Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH3NH3)2MnCl4, from 10 to 300k

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 ΔH_t = 727±5 J mol^?1 and ΔS_t = 7.76±0.05 J K^?1 mol^?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔH_t = 116±1J mol^?1 and ΔS_t = 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.     

Anomalous states of the crystal structure of (Rb0.1(NH4)0.9)2SO4 solid solutions in the temperature range 4.2–300 K

Crystals of (Rb_0.1(NH₄)_0.9)₂SO₄ solid solutions are studied using x-ray diffractometry. It is revealed that the temperature dependence of the lattice parameter a exhibits an anomalous behavior, namely, the “invar effect” at temperatures above the ferroelectric phase transition point T _c and an anomalous increase in the temperature range from T _c to the liquid-helium temperature. An anomalous increase in the lattice parameter a and an increase in the intensity of Bragg reflections with a decrease in the temperature are interpreted within the model of the coexistence of two sublattices hypothetically responsible for the ferroelectric phase transition. A series of superstructure reflections observed along the basis axes corresponds to a sublattice formed in the matrix of the host structure. Analysis of the ratio between the lattice parameters of these structures allows the inference that, in the temperature range 4.2–300 K, the structure of the crystal under investigation can be considered an incommensurate single-crystal composite.     

Lattice mediated interactions and ferroelectric anomalies in the crystal (CH3NH3)5Bi2Cl11 (PMACB)

Abstract

A two-sublattice compressible pseudospin model has been constructed to describe the behaviour of the spontaneous polarisation of PMACB in the temperature region comprising the ferroelectric phase transition (T=307K) and the isomorphous anomaly (T= 180 K). A strong coupling of pseudospins with a “pancake” strain ?(2ε₃₃ ? ε₁₁ - ε₂₂) /3 resulting from the comparison of the model with the experimental spontaneous polarisation agrees with the existing data on the dielectric susceptibility, and on the specific heat. The ferroelectric phase transition then turns out to be close to a tricritical point. The numerical values of the parameters of the model show how the ferroelastic phase transition (T=260K) can occur in this crystal without any discernible effect on the specific heat and on the dielectric susceptibility.     

Experimental study of the energy dependence of the reaction rate coefficients of the reaction CH 3 + with N2O and NH3

The kinetics of the reaction of CH ₃ ⁺ ions with N₂O and NH₃ has been investigated in He and Ar buffers using Selected Ion Flow Drift Tube technique (SIFDT). Both studied reactions proceed with nearly collisional rate at near thermal energies. The rate coefficient of the reaction of CH ₃ ⁺ with N₂O is decreasing more than one order of magnitude (from 1.2×10^?9cm³s^?1 up to 8×10^?11cm³s^?1) with reactant ion/reactant neutral average centre-of-mass kinetic energy (E _r) increasing from near thermal up to 2 eV. The dominant product of this reaction is the isomer HCO⁺ (≥94%) and the minor product is CH₃O⁺. The reaction of CH ₃ ⁺ with NH₃ has two binary channels with the dominant product ion CH₂NH ₂ ⁺ (>70%) and the minor product ion NH ₄ ⁺ (≈10%) and three body association channel with product CH₃NH ₃ ⁺ (≈20% at 0.32 Torr). The rate coefficient of this reaction is decreasing nearly by one order of magnitude with increasingE _r from near thermal up to 0.8eV, forE _r>0.8 eV the rate coefficient increases with increasingE _r. The experimental results are interpreted in terms of a simple model assuming the reactions to proceed via the formation of long-lived collision complexes.     

Thermodynamic properties of (NH4)2KGaF6 elpasolite

The heat capacity of (NH₄)₂KGaF₆ elpasolite is measured in the temperature range from 80 to 350 K. A sequence of three phase transitions at T ₁=288.5 K, T ₂=250 K, and T ₃=244.5 K is revealed, and the thermodynamic characteristics of these transitions are determined. The influence of hydrostatic pressure on the phase transition temperature is investigated. The results obtained are discussed within the model of orientational ordering of NH ₄ ⁺ and GaF ₆ ^3? ionic groups.     

PMR studies of molecular motions,phase transitions and quantum tunnelling in NH4SnCl3 and N(CH3)4SnCl3

Molecular reorientation and low temperature relaxation effects of NH⁺ ₄ ion and the effect of CH₃ substitution (in place of H) are investigated by proton spin lattice relaxation time (T₁) measurements at 10 MHz in NH₄SnCl₃ and N(CH₃)₄SnCl₃ in the temperature range 4.2 K upto the melting points of the compounds (? 440 K). Phase transitions around 360 K in NH₄SnCl₃ and around 361 and 116K in N(CH₃)₄SnCl₃ have been observed. In NH₄SnCl₃, the high temperature minimum at 330.5 K is attributed to the translational diffusion of the NH⁺ ₄ ions, while the other T₁, minima at 103.5, 60 and 50 K are ascribed to the reorientations of the NH⁺ ₄ ion about the C₂ and C₃ axes. The low temperature minimum at 13.5 K is attributed to rotational tunnelling of the NH⁺ ₄ ions. In N(CH₃)₄SnCl₃, in addition to the high temperature minima at 212.2 and 182.6 K due to N(CH₃)₄ tumbling and CH₃ reorientation, a temperature independent T₁ behaviour between 83 and 31 K is observed, below which T₁ decreases and tends to go through a minimum around 5 K. This low temperature minimum is attributed to rotational tunnelling of the CH₃ groups. The motional parameters and tunnel frequencies are estimated.     

