2. Phase and structural stability of ICs

Electron concentration (band filling) is an important parameter governing phase stability of ICs. We have been investigating the effect of band filling in numerous quasibinary materials. In the system CaMg xAl 2-x (0 < x < 2) we obtained the sequences of Laves-phase structures cubic MgCu 2 hexagonal MgNi 2 hexagonal MgZn 2 type with increasing Mg content. In systems T xT’ 1-xSn 2 (T = Ti - Co; T’ = V - Ni) we could reveal the sequence of structures CuMg 2 NiMg 2 CuAl 2 CoGe 2 with increasing electron concentration. In the crystal structures of the materials T 8Ga 41-xZn x (T = V, Cr, Mn) we observed an intriguing segregation of the Zn and Ga atoms leading to the formation of separated Zn 13 cluster entities. These clusters correspond almost to ideal centered cuboctahedra or small pieces of fcc metal. Strong ordering tendencies between Zn and Ga or In were also observed in the systems CoGa 3-xZn x and CoIn 3-xZn x.

Fig.2: Structure of Mn 8Ga 27.4Zn 13.6 showing the distribution of Ga (green) and Zn atoms (blue circles, yellow cluster entities). Mn atoms are depicted as red circles.

S. Ponou , N. Müller, T. F. Fässler, U. Häussermann “KBi 2-xPb x: A Zintl-Phase Evolving from a Distortion of the Cubic Laves-Phase Structure” Inorg. Chem., 44 (2005), 7423.

S. Amerioun, T. Yokosawa, S. Lidin , U. Häussermann “Phase Stability in the Systems AeAl 2-xMg x (Ae = Ca, Sr, Ba): Electron Concentration and Size Controlled Variations Based on the Laves Phase Structural Theme” Inorg. Chem., 43 (2004), 4751.

S. Amerioun, S. I. Simak, U. Häussermann “Laves Phase Structural Changes in the System CaAl 2-xMg x” Inorg. Chem., 42 (2003), 1467.

U. Häussermann, S. Amerioun, L. Eriksson, C.-S. Lee, G. J. Miller “The s-p Bonded Representatives of the Prominent BaAl 4 Structure Type: A Case Study on Structural Stability of Polar Intermetallic Network Structures” J. Am. Chem. Soc., 124 (2002), 4371.

P. Viklund, C. Svensson, S. Hull , P. Berastegui, S. I. Simak, U. Häussermann “From V 8Ga 36.9Zn 4.1 and Cr 8Ga 29.5Zn 11.5 to Mn 8Ga 27.4Zn 13.6: A Remarkable Onset of Zn-Cluster Formation in an Intermetallic Framework” Chem. Eur. J., 7 (2001), 5143.

U. Häussermann, P. Viklund, C. Svensson, S. Eriksson, P. Berastegui, S. Lidin “The Hume-Rothery Compound Mn 8Ga 27.4Zn 13.6: Separated Zn 13-Clusters Interspersed in a Primitive Cubic Host Lattice” Angew. Chem. Int. Ed., 38 (1999), 488.

U. Häussermann, S. I. Simak, I. A. Abrikosov, B. Johansson, S. Lidin “Electronically Induced Phase Transitions in Ternary Transition Metal Distannide Systems” J. Am. Chem. Soc., 120 (1998), 10136.

High-pressure structural relationships have been investigated and rationalised for group 13 (B, Al , Ga , In) and group 15 (P, As, Sb, Bi) elements. The heavier group 15 elements transform to a most unusual incommensurate high-pressure modification, which stimulated a detailed investigation of the high-pressure behaviour of the solid solution Bi 1-xSb x. An ongoing study is the systematic investigation of the high-pressure behaviour of s-p bonded polar ICs with the composition AeE 2 (Ae = Ca, Sr, Ba; E = Zn, Al , Ga , In). From theoretical calculations we predict valence electron concentration controlled structural transitions to the topologically close-packed Laves Phase structures.

Fig.3: (a) High-pressure structural behaviour of the group 15 elements. (b) Electron density distribution in high pressure Sb with the Bi-III modification showing the pairing of atoms in the guest structure.

U. Häussermann, O. Degtyareva, A. S. Mikaylushkin, K. Söderberg, S. I. Simak, M. I. McMahon, R. J. Nelmes, R. Norrestam “Alloys Bi 1-xSb x under High-Pressure: Effect of Alloying on the Incommensurate Bi-III Type Composite Structure” Phys. Rev. B, 69 (2004), 134203.

U. Häussermann “High-Pressure Structural Trends of Group 15 Elements: Simple Packed Structures versus Complex Host-Guest Arrangements” Chem. Eur. J., 9 (2003), 1471.

U. Häussermann, B. Johansson, R. Ahuja, S. I. Simak “Metal-Nonmetal Transition in the Boron Group Elements” Phys. Rev. Lett., 90 (2003), 065701.

U. Häussermann, K. Söderberg, R. Norrestam “A Comparative Study of the High-Pressure Behaviour of As, Sb, and Bi” J. Am. Chem. Soc., 124 (2002), 15359.