Krishnendu Buxi

The insufficient X-ray scattering contrast and similar neutron scattering cross-sections of Fe and Ni render the accurate determination of ‘coloring scheme’ ambiguous in the vacancy ordered 4 ³ −superstructure of CsCl-related phase Fe _1.5 Ni _1.5 Ga ₄ . Therefore, to overcome this limitation, X-ray resonance diffraction near the K-absorption edge of Fe has been utilized for the accurate site occupancy pattern of Fe and Ni in the structure of Fe _1.5 Ni _1.5 Ga ₄ . Measurements on three independently annealed samples with progressively longer annealing times revealed no evidence of atomic ordering, instead, Fe prefers to partially occupy the Ni site (48𝑔 Wyckoff site) in the structure of binary Ni ₃ Ga ₄ (𝐼𝑎3̅𝑑 space group, 𝑎 =11.52 Å). The vacancy ordering present at the 16𝑏 site drives the lattice distortion in the structure. Upon heating to ~955 K, the room-temperature (RT) structure transforms to the 2.9 Å CsCl-type structure (𝑃𝑚3̅𝑚 space group), where Fe, Ni and vacancy are randomly distributed suppressing the lattice distortions. DFT based calculations on the hypothetical ordered model predict a ferromagnetic ground state with non-magnetic and antiferromagnetic states lying very close to it in energy. This near-degeneracy points to a highly complex magnetic energy landscape with competing ferromagnetic and antiferromagnetic interactions, which explains the experimentally observed spin-glass behavior.

This investigation focuses on the high temperature synthesis and characterization using the single crystal and powder X-ray diffraction analysis of a partly disordered ternary intermetallic phase Cr _1−???? Ni ₂ Ga ₄ , 𝛿 ≈0.2 within the CsCl-related 𝑇 ₃ Ga ₄ family. With increase in annealing time during synthesis, the vacancy-disordered cubic (𝐹𝑚3̅𝑚, 𝑎 = 5.77 Å) phase transforms into a vacancy-ordered tetragonal phase (𝐼4 ₁ /𝑎𝑚𝑑, 𝑎 = 5.77 Å, 𝑐 = 11.54 Å) with three perpendicular twin domains. The superstructure reflections corresponding to vacancy ordering can be observed with both the single crystal and powder X-ray diffraction. Although the phase could not be separated in its pure form, the DFT simulations on the tetragonal model suggest the phase is suspectable to antiferromagnetic ordering. The partial occupancy of the Cr is rationalized by the antibonding populations near the fermi level. Furthermore, the variations in atomic sizes and electronegativities among the first-row transition metals provide additional insight to rationalize the structural distinctions of Co, and Fe containing analogues with the Cr-containing phase in the broader CsCl type/related 𝑇 ₃ Ga ₄ family and providing a rational pathway to discover novel intermetallic phases.

This investigation focuses on the solid-state synthesis and a detailed structural and phase analytical investigation of the CoxNi3–xGa4 series along a constant Ga compositional line within Co–Ni–Ga phase space. Combined analysis of single crystal X-ray diffraction and neutron powder diffraction of CoxNi3–xGa4, x = 1.5 reveals a random distribution of Co and Ni over a single Wyckoff site, which contrasts with previous report suggesting atomic ordering. Phase analyses via X-ray powder diffraction indicates a vacancy order–disorder transition as the Co concentration increases beyond a critical limit. DFT calculation predicted a semimetallic and nonmagnetic ground state, supported by the absence of magnetic peaks or intensity enhancement at 10 K neutron data compared to 300 K. Interestingly, Peierls-like distortions are observed within the Ga substructure in the vacancy-ordered phase, whereas such distortions are absent in the vacancy-disordered phase. These distortions result in periodic charge localization and exhibit subtle alterations between room temperature and 180 K. Magnetic measurements further reveal cluster spin-glass behavior with a high coercivity. The nontrivial spin dynamics, coupled with large coercivity, makes this phase a compelling candidate for multifunctional applications.

The insufficient X-ray scattering contrast and similar neutron scattering cross-sections of Fe and Ni render the accurate determination of ‘coloring scheme’ ambiguous in the vacancy ordered 4 ³ −superstructure of CsCl-related phase Fe _1.5 Ni _1.5 Ga ₄ . Therefore, to overcome this limitation, X-ray resonance diffraction near the K-absorption edge of Fe has been utilized for the accurate site occupancy pattern of Fe and Ni in the structure of Fe _1.5 Ni _1.5 Ga ₄ . Measurements on three independently annealed samples with progressively longer annealing times revealed no evidence of atomic ordering, instead, Fe prefers to partially occupy the Ni site (48𝑔 Wyckoff site) in the structure of binary Ni ₃ Ga ₄ (𝐼𝑎3̅𝑑 space group, 𝑎 =11.52 Å). The vacancy ordering present at the 16𝑏 site drives the lattice distortion in the structure. Upon heating to ~955 K, the room-temperature (RT) structure transforms to the 2.9 Å CsCl-type structure (𝑃𝑚3̅𝑚 space group), where Fe, Ni and vacancy are randomly distributed suppressing the lattice distortions. DFT based calculations on the hypothetical ordered model predict a ferromagnetic ground state with non-magnetic and antiferromagnetic states lying very close to it in energy. This near-degeneracy points to a highly complex magnetic energy landscape with competing ferromagnetic and antiferromagnetic interactions, which explains the experimentally observed spin-glass behavior.

