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CaAg – SrAg – BaAg: Zwischen CrB‐ und FeB‐Strukturtyp
Author(s) -
Harms Wiebke,
Burggraf Fabian,
Daub Michael,
Dürr Ines,
Röhr Caroline
Publication year - 2008
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.200800225
Subject(s) - stacking , orthorhombic crystal system , crystallography , crystal structure , valence electron , chemistry , ternary operation , atom (system on chip) , hexagonal crystal system , structure type , valence (chemistry) , space group , x ray crystallography , electron , diffraction , physics , organic chemistry , optics , quantum mechanics , computer science , programming language , embedded system
CaAg – SrAg – BaAg: Between CrB and FeB Structure Type Depending on both electronic (valence electron numbers) and geometric (atom size ratios) characteristics of the contributing elements, the 1:1 compounds A II M of the heavier alkaline earth elements A and electron‐rich transition metals M form the well known CrB or FeB structure types. Both structure types exhibit M zig‐zag chains, which are stacked in different orientations against each other. In systematic studies of the pseudo‐binary sections Ca x Sr 1−x Ag and Sr x Ba 1−x Ag three new ternary phases with different stacking variants between the CrB (cubic stacking, c ) and the FeB (hexagonal stacking, h 2 ) structure type have been prepared and determined on the basis of single crystal x‐ray data. Starting from CaAg (CrB type, stacking c ), up to 25 percent of Sr (i.e. in Ca 0.75(2) Sr 0.25(2) Ag: orthorhombic, space group Cmcm , a = 413.4(1), b = 1157.8(3), c = 467.4(2) pm, Z = 4, R 1 = 0.0453) can be substituted without a change in the structure type. At a 1:1 ratio of Ca and Sr, a new structure type with stacking sequence ( hc 5 ) 2 is formed (Ca 0.48(3) Sr 0.52(3) Ag: space group Pnma , a = 4800.1(1), b = 471.8(12), c = 624.11(9) pm, Z = 24, R 1 = 0.0881). Further reduced cubic stacking (( hc 2 ) 2 ) is observed in the Sr rich region with Ca proportions between 39 and 25(2) % (space group Pnma , x = 0.39(1)/0.25(2): a = 2412.2(3)/2434.67(4), b = 472.75(5)/473.97(8), c = 626.17(7)/629.2(3) pm, Z = 12, R 1 = 0.0430/0.0489) where the HT‐TbNi structure type is formed. The stability range of the SrAg structure type (( hc ) 2 ) starts at a Ca content of 5 % (Ca 0.05 Sr 0.95 Ag: space group Pnma , a = 1643.1(2), b = 476.79(7), c = 636.27(9) pm, Z = 8, R 1 = 0.0318). From a new second modification of SrAg (space group Pnma , a = 4131.5(4), b = 478.79(6), c = 638.81(6) pm, Z = 20, R 1 = 0.0394) up to a Ba proportion of 8 percent, the α‐CaCu structure type with sequence ( hch 2 c ) 2 is formed. The stability range of the FeB structure with purely hexagonal stacking extends from Sr 0.84(1) Ba 0.16(1) Ag ( a = 830.78(12), b = 479.35(8), c = 644.93(11) pm, Z = 4, R 1 = 0.0413) to the border phase BaAg. In an accompanying computational study, the chemical bonding in the binary compounds CaAg, SrAg and BaAg is analyzed using FP‐LAPW band structure methods. Together with geometrical criteria, the results are used to compare and discuss the stability of the different stacking variants of the polar intermetallic 1:1 phases inbetween the CrB and the FeB structure type.