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1.
(CnH2n+1NH3)2MnCl4 consist of perovskite-type layers sandwiched between hydrocarbon radical layers. New phase transitions were found between liquid nitrogen and decomposition temperature in these compounds by differential thermal analysis and hot stage microscopy. A systematic research of the lattice dynamics, the transition mechanisms, their influence on physical properties and their dependence on the various possibilities of molecular engineering offered in these quasi two-dimensional perovskites was started.  相似文献   

2.
The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔHt= 586 ± 2 J mol?1; ΔSt = 5.47 ± 0.02 J K?1mol?1) and at T =164.3 ± (ΔHt = 496 ± 7 J mol?1; ΔSt =3.29 ± 0.05 J K?1mol?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔHt= 1472.3 ± 0.1 J mol?1; ΔSt = 13.956 ± 0.001 J K?1mol?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔHt = 982 ± 4 J mol?1 ΔSt = 6.16 ± 0.02 J K? mol?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔHt = 598 ± 5 J mol?1, ΔSt = 3.85 ± 0.03 J K?1 mol?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH3CH2CH2NH3)2MnCl4 magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.  相似文献   

3.
The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH3NH3)2MnCl4, has been measured over the range 10K <T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with ΔHt = 727±5 J mol?1 and ΔSt = 7.76±0.05 J K?1 mol?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔHt = 116±1J mol?1 and ΔSt = 0.451±0.004 J K?1mol?1. These results are discussed in the light of recent measurements on (CH3NH3)2CdCl4, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type.In addition to the structural phase transitions, (CH3NH3)2MnCl4 also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.  相似文献   

4.
A second order phase transition between the space groups D182h and D174hleads to the high temperature tetragonal phase of (CH3NH3)2MnCl4. Similar transitions to tetragonal phases exist also in the ethyl- and propyl-compounds. Transition temperature increase with an increasing carbon chain length. Very low ΔH- and ΔS-values are compatible with a transition model obtained from nuclear resonance experiments. Further thermoanalytical results bear evidence on the complex role of alkyl-ammonium groups.  相似文献   

5.
We used optical birefringence, X-ray and neutron diffraction methods with single crystals to study the structural phase transitions of the perowskite-type layer structures of (CH3NH3)2MeCl4 with Me=Mn, Fe. The Mn-compound shows the following structural transitions at 394 K — a continuous order-disorder phase transition from tetragonal symmetry I4mmm to orthorhombic space group Abma (Cmca in reference 10); at 257 K — a discontinuous transition to a second tetragonal modification; at 95 K — a discontinuous transition to a monoclinic phase. For the Fe-compound the corresponding transition temperatures are 328 K and 231 K, respectively. A low temperature monoclinic phase could not be observed. The lattice parameters of the different modifications were determined as a function of temperature. The temperature dependent course of the order parameter has been investigated for the order—disorder transition. For both compounds, all the methods used gave the same value for the critical exponent of β = 0.315.  相似文献   

6.
1H nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques were employed to study the perovskite-type layered structure compound (C18H37NH3)2MnCl4 undergoing structural phase transitions. The spin relaxation was found to sensitively reflect the two-dimensional electron spin diffusion.  相似文献   

7.
The magnetic susceptibility of the layered compounds (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2 has been measured in the range 80 < T < 300 K. The results follow a Curie-Weiss behavior in the range 120 < T < 300 K but are field dependent for T < 120 K. The results are interpreted in terms of a two-dimensional antiferromagnetic interaction which is canted. A comparison with the corresponding pure chloride compounds is given.  相似文献   

8.
The crystal structures of (NH+4)Zr2(PO4)3 and (H3O+)Zr2(PO4)3 have been determined from neutron time-of-flight powder diffraction data obtained at 15 K. Both compounds are rhombohedral, R3c, with cell parameters a=8.7088(1) and c=24.2197(4) Å for the ammonium compound and a=8.7528(2), c=23.6833(11) Å for the hydronium compound. In both cases the ions are completely localized in the type I cavities and hydrogen bonded to lattice oxygens. The measured unit cell parameters are relatively large for this class of compounds but the entrance ways into the cavities are still too small to allow for unrestricted movement of the ions. Thus the low conductivity of the hydronium ion is related to this and other structural features.  相似文献   

