We know the band gap of Perovskite materials is due to electronegativity between B cation and X anion(in ABX3 compound). We can change band gap and opto-electronic properties by changing structure which is again due to the play of B and X. So I want to know the role of A cation in ABX3 type perovskite materials.
A helps in self assembly of BX3 group to give a layered material.
[1] "A' metal ion size is useful in governing regularities in formation of perovskites compounds and use it to further guide the exploration of new materials.
[2]In early 1920s,Goldschmidt has proposed
“tolerance factor” ( t=rA +rO / √2 ( rB+rO)
where rA, rB and rO are the ionic radii of A, B and O, respectively to study the stability of perovskites.
[3] Geometrically, for an ideal perovskite, the ratio of D (A–O), the bond length of A–O bond, to D (B–O),the bond length of B–O bond is √2:1.Thus, if the bond length is roughly assumed to be the sum of two ionic radii,the “t” value of an ideal perovskite should be equal to 1.0.However, Goldschmidt found that, as an experimental fact, “t” values of most cubic perovskites are in the range of 0.8–0.9,and distorted perovskites occur in somewhere wider range of “t”.
[4]Goldschmidt’s tolerance factor "t " has been widely accepted as a criterion for the formation of the perovskite structure, a number of researchers have used it to discuss the perovskite stability, including oxides, fluorides, chlorides. Up to now, almost all known perovskite have “t” values in the range of 0.75–1.00.
REF:
Journal of Alloys and Compounds 372 (2004) 40–48
Formability of ABO3 perovskites
Chonghe Li, Kitty Chi Kwan Soh , Ping Wu
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