Growth of Srb4o7 Crystal Fibers Along the C-Axis by Micro-Pulling-Down Method

Article Growth of SrB4O7 Crystal Fibers along the c-Axis by Micro-Pulling-Down Method

Ryouta Ishibashi, Harutoshi Asakawa * and Ryuichi Komatsu

Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8511, Japan; [email protected] (R.I.); [email protected] (R.K.) * Correspondence: [email protected]; Tel.: +81-836-85-9631

Abstract: SrB4O7 (SBO) receives much attention as solid-state ultraviolet lasers for micro-machining, photochemical synthesis, and laser spectroscopy. For the application of SBO, the SBO crystals require the control of twinning to amplify the conversion light. We also expected that the inhibitation of the

SrB2O4 appearance was essential. Here, we show the growth of SBO crystals along the c-axis through the micro-pulling-down method while alternating the application of electric ﬁelds (E). Without the application, single crystals were grown. At E = 400 V/cm no needle domains of SrB2O4 inside SBO crystals existed; however, composition planes were formed and twin boundaries did not appear. In contrast, the inversion of surface morphology emerged, and the convex size was especially large at 1000 V/cm. These results demonstrate that convection is generated perpendicular to the growth front by alternating the application of electric ﬁelds. This surface morphological change contradicts the conventional concept of growth through the micro-pulling-down method. The distance from seed

crystals vs. grain density plot also showed that the density did not decrease with a sufﬁcient slope. Consequently, we concluded that the selection of the c-axis as growth faces is not fruitful to fabricate

Citation: Ishibashi, R.; Asakawa, H.; twins, and the selection of the growth condition, under which geometrical selection strongly affects,

Komatsu, R. Growth of SrB4O7 is the key. Crystal Fibers along the c-Axis by Micro-Pulling-Down Method. Keywords: SrB4O7; micro-pulling-down method; melt growth; electric ﬁeld; borate; oxide Crystals 2021, 11, 987. https:// doi.org/10.3390/cryst11080987

Academic Editors: Kheirreddine 1. Introduction Lebbou, Harutoshi Asakawa and SrB4O7 (SBO) belongs to an orthorhombic structure with the space group of Pmn21 Sławomir Grabowski (a = 4.4273 Å, b = 10.714 Å, c = 4.2359 Å) [1,2]. SBO crystals also show high nonlinear- ity [3–7] and are transparent down to the deepest ultraviolet wavelength (125 nm) [4–9]. Received: 10 July 2021 Hence, SBO receives much attention as solid-state ultraviolet lasers for semiconductor Accepted: 18 August 2021 Published: 20 August 2021 photolithography, micro-machining, photochemical synthesis, laser spectroscopy, and photoemission spectroscopy.

Publisher’s Note: MDPI stays neutral For the application of SBO, the SBO crystals require the appearance of the periodic with regard to jurisdictional claims in twin structure, which is so-called quasi-phase-matching (QPM) structures. The QPM is the published maps and institutional affil- structure in which the sign of the nonlinear optical coefficient is periodically reversed and iations. enabled to compensate the phase shift of the second harmonic generation [10,11]. Hence, the conversion light is, generally, amplified owing to the QPM structure. It is well-known that QPM structures in ferroelectric oxide materials can be formed by the application of an external electric field such as the poling method [10,12–15] and hot-pressing method [16]. However, since borates are non-ferroelectric crystals, with the utilization of the poling Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. method and the hot-pressing method it is difficult to control twin boundary orientation. This article is an open access article Aleksandrovsky et al., for the first time, reported twinned structures of SBO crys- distributed under the terms and tals [17,18]. They succeeded in growing partially twinned SBO crystals by the Czochralski conditions of the Creative Commons method (CZ). We expect that to control the formation of twin boundaries, the active incor- Attribution (CC BY) license (https:// poration of defects is further required by combining the growth under high temperature creativecommons.org/licenses/by/ gradient G with the periodic application of electric fields. The growth by the micro-pulling- 4.0/). down method (µ-PD method) can be a suitable method. The orientation of the ions in

Crystals 2021, 11, 987. https://doi.org/10.3390/cryst11080987 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 987 2 of 12

the melt and the dipoles in the crystals can be controlled by alternating the application of electric fields. The temperature gradient range of the µ-PD method is 1–2 units larger than that of the CZ method because of the small hot zone [19–21], and electric fields can be applied homogeneously. The incorporation of homogenous composition in crystals is also possible owing to a high temperature gradient. Actually, in the case of the ferroelectric LiNbO3 (LN), Uda et al. succeeded in the domain inversion by applying an external electric current during the growth through the µ-PD method [22]. Recently, we tried to grow twinned SBO crystals along the b-axis during the application of electric fields by the µ-PD method [23]. Then, our previous study explored that the growth of SBO by the µ-PD method via alternating the application of opposing electric fields (E) exhibits two mechanisms. At E = 400 V/cm needles of SrB2O4 appeared inside SBO fibers. At E= 1000 V/cm, new domains of SBO grew from the seed crystals, and growth twins were, interestingly, formed perpendicular to growth fronts. The twin face was (100) face. Thus, we revealed new challenges that the twin boundary is formed perpendicular to the growth front. Furthermore, we found the inversion of surface shapes during the growth even by the µ-PD method. We can explain the relationship between the two mechanisms and the inversion of surface shapes, using the concept of convection. We expect that the inhibitation of the SrB2O4 appearance by the convection is the key for the fabrication of twins. Furthermore, the growth conditions, under which defects are further incorporated, are necessary for the formation of twins, as described so far. The orientation of the growth face is crucial for the fabrication of twins as well. Here we show the growth of SBO crystal fibers along the c-axis by the µ-PD method.

