The structural stability, elastic constants and electronic structure of Al-Sr intermetallics by first-principles calculations

The lattice constants, enthalpies of formation, elastic constants and electronic structures of Al-Sr intermetallics have been calculated by first-principles method within generalized gradient approximation. The calculated lattice constants and enthalpies of formation are in good agreement with experimental and other theoretical results. The polycrystalline bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also estimated from the calculated single crystalline elastic constants. The total and partial electronic densities of state for the intermetallics were obtained, and the results indicated that Al₂Sr-oI is more stable than Al₂Sr-cF. Finally, longitudinal, transverse and average sound velocities and Debye temperature are estimated.     

过渡金属铝化物FeAl和CoAl弹性性质的第一性原理研究

本文通过第一性原理计算(密度泛函理论结合均匀形变方法)得到过渡金属铝化物FeAl和CoAl的二阶和三阶弹性常数,这些弹性常数是通过拟合计算出的能量与应力关系得到的。计算结果和理论数据及实验值符合的很好。接下来本文又研究了FeAl和CoAl在不同压强下的弹性性质。不同压强下的弹性常数Cij,体模量B,剪切模量G,泊松比σ 也成功的得到了。B/G比值和柯西压强 PC 都表明在零压下FeAl和CoAl表现出脆性。在压强小于60GPa的情况下,增大压强可以增强它们的韧性,但它们始终表现为脆性。     

Preparation and crystal structure of ternary rare-earth platinum metal aluminides R2T3Al9 (T=Rh, Ir, Pd) with Y2Co3Ga9-type structure and magnetic properties of the iridium compounds

The ternary aluminides R₂Rh₃Al₉ (R=Y, La-Nd, Sm, Gd-Tm, Lu), R₂Ir₃Al₉ (R=Y, La-Nd, Sm, Gd-Lu), and R₂Pd₃Al₉ (R=Y, Gd-Tm) have been prepared by arc melting of the elemental components with an excess of aluminum and dissolving the aluminum-rich matrix in hydrochloric acid. They crystallize with Y₂Co₃Ga₉-type structure: Cmcm, Z=4. The crystal structures of Ho₂Rh₃Al₉ and Er₂Ir₃Al₉ have been refined from single-crystal X-ray data; Ho₂Rh₃Al₉: a=1316.8(3) pm, b=760.2(2) pm, c=933.7(2) pm, R=0.044 for 255 structure factors and 27 variables; Er₂Ir₃Al₉: a=1313.8(2) pm, b=758.5(1) pm, c=933.8(2) pm, R=0.057 (392 F values, 27 variables). The structure may be viewed as consisting of atomic layers of the compositions A=R₂Al₃ and B=T₃Al₆ which alternate in the sequence ABAB along the z direction. Approximately 33% and 27% of the A layers were found to be misplaced in the crystals investigated for Ho₂Rh₃Al₉ and Er₂Ir₃Al₉, respectively. The magnetic properties of most iridium-containing compounds have been determined with a superconducting quantum interference device magnetometer. The yttrium and the lanthanum compounds show Pauli paramagnetism, others reflect the magnetic behavior of the rare-earth components. The magnetic ordering temperatures are all lower than 20 K.     

Ternary Aluminides with the Ideal Composition A2Pt6Al15 (A = Y,Gd—Tm,Zr)

