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Kakeru Shimotsukasa, Daisuke Urushihara, Toru Asaka, [Tohru S. Suzuki](https://orcid.org/0000-0001-9458-6863), Koichiro Fukuda

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[Enhancement of lanthanum ion conductivity by (40−2) plane orientation of polycrystalline La<sub>4</sub>Ga<sub>2</sub>O<sub>9</sub>](https://mdr.nims.go.jp/datasets/25c54449-cd5c-4b7b-bcbc-bbafe54fad1f)

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Enhancement of lanthanum ion conductivity by ($40\bar{2}$) plane orientation of polycrystalline La4Ga2O9FULL PAPEREnhancement of lanthanum ion conductivityby (40�2) plane orientation of polycrystalline La4Ga2O9Kakeru Shimotsukasa1, Daisuke Urushihara1, Toru Asaka1, Tohru S. Suzuki2 and Koichiro Fukuda1,³1Division of Advanced Ceramics, Nagoya Institute of Technology, Nagoya 466–8555, Japan2Optical Ceramics Group, Research Center for Electronic and Optical Materials, National Institute for Materials Science,Tsukuba 305–0047, JapanTo enhance the conductivity of La3+, the (40�2) plane aligned polycrystalline La4Ga2O9 (space group P21/c) wasprepared by sintering at 1673K for 2 h after colloidal processing under a high magnetic field of 12T. Thetextured polycrystal was characterized by X-ray diffraction and impedance spectroscopy with respect to thegrain alignment direction. The (40�2) plane normal of each constituent crystal grain of the polycrystal was foundto be almost parallel to the applied magnetic field. The texture fraction of (40�2), expressed as the Lotgeringfactor f40�2, was 0.235. A comparison was made of the conductivities parallel (·¬) and perpendicular (·¦) to thealigned plane normal at temperatures ranging from 673 to 1073K. These were also compared with the conduc-tivity (·r) of randomly grain-oriented La4Ga2O9 polycrystal. The ·¬, ranging from 1.63 © 10¹7 S cm¹1 at 623Kto 8.52 © 10¹4 S cm¹1 at 1073K, demonstrated the highest value at each temperature, followed by ·r and ·¦ inthat order. The ·¬/·¦ ratios ranged from 10.0 at 673K to 15.2 at 1073K, and the ·¬/·r ratios ranged from 6.7 at873K to 8.3 at 1073K. Since the a-axis is almost parallel to the (40�2) plane normal, the enhanced La3+conductivity of the La4Ga2O9 polycrystal has confirmed, for the first time, the prediction by the bond valenceenergy landscape method in the literature that La3+ conduction is preferential along the a-axis.Key-words : Lanthanum ion conductor, Grain alignment, Anisotropy, Impedance spectroscopy[Received August 4, 2025; Accepted August 29, 2025]1. IntroductionSolid electrolytes, in which multivalent cations conductat relatively high rates, are capable of high-density chargetransfer and are expected to be used in high-performanceelectrochemical devices such as high-capacity rechargeablebatteries.1–5) One of the factors that restricts the practicalapplication of ceramics electrolytes is the comparativelyelevated operating temperatures when compared with poly-mer electrolytes. It may therefore be employed as a moreadvanced energy storage battery to supersede sodium-sulphur batteries, which presently function at 573K.6) Insodium-sulphur batteries, the monovalent Na+ conductsthrough the ¢-alumina solid electrolyte. In ceramics elec-trolytes that conduct trivalent cations, such as La3+ forexample, a single cation possesses the capacity to carrythree times the amount of charge as Na+.The bond valence (BV) energy landscape method7,8) hasrevealed the potential for one-dimensional conduction oftrivalent rare earth ions (RE3+) along the a-axis in thecuspidine-type compounds RE3+4Ga2O9 (RE = La, Pr, Nd,Sm) and RE3+4Al2O9 (RE = Y, Nd, Eu, Tb, Lu).9) Indeed,La3+ conduction has been experimentally confirmed in therandomly grain-oriented polycrystalline La4Ga2O9 with arelatively high transference number of 0.992. The crystalstructure of La4Ga2O9 (space group P21/c) was first deter-mined by Kasunič et al. in 2009,10) who elucidated thepresence of four La sites, two Ga sites, and nine O sites,with all identical site symmetry of Wyckoff position 4e.The fractional coordinates of these fifteen