2. Materials and Methods The µ-PD furnace used in this study was the same as in our previous studies [23–26]. Figure1A–C show a hot zone of the µ-PD furnace. A platinum crucible with a rectangular nozzle (5 mm × 1 mm × 2 mm (height)), which has a 0.4 mm cylindrical hole at the center of the base, was used. To set a temperature gradient (G) in crystals, the platinum crucible was surrounded by the refractory [23]. SBO ceramic powders with stoichiometric composition were prepared, and the raw material was melted in the platinum crucible by applying DC power. The temperature near the bottom of a nozzle remained at 1050 ◦C during the growth. Afterheaters were not placed in the furnace to carry out a high temperature gradient. Then, Pt electrodes were immersed in the melt. The tip of the Pt electrodes was placed on ca. 5 mm from the bottom of the nozzle. Seed crystals were fixed on an aluminum rod by Pt wire. The orientation of the seed crystals was confirmed by X-ray diffraction, transmittance electron microscopy (TEM), and the selected area electron diffraction (SAED). Then, they were attached to the bottom of the platinum nozzle, and were pulled down in air along the c-axis. Figure1D shows the growth conditions. The photographs corresponded to the summary of our previous study, which grew at 50–55 K/mm [23]. At that time, the pulling- down direction was the b-axis. In this study, the G of 70–75 K/mm just below the nozzle, and larger than in our previous study [23] was selected. Before the growth, the temperature profile from the bottom of the nozzle along the pulling-down direction was confirmed by a thermocouple. Then, the temperature gradient of 0.5–1 mm away from the nozzle was determined by the linear approximation [25]. These values would correspond to the intrinsic temperature gradient in the crystals near the interface to understand the crystal growth. A pulling-down rate was also 0.02 mm/min. During the growth, various voltages were applied to the seed crystals through Pt wires and Pt electrodes. During the growth, various voltages of E in the range of 0–1400 V/cm were applied to the growth fronts of seed crystals through Pt electrodes [23]. The external voltage was switched every 30 min through a power supply. Crystals 2021, 11, x FOR PEER REVIEW 3 of 13

seed crystals through Pt electrodes [23]. The external voltage was switched every 30 min through a power supply. After the growth, the fiber crystals were fixed in resin, and test pieces were polished. Then the fibers were observed by polarized optical microscopy (BX51, Olympus, Japan). During the growth, photographs of the meniscus just below the die were taken by a camera (SZ-31MR, Olympus, Japan). To determine the crystal faces of the boundaries in the test pieces, the fibers were cut off. The cross sections of fiber crystals were also processed by focused ion beam (FIB) (JIB-4000, JEOL, Japan), and the orientation of domain boundaries in crystal fibers was evaluated by TEM and SAED. The TEM/SAED observation was carried out, using a JEOL JEM-2100 electron microscope at an acceleration voltage of 200 kV. The camera length was 80 cm. The thickness of the samples, which were obtained by FIB, was ca. 200 ± 20 nm. The sample widths were ca. 5 μm, and the samples almost had no gradient at the surface. The brightness and contrast of the images were adjusted, using a software (ImageJ) [27]. The reciprocal lattice was evaluated and the zone axes were finally obtained, using the ReciPro software which can calculate the Crystals 2021, 11, 987 relation between the orientation of the crystals and 2D diffraction patterns [28,29].3 of 12In addition, the schematic drawing of the crystal structures was illustrated by a software (VESTA) [30].

FigureFigure 1.1. GrowthGrowth byby thethe micro-pulling-down method with applying applying electric electric fields. fields. (A (A) )Schematic Schematic drawing of the setup; (B) bottom of the Pt crucible; (C) side view of the Pt crucible; (D) growth drawing of the setup; (B) bottom of the Pt crucible; (C) side view of the Pt crucible; (D) growth condition (open circles) of this study. A white arrow corresponds to the pulling-down direction condition (open circles) of this study. A white arrow corresponds to the pulling-down direction (c-axis). Seed crystals were fixed to the alumina rod by Pt wire. The other Pt wire was immersed (intoc-axis). melt Seed of SBO. crystals The werephotographs fixed to in the (D) alumina show the rod summary by Pt wire. of our The previous other Pt study wire was [23], immersed and these intocorrespond melt of SBO.to the The growth photographs along the in b-axis. (D) show At E the≧ summary400 V/cm ofneedles our previous of SrB2O study4 appeared [23], and inside these correspondSBO fibers, toand the at growthE ≧ 1000 along V/cm the growthb-axis. twins At E =of 400{100} V/cm faces needleswere formed of SrB perpendicular2O4 appeared to inside SBOgrowth fibers, fronts. and Figure at E = 10001 reprinted V/cm growthwith permission twins of{100} fromfaces Sho Inaba, were formed Ryouta perpendicular Ishibashi, Harutoshi to growth fronts.Asakawa, Figure Takaaki1 reprinted Machida, with Yuki permission Hamada, from and Sho Ryuichi Inaba, Komatsu, Ryouta Ishibashi,Effects of the Harutoshi Application Asakawa, of TakaakiElectric Machida,Fields on Yukithe Growth Hamada, of SrB and4O Ryuichi7 Crystals Komatsu, by the Micro-Pulling-Down Effects of the Application Method, of Electric Cryst. Fields Growth Des. 2021, 21, 1, 86–93. Copyright 2020 American Chemical Society. on the Growth of SrB4O7 Crystals by the Micro-Pulling-Down Method, Cryst. Growth Des. 2021, 21, 1, 86–93. Copyright 2020 American Chemical Society. 3. Results AfterFigure the 2 growth,shows the the fiberSBO crystalscrystal fibers were fixedgrow inn resin,without and the test application pieces were of polished. electric Thenfields. the Although fibers were the observedcrystal has by rough polarized surfac opticales, the microscopy sample was (BX51, almost Olympus, transparent. Japan). The Duringtest piece the was growth, observed photographs under crossed of the meniscus Nicols, justand belowsymbols the in die Figure were taken2C correspond by a camera to (SZ-31MR,the crystallographic Olympus, orientation Japan). To determineof the seed the crystals. crystal The faces crystal of the showed boundaries uniform in the polar- test pieces,ized light. the fibersIn addition, were cut the off. boundary The cross between sections the of fiberseed crystals wereand grown also processed crystals did by focusednot exist. ion Hence, beam (FIB)we confirmed (JIB-4000, JEOL,that the Japan), orientation and the orientationof seed crystals of domain remained boundaries in the ingrown crystal crystal. fibers Since was SBO evaluated is too hard by TEM and andfragil SAED.e, polishing The TEM/SAED the test piece observation was difficult. was By carriedpolishing, out, the using test a JEOLpiece JEM-2100received scratches, electron microscope and there atwere an acceleration cracks. Sometimes voltage ofthe 200 sam- kV. The camera length was 80 cm. The thickness of the samples, which were obtained by FIB, was ca. 200 ± 20 nm. The sample widths were ca. 5 µm, and the samples almost had no gradient at the surface. The brightness and contrast of the images were adjusted,

using a software (ImageJ) [27]. The reciprocal lattice was evaluated and the zone axes were ﬁnally obtained, using the ReciPro software which can calculate the relation between the orientation of the crystals and 2D diffraction patterns [28,29]. In addition, the schematic drawing of the crystal structures was illustrated by a software (VESTA) [30].

