# Fileset

[Manuscript.pdf](https://mdr.nims.go.jp/filesets/2871d5b9-e3a7-434d-8834-c9a1aa457a5f/download)

## Creator

[Yueshen Zhou](https://orcid.org/0009-0009-5327-9731), [Dongsheng Yuan](https://orcid.org/0000-0001-9650-2272), [Kiyoshi Shimamura](https://orcid.org/0000-0001-6502-8731)

## Rights

[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Bulk crystal growth and characterization of non-centrosymmetric single crystal CaTa4O11](https://mdr.nims.go.jp/datasets/2292a9f0-adf8-41a8-a82a-e968d7ad4dd9)

## Fulltext

ARTICLE   Please do not adjust margins Please do not adjust margins Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x   Bulk crystal growth and characterization of non-centrosymmetric single crystal CaTa4O11 Yueshen Zhou,ab Dongsheng Yuan*a  and Kiyoshi Shimamura*ab Bulk single crystals of non-centrosymmetric CaTa4O11 were grown successfully by the floating-zone method for the first time, despite that the already reported CaO-Ta2O5 phase diagram indicates its incongruent melting behavior. The as-grown crystal rods (Ф 5-6 mm) have a black and a colorless transparent appearance grown in Ar and O2 atmospheres, respectively. Subsequently, O2 atmosphere was selected to optimize its bulk crystal growth. The CaTa4O11 single crystals show a good crystallinity with the evidence of a symmetric X-ray rocking curve for 004 diffraction with a FWHM of 72’’, although the fabrication technique has the drawback of an extremely high thermal gradient for crystal growth. For this new hexagonal bulk single crystal, its anisotropic optical, dielectric, and thermal properties were characterized. The crystal has a transmittance of ~78% from the cut-off edge 350 nm till 800 nm. When excited by both 254 nm UV and X-ray, it shows a broad band emission with a peak wavelength around 427 nm. The dielectric constants ε11 and ε33 were found to be 30 and 52.3, respectively at 1 kHz. Meanwhile, the thermal conductivity along the c-axis is 5.69 Wm-1K-1, which is 1.8 times higher than that along the a-axis, which measures 3.19 Wm-1K-1 at room temperature. Introduction Single crystals of tantalates have garnered great attention across virous applications including nonlinear optics, scintillation, piezoelectricity/pyroelectricity, etc [1-4]. Among these, several single-crystal alkaline-earth tantalates have been grown by the conventional melting techniques such as Czochralski, Bridgman and floating-zone methods to explore their properties and applications. For instance, MTa2O6 (where M represents Mg, Ca, and Sr) crystals were studied as laser active media and microwave resonators [5-8]. The acoustic response of Sr2Ta2O7 single crystals was investigated for employment in ferroelectric random-access memories [9]. As potential scintillators for high-energy X-ray radiography, Mg4Ta2O9 single crystals were found to show bright intrinsic radioluminescence, promoting alkaline-earth tantalates as promising intrinsic scintillator candidates [10,11].     The CaO-Ta2O5 system was reported by K. T. Jacob et al. to have four stoichiometric compounds: CaTa4O11, CaTa2O6, Ca2Ta2O7, and Ca4Ta2O9 [12]. To explore functional properties in this family, their single crystals in bulk form are highly demanded.  Despite this binary phase diagram was already established via the synthesis method of high-temperature solid-solid reaction as reproduced in Fig. 1 [13], the melting behaviours of the stoichiometric compounds were still pending questions until their melting-growth study.                Fig. 1. The reported CaO-Ta2O5 phase diagram above 1200 °C [13].      