Laves phase

Laves phases exhibit intrinsic properties [68] that suggest their potential applications in superconducting materials, magnetic materials and metal hydride batteries [4, 69, 70]. Thousands of topologically close-packed compounds are classified within this family, with hexagonal MgZn2

(C14), cubic MgCu2 (C15), and hexagonal MgNi2 (C36) structure types [14, 71]. The phase stability and structures of Laves structures are strongly affected by two factors: (i) atomic size (for binary Laves systems AB2, the ideal radius ratio of the two atoms is ca. 1.225 and values of 1.05–1.68 occur in many surveys) [72], and (ii) concentration of valence electrons. For Mg(Cu/Al)2 [73] and Ca(Al/Li)2 [74] systems, the structures vary between C15, C36 and C14 types, accompanied by changes in the number of valence electrons [75]. For s-p bonded Laves compounds CaAl2–xMgx, the structural transformation has been further studied by first-principles calculations considering especially moments of the electronic density of states on structure stability [76].

HfAl2 [77] was reported as a Laves phase with the MgZn2 structural type. The Al atom is replaceable by other metallic elements to form ternary compounds of the MgCu2 {e.g. HfAl1.65(Au, Cu, Ni)0.35 [78, 79], HfAl1.7Co0.3 [80], HfAg0.425Al1.575 [81], HfAl1.5Zn0.5 [82], HfAl0.1Ni1.9 [83]} and MgNi2 (HfPd0.33Al1.67, HfPt0.36Al1.64 [78]) types. No previous reports describe successive variations of the Laves structures in Hf–Cu–Al system. In this study, we systematically investigated the

changes in the Laves structures of the ternary system HfAl2–xCux to determine the ranges of homogeneity and to characterize its new phases. We obtained several new phases that feature a measurable phase width and structural transformations triggered through substitution of Al atoms by Cu atoms. Here we report the synthesis, structural characterization, electrical resistivities, and electronic structures of three Laves phases within the system Hf–Cu–Al.

1.4.1.2 Sc₁₁Ir₄ phase

Hundreds of compounds comprising elements throughout the periodic table adopt [14] the structural type Th6Mn23 [84]. Two possible compositions – A6B16C7 or A6(B, C)2 – are observed in a ternary system, where A is an early transition metal or lanthanide, and B and C are elements from groups 7 to 15. Research on M6Fe16Si7 (M = Ta, Nb) [85], Mn6Ni16Si7 [86], U6Fe16Si7 [87], La6Mg22Al [88], Y6Fe23–xCrx [89] include crystallographic measurements implemented with X-ray and neutron diffraction. The Th6Mn23 prototype contains an interstitial position that can be either empty or filled with an atom of appropriate size. According to the literature, the filled compound of type Th6Mn23, known also as structural type Sc11Ir4 [90], occurs mostly in silicides or aluminides, including U6Fe16Si7C [91] and Ln3Pd8Sb4 (Ln = Y, Gd, Tb, Dy, Ho, Er, Tm) [92].

Few ternary phases have been reported in the Hf–Cu–Al system [78, 92, 93] . During our investigation on structural variations of Laves phases HfAl2–xCux [94], we obtained a compound characterized as Hf6Cu16Al7.58, similar to ZrCu16Al7 [95]. Unlike the reported aluminides such as

Ti26.88Fe28Al65.12 [96], Ti45.7Co30.4Al43.9 [97], and Ti37.9Ru28Al54.1 [98] , which feature a large proportion of Al and a mixed occupancy of various metals in the 4b site, Hf6Cu16Al7.58 as synthesized contains little aluminium and the 4b site is filled partially with Al. Here we report the synthesis, structural characterization, electrical resistivity and electronic structure of a compound of type Sc11Ir4 in the Hf–Cu–Al system.

