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Kaoru Kouzu, Shigeru Okada, Takeshi Hagiwara, Akiko Nomura, Kunio Yubuta, [Takao Mori](https://orcid.org/0000-0003-2682-1846), Toetsu Shishido, Akira Yoshikawa

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[Syntheses and some properties of solid solution Yb(Al,T)B4 (T = Mo or W) compounds](https://mdr.nims.go.jp/datasets/338f13f3-0f9a-4fd6-9d70-8890e51ccf8a)

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Crystal Structure of a Cu3Au-type compound CeRh3B0Syntheses and some properties of solid solution Yb(Al,T)B4 (T = Mo or  W) compoundsKaoru Kouzua, Shigeru Okadaa,b, Takeshi Hagiwarab, Akiko Nomurac, Kunio Yubutad, Takao Morie, Toetsu Shishidoc and Akira Yoshikawac,a Department of Science and Engineering, Kokushikan University, 4-28-1 Setagaya, Setagaya-ku, Tokyo 154-8515, Japanb Research Institute for Engineering, Kanagawa University,3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan. c Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, JapandDepartment of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japane International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan*Corresponding author:Department of Science and Engineering, Kokushikan University, Tokyo 154-8515, JapanTel: +81-3-5481-3279E-mail: kouzu@kokushikan.ac.jpAbstractRAlB4 (R = rare earth) have attracted increasing attention with recent reports of interesting structural, chemical and physical properties. The authors developed and we investigated the formation of the solid solution Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mn) compounds in which the Al position of YbAlB4 is substituted with T atoms using the Al flux method. From the result of XRD patterns, Yb(Al0.995 T0.005)B4 were obtained as a single phase. In this study, we investigated the formation of the solid solution Yb(Al0.995T0.005)B4 (T = Mo or W) compounds. The resulting crystals were examined for lattice constants and chemical composition, in addition, hardness values by micro-Vickers hardness and oxidation resistance by heating in air with a DTA-TG apparatus. Comparing the lattice constants of YbAlB4 and Yb(Al0.995T0.005)B4, When Mo or W were included, the values tended to increase. The hardness values of Yb(Al0.995Mo0.005)B4 was 11.0(±0.7) GPa, of Yb(Al0.995W0.005)B4 was 15.7(±1.9) GPa. The oxidation onset temperature in the heating of Yb(Al, T)B4 (T = Mo, W) is about 100 K higher than YbAlB4.Keywords: Al flux method, Yb(Al,T)B4 (T = Mo, W) compounds, micro-Vickers hardness, TG-DTA1. IntroductionRAlB4 (R = rare earth) [α-type, Pbam][1,2] have attracted increasing attention with recent reports of interesting structural, chemical and physical properties [3-6]. RAlB4 has the same structure as that postulated for YCrB4. Fig. 1 shows a perspective view of the YCrB4-type structure. The linkage of boron atoms in the structure of YCrB4-type is made up by five- and seven-membered rings. As in the AlB2-type, the structure of RAlB4 is built up by two-dimensional boron networks sandwiched between metal layers. These boron atoma reside in the intersitial sites of trigonal prisms formed by the Y and Cr atoms. The authors developed and synthesized in obtaining a new compound, Tm(Al,T)B4, in which small amounts of Fe, Cr, Mn, Mo, and W atoms are dissolved in the Al atoms of the TmAlB4 crystal using the Al flux method [7,8]. From Table 1, we can see that when Mo atoms dissolve in the Al atoms of Tm(Al1-xMox)B4 from the XRD patterns, a solid solution is