共查询到20条相似文献,搜索用时 15 毫秒
1.
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 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
2.
K. Strobel J. Frank K. Rosshirt R. Geick 《International Journal of Infrared and Millimeter Waves》1980,1(2):295-307
We have studied the
magnetic excitations of (CH3NH3)2MnCl4 in the antiferromagnetic and in the spin-flop regimes by means of magnetic resonance in the millimeter range (60–100 GHz). Rather odd line shapes of the resonance absorption line for narrow lines are explained as interference effects between the resonant and the non-resonant circular wave in the sample. For the antiferromagnetic resonance (AFMR) and for the paramagnetic resonance (EPR, above the Néel temperature), we have also studied the line width as a function of temperature. 相似文献
3.
R. Geick O. Hofmann G. Wagner 《International Journal of Infrared and Millimeter Waves》1980,1(2):285-294
The dielectric constant of (CH3NH3)2 CdCl4 has been measured as a function of temperature in the submillimeter range. Freshly grown samples with a wide domain structure at room temperature show a pronounced dichroism in this spectral range which is caused by the orientation of the molecules in the room temperature orthorhombic phase and their response to the electromagnetic wave. 相似文献
4.
E.H. Bocanegra M.J. Tello M.A. Arriandiaga H. Arend 《Solid State Communications》1975,17(10):1221-1222
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.
M.A. White N.W. Granville N.J. Davies L.A.K. Staveley 《Journal of Physics and Chemistry of Solids》1981,42(11):953-965
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
6.
Measurements of Vickers microhardness (MHv) as a function of temperature has been carried out on [N(CH3)4]2CuCl4 single crystals. The suitability of this technique for the determination of transition temperatures in the paraelastic-incommensurate→ferroelastic (commensurate) phase transitions sequence is shown. 相似文献
7.
C.C. BecerraA. Paduan-Filho 《Solid State Communications》2003,125(2):99-101
The temperature dependence of the ac susceptibility (χ) at constant applied magnetic field was investigated in the paramagnetic region of the quasi-2D ferromagnet (CH3NH3)2CuCl4. Above the Curie temperature (TC=8.85 K) a maximum in the χ(T,H) curves was observed at Tm(H). The temperature at the maximum increases with increasing applied field. This anomaly is related to short range fluctuations close the order transition. The behavior of Tm(H) is governed by the gap exponent of the scaling function (Δ=γ+β). We found Δ=2.2±0.1 in very good agreement with the previously known values of γ and β. 相似文献
8.
(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
9.
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
10.
11.
The mixed compound (CH3NH3)2Cu(Cl0.1Br0.9)4 which has random bonds of ferromagnetic and antiferromagnetic interactions has been investigated by the pulsed NMR of 1H, 63,65Cu and 79,81Br. The temperature dependence of the line width of 1H showed the existence of the magnetic phase transition at 15 K in the applied field of 4 kOe. The moments of Cu2+ have been shown to lie within the c-plane and have some randomness from the field dependence of the NMR spectrum and the spin-echo decay time of the copper and the bromine nuclei. 相似文献
12.
R. Mokhlisse M. Couzi N.B. Chanh Y. Haget C. Hauw A. Meresse 《Journal of Physics and Chemistry of Solids》1985,46(2):187-195
The structural phase transitions of (C3H7NH3)2CdCl4 (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 CrystB34, 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. 相似文献
13.
R. Jakubas 《Solid State Communications》1985,53(11):1023-1027
The complex dielectric permittivity ?(ω) of [N(CH3)4]2CoCl4 and [N(CH3)4]2ZnCl4 along the a-axis was measured between 0.35 MHz and 100 MHz. It has been found that for both substances the relaxation frequencies are about 5 MHz at Tc. The dielectric relaxation of both substances could be described by a polydispersive process β = 0.74 in the vicinity of Tc. However, for the temperature region of (T?Tc) > 0.6 for [N(CH3)4]2CoCl4 the dielectric absorption seems to be rather monodispersive. 相似文献
14.
F. Milia 《Solid State Communications》1984,51(8):625-626
The Cl35 nuclear quadrupole resonance spectra of (CH3NH3)HgCl3 have been measured between -150°C and + 100°C. The spectra clearly show that a structural phase transition of first order takes place around Tc? 60°C. The transition may be related to a disordering of the CH3NH3 groups which are reorienting both above and below Tc. The positive temperature coefficient of the Cl NQR frequency, dv/dT may be also explained by the CH3NH3 motion. 相似文献
15.
The complex electrical impedance of Na3H(SO4)2 along the bm-axis has been measured from 25°C to 316°C in the frequency range 4 kHz–40 MHz. The temperature dependence of the electrical conductivity shows remarkable changes in the temperature range 160°C–260°C. The sample crystal becomes a fast ionic conductor above 260°C. The conduction mechanisms of proton and sodium ions in the different phases are analyzed in detail with respect to the structural features of the sample crystal. 相似文献
16.
A. El-Korashy 《Solid State Communications》2005,135(5):298-303
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.
The a.c. magnetic susceptibility (χ) was measured for (C6H5CH2NH3)2CuCl4 (φC1Cl) above 4.2K, and two-consecutively anomalous behaviours of χ were observed at 6K and 6.9K. Brief discussions about these phenomena were given in relation to the two-dimensionality of φC1Cl system. 相似文献
18.
H. Shimizu N. Abe N. Kokubo N. Yasuda S. Fujimoto T. Yamaguchi S. Sawada 《Solid State Communications》1980,34(5):363-368
The hydrostatic pressure effect on the stability of phases I, II, III and IV in {N(CH3)4}2FeCl4 was studied by dielectric and DTA measurements in the temperature and pressure region of -40-30°C and 0–2 kbar. The pressure-induced new phase V found at applied pressures between 0.3 and 1.0 kbar was confirmed to be ferroelectric by the observation of D-E hysteresis loop. These results are compared with those of {N(CH3)4}2XCl4 (X: Co, Zn and Mn) in this paper. 相似文献
19.
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
20.
(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