3. Results Figure2 shows the SBO crystal fibers grown without the application of electric fields. Although the crystal has rough surfaces, the sample was almost transparent. The test piece was observed under crossed Nicols, and symbols in Figure2C correspond to the crystallographic orientation of the seed crystals. The crystal showed uniform polarized light. In addition, the boundary between the seed crystals and grown crystals did not exist. Hence, we confirmed that the orientation of seed crystals remained in the grown crystal. Since SBO is too hard and fragile, polishing the test piece was difficult. By polishing, the test piece received scratches, and there were cracks. Sometimes the samples also partially lacked. When the stage was rotated by 45◦, the polarized light was uniformly extinguished. This result demonstrates that the sample is a single crystal. Crystals 2021, 11, x FOR PEER REVIEW 4 of 13

Crystals 2021, 11, 987 4 of 12 ples also partially lacked. When the stage was rotated by 45°, the polarized light was uniformly extinguished. This result demonstrates that the sample is a single crystal.

FigureFigure 2.2. SBOSBO crystalcrystal fibersfibers growngrown withoutwithout thethe applicationapplication ofof voltage:voltage: ((AA)) externalexternal formform ofof SBOSBO crystals; (B) SBO crystals observed by polarized optical microscopy; (C) the test piece of (B) rotated crystals; (B) SBO crystals observed by polarized optical microscopy; (C) the test piece of (B) rotated by 45°. A white arrow corresponds to the pulling-down direction. Symbols in (C) correspond to by 45◦. A white arrow corresponds to the pulling-down direction. Symbols in (C) correspond to the the crystallographic orientation of seed crystals. crystallographic orientation of seed crystals. In this study, we separated and considered the growth conditions into E < 1000 and In this study, we separated and considered the growth conditions into E < 1000 and 1000 ≦ E to understand the growth mechanism. Figure 3 shows SBO crystal fibers 1000 5 E to understand the growth mechanism. Figure3 shows SBO crystal fibers grown at 400grown V/cm. at 400 The V/cm. fibers The were fibers slightly were opaque slightly owing opaque to rough owing surfaces. to rough The surfaces. alternating The timing alter- ofnating electric timing fields of corresponded electric fields to corresponded the increase intothe the radius increase of thein the fiber. radius At the of tipthe offiber. fibers, At nothe change tip of fibers, in the radiusno change was observedin the radius since wa thes observed electrode ofsince the the Pt wireelectrode became of awaythe Pt from wire thebecame growth away front from of the the seed growth crystals. front Just of abovethe seed the crystals. surface ofJust the above seed crystal,the surface additional of the seed crystal, additional domains appeared and grew (Figure 3B). In contrast, needle domains appeared and grew (Figure3B). In contrast, needle domains of SrB 2O4 did not appear,domains and of theSrB inside2O4 did of not the appear, fibers was and not the uniform. inside of The the number fibers ofwas domains not uniform. decreased The withnumber a distance of domains from thedecreased seed crystals with a owing distance tothe from geometrical the seed crystals selection. owing In the to tip the of geo- the fiber,metrical two selection. domains survivedIn the tip (Figure of the 3fiber,C). Figure two 3domainsD shows survived the cross-sectional (Figure 3C). view Figure of the 3D fibershows in the Figure cross-sectional3B,C. The two view domains of the fiber were in confirmed. Figures 3B,C. Then, The the two portions domains of thewere black con- linesfirmed. were Then, processed the portions by FIB of to the determine black lines the crystallographicwere processed faceby FIB of theto determine boundary bythe TEM/SAED.crystallographic face of the boundary by TEM/SAED. FigureFigures3F,G 3F,G show show electron electron diffraction diffraction patterns patterns of the twoof domainsthe two neardomains the boundary. near the Inboundary. Figure3F, In reciprocal Figure 3F, vectors reciprocal of OF, vectors OF 0, and of FFOF,0 wereOF′, ca.and 2.3, FF′ 2.4, were and ca. 3.3 2.3, nm 2.4,−1, respec-and 3.3 tively.nm−1, respectively. In contrast, reciprocal In contrast, vectors reciprocal of OG, vectors OG0, and of OG, GG0 wereOG′, and ca. 2.5, GG 2.3,′ were and ca. 3.3 2.5, nm −2.3,1, respectively.and 3.3 nm− The1, respectively. spatial resolution The spatial of the measurementresolution of the was measurement calculated. Consequently, was calculated. the errorsConsequently, of the reciprocal the errors vectors of the were reciprocal± 0.17 vectors nm−1. Thewere ReciPro ± 0.17 calculationnm−1. The ReciPro shows thatcalcula- the crystallographiction shows that orientationsthe crystallographic of OF, OF orientations0, and FF0 corresponded of OF, OF′, and to [100], FF′ corresponded [001], and [101 to], respectively.[100], [001], Theand crystallographic[101], respectively. orientations The crystallographic of OG, OG0, andorientations GG0 also correspondedof OG, OG′, and to [100],GG′ also [011], corresponded and [111], respectively. to [100], [011], The and calculation [111], respectively. of the zone axesThe demonstratedcalculation of the that zone the orientationaxes demonstrated perpendicular that the to orientation the paper inperpen Figuredicular3F,G were to the [ 0paper10] and in [Figures011], respectively. 3F,G were Thus,[010] theand boundary [011], respectively. in Figure3D Thus, was the the composition boundary in plane Figure given 3D by was [0 10] the and composition [011]. planeThe given next by SBO [010 crystal] and [ fibers011]. were grown at 800 V/cm (Figure A1). The fibers were slightly opaque owing to rough surfaces. Just above the surface of the seed crystal additional domains appeared and grew. The geometrical selection also occurred. In contrast, needle domains of SrB2O4 did not appear as well as at 400 V/cm. Thus, no SrB2O4 appeared even at low E. Furthermore, we determined that the composition plane of [430] and [011] was formed perpendicular to the growth front. Figure4 shows SBO crystal fibers grown at 1000 V/cm. The fibers were slightly opaque owing to rough surfaces. When alternating electric fields, the radius of the fiber slightly increased. Just above the surface of the seed crystal, additional domains appeared and grew. Figure4D shows the cross-sectional view of the fiber. The two domains existed.

