Magnetic susceptibility studies of (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2

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.     

Dielectric dispersion in the incommensurate phases of [N(CH3)4]2CoCl4 and [N(CH3)4]2ZnCl4

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 T_c. The dielectric relaxation of both substances could be described by a polydispersive process β = 0.74 in the vicinity of T_c. However, for the temperature region of (T?T_c) > 0.6 for [N(CH₃)₄]₂CoCl₄ the dielectric absorption seems to be rather monodispersive.     

Birefringence,X-ray and neutron diffraction measurements on the structural phase transitions of (CH3NH3)2 MnCl4 and (CH3NH3)2FeCl4

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 $\text{I4}\text{mmm}$ 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.     

Brillouin scattering of N(CH3)4 ZnCl4

The elastic behaviour with temperature near the commensurate incommensurate phase transition of [N (CH₃)₄ Zn Cl₄] have been investigated by Brillouin scattering and compared to ultrasonic results. Elastic and attenuation anomalies observed at 8 MHz are not found at Brillouin frequency. Photoelastic coefficients have been measured.     

H NMR study of N(CH3)4H(ClF2CCOO)2

The proton spin–lattice relaxation times and ¹H NMR second moments were measured over a wide range of temperature. The results were compared with those of the ¹⁹F NMR relaxation that we obtained earlier. For both nuclear species, the evolution of the longitudinal magnetizations with time is observed to be strongly bi-exponential and were in good quantitative agreement with the cross-relaxation theory.     

Crystal growth, spectroscopic and crystal field studies of [N(CH3)4]2MnCl4 and [N(CH3)4]2CoCl4 single crystals in the paraelectric phase

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.     

Low temperature far IR spectra of (CH34)N NiCl3, (CH3)4N NiBr3 and (CD3)4N NiBr3

The known phase transition in (CH₃)₄N NiCl₃, at 171 K, has been characterised by far IR spectroscopy. The transition is explained as due to a formation of weak labile C-H,…, Cl hydrogen bonds at low temperatures, restricting the “free” internal rotations of the methyl groups and perhaps at the same time ordering the orientations of the tetramethylammonium ions. No similar transition in (CH₃)₄N NiBr₃ was found.     

Magnetic susceptibility of (IPA)2FeCl4, (IBuA)2FeCl4 and (IPA)2CuCi4 (IPA = isopropylammonium ion and IBuA = isobutylammonium ion)

The magnetic susceptibility of (IPA)₂FeCl₄, (IBuA)₂FeCl₄ and (IPA)₂CuCl₄, where IPA = (CH₃)₂CHNH⁺₃ and IBuA = (CH₃)₂CHCH₂NH⁺₃, were measured in the temperature range form 80 K up to a temperature near the melting points of the samples. The results obtained were interpreted in termw of a canted two-dimensional antiferromagnet. The variation of the magnetic susceptibility with the change in the composition was also discussed.     

Magnetic resonances in (CH3NH3)2MnCl4

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.     

Magneto-optical measurement on the layer type magnet (CH2)2(ND3)2MnCl4

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 T_N. In theffield dependent measurements the spin flop at H_SF = 33.6 ± 1 kOe (T = 4K) could clearly be detected.     

Magnetic susceptibility of (CH3NH3)2FeCl3Br: An example of a canted spin system

Magnetic susceptibility of (CH₃NH₃)₂FeCl₃Br is measured in the temperature range 80–200°K. A sharp peak characteristic of a spin canted system is found at T_N(H=0)=98° K. Magnetic field dependence of the susceptibility and the effect of the halide ion size on the canted spin is discussed.     

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

The dielectric permittivity (?) of TMA-ZC single crystals was measured along the mean crystallographic axes a, b and c, in a temperature range from 273 to 340 K. The ?-T relationship exhibited peak values at T_i=296 K and T_c1=279 K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j-E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.     

