Site preference, magnetism and lattice vibrations of intermetallics M7−xTxB3 (M = Rh, Ru; T = Fe, Co)
Introduction
The crystal chemistry and magnetism of metal-rich borides have been intensely studied in recent years [1]. The targeted phases comprise those crystallizing with the Th7Fe3 structure type [2] whose general formula M7−xTxB3 (M = Ru, Rh; T = Cr, Mn, Fe, Co, Ni) [3], [4], [5], [6], [7], [8] implies a substitution of a 4d transition metal M by a 3d element T in the binary phase M7B3. The phase FeRh6B3 is a new – likely ferromagnetic – boride with a peculiar site preference upon internal Fe/Rh substitution, because the iron atom prefers to enter only one (6c) of the three available crystallographic rhodium sites (6c, 6c and 2b), and the T occupancy of this very 6c site is only 1/3 [8]. If Fe atoms were to replace all rhodium atoms at the 6c site, the thus derived hypothetical formula would arrive at “Fe3Rh4B3”. Likewise, a 2/3 occupancy of the 6c site by iron would lead to the hypothetical formula “Fe2Rh5B3”, which is analogous to the previously reported ternary compound Re5W2B3 [9]. Unfortunately, the structure of the latter compound has not yet been determined in detail. Puzzlingly, this has not been tried before such that we do not know whether or not iron replaces ruthenium (and at which site) in the Ru7B3 structure. Ru7B3 is remarkable by itself because it represents the only structurally well-characterized binary boride of the Th7Fe3 structure type [10] up to now.
In this work, we will focus our attention on phase stability and site preference of M7−xTxB3 in order to thoroughly investigate its structural properties. Moreover, we will support our simulated findings by first-principles calculations of the density functional theory (DFT) type. Last, we will try to predict some properties related to lattice vibration.
Section snippets
Method for obtaining the pair potentials
The acquisition of inter-atomic potentials forms the basis for this research. On the basis of Möbius inversion in the number theory, Chen et al. proposed a concise inversion theorem to obtain intermetallic pair potentials from the cohesive energy curves [11], [12]. The cohesive energy curves can be obtained by either first-principle calculations or from experimental data. Chen's lattice inversion technique has been described in previous work [13], [14], [15], [16], [17], [18], [19].
There are
Structural properties
The crystal structure of Ru7B3 was determined by Aronsson [10] in the late 1950s. It was found that the lattice is hexagonal with a space group P63mc. There are 20 atoms in one unit cell with effective coordinate RuI(6c1), RuII(6c2), RuIII(2b), and B(6c), respectively. Based on any existing experimental structure close to Th7Fe3 type with the P63mc space group, for example, the atom position and symmetry of Ref. [10], [25], initial M7B3 (M = Ru, Rh) structure is constructed within Accelrys
Lattice vibrations of intermetallics M7−xTxB3
Phonon density of states reflects the lattice dynamic properties, from which some important thermodynamic parameters can be derived. In this section, with the inverted interatomic potentials, the total phonon densities of states as well as the partial density of states of different elements for M7−xTxB3 intermetallic borides are evaluated in a crystal cell including 20 atoms based on the lattice theory. Fig. 5 shows the total and partial density of states of M7−xTxB3 intermetallics,
Conclusion
From the present study, it results that the influence of T substitution for M on the structural properties of M7−xTxB3 can be divided into two parts: (I) the dopant element T can stabilize M7−xTxB3 with the hexagonal Th7Fe3-type structure in the rang of 0 < x < 1.50 and 3.88 < x < 7.00; (II) the dopant T atoms strongly prefer 2b sites and the order of site preference is 2b, 6c1 and 6c2. The calculated structure parameters are in good agreement with experiments. We have also calculated the
Acknowledgments
The work was supported by the National Natural Science Foundation of China (Grant No. 50971024), the 973 Project in China (No. 2011CB606401) and Basic Theoretical Research Foundation of Metallurgical Engineering Research Institute of University of Science and Technology Beijing.
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