Ultrasonic Studies of (CH3)2NH2Al(SO4)2 · 6H2O crystals irradiated by γ quanta and electrons

The temperature dependence of the longitudinal-ultrasound velocities in (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystals was studied using the echo-pulse technique in the 90–300 K range. The measurements were carried out along mutually perpendicular crystallographic directions X, Y, Z on samples both unirradiated and irradiated to various doses by γ quanta and an electron beam. The ultrasound velocity V in this crystal was shown to be anisotropic, with V _YY >V _XX >V _ZZ . The V _XX =f(T), V _YY =f(T), and V _ZZ =f(T) curves exhibit anomalies in the form of breaks at the ferroelectric phase transition (PT) at T _c1=152 K, as well as in the region of T _c2=218 K. It was established that as the irradiation dose increases, the PT temperature T _c1 decreases and the anomalies in the temperature dependences of the ultrasound velocities are smeared.     

Vibrational studies of two phosphotellurates: Te(OH)62(NH4)2HPO4 and Te(OH)6Na2HPO4· H2O

IR and Raman spectra of the two phospsotellurates Te(OH)₆2(NH₄)₂HPO₄ and Te(OH)₆Na₂HPO₄· H₂O have been analysed on the basis of vibrations of HPO₄²⁻, TeO₆, NH₄⁺ and H₂O groups. It has been found that the NH₄⁺ ion rotates freely in the crystalline lattice. The splitting of the non-degenerate v_sPO₃, mode into two components suggests the possibility of resonance interaction between the HPO²⁻₄ ions in Te(OH)₆Na₂HPO₄· H₂O. The non-existence of H₃O⁺ ion in this crystal was also noticed.     

Thermal stability and photoluminescence property of hexagonal MoO3·0.55H2O microrods

The metastable phase of well-faceted, hexagonal, prism-like molybdenum oxide hydrate (MoO₃·0.55H₂O) was successfully synthesized by evaporating molybdic acid solution prepared through cation membrane electrolysis of Na₂MoO₄·2H₂O aqueous solution. The obtained crystals were characterized by X-ray diffraction (XRD), thermogravimetric (TG), scanning electronic microscopy (SEM) and photoluminescence (PL) spectrophotometry. The as-prepared MoO₃·0.55H₂O rods were of 2–4 μm in width and 5–12 μm in length. The MoO₃·0.55H₂O microrods displayed photoluminescence properties at room temperature and were transformed into stable orthorhombic α-MoO₃ after air annealing at 380 °C. Moreover, the influence of temperature factor on the phase transformation process, morphology and photoluminescence properties of MoO₃·0.55H₂O was investigated in detail.     

Incommensurate Phases of the MgSiF6·6H2O Crystals: EPR and Group-Theoretical Studies

The group-theoretical study of the structural phase transition to incommensurate state of MgSiF₆·6H₂O crystals, revealed by the electron paramagnetic resonance (EPR) method, as well as analysis of the EPR results, are presented. The consideration of temperature dependences of Mn²⁺ admixture ion EPR spectrum symmetry and parameters leads to the conclusion that at T _i1 = 370 ± 0.3 K they undergo second-order structural phase transition to incommensurately modulated state, the order parameter of this transition may be the angle of [Mg(H₂O)₆]²⁺ octahedra rotation around crystal C ₃ axis. At temperature decreasing below T _i1 the gradual transformation of plane-wave modulation of lattice displacements into soliton mode occurs, which is interrupted by the first-order phase transition at T _i2 = 343 ± 0.3 K accompanied by abrupt decrease in modulation amplitude. At T _c = 298.5 ± 0.3 K the first-order improper ferroelastic phase transition into monoclinic phase occurs. The group-theoretical analysis of the phase transition at T _i1 in the investigated crystals, carried out for the first time, has shown that the existence of the incommensurately modulated phase is conditioned by the fundamental reasons (presence of Lifshitz invariant). The conclusions of this analysis on the nature of order parameter, the structural motifs of incommensurate phase and the possible character of temperature evolution of the structure are in agreement with the EPR investigation data.     

