Perovskite-type Layer Compound Research Articles

Subsolidus phase diagram of binary system in the perovskite type layer compounds (n-CnH2n+1NH3)2MnCl4

The thermotropic phase transitions in the perovskite type layer compound (n-C10H21NH3)2MnCl4 and (n-C14H29NH3)2MnCl4 were synthesized and, at the same time, a series of their mixtures C10Mn-C14Mn were prepared. The experimental binary phase diagram of C10Mn-C14Mn was established by differential thermal analysis (DTA), IR and X-ray diffraction. In the phase diagram new material (n-C10H21NH3)(n-C14H29NH3)MnCl4 and two eutectoid invariants were observed, two eutectic points temperatures are about 29.8 and 27.9°C. Contrasting other similar system, there are three noticeable solid solution ranges (α, β, γ) at the left and right boundary and middle of the phase diagram.

Subsolidus phase diagram of binary system of thermotropic phase transitions compounds ( n-C nH 2 n+1 NH 3) 2MnCl 4 ( n = 12, 14, 16)

The thermotropic phase transitions in the perovskite type layer compound ( n-C n H 2 n+1 NH 3) 2 MnCl 4 ( n = 12, 14, 16) were studied and a series of their mixtures were prepared. The low temperature crystal structures of the pure salts are characteristic of the piling of sandwiches in which a two-dimensional macro-anion MnCl 4 2− is sandwiched between two alkylammonium layers. These layers become conformationally disordered in the high temperature phases. The subsolidus binary phase diagrams of ( n-C 12H 25NH 3) 2MnCl 4–( n-C 14H 29NH 3) 2MnCl 4 and ( n-C 14H 29NH 3) 2MnCl 4–( n-C 16H 33NH 3) 2MnCl 4 were established by differential thermal analysis (DTA) and X-ray diffraction (XRD). In each phase diagram, an intermediate compound and two eutectoid invariants were observed. There are three noticeable solid solution ranges ( α, β, γ) at the left boundary, right boundary and middle of the phase diagram.

Nonisothermal Kinetics of the Solid‐Solid Phase Transitions in the Perovskite Type Layer Compounds (CnH2n+1NH3)2ZnCl4

AbstractNonisothermal kinetics of the solid‐solid phase transitions in the perovskite type layer compounds (CnH2n+1NH3)2ZnCl4 has been determined for the hydrocarbon chain length from even number 12 to 18 of carbon atoms. The activation energy Ea was calculated by the Kissinger and Ozawa methods from the curves of Differential Scanning Calorimetry (DSC) measurements at different heating rates. The results indicated that the values of Ea of the phase transitions decreased with increasing chain length and showed a linear dependence on the number of carbon atoms, which was caused by that the order and rigidity of the hydrocarbon chain were decreased with increasing the carbon atoms. The two methods are in agreement with each other. The reaction order n of all materials is one that remains constant independently of the heating rate.

Phase transition in the bidimensional compound [C6H5(CH2)2NH3]2CdCl4

Bis phenyl 2 ethylammonium lead tetrachloride ([C6H5(CH2)2NH3]2CdCl4) is a new member of the family of perovskite-type layer compounds. It exhibits a structural phase transition This transition is characterised by DSC measurements, X-ray diffraction, dielectric measurements and Raman scattering on polycrystalline samples. The space groups and the cell parameters of phases I and II were determined by X-ray diffraction on single crystals and powder samples. The phases I and II have the same space group Pcab with Z = 4. The transition is also of first order and involves both organic action motions and CdCl6 octahedral deformations. This transitional behaviour and the crystal dynamics will be discussed and compared with those found in the literature for homologous compounds.