Crystals 2021, 11, 987 5 of 12 Crystals 2021, 11, x FOR PEER REVIEW 5 of 13

FigureFigure 3. 3.SBO SBO crystal crystal fibers fibers grown grown at at 400 400 V/cm: V/cm: ((AA)) externalexternal formform ofof SBOSBO crystalcrystal fibers;fibers; (B(B,C,C)) po- po- larized optical microscope image of the SBO crystal taken under crossed Nicols; (D) larized optical microscope image of the SBO crystal taken under crossed Nicols; (D) cross-sectional cross-sectional view of the crystal fiber at the white-arrowhead position of (A) (a portion between view of the crystal fiber at the white-arrowhead position of (A) (a portion between (B,C)); (E) parts (B,C)); (E) parts processed by FIB for the TEM/SAED observation (black lines); (F) electron diffrac- processedtion pattern by FIBof the for domain the TEM/SAED at the left observation of (E); (G) (black electron lines); diffraction (F) electron pattern diffraction of the patterndomain of at the the domainright ofat (E the). Symbols left of (E in); ( AG)) correspond electron diffraction to the crystallographic pattern of the domain orientation at the of rightseed crystals. of (E). Symbols A white inarrow (A) correspond shows the pulling-down to the crystallographic direction. orientationAlternating oftiming seed crystals.of electric A fields white corresponds arrow shows to solid the pulling-downlines (electrodes direction. in the melt; Alternating +; growth timing fronts: of electric −) and fieldsdashed corresponds lines (electrodes to solid in lines the melt: (electrodes −; growth in thefronts: melt; +). +; growth fronts: −) and dashed lines (electrodes in the melt: −; growth fronts: +).

FigureThe next4F,G SBO show crystal electron fibers diffractions were grown of theat 800 two V/cm domains (Figure on A1). the whiteThe fibers lines were in Figureslightly4E opaque. In Figure owing4F, to reciprocal rough surfaces. vectors Just of OF,above OF the0, and surface FF0 ofwere the ca.seed 2.2, crystal 2.3, addi- and 3.1tional nm− domains1, respectively. appeared In contrast, and grew. reciprocal The geomet vectorsrical of selection OG, OG0, also and occurred. GG0 were ca.In contrast, 3.2, 5.4, −1 0 andneedle 5.3 nmdomains, respectively. of SrB2O4 The did ReciPro not appear calculation as well shows as at the400 orientations V/cm. Thus, of no OF, SrB OF2O, and4 ap- 0 FFpearedcorresponded even at low to [100], E. Furthermore, [001], and [1 we01], determined respectively. that In addition, the composition the orientations plane of of [ OG,430] 0 0 OGand, and[011 GG] wascorresponded formed perpendicular to [112], [1 to01 ],the and growth [211], front. respectively. Consequently, from the calculationFigure of 4 zone shows axes, SBO we determinedcrystal fibers that grown the crystallographic at 1000 V/cm. orientation The fibers perpendicular were slightly toopaque the paper owing in Figure to rough4F,G surfaces. were [ 01 When0] and alternat [131], respectively.ing electric Thus,fields, we the can radius determine of the thatfiber theslightly boundary increased. corresponded Just above to the the composition surface of planethe seed of [0 crystal,10] and additional [131]. domains ap- pearedFigure and A2 grew. shows Figure SBO 4D crystal shows fibers the growncross-sectional at 1400 V/cm. view of The the fibers fiber. were The slightlytwo do- opaque.mains existed. Just above the surface of the seed crystal, additional domains appeared and grew. The geometricalFigures 4F,G selection show waselectron observed diffractions as well. of The the boundary two domains in the cross-sectionon the white was lines the in compositionFigure 4E. In plane Figure of [314F,2] reciprocal and [011]. vectors Thus, no of twinOF, OF boundary′, and FF appeared′ were ca. in 2.2, this 2.3, study. and 3.1 nm−In1, respectively. contrast, we In attempted contrast, toreciprocal determine vectors whether of OG, the OG domains′, and remainedGG′ were inca.the 3.2, ori- 5.4, entationand 5.3 ofnm the−1, seedrespectively. crystals exceptThe ReciPro without calculation the application. shows However,the orientations it was difficultof OF, OF to′, determineand FF′ corresponded the relationship to between[100], [001], the pulling-down and [101], respectively. direction and In the addition, electron the diffraction orienta- patternstions of owingOG, OG to′ the, and error GG observed′ corresponded through to the[112], processing [101], and of test[211], pieces. respectively. Conse- quently, from the calculation of zone axes, we determined that the crystallographic orientation perpendicular to the paper in Figures 4F,G were [010] and [131], respectively. Thus, we can determine that the boundary corresponded to the composition plane of [010] and [131].

