Zintl Phases Foad Tehrani Najafian The concept of the ionic compound of main group elements is that one atom gives electron(s) to another atom with the result that both become ions (cation and anion) and both reach an octet (e.g. NaCl). The electronegativity of the constituting elements has a large difference. In covalent compounds the atoms share electrons to reach the octet (e.g. CH4). The electro- negativity of the constituting elements is comparable. For a lot of compounds in which the constituting elements have intermediate differences of the electronegativities these two concepts cannot explain the composition and structure. For this compounds Eduard Zintl developed a new concept. What are the Zintl phases? In Zintl phases one element is an electropositive element from the first or second group of the periodic table (alkali or alkaline earth metal), and the second one is a less electropositive element from the groups 13-15. The more electro negativity partner in the Zintl phase’s gets a certain number of electrons from the electropositive one. To reach the octet rule polyanions are formed. Depending on the VEC (see below) the electronegative partner behaves as an element of another group of the periodic table. This means that the anionic structure is related to the structure of an element of this group. What means the VEC? VEC is the valence electrons concentration of the anions in the Zintl phase compounds We can determine the VEC as follows: VEC =(m× e(M) + n× e(A))/n e(M), e(A) are the number of valence electrons of the constituting elements m is the number of the cations, n the number of anions per formula VEC is a very important value for us to determine the group to which the anion belongs. We can use the VEC to calculate the number of covalent bonds of the anions: b(AA) =8-VEC These two values can give us an idea about the Zintl phase compound structure. Examples: NaTl: VEC(Tl) = m × e(M) + n × e(A)/n = (1 × 1 + 1 × 3)/1 = 4 (Tl- behaves like an element of group 14 and forms a diamond-like structure) b(Tl) = 8 - 4 = 4 (that mean that every Tl has 4 covalent bonds) . Be3Si4: VEC (Be3Si4) = (3 × 2 + 4 × 4)/4 = 22/4 = 11/2 b(Be3Si4) = 8 - 11/2 = 5/2 This average value of 2,5 covalent bonds per Si atoms results when half of the Si atoms have 6- 3 covalent bonds and the other 2 have just 2 covalent bonds (the famous “butterfly ion” (Si4) is formed. Properties: • Most Zintl-phase compounds are brittle deeply colored. • They are semiconductors. • They are very sensitive against water and oxygen. • They have a fixed composition. Syntheses: The Zintl phase compounds are typically synthesized by reducing elements of group 13-15 in liquid ammonia in the presence of alkaline or alkaline earth metals. Another possibility is a direct synthesis from elements. Questions: 1) Calculate the VEC for silicon in Na4Si4? Give two possible structure of the anionic part of this Zintl phase. 2) Calculate the VEC for arsenic in CaAs and for germanium in BaGe and propose anionic structures, respectively. .