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[Zhijun Li](https://orcid.org/0009-0006-6314-410X), Xun Kang, [Xuan Liang](https://orcid.org/0000-0002-1062-4103), [Alexei A. Belik](https://orcid.org/0000-0001-9031-2355), [Masao Arai](https://orcid.org/0000-0003-0088-5649), [Kazunari Yamaura](https://orcid.org/0000-0003-0390-8244), Rintaro Oshikiri, Asuka Ishikawa, Takafumi D. Yamamoto, Shintaro Suzuki, Ryuji Tamura

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This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic Chemistry, copyright © 2025 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.inorgchem.4c05010.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Crystal Structure and Physical Properties of Au<sub>4</sub>Al-Type Suboxides in the Ti–Rh–Si–O and Ti–Ir–Si–O Systems](https://mdr.nims.go.jp/datasets/ae5afc42-35d3-47d1-887d-67aec3f2843c)

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Crystal Structure and Physical Properties of Au4Al-Type Suboxides in the Ti-Rh-Si-O and Ti-Ir-Si-O SystemsZhijun Li,1,2,* Xun Kang,1,2 Xuan Liang,1,2 Alexei A. Belik,1 Masao Arai,3 Kazunari Yamaura1,2,* Rintaro Oshikiri,4 Asuka Ishikawa,4 Takafumi D. Yamamoto,4 Shintaro Suzuki,4,# Ryuji Tamura 41 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan2 Graduate School of Chemical Sciences and Engineering, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan3 Center for Basic Research on Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan4 Department of Materials Science and Technology, Tokyo University of Science, 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, JapanAbstractThis study reports the first successful synthesis and comprehensive characterization of Ti3RhSiO and Ti3IrSiO, the first members of Ti-based Au4Al-type materials. These compounds crystallize in the cubic space group P213 (No. 198) with refined lattice parameters of 6.75362(2) Å and 6.75524(1) Å, respectively. Oxygen plays a crucial role in stabilizing the cubic phase, as confirmed by synchrotron X-ray diffraction and the absence of this phase in oxygen-free samples. First-principles calculations and resistivity measurements reveal a robust multiband metallic character, with Ti-3d and Rh-4d (or Ir-5d) orbitals contributing significantly at the Fermi level. Specific heat analysis highlights weak anharmonic lattice vibrations, while magnetoresistance measurements demonstrate negligible field sensitivity. These unique structural and electronic properties provide valuable insights for the development of Ti-based Au4Al-type compounds, opening new avenues for fundamental materials research. 1. IntroductionNon-centrosymmetric β-Mn-type structures have recently attracted considerable interest due to their potential to host skyrmions and superconductivity—key phenomena for advanced technologies such as spintronic devices and high-density, low-power magnetic memory systems.1 These cubic structures crystallize in the isomorphous space groups P4132 or P4332 and feature two distinct atomic sites, 8c and 12d.2 Notably, β-Mn-type compounds typically incorporate both metallic and non-metallic elements. Examples include carbon- or nitrogen-containing systems such as W4IrC1-x 3 and Rh2Mo3N.4 However, β-Mn-type structures stabilized by oxygen remain unexplored. Oxygen is known to enhance various alloy properties, including mechanical toughness 5 and superconductivity,6 motivating the exploration of oxygen-stabilized β-Mn-type materials. In this study, we investigate a related structure: the Au4Al-type, an ordered variant of the β-Mn-type structure that crystallizes in the cubic P213 space group.7 The Au4Al-type structure introduces additional symmetry breaking by splitting the 8c site into two distinct 4a sites, enabling ordered elemental occupations. This ordering can significantly influence electronic and magnetic properties, as seen in compounds such as (Cr,Ni)4-xTxSi (T = Cu, Fe, Pd),8 Mn3Ir(Si1-xGex),9 Al25Ti55Pt20,10 and (Ni,Fe)4P.11 Here, we report the first synthesis and characterization of Ti-based Au4Al-type suboxides, specifically Ti3RhSiO and Ti3IrSiO. These compounds establish a new family of oxygen-stabilized β-Mn-type materials with distinct structural and electronic properties. Through a comprehensive investigation of their structural and magnetic characteristics, we aim to broaden the chemical and physical diversity of β-Mn-type and related suboxides. Our findings provide new insights into their potential for novel electronic states and enhanced structural stability.2. ExperimentalSample Preparation: Polycrystalline samples of Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO were synthesized using an arc-melting method. Stoichiometric amounts of Ti (99.99%, Kojundo Chemical Laboratory Co., Ltd.), Rh (≥99.95%, Furuya Metal Co., Ltd.), Ir (99.99%, source unspecified), Si (99.999%, Kojundo Chemical Laboratory Co., Ltd.), and SiO2 (99.999%, Soekawa Chemical Co., Ltd.) were accurately weighed and thoroughly mixed in an agate mortar. The homogeneous mixtures were pressed into pellets and arc-melted multiple times in an argon atmosphere using a commercial arc furnace (ACM-C01, DIAVAC Limited, Japan). After melting, the samples were cooled to room temperature within the furnace. A portion of the samples was ground into powder for X-ray powder diffraction measurements, while the remaining pieces were reserved for further characterization.Structural Characterization: Powder X-ray diffraction (XRD) data were collected at room temperature using a RIGAKU MiniFlex600 diffractometer with Cu-Kα radiation (2θ range: 3–80°, step size: 0.02°, scan speed: 10°/min) to assess phase purity. High-resolution synchrotron XRD was performed at beamline BL02B2 of SPring-8 (Japan) for detailed structural analysis.12,13 The powdered samples were loaded into Lindemann glass capillaries (0.2 mm inner diameter), and the incident X-ray wavelength was set to 0.61974 Å. CeO2 benchmark standard was used for wavelength calibration. Rietveld refinements of the synchrotron XRD patterns were conducted using the RIETAN-VENUS software package.14,15 Magnetic Measurements: Magnetic properties were measured using a SQUID magnetometer (MPMS3, Quantum Design Japan, Inc.). Magnetic susceptibility (χ) was recorded in both zero-field-cooled (ZFC) and field-cooled (FC) modes under an applied magnetic field (H) of 10 kOe, over a temperature range of 2–300 K. Isothermal magnetization (M) as a function of H was measured at selected temperatures (10 K, 100 K, and 300 K) with fields up to 70 kOe. Corrections for sample holder contributions were applied using baseline data obtained under identical conditions. Heat Capacity and Electrical Resistivity: Specific heat capacity (Cp) and electrical resistivity (ρ) were measured using a Physical Property Measurement System (PPMS, Quantum Design Japan, Inc.) over the temperature range of 1.8–300 K. Cp was determined using the thermal relaxation method, with Apiezon N grease ensuring thermal contact. Resistivity measurements employed a standard four-probe method, with platinum wires (50 μm diameter) and silver paste used for electrical connections. A constant current of 5 mA was applied during measurements. First-Principles Calculations: First-principles calculations were performed using the full-potential augmented plane wave and local orbital method, as implemented in the WIEN2k package.16,17 The generalized gradient approximation (GGA) within density functional theory (DFT) was employed using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.18 Muffin-tin radii were set to 2.0, 2.44, 2.1, and 1.81 atomic units for Ti, Rh, Si, and O, respectively. The cutoff wave number Kmax was chosen to satisfy RmaxKmax = 7, where Rmax is the largest muffin-tin radius of 2.44 for Rh. Integration over the Brillouin zone was performed using the tetrahedron method with a 16 × 16 × 16 k-point mesh, resulting in 176 inequivalent k-points in the irreducible Brillouin zone. Spin-orbit interaction (SOI) was included in the self-consistent calculations.  3. Results and discussion3.1 Formation and Oxygen Stabilization of Au4Al-Type Suboxides Over the past several decades, extensive research has focused on oxygen-stabilized titanium-based alloys, commonly known as suboxides.19–22 Notable examples include Ti4Co2O, Ti4Rh2O, and Ti4Ir2O, which exhibit superconductivity at temperatures below 5.4 K.21 These suboxides demonstrate the potential for achieving unique electronic and structural properties through oxygen incorporation. Similarly, the Ti-Cr-Si-O system is recognized for its high oxygen content and the presence of an icosahedral phase.23–32 Neutron diffraction studies have identified its composition as Ti88Cr34Si24O23, featuring Mackay clusters within a 1/1 approximant crystal.27 These findings underscore the critical role of oxygen in stabilizing complex structural motifs and provide a strong foundation for exploring similar behaviors in other alloy systems.In our experiments within the Ti-M-Si-O system, we found that introducing oxygen led to the formation of new stable compounds when M = Rh or Ir. Unlike previously reported cases with M = Cr, Mn, and Fe,23–32 these compounds do not adopt the 1/1 approximant structure. Instead, they crystallize in the Au4Al-type suboxide structure. Notably, these suboxides feature icosahedral clusters—a structural motif commonly associated with alloy quasicrystals—which suggests the potential for discovering oxide quasicrystals within this new material family. This intriguing possibility presents promising avenues for further exploration.To optimize the nominal composition