BEI image of thin section perpendicular to growth direction, close to the interface between seed as langatate (LTG, La3Ta0.5Ga5.5O14) and grown crystal. (c) Enlarged view of (b).
3.2. Occurrence of secondary phase in
μ-PD grown crystal
A colorless and transparent CTGS single crystal with a mass of 120 mg, a length of 7 mm, and a diameter of 2 mm was obtained via the μ-PD technique [Fig. 3(a)] using a langatate single crystal as a seed. To evaluate the degree of homogeneity of the crystal and to determine whether any secondary phases were present in the CTGS crystal, a back-scattered electron image (BEI) of a cross section of the obtained crystal was obtained. The specimen was cut from the top portion of the obtained CTGS crystal, which is the initial stage of the growth, and mirror polished. BEI analysis revealed secondary phases in the matrix as white spheres [Fig. 3(b)]. The bright region enclosed by the white dashed line is the langatate (LTG, La3Ta0.5Ga5.5O14) seed crystal. Fig. 3 (c) shows a more highly magnified BEI image of the area where CTGS contacts the langatate seed crystal. The compositions of the matrix and secondary phases were determined via electron-probe microanalysis (EPMA), and the secondary phase was identified as a Ca–Ta–Ga oxide (CTG:Ca3Ta2Ga4O14). The compositions of the matrix CTGS and secondary CTG phases are presented in Table 1. The atomic ratios were normalized in such a way that their values were the same as the chemical formula of CTGS when it has stoichiometric composition. The matrix phase of CTGS was rich in Si and poor in Ga and Ta compared to the s-CTGS (Ca:Ta:Ga: Si ¼3:1:3:2). It should be noted that CTG appeared in the very early stage of CTGS crystallization. Many groups have reported that CTGS crystals grown using the Czochralski (Cz) technique do not appear to have secondary phases [23–25]. In addition, Yokota et al. [26] observed a secondary phase in the peripheral portion of a CTGS crystal grown from a stoichiometric melt using the μ-PD technique, which is similar to the result obtained in the present study.
mechanism. Incongruency of a material yields a primary phase different from the host phase during the early stage of the cooling process. However, as shown in Fig. 2, a pair of endothermic and exothermic peaks for melting and solidification of CTGS without any other reaction peaks indicates that CTGS is congruent. The eutectic reaction yields a secondary phase when the melt reaches the eutectic point. As reported by Zhao et al. [27], the stoichiometric composition of CTGS was found to be outside the solid-solution region. The schematic phase diagram for the CTGS– CTG binary system is shown in Fig. 4. During CTGS crystallization from the stoichiometric melt, the composition of the melt travels along the liquidus line, and the secondary CTG phase appears when the composition of the melt reaches the eutectic point (e). However, such a secondary phase induced by the eutectic reaction only forms during the late stage of crystal growth. In the present study, the CTG phase was observed during the beginning stage of crystal growth [Fig. 3(b), (c)], which indicates that it was not formed due to the eutectic reaction. If the phase diagram shown in Fig. 4 is plausible, CTG would appear at the final solidification stage by the Cz technique. However, no report of the secondary phase was found in references [23–25]. We think that this is due in part to the low solidification fraction of the Cz technique (in general 1/3 to 1/2). Thus, the formation of the secondary phase of CTG can be explained by liquid immiscibility. Liquid immiscibility is the coexistence of two liquids with different compositions in equilibrium with each other. It has been studied as an important
3.3. Liquid immiscibility in CTGS melts Three possible mechanisms for the growth of CTGS from the stoichiometric melt with secondary phase were considered: incongruency, eutectic reaction, and liquid immiscibility. Based on the experimental results, we examined the validity of each Table 1 Composition of the host CTGS and the secondary CTG phases in CTGS crystal grown by the μ-PD technique. Values indicate atomic ratios. Elements
Matrix phase:CTGS
Secondary phase:CTG
Ca Ta Ga Si O
2.94 0.99 2.53 2.54 15.18
3.53 2.12 3.19 0.16 15.44
Fig. 4. Schematic phase diagram for the pseudo-binary CTGS-CTG system assuming a eutectic reaction.
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Fig. 5. Schematic phase diagram for the pseudo-binary CTGS-CTG system showing the liquid immiscibility gap in the CTGS melt; (a) cooled down from 1490 °C and (b) cooled down from 1650 °C.
differentiation mechanism in silicate melts [28–30]. The spherical shape of the secondary phase [Fig. 3(b)] is typical of the shape of a pre-solidified melt embedded in a host melt for liquid immiscibility. Although liquid immiscibility has been reported in langasite-type crystals with three elements [6], it is not known for those containing four elements. When the melt with a stoichiometric CTGS composition was cooled down from 1490 °C, liquid–liquid phase separation possibly occurred prior to initiation of the solidification process, leading to differentiation into Si-poor and Si-rich melts [Fig. 5(a)]. CTGS subsequently crystallized from the Si-rich melt, while the CTG secondary phase precipitated from the Si-poor melt. However, the melt cooled from 1650 °C did not experience such a phase separation but directly reached the CTGS liquidus line [Fig. 5(b)]. This would be due to the difference of thermal history of DTA measurements for heating up to 1490 and 1650 °C. The measurement heating up to 1490 °C stays longer time in the liquid immiscibility region than that of the 1650 °C one. Since the time is required to reach equilibrium for the liquid immiscibility state, we infer that this difference is attributed to the fact that the liquid immiscibility was observed only for the 1490 °C measurement. The immiscibility of the liquid is metastable and easily suppressed by stirring the melt, which is conducted during Cz growth. In contrast, there is no stirring of the melt in the μ-PD technique and the melt flow is not significant [31,32], thus, a static state of the melt is obtained. Liquid immiscibility can occur under such a situation. In addition, the lower solidification rate of Cz causes the secondary phases to be hidden in the melt that remains separate from the growing crystal. However, with the μ-PD method, all the melt is consumed, so secondary phases cannot stay hidden. For the growth of single CTGS crystals, stability against liquid immiscibility in the melt is found to be important. The occurrence of liquid immiscibility of CTGS growth indicates that it could happen in other ordered four-element langasite crystals. 4. Conclusion The congruency and liquid immiscibility of CTGS grown from
the stoichiometric melt were investigated. The formation mechanism of the CTG secondary phase was also clarified. CTGS was shown to be a congruently melting material, with only a pair of melting and solidification peaks observed in the DTA curves. Considering that langasite-type crystals with three elements are known to be incongruently melting materials, CTGS, which is a langasite-type crystal with four elements, is advantageous for crystal growth. CTGS crystal grown by the μ-PD technique contains a secondary phase that was determined to be CTG poor in Si. Using EPMA, the composition of the host CTGS phase was determined to be rich in Si and poor in Ga and Ta. Considering the early crystallization of the CTG phase, liquid immiscibility might have occurred in the s-CTGS melt near 1490 °C, and Si-rich and Si-poor melts were formed. The host CTGS crystal solidified from the Si-rich melt that belonged to the solid-solution region, whereas the CTG secondary phase precipitated from the Si-poor melt. Even a crystal that grows congruently produces secondary phases when liquid immiscibility occurs in a melt.
Acknowledgements This work was supported in part by JSPS KAKENHI Grant No. 24360002.
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