J. Zaleski

Phase transitions in (CH3NH3)3Bi2I9 (MAIB)

Abstract Crystals of tris (methylammonium) nonaiododibismuthate were grown which turned out to be isomorphous with Cs3Bi2I9 and (CH3NH3)3Sb2I9. Two phase transitions, at 223 (second order) and 142 K (first order), were revealed. The low temperature phase transition is characterized by a strong dielectric anomaly. The pyroelectric behaviour could indicate a ferrielectric ordering below 142 K.

Properties and structural phase transitions of (CH3NH3)3Sb2I9

Abstract The tris (methylammonium) nonaiododiantimonate (III) crystals were grown. The preliminary X-ray diffraction studies showed that at room temperature they are isomorphous with Cs3Sb2I9. The crystals are hexagonal, space group P63/mmc with the unit cell parameters; a = 8.543 (1), c = 21.520 (7) a, Z = 2, V = 1360 A3. The DSC studies revealed two structural phase transitions at T c1 = 147 K, and T c2 = 111 K. The dielectric studies showed only one anomaly at T c1 = 147 K. The high temperature phase behaves like a plastic one with respect to a freedom of rotational motions of methylammonium cations. The dielectric and pyroelectric properties seem to indicate antiferroelectric ordering below T c1.

Structure, phase transition and molecular motions in (C5H5NH)BiCl4

The crystal structure at 293 K of the new pyridinium compound, (C5H5NH)BiCl4, has been determined by X-ray diffraction as monoclinic, space group Cc, Z = 4. The crystal is built up of one-dimensional (BiCl4−)n chains and pyridinium C5H5NH+ cations. A structural phase transition of first-order type is detected by differential scanning calorimetry (DSC) and dilatometric measurements at 114/110 K (on heating–cooling, respectively). Proton NMR second moment and spin–lattice relaxation time studies confirmed the order–disorder mechanism of the phase transition at 110 K. It was connected with the reorientational motion of the pyridinium cation. Dielectric investigations reveal absorption and dispersion in the audio-frequency region in both the high and low temperature phases. The experimental results were analysed in the high temperature phase in terms of the Havriliak–Negami formula. In the low temperature phase the Cole–Cole relation for a single relaxator was applied. Above the phase transition point the macroscopic relaxation time exhibits non-Arrhenius behaviour, whereas below Tc it is perfectly described by a pure Arrhenius relation.

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

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 A, Z = 4, V = 3188 A3. 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 polarizin...

Structure and phase transitions in the ferroelastic [C(NH2)3]3Bi2Br9 crystal

Differential scanning calorimetry, dilatometric, dielectric and linear birefringence measurements have been used to study the ferroelastic [C(NH2)3]3Bi2Br9 crystal. The x-ray studies showed that it crystallizes at room temperature in the monoclinic symmetry, space group P21/m. The crystal undergoes a complex sequence of phase transitions: at 311 K, 333.5 K, 350 K, 415 K and 425 K. All phase transitions were found to be of first order type. The ferroelastic domain structure is maintained from room temperature up to 425 K. The temperature measurements of the linear birefringence and optical observations suggest the tetragonal symmetry of the parent paraelastic phase above 425 K.

Calorimetric, dielectric, infrared spectra and thermal expansion studies of structural phase transitions in ((CH3)2CHNH3)2MX5 (M=Sb, Bi; X=Cl, Br) crystals

Differential scanning calorimetric, thermal expansion, dielectric dispersion, infrared and preliminary x-ray diffraction studies on ((CH3)2CHNH3)2MX5 (M=Sb, Bi; X=Cl, Br) crystals are reported. All the studied salts are isomorphous, crystallizing in monoclinic symmetry. They undergo low-temperature structural phase transitions of first and second-order type: ((CH3)2CHNH3)2SbBr5 at 171 and 180 K, ((CH3)2CHNH3)2BiCl5 at 164 K, ((CH3)2CHNH3)2BiBr5 at 155 and 133 K. Dielectric dispersion studies in all isopropylammonium crystals between 100 Hz and 1 MHz reveal a low-frequency relaxation process described by the Cole-Cole formula with alpha approximately=0.15-0.20 in the phase transition temperature region. All phase transitions are likely to be due to the motion of the isopropylammonium cations.