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Figure 4. SBO crystal fibers grown at 1000 V/cm: (A) external form of SBO crystal fibers; (B,C) po- Figure 4. SBO crystal fibers grown at 1000 V/cm: (A) external form of SBO crystal fibers; (B,C) polarized optical microscope image of the SBO crystal taken under crossed Nicols; (D) larized optical microscope image of the SBO crystal taken under crossed Nicols; (D) cross-sectional cross-sectional view of the crystal fiber at the white-arrowhead position of (A) (a portion between view(B,C of)); the(E) parts crystal processed fiber at the by white-arrowheadFIB for the TEM/SAED position observation of (A) (a portion (white betweenlines); (F) ( Belectron,C)); (E )diffrac- parts processedtion pattern by FIBat the for left the of TEM/SAED (E); (G) electron observation diffraction (white pattern lines); at (F the) electron right of diffraction (E). Symbols pattern in (A) at thecor- leftrespond of (E); to (G )the electron crystallographic diffraction patternorientation at the of right seed of crystals. (E). Symbols A white in ( Aarrow) correspond corresponds to the to crys- the tallographicpulling-down orientation direction. of Alternating seed crystals. timing A white of electric arrow correspondsfields was drawn to the by pulling-down solid lines (electrodes direction. Alternatingin the melt: timing +; growth of electric fronts: fields −) and was dashed drawn lines by solid (electrodes lines (electrodes in the melt: in the−; growth melt: +; fronts: growth +). fronts: −) and dashed lines (electrodes in the melt: −; growth fronts: +). Figure A2 shows SBO crystal fibers grown at 1400 V/cm. The fibers were slightly opaque.Figure Just5 shows above the the relation surfac betweene of the theseed geometrical crystal, addi selectiontional anddomains growth appeared conditions. and Heregrew. we The defined geometrical fiber width selection over was average observed domain as well. width The as domainboundary density in theat cross-section a distance fromwas the composition surfaces of seed plane crystals. of [312 To] and compare [011]. Thus, the effects no twin of the boundary growth appeared conditions, in wethis alsostudy. analyzed the data (filled squares) in our previous study [23]. With a distance from the surfacesIn ofcontrast, the seed we crystals, attempted the domain to determine density whether decreased. the Furthermore, domains remained when comparing in the ori- openentation squares of the with seed filled crystals squares, except at 75 without K/mm th thee application. domain density However, decreased it was slower difficult than to atdetermine 55 K/mm. the When relationship comparing between open circlesthe pullin withg-down filled squares,direction at and 400 the V/cm electron the domain diffrac- densitytion patterns decreased owing earlier to the than error at observed 1000 V/cm. through In general, the processing the geometrical of test selectionpieces. means that whenFigure the 5 crystalshows facesthe relation of preferential between growth the geometrical are oriented selection fully perpendicular and growth tocondi- the growthtions. Here front, we such defined domains fiber survive. width Then,over av theerage crystal domain faces width at the sideas domain of the preferentialdensity at a domainsdistance remain.from the Hence,surfaces the of resultsseed crystals demonstrate. To compare that the the condition, effects of underthe growth which condi- the geometricaltions, we also selection analyzed strongly the data occurs, (filled is thesquares) key to in the our growth previous of SBO study crystals [23]. With with twina dis- faces.tance Iffrom we selectedthe surfaces under of mildthe seed growth crystals, conditions, the domain the preferential density decreased. growth of Furthermore, (001) faces wouldwhen providecomparing the open facet ofsquares the intrinsic with filled domains squares, such at as 75 (100) K/mm face the and domain (010) face. density decreased slower than at 55 K/mm. When comparing open circles with filled squares, at 400 V/cm the domain density decreased earlier than at 1000 V/cm. In general, the geometrical selection means that when the crystal faces of preferential growth are oriented fully perpendicular to the growth front, such domains survive. Then, the crystal faces at the side of the preferential domains remain. Hence, the results demonstrate that the

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condition, under which the geometrical selection strongly occurs, is the key to the growth of SBO crystals with twin faces. If we selected under mild growth conditions, the Crystals 2021, 11, 987 7 of 12 preferential growth of (001) faces would provide the facet of the intrinsic domains such as (100) face and (010) face.

Figure 5. Distance fromfrom seedseed crystalscrystals vs.vs. graingrain densitydensity plots.plots. OpenOpencircles: circles:75 75 K/mm K/mm (400(400 V/cm);V/cm); open squares: 73.4 K/mm (1000 V/cm); filled squares: 55 K/mm (1000 V/cm) [23]. A solid line cor- open squares: 73.4 K/mm (1000 V/cm); ﬁlled squares: 55 K/mm (1000 V/cm) [23]. A solid line responds to the existence of two domains in fiber crystals. corresponds to the existence of two domains in ﬁber crystals.

Figure 66 shows shows thethe shapesshapes ofof thethe meniscusmeniscus between between SBOSBO crystals crystals andand the the melt melt to to understand the growth mechanism.mechanism. The surfacesurface morphology without the applicationapplication ofof electric ﬁeldsfields waswas concaveconcave (Figure (Figures6A,B). 6A,B). The The result result was was the the same same as the as growththe growth along along the bthe-axis b-axis shown shown in our in previous our previous study. study. In general, In general, the limited the processlimited ofprocess the melt of growththe melt is thegrowth removal is the of removal the latent of heat, the latent and the heat, heat an ﬂowd the corresponds heat flow corresponds to the surface to morphology the surface inmorphology the melt growth. in the Hence,melt growth. this result Hence, demonstrates this result thatdemonstrates the latent heatthat ofthe crystal latent growthheat of Crystals 2021, 11, x FOR PEER REVIEWremainscrystal growth at the centerremains of theat the growth center front. of the Since growth the centerfront. Since was hotter the center than thewas peripherical hotter than8 of 13

region,the peripherical this result region, should this be result valid. should be valid. On the other hand, the application of electric fields provides surface-morphological changes. Figures 6C–F show the growth along the b-axis at 55 K/mm and 1000 V/cm. When the electrodes in the melt and growth fronts became + and −, respectively, a convex shape was generated at the center of the growth front. In 30 min, the surface morphology was back to concave. At 75 K/mm and 400 V/cm, when the electrodes in the melt and growth fronts during the growth along the c-axis became − and +, respectively, a convex shape was formed at the center of the growth front (Figures 6G,H). In 30 min, the surface morphology of the SBO was also back to concave (Figures 6I,J). At 75 K/mm and 1000 V/cm, the morphological change was observed as well (Figures 6K–N). These results demonstrate that when the electrodes in the melt and growth fronts became + and −, respectively, the latent heat flowed from the top of the convex shape to the peripheral parts. In other words, the convex shape demonstrates that convection is compulsorily generated perpendicular to the growth front by alternating the application of electricFigure 6.fields. Interfaces The convectionbetween SBO was crystals also andirrespective melt just belowof crystal a nozzle. faces. ( A,B): 70.6 K/mm, 0 V/cm; Figure 6. Interfaces between SBO crystals and melt just below a nozzle. (A,B): 70.6 K/mm, 0 V/cm; (C,DFurthermore,): 55 K/mm, 1000 the V/cm convex (melt: shape +; growth: with a −size); (E larger,F) 55 K/mm,than Figures 1000 V/cm 6C,D (melt: were +; observed.growth: −); (C,D): 55 K/mm, 1000 V/cm (melt: +; growth: −); (E,F) 55 K/mm, 1000 V/cm (melt: +; growth: −); Since(G,H )the 75 K/mm,pulling-dow 400 V/cmn rate (melt: corresponds +; growth: −to); the(I,J) growth75 K/mm, rate, 400 and V/cm the (melt: growth +; growth: rate was −); (notK,L ) − − (changedG73.4,H) K/mm, 75 K/mm,in this1000 400 study.V/cm V/cm (melt:In (melt:this −; study,growth: +; growth: the +); surface(M);, (NI,)J )73.4 75kinetics K/mm, K/mm, of 4001000 the V/cm growthV/cm (melt: (melt: should +; +; growth: growth: be gov- −);). (ernedK(B,L,D) 73.4,F by,H, K/mm,Jthe,L,N application) were 1000 the V/cm same of (melt: electricas (−A;,C growth: ,Efields,G,I,K +);and,M ()M respectively,temperature,N) 73.4 K/mm, and gradients. 1000were V/cm emphasized Therefore, (melt: +; on growth: thesethe in- −resultsterfaces.). (B,D suggest, F(,CH–,JF,)L ,correspond Nthat) were a high the to same temperaturethe asdata (A in,C ,Eour ,Ggrad ,Iprevious,Kient,M) respectively,accelerates study, which andthe grew wereconvection along emphasized the formed b-axis on the[23].by interfaces.alternatingSolid and (dashedC the–F) correspondapplication lines show to ofoutlines the electric data inof ourfields.the previousinte rface study,between which the grewSBO crystals, along the melt,b-axis and [23 ].the Solid noz- andzle. dashed lines show outlines of the interface between the SBO crystals, melt, and the nozzle.

