Physical properties of cage-like compound UB₁₂

Abstract

Boron and uranium form three metallic borides having the chemical formulae UB 2 , UB 4 and UB 12 . In this study, we present the temperature variations of magnetic susceptibility, specific heat, electrical resistivity (performed in magnetic fields of 0 and up to 9 T), thermoelectric power and thermal conductivity measured on the bulk sample of UB 12 . This dodecaboride behaves as a typical metal, being a Pauli paramagnet and exhibiting a large variety of physical properties due to its specific close-packed structure containing B 12 groups. We describe also an uncommon phenomenon observed in UB 12 , that is, a fairly large scattering of the experimental resistivity data under application of a magnetic field at low temperatures and its systematic vanishing during heating of the sample. This effect is probably caused by inharmonious movement (rattling) of the uranium atoms inside the oversized coordination cage, B 24 , reflected by applying the magnetic field. The specific heat, resistivity, thermoelectric power and heat transport data have been analysed in the framework of the low-frequency Einstein modes, which are mainly responsible for the phonon spectra behaviour in the system studied here.

Anomalous rattling and single crystalline properties of the caged compound URu2Al10

“…10(a) and 10(b) for the Fe-and Rubased compounds, respectively. Thus, these figures present the results obtained for a temperature region of below about 16 K. As expected based on our earlier results obtained for UB 12 [11] and ThFe 2 Al 10 [28], UFe 2 Al 10 and URu 2 Al 10 also reveal a new phenomenon under an applied field of 9 T, for example. This phenomenon has already been described by the current authors, and is associated with a large scattering of the experimental resistivity points at low temperatures after applying an external magnetic field.…”

Synthesis and Characterization of Single-Phase Metal Dodecaboride Solid Solutions: Zr1–xYxB12 and Zr1–xUxB12

“…Although exhibiting color, all dodecaborides are considered to be metals and not semiconductors. 30,79 Therefore, the existence of color can be attributed to a charge-transfer (similar to KMnO4, Figure S21) from the boron cage to the metal; and changes with the oxidation state of the metal: Y 3+ , Zr 4+ , and U 5+/6+ (Figure S20). 63 The diffuse-reflectance spectra for alloys with a composition of (Zr1-xYx):13B ( Figure S20) show a change in the maxima of pseudo-absorbance (after Kubelka-Munk transformation) [80][81][82][83][84][85] shift from 594 (ZrB12, violet) to 730 (YB12, blue) nm, closely following Vegard's law 64 values ( Table 1 and Figures 4 and S19).…”

“…1,[10][11][12] Metal boride structures range from metal rich subborides 13 (M4B) to mono-, 14,15 di- 16,17 and tetraborides, [18][19][20] to boron rich borides: dodecaborides 21 (MB12) and higher borides 22 (MB66), and β-rhombohedral boron doping phases 23,24 (MB50-100); as well as ternary and multinary metal borides. 1,25 Of the aforementioned boride families, metal dodecaborides are interesting due not only to having superior mechanical properties (superhardness), [26][27][28][29] but also interesting optical (color) [26][27][28] and electronic (superconductivity) 30 properties, as well as good oxidation resistance. 26 Metal dodecaborides can crystallize in two different structures: cubic-UB12 ( 3 ̅ ) and tetragonal-ScB12 (I4/mmm) (Figure 1).…”

Comparative studies of the cage systems ThFe2Al10 and UFe2Al10

“…Hence, any rattling in such anisotropic systems is possible predominantly along the c-axis. The first example of behaviour mentioned above was observed in the magnetoresistance of metallic UB 12 [40], which belongs to cagetype compounds. An unveiling of such a phenomenon was possible only in the study of metallic caged uranium compounds due to an existing interaction between the rattling vibration of a low frequency (Θ E ≈ 20 K) and the conduction band, as described in the literature.…”

“…Interestingly, such behaviour was not observed in the case of isostructural UFe 2 Al 10 , where, however, the uranium atom has an ionic character and the chemical bonding with the cage atoms is considerably stronger than that in Th-based aluminide [15]. The phenomenon of strong scattering of the resistivity data at low temperatures under a magnetic field was reported for the first time for the dodecaboride UB 12 [40], which is also a cage compound, where the metallic uranium central atom is surrounded by an isotropic framework of 24 boron atoms. This finding sheds more light onto the cubic crystal structure of the dodecaborides, where the Einstein oscillators are sufficiently free around their sites due to a very weak chemical bonding of the uranium atom with its surroundings.…”