Electron paramagnetic resonance of gamma irradiated [(CH3)4N]InCl4 and [(CH3)4N]2CdCl4 single crystals

Gamma irradiated [(CH₃)₄N]InCl₄ and [(CH₃)₄N]₂CdCl₄ single crystals were investigated by electron paramagnetic resonance at ambient temperature, and it has been found that both compounds indicate the existence of (CH₃)₃N⁺ radicals. The g factors were found to be isotropic, and the hyperfine constant for H atoms was measured as 2.86 mT and is isotropic for this radical in these substances. The hyperfine coupling constant of the N nucleus with the hole in (CH₃)₃N⁺ in [(CH₃)₄N]InCl₄ was found to be anisotropic with the A_zz=2.92, A_yy=1.62 and A_xx=1.40 mT. From these, it has been revealed that the C_3v-axis of (CH₃)₃N⁺ radical performs rotational or jumping reorientational motions around a fixed axis, in addition to the rotations of protons in CH₃ groups and the rotational motions of CH₃ groups around the C_3v-axis of the radical. The g, and the hyperfine coupling factors of the N nucleus were isotropic in (CH₃)₃N⁺ in [(CH₃)₄N]₂CdCl₄. This indicates the motional behaviour of the radical in this compound is as in a liquid. This isotropic behaviour of the hyperfine coupling constants was found to be same until the attainable lowest temperature of 113 K in our laboratory.     

Dichroism of (CH3NH3)2CdCl4 in the farinfrared

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.     

A study on the structural phase transitions of ferroelastic [N(CH3)4]2CuCl4 from microhardness measurements

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.     

Electron paramagnetic resonance of Mn IN (NH4)2Co2(SO4)3 and (NH4)2Ni2(SO4)3 single crystals

EPR studies are reported on single crystals of ammonium cobalt sulphate and ammonium nickel sulphate containing Mn²⁺ ions. In each case only one magnetic complex of Mn²⁺ ion is found. The resonance lines in the case of Mn²⁺ doped ammonium nickle sulphate are characterised by a strong angular dependence of line intensities. The resonance lines in both the cases are fitted to a spin-Hamiltonian corresponding to orthorhombic symmetry.     

Microwave spectrum of (CH3)3CCN-SO3

Eight rotational transitions of the complex (CH₃)₃CCN-SO₃ have been recorded using pulsed-nozzle Fourier transform microwave spectroscopy and a series of ab initio calculations has been performed. The complex is a symmetric top with free or nearly free internal rotation of the SO₃ and (CH₃)₃CCN subunits. The nitrogen-sulfur bond distance is determined to be 2.394(19) Å. Calculations at the MP2/aug-cc-pVTZ level/basis, which are in excellent agreement with the experimental results, give a binding energy of 11.0 kcal/mol relative to (CH₃)₃CCN and SO₃. Physical properties of the system, including N-S bond length, N-S-O angle, binding energy, and the degree of electron transfer (obtained from Townes and Dailey analysis of the ¹⁴N nuclear quadrupole coupling constant) are compared with those of similar complexes. The proton affinity of the base is a useful parameter for ordering complexes in the series.     

Nuclear magnetic resonance in [N(CH3)4]2CoCl4 single crystals: transferred hyperfine interaction and spin-lattice relaxation rate

The molecular susceptibility and paramagnetic shift of [N(CH₃)₄]₂CoCl₄ single crystals were measured, and from these experimental results we obtained the transferred hyperfine interaction, H_hf, due to the transfer of spin density from Co²⁺ ions to [N(CH₃)₄]⁺ ions. The transferred hyperfine interaction can be expressed as a linear equation, with H_hf increasing with increasing temperature. The remarkable change in H_hf near T_c5 (=192 K) corresponds to a phase transition. The proton spin-lattice relaxation times of [N(CH₃)₄]₂CoCl₄ single crystals were also investigated, and it was found that the relaxation process can be described by a single exponential function. The variation of the relaxation time with temperature undergoes a remarkable change near T_c5, confirming the presence of a phase transition at that temperature. From the above results, we conclude that the increase in H_hf with increasing temperature is large enough to allow the transfer of spin density between Co²⁺ ions and the nuclear spins of the nonmagnetic [N(CH₃)₄]⁺ ions in the lattice, and thus the increase in the relaxation time with temperature is attributed to an increase in the transferred hyperfine field.     

Impurity-induced magnetic order in the mixture of two spin gap systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3

The ground state of the solid solution of the two spin gap systems (CH₃)₂CHNH₃CuCl₃ and (CH₃)₂CHNH₃CuBr₃ has been investigated by ¹H NMR. The existence of a magnetic ordering in the sample with the Cl-content x=0.85 was clearly demonstrated by a drastic splitting in a resonance line at low temperatures below T_N=13.5 K. The observed NMR spectra in the ordered state was qualitatively consistent with the simple antiferromagnetic state.