CeCr2Al20-type Structure Research Articles

Synthesis and properties of AxV2Al20 (A = Th, U, Np, Pu) ternary actinide aluminides

Polycrystalline samples of AxV2Al20 (A = Ce, Th, U, Np, Pu; 0.7 ≤ x ≤ 1.0) actinide intermetallics were synthesized using the arc-melting method. Crystal structure studies were performed by means of powder x-ray diffraction and the Rietveld refinement method. All studied compounds crystallize in the CeCr2Al20-type structure (space group Fd-3m, no. 227) with the actinide or Ce atom located in the oversized cage formed by Al atoms. Comparison of the crystallographic results with the reported data for LnV2Al20 (Ln = lanthanides) counterparts reveals distinctly different behavior of the lanthanide- and actinide-bearing compounds. This difference is suggested to be caused by fairly localized character of the 4f electrons, whereas itinerant character of the 5f electrons is likely seen for U- and Np-containing phases. Magnetic susceptibility and specific heat measurements did not reveal any magnetic ordering in U0.8V2Al20, Np0.8V2Al20 and Pu0.8V2Al20 down to 2.5 K. A small anomaly in low-temperature specific heat of Ce0.8V2Al20, U0.8V2Al20, and Np0.8V2Al20 is observed, likely arising from a low-energy Einstein mode.

Low temperature properties of ZrTr2Zn20 (Tr = transition metal) with a pyrochlore lattice

Single crystals of ZrTr2Zn20 (Tr = Mn, Co, Ru) with a cubic CeCr2Al20-type structure (space group Fd-3m) were grown by a Zn-flux method. Electrical resistivity ρ(T) of ZrTr2Zn20 shows typical metallic behavior with the residual resistivity ratio (RRR) of 1.1 (Mn), 8.4 (Co) and 10 (Ru). Magnetic susceptibility χ(T) of ZrRu2Zn20 shows large diamagnetism. χ(T) of ZrTr2Zn20 (Tr = Mn, Co) clearly shows Curie-Weiss behavior, indicating that Mn/Co spins are really active as local magnetic moments or that a large spin fluctuation exists. χ(T) of ZrMn2Zn20 increases ferromagnetically at around 20 K. Because the anti-ferromagnetic correlation with active localized spins exists in ZrCo2Zn20, geometrical frustration of the pyrochlore lattice of Co is expected. However, specific heat C(T) of ZrCo2Zn20 shows no upturn at low temperatures and Sommerfeld coefficient γ is evaluated to be 24.8 mJ/(mol K2). We consider that the localized Co spins are insufficient to cause anomaly originated from the geometrical frustration, or the Co spins exist in a disordered arrangement of excess Co.

La substitution effect on the magnetic phase transition of SmTi2Al20

Powder X-ray and specific heat measurements were performed on (SmxLa1-xTi2Al20 (x = 0.2-1) single crystals grown by the Al self-flux method. All samples crystallize in the CeCr2Al20-type structure with Fd¯3m symmetry and the lattice parameter changes almost linearly with respect to the Sm concentration. From a λ-type peak appearing in the specific heat, it was found that the antiferromagnetic phase transition occurring at 6.5 K for x = 1 remains in the large Sm-concentration range. The transition temperature decreases almost linearly depending on the Sm concentration and goes to zero at the finite Sm concentration value of ∼ 0.1, indicating the possible existence of a quantum critical point. The robustness of the phase boundary against the magnetic field remains even in the La-substituted system. This behavior is in marked contrast to the field-sensitive behavior observed in Ce-based heavy fermion compounds.

X-ray absorption spectroscopy and novel electronic properties in heavy fermion compounds YbT2Zn20 (T: Rh and Ir)

YbT2Zn20 (T: Rh and Ir), which crystallizes in the cubic CeCr2Al20-type structure, is a member of the well-known heavy fermion compounds, indicating a huge electronic specific heat coefficient γ ≊ 500 mJ/(K2·mol). We have measured temperature and magnetic field dependences of Yb valence in YbT2Zn20(T: Rh and Ir) at ambient pressure by the Lm edge x- ray absorption spectroscopy in order to investigate the valence state of Yb 4f electrons in these compounds. It is revealed that the Yb valence in both compounds significantly decreases with temperature below about 100 K and increases with increasing magnetic field at low temperatures in contrast to the case of YbCo2Zn20.