independentcrystallographic sites were standardized according to therules developed by Parthé and Gelato11) and the results arepresented in Table S1.9) The crystal structure drawn usingthe standardized fractional coordinates consists of the 6- to8-coordinated La polyhedra [La1O7], [La2O6], [La3O8]and [La4O7], and the 4-coordinated Ga tetrahedra [Ga1O4]and [Ga2O4] (Fig. S1). The BV energy isosurface for La3+has demonstrated that only La3+ at the La1 and La2 sitescontributes to the conduction along the a-axis, while thatat La3 and La4 does not.9) When the polycrystallineLa4Ga2O9 was electrolyzed at 10V and 1073K, the reac-tion La4Ga2O9 ¼ 2LaGaO3 + La2O3 occurred, producingthe crystalline LaGaO3 on the anode side and the La2O3-rich deposit on the cathode side. The formation of thesetwo compounds has indicated that precisely half of the Laions that constitute La4Ga2O9 contribute to conduction,which is in complete accordance with the conductionmechanism predicted by the BV energy landscape method.In addition to La4Ga2O9, the only other compound reported³ Corresponding author: K. Fukuda; E-mail: fukuda.koichiro@nitech.ac.jpJournal of the Ceramic Society of Japan 133 [11] 657-662 2025DOI https://doi.org/10.2109/jcersj2.25106 JCS-Japan©2025 The Ceramic Society of Japan 657This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.https://doi.org/10.2109/jcersj2.25106https://creativecommons.org/licenses/by/4.0/as a La3+ conductor is La1/3Zr2(PO4)3.12) In this particularNASICON-type compound, La3+ conducts in three dimen-sions, indicating that the orientation of this polycrystallinematerial does not enhance conductivity. However, it hasbeen anticipated that there will be a further improvementin the conductivity of La3+ in the oriented polycrystallineLa4Ga2O9, in which each crystal grain comprising the poly-crystal is almost oriented in the a-axis direction.In ceramics, the orientation of constituent crystal grainshas been demonstrated to exert a substantial influence onthe material properties. The anisotropy of the physicalproperties of individual grains can be significantly en-hanced by uniaxial alignment, which can improve theoverall performance of the ceramics. A variety of meth-odologies have therefore been employed to orientate thecrystal grains that comprise ceramics, including templatedgrain growth,13,14) hot forging,15) reactive diffusion,16) andsintering after colloidal processing under a strong mag-netic field.17,18) The latter method exploits the phenome-non whereby single crystal particles of weakly magneticmaterials with very low magnetic susceptibility, such asdiamagnetic and paramagnetic crystals, rotate in a strongmagnetic field generated by a superconducting magnet.19)The CaAl4O7 crystal (space group C2/c) has been dem-onstrated to conduct Ca2+.20) Furthermore, the magnetictorque generated in a strong magnetic field has been foundto align the b-axis in the direction of the applied magneticfield.21) This property has been successfully employed toprepare the b-axis aligned CaAl4O7 polycrystal by meansof sintering following colloidal processing under a strongmagnetic field.21) The anisotropy of the Ca2+ conductivityof the fabricated textured polycrystal has been consistentwith the conduction pathways being parallel to the ©101ªdirection, as indicated by the BVenergy isosurface of Ca2+.In the present study, the La3+ conduction anisotropy ofLa4Ga2O9 has been demonstrated for the first time in the(40�2) plane aligned polycrystal fabricated by sinteringafter the application of a strong magnetic field. As the(40�2) plane normal is almost parallel to the a-axis, whichis the conduction direction depicted by the BV energyisosurface for La3+, the conductivity along this direction ofthe textured polycrystal has been enhanced in compari-son to the conductivity of the randomly grain-orientedLa4Ga2O9 polycrystal.2. Experimental2.1 MaterialsThe reagent-grade chemicals of La2O3 (99.99%, KishidaChemical Co. Ltd., Osaka, Japan) and Ga2O3 (99.99%,Kojundo Chemical Laboratory Co. Ltd., Saitama, Japan)were utilized as starting materials and weighed in themolar ratio [La2O3:Ga2O3] = [2:1]. The starting powderswere well mixed using a planetary ball mill (P-7, Fritsch,Idar-Oberstein, Germany), heated at 1573K for 20 h, fol-lowed by quenching in air. The slightly sintered poly-crystalline material thus obtained was subjected to pul-verization using the ball mill in order to produce the finepowder consisting exclusively of La4Ga2O9. The powderspecimen (designated S-A) was sieved using a sieve with a20¯m mesh opening and then further ground using theball mill to prepare the fine powder, a part of which wasobserved by a scanning electron microscope (SEM: modelJSM-6500F, JEOL Ltd., Tokyo, Japan). The outer shape ofthe 1264 crystal grains in the SEM image was analyzedusing the AI Particle Analyzing Software (AIPAS, BLUETAG Co., Ltd. Tokyo, Japan) to obtain the particle sizedistribution (Fig. S2). The particle size corresponding to acumulative frequency of 50% (D50) was found to be 0.85¯m, with a maximum particle size of approximately 5¯mor less. A part of the powder sample S-A was dispersedthrough the process of mutual electrostatic repulsion indistilled water, with the addition of an appropriate amountof polymeric dispersant. A static magnetic field of 12Twas applied in a parallel manner to the casting directionof the well-dispersed suspension during the slip-castingprocess. Previous literature provides comprehensive de-tails regarding the high magnetic field apparatus and thepreparation procedure for the suspension.17) The crystalparticles of La4Ga2O9 in the suspension were oriented inthe strong magnetic field, suggesting that La4Ga2O9 is dia-magnetic like Al2O3.18) The resulting green body was thenheated at 1673K for 2 h in the absence of the magneticfield to prepare the disc-shaped sintered polycrystal(termed S-B) with a diameter of 10.6mm and a thicknessof 2.0mm. The table plane was perpendicular to the direc-tion of the applied magnetic field [Fig. S3(a)].The residual part of sample S-A was compressed into apellet, heated at 1673K for 20 h, followed by quenching inair to obtain the disc-shaped sintered La4Ga2O9 polycrystal(termed S-C) with random grain orientation (¤5.5mm and2.0mm thick).2.2 CharacterizationThe resulting sintered polycrystal of S-B was cut intotwo pieces using a diamond saw perpendicular to thetable plane [Fig. S3(b)]. X-ray diffraction (XRD) patternswere collected separately from the table plane and thecross-sectional surface using a diffractometer with Bragg-Brentano geometry (model X’Pert PRO Alpha-1,PANalytical B.V., Almelo, The Netherlands). The incidentCuK¡1 beam was maintained at a constant irradiationlength of 5mm on the sample surface by an automaticdiverging slit. Since the X-ray beam width was 5mm, thearea analyzed was approximately 25mm2. Continuousscanning was used with an experimental 2ª range from10.0 to 88.0°. The integrated reflection intensities wereextracted by the Le Bail method22) using the computerprogram RIETAN-FP.23) The XRD pattern with 10.0° ¯2ª ¯ 80.8° was obtained from the table plane of the disc-shaped sintered polycrystal of S-C. The integrated inten-sities of individual reflections were derived by the Le Bailmethod.The X-ray powder diffraction (XRPD) pattern of S-Awas collected in the 2ª range of 10.0–88.0°, and the unitcell dimensions were refined by the Le Bail method usingthe computer program RIETAN-FP. The RIETAN-FPShimotsukasa et al.: Enhancement of lanthanum ion conductivity by (40�2) plane orientation of polycrystalline La4Ga2O9JCS-Japan658program was also utilized for the simulation of the XRDintensities of polycrystalline models with random andaligned crystal orientation, based on the standardizedstructural parameters of La4Ga2O9 (Table S1). We havedeveloped two polycrystalline models. The first model,designated model 1, consists of the aligned platelet crys-tals with a well-developed (40�2) cleavage plane, and thesecond model, designated model 2, consists of the alignedneedle-like crystals elongated along the (40�2) planenormal. Following the requirement of the March-Dollasefunction24,25) built into RIETAN-FP, the preferred orienta-tion vector was set to be a reciprocal