Ternary rare earth platinum aluminides were prepared by arc‐melting of the elemental components followed by annealing in a high‐frequency furnace. Their crystal structure was determined for the yttrium compound from four‐circle X‐ray diffractometer data. It has hexagonal symmetry with a = 428.1(1) pm, c = 1638.3(3) pm, space group P6₃/mmc, and was refined to a conventional residual of R = 0.018 for 325 F values and 19 variable parameters. Of the five crystallographic positions, the yttrium position and one of the three aluminum positions show partial occupancies corresponding to the composition Y_1.357(3)Pt₄Al_9.99(2) with the Pearson symbol hP20 — 4.65. These partially occupied sites are that close to each other that at best only one can be fully occupied. A model for an ordered distribution of occupied and unoccupied Y and Al sites requires a √3 larger a axis with the Pearson symbol hP20 — 4.67 for the subcell, very close to the experimental result. Corresponding superstructure reflections could be observed on an image‐plate single‐crystal diffractometer only in the form of diffuse streaks. The compound has the ideal composition Y₂Pt₆Al₁₅ with Z = 2 for the superstructure. This corresponds to the formula Y_1.33Pt₄Al₁₀ with Z = 1 for the subcell. The compounds A_1.33Pt₄Al₁₀ with A = Gd, Tb, Dy, Ho, Er, Tm were found to be isotypic with that of the yttrium compound. This structure is closely related to or isotypic with, respectively, those of Yb₂Fe₄Si₉, Sc_1.2Fe₄Si_9.8, Ce_1.2Pt₄Ga_9.8, Ce₂Pt₆Ga₁₅, Tb_0.67Ni₂Ga_5—xSi_x, RE_0.67Ni₂Ga_5—xGe_x> (with RE = Y, Sm, Ho), and Gd_0.67Pt₂Al₅, reported in earlier investigations. The new compound Zr_1.00(1)Pt₄Al_10.22(3) has nearly the same hexagonal structure with a = 426.1(1) pm and c = 1622.8(3) pm. It was refined from four‐circle diffractometer data to a residual of R = 0.021 for 288 structure factors and 19 variable parameters.     

Die Aluminidiodide La24Al12I21 und La10Al5I8: Verbindungen mit intermetallischen La‐Al‐Bereichen und La‐Al‐Clustern

The Aluminide Iodides La₂₄Al₁₂I₂₁ and La₁₀Al₅I₈: Compounds with Intermetallic La‐Al Fractions and La‐Al Clusters Reacting pieces of La, LaI₃ and Al filings (molar ratio 22 : 8 : 15) at 800 °C–825 °C results in La₂₄Al₁₂I₂₁ (70 % yield) together with La₁₀Al₅I₈ (10 % yield), besides known La₃Al₂I₂ and La₂Al₂I. Both new compounds form golden coloured needles. La₁₀Al₅I₈ is brittle, whereas La₂₄Al₁₂I₂₁ is shaped as hair‐like easily deformable bundles. Both are monoclinic, space group C2/m, La₂₄Al₁₂I₂₁ with a = 35.753(7) Å, b = 4.327(1) Å, c = 27.442(6) Å, β = 116.62(3)° and La₁₀Al₅I₈ with a = 19.649(1) Å, b = 4.296(1) Å, c = 18.0290(1) Å and β = 96.67(3)°. The La atoms form trigonal prisms condensed into double chains along [010]. The La prisms are centered by Al atoms which form Al₆ rings connected into chains. The La‐Al strands are surrounded by I atoms in La₂₄Al₁₂I₂₁, whereas in La₁₀Al₅I₈ they are connected to form corrugated sheets separated by close packed layers of I atoms together with Al atoms. The octahedral voids around the Al atoms are occupied by La atoms, and such La₆Al clusters are connected via opposite edges to octahedra chains along [010].     

Die Verbindung La5Br4Al4 und ihre topologische Verwandtschaft mit dem Ln3ClGa4‐Typ

The Compound La₅Br₄Al₄ and the Topological Relation with the Ln₃ClGa₄ Structure Type The compound La₅Br₄Al₄ can be prepared from La metal, LaBr₃ and Al filings at 950 °C with a yield of 20 %. It crystallizes tetragonally in I4/mcm with a = b = 8.292(1)Å, c = 20.122(4)Å. In the crystal structure 3 sheets of Br/La, La/Al, and La/Br atoms, respectively, are stacked along [001] and are connected by single layers of Br atoms. The Al atoms form undulated nets of condensed Al₈ and Al₄ rings. The La atoms are arranged in tetragonal antiprisms, centered by the second kind of crystallographically different La atoms. These units are connected to sheets. In La₃ClGa₄ the identical 3 sheets formed by La, Ga, Cl atoms are present, however, not separated by the additional layers of Br atoms as in La₅Br₄Al₄.