La_n(n+1)+xNi_n(n+5)+ySi_{(n+1)(n+2)–z}: A Symmetric Mirror Homologous Series in the La–Ni–Si System

Abstract

A series of four homologous silicides have been discovered during systematic explorations in the central part of the La–Ni–Si system at 1000 °C. All compounds La12.5Ni28.0Si18.3 (n = 3; a = 28.8686(8), c = 4.0737(2) Å, Z = 3), La22.1Ni39.0Si27.8 (n = 4; a = 20.9340(6), c = 4.1245(2) Å, Z = 1), La32.9Ni49.8Si39.3 (n = 5; a = 24.946(1), c = 4.1471(5) Å, Z = 1), and La44.8Ni66.1Si53.4 (n = 6; a = 28.995(5), c = 4.158(1) Å, Z = 1) crystallize in the hexagonal space group P63/m and can be generalized according to La n(n+1)+x Ni n(n+5)+y Si(n+1)(n+2)–z with n = 3–6. Their crystal structures are based on AlB2-type building blocks, fused La-centered Ni6Si6 hexagonal prisms, yielding larger oligomeric equilateral domains with the edge size equal to n. The domains extend along the c axis and show checkered ordering of the cationic and anionic parts, while all their atoms are located on mirror planes. La n(n+1)+x Ni n(n+5)+y Si(n+1)(n+2)–z can be considered as a mirror series to the La-rich La(n+1)(n+2)Ni n(n–1)+2Si n(n+1), where an exchange of the formal cationic and anionic sites, i.e., La and Si, occurs. The La–Ni–Si system is the first system where two such analogous series have been observed.

The Ce–Ni–Si System Revisited: More Homologue Compounds?

“…The same situation is also observed for the La-analogue phase, where we also obtained a single crystal with a smaller unit cell volume. The refined composition (La 22.4 Ni 36.9 Si 28.2 ) is slightly lower in nickel and richer in La compared to the result of Grilli et al…”

“…M1 sites consist of Ni and Si atoms with small occupancies, while the M3 sites are filled with Ce and Si with rather high occupancy, with their sum occ­(Ce+Si) in M3 close to unity (for details, see Table S1). Such disorder with split positions is also observed in the La-analogue phases with n = 3–6 . The homogeneity range of this phase is related to the site occupancy in the disordered region.…”

“…In contrast to the La–Ni–Si system for which four members of this series were observed, only two members were discovered in the Ce–Ni–Si system, with n = 4 and 5. Members with n = 3 would have a composition close to Ce21.2Ni47.7Si31.1 (in atom %); however, this composition was neither observed in the 800 °C or in the 1000 °C isothermal section of the Ce–Ni–Si system.…”

“…Rietveld refinement using the data set of Tb 15 Ni 28 P 21 yielded a final R I value of 4.1%. It should be noted that a Rietveld refinement using the data set from Grilli et al resulted in a practically similar R I value; however, some atomic sites in the disordered area could not be refined reliably. In any case, the details of the atomic distribution around the 6 3 -screw axis could not be well resolved from the XRPD data.…”

“…Moreover, during this work, we also solved a hitherto unknown crystal structure, Ce 3 Ni 4 Si 2 (formerly CeNi 1.3 Si 0.7 ). Interestingly, all phases studied in this work are part of some homologue series, one of which was recently discovered in the La–Ni–Si system with the general formula of RE n ( n +1)+ x Ni n ( n +5)+ y Si ( n +1)( n +2)– z …”