Comparative Studies of Phase Transitions of Alkyl Chains in (C6H13NH3)2PbCl4, (C9H19NH3)2PbCl4 and (C9H19NH3)2CdCl4

Structural phase transitions have been studied in the perovskite-type layer compounds (C 6 H 13 NH 3 ) 2 PbCl 4 and (C 9 H 19 NH 3 ) 2 PbCl 4 using differential scanning calorimetry, X-ray diffraction and spectroscopic techniques. The results show that these structural phase transitions are related to the dynamics of the alkylammonium ions and to deformation of PbCl 6 octahedra. The phases stable at low temperature are ordered and the alkylammonium chains are in extended configuration with a gauche bond in the proximity of the NH 3 group. The minor transition in these compounds corresponds to the formation of gauche forms (G) in the vicinity of the methyl group. The major transition in these two lead compounds has been attributed to chain melting. The two structural phase transitions in the perovskite layer structure compound (C 9 H 19 NH 3 ) 2 CdCl 4 have been studied by DSC anti infrared spectroscopic techniques. The main transition corresponds to the onset of chain melting at the same time as the dynamic twofold rotational disordering of the chains. The minor transition corresponds to a further melting of the chains. From the study of (C 6 H 13 NH 3 ) 2 PbCl 4 , (C 9 H 19 NH 3 ) 2 PbCl 4 and (C 9 H 19 NH 3 ) 2 CdCl 4 , it is suggested that the number of phase transitions and their nature depend on the length of the alkylammonium chains and on the nature of the metal atom.

Differential scanning calorimetric x-ray diffraction and spectroscopic studies of phase transitions in the bidimensional compound

Four crystalline phases (called III, II, and I) have been observed in the perovskite-type layer compound using differential scanning calorimetry, x-ray diffraction and spectroscopic studies. The crystallographic evolution with increasing temperature appears to be triclinic . An intermediate disordered phase II', stable in a narrow temperature range and structurally similar to phase II, has also been observed, such that the transformation proceeds, in fact, two steps . Phase III, stable at low temperature, has an ordered structure. The phase transition , which is of first-order type, corresponds to an order - disorder mechanism involving the organic part of the structure (alkylammonium chains) and deformation of octahedra. The phase transition , which is of second-order type, is related to the arrangement of octahedra of perovskite layers. The transition is of first-order type and involves conformational disorder of the chains. In phase I chain melting sets in.

Differential scanning calorimetric, x-ray diffraction, and spectroscopic studies of phase transitions in the two-dimensional compound (C6H13NH3)2PbCl4

Five structural phase transitions have been observed in the perovskite-type layer compound (C6H13NH3)2PbCL4 using differential scanning calorimetry, X-ray diffraction, and vibrational analysis. Phase IV, stable at low temperature, has an ordered structure. The phase transition IV → III', which is of first-order type, corresponds to an order-disorder mechanism involving the organic part of the structure (alkylammonium chains) coupled with deformation of PbCl6 octahedra. The phase transition III' → III, which is of second-order type, is related to the arrangement of the mineral matrix (octahedra of perovskite layers). The sequence of transitions III → II' → II is characterized by a weak thermal anomaly corresponding to a second-order or nearly second-order transformation. It is suggested that phase II' could be incommensurate. The II → I transition is of first-order type and involves conformational disorder of the chains. In the phase I chain melting sets in, it mainly consists in the introduction of kinks GTG' in the alkylammonium ions.

X-ray and Raman studies of the re-entrant phase and phase transitions in the perovskite-type layer compound bis(n-propylammonium) lead tetrabromide

Bis(n-propylammonium) lead tetrabromide (C3H7NH3)2PbBr4) is a new member of the family of perovskite-type layer compounds. It exhibits two structural phase transitions according to the phase sequence: phi (172 K) to to epsilon (283 K) to delta . These transitions are characterized by x-ray diffraction and Raman scattering. The epsilon -phase is embedded between two isomorphic phases phi , and delta ; the latter exhibits monoclinic cell symmetry, and the space group is P21/a (Z=2). The room-temperature phase delta is disordered. (p is the most ordered phase. The epsilon to phi transition is characterized by a noticeable decrease in the interlayer distance and a distortion of the PbBr6 octahedra. This transitional behaviour will be discussed and compared with those found in the literature for homologous compounds; the re-entrant phenomenon and other points of similarity suggest for instance an incommensurate structure for the epsilon -phase.