1.4.2 Hf–Al–Sb system

Ternary intermetallics of T5AxB3-x (T = early transition metal; A, B = group 13-15 elements) have been extensively investigated [14] because of their rich structural chemistry and tunable electronic properties. Much has been reported about the hexagonal Mn5Si3 structure with interesting physical properties including solid strength and hardness of silicide, aluminide, beryllide and chromide compounds [99], the stability of Nb5Si3 at high temperature [100], and the ability of Y5Si3 and ternary Y5(Si,Ge)3 to store hydrogen [101]. Structures in this family are correlated with similar polyhedra, that the Mn5Si3 type was composed of edge-shared trigonal prisms arranged along the [001] direction, whereas the prisms formed sheets of trigonal columns about (101) in the Y5Bi3-type structure [102]. Many ternary phases, such as in (Ti, Zr, Hf)-M-Sb systems, adopt W5Si3 structural form with substituted transition-metal atoms (M), including Ti5CuSb2 [103], Zr5Cu0.45Sb2.55 [104], Zr5M0.5Sb2.5 (M = Fe, Co, Ni, Ru, Rh) [105], T5M1–xSb2+ x (T = Ti, Zr, Hf; M = Fe, Co, Ni, Cu, Ru, Rh, Pd, Cd) [106], Hf5M1–xSb2+x (M = V, Cr, Mn, Fe, Co, Ni, Cu) [107]. The transition metals show

a preference to occupy the 4a site according to the Brewer-Engel rules [108], and inhibition of local arrangement of Sb-Sb separation [109].

Besides these phases, compounds of W5Si3 type comprising early transition-metal and main- group elements are less available, only M5(Sn, Ga)3 (M = Nb, Ta) [110, 111], Ti5XSb2 (X = Al, Ga, In) [112], Ti5Si1.3Sb1.7 [113] have been explored. To understand the structural diversity and their physical properties, we undertook exploratory experiments in the ternary Hf-Al-Sb system about which is less known than other phases.

We obtained several new phases that feature a measurable phase width and variation of structural types in the sequence Mn5Si3 Æ W5Si3 Æ Y5Bi3 as Al atoms were gradually replaced by Sb atoms in Hf5Al3–xSbx reactions. Herein, we report their syntheses, crystal structures, band structures and physical properties.

1.4.3 Hf–Ni–Ga system

Intermetallic compounds of the ternary systems A–Ni–Ga (where A is a rare earth, alkali, alkaline earth, or early transition element) exhibit many different structures [14]. Among these phases, most belong to RE–Ni–Ga systems (RE = rare-earth metal), such as RE3Ni6Ga2 [114], RENiGa3 [115], RE4Ni2Ga17 (RE = Ce, Nd) [116], Eu3Ni4Ga4 [117], RE15Ni96–xGax [118], and Yb4Ni10+xGa21–x [119].

In addition to their flexible compositions, these systems have been studied extensively for their phase transitions [120], the valence behavior of the electropositive atom [121], band-structures [122]

and intrinsic magnetic properties [123-125]. In contrast, only a few compounds have been reported that comprise elements of groups 1–3. For example, CaNi2Ga3 of BaZn5 type, deformed from a CaCu5 structure [126]; Na10NiGa10 featuring a three-dimensional net composed of [Ga10Ni]^10–

gallium clusters [127]; Sc5(Ni, Ga)1.925 built through condensation of cuboctahedral and tricapped trigonal prisms [128]; and Y(Ni, Ga)2, which transforms its structure between CeCu2 and CaIn2

types for various compositions [129].

For TM–Ni–Ga systems (TM = Ti, Zr, Hf), several compounds have been synthesized and their structures determined, including TMNiGa [130, 131], TMNi2Ga [132], Ti4Ni2Ga3 [133], Hf(Ni, Ga)3 [134] and HfNiGa2 [135]. The unique structures revealed in compounds TM6Ni8Ga15 [136]

and HfNi2.15Ga3.85 [137] led us to explore new phases featuring large contents of nickel and gallium atoms. We have undertaken a systematic synthesis of new compounds in the ternary M–Ni–Ga system (M = Zr, Hf). Here, we report the synthesis, structures and calculated electronic structures of the intermetallic compounds Hf13.0Ni40.8Ga30.9 and Zr13.0Ni40.6Ga31.0. Each phase adopts hexagonal structure of Y13Pd40Sn31-type with a complicated framework and atomic sites [138], and we discuss these structures in relation to those determined previously for CaCu5-corresponding phases.