9.
This paper describes the synthesis and characterization of organic–inorganic layered perovskite compounds, (CnH2n+1NH3)2PbI4 (n=4, 5, 7, 8 and 9). The effect of the number of carbon atoms on luminescence properties has been examined. Thin films of microcrystalline (CnH2n+1NH3)2PbI4 fabricated by spin-coating are highly oriented, with the c-axis perpendicular to the substrate surface. Temperature-dependent optical absorption spectra reveal that (CnH2n+1NH3)2PbI4 films (n=4, 7, 8 and 9) show the structural phase transitions. The excitonic structures of (CnH2n+1NH3)2PbI4 vary with the number of carbon atoms of the alkyl chain length. At low temperatures below 100 K, the lowest-energy free-exciton band of (CnH2n+1NH3)2PbI4 (n=7, 8 and 9) split into three fine-structure levels. In contrast to (CnH2n+1NH3)2PbBr4 films, (CnH2n+1NH3)2PbI4 (n=7, 8 and 9) shows no triplet exciton emission, but it shows the Stokes-shifted emission from bound excitons.  相似文献   

10.
By neutron diffraction and susceptibility measurements the crystallographic and magnetic structures of (CH3NH3)2MnCl4 and (CD3ND3)2MnCl4 have been investigated. These compounds belong to a system of quasi two-dimensional Heisenberg antiferromagnets. At 96K a structural first order phase transition from the tetragonal high-temperature structure to an orthorhombic low-temperature structure was found. A magnetic phase transition from a two-dimensional antiferromagnetic preorder to a three-dimensional magnetic order occurs at 44.5K. The three-dimensional magnetic structure is characterized by antiferromagnetic (MnCl4)2- — layers perpendicular to the c-axis, with a ferromagnetic coupling between interacting next nearest (MnCl4)2- layers. The magnetic moments of the Mn-ions lie in the antiferromagnetic planes.  相似文献   

11.
The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH3(CH2)5NH3SO4 1.5H2O (DAP-S) and NH3(CH2)9NH3SO4·H2O (DAN-S). DAP-S is monoclinic P21/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å3; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P21/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å3 and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal.  相似文献   

12.
Bulk (1 0 0) n-GaSb surfaces have been treated with a sulphur based solution ((NH4)2S/(NH4)2SO4) to which sulphur has been added, not previously reported for the passivation of GaSb surfaces. Au/n-GaSb Schottky barrier diodes (SBDs) fabricated on the treated material show significant improvement compared to that of the similar SBDs on the as-received material as evidenced by the lower ideality factor (n), higher barrier height (?b) and lower contact resistance obtained. Additionally, the reverse leakage current, although not saturating, has been reduced by almost an order of magnitude at −0.2 V. The sample surfaces were studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The native oxide, Sb–O, present on the as-received material is effectively removed on treating with ([(NH4)2S/(NH4)2SO4]+S) and (NH4)2S. Analysis of the as-received surface by XPS, prior to and after argon sputtering, suggests that the native oxide layer is ≤8.5 nm.  相似文献   

13.
(CnH2n+1NH3)2 CdCl2Br2 compounds with n≥10 show several solid-solid phase transitions before descomposition takes place at 436 K. The results show a different behaviour between odd or even members of the series studied. The sequence of phase -- transitions can be interpreted as a dynamic order-disorder mechanism followed by a melting of the alkyl chains.  相似文献   

14.
The K absorption-edge spectra of the ligand chlorine ion in square-planar complex compounds cis- and trans-[Pt(NH3)2Cl2], trans-[Pd(NH3)2Cl2], and (NH4)2PdCl4 are reported and discussed in connection with the chlorine K absorption spectra of K2PtCl4 and K2PdCl4, reported previously. The observed chemical shift of a white line at the absorption threshold is interpreted in terms of the difference of the ligand-field splitting of electronic states for metal ions. The white line is attributed to the electronic transition from the Cl? ls level to the lowest unoccupied antibonding molecular orbital (MO), which is specified by a MOb1g1) in the square-planar complex with D4h symmetry. The other absorption structures are regarded as continuum “shape resonances” of the outgoing electron trapped by the cage of the surrounding atoms. The effect of geometrical isomerism is found in the chlorine K absorption spectra of cis- and trans-[Pt(NH3)2Cl2].  相似文献   

15.
Dielectric and thermocurrent measurements have been carried out on (NH4) 3AlF6 and (NH4) 3FeF6 ceramic samples. A maximum of permittivity is observed close to the transition temperature (TT(NH4) 3AlF6 = 217K; TT(NH43FeF6 = 264K. In the low-temperature phase a polarization current of about 10-9A is obtained and can be reversed when the sign of the polarization field is changed, a property which could correspond to a ferroelectric behavior. However, no pyroelectric current is detected when the temperature decreases from TT. Another hypothesis, based on a field-induced polarization, has been considered : the depolarization current could be due to charge displacements from potential minima favored by rising temperature. In any way, the low-temperature phase is characterized by a remanent polarization.  相似文献   

