The perovskite-type layer structure compound (CH3NH3)2CdCl4 exhibits an unusual phase sequence: 






The phase transitions are governed by changes both in the motion of CH3NH3 groups and the hydrogen bonding scheme. Structural data are given for the tetragonal high-temperature and the monoclinic low-temperature phase. The whole phase sequence is discussed in detail.



Die perowskitartige Schichtstruktur-Verbindung (CH3NH3)2CdCl4 zeigt eine ungewohnliche Phasenfolge: 






Die Phasenumwandlungen werden durch Anderungen sowohl im Bewegungsverhalten der CH3NH3-Gruppen als auch im Wasserstoff-Bindungsschema bewirkt. Fur die tetragonale Hochtemperaturphase und die monokline Tieftemperaturphase werden die Strukturdaten bestimmt. Zusammenfassend wird die komplette Phasenfolge analysiert.

X-ray study of structural phase transitions in the perovskite-type layer compound (CH3NH3)2CdCl4

Perovskite type layer-structure compounds of the formulas (Cn H2n+1 NH3)2·MCl4 and NH3 (CH2)n NH3MCl4 (M = Cd, Mn, Cu, Fe, n = 1–18) exhibit a variety of structural phase transitions which are connected with the dynamics of the alkyl-ammonium or alkylenediammonium chains respectively. Two classes of phase transitions have been observed; Order-disorder transitions of the rigid hydrocarbon chains and conformational transitions of the chains. Typical examples of both classes are presented and their chain dynamics are discussed.

Structural phase transitions in perovskite layer structures

The crystal structure of (CH3NH3)2MnCl4 (MAMC) is determined by neutron diffraction at 188 K in the low temperature tetragonal phase (P42/ncm). The sequence of structural phase transitions of MAMC can be discussed in terms of NH … Cl bridges between NH3 and certain Cl−-ions. The order of the room temperature orthorhombic structure is determined by one dominant H-bond and that of the low temperature tetragonal structure by three equivalent H-bridges.



Mit Hilfe der Neutronenbeugung wird die Kristallstruktur von (CH3NH3)2MnCl4 (MAMC) bei 188 K in der tetragonalen Tieftemperaturphase (P42/ncm) bestimmt. Die Folge der strukturellen Phasenubergange von MAMC kann entsprechend den NH … Cl Brucken zwischen NH3 und bestimmten Cl−-Ionen diskutiert werden. Die Ordnung der orthorhombischen Zimmertemperaturstruktur wird durch eine dominante H-Bindung bestimmt und diejenige der tetragonalen Tieftemperaturstruktur durch drei gleichwertige H-Brucken.

On the importance of hydrogen bonding for the structural phase transitions in (CH3NH3)2MnCl4. A single‐crystal neutron diffraction study

The low-frequency (0 to 500 cm−1) Raman spectra are recorded at different temperatures between 40 and 413 K through the different structural modifications: P21/c P42/ncm Cmca I4/mmm. The results agree with previous NMR data and show that the three higher-temperature tetragonal and orthorhombic phases exhibit specific disorder with respect to the orientation of the methylammonium cation while the monoclinic low-temperature modification is ordered. A soft-mode behaviour is observed in the intermediate P42/ncm and Cmca phases, associated with the high-temperature second-order Cmca I4/mmm transition. A discussion of the unusual phase sequence in these materials is tentatively given, in view of previously proposed mechanisms for the phase transitions.



Les spectres Raman de basse frequence (0 a 500 cm−1) ont ete enregistres a differentes temperatures comprises entre 40 et 413 K, dans les differentes phases cristallines: P21/c P42/ncm Cmca I4/mmm. Les resultats sont en accord avec des donnees RMN anterieures et montrent que les trois phases de plus haute temperature (tetragonales et orthorhombique) presentent un desordre specifique dans l'orientation des cations methylammonium, alors que la phase monoclinique de basse temperature est ordonnee. Un comportement de mode mou est associe a la transition du 2e ordre Cmca I4/mmm de haute temperature observe dans les phases inter-mediaires P42/ncm et Cmca. Une discussion de la sequence de phase inhabituelle de ces materiaux est faite sur la base des mecanismes de transitions de phase proposes anterieurement.

Low‐frequency raman spectra and phase transitions in perovskite‐type layer compounds. (CH3NH3)2MnCl4 and (CH3NH3)CdCl4

Lattice dynamics of the high-Tc superconductor La2−xMxCuO4

The authors have measured the phonon dispersion curves of the fluoride perovskite materials KMnF3 and KZnF3 by means of coherent neutron inelastic scattering. The aim of this investigation was to provide the phonon spectrum of KMnF3 which undergoes structural phase transitions and which has been studied by various methods in the last decade and, for comparison, of KZnF3 in which no phase transitions occur. In addition they have found an anomalous temperature dependence of the R15' and M2 modes in KZnF3 which are the modes that soften near the phase transitions of KMnF3. This indicates that there is in general a tendency towards an instability against octahedra rotations in all fluoride perovskites. For the analysis of the phonon spectra, they have employed a lattice dynamical shell model under various conditions. In some of the models, they have successfully used interionic potentials which were derived from experimental data in other materials for some of the short-range interactions. Thus it could be shown that these potentials are also valid in the fluoride perovskites with different interionic distances. Furthermore, the experimental data are compared with the results of infrared spectroscopy and Raman studies at low temperatures as a test of the assignment of optically determined phonon frequencies to normal modes which are at the zone centre Gamma at low temperatures and at zone-boundary points in the cubic perovskite structure.

http://iopscience.iop.org/article/10.1088/0022-3719/15/32/015/pdf

Lattice dynamics, lattice instabilities and phase transitions in fluoride perovskites

The far-infrared reflectivity of (CH3NH3)3MnCl4(=MAMC) and of (CH3NH3)2FeCl4(=MAFC) single crystals was studied at several temperatures. By means of a Kramers-Kronig analysis, the frequencies of the long-wavelength infrared-active phonons were determined. Because of the considerable gap (300-900 cm-1) between the external and the internal modes of the organic ions these can be treated as rigid bodies at far-infrared frequencies. In addition, the authors present the results of a structure analysis of MAMC above and below the continuous phase transition at 394K which turns out to be of the order-disorder type. In the high-temperature tetragonal modification, three strong and two weak IR-active modes are observed for E perpendicular to c.

G. Heger

Papers

Lattice dynamics, lattice instabilities and phase transitions in fluoride perovskites

Lattice dynamics in perovskite-type layer structures. FIR studies of (CH3NH3)2MnCl4 and (CH3NH3)2FeCl4