Chapter 2

Experiments

2.1 Synthesis

2.1.1 Hf–Cu–Al system

2.1.1.1 Laves phase

To seek new phases in the ternary system Hf–Cu–Al, Hf ingot (99.9%, Alfa Aesar), Al powder (99.99%, Alfa Aesar) and Cu powder (99.999%, Alfa Aesar) were combined in stoichiometric ratios (total mass: ca. 0.35 g) in a glove box under an atmosphere of N2. Alloys of the type HfAl2–xCux

(nominal compositions: x = 0, 0.1, 0.2, …, 2.0) were effected on arc-melting samples on a water-cooled copper hearth under an Ar atmosphere. The samples were melted three times to ensure homogeneity; loss of mass was controlled to less than 3%. After these reactions, each compound was sealed in an evacuated silica tube and annealed at 1073 K for five days.

1.4.1.2 Sc11Ir4 phase

Hf ingot (99.9%, Alfa Aesar), Al powder (99.99%, Alfa Aesar) and Cu powder (99.999%, Alfa Aesar) were used to seek in the ternary system Hf–Cu–Al. The elements were combined in

stoichiometric ratios (total mass: ca. 0.35 g) in a glove box under an atmosphere of N2 and cold pressed into pellets. The reactants were loaded in an alumina crucible and sealed within an evacuated fused-silica tube. The tube was heated from 300K to 1273K over 12 h; the latter temperature was maintained for 6 h before cooling to room temperature. The samples revealed a silver surface after being crushed into pieces; they were insensitive to air or moisture. Further crystal analyses, using powder X-ray diffraction, were performed after the target phases were obtained in pure form.

2.1.2 Hf–Al–Sb system

The starting materials for seeking new phases in the ternary system Hf-Al-Sb including hafnium ingot (99.9 %, Alfa Aesar), aluminum powder (99.99 %, Alfa Aesar) and antinomy powder (99.999

%, Alfa Aesar). The samples of total mass ~0.35 g in stoichiometric ratio with an excess of 0.05 g to 0.15g antimony were cold pressed into pellets within glove box of nitrogen atmosphere. Samples of Hf5Al3-xSbx (nominal compositions x = 0, 0.5, 1.0, …, 3.0) were effected on arc-melting samples on a water-cooled copper hearth in an argon atmosphere. The samples were melted three times to ensure homogeneity and loss of mass was controlled to be less than 3 % of stoichiometric weight.

After reaction, each sample was sealed in an evacuated silica tube and annealed at 1073 K for five days.

2.1.3 Hf–Ni–Ga system

To obtain new Hf-Ni-Ga phases, samples of a Hf ingot (99.9%, Alfa Aesar), Zr ingot (99.9%, Alfa Aesar), Ni powder (99.99%, Alfa Aesar), and Ga (99.999 %, Alfa Aesar) were combined (total mass ca. 0.35 g) with the elements in stoichiometric ratios in a glove box under an atmosphere of N2. The reactions were effected through arc-melting of the samples on a water-cooled copper hearth under an Ar atmosphere. The samples were melted three times to ensure homogeneity; loss of mass was controlled to less than 3%. After these reactions, each sample was sealed in an evacuated silica tube and annealed at 1073 K for five days. The samples revealed a silver surface after being crushed into pieces; they were insensitive to air or moisture. Further crystal analyses, using powder X-ray diffraction, were performed after the target phases were obtained in pure form. The same processes were duplicated for the synthesis of the Zr compound.

2.2 Characterization

Powder X-ray diffraction data were collected on a Bragg–Brentano–type powder diffractometer (Bruker D8 Advance; operated at 40 kV and 40 mA; Cu Kα; λ = 1.5418 Å). For phase identification, all patterns were measured in a 2θ range from 5 to 60° with a step size of 0.05° and a counting period of 1 s/step. The patterns of the MgNi2-type compounds were similar to those for MgZn2-type compounds, but could be differentiated their (015) peak at angle of 2θ of 33°. Energy dispersive

spectra (SEM/EDX, Hitachi S-4700I high-resolution scanning electron microscope) were recorded using small pieces of samples to confirm that the levels of component elements were close to their weighted-in compositions.