of the stable phase, we initially synthesized compounds Ti88Rh34Si24-x(SiO2)x using an arc furnace, based on bulk compositions identified in the Ti-Cr-Si system. By testing various values of x, we determined that x = 12 minimized impurity formation and yielded reproducible results (Fig. 1).When synthesis was performed without an oxygen source, the primary cubic phase did not form, confirming that oxygen is essential for structural stabilization. This indicates that oxygen plays a crucial role beyond merely acting as an impurity or byproduct; it is integral to the formation and stabilization of the cubic lattice. These results align with previous studies on the Ti-Cr-Si-O system, further emphasizing the necessity of oxygen for structural stability. Interestingly, the cubic lattice constant remained unchanged across all samples, regardless of the initial oxygen content (Fig. S1). This observation suggests that oxygen atoms are incorporated in a fixed, stoichiometric manner within the lattice rather than in variable concentrations. These findings strongly indicate that oxygen is an essential structural component in the Ti-Rh-Si-O system, stabilizing the cubic lattice without altering its dimensions.The cubic phase, initially synthesized with the nominal composition Ti88Rh34Si12(SiO2)12, was further analyzed using synchrotron XRD, revealing that the actual chemical composition is more accurately represented as Ti3RhSiO. Based on this finding, subsequent syntheses were adjusted to the nominal composition Ti3RhSiO, leading to a significant improvement in the sample quality for the oxygen-containing phase (Fig. S2). In contrast, the cubic phase was completely absent in the oxygen-free sample, Ti3RhSi, underscoring the indispensable role of oxygen in structural stabilization. These results further confirm that oxygen is crucial for the formation of the cubic phase. A purified sample with the correct oxygen content was subjected to synchrotron XRD for detailed structural analysis, as described in the following section. Figure 1: X-ray diffraction patterns of samples prepared with nominal compositions Ti88Rh34Si24-x(SiO2)x for varying values of x (0, 4, 8, 12, and 16), measured using Cu-Kα radiation. The patterns confirm the formation of the cubic phase upon the addition of an oxygen source. Notably, samples with higher x values display additional peaks (marked with *), which are likely associated with TiO1-δ impurities. 3.2 Crystal and Electronic StructuresThe Rietveld refinements of the synchrotron XRD patterns for Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO, measured at room temperature (296–300 K), are shown in Fig. 2. These refinements were performed using Mn3IrGe as the structural reference model.33 The analysis confirms that all samples crystallize in a cubic structure, specifically in the space group P213 (No. 198). The refined lattice constants are: Ti3RhSiO, a = 6.75362(2) Å; Ti3Rh0.5Ir0.5SiO, a = 6.75398(1) Å; and Ti3IrSiO, a = 6.75524(1) Å. The analysis confirms that Ti atoms fully occupy the 12b Wyckoff positions, while Rh(Ir), Si, and O atoms are located at the 4a sites. The final crystallographic parameters for each sample are summarized in Table 1. In preliminary refinements, we investigated potential metal mixing, similar to that observed in the 1/1 crystal approximant Ti88Cr34Si24O23.28 However, no significant evidence of metal mixing was found. Consequently, atomic sites were assigned to specific atoms without mixing in the final refinements. For the oxygen site, we initially considered the possibility of non-stoichiometry, but neither preliminary analysis nor synthesis results supported this hypothesis. Therefore, in the final refinements, the oxygen site was fixed as fully occupied. Figure 2: Rietveld analysis of synchrotron X-ray diffraction patterns for powders of (a) Ti3RhSiO, (b) Ti3Rh0.5Ir0.5SiO, and (c) Ti3IrSiO, measured at room temperature (296–300 K). Each panel displays the observed pattern (crosses), calculated pattern (solid green lines), and difference profile (blue curves). Vertical bars indicate the Bragg reflection positions for each phase. The refinement results and fit indices are summarized in Table 1, confirming the high quality of the Rietveld analysis. Relatively large Rwp and Rp values for the Ti3RhSiO sample are attributed to the presence of impurities. Including regions with impurity peaks results in agreement factors of Rwp = 18.67% and Rp = 10.89%. However, excluding these regions significantly improves the agreement factors to Rwp = 10.21% and Rp = 7.27%. Given the nearly identical crystal structures of the three samples, only one representative structural image is shown in Fig. 3 to avoid redundancy. In this structure, Ti and Rh atoms form an icosahedral framework around the central Si