lattice vector, 4a* +0b* ¹ 2c*, perpendicular to the (40�2) plane for both poly-crystalline models. The parameter r, representing the effec-tive sample compression in model 1 (r ¯ 1), was set to0.1, 0.5, and 1 with decreasing degree of orientation. In thecrystalline model exhibiting no preferred orientation, r isequal to one. In model 2 (r ² 1) the parameter r, which isrepresentative of the extension due to preferred orientation,was set to 5 and 1.8, with the orientation degree higher forthe former than for the latter.The texture fraction in the (40�2) plane was determinedby the Lotgering method.26) This process involved thecollection of hkl reflection intensities from the table planeof the textured specimen and the simulation of hkl reflec-tion intensities of the polycrystalline model with randomgrain orientation. The Lotgering factor f40�2 is defined bythe following equation:f40�2 ¼ ðP40�2 � P0Þ=ð1� P0Þ ð1ÞwhereP40�2 ¼ ð� I40�2Þ=ð� IhklÞ;andP0 ¼ ð� I040�2Þ=ð� I0hklÞ:The Ihkl and I0hkl represent the observed and the simu-lated intensities of hkl peaks, respectively. The f40�2-valueswere also derived using Eq. (1) for the simulated XRDpatterns of the polycrystalline model 1 with differentdegrees of orientation.One of the cut specimens of S-B was subjected tofurther division into two pieces using a diamond saw. Onepiece was then mechanically polished parallel to the orig-inal table plane using SiC abrasive paper, followed by amirror-polished finish with diamond paste, to prepare thethin plate electrolyte with a thickness (L) of 0.202 cm,surface area (S) of 9.31 © 10¹2 cm2, and shape factor (L/S)of 2.17 cm¹1. This specimen was designated electrolyte 1.The other piece was polished parallel to the original cross-sectional surface to make the thin plate electrolyte (des-ignated electrolyte 2) with L/S = 2.81 cm¹1 (L = 19.60 ©10¹2 cm and S = 6.98 © 10¹2 cm2). The preparation of theplate electrodes involved the application of platinum pasteto both sides of electrolyte 1 and electrolyte 2, followed byheating at 1273K. During this process, the paste decom-posed and the platinum residue hardened to form theplatinum electrode. Complex impedance data were col-lected using an impedance analyzer (IM3570, HIOKI E. E.Co., Nagano, Japan) at temperatures ranging from 623 to1073K over the frequency range of 4 to 5MHz in air. Acalibrated type K thermocouple was positioned in closeproximity to the sample to ensure precise temperaturemeasurements. The impedance spectra were subjected to anon-linear least squares fitting process utilizing ZViewsoftware.27) In the adopted equivalent circuit, the ele-ments corresponding to conductor-derived equipment (eq),bulk, and grain boundary (gb) are connected in series asReq(RbulkQbulk)(RgbQgb), where R is the resistance in par-allel with the constant phase element Q.The table planes of sample S-C were mirror-polishedwith diamond paste, thus producing a disc-shaped electro-lyte for the collection of complex impedance data. Sincethe distance between electrodes L was 19.60 © 10¹2 cmand the electrode area S was 8.49 © 10¹1 cm2, the shapefactor L/S was determined to be 2.31 © 10¹1 cm¹1. Theplatinum electrodes were prepared on the polished tableplanes according to the previously outlined method.The calculation of the spatial distribution of the BVenergy for La3+ in the crystal structure of La4Ga2O9 wasperformed using a computer program PyAbstantia.28) TheBV energy isosurface and the ball-and-stick structuralmodel, as well as the arrangement of the (40�2) plane rela-tive to the BV energy isosurface for La3+, were visualizedusing a software VESTA.29)3. Results and discussion3.1 (40�2) plane aligned polycrystal and ori-entation degreeThe refined unit-cell dimensions of the La4Ga2O9 sam-ple S-A were in fair agreement with the reported values inthe literature (Table S2).9,10) The refinement result wassatisfactory, as evidenced by the relatively low R and Svalues of Rwp = 6.833%, Rp = 5.1240%, and S = 1.1545,with no reflections from any impurities (Fig. S4). As illus-trated in Fig. 1, the interior of the BV energy isosurfacecorresponds to the