This investigation focuses on the solid-state synthesis and a detailed structural and phase analytical investigation of the Co _???? Ni _3−???? Ga ₄ series along a constant Ga compositional line within Co⎼Ni⎼Ga phase space. Combined analysis of single crystal X-ray diffraction and neutron powder diffraction of Co _???? Ni _3−???? Ga ₄ , 𝑥 = 1.5 reveals a random distribution of Co and Ni over a single Wyckoff site, which contrasts with previous report suggesting atomic ordering. Phase analyses via X-ray powder diffraction indicates a vacancy order-disorder transition as the Co concentration increases beyond a critical limit. DFT calculation predicted a semi-metallic and nonmagnetic ground state, supported by the absence of magnetic peaks or intensity enhancement at 10 K neutron data compared to 300 K. Interestingly “Peierls-like” distortions are observed within the Ga substructure in the vacancy-ordered phase, whereas such distortions are absent in the vacancy-disordered phase. These distortions result in periodic charge localization and exhibit subtle alterations between room temperature and 180 K. Magnetic measurements further reveal cluster spin-glass behavior with a high coercivity. The non-trivial spin dynamics, coupled with large coercivity, makes this phase a compelling candidate for multifunctional applications.

This investigation focuses on the solid-state synthesis and a detailed structural and phase analytical investigation of the Co _???? Ni _3−???? Ga ₄ series along a constant Ga compositional line within Co⎼Ni⎼Ga phase space. Combined analysis of single crystal X-ray diffraction and neutron powder diffraction of Co _???? Ni _3−???? Ga ₄ , 𝑥 = 1.5 reveals a random distribution of Co and Ni over a single Wyckoff site, which contrasts with previous report suggesting atomic ordering. Phase analyses via X-ray powder diffraction indicates a vacancy order-disorder transition as the Co concentration increases beyond a critical limit. DFT calculation predicted a semi-metallic and nonmagnetic ground state, supported by the absence of magnetic peaks or intensity enhancement at 10 K neutron data compared to 300 K. Interestingly “Peierls-like” distortions are observed within the Ga substructure in the vacancy-ordered phase, whereas such distortions are absent in the vacancy-disordered phase. These distortions result in periodic charge localization and exhibit subtle alterations between room temperature and 180 K. Magnetic measurements further reveal cluster spin-glass behavior with a high coercivity. The non-trivial spin dynamics, coupled with large coercivity, makes this phase a compelling candidate for multifunctional applications.

The Ga-rich region of the Fe-Ni-Ga ternary system was investigated, by exploring a line of compositions Fe_xNi_3-xGa₄, with 0.5 ≤ x ≤ 2.5. The single-phase cubic material was found only at the composition Fe_1.5Ni_1.5Ga₄ and its immediate vicinity, representing a new phase in the Fe-Ni-Ga diagram. The homogeneity range of this phase was estimated by additionally exploring a set of compositions Fe_xNi_yGa_z around the central composition Fe_1.5Ni_1.5Ga₄. The structural model was constructed based on the structure of the binary Ni₃Ga₄ parent phase, which crystallizes in the cubic Ia 3̅ d space group. We have considered that by substituting Fe for Ni, the Ia 3̅ d structure is preserved, with the Fe and Ni being statistically distributed at their 48g Wyckoff site. The possibility of a symmetry-reduced chiral structural model I4₁32 driven by chemical ordering of Fe and Ni cannot be entirely ruled out on the basis of the crystallographic study. The magnetic study of the Fe_1.5Ni_1.5Ga₄ phase has revealed that the material forms a spin glass phase below the spin freezing temperature T_f ≈ 9 K. Since the spin glass ordering of the Fe and Ni magnetic moments is compatible with their random distribution, the magnetic study supports the disordered cubic Ia 3̅ d model.

The γ‐brass type phase in the Ag–Zn system has been synthesized by a high‐temperature method, and the crystal structure has been determined by the single‐crystal X‐ray diffraction studies. The phase Ag _{5+ δ} Zn _{8‐ δ} adopts a cubic space group I 3 m with a cell parameter of ≈9.35 Å. Like the γ‐brass type phases, the structure can be seen as the combination of two similar 26 atom‐γ‐cluster present at (0, 0, 0) and (½, ½, ½) of the unit cell. Unlike the other members of the cubic I ‐centred γ‐brass/γ‐brass type phases, the structure of Ag _{5+ δ} Zn _{8‐ δ} is heavily disordered and accommodates substitutional disorder between Ag and Zn in all four crystallographic sites of the 26 atom‐γ‐cluster. The crystallographic work on the “coloring” of the Ag and Zn atoms among the different crystallographic sites of the Ag _{5+ δ} Zn _{8‐ δ} reveals that the site distribution pattern of Ag and Zn atoms is allied to the Cu ₅ Zn ₈ γ‐brass structures. This “coloring scheme” is rationalized by electronic structure calculations on the ordered model “Ag ₅ Zn ₈ ” from the experimentally determined structure of Ag _{5+ δ} Zn _{8‐ δ} . The pronounced pseudo‐gap near the Fermi level for Ag _{5+ δ} Zn _{8‐ δ} indicates that the phase is stabilized by the Fermi sphere‐Brillouin zone‐induced Hume–Rothery mechanism.