CaTa2O6 and Ca2Ta2O7 were observed to show polymorphs:  orthorhombic & tetragonal & cubic structures for the former and trigonal & monoclinic for the latter, thus their crystal growths require quenching to avoid the corresponding phase transitions [14,15]. C. R. Ferrari et al. used the laser-heated pedestal growth (LHPG) method that can achieve rapid cooling but can only obtain thin and small samples in a diameter of ~700 μm, not to mention bulk single crystals over 5 mm in size  [6]. Regarding CaTa4O11 and Ca4Ta2O9, the phase diagram indicates their incongruent melting behaviors. To the best of our knowledge, there has been no report of single-crystal form of Ca4Ta2O9. What interests us is the Ta-richest compound here, CaTa4O11, which shows a great potential as a scintillator due to its high density (7.57 g/cm3) and effective atomic number (68.4). a. National Institute for Materials Science, Tsukuba 305-0044, Japan. Email: YUAN.dongsheng@nims.go.jp, SHIMAMURA.Kiyoshi@nims.go.jp b. Graduate School of Advanced Science and Engineering, Waseda University, Shinjuku, Tokyo 169-8555, Japan.  mailto:YUAN.dongsheng@nims.go.jpARTICLE Journal Name 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Although Akashi et al. mentioned the synthesis of crystal samples using the floating-zone technique in 1969 [16], however, no subsequent reports on either bulk single crystal growth or the basic physical properties of CaTa4O11 were seen. To answer the question that CaTa4O11 melts incongruently as reported in the phase-diagram [13] or not and further investigate potential properties of CaTa4O11 single crystals, we studied its crystal growth using the Xenon lamp floating zone method.      In this work, CaTa4O11 bulk single crystal growth in different atmospheres were investigated. The optimized high-quality crystal was characterized in terms of structural, anisotropic optical, thermal, and dielectric properties.   Experimental Crystal growth Polycrystalline CaTa4O11 rods were fabricated through the traditional solid-state reaction method using high-purity (99.99%) Ta2O5 and CaCO3 raw materials. Initially, the stoichiometric powders of raw materials were weighed and mixed thoroughly. A follow-up calcination was carried out at 1100 °C for 10 hours in air. After calcination, the powders were packed into a rubber tube and then pressed isostatically under 300 MPa. The pressed rods were subsequently sintered in air at 1300 °C for 10 hours, resulting in the production of dense ceramic rods.     Single-crystal growth of CaTa4O11 was conducted using a Xenon lamp floating-zone furnace. The growth rate was 6 mm/h. During the growth process, the feed rod rotated at a speed of 26 rpm, while the seed rod at 13 rpm in the opposite direction. Both argon (Ar) and oxygen (O2) atmospheres were employed to optimize the crystal quality.  Characterization  The powder X-ray diffraction analysis was conducted by a Rigaku SmartLab3 diffractometer (Cu Kα X-ray radiation with a wavelength of 1.54059 Å). The crystal quality was evaluated by X-ray rocking curve measurement on a c-oriented sample, using a high-resolution X-ray diffractometer of PANalytical X’Pert PRO. Optical transmittance were measured with a JASCO spectrometer V-570. Photoluminescence (PL) spectrum was acquired using a JASCO FP-8650FDA fluorescence spectrometer. An X-ray generator was used for measuring the X-ray excited luminescence spectrum [17]. Dielectric constants were determined at a frequency of 1 kHz utilizing an LCR meter (Hioki IM3536). The Dulong-Petit law was applied to estimate the specific heat capacity (Cp) [18], while thermal diffusivity measurements were carried out through a laser flash method using an LFA 467 instrument from NETZSCH.  Results and discussion Crystals and Structure Figure 2 shows the images of single-crystal CaTa4O11 grown in Ar and O2 atmosphere. CaTa4O11 crystal grown in Ar presents a black color as shown in Fig. 2(a). This originates from the reduction of Ta5+ to Ta4+ at high temperature in an oxygen-deficient atmosphere and correspondingly the formation of oxygen vacancies [5]. Conversely, growing the crystal in an O2 atmosphere effectively eliminates the black color, resulting in a colorless and transparent single crystal, as shown in Fig. 2(b).  