Monoclinic-orthorhombic phase transitions in A₂BX₄ single crystals with Sr₂GeS₄-type structure

Abstract

The structural phase transitions including the monoclinic-orthorhombic transition (α-β transition) in K₂ZnI₄, Rb₂CoI₄ and Rb₂MnI₄ are investigated by means of X-ray diffraction. These compounds take the Sr₂GeS₄-type structure at room temperature. In Rb₂CoI₄ and Rb₂MnI₄, the α-modification is transformed to the β-modification (β-K₂SO₄-type structure) at 515 K and 510 K, respectively. In the β-modification of Rb₂MnI₄, it is found that the transition from the normal phase (space group Pmcn) to incommensurate one (modulation along c*) takes place at around 520 K. On the other hand, the α-β transition is not observed up to the melting point in K₂ZnI₄. We discuss the α-β transition on the basis of the ratio the ionic radius of A⁺ to the average bond length B-X in tetrahedral BX₄ ^2- ion. It is found that the compounds in this family are classified into three types by the ratio.

Key Words:

• X-ray diffraction
• Sr₂GeS₄-type structure
• β-K₂SO₄-type structure
• α-β transition