Structure ZrBeSi

The scheme of the sequence of unit cell sections of Ni2In is shown here together with that of its (second kind) derivative structure ZrBeSi (Fig. 7.39). 

Figure 7.39. Sequences of characteristic sections of the hP6-Ni2In and ZrBeSi structures. Four adjacent cells are shown.

Early investigations of the PrNiSb compound showed that it had the AlB2-type structure with a = 0.4376, c = 0.4053 (Pecharsky et al., 1983a X-ray powder analysis). For experimental procedure, see ScNiSb. At variance with these data, Hartjes and Jeitschko (1995) suggested a ZrBeSi type structure, a = 0.4375, c = 0.8123 from X-ray powder diffraction. For experimental details, see LaNiSb. 

Eu-Cu-Sb. EuCuSb was found to crystallize with the ZrBeSi type structure with lattice parameters of a = 0.4512, c = 0.8542 (Tomuschat and Schuster, 1981 X-ray photographic powder method). 

The structure types Caln2, ZrBeSi, NdPtSb, LiGaGe, CaLiSn,... 

If the T3Pb3 layers are planar, e.g. YbHgPb (Merlo et al., 1993), the ZrBeSi type (Nielsen and Baenziger, 1954) occurs. This structure type mostly occurs when the rare earth atoms have a large radius. The stacking sequence of the T3Pb3 hexagons is similar in the three structure types, i.e. AB AB. 

Smaller A components like Yb favour the formation of structures with tetrahedral or distorted tetrahedral coordination of the transition metal. This trend is obvious from the compoimds reviewed here, where representatives with AIB2 or ZrBeSi structures (planar TaXa hexagons) are scarce, but occur frequently in Eu7X compounds due to the larger europimn atoms (Pottgen and Johrendt 2000). 

A phonon contribution with an additional cubic term can be used to analyze the resistivity behavior of the metallic bismuthides YbAgBi and YbAuBi (Merlo et al. 1995). An anomaly occurs in the temperature dependence of the resistivity of YbCuBi at about 375 K, most likely resulting from a structural phase transition from the high-temperature ZrBeSi to the low-temperature NdPtSb modification. The anomaly in the resistivity is paralleled by a drop of the da ratio of the lattice parameters in the same temperature range. 

The only information on the interaction of europium with zinc and germanium is due to the work of Pottgen (1996) who reported the crystal structure of EuZnGe (ZrBeSi type, a = 0.4372, c = 0.8604 X-ray powder diffraction). 

Fig. 3. Trigonal-prism stacking variants (a) ZrBeSi = ordered AIB type (b) hypothetical M3X4 structure (c)3 -MoN (d)fi -WN (e) rhombohedral 8-WN and (f) WC structure.

Double layers derived from the ZrBeSi structure by omitting, say, the atoms at the Be sites are cubically close packed in AcAbABaBcBCbCaC... 

Number 1 corresponds to AIB2 structure type 2 - ZrBeSi 3 - MgAgAs 4 - Caln2 5 - TiNiSi 6 - LiGaGe 7-KHg2 8-CeScSi. 

Fig. 15. Structures of (a) low- (LiGaGe-type) and (b) high-temperature YbCuBi (ZrBeSi-type).

Structure type ZrBeSi (Nielsen and Baenziger 1954 table 54) has space group Pfis/mmc, a = 0.4248, c = 0.1199 (for CePdP) (Johrendt and Mewis 1990). 

Lux et al. (1991a) determined the y-EuPtP structure by a single-crystal method at 180 K (/J = 0.044). In y-EuPtP both Pt and P atoms are displaced from the ideal positions with z = l/4 this leads to a decreased symmetry compared to ST ZrBeSi. One can consider y-EuPtP as a superstructure of Caln2 (landelli 1964). Interatomic distances are 5Eui Pt = 0.3082 Sguz-Pt = 0.3204 6eui-p = 0.3091 eu2-p = 0.3195 6pt p=0.2370. 

---