Solid State and Structural Chemistry Unit, Indian Institute of Science
○Tayur N Guru Row Diptikanta Swain

The occurrence of a phase transition in a solid alters several material properties. Measurement of a particular property across the phase transition using sophisticated techniques provides insights into the nature of phase transition. Ferroic phase transitions in crystals usually involve a change of the space group symmetry. The langbeinite family of sulfates is well known for their ferroelectrics and ferroelastic phase transition with temperature. Langbeinites are categorized into three types, type-I, type-II and type-III. Type-I langbeinites exhibit series of phase transition from cubic to orthorhombic and to monoclinic crystal systems, Type-II exhibits phase transition from cubic to orthorhombic modifications and Type-III does not exhibit any phase transition with the temperature in the range 293K to 80K. We have investigated two new compounds in the langbeinite family, Rb₂Mn₂(SO₄)₃ and (NH₄)₂Mn₂(SO₄)₃ at various temperature to cover the entire range of phase transition. Rb₂Mn₂(SO₄)₃ shows a phase transition from cubic (P2₁3) at 293K to orthorhombic (P2₁2₁2₁) at 80K via a monoclinic (P2₁) 140K crystal system, where as (NH₄)₂Mn₂(SO₄)₃ shows a phase transition from cubic(P2₁3) at 293K to monoclinic (P2₁) at 120K via orthorhombic (P2₁2₁2₁) at 80K crystal system. The features of the crystal structures across the phase transition regime will be presented.