4. DiscussionOn the other hand, the application of electric ﬁelds provides surface-morphological changes.We revealed Figure6C–F that show unfortunately, the growth the along growth the alongb-axis the at 55c-axis K/mm is not and proper 1000 V/cm.for the When the electrodes in the melt and growth fronts became + and −, respectively, a convex formation of twins. As shown in Figure 7, only the (100) faces include the twin faces in shape(100), was(010), generated and (001) at faces. the center Hence, of thewe growthexpected front. that In twin 30 min, boundaries the surface perpendicular morphology to wasthe backgrowth to concave.fronts would At 75 appear. K/mm However, and 400 V/cm,only composition when the electrodes planes were in theformed melt under and the growth conditions. Twin boundaries perpendicular to the growth fronts did not appear unlike the previous study [23].

Figure 7. Crystal structure of SBO [6]. (A): (100) face perpendicular to the paper; (B): (010) face perpendicular to the paper; (C): (001) face perpendicular to the paper. Green, red, blue balls correspond to strontium, oxygen, and boron atoms, respectively. Symbols in the individual drawings show the crystallographic orientation. Dashed lines depict the position of mirror planes.

Now we focus on the frequency of the geometrical selection to understand the reason. During the geometrical selection, the growth faces, which show fast crystal growth, preferentially survive. We expect that some domains in Figures 3F, 4F, A1F, and A2F

Crystals 2021, 11, x FOR PEER REVIEW 8 of 13

Crystals 2021, 11, 987 8 of 12

growth fronts during the growth along the c-axis became − and +, respectively, a convex shape was formed at the center of the growth front (Figure6G,H). In 30 min, the surface morphology of the SBO was also back to concave (Figure6I,J). At 75 K/mm and 1000 V/cm, the morphological change was observed as well (Figure6K–N). These results demonstrate that when the electrodes in the melt and growth fronts became + and −, respectively, the latent heat ﬂowed from the top of the convex shape to the peripheral parts. In other words, the convex shape demonstrates that convection is compulsorily generated perpendicular

to the growth front by alternating the application of electric fields. The convection also emergedFigure 6. Interfaces irrespective between of crystal SBO facescrystals of and growth melt fronts.just below a nozzle. (A,B): 70.6 K/mm, 0 V/cm; (C,DFurthermore,): 55 K/mm, 1000 the V/cm convex (melt: shape +; growth: with a−); size (E,F larger) 55 K/mm, than Figure1000 V/cm6C,D (melt: were +; observed. growth: −); (G,H) 75 K/mm, 400 V/cm (melt: +; growth: −); (I,J) 75 K/mm, 400 V/cm (melt: +; growth: −); (K,L) Since the pulling-down rate corresponds to the growth rate, and the growth rate was not 73.4 K/mm, 1000 V/cm (melt: −; growth: +); (M,N) 73.4 K/mm, 1000 V/cm (melt: +; growth: −). changed in this study. In this study, the surface kinetics of the growth should be governed (B,D,F,H,J,L,N) were the same as (A,C,E,G,I,K,M) respectively, and were emphasized on the in- byterfaces. the application (C–F) correspond of electric to the fields data andin our temperature previous study, gradients. which grew Therefore, along thesethe b-axis results [23]. suggestSolid and that dashed a high lines temperature show outlines gradient of the inte acceleratesrface between the convection the SBO crystals, formed melt, by alternatingand the noz- thezle. application of electric fields.

4.4. DiscussionDiscussion WeWe revealedrevealed thatthat unfortunately,unfortunately, thethe growthgrowth alongalong thethec c-axis-axis isis notnot properproper forfor thethe formationformation ofof twins.twins. AsAs shownshown inin FigureFigure7 ,7, only only the the (100) (100) faces faces include include the the twin twin faces faces in in (100),(100), (010), (010), and and (001) (001) faces. faces. Hence, Hence, we we expected expected that that twin twin boundaries boundaries perpendicular perpendicular to the to growththe growth fronts fronts would would appear. appear. However, However, only only composition composition planes planes were were formed formed under under the growth conditions. Twin boundaries perpendicular to the growth fronts did not appear the growth conditions. Twin boundaries perpendicular to the growth fronts did not ap- unlike the previous study [23]. pear unlike the previous study [23].

FigureFigure 7.7.Crystal Crystal structure structure of of SBO SBO [6 ].[6]. (A ):(A (100)): (100) face face perpendicular perpendicular to the to paper;the paper; (B): (010)(B): (010) face per-face perpendicular to the paper; (C): (001) face perpendicular to the paper. Green, red, blue balls cor- pendicular to the paper; (C): (001) face perpendicular to the paper. Green, red, blue balls correspond respond to strontium, oxygen, and boron atoms, respectively. Symbols in the individual drawings to strontium, oxygen, and boron atoms, respectively. Symbols in the individual drawings show the show the crystallographic orientation. Dashed lines depict the position of mirror planes. crystallographic orientation. Dashed lines depict the position of mirror planes. Now we focus on the frequency of the geometrical selection to understand the rea- Now we focus on the frequency of the geometrical selection to understand the reason.son. DuringDuring thethe geometricalgeometrical selection,selection, thethe growthgrowth faces,faces, whichwhich showshow fastfast crystalcrystal growth,growth, preferentiallypreferentially survive.survive. WeWe expect expect that that some some domains domains in in Figure Figures3F, Figure 3F, 4F,4F, A1F, Figure and A1 A2FF, and Figure A2F have a relatively large area in the domains, and these can keep the crystal orientations of seed crystals according to the geometrical selection mechanism. However, our growth condition does not strongly provide the geometrical selection, compared with our previous study (Figure5). Since the growth rate of (100) faces can be slower than Crystals 2021, 11, 987 9 of 12