Single Crystal Growth and Electronic State of UPd2Cd20

We succeeded in growing single crystals of UPd2Cd20 by the Cd self-flux method. This compound crystallizes in the cubic CeCr2Al20-type structure. From the low-temperature electrical resistivity and Kadowaki-Woods relation, the electronic specific heat coefficient γ is estimated to be about 400mJ/mol ·K2. The magnetic susceptibility of UPd2Cd20 indicates a broad max-imum at 6.6K and a rapid decrease at 5.0K, which correspond to the formation of the heavy fermion state and the antiferromagnetic transition, respectively. These experimental results strongly suggest that UPd2Cd20 is a new heavy fermion antiferromagnet.

Structure and physical properties of RT2Cd20 (R=rare earth, T=Ni, Pd) compounds with the CeCr2Al20-type structure

Eleven new compounds, R Ni2Cd20 (R=Y, La–Nd, Sm, Gd, Tb) and R Pd2Cd20 (R=Ce, Pr, Sm), were grown as single crystals in high temperature cadmium-rich solutions. They crystallize in the cubic CeCr2Al20-type structure (Fd3¯m, Z=8) as characterized by measurements of powder X-ray diffraction. Electrical resistivity, magnetization, and specific heat measurements were performed on R Ni2Cd20 (R=Y, La–Nd, Sm, Gd, Tb) single crystals. Whereas YNi2Cd20 and LaNi2Cd20 exhibit unremarkable metallic behavior, when magnetic moments from localized 4f electron states (Gd3+–Tb3+) are embedded into this host, they exhibit ferromagnetic order with values of the Curie temperature TC for R Ni2Cd20 (R=Gd, and Tb) which scale with the de Gennes factor.

Metamagnetic Behavior and Effect of Pressure on the Electronic State in Heavy-Fermion Compound YbRh2Zn20

We measured the magnetic susceptibility, high-field magnetization, magnetoresistance, specific heat, and Hall coefficient, together with the electrical resistivity under high pressures and magnetic fields, for the heavy-fermion compound YbRh2Zn20 with the cubic CeCr2Al20-type structure. The metamagnetic behavior was observed at Hm = 65 kOe for the magnetic field along the direction below Tχmax = 5.3 K, at which the temperature of the magnetic susceptibility indicates a broad maximum. The coefficient A of the T2 dependence of electrical resistivity ρ=ρ0 + AT2 and the electronic specific heat coefficient C/T possess a broad maximum at Hm. The metamagnetic field Hm is found to decrease with increasing pressure and to become zero at a critical pressure Pc ≃5.2 GPa. Correspondingly, the A value increases drastically in magnitude, and the Fermi liquid relation is no longer satisfied at 5.2 GPa in zero magnetic field, implying a non-Fermi liquid state. On the other hand, at magnetic fields of H > 20 kOe, t...

Characteristic magnetic phase diagram in heavy-fermion compound YbCo2Zn20

We studied the magnetic field versus temperature phase diagram of the heavy-fermion compound YbCo2Zn20 with the cubic CeCr2Al20-type structure by measuring the specific heat, ac-susceptibility, and electrical resistivity under magnetic fields. In a low magnetic field region, a metamagnetic behavior was observed at Hm = 6 kOe for three principal field directions below Tχmax = 0.32 K, at which the temperature dependence of ac-susceptibility shows a local broad maximum. Two crossover lines were found below and above Hm. At high magnetic fields, the heavy-fermion state was rapidly suppressed with increasing magnetic field, and the field-induced ordered phase appeared in a very limited field direction around H || ⟨111⟩ above the transition field HQ= 60 kOe in the temperature range below TQ = 0.6 K.

Mixed-metal flux synthesis of quaternary RMn 2Tr xZn 20−x compounds with Tr=Al, In

Eighteen new intermetallic compounds RMn 2 Tr x Zn 20− x (2< x<7; R=rare-earth metal; Tr=Al, In) were synthesized using low-melting mixtures of ( Tr/Zn) as a solvent. Structural refinement using single-crystal X-ray diffraction data shows that the compounds are substituted variants of the cubic CeCr 2Al 20-type structure ( Fd-3 m, Z=8; unit cell parameters vary from a=14.1152(3)Å for YbMn 2Al 5.3Zn 14.7 to a=14.8125(4)Å for SmMn 2In 5.9Zn 14.1). The Zn and Tr elements show site preferences in the indium compounds, but not in the aluminum analogs. The substitution of trielide element for zinc modifies the valence electron count of the compounds to allow for the incorporation of Mn into the structure. Magnetic susceptibility data show no evidence of magnetic ordering down to 3 K.