formed in the range of x = 0.002 - 0.030, but when W atoms dissolve, the solid solution Tm(Al1-xWx)B4 is only formed in the narrow range of x = 0.002 - 0.010. Furthermore, when x = 0.030, MoAlB is produced when Mo atoms dissolve, but WAlB is not observed when W atoms dissolve [9]. Recently, we reported on the crystal growth and the micro-hardness of Yb(Al1-x,Tx)B4 (T = Fe, Cr, Mn) compounds [7-9]. Based on previous results, an Al atoms to T atoms ratio of x = 0.005 was used. However the question of crystal growth and the properties of new compounds Yb(Al0.995T0.005)B4 (T = Mo, W) are still not elucidated completely: there is no report on crystal growth and physical properties. In this work we report on the experimental conditions for growing crystals of the solid solution Yb(Al0.995T0.005)B4 (T = Mo, W) compounds from the high temperature Al and T mixed solution.In addition, measurements obtained crystals were examined for lattice constants and chemical compositions, the hardness values using a micro-Vickers hardness tester at room temperature were examined, and the oxidation resistance by heating in air using the TG-DTA apparatus. We report this result in comparison with previously reported Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mn) [7-9].2. ExperimentalThe reagents used to prepare the solid solution Yb(Al0.995T0.005)B4 (T = Mo, W) compounds were ytterbium oxide (Yb2O3 purity: 99.9 %), crystalline boron (B < 200 mesh) (purity: 99 %) powder, aluminum metal (Al purity: 99.99 %) button, molybdenum metal (Mo purity: 99.9 %) powders, tungsten metal (W purity: 99.9 %) powders. For the flux atomic ratios of B/Yb2O3 = 9.0 (Yb2O3 + 9B → 2YbB3 + 3BO), and mixed flux Al1-xTx (referred to as “flux”). The mixtures of the materials in proper atomic ratios were placed in an α-Al2O3 crucible and heated in Ar gas. The amounts of Yb2O3 and B in the starting materials were fixed. Al and Mo or W metals were added to each mixture at a mass ratio of 93 - 95 mass%. We report the experimental conditions for growing crystals of the Yb(Al,T)B4 (T = Mo, W) from the high temperature Al and T mixed solution. The temperature of the furnace was raised at a rate of 300 K/h to 1773 K, kept constant for 5 h, and then lowered slowly to room temperature at a rate of 50 K/h. Under a stereomicroscope crystal were selected suitable for chemical analyses and measurements of micro-Vickers hardness, and TG-DTA. The phase analysis and determination of lattice constants were carried out using a powder X-ray diffractometer (XRD) (Rigaku, Rint-2000). The morphology, size, chemical composition and impurity content of the crystals were observed by the scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) (Horiba Co, EMAX-2770) and electron probe microanalysis (EPMA) (JEOL, JXA-8621MX). Hardness measurements on as-grown Yb(Al,T)B4 crystals were done using a Vickers diamond indenter at room temperature. A load of 1.96 N was applied for 20 seconds on about ten positions in relatively large planes of each crystal, and the values obtained were averaged. The crystals were powdered in an Al2O3 mortar and then heated to 1200 K in air using the differential thermal analyses and thermogravimetric (TG-DTA, Seiko Instrument) apparatus for oxidation resistance. After inserting the sample into the quartz cell, it was heated in air at a constant rate of 10 K·min-1. 