Powder X-ray diffraction analysis confirmed that the as-grown crystals are single-phase hexagonal, with profiles matching the ICSD reference (01-070-0847), as illustrated in Fig. 3 [16].          Fig. 2. Images of the CaTa4O11 crystals grown from (a) Ar and (b) O2. The black arrows show the direction of the crystal growth.              Fig. 3. Powder X-ray diffraction patterns of the as-grown single-crystal CaTa4O11.       The X-ray rocking curve of 004 diffraction peak is shown in Fig. 4. The curve is symmetric and has a full width at half maximum (FWHM) of 72′′, indicating a good crystallinity of the as-grown crystals.                Fig. 4. X-ray rocking curve of O2-grown CaTa4O11 crystal. Journal Name  ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3  Please do not adjust margins Please do not adjust margins Given the absence of reliable crystal structure reports from high-quality CaTa4O11 bulk single crystals, here the crystal structure was analyzed. The structure and refinement data of CaTa4O11 are listed in Table 1 with a more reasonable R-factor than all reported values [16, 19]. The corresponding cif file is given in the ESI‡. CaTa4O11 crystallizes in the hexagonal system with the space group of P6322 (a = b = 6.2127 Å, c = 12.2638 Å). Fig. 5(a) shows an asymmetric unit and the corresponding bonding lengths of CaTa4O11. One of the Ta atoms, designated as Ta2, is coordinated by seven oxygen atoms, forming a pentagonal bipyramid. The remaining Ta (Ta1) and Ca atoms are surrounded by six and eight oxygen atoms, respectively. Fig. 5(b) shows the crystal structure of CaTa4O11 single crystal projected to <110> direction. We can see that it contains an alternant structure with a layer formed by Ta2 pentagonal bipyramids and another layer formed by Ta1 and Ca atoms where both layers are parallel to the c-plane.  Table 1 Crystal data and structure refinement of CaTa4O11 single crystal Empirical formula CaTa4O11 Formula weights (g mol-1) 939.88 Temperature (K) 297 Crystal system, space group Hexagonal, P6322   Unit cell dimensions (Å) a=b=6.2127 (2), c=12.2638(4) V (Å3) 409.94 (3) Density (g cm-3) 7.614 Z 2 F000 800 Absorption coefficient (mm-1) 53.885 Theta range (deg.) 3.753-29.169 Limiting indices -8  h  7, -6  k  8, -15  l  15 Reflection number 5531 Unique/collected reflection number 355/349 Rint 0.0402 Final R indices [I > 2(I)] 0.0134/0.0299 R indices (all data) 0.0141/0.03 Goodness-of-fit 1.276 Largest diff. peak and hole (e/Å3) 0.942/-0.931              Fig. 5 (a) An asymmetric unit formed by one TaO6 group, one TaO7 group, and the Ca atom in eightfold oxygen coordination, together with their corresponding interatomic distances, (b) crystal structure of CaTa4O11 single crystal projected to <110> direction.  Optical and luminescence properties The transmission spectra of CaTa4O11 single crystals obtained in an O2 atmosphere for the (010) and (001) planes are displayed in Fig. 6(a). Both exhibit a transmittance of approximately 78% across the wavelength range from 350 to 800 nm. To determine its optical band gap, the Tauc formula (h)1/n = A1/n (h-Eg) was applied, and a better fitting was achieved with n = 1/2, as illustrated in Fig. 6(b) [20]. This finding suggests that it has a direct band gap of about 4.5 eV. Fig. 6(c) presents the excitation and emission spectra where a dominant excitation band appears at 255 nm and the corresponding emission band shows a peak wavelength at 427. It can be seen in the fitted emission spectrum that the dominant emission broad band was deconvoluted into only one peak as shown in Fig. S1‡. According to the previous research, this might originate from the Ta-O charge-transfer transition associated with the TaO6/TaO7 polyhedrons in CaTa4O11 [21]. Under X-ray excitation, CaTa4O11 also exhibits a band with a peak wavelength around 427 nm, as depicted in Fig. 6(d). The similarity of the spectrum to the PL one implies the same emission center for both UV and X-ray excitation.                              