conduction path of La3+. This figureclearly demonstrates that La3+ in only the La1 and La2sites conducts in the a-axis direction, while La3+ in theLa3 and La4 sites does not contribute to conduction. Thisis in full agreement with the conclusion of the precedingstudy.9) The angle at which the a-axis intersects the (40�2)plane was determined using the refined lattice constantsand found to be nearly perpendicular to the (40�2) plane at89.406(3)°. It should be noted here that if La3+ conductsalong the a-axis almost perpendicularly to the (40�2) plane,then the closer the f40�2-value is to 1, the higher the ionicconductivity is expected to be.The XRD pattern taken from the table plane of thetextured sample actually demonstrated the reflections 22�1and 62�3 in addition to the most intense 40�2 reflection, andthe f40�2-value was determined to be 0.235 [Fig. 2(a)].When the (40�2) planes of the crystal grains comprising thepolycrystal are fully oriented, the f40�2-value becomes 1,and only the 40�2 reflection will be observed, as demon-strated by the simulated XRD pattern of the polycrystal-Journal of the Ceramic Society of Japan 133 [11] 657-662 2025 JCS-Japan659line model 1 with f40�2 = 1.00 (r = 0.1) [Fig. S5(a)]. Thesimulations also show that, as the degree of (40�2) plane ori-entation decreased with f40�2 = 0.20 (r = 0.5), the reflec-tions other than 40�2, such as 22�1 and 62�3, become moreprominent [Fig. S5(b)]. This simulated XRD pattern wassimilar in profile shape to the XRD pattern of the texturedsample, so they showed the comparable f40�2-values. Asdemonstrated in Fig. 2(a) inset and Fig. S5(c), the simu-lated XRD patterns for f40�2 = 0 (r = 1), corresponding tothe XRD pattern of randomly grain-orientated La4Ga2O9polycrystal, show that the reflection indices of the strongestpeak are 22�1with the 40�2 reflection still clearly recognized.Therefore, the texture fraction of the (40�2) plane is alsoquantified by the integrated intensity ratio of I40�2=I22�1. TheI40�2=I22�1 ratio for the randomly grain-oriented polycrystal-line model 1 (f40�2 = 0 and r = 1) is 0.184 [Fig. S5(c)]. Forthe current textured specimen, the integrated intensity ofthe 40�2 reflection exhibited a significant increase relative tothat of the 22�1 reflection, resulting in a substantially higherI40�2=I22�1 ratio of 5.34 [Fig. 2(a)].In the event of the (40�2) plane normal of each crystalgrain comprising the polycrystal being perfectly oriented,the XRD pattern should exhibit an absence of 40�2 reflec-tion, with only the 0kl reflections being observed. This isFig. 1. Isosurface of BV energy for La3+ (Isovalue ¹1.60 eV,Emin ¹8.53 eV) showing the infinite connectivity of the La3+conduction pathway along the a-axis in La4Ga2O9. The anglebetween the a-axis and the (40�2) plane is 89.406(3)°, indicatingthat the conduction pathway and the (40�2) plane are nearlyorthogonal. The parallelepiped drawn with solid lines repre-sents the unit cell [a = 0.79778(3) nm, b = 1.12049(4) nm, c =1.16198(4) nm, and ¢ = 109.482(2)°]. The structural model andisosurface visualized using a computer program VESTA.28)Fig. 2. The fitting results of the X-ray diffraction patterns (red symbols: +) of the disc-shaped sinteredpolycrystal. The irradiated surface areas correspond to the table plane in (a) and the cross section in (b). In eachfigure, the calculated pattern and the possible locations of the Bragg reflections are indicated by solid lines at thetop and vertical bars at the bottom, respectively, while the difference curve is shown at the bottom of the figure.Inset in (a): The simulated X-ray diffraction pattern of the randomly grain-oriented La4Ga2O9 polycrystal. Inset in(b): Magnified view for 40° ¯ 2ª ¯ 50°.Shimotsukasa et al.: Enhancement of lanthanum ion conductivity by (40�2) plane orientation of polycrystalline La4Ga2O9JCS-Japan660demonstrated by the simulated XRD pattern of the poly-crystalline model 2 with r = 5 [Fig. S6(a)]. Conversely, inmodel 2 with the lower orientation degree of r = 1.8, the40�2 reflection is clearly observed [Fig. S6(b)], although itsdiffraction intensity is extremely low. The XRD