3. Results and DiscussionThe XRD patterns of the obtained Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W) compounds, Al0.89TmB3.75 (ICDD card: No.48-1678) and MoAlB (ICDD card: No.04-0787) are shown in Fig. 2. From this, the solid solution Yb(Al,T)B4 (T = Fe, Cr, Mo, Mn, W) has similarity to the ICDD card. A single phase Yb(Al,T)B4 (T = Fe, Cr, Mo, Mn, W) compounds were synthesized from mixed fluxes of Al and T metals. And In the case of Yb(Al, Mo)B4, the pattern MoAlB was confirmed in addition to Yb(Al, Mo)B4. However, no WAlB was observed from Yb(Al,W)B4. The obtained crystals are thick tetragonal crystals in the case of Yb(Al, Mo)B4, which may occur together with needle-like MoAlB crystals, whereas in the case of Yb(Al, W)B4, they form as single-phase polyhedral crystals with complex morphology in Fig.3. The relationship between the lattice constants of YbAlB4 [10] and Yb(Al,T)B4 (T = Fe, Cr, Mo, Mn, W)　with metal atoms in solid solution is shown in Fig. 4. Comparing the lattice constants and unit cell volumes of YbAlB4 and Yb(Al,T)B4 (T = Mo, W), larger than YbAlB4 when Mo or W is dissolved. This suggests that this is related to the ionic radius of the metal atoms dissolved in YbAlB4. This is presumably due to the incorporation of Mo ((+3) 6 (CN) 0.69 Å), (+4) 6 (CN) 0.65 Å, (+5) 6 (CN) 0.61 Å) or W ((+4) 6 (CN) 0.66 Å, (+5) 6 (CN) 0.62 Å, (+6) 6 (CN) 0.60 Å))[11], which are larger than the Al ((+3) 6 (CN) 0.54 Å) [11] in Yb(Al,T)B4. The chemical analyses of Yb(Al,T)B4 (T = Fe, Cr, Mo, Mn, W) compounds using EPMA measurements are shown in Table 2. From these date, it was found that Yb(Al,T)B4 (T = Mo, W) with T as a solid solution contained about 40% of each T metal. Previous reports have shown that compounds containing Fe have a low ratio of Fe atoms, whereas compounds containing Cr have a high ratio of Cr atoms. From these results, it can be understood that many metal atoms form solid solutions in YbAlB4.The micro-Vickers hardness date of Yb(Al,T)B4 (T = Fe, Cr, Mo, Mn, W) compounds are given in Table 3 together with those for YbAlB4. The values of the micro-Vickers hardness for Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W) was in the range of 11.0(±0.7) - 15.7(±1.9) GPa. No significant dependency of hardness on the metals was observed. The hardness of RAlB4 is 14.5 GPa for TmAlB4, 14.2 GPa for YbAlB4, and 14.0 GPa for LuAlB4 [12-14]. When these were compared with the hardness values ​​of Yb(Al, T)B4, the values ​​were comparable and within the error range. The hardness of AlB2-type alloys, which have a structure similar to that of RAlB4, has been reported for TiB2 (33 GPa), ZrB2 (22 GPa), VB2 (27 GPa), MoB2 (21 - 24 GPa), and CrB2 (26 - 29 GPa) [15]. These compounds are formed by six-membered rings made of boron atoms, and the units made of boron and metal atoms form a layered structure with each other. Comparing these structures, it is suggested that the hardness of the RAlB4- type is different due to the difference in the units in which boron atoms form five- and seven-membered rings. The oxidation process of Yb(Al,T)B4 (T = Mo, W)compounds were studied at temperatures below 1200 K by TG-DTA analyses in air atmosphere. The results for TG-DTA curve using Yb(Al,T)B4 (T= Mo, W) shown in Fig.5. Table 4 shows the exothermic reaction observed from the DTA curves and the oxidation initiation temperature obtained from the TG curves of Yb(Al,T)B4 (T = Fe, Mo, Mn, W). The DTA results showed that the large exothermic peaks were located around 1000–1060 K, which was almost the same. The oxidation initiation temperature of YbAlB4 is 863 K [5], however, it was found that the oxidation onset temperature of the solid solution Yb(Al,T)B4 (T = Fe, Mo, Mn, W) were about 100 K or more higher than that of YbAlB4. After heating of the samples, XRD measurements were not possible because the oxidation products reacted with quartz.4. ConclusionsThe results of an attempt to synthesize the Yb(Al,T)B4 (T = Mo or W) compounds using the Al flux method are shown below.1) A single phase Yb(Al0.995T0.005)B4 (T = Mo,W) compounds were synthesized from mixed fluxes of Al and T metals. 