ARTICLE Journal Name 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins                            Fig. 6. (a) Transmission spectra of CaTa4O11, (b) the variation of (αh)2 vs. photon energy (h), (c) PL excitation and emission spectra of CaTa4O11, (d) X-ray luminescence spectrum.  Dielectric constants CaTa4O11 has two independent dielectric constants and were measured at 1kHz as ε11 = 30 and ε33 = 52.3. Table 2 lists the dielectric constants of CaTa4O11 crystal, in comparison with -quartz, and some other tantalates crystals. The dielectric constant ε33 of CaTa4O11 is larger than those of α-quartz, Ca3TaGa3Si2O14, and LiTaO3, but lower than that of SrTa2O6.  Table 2 Dielectric constants of CaTa4O11 crystal in comparison with -quartz, Ca3TaGa3Si2O14, LiTaO3 and SrTa2O6 crystals Crystal ε11 ε33 CaTa4O11 30 52.3 -quartz [22] 4.5 4.6 Ca3TaGa3Si2O14 [23] 16.3 21.6 LiTaO3 [24] 54 43 SrTa2O6 [8] 108 128  Thermal conductivity The thermal conductivity can be calculated by the equation: k = ρCp, where k, , ρ and Cp denote the thermal conductivity, thermal diffusivity coefficient, density, and specific heat capacity of the crystal, respectively [25]. The thermal conductivity kij of a crystal is also a symmetrical second-rank tensor like the electric permittivity, and for point group 622, the matrix controlled by Neumann’s principle is the following form [26, 27]:            So, the thermal diffusivity coefficients of the CaTa4O11 crystal were measured along the a-axis and c-axis by a laser flash method, and the results are shown in Fig. 7(a). The specific heat capacity was estimated by the Dulong-petit law, utilizing the equation Cp = 3rR/M, where r = 16 is the number of atoms in one formula, with the gas constant R being 8.314 J/(k·mol), and M for molar mass being 939.86 g/mol. The calculated Cp is approximately 0.42 J/(k·g). Note that the Dulong-petit law is not a sufficiently accurate measure, and the calculated Cp value might be a little bit higher than the real ones [18]. From the Fig. 7(b), we can see that as the temperature rises, the thermal conductivity of the CaTa4O11 crystal decreases. This phenomenon can be attributed to heightened phonon scattering due to vigorous atomic vibrations and the increased average interatomic distance resulting from lattice expansion. The thermal conductivity anisotropy, with a higher value along the c-axis, can be attributed to the alternant layer structure along this axis which promotes stronger atomic bonding and consequently increases thermal conductivity [28].                             Fig. 7. Temperature dependence of the (a) thermal diffusivity, (b) thermal conductivity of single crystal CaTa4O11.   Journal Name  ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5  Please do not adjust margins Please do not adjust margins Conclusions For the first time, high-quality bulk single crystals of CaTa4O11 were grown by the floating-zone method, and thus the current phase-diagram was revised for the stoichiometry CaO∙2Ta2O5, showing a congruently melting behavior. The crystal became black in an Ar atmosphere due to oxygen vacancies and recovered to the intrinsic colourless when grown in an O2 atmosphere. Its crystallographic quality was characterized with a FWHM of 72′′ in XRC. The O2-grown crystals has a transmittance of approximately 78% and a broad band emission centered around 427 nm when excited under UV and X-ray. Given its high stopping power of X- and γ-ray, this single crystal holds promise as a potential host for scintillation applications when it is properly activated. The dielectric constants ε11 and ε33 are 30 and 52.3, respectively, at 1 kHz. Additionally, the thermal conductivity along the c-axis is 1.8 times higher than along the a-axis, measuring 5.69 Wm⁻¹K⁻¹ and 3.19 Wm⁻¹K⁻¹ at room temperature, respectively.   