atom, with oxygen atoms distributed throughout the surrounding lattice. Additionally, the temperature dependence of the lattice parameter and unit cell volume for cubic Ti3RhSiO is shown in Fig. S3. Both the lattice parameter (a) and unit cell volume (V) exhibit gradual thermal expansion with increasing temperature, while the thermal expansion coefficient remains nearly linear up to 800 K. This behavior demonstrates stable thermal expansion and structural integrity at elevated temperatures, underscoring the critical role of oxygen in stabilizing the cubic lattice and ensuring its robustness over a wide temperature range. Table 1: Structural parameters of Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO, measured at room temperature (296–300 K) using synchrotron X-ray diffraction with a wavelength of 0.61974 Å. All compounds crystallize in the cubic P213 space group. atom Wyckoff position g x y z Biso (Å2) Ti3RhSiO, a = 6.75362(2) Å Ti 12b 1 0.12940(37) 0.20449(28) 0.45789(30) 0.406(21) Rh 4a 1 0.07421(14) = x = x 0.417(16) Si 4a 1 0.67901(49) = x = x 0.352(78) O 4a 1 0.3783(14) = x = x 1.20(21) Ti3Rh0.5Ir0.5SiO, a = 6.75398(1) Å Ti 12b 1 0.13082(20) 0.20433(17) 0.45690(18) 0.529(15) Rh/Ir 4a 0.5/0.5 0.07398(5) = x = x 0.327(5) Si 4a 1 0.67791(30) = x = x 0.509(45) O 4a 1 0.38080(71) = x = x 0.64(11) Ti3IrSiO, a = 6.75524(1) Å Ti 12b 1 0.13148(19) 0.20420(17) 0.45678(18) 0.558(15) Ir 4a 1 0.07400(4) = x = x 0.471(4) Si 4a 1 0.67905(28) = x = x 0.254(36) O 4a 1 0.38190(65) = x = x 0.45(10)Note: The calibrated X-ray wavelength was 0.61974 Å. The space group for all samples was P213 (No. 198) with Z = 4, and g represents the occupation factor. For Ti3RhSiO: lattice parameter a = 6.75362(2) Å, unit-cell volume V = 308.0419(18) ų, R indices: Rwp = 18.67%, Rp = 10.89% (Rwp = 10.21% and Rp = 7.27% when regions with impurities are excluded). For Ti3Rh0.5Ir0.5SiO: a = 6.75398(1) Å, V = 308.0914(7) ų, Rwp = 8.62%, Rp = 5.65% (Rwp = 6.81% and Rp = 5.07% when regions with impurities are excluded). For Ti3IrSiO: a = 6.75524(1) Å, V = 308.2637(6) ų, Rwp = 6.97%, Rp = 5.08% (Rwp = 6.28% and Rp = 4.83% when regions with impurities are excluded).  Figure 3: Crystal structure of Ti3RhSiO (Au4Al-type structure) viewed from different orientations. Solid lines indicate the cubic unit cell. (Top left) A single unit cell showing the icosahedral framework formed by Ti and Rh atoms surrounding the Si atom. (Top right) Polyhedral representation highlighting the arrangement and connectivity of the icosahedral frameworks, with blue spheres indicating oxygen positioned among the icosahedra. (Bottom) An alternative view showing the network of polyhedra formed by Ti atoms surrounding oxygen within the crystal lattice. Based on the experimentally obtained structural parameters, we conducted first-principles calculations to gain deeper insights into the electronic structure of Ti3RhSiO. The total and partial densities of states (DOS) are shown in Fig. 4a, with the vertical line marking the Fermi energy (EF). The lowest panel represents the total DOS, while the upper panels illustrate the partial contributions from each atomic species. Ti3RhSiO is confirmed to be a multiband metal, with states at the Fermi level primarily contributed by Ti-3d and Rh-4d orbitals, with minimal contributions from Si and O. The limited involvement of oxygen near EF suggests its primary role as an oxide ion, enhancing structural stability rather than significantly contributing to the electronic states at the Fermi level. Figures 4b and 4c present the energy band structure of Ti3RhSiO along high-symmetry lines in the Brillouin zone, with and without SOI. Several bands cross the Fermi level in multiple directions, confirming robust metallic conductivity. This observation aligns with the experimental resistivity behavior discussed in the following section. Due to the lack of inversion symmetry in this material, SOI lifts the two-fold degeneracy of the energy bands. Nonetheless, the overall band structure remains largely unchanged. Interestingly, some of the band crossings observed in the dispersion suggest a potential topological character of the bulk electronic states. However, the multiband metallic nature of Ti3RhSiO may obscure these topological features, making them difficult to identify definitively. Further investigations, such as surface-sensitive techniques or advanced theoretical analyses, are required to explore this possibility in greater detail.The predicted electronic structure for the Co phase, shown in Fig. S4, is expected to resemble that of the Rh and Ir phases. However, the compound with M = Co has not yet been experimentally confirmed or synthesized in this study. Figure 4: (a) Total and partial density of states (DOS) for Ti3RhSiO. The energy zero is set at the Fermi