Phase transition behavior in the perovskite-type layer compound (n-C 12H 25NH 3) 2CuCl 4

The solid state phase transitions in the perovskite-type layer compound ( n-C 12H 25NH 3 2CuCl 4, have been studied by X-ray diffraction, differential scanning calorimetry and i.r. spectroscopy. Two-step reversible phase transitions are observed at T c 1 = 327.9 K and T c 2 = 335.7 K on heating whereas at T c 1 = 324.4 K and T c 2= 330.1 K on cooling, accompanied with the increase of interlayer distance ( Δ d 1≅ 1.8 A ̊ and Δ d 2≅ 0.85 A ̊ , respectively). In the first transition important melting of the hydrocarbon chains occurs together with the flipping of polar NH 3 groups. Defects of the gt 3 g′ and end gauche forms are found. The weakening of the NH-Cl hydrogen bonds is also observed. Further conformational changes of the hydrocarbon part are dominantly observed in the second phase transition. As the interlayer spacing is increased, only short gtg ' kinks are allowed. The mechanism of conformational transition for this compound can be therefore suggested as follows: gt 9 or tgt 8 → 2g − 2 kink (−gt 3g′−) → 2g −1 kink (−gtg′−) .

DSC, X-ray diffraction and Raman studies of high-temperature phase transitions in the perovskite-type layer compound butylenediammonium lead tetrachloride

Butylenediammonium lead tetrachloride exhibits three structural phase transitions according to the sequence I-(324 K) to gamma to (341 K) to beta to (420 K) to alpha . These transitions are characterized by DSC measurements and Raman scattering on polycrystalline samples. The space groups and the cell parameters of I, gamma and beta phases were determined by X-ray diffraction on single crystals and powder samples. The three phases I, gamma and beta have the same space group P21/c with Z = 4. The first-order phase transition at 324 K is driven by orientational and conformational motions of the butylenediammonium chains. The transition at 341 K is also of first-order type and involves both organic cation motions and PbCl6 octahedra deformations. The transition at 420 K was evidenced by Raman scattering; it leads to a plastic phase similar to that observed in the homologous monoammonium compound (C3H7NH3)2PbCl4. This transitional behaviour and the crystal dynamics will be discussed and compared with those of homologous compounds.

Raman scattering in K1-xRbxAlF4: evidence of a new phase

A Raman scattering study of the quasi-reversible structural phase transitions observed below room temperature in the perovskite-type layer compounds K1-xRbxAlF4 with x=0.06, 0.09 and 0.15 is reported. The Raman spectra were analysed with the help of the result of a calculation of the phonon spectrum in the ideal (aristotypic) phase (phase I, P4/mmm symmetry). For all the studied compounds, the spectra collected at room temperature are consistent with the P4/mbm symmetry (phase II), resulting from the condensation of the M3 mode. The phase IIIa Raman spectra for x=0.06 and x=0.09 are explained with the P21/m space group and the phase II-phase IIIa transition can be imputed to the condensation of two components of the degenerate X13 mode plus one of the components of the degenerate M19 mode. The Raman spectra of the K0.85Rb0.15AlF4 compound, on lowering the temperature, reveal the existence of a new phase (IIIb) between phase II and phase IIIa An assignment to vibrational modes, in this phase IIIb, is possible in the Pmmn space group and so the phase IIIa-phase IIIb transition could be attributed to the condensation of the X13 mode like in pure RbAlF4. The non-reversible martensitic phase transition of the pure compound KAlF4 is no longer observed for x=0.15.