Ternary Intermetallics with High Zinc Content: TT′2Zn20 (T=Zr, Hf, Nb; T′=Mn, Fe, Ru, Co, Rh, Ni) with CeCr2Al20-Type Structure

Fourteen new compounds TT′2Zn20 were prepared by reaction of the elemental components. They crystallize with the cubic CeCr2Al20-type structure (space group Fd3m, Z=8), which was refined from single-crystal X-ray data of HfRu2Zn20 (a=1406.9(1) pm) and NbFe2Zn20 (a=1387.9(1) pm) to conventional residuals of R=0.018 and R=0.022 for 268 and 144 structure factors, respectively, and 17 variable parameters each. The structure of the compound NbCo2Zn20 (a=1382.2(1) pm) was refined from X-ray powder data to RF=0.031. The occupancy parameters have practically the ideal values for all five atomic sites of the three structures. The largest voids in this structure are octahedral with a size similar to those in the structure of elemental zinc.

Ternary Rare Earth Transition Metal Zinc Compounds RT2Zn20 with T = Fe, Ru, Co, Rh, and Ni

Forty eight new compounds RT2Zn20 were prepared by annealing cold-pressed pellets of the elemental components in an argon atmosphere. They crystallize with the cubic CeCr2Al20 type structure (Fd3̅̅m , Z = 8), which was refined from single-crystal diffractometer data of TbFeiZn20 (a = 1411.1(1) pm ), YRu2Zn20 (a = 1422.6(1) pm ), DyRu2Zn20 (a = 1422.1(1) pm), GdCo2Zn20 (a = 1406.0(1) pm ), DyRh2Zn20 (a = 1418.2(1) pm ), and TmNi2Zn20 (a= 1401.6(1) pm) to conventional residuals varying betw een R = 0.011 and R - 0.024. The com pounds have a tendency for tw inning, thus m im icking hexagonal sym metry, with the cubic [111] axis as the axis w ith the pseudohexagonal symmetry. M inor inconsistencies in the cell volum es of these com pounds indicate slight deviations from the ideal com position. N evertheless, the five atom ic sites of this structure w ere found to be fully occupied w ithin the error lim its w ith the exception of one zinc site of TmNi2Zn20. The coordination for the site of the rare earth atom s is a Frank-K asper polyhedron with coordination num ber (CN) 16. The transition metal atom s occupy a site w ith icosahedral zinc coordination (CN 12). Two of the three zinc sites are in pentagonal prism atic coordination of zinc atom s, capped by rare earth and/or transition metal atom s (CN 12), w hile the third zinc site has 12 zinc neighbors form ing a hexagonal prism , w hich is capped by tw o rare earth atom s (CN 14).

Ternary Aluminides AT2Al20 (A = Rare Earth Elements and Uranium; T = Ti, Nb, Ta, Mo, and W) with CeCr2Al20-Type Structure

Of the title compounds 28 were prepared for the first time. They crystallize with a cubic structure (space group Fd3m, Z = 8) similar to the structures of Mg3Cr2Al18 and ZrZn22 with an atom distribution first proposed for CeCr2Al20. Single-crystal X-ray data for CeTi2Al20 (a = 1470.5(1) pm) and CeMo2Al20 (a = 1457.5(1) pm) were refined to residuals of R = 0.017 and R = 0.012 for 17 variables from 302 and 289 structure factors, respectively. The atoms occupy five atomic sites, which are fully occupied in the titanium compound. For the molybdenum compound a considerable deviation from the ideal composition was observed for the crystal used for the structure refinement; nevertheless, it is argued that the homogeneity range might include the ideal composition. The cerium and the transition metal atoms are surrounded by 16 and 12 aluminum atoms, respectively, which correspond to Frank-Kasper polyhedra. In contrast, the aluminum atoms have coordination polyhedra formed by pentagonal and hexagonal prisms of aluminum atoms, which are capped by the cerium and the transition metal atoms.