2) Comparing the lattice constants of YbAlB4 and Yb(Al,T)B4, the lattice constants of Yb(Al,T)B4 (T = Fe, Cr, Mo, Mn, W) tends to be smaller when Cr, Fe or Mn metals were contained, and tends to be larger when Mo or W metals were contained.3) The hardness of Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W) was in the range of 11.0(±0.7) to 15.7(±1.9) GPa. The hardness values of Yb(Al,T)B4 were almost the same value as YbAlB4.4) The oxidation onset temperature by heating of Yb(Al,Mo)B4 were 972 K and Yb(Al, W)B4 were 987 K. The oxidation onset temperature in the heating of Yb(Al,T)B4 (T = Mo, W) is about 100 K higher than YbAlB4. AcknowledgementsFor EPMA measurements, we received the support pf Mr. Marita, Technical Staff of the Institute for Materials Research, Tohoku University. The authors are indebted to IMR in Tohoku University for their technical contributions. K.K was partly supported by the GIMRT program (202212-CRKKE-0020) at the Institute for Materials Research, Tohoku University, Japan. K.Y. was partly supported by JSPS KAKENHI Grants number (Nos. JP19K05643, No. JP20H05258 and JP23K04373) and the GIMRT program (Nos. 202211-RDKGE-0008 and 202311-RDKGE-0001) at the Institute for Materials Research, Tohoku University, Japan. References[1] S. Okada, T. Shishido, T. Mori, K. Kudou, K. Izumi, T. Lundström, K. Nakajima: J. Alloys and Compounds, 408-412, 547-550(2006).[2] T. Mori, T. Shishido, K. Nakajima, S. Okada, K. Kudou, K. Kiefer, K. Siemensmeyer: J. Physics: Conference Series, 200, 012127 (2010).[3] T. Mori, J. Solid State Chem., 275, 70-82 (2019).[4] T. Yamasaki, K. Kouzu, A. Nomura, S. Okada, T. Shishido, K. Yubuta, A. Yoshikawa, T. Mori: Transactions of the Kokushikan University School of Science and Engineering, No.11, 49-53 (2017).[5] K. Kouzu, T. Yamasaki, S. Okada, A. Nomura, K. Yubuta, T. Shishido, A. Yoshikawa, T. Mori, P.  Rogl, J. Flux Growth, 13, 1, 2-6 (2018). [6] T. Mori, H. Borrmann, S. Okada, K. Kudou, A. Leithe-Jasper, U. Burkhardt, Yu. Grin: Phys. Rev., B 76, 064404 1, 1-10 (2007).[7] K. Kouzu, T. Yamasaki, S. Okada et al., J. Jpn. Soc. Powder Powder Metallugy, 66, 525-529 (2019).[8] K. Kouzu, S. Okada et al., J. Jpn. Soc. Powder Powder Metallugy, 70, 461-465 (2023).[9] S. Okada, K. kouzu et al., Solid State Phenomena, 289, 65-70 (2019).[10] S. Okada, K. Kudou et al., JJAP Series 10,136-137 (1994).[11] David R. Lide Ed., “CRC Handbook of Chemistry and Physics”, 76th Edition 1995-1996, CRC Press, New York, pp.12-14~12-15.[12] K. Kouzu, T. Yamasaki, S. Okada, T. Mori, Q. Guo, T. Shishido, K. Yubuta, A. Nomura, A. Yoshikawa, P. Rogl: Solid State Phenomena, Vol.289, 120-126 (2019).[13] K. Kouzu, T. Yamasaki, S. Okada, T. Mori, Q. Guo, K. Yubuta, A. Nomura, T. Shishido, A. Yoshikawa, P. Rogl: J. Jpn. Soc. Powder Powder Metallurgy, 66, 525-529 (2019).[14] S. Okada, Y. Yu, T. Lundström, K. Kudou, T. Tanaka, Jpn. J. Appl. Phys., Vol.35, No.9 ,4718-4723 (1996).