An infrared spectroscopic study of the structural phase transition in the perovskite-type layer compound [ n-C 16H 33NH 3] 2CoCl 4

The solid-solid phase transitions in the perovskite-type layer compound [ n-C 16H 33NH 3] 2CoCl 4 have been studied by infrared spectroscopy. A new phase transition at 340 K was found by comparison with differential scanning calorimetry results. A temperature dependence study of the infrared spectra provides evidence of the occurrence of structural phase transitions related to the dynamics of the alkylammonium ions and hydrogen bonds. The main transition at 374 K corresponds to the conformational order-disorder change in the chain, which probably couples with reorientational motions of the NH 3 polar heads. GTG or GTG′ defects appear in the high temperature disordered phase.

Electronic structure of tetragonal BaK2-xBaxBi2O7 and related layer-type bismuthates.

The electronic structure of body-centered-tetragonal ${\mathrm{BaK}}_{2}$${\mathrm{Bi}}_{2}$${\mathrm{O}}_{7}$, an ordered prototype of the recently synthesized perovskite-type layer compound ${\mathrm{BaK}}_{1.3}$${\mathrm{Ba}}_{0.7}$${\mathrm{Bi}}_{2}$${\mathrm{O}}_{7}$, has been calculated with the use of the linear augmented-plane-wave method. In rigid-band terms, the results predict metallic properties for the entire ${\mathrm{BaK}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{Bi}}_{2}$${\mathrm{O}}_{7}$ alloy series, with ${\mathit{E}}_{\mathit{F}}$(x) rising with increasing x to gradually fill a pair of nearly two-dimensional (sp\ensuremath{\sigma}${)}^{\mathrm{*}}$ conduction bands. For compositions xg0.8, the onset for filling the upper (sp\ensuremath{\sigma}${)}^{\mathrm{*}}$ subband produces a sharp increase in the density of states. The resulting value (per Bi) is about 50% greater than that for cubic ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{K}}_{\mathit{y}}$${\mathrm{BiO}}_{3}$ near the composition (y\ensuremath{\approxeq}0.3--0.4) where ${\mathit{T}}_{\mathit{c}}$ (\ensuremath{\sim}30 K) is a maximum. This suggests that a slightly increased Ba composition (x\ensuremath{\sim}0.8) should provide more favorable conditions for possible superconductivity in these ${\mathrm{BaK}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{Bi}}_{2}$${\mathrm{O}}_{7}$ alloys than those predicted for the nonsuperconducting x\ensuremath{\sim}0.7 phase that has been prepared thus far.

An infrared spectroscopic study of the structural phase transition in the perovskite-type layer compound [ n-C 16H 33NH 3] 2CoCl 4

The solid-solid phase transitions in the perovskite-type layer compound [ n-C 16H 33NH 3] 2CoCl 4 have been studied by infrared spectroscopy. The new phase transition at 340 K was found by the comparison of DSC results. A temperature dependence study of the i.r. spectra provides the evidence for the occurrence of a structural phase transition related to the dynamics of the alkylammonium ions and hydrogen bonds. The main transition at 374 K corresponds to the conformational order-disorder change of the chain which probably couples with reorientational motions of the NH 3 polar heads. The GTG or GTG' defects appear in the high temperature disordered phase.

The Jahn-Teller effect and orientational order in (CnH2n+1NH3)2CuCl4, n=1, 2, 3

Crystal optical and X-ray diffraction studies show that the high-temperature phase I of the perovskite-type layer-compounds (CnH2n+1NH3)2CuCl4 with n=1, 2, 3 is Jahn-Teller distorted orthorhombic with space group Bbcm (D2h18). As a consequence, the order of the CnH2n+1NH3 group leads to two configurations, realised in a monoclinic phase II with space group P21/c (C2h5) and an orthorhombic phase III with Pbca (D2h15). The transition sequences found above 300 K depend on n: I from Tcl=348 K to II for n=1; I from Tcl=364 K from II from Tc2h=356 K, Tc2c=338 K to III for n=2; and I from Tcl=434 K to III for n=3. The domain phenomena at Tcl and Tc2 point to a subtle balance between the tilt and order configurations of phases II and III. Birefringence and lattice constraints indicate a very loose inter-layer contact in phase I.