16.
Single crystals [N(CH3)4]2MnCl4 and [N(CH3)4]2CoCl4 were grown by the slow evaporation technique from the super-saturated solutions. The samples obtained were undergone the X-ray and spectroscopic studies. Absorption spectra in the paraelectric phase at T=303 K have been recorded using the Shimadzu 160A double beam automatic scanning spectrophotometers. On the basis of the exchange charge model and Racah theory the crystal field parameters and Racah parameters have been calculated; all absorption bands for both crystals were given an assignment.  相似文献   

17.
A group theoretical analysis of the second-order structural phase transition in (CH3NH3)2MnCL4 at 394°K and of similar transitions in ethyl and propyl compounds (D174hD182h) was performed. The soft mode transforms according to the τx5-irreducible representation at the X-point of the Brillouin-zone boundary and its eigenvector is discussed. The transition is of the order-disorder type and is caused by a slowing down of the hindered rotation of NH3-groups. Knowing the symmetry of the order parameter, a thermodynamic potential expansion was constructed and expected anomalies in material constants around the transition temperature are briefly discussed. The high temperature phase transitions in analogous copper compounds are explained as a sequence D174hD182hD152h. The second of these phase transitions is driven by a soft mode transforming as the τY7-representation at the D182h Brillouin zone boundary.  相似文献   

18.
Middle infrared absorption, Raman scattering and proton magnetic resonance relaxation measurements were performed for [Zn(NH3)4](BF4) in order to establish relationship between the observed phase transitions and reorientational motions of the NH3 ligands and BF4 anions. The temperature dependence of spin-lattice relaxation time (T1(1H)) and of the full width at half maximum (FWHM) of the bands connected with ρr(NH3), ν2(BF4) and ν4(BF4) modes in the infrared and in the Raman spectra have shown that in the high temperature phase of [Zn(NH3)4](BF4)2 all molecular groups perform the following stochastic reorientational motions: fast (τR≈10−12 s) 120° flips of NH3 ligands about three-fold axis, fast isotropic reorientation of BF4 anions and slow (τR≈10−4 s) isotropic reorientation (“tumbling”) of the whole [Zn(NH3)4]2+ cation. Mean values of the activation energies for uniaxial reorientation of NH3 and isotropic reorientation of BF4 at phases I and II are ca. 3 kJ mol−1 and ca. 5 kJ mol−1, respectively. At phases III and IV the activation energies values for uniaxial reorientation of both NH3 and of BF4 equal to ca. 7 kJ mol−1. Nearly the same values of the activation energies, as well as of the reorientational correlation times, at phases III and IV well explain existence of the coupling between reorientational motions of NH3 and BF4. Splitting some of the infrared bands at TC2=117 K suggests reducing of crystal symmetry at this phase transition. Sudden narrowing of the bands connected with ν2(BF4), ν4(BF4) and ρr(NH3) modes at TC3=101 K implies slowing down (τR?10−10 s) of the fast uniaxial reorientational motions of the BF4 anions and NH3 ligands at this phase transition.  相似文献   

19.
The linear birefringence (LB) of the antiferromagnet (CH2)2(ND3)2MnCl4 has been measured as a function of temperature and in magnetic fields up to 100 kOe. The temperature dependence of the LB points to a pronounced two dimensional magnetic behaviour. No anomaly corresponding to the effect of three dimensional ordering could be detected at TN. In theffield dependent measurements the spin flop at HSF = 33.6 ± 1 kOe (T = 4K) could clearly be detected.  相似文献   

20.
The series of the hybrids (CnH2n+1NH3)2FeCl4, where n=2, 4, 6, 8, 10 and 12, were synthesized in HCl solutions. The optimum conditions of synthesis were investigated including the sequence of adding reactants, the stoichiometric ratio of the reactants, the reaction time and the drying procedures. The results demonstrate that the order of adding reactants, the drying temperature and drying time have little effect on the structures of the products. But for the hybrids of n?10, longer reaction time is needed to form the layered structures. The X-ray diffraction (XRD) patterns and the images of scanning electron microscopy of the products demonstrate that the hybrids crystallize well with typical layered structures. Chemical analysis of C, H and N confirms that the hybrids consist of the elements in the ratio of the molecular formulas. The d values, the interlayer distance between the two adjacent inorganic sheets of each hybrid, are obtained from the XRD patterns and they suggest that the organic chains interdigitate between the inorganic sheets. The longer the organic chains are, the more interdigitations there are. This can be explained by the different interaction strengths between the organic species when the chain lengths change. These different interactions can also explain the phenomena observed in the synthesis and the decomposing temperatures of the hybrids.  相似文献   

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