[15] T. Tanaka et al., Fundamentals and Application of Boron, Borides and Related Materials”, CMC Co., 103 (2008).Table 1 Growth conditions of Tm(Al1-xTx)B4 crystals obtained from Al1-xTx (x = 0.002 - 0.050) flux (T = Mo [4] or W). Table 2 Results for chemical analyses of the Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W)　 compounds obtained from Al0.995T0.005 flux. Table 3 Results for micro-Vickers hardness of Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W) and YbAlB4 [5] compounds.Table 4 The results for measurement TG-DTA of Yb(Al0.995T0.005)B4 (T = Fe, Mo, Mn, W) compounds.Fig. 1 Crystal structure of YCrB4.Fig. 2 XRD patterns of samples obtained from Yb(Al0.995T0.005)B4 compounds systems.〇: Al0.89TmB3.75 (ICDD card of No. 48-1678), ×:MoAlB (ICDD card of No. 04-0787)Fig. 3 Stereomicroscope photographs of Yb(Al0.995T0.005)B4 (T = Mo or W) crystals.(a) Yb(Al0.995Mo0.005)B4　crystals.(b) Yb(Al0.995W0.005)B4 crystals.Fig. 4 Relationships for lattice constants of YbAlB4 [10] and Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W) compounds. Fig. 5 TG-DTA analyses of (a) Yb(Al0.995Mo0.005)B4 and (b) Yb(Al0.995W0.005)B4 crystals heated in air. Sample was heated in air at a constant rate of 10 K·min-1.0.002 Al0.998Mo0.002TmB4, Tm(Al1-xMox)B40.005 Al0.995Mo0.005Tm(Al1-xMox)B40.010 Al0.990Mo0.010Tm(Al1-xMox)B40.030 Al0.970Mo0.030MoAlB, unknown phaseTm(Al1-xMox)B40.050 Al0.950Mo0.050MoAlB, unknown phase0.002 Al0.998W0.002TmB4, Tm(Al1-xWx)B40.005 Al0.995W0.005Tm(Al1-xWx)B40.010 Al0.990W0.010Tm(Al1-xWx)B40.030 Al0.970W0.030unknown phase0.050 Al0.950W0.050WAl4, unknown phaseAtomicratio (x)Phases identifield(from XRD)Al1-xWx fluxAtomicratio (x)Al1-xMo fluxPhases identifield(from XRD)Fig. 1 Crystal structure of YCrB4.Table 1 Growth conditions of Tm(Al1-xTx)B4 crystals obtained from Al1-xTx (x = 0.002 - 0.050) flux (T = Mo [4] or W). Fig. 2 XRD patterns of samples obtained from Yb(Al0.995T0.005)B4 compounds systems.〇: Al0.89TmB3.75 (ICDD card of No. 48-1678), ×:MoAlB (ICDD card of No. 04-0787)(a)                     (b)Fig. 3 Stereomicroscope photographs of Yb(Al0.995T0.005)B4 (T = Mo or W) crystals.(c) Yb(Al0.995Mo0.005)B4　crystals.(d) Yb(Al0.995W0.005)B4 crystals.Fig. 4 Relationships for lattice constants of YbAlB4 [10] and Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W) compounds. Table 2 Results for chemical analyses of the Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W)　 compounds obtained from Al0.995T0.005 flux. Yb Al T TotalYb(Al,Fe)B450.2 36.1 13.7 100 Yb(Al0.7Fe0.3)B4Yb(Al,Cr)B449.6 20.8 29.6 100 Yb(Al0.4Cr0.6)B4Yb(Al,Mo)B450.5 31.1 18.4 100 Yb(Al0.6Mo0.4)B4Yb(Al,Mn)B456.4 23.3 20.3 100 Yb(Al0.5Mn0.5)B4Yb(Al,W)B449.2 31.5 19.3 100 Yb(Al0.6W0.4)B4Yb(Al0.995T0.005)B4Atomic ratio (at %)obtained from EPMAChemical composition ofYb(Al1-xTx)B4Table 3 Results for micro-Vickers hardness of Yb(Al0.995T0.005)B4 (T = Fe, Cr, Mo, Mn, W) and YbAlB4 [5] compounds.Micro-Vickers hardness (GPa)Yb(Al, Fe)B415.7(±1.5)Yb(Al, Cr)B413.3(±2.7)Yb(Al, Mo)B411.0(±0.7)Yb(Al, Mn)B413.0(±0.6)Yb(Al, W)B415.7(±1.9)YbAlB4 [10] 14.2(±1.4)Yb(Al0.995T0.005)B4(a) DTA-TG curve of Yb(Al0.995Mo0.005)B4(b) DTA-TG curve of Yb(Al0.995W0.005)B4Fig. 5 TG-DTA analyses of (a) Yb(Al0.995Mo0.005)B4 and (b) Yb(Al0.995W0.005)B4 crystals heated in air. Sample was heated in air at a constant rate of 10 K·min-1.Table 4 The results for measurement TG-DTA of Yb(Al0.995T0.005)B4 (T = Fe, Mo, Mn, W) compounds.Yb(Al0.995T0.005)B4Oxidationstart (K)Yb(Al0.995Fe0.005)B41000,1046 971Yb(Al0.995Mo0.005)B41031,1051 972Yb(Al0.995Mn0.005)B41001,1033 980Yb(Al0.995W0.005)B4987Exothermicpeak (K)1056YCrBPAGE  1