A Phenomenological Theory of the Incommensurate Phase Transition in (C3H7NH3)2MnCl4

The model for the reentrant phase transition, including an incommensurate phase as the sandwiched phase, is substantiated for the phase transition sequence in the perovskite-type layer compound bis-n-propylammonium tetrachloromanganate (C 3 H 7 NH 3 ) 2 MnCl 4 (PAMC). By detailed considerations of the symmetry aspect of the phase transitions concerned, a thermodynamic potential functional is obtained to reproduce the phase sequence, where the pseudo-Lifshitz invariant plays an important role.

Polymorphism in the Bidimensional compound ( n-C 16H 33NH 3) 2CdCl 4

Four crystalline phases have been observed in the perovskite-type layer compound ( n-C 16H 33NH 3) 2CdCl 4 using X-ray diffraction, differential scanning calorimetry and vibrational analysis. Phase IV, stable at low temperature, has a monoclinic ordered structure. The two intermediate phases III and II are orthorhombic and the high temperature form I is tetragonal. In the last case, the diffraction pattern shows the presence of a diffuse scattering phenomenon corresponding to the bidimensional melting of only the organic part of the structure (C 16H 33NH 3 chains). Entropy values of the order-disorder process are best interpreted by a disorder similar to that of the molten state of the n-paraffins. The largest enthalpies are measured at the (IV-III) and (II-I) transitions; they correspond to the two discontinuities of the interlayer spacing and are related to a modification of the organic chain length due to the occurrence of kink defects ( GT 2 n+1 G′), and a decrease of the number of trans forms between the G and G′ bonds, respectively.

Crystal optical investigations of (C3H7NH3)2CuCl4

Abstract The temperature dependence of the optical birefringence within the cleavage plane of the perovskite-type layer compound (C3H7NH3)2CuCl4 was measured in the region 290-450 K. The tetragonal-to-orthorhombic α - (3 ferroelastic phase transition occurs at T1=438 K and is of distinct first-order character. The α = (3 phase boundary could be observed. In the range ∼ 379 = ∼ 408 K, Δn(T) curve exhibits clear deviation from regular dependence which is similar to the anomalous birefringence behaviour connected with the well-known incommensurate reent-rant γ-phase of the manganium analogue (C3H7NH3)2MnCl4. The principal refractive indices were also measured.

On the incommensurate re-entrant high-symmetry phase sequence in (C3H7NH3)2MnCl4. I. Microscopic considerations, model calculations and some macroscopic experiments

The perovskite-type layer compound (C3H7NH3)2MnCl4 exhibits an unusual phase sequence with two incommensurate phases. The high-temperature incommensurate phase is the only one known where the amplitude of the modulation wave vanishes on lowering the temperature and the high-symmetry phase re-enters. Microscopic considerations indicate that incommensurability and re-entrant behaviour are due to the thermally activated disorder of the propylammonium chains. A previous model free energy is discussed and calculations of the thermal expansion, heat capacity and elastic modulus are compared with experiments. The model describes well the main features of the phase sequence. However, it does not account for the elastic instability connected with the re-entrant phase transition.

Raman scattering and X-ray diffraction study of structural phase transitions in the perovskite-type layer compound (C 3H 7NH 3) 2CdCl 4

The structural phase transitions of (C 3H 7NH 3) 2CdCl 4 (PACC) have been studied by means of Raman scattering. X-ray diffraction and DSC measurements It is shown that the order-disorder phase transition Abma ↔ Pbca occurs at 156 K and not at 183 K as previously proposed by Chapuis ( Acta Cryst B34, 1506 (1978)) Apparently, the transition at 183 K does not change the Abma space group; it is suggested that it could be related to the occurrence of an incommensurate phase Another structural transformation of PACC is detected at 114 K, but no conclusion can be given as far as the structure of the low-temperature phase is not determined.