patternobtained from the cross-sectional surface of the cut sampleexhibited a very weak 40�2 reflection, with the 0kl reflec-tions being predominantly observed [Fig. 2(b)]. This find-ing is consistent with the simulated XRD pattern of thepolycrystalline model 2 with r = 1.8. It is noteworthy thatthe crystal grains comprising the polycrystalline model 2have an overall random orientation of their b-axis direc-tion around the (40�2) plane normal. Therefore, the closesimilarity between the observed XRD pattern [Fig. 2(b)]and the simulated XRD pattern [Fig. S6(b)] suggests thatthe textured sample is composed of the crystal grains withrandomly oriented b-axis directions around the common(40�2) plane normal.The f40�2-value obtained from the XRD intensities of theS-C polycrystal (Fig. S7) according to Eq. (1) was 0.01.This confirms that the S-C is composed of the La4Ga2O9crystals with random grain orientation.3.2 Anisotropy of La3© conductionThe bulk conductivity of electrolyte 1 (·¬) correspondsto the La3+ conductivity parallel to the direction in whichthe (40�2) plane normal of the constituent crystal grains aremoderately oriented at f40�2 = 0.235, while the bulk con-ductivity of electrolyte 2 (·¦) corresponds to the La3+conductivity perpendicular to this direction (Fig. 3). The·¬ and ·¦ values at 623–1073K were derived from thecorresponding Rbulk values at each temperature of electro-lyte 1 and electrolyte 2, respectively (Figs. S8 and S9).The ·¬-value exhibited a steady increase from 1.63 © 10¹7S cm¹1 at 623K to 8.52 © 10¹4 S cm¹1 at 1073K with in-creasing temperature. The activation energy of conduction(E1) was determined to be 1.16(2) eV from the Arrheniusplot. The ·¦-value also increased steadily from 5.92 ©10¹8 to 5.62 © 10¹5 S cm¹1 with increasing temperaturefrom 673 to 1073K, with the E2-value of 1.14(1) eV. Thebulk conductivity of S-C (·r) was determined from thecorresponding Rbulk values at each temperature (Fig. S10).It increased steadily from 2.08 © 10¹8 S cm¹1 (623K) to1.03 © 10¹4 S cm¹1 (1073K) with increasing temperature,with the Er-value of 1.15(1) eV. The activation energies,E1, E2, and Er, were found to be almost identical amongthe three. It is hypothesized that these values correspond tothe activation energy required for the conduction of La3+,occupying the La1 and La2 sites, in the a-axis direction.On the other hand, the value of the activation energy EBV,as estimated by the BVenergy landscape method, has beendetermined to be 6.55 eV.9) Therefore, the value of thescaling factor required to convert the EBV value to theactual activation energy value is approximately 0.176(= 1.15/6.55).If the direction of La3+ conduction were restricted tothe (40�2) normal direction, for the perfectly (40�2) planealigned polycrystal with f40�2 = 1, the ·¦-value should bezero. However, the actual f40�2-value was found to be equalto 0.235, strongly suggesting that the conduction pathwayscontributing to ·¦ are formed within the present texturedpolycrystal. When compared at the same temperatures, the·¬-values were about 10.0 (673K) to 15.2 (1073K) timeshigher than the ·¦-values. This provides strong evidencethat the conduction of La3+ in the crystal structure ofLa4Ga2O9 occurs predominantly along the a-axis. The con-ductivity of the randomly grain-oriented polycrystal (·r)of La4Ga2O9 was intermediate between ·¬ and ·¦, withthe ·¬/·r ratios ranging from 6.7 (873K) to 8.3 (1073K).A comparison of the ·r values of La1/3Zr2(PO4)312) withthose of ·¬ at each temperature from 623 to 973K revealedthat the latter were approximately 10.5 (623K) to 132(973K) times higher than the former.Observation of the crystal structure of La4Ga2O9 fromthe reciprocal c-axis (c*-axis) direction reveals a layeredstructure (layer thickness of approximately 4.0 nm) parallelto the (40�2) plane, which is nearly perpendicular to the a-axis (Fig. S11). This layer is arranged in two pieces for theunit cell period along the a-axis direction, forming theentire crystal structure. The observed alignment of the(40�2) layer with the direction of the applied magnetic fieldindicates that the magnetic field