Investigation on structural Phase Transition in Langbeinites

Solid State and Structural Chemistry Unit, Indian Institute of Science
â—‹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.