Conflicts of interest There are no conflicts to declare.  Acknowledgements The authors would like to sincerely thank Mr. Satoshi Yamamoto for his kind support with crystal polishing and Dr. Yoshitaka Matsushita for his support of crystal structure analysis. This work was supported by "Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Proposal Number JPMXP1223NM5337.  Notes and References ‡ Electronic supplementary information.  [1] A. Hossain and M. H. Rashid, IEEE Trans. Ind. Appl. 1991, 27, 824–829. [2] J. G. Gualtieri, J. A. Kosinski, and A. Ballato, IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 1994, 41, 53–59. [3] R. Dou, Q. Zhang, J. Gao, Y. Chen, S. Ding, F. Peng, W. Liu and D. Sun, Crystals, 2018, 8, 1-21. [4] E. D. Bourret, D. M. Smiadak, R. B. Borade, Y. Ma, G. Bizarri, M. J. Weber and S. E. Derenzo, J. Lumin., 2018, 202, 332-338. [5] M. Higuchi, K. Ando, J. Takahashi and K. Kodaira, JCS-Japan, 1993, 101, 118-120. [6] C. R. Ferrari, A. S. S. de Camargo, L. A. O. Nunes and A. C. Hernandes, J. Cryst. Growth, 2004, 266, 475-480. [7] A. S. S. de Camargo, C. R. Ferrari, A. C. Hernandes and L. A. O. Nunes, J. Phy Condens. Matter, 2004, 16, 5915-5923. [8] I. Tanaka, Y. Sato and H. Kojima, J. Cryst. Growth, 1990, 99, 837-840. [9] A. Hushur and S. Kojima, Mater. Sci. Eng. A, 2006, 442, 35-38. [10] D. Yuan, F. Moretti, D. Perrodin, G. Bizarri, T. Shalapska, C. Dujardin and E. Bourret, CrystEngComm, 2022, 22, 3497-3504. [11] T. Hayashi, K. Ichiba, D. Nakauchi, K. Watanabe, T. Kato, N. Kawaguchi and T. Yanagida, J. Lumin, 2023, 255. 119614. [12] K. T. Jacob and A. Rajput, J. alloys Compd, 2015, 620, 256-262.  [13] D. A. Reeve, J. Less-Common Met, 1969, 17, 215-222. [14] N. G. Teixeira, R. L. Moreira, R. P. S. M. Lobo, M. R. B. Andreeta, A. C. Hernandes and A. Dias, Cryst. Growth Des., 2011, 11, 5567-5573. [15] I. E. Grey, R. S. Roth, W. G. Mumme, J. Planes, L. Bendersky, C. Li and J. Chenavas, J. Solid State Chem, 2001, 161, 274-287. [16] M. Isobe, F. Marumo and S. Iwai, Acta Cryst., 1975, B31, 908-910. [17] T. Yanagida, M. Koshimizu, G. Okada, T. Kojima, J. Osada and N. Kawaguchi, Opt. Mater., 2016, 61, 119-124. [18] S. Tanusilp, Y. Ohishi, H. Muta, S. Yamanaka, A. Nishide, J. Hayakawa and K. Kurosaki, Phys. Status Solidi PRL, 2017, 12, 1700372. [19] L. Jahnberg, J. Solid State Chem., 1970, 1, 454-462. [20] P. Makula, M. Pacia and W. Macyk, J. Phys. Chem. Lett., 2018, 9, 6814-6817. [21] L. Qin, D. Wei, Y. Huang, S. I. Kim, Y. M. Yu and H. J. Seo, Inorg. Chem., 2013, 52, 10407-10413. [22] W. Dong, Y. Sun, B. Wang, M. Zhu, J. Li, X. Xu and J. Wang, Cryst. Growth Des. 2022, 22, 4243−4249. [23] X. Fu, E. G. Villora, Y. Matsushita, Y. Kitanaka, Y. Noguchi, M. Miyayama, K. Shimamura and N. Ohashi, RSC Adv., 2017, 7, 56697-56703. [24] Roditi International, Lithium Tantalate, https://roditi.com/SingleCrystal/Lithium-Tantalate/LiTaO3 Properties.html, (accessed June 2, 2024). [25] W. Zhang, X. Tao, C. Zhang, and M. Jiang, Cryst. Growth Des. 2009, 9, 2633-2636. [26] R. E. Newnham, 'Tensors and physical properties', Properties of Materials: Anisotropy, Symmetry, Structure (Oxford, 2004; online edn, Oxford Academic, 12 Nov. 2020). [27] R. Mckinney, P. Gorai, E. Toberer and V. Stevanovic, Chem. Mater., 2020, 32, 3320-3321. [28] S. Mukhopadhyay, L. kindsay and D. J. Singh, Sci. Rep., 2016, 6, 37076.