A new ferroelectric compound: Methylammonium trichloromercurate (CH3NH3HgCl3)

Measurements of the dielectric constants revealed a ferro- to paraelectric transition in methylammonium trichloromercurate (CH₃NH₃HgCl₃). Thermal analysis by differential scanning calorimetry indicates a transition temperature of Tc ≈ 60°C. The structures of both phases were determined from single crystal X-ray measurements. The ferroelectric phase crystallizes at room temperature in the polar trigonal space group $\text{P}\text{3}{}_2\text{(a = b =}\text{7.8117(3)}\text{, c =}\text{9.826(3)}\text{A}\text{?}\text{, Z =}\text{3}\text{)}$. The refinement of the ferroelectric structure included the fractional contribution of the two domains present. The paraelectric phase has monoclinic symmetry (space group C2 with $\text{a =}\text{13.816(2)}\text{, b =}\text{7.880(1)}\text{, c =}\text{9.734(3)}\text{A}\text{?}\text{, β =}\text{90.49(5)°}\text{, Z =}\text{6}\text{)}$ and contains an almost completely disordered methylammonium group while order with pronounced thermal motion is observed in the ferroelectric phase.     

Structural characterization,thermal, vibrational properties and molecular motions in perovskite-type diaminopropanetetrachlorocadmate NH3(CH2)3NH3CdCl4 crystal

The work presents a detailed analysis of the sequencing of the structural phase transitions in NH₃(CH₂)₃NH₃CdCl₄ crystal by differential scanning calorimetry (DSC), X-ray, infrared, far infrared and Raman spectroscopy. DSC studies have shown that in analyzed crystal occurring one reversible continuous phase transition at 375/374 K (on heating/cooling). Observed in Nujol and Fluorolube mulls in the wide temperature range between 296 K and 413 K spectral changes through the structural phase transition can be attributed to an onset of motion of cations. An assignment of some bands due to internal modes has been also proposed.     

Effect of electron irradiation on the heat capacity of [NH2(CH3)2]2 · CuCl4 in the phase transition region

The specific heat of crystalline [NH₂(CH₃)₂]₂ · CuCl₄ in the ferroelectric phase, both nonirradiated and irradiated by electrons, was measured calorimetrically. The temperature region of existence of the ferroelectric phase was shown to broaden under electron irradiation. The existence of an incommensurate phase above the Curie point T _c1 in the crystal was confirmed. The phase-transition sequence observed in [NH₂(CH₃)₂]₂ · CuCl₄ is shown to be described by a phenomenological model for A ₂ BX ₄-type ferroelectrics with an organic cation.     

An EPR study of a triplet state center in (NH4)2SO4

Heating ammonium sulphate to near its dissociation temperature and then quenching to room temperature produces a stable triplet state center that is identified as NH₂⁺. The symmetry of the EPR spectra indicates that the center is located in two non equivalent sites with each of these sites having slightly different EPR parameters and two measurably differents orientations. Thus the center is apparently occupying the positions of the NH₄⁺ groups in the perfect lattice. A study is made of this center both at room temperature and also well below the ferroelectric phase transition of (NH₄)₂SO₄. It is found that above and below the transition temperature (?50°C) there is very little temperature dependence of the EPR spectra but at the phase transition there is an abrupt change in the spectra. The number of detectable centers doubles below the phase transition indicating that the inversion symmetry of the NH₂⁺ sites is eliminated at the transition temperature.     

81Br NQR Study of [NH3(CH2) n NH3]CdBr4 (n = 4 and 5) and [NH3(CH2) n NH3]ZnBr4 (n = 5 and 6)

⁸¹Br NQR frequencies and differential scanning calorimetry (DSC) were measured as a function of temperature. [NH₃(CH₂)₄ NH₃]CdBr₄ (1) and [NH₃(CH₂)₅NH₃]CdBr₄ (2) showed a doublet and quartet ⁸¹Br NQR spectrum, respectively. [NH₃(CH₂)₅NH₃]ZnBr₄ (3) and [NH₃(CH₂)₆NH₃]ZnBr₄ (4) exhibited a four-line ⁸¹Br NQR spectrum. From the NQR results, it is inferred that (1) and (2) consist of infinite two-dimensional sheets of corner-sharing CdBr₆ octahedra, whereas (3) and (4) have isolated [ZnBr₄]²⁻ tetrahedra. All of the crystals except (1) showed at least one structural phase transition above 380 K.     