energy (EF), indicated by the vertical line at 0 eV. The partial DOS highlights contributions from Ti, Rh, Si, and O atoms, with states near EF primarily dominated by Ti-3d and Rh-4d orbitals, while contributions from Si and O are minimal. (b) and (c) display the electronic band structures with and without spin-orbit coupling, respectively, along high-symmetry directions (Γ, X, M, R) in the Brillouin zone. Both band structures confirm metallic behavior, with several bands crossing EF. 3.3 Magnetic properties The temperature dependence of χ for Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO, measured under applied magnetic fields of up to 10 kOe, is shown in Figs. 5a–5d. For the Ti3RhSiO sample at 10 kOe, the ZFC and FC curves closely overlap and exhibit minimal temperature dependence across the 2–300 K range. This uniform behavior is characteristic of paramagnetic materials, confirming the absence of magnetic ordering within the measured temperature range. Since these materials exhibit metallic conductivity, Pauli paramagnetism is believed to be the dominant mechanism rather than Van Vleck paramagnetism. The calculated DOS at the Fermi level is 7.2 states eV-1 cell-1, corresponding to a theoretical susceptibility of 5.8 × 10-5 emu mol-1 Oe-1. However, this value does not fully account for the experimentally observed temperature-independent χ, suggesting an enhancement in χ. This enhancement could potentially arise from impurities or lattice defects in the metallic structure, which may modify the local electronic states and increase susceptibility.34 Additionally, relatively strong Coulomb interactions between electrons could also contribute to the enhanced susceptibility.35 Further investigations are required to clarify and quantify these contributions. For Ti3IrSiO and the solid solution compounds, a similar overall temperature dependence of χ was observed. However, a slight increase in susceptibility upon cooling suggests that an increase in the electronic DOS near the Fermi level may contribute. Trace amounts of impurities or secondary phases may also lead to deviations from ideal Pauli paramagnetic behavior. Additionally, in some metallic systems, weak ferromagnetic interactions or spin fluctuations can influence χ at low temperatures, potentially contributing to the observed temperature-dependent features. In contrast, these low-temperature-dependent features are absent in the Ti3RhSiO compound. Further studies are needed to fully understand and explain these behaviors. Figure 5e presents the inverse susceptibility as a function of temperature for the compounds. The absence of Curie-Weiss behavior (except at very low temperatures) indicates a lack of significant localized spin moments. The slight Curie-Weiss-like behavior observed at low temperatures is likely due to unidentified magnetic impurities or other potential contributions, as discussed in the preceding paragraphs.Figures 5f–5h present M as a function of H at 10 K, 100 K, and 300 K. All three compounds exhibit linear M–H curves without hysteresis, confirming their paramagnetic nature across the measured temperatures. The absence of magnetic transitions or saturation at high fields further supports this conclusion. Small steps near zero field in the M–H curves are observed, likely due to minor impurity phases. However, these impurities have a negligible impact on the overall paramagnetic response, as evidenced by the predominantly linear magnetization behavior. These results confirm that Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO exhibit robust, enhanced Pauli-like paramagnetism, with no evidence of long-range magnetic ordering or significant field-dependent magnetic phenomena. Figure 5: (a–c) Temperature dependence of magnetic susceptibility (χ) measured under applied magnetic fields of 10 kOe and 3 kOe for Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO, respectively. Zero-field-cooled (ZFC) and field-cooled (FC) measurements exhibit minimal temperature dependence across the 2–300 K range. (d) Temperature dependence of χ measured under an applied magnetic field of 300 Oe for the same compounds. (e) Inverse magnetic susceptibility (χ-1) plots derived from the data. (f–h) Magnetization (M) as a function of magnetic field (H) at 10 K, 100 K, and 300 K for Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO, respectively.3.4 Heat Capacity and Electrical Resistivity Cp measurements were conducted for each sample under zero-field conditions (H = 0 kOe) over the temperature range of 2–300 K during cooling. Figures 6a–6c present the Cp/T values, which exhibit a smooth trend across the entire temperature range. Minor fluctuations at higher temperatures are likely due to technical noise, attributed to the grease used to ensure thermal contact during the experiment. To further examine the low-temperature specific heat behavior, the Cp/T versus T2 data were fitted using a model that accounts for both lattice (Clatt) and electronic (Cel) contributions, expressed as Eq. (1):      .        (1)Here, the lattice contribution is represented by Eq. (2):      ,        (2)while the electronic (metallic) contribution is described by Eq. (3):      .          (3)In this context, γ represents the electronic specific heat coefficient, while β1 is a constant related to the Debye temperature (ΘD). The fitted values for γ, β1, ΘD (derived from β1), and β2 are summarized in Table 2, offering valuable insights into the thermal and electronic properties of these materials. This analysis helps to distinguish the metallic and lattice contributions, further corroborating the multiband metallic nature of the studied compounds. To further investigate the lattice contributions, the Debye–Einstein model was applied, analyzing the specific heat data in the form of (Cp−γT)/T3 vs. T, as shown in Figs. 6d–6f. The fitting was performed using Eq. (4):36     .  (4)This equation combines the Debye and Einstein models to provide a more detailed interpretation of the lattice-specific heat. Here, TD is the Debye temperature, TE is the Einstein temperature, NA is Avogadro’s constant, and kB is Boltzmann’s constant. nD and nE are scale factors corresponding to the numbers of vibrating modes per formula unit in the Debye and Einstein models, respectively. It is important to distinguish between ΘD and TD: ΘD refers to the Debye temperature derived directly from low-temperature specific heat data using the lattice coefficient β1. In contrast, TD is the Debye temperature obtained from fitting the specific heat data using the Debye–Einstein model, which accounts for contributions over a broader temperature range.The fitting results in these figures align well with the experimental data across the temperature range of 2–300 K. Key parameters such as TD, TE, and the scale factors nD, and nE are summarized in Table 2, providing valuable insights into the vibrational dynamics and lattice stability of these compounds. Notably, the data suggest the presence of phonon excitations associated with anharmonic vibrational modes below 50 K, as indicated by the bell-shaped feature attributed to the Einstein contribution to the heat capacity.37While this feature is reminiscent of that observed in β-pyrochlore oxides such as KOs2O6, its peak magnitude in the suboxides is less than one-tenth of that in KOs2O6.37 This disparity suggests that the anharmonic “rattling” vibrations within the icosahedra are significantly weaker in these materials.38  The comprehensive analysis of the specific heat data highlights the contributions of both lattice and electronic components in these compounds. The successful application of the Debye–Einstein model across a broad temperature range confirms that specific heat behavior is predominantly governed by the interaction of these vibrational modes. These findings deepen our understanding of the thermal properties of Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO, reinforcing their structural stability and unique vibrational dynamics. Figure 6: (a–c) Temperature dependence of specific heat divided by temperature (Cp/T) under zero magnetic field (H = 0 kOe) for Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO, respectively. Insets display fitted curves for the low-temperature region, showing Cp/T as a function of the square of temperature. (d–f) show plots of the adjusted specific heat data with Debye–Einstein model fitting on a logarithmic scale for these three samples.  Table 2: Thermodynamic parameters for Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO. Parameters include the electronic specific heat coefficient (γ), lattice specific heat coefficients (β1 and β2), and Debye temperature (ΘD) obtained from low-temperature specific heat measurements. Additionally, Debye temperature (TD), Einstein temperature (TE), and scale factors (nD and nE) were derived from the Debye–Einstein model. ΘD refers to the Debye temperature derived from β1, while TD represents the fitted Debye temperature over a broader range.   γ (mJ mol-1 K-2) β1 (J mol-1 K-4) [ ΘD (K) ] β2 (J mol-1 K-6) Ti3RhSiO 9.3(2) 7.3(2)×10-5 [ 542(1) ] 5.3(2)×10-8 Ti3Rh0.5Ir0.5SiO 6.8(3) 8.6(3)×10-5 [ 514(1) ] 4.2(6)×10-8 Ti3IrSiO 6.3(6) 14.7(5)×10-5 [ 430(1) ] 6.1(3)×10-8  TD (K) TE (K) nD nE Ti3RhSiO 470(23) 203(10) 4.5(6) 0.77(14) Ti3Rh0.5Ir0.5SiO 452(13) 181(6) 4.5(4) 0.55(6) Ti3IrSiO 416(27) 145(11) 4.5(8) 0.37(9)Figure 7 presents the temperature dependence of ρ for Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO. All three compounds exhibit metallic behavior, with ρ decreasing progressively as temperature decreases from 300 K, consistent with typical metallic conductivity. This observation aligns well with the multiband metallic character predicted by the DOS and band-structure calculations. Additionally, the close overlap of heating and cooling data demonstrates excellent reproducibility and measurement stability.  