that of high indexes faces, the facets of (100) faces cannot survive perpendicular to the growth front. In contrast, a high temperature gradient accelerates the convection formed by alternating the application of electric fields, and the appearance of the SrB2O4 was further inhibited in the broad region of the temperature gradient. Hence, the utilization of high temperature gradients can be useful for the fabrication of twins. In addition, it is well-known that the growth by µ-PD method does not exhibit the generation of the convection even in the growth during the application of electric fields [21,26,31,32]. Many researchers strongly believe that borates, which show extremely high viscosity, grow without convection. There- fore, the finding contradicts the conventional concept of growth by the µ-PD method. The convection also emerged irrespective of crystal faces of growth fronts. However, from the viewpoint of twin formation, effects of the geometrical selection and the convection compete under high temperature-gradient conditions. We need to consider such two factors when the growth conditions are selected. In addition, our growth conditions are different from the previous LN study [22]. The pulling-down rate in the previous study was 10 times larger than that in our case. We need to reconsider the pulling- down rate conditions. In the future, we are going to systematically investigate the effects of crystallographic orientation and temperature gradient. Afterheaters will be set in the furnace, and the growth at a low temperature gradient will be essential.

5. Conclusions In this study, we attempted to grow SBO crystal fibers along the c-axis by the µ-PD method via applying external electric fields. Without the application of electric fields, single crystals were grown. At E = 400 V/cm, no needle SrB2O4 crystals inside SBO crystal fibers appeared in the growth. However, composition planes were formed, and twin boundaries did not emerge. In contrast, the inversion of surface morphology was observed. This result demonstrates that the convection appears by alternating the application of the electric fields. The convection also emerges irrespective of crystal faces of growth fronts. Since it is well-known that growth by the µ-PD method does not exhibit the generation of the convection, even in the growth during the application of electric fields [21,26,31,32], the result contradicts the conventional concept of the growth by µ-PD method. In particular, the convex size became large at 75 K/mm and 1000 V/cm. The comparison with our previous study [23] suggested that the convection can be accelerated by a relatively high temperature gradient. To understand the reason for the failure, we verified how often the geometrical selection emerged. Consequently, the distance from seed crystals vs. grain density plot showed that a decrease in the density with a distance was slower than our previous study [23]. Generally, the geometrical selection means that when the crystal faces of preferential growth are oriented fully perpendicular to the growth front, such domains survive. Then, the crystal faces at the side of the preferential domains remain. Namely, the geometrical selection sufficiently did not appear under our growth conditions. Therefore, we concluded that the selection of the c-axis as growth faces is not fruitful to fabricate twins, and the selection of the growth condition, under which geometrical selection strongly affects, is the key. A high temperature gradient can also play an important role in the assist of convection. We are going to systematically investigate the effects of crystallographic orientation and temperature gradient in the future.

Author Contributions: H.A. and R.K. designed the research performed by R.I. and H.A; R.K. pro- duced the experimental system. H.A. wrote the paper. The authors declare no conﬂict of interest. All authors have given approval to the ﬁnal version of the manuscript. Funding: This research received no external funding. Data Availability Statement: Not applicable. Acknowledgments: This work was supported by YAMAGUCHI UNIVERSITY FUND. The authors thank N. Harada, K. Yuki, Y. Tsugeno, and N. Takada (Yamaguchi Univ.) for their technical support of TEM/SAED observation. Crystals 2021, 11, 987 10 of 12

Conﬂicts of Interest: The authors declare no conﬂict of interest. Crystals 2021, 11, x FOR PEER REVIEW 10 of 13

Abbreviations

Acknowledgments:SBO: SrB4O This7; µ -PDwork method,was supported micro-pulling-down by YAMAGUCHI UNIVERSITY method; FUND. QPM, The quasi-phase-matching; au- LiNbOthors thank3, LN; N. Harada, DC power, K. Yuki, directY. Tsugeno, current and N. power; Takada (Yamag FIB, focuseduchi Univ.) ionfor their beam; technical SAED, selected-area electronsupport of diffraction;TEM/SAED observation. TEM, transmitted electron microscopy; XRD, X-ray diffraction. Conflicts of Interest: The authors declare no conflict of interest.

AppendixAbbreviations: A SBO: SrB4O7; μ-PD method, micro-pulling-down method; QPM, quasi-phase-matching; LiNbO3, LN; DC power, direct current power; FIB, focused ion beam; SAED, selected-areaThe fibers electronwere diffraction; slightly TEM, transm opaque.itted electron Although microscopy; we XRD, did X-ray not diffraction understand the reason, the change in the radius of the fiber by alternating timing of electric fields did not strongly emerge.Appendix InA Figure A1B, just above the surface of the seed crystal, additional domains appeared.The fibers The were number slightly of opaque. domains Although decreased we did with not understand a distance the from reason, the the seed crystals owing tochange the geometricalin the radius of selection. the fiber by As alternatin showng timing in Figure of electric A1C, fields at the did endnot strongly of the growth only one emerge. In Figure A1B, just above the surface of the seed crystal, additional domains domainappeared. survived. The number In of contrast, domains decreased the two with domains a distance were from confirmed the seed crystals in the ow- cross-section. Then, theing partsto the ofgeometrical the white selection. lines were As shown measured in Figure by A1C, electron at the diffraction end of the growth as well. only Reciprocal vectors ofone OF, domain OF0 ,survived. and FF In0 were contrast, ca. the 7.5, two 2.4, domains and were 8.1 nm confirmed−1, respectively. in the cross-section. In contrast, OG, OG0, andThen, GG the0 partscorresponded of the white tolines ca. were 2.3, measur 2.5, anded by 3.5 electron nm−1 ,diffraction respectively. as well. The Recip- ReciPro calculation rocal vectors of OF, OF′, and FF′ were ca. 7.5, 2.4, and 8.1 nm−01, respectively.00 In contrast, shows the crystallographic orientations of OF, OF−1 , and FF were [340], [001], and [341], OG, OG′, and GG′ corresponded to ca. 2.3, 2.5, and 3.5 nm , respectively.0 The ReciPro0 respectively.calculation shows The the crystallographic crystallographic orientations orientations of OF, of OF OG,′, and OG FF′′, were and [340], GG [001],were [100], [011], and [and111 ],[34 respectively.1], respectively. the The calculation crystallographic of zoneorientations axes demonstratedof OG, OG′, and thatGG′ were the crystallographic orientation[100], [011], and perpendicular [111], respectively. to the the paper calculation in Figure of zone A1 axesF,G demonstrated were [430 ]that and the [0 11], respectively. Thus,crystallographic the boundary orientation was perpendicular the composition to the paper plane. in Figures A1F,G were [430] and [011], respectively. Thus, the boundary was the composition plane.