would generate a magnetictorque on the (40�2) layer, causing the normal direction ofthe (40�2) layer to align with the direction of the magneticfield. The crystalline particle size has the potential to influ-ence the effect of torque rotation of the particles. It can behypothesized that smaller particles are more susceptible tothe effects of magnetic torque. However, further researchis needed to clarify the details of the particle alignmentprocess caused by the application of a strong magneticfield.The BV energy landscape method was the first to dem-onstrate the possibility of RE3+ conduction along the a-axis in the cuspidine-type compounds RE3+4Ga2O9 (RE =Fig. 3. Anisotropy of La3+ conduction in the (40�2) planealigned La4Ga2O9 polycrystal. The bulk conductivity ·¬ (elec-trolyte 1) is parallel to the aligned plane normal, and that of·¦ (electrolyte 2) is perpendicular to it. The bulk conductivitiesfor randomly grain-oriented polycrystals (·r) are shown forLa4Ga2O9 and La1/3Zr2(PO4)3.Journal of the Ceramic Society of Japan 133 [11] 657-662 2025 JCS-Japan661La, Pr, Nd, Sm) and RE3+4Al2O9 (RE = Y, Nd, Eu, Tb,Lu).9) In the present study, the conduction anisotropy ofLa3+ in La4Ga2O9 has been experimentally confirmed forthe first time, providing strong support for the hypothesisthat the conduction direction is primarily along the a-axis.In the sample prepared for this study, the f40�2-value was0.235, which is considerably less than 1. The precise causeof this remains to be elucidated; however, it is conceivablethat the dispersion of the crystal particles in the suspensionduring the application of the magnetic field was inade-quate. If a sample with the highly (40�2) plane orientationcan be obtained in which the f40�2-value is close to 1, thena further improvement in ionic conductivity can be expect-ed. This objective could be realized through the improve-ment of the suspension dispersion process by optimizingthe particle size distribution of the powder and/or theamount of polymeric dispersant added.4. ConclusionThe (40�2) plane aligned polycrystalline La4Ga2O9 (spacegroup P21/c) was prepared by sintering at 1673K for 2 h,following colloidal processing under the high magneticfield of 12T. In the course of the slip-casting process, thedispersed particle suspension exhibited a tendency to alignthe normal direction of each grain’s (40�2) plane with thestatic applied magnetic field, which was oriented parallel tothe direction of particle settlement. Therefore, followingsintering in the absence of a magnetic field, the resultingpolycrystal exhibited a moderate (40�2) plane orientation,as evidenced by the Lotgering factor f40�2 of 0.235. Theconductivities parallel (·¬) and perpendicular (·¦) to thealigned plane normal were compared at temperatures rang-ing from 673 to 1073K, together with the conductivity(·r) of randomly grain-oriented polycrystal. The ·¬-valueincreased steadily from 1.63 © 10¹7 to 8.52 © 10¹4 S cm¹1with increasing temperature from 623K to 1073K. Theactivation energy for conduction was determined to be1.16(2) eV. When compared at the same temperatures, ·¬was the highest, followed by ·r and then ·¦ in that order.The present study has confirmed for the first time theanisotropy of La3+ conduction in La4Ga2O9, and theobtained results were consistent with the preferential a-axisconduction predicted by the BV energy landscape method.If the polycrystals with the higher degree of (40�2) planeorientation can be prepared by the improved colloidalprocessing under high magnetic field, a further enhance-ment of the conductivity would be expected.Appendix A. Supporting informationSupplementary data associated with this article can be foundin the on line version.References1) L. F. O’Donnell and S. G. Greenbaum, Batteries 7, 3(2021).2) Y. Liang, H. Dong, D. Aurbach and Y. Yao, Nat. Energy5, 646 (2020).3) A. Ponrouch, J. Bitenc, R. Dominko, N. Lindahl, P.Johansson and M. R. Palacín, Energy Storage Mater.20, 253 (2019).4) N. Imanaka and S. Tamura, B. Chem. Soc. Jpn. 84, 353(2011).5) N. Imanaka, J. Ceram. Soc. 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