At low temperatures, a slight drop in ρ is observed, attributed to the presence of a known superconducting impurity phase. While this impurity does not significantly affect the metallic character of the main phase, it contributes to the resistivity drop near 2.5 K. Possible candidates for this impurity phase include Ti4Ir2O (Tc = 5.4 K), 22 TiIrSi (Tc = 1.4 K), 39 and Ti3Ir (Tc = 5.1 K). 40 However, magnetic measurements under a weak magnetic field (≤100 Oe) on the same pellet used for resistivity measurements revealed only a very weak superconducting signal. This suggests that the observed superconductivity likely originates from an impurity phase closely related to one of these known compounds. Furthermore, resistivity measurements on a sample from a different batch showed no superconducting behavior, further supporting the hypothesis that the superconductivity arises from an impurity rather than being an intrinsic property of the main phase. The inset of Fig. 7 presents the magnetoresistance (MR) at 5 K as a function of H for the three samples. The MR values remain notably small across the entire field range (−70 to 70 kOe), indicating that the electrical transport properties are largely unaffected by external magnetic fields. This minimal MR response suggests the absence of significant spin-dependent scattering mechanisms or other field-induced magnetic effects in these materials. These findings further confirm the robust metallic nature of Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO, demonstrating their weak dependence on magnetic fields in transport properties. This underscores the stability and reliability of their metallic behavior under varying external conditions. Figure 7: Temperature dependence of electrical resistivity (ρ) for Ti3RhSiO, Ti3Rh0.5Ir0.5SiO, and Ti3IrSiO. All three compounds exhibit metallic behavior, with resistivity gradually decreasing as the temperature decreases from 300 K. The heating data closely follow the cooling trace on this scale. The drop in resistivity at low temperatures is attributed to a known superconducting impurity phase. The inset shows the magnetoresistance (MR) at 5 K as a function of the magnetic field (H), indicating minimal MR across the measured field range.3.5 Oxygen-Stabilized Icosahedral Structures and Their Electronic ImplicationsOxygen-stabilized icosahedral quasicrystals and 1/1 approximants have been reported in the Ti-Cr-Si-O and Ti-Mn-Si-O systems, even before oxide quasicrystals were formally discovered.41 Although these suboxides do not exhibit notable magnetic ordering, they hold potential for uncovering valuable properties through further materials development. Motivated by these possibilities, we have explored new suboxides in the Ti-M-Si-O system, aiming to expand their structural and functional diversity. Interestingly, our exploration of the Ti-M-Si-O system revealed that stable Au4Al-type suboxides formed only when M = Rh or Ir, rather than 1/1 approximants. These compounds exhibit icosahedral clusters, a structural motif commonly associated with alloy quasicrystals. While no direct evidence of quasicrystalline order was detected, the presence of these clusters highlights the potential for discovering new quasicrystals within this material family. Electronic DOS calculations reveal that Ti3RhSiO exhibits a metallic character, with Ti-3d and Rh-4d orbitals contributing significantly to the states at the Fermi level. Band structure calculations further support this metallic nature, revealing multiple band crossings at the Fermi level. In contrast, oxygen plays a minimal role near the Fermi level, acting primarily as a structural stabilizer in the form of an oxide ion.Using the ionic model of oxygen, we calculated the electron-per-atom ratio (e/a) for Ti3RhSiO and Ti3IrSiO, obtaining values of 1.50 and 1.67, respectively. These values approach ranges known to stabilize certain types of icosahedral quasicrystals: e/a = 1.7–1.9 for Mackay-type, e/a = 2.0–2.2 for Bergman-type, and e/a = 2.0–2.15 for Tsai-type icosahedral quasicrystals.42 This proximity suggests that structural stability could be further enhanced through geometric adjustments, such as tuning atomic size ratios to improve cluster packing efficiency.43 4. ConclusionIn this study, we successfully synthesized and characterized Ti3RhSiO and Ti3IrSiO, marking the first experimental confirmation of Ti-based Au4Al-type compounds. These materials adopt an Au4Al-type structure featuring oxygen-stabilized icosahedral units. Our