Figure A1. SBO crystal fibers grown at 800 V/cm: (A) external form of SBO crystal fibers; (B,C) po- Figurelarized A1.opticalSBO microscope crystal fibers image grown of the at SBO 800 V/cm:crystal taken (A) external under crossed form of Nicols; SBO crystal(D) fibers; (B,C) po- larizedcross-sectional optical view microscope of the crystal image fiber at of the the white SBO arrowhead crystal takenposition under of (A) ( crossedA portion Nicols;between (D) cross-sectional view(B,C)); of (E the) parts crystal processed fiber by at FIB the forwhite the TEM/SAED arrowhead observation position (white of (lines);A)(A (Fportion) electron betweendiffrac- (B,C)); (E) parts processed by FIB for the TEM/SAED observation (white lines); (F) electron diffraction pattern of the domain at the left of (E); (G) electron diffraction pattern of the domain at the right of (E). Symbols in

(A) correspond to the crystallographic orientation of seed crystals. A white arrow corresponds to the pulling-down direction. Alternating timing of electric ﬁelds corresponded to solid lines (electrodes in the melt, +; growth fronts, −) and dashed lines (electrodes in the melt, −; growth fronts, +). Crystals 2021, 11, x FOR PEER REVIEW 11 of 13

tion pattern of the domain at the left of (E); (G) electron diffraction pattern of the domain at the right of (E). Symbols in (A) correspond to the crystallographic orientation of seed crystals. A white Crystals 2021, 11, 987 arrow corresponds to the pulling-down direction. Alternating timing of electric fields11 corre- of 12 sponded to solid lines (electrodes in the melt, +; growth fronts, −) and dashed lines (electrodes in the melt, −; growth fronts, +).

TheThe fibers fibers were were slightly slightly opaque. opaque. The alternatingThe alternating timing timing of electric of electric fields corresponded fields corre- tosponded the increase to the in increase the radius in the of theradius fiber. of Inthe Figure fiber. A2In B,Figure just aboveA2B, just the above surface the of surface the seed of crystal,the seed additional crystal, additional domains appeared.domains appeared. The number The of number domains of decreaseddomains decreased with a distance with a fromdistance the seedfrom crystals the seed owing crystals to the owing geometrical to the selection.geometrical Then, selection. the pieces Then, in thethe directionpieces in ofthe the direction individual of the white individual lines were white measured lines we byre electron measured diffraction. by electron Reciprocal diffraction. vectors Recip- of 0 0 −1 OF,rocal OF vectors, and FF of wereOF, OF ca.′, 3.4,and 2.8, FF and′ were 5.1 ca. nm 3.4,, 2.8, respectively. and 5.1 nm In− contrast,1, respectively. reciprocal In contrast, vectors 0 0 −1 ofreciprocal OG, OG ,vectors and GG of wereOG, ca.OG 2.4,′, and 2.5, GG and′ were 3.5 nm ca. 2.4,, respectively. 2.5, and 3.5 The nm ReciPro−1, respectively. calculation The 0 0 showsReciPro the calculation crystallographic shows orientations the crystallographic of OF, OF , andorientations FF corresponded of OF, OF to′ [,11 and1], [021],FF′ corre- and 0 0 [1sponded30], respectively. to [111], The[021], orientations and [130], of respectively. OG, OG , and The GG orientationsalso corresponded of OG, toOG [001],′, and [110], GG′ andalso [ 11corresponded1], respectively. to [001], Consequently, [110], and the[111 calculation], respectively. demonstrated Consequently, that the the orientation calculation perpendiculardemonstrated tothat the the paper orientation in Figure perpendicu A2F,G werelar [21 to3 ]the and paper [110], in respectively. Figures A2F,G Thus, were the boundary[213] and was[110 the], respectively. composition Thus, plane. the boundary was the composition plane.

FigureFigure A2. A2.SBO SBO crystal crystal fibers fibers grown grown at at 1400 1400 V/cm: V/cm: (A ()A external) external form form of SBOof SBO crystal crystal fibers; fibers; (B,C )(B po-,C) polarized optical microscope image of the SBO crystal taken under crossed Nicols; (D) larized optical microscope image of the SBO crystal taken under crossed Nicols; (D) cross-sectional cross-sectional view of the crystal fiber at the white arrowhead position of (A) (A portion between view of the crystal fiber at the white arrowhead position of (A)(A portion between (B,C)); (E) parts (B,C)); (E) parts processed by FIB for the TEM/SAED observation (white lines); (F) electron diffrac- processedtion pattern by at FIB the for left the of TEM/SAED (E); (G) electron observation diffraction (white pattern lines); at (F the) electron right of diffraction (E). Symbols pattern in (A) at thecor- leftrespond of (E); to (G )the electron crystallographic diffraction patternorientation at the of rightseed ofcrystals. (E). Symbols A white in ( Aarrow) correspond corresponds to the to crys- the tallographicpulling-down orientation direction. of Alternating seed crystals. timing A white of electric arrow correspondsfields was drawn to the by pulling-down solid lines (electrodes direction. Alternatingin the melt, timing +; growth of electric fronts, fields −) and was dashed drawn lines by solid(electrodes lines (electrodes in the melt, in − the; growth melt, +; fronts, growth +).fronts, −) and dashed lines (electrodes in the melt, −; growth fronts, +). References References 1. Block, S.; Perloff, A.; Weir, C.E. The crystallography of some M2+ borates. Acta Crystallogr. 1964, 17, 314–315. [CrossRef] 2. Krogh-Moe, J.; Magnéli, A.; Seip, R.; Seip, H.M. The Crystal Structure of Strontium Diborate, SrO·2B2O3. Acta Chem. Scand. 1964, 18, 2055–2060. [CrossRef] Crystals 2021, 11, 987 12 of 12

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