The structure, phase transition and molecular dynamics of [C(NH2)3]3[Sb2Br9]

The crystal structures of [C(NH2)3]3[Sb2Br9] (Gu3Sb2Br9) at 300 K and of [C(NH2)3]3[Sb2Cl9] (Gu3Sb2Cl9) at 90 and 300 K are determined. The compounds crystallize in the monoclinic space group: C 2/c. The structure is composed of Sb2X93− (X = Cl, Br) ions, which form two-dimensional layers through the crystal, and guanidinium cations. In Gu3Sb2Br9 the structural phase transformation of the first-order type is detected at 435/450 K (on cooling/heating) by the DSC and dilatometric techniques. The dielectric relaxation process in the frequency range between 75 kHz and 5 MHz over the low temperature phase indicates reorientations of weakly distorted guanidinium cations. The proton 1H NMR second-moment and spin–lattice relaxation time, T1, temperature runs for the polycrystalline Gu3Sb2Br9 sample indicate a complex cation motion. A significant dynamical non-equivalence of two guanidinium cations was found. The possible mechanism of the phase transition in Gu3Sb2Br9 is discussed on the basis of the results presented.

Crystal structure and phase transitions of [(C2H5)4N]6Bi8Cl30

Abstract A new [(C 2 H 5 ) 4 N] 6 Bi 8 Cl 30 crystal of the family of alkylammonium halogenobismuthates was grown. X-ray diffraction studies showed that the crystals are monoclinic, space group C 2/ m with a = 20.117(5), b = 12.682(3), c = 20.396(5) A, β = 93.03(3), Z =2. The lattice consists of (C 2 H 5 ) 4 N + cations and a new type of Bi 8 Cl 6− 30 anion. Dielectric studies revealed two closely-lying structural phase transitions around 241 K (on cooling). They were interpreted as due to a freezing of the rotational motions of tetraethylammonium cations.

Crystal structure and X-ray investigation of phase transition of tetraethylammonium tetrachloroantimonate N(C2H5)4SbCl4

Abstract The anionic sublattice of N(C2H5)4SbCl4 consists of isolated Sb4Cl4- 16 units built of four SbCl3- 6 distorted octahedra. There are two crystallographically inequivalent disordered tetraethylammonium cations in crystal structure. The disorder is realized by presence of two positions for all methylene carbon atoms. N(C2H5)4SbCl4 has at 298 K orthorhombic Fddd space group with a = 12.824 (3), b = 24.050 (4), c = 39.739 (4) A, V = 12256 A3, Z = 32. Refinement of the atomic parameters by the least square methods gave R = 0.040, wR = 0.036 for 1830 independent reflections with F > 4[sgrave](F). Temperature dependencies of lattice parameters between 170 K and 310 K were determined. The presence of the phase transition of first order at Tc = 272 K was confirmed. The broadening and then splitting of the reflections below 272 K was observed which indicates that this transition take place to phase of lower than orthorhombic symmetry.

Structure, phase transitions and molecular motions in ferroelastic (C4H8NH2)SbCl6·(C4H8NH2)Cl

The crystal structure at 293 K of the new pyrrolidinium chloroantimonate (V) analogue, (C4H8NH2)SbCl6(C4H8NH2)Cl, has been determined by x-ray diffraction as monoclinic, space group P21/c, Z = 8. The crystal is built up of isolated SbCl6- anions, two types of inequivalent pyrrolidinium cation and isolated Cl- ions. It undergoes five solid-solid phase transitions: at 351/374 K of first-order type (cooling/heating, respectively), at 356 and 152 K second order and at 135/141 and 105/134 K first order, detected by differential scanning calorimetry, dilatometric and dielectric measurements. The ferroelastic domain structure appears between 152 and 135 K. The proton nuclear magnetic resonance second moment and spin-lattice relaxation time of polycrystalline samples were studied over the temperature range 27-410 K. The order-disorder mechanism of the phase transitions at 105 and 374 K connected with the reorientational motion of the pyrrolidinium cations has been confirmed.