Successive phase transitions and ferroelasticity of [(CH3)2NH2]3Bi2I9

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Abstract
Preliminary X-ray diffraction studies show that at room temperature [(CH3)2NH2]3Bi2I9 (DMAIB) is isostructural with Cs3Sb2I9. The crystals are orthorhombic, space group Pnam with unit cell parameters a = 22.565, b = 15.549 and c = 9.088 Å, Z = 4, V = 3188 Å3. Observations under a polarizing microscope at room temperature show that DMAIB crystal is ferroelastic. The ferroelasticity can be explained as a consequence of a hexagonal to orthorhombic 6/mmmFmmm phase transition. The successive structural phase transitions in DMAIB were studied by dielectric and DSC methods and polarizing microscope observations. DMAIB undergoes four transitions at 304 K (I⇒ II), 285 K (II ⇒ III), 230 K (III ⇒ IV) and 195 K (IV ⇒ V). The I ⇒ II and II ⇒ III transitions are of first order while the III ⇒ IV and IV ⇒ V are of second order. The III ⇒ IV transition is not revealed by DSC while the IV ⇒ V is not observed in a ⇒α(T) plot. The successive stages of the ferro-paraelastic (II ⇒ I) transition are shown in polarizing microscope photographs. The II-I phase boundary forms a 46 ± 0.5 deg. angle to the [001] direction of the low-temperature orthorhombic phase.