Br NQR relaxation and successive phase transitions of CH3NH3HgBr3

The measurement of ⁸¹Br NQR in CH₃NH₃HgBr₃ has been carried out in the temperature range between 80 and 300 K using a pulse NQR method. The temperature dependence of ⁸¹Br NQR frequencies in CH₃NH₃HgBr₃ has revealed that it undergoes three characteristic successive phase transitions at T?=?123, 184 and 239 K. The phase transition temperature at T?=?239 K is the second-order type, whereas those at T?=?184 and 123 K are the first-order nature of the phase transitions. Each phase transition seems to be closely related to the motions of methyl ammonium cation as a partial or whole. The enhancement of 1/T ₁ at T?=?239 K indicates the onset of the molecular motion of the cation as a whole with increasing temperatures.     

Thermal and optical properties of the ferroelectric (C3N2H5)5Bi2Cl11 crystal

Thermal (specific heat) and optical (linear birefringence) studies were performed for a new ferroelectric crystal (C₃N₂H₅)₅Bi₂Cl₁₁. Two phase transitions were confirmed and described. The first-order paraelastic-ferroelastic phase transition at 360 K was studied with a polarizing microscope. The continuous second-order phase transition at 165 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of an order of magnitude higher than the closely related ferroelectric crystal; MAPCB — (CH₃NH₃)₅Bi₂Cl₁₁. Thermal parameters (such as the excess enthalpy and the excess entropy ) of the continuous transition were estimated and discussed. The ‘two-site’ model describing the motion of three of the five imidazolium cations, which is proposed from the structural studies, is fully confirmed by the data from the ac-calorimetric measurements.     

The effect of hydrostatic pressure on the ferroelectric phase transition in [NH2(CH3)2]3[Sb2Cl9]

The effect of hydrostatic pressure on the ferroelectric phase transition temperature in [NH₂(CH₃)₂]₃[Sb₂Cl₉] (DMACA) has been studied by electric permittivity measurements at pressures up to 400 MPa. The pressure-temperature phase diagram is given. The phase transition temperature (T_c) increases with increasing pressure up to 150 MPa, passes through a maximum and then decreases with a further increase of pressure. The unexpected nonlinear decrease in T_c with pressure increasing above 150 MPa suggests that the mechanism of ferroelectric phase transition in DMACA is different from hitherto assumed.     

Structural aspects of the dielectric anomaly in SC(NH2)2 crystals at 161 K

The residual incommensurate modulations in the ferroelectric phase of the SC(NH₂)₂ thiourea are investigated using single-crystal x-ray diffractometry. It is found that the structural states of the ferroelectric phase (T<169 K) depend on the thermal prehistory of the crystal in the temperature range of existence of the incommensurate phase (202-169 K). The dielectric anomaly observed at T _x=161 K is justified structurally.     

Dielectric measurements,X-ray diffraction and raman studies of mixed caesium-ammonium mercury chloride: Cs0.7(NH4)0.3HgCl3

Measurements of the dielectric constants have revealed a transition at T=449K in Cs_0.7(NH₄)_0.3HgCl₃. This transition was confirmed by X-ray diffraction and Raman scattering on polycrystalline samples. The room-temperature phase is ordered and exhibits trigonal symmetry (space group P3₂ with the unit cell dimensions a = 13.295(11) Å; c = 9.419(8) Å). Transport properties in this material appear to be due to the high mobility of NH⁺ ₄ andCs⁺.     

Phase Transition in the Compound NH3(CH2)5NH3TlCl5

Penthylene diammonium pentachlorothallate(III) exhibits a phase transition at 316 K <artwork name="GPHT21157eu1"> This transition has been characterized by optical birefringence, dielectric measurements differential scanning calorimetry and spectroscopic measurements on polycrystalline samples. The space group and the cell parameters of phase I were determined by X-ray diffraction from single crystals. Phase I has space group P2₁2₁2₁, with Z = 4, a = 7.696(3), b = 13.2890(17) and c = 13.503(18) Å. The transition is both displacive and order-disorder involving mainly conformational changes of the organic chain [NH₃(CH₂)₅NH₃]²⁺ coupled with distortion of the TlCl₆ octahedra. This behaviour and the crystal dynamics will be discussed and compared with those found in the literature for homologous compounds.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

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 (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =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 (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 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; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 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; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 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.     

Dielectric properties of the nanoporous MCM-41 matrix filled with the (NH4)2SO4 ferroelectric

Variations with temperature of the linear dielectric permittivity and amplitude of the third harmonic were studied for nanoporous MCM-41 matrices with 4.0-nm channel pores filled with the (NH₄)₂SO₄ ferroelectric, in comparison with bulk ammonium sulfate. The measurements were performed upon heating and cooling in the temperature range from 100 K to room temperature. A noticeable shift to low temperatures (by approximately 25 K) for the ferroelectric phase transition in the MCM-41/(NH₄)₂SO₄ nanocomposite as compared to bulk (NH₄)₂SO₄ was revealed. The temperature hysteresis observed at the phase transition in the nanocomposite was approximately 2 K which is close to that in bulk ammonium sulfate. The significant decrease of the transition temperature in nanostructured ammonium sulfate agrees with the theoretical predictions based on the Landau and Ising models of the size effect on the ferroelectric phase transition in isolated small particles.