findings clearly demonstrate that oxygen plays a critical role in stabilizing the cubic phase. Arc-melting experiments consistently confirmed that only oxygen-containing samples yielded the stable cubic structure, while oxygen-free samples failed to form the targeted phase.The Au4Al-type structure is an ordered variant of the non-centrosymmetric β-Mn-type structure, which has garnered significant interest for its potential to host skyrmions and exhibit superconductivity. A key feature of this structure is the splitting of the 8c Wyckoff position into two 4a sites, enabling ordered site occupations. This configuration allows for precise control over electronic and magnetic properties, while the resulting symmetry-breaking mechanism may significantly influence critical physical attributes, including optical and electronic responses.Electronic structure calculations confirm that Ti₃RhSiO exhibits a metallic character, with Ti-3d and Rh-4d (or Ir-5d) orbitals contributing significantly at the Fermi level. Oxygen primarily acts as a structural stabilizer in the form of an oxide ion. The metallic nature was further validated by experimental resistivity measurements, which demonstrated stable metallic conductivity across the entire temperature range. Additionally, the proximity of the electron-per-atom ratios (e/a) for Ti3RhSiO and Ti3IrSiO to ranges known for icosahedral quasicrystals highlights the potential for enhancing structural stability through geometric adjustments, such as optimizing atomic size ratios to improve cluster packing efficiency. Although no direct evidence of quasicrystalline order was observed, the presence of icosahedral clusters in these materials highlights their potential for the discovery of new oxide quasicrystals. These results establish Ti3RhSiO as a promising prototype for Ti-based Au4Al-type compounds, offering exciting possibilities for hosting novel electronic states, including quasicrystalline and skyrmion-hosting properties. In summary, this work not only highlights the unique structural and electronic properties of Ti3RhSiO and Ti3IrSiO but also opens new avenues for exploring Ti-based Au4Al-type materials. Future research will focus on expanding the compositional range, probing localized magnetic moments, and investigating novel properties, such as potential superconductivity and skyrmion-hosting states, contributing to both fundamental understanding and practical applications in spintronic devices. AUTHOR INFORMATIONCorresponding Author* Zhijun Li and Kazunari YamauraQuantum Solid State Materials GroupResearch Center for Functional MaterialsNational Institute for Materials Science1-1-Namiki, Tsukuba, Ibaraki 305-0044, JapanTEL: +81-29-860-4658E-mail: Li.Zhijun@nims.go.jp and YAMAURA.Kazunari@nims.go.jpPresent Address# Shintaro SuzukiDepartment of Physical Sciences, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan NotesThe authors declare no competing financial interests. Author ContributionsThe manuscript was written with contributions from all authors. All authors have approved the final version of the manuscript. Funding SourcesMANA is supported by the World Premier International Research Center Initiative (WPI) of MEXT, Japan. This research was partly funded by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) (Grant Nos. JP19H05817, JP19H05818, JP22H04601), the JSPS Bilateral Program (Grant No. JPJSBP120237714), and the Japan Science and Technology Agency (JST), CREST, Japan (Grant No. JPMJCR22O3).Supporting Information fileThe Supporting Information includes the lattice parameter of the cubic phase for samples with nominal compositions Ti88Rh34Si24-x(SiO2)x, plotted as a function of x. X-ray diffraction patterns of Ti3MSiO (M = Rh, Rh0.5Ir0.5, Ir) were measured using Cu-Kα radiation. The temperature dependence of the lattice parameter and unit cell volume for cubic Ti3RhSiO is provided. The DOS for Ti3MSiO (M = Co, Rh, Ir) is shown as a function of energy. 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Icosahedral Quasicrystal and 1/1 Cubic Approximant in Au–Al–Yb Alloys. Philosophical Magazine 2011, 91 (33), 4218–4229. https://doi.org/10.1080/14786435.2011.608732.For Table of Contents OnlyTOC synopsis: The first successful synthesis and detailed characterization of Ti3RhSiO and Ti3IrSiO, pioneering Ti-based Au4Al-type compounds, are reported. These materials crystallize in the cubic P213 space group and exhibit multiband metallic behavior. The critical role of oxygen in stabilizing the cubic phase is established. Electronic structure and specific heat analyses reveal unique properties, paving the way for further exploration of Ti-based Au4Al-type materials, despite their limited current applications.2image1.tiffimage2.tiffimage3.tiffimage4.tiffimage5.tiffimage6.tiffimage7.tiffimage8.tiff