Structure and magnetotransport properties of the new quasi-two-dimensional molecular metal β″-(BEDT-TTF)4H3O[Fe(C2O4)3] · C6H4Cl2

The β″-(BEDT-TTF)₄A^I[M^III(C₂O₄)₃] · G(A^I=NH ₄ ⁺ , H₃O⁺, K⁺, Rb⁺; M^III=Fe, Cr; G = “guest” solvent molecule) family of layered molecular conductors with magnetic metal oxalate anions exhibits a pronounced dependence of the conducting properties on the type of neutral solvent molecules introduced into the complex anion layer. A new organic dichlorobenzene (C₆H₄Cl₂)-containing conductor of this family, namely, β″-(BEDT-TTF)₄H₃O[Fe(C₂O₄)₃] · C₆H₄Cl₂, is synthesized. The structure of the synthesized single crystals studied by X-ray diffraction is characterized by the following parameters: a = 10.421(1) Å, b= 19.991(2) Å, c= 35.441(3) Å, β = 92.87(1)°, V= 7374(1) ų, space groupC2/c, and Z = 4. In the temperature range 0.5&;2-300 K, the conductivity of the crystals is metallic without changing into a superconducting state. The magnetotransport properties of the crystals are examined in magnetic fields up to 17 T at T = 0.5 K. In fields higher than 10 T, Shubnikov-de Haas oscillations are detected, and the Fourier spectrum of these oscillations contains two frequencies with maximum amplitudes of about 80 and 375 T. The experimental results are compared with the related data obtained for other phases of this family. The possible structural mechanisms of the effect of a guest solvent molecule on the transport properties of the β″-(BEDT-TTF)₄A^I[M^III(C₂O₄)₃] · G crystals are analyzed.     

[Fe3O(Ala)6(H2O)3](ClO4)7和[Fe3O(Gly)6(H2O3)(NO3)7)·3H2O]顺磁共振谱及变温磁化率研究

本文通过对[Fe 3O(Ala) 6(H 2O) 3](ClO 4) 7和[Fe 3O(Gly) 6(H 2O) 3](NO 3) 7·3H 2O的ESR谱的解析及变温磁化率的研究,得出它们的ESR谱具有各向同性的特点;朗德因子分别为2.019和1.997;两种配合物中铁离子间有反铁磁相互作用.     

Low-temperature x-ray diffraction studies of the crystallographic parameters and thermal expansion of (CH3)2NH2Al(SO4)2 · 6H2O crystals

The a, b, c, and β crystallographic parameters of the (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal (DMAAS) have been measured by x-ray diffraction in the 90–300-K temperature range. The thermal expansion coefficients along the principal crystallographic axes α_a, α_b, and α_c have been determined. It was shown that, as the temperature is increased, the parameter α decreases and b increases, whereas c decreases for T<T _c (where T _c is the transition temperature) and increases for T>T _c, so that one observes a minimum in the c=f(T) curve in the region of the phase transition (PT) temperature T _c ～ 152 K. The thermal expansion coefficients α_a, α_b, and α_c vary in a complicated manner with increasing temperature, more specifically, α_a and α_c assume negative values at low temperatures, and the α_a=f(T), α_b=f(T), and α_c=f(T) curves exhibit anomalies at the PT point. The crystal has been found to be substantially anisotropic in thermal expansion.     

Synthesis of single crystalline (NH4)2V6O16·1.5H2O nest-like structures

Novel nest-like (NH₄)₂V₆O₁₆·1.5H₂O structures made of nanobelts have been synthesized by a facile hydrothermal approach. The powder X-ray diffraction pattern of the sample reveals the monoclinic crystalline phase of (NH₄)₂V₆O₁₆·1.5H₂O. The scanning electron microscopy images of the sample obtained at 130 °C for 3 days exhibit nest-like morphology. The transmission electron microscopy result reveals that the nanobelts have a smooth surface. The selected area electron diffraction pattern of the nanobelts indicates single crystalline nature. The two major weight losses occur in thermogravimetric analysis which correspond to the removal of water and ammonia molecules. Further, calcination of the (NH₄)₂V₆O₁₆·1.5H₂O product results in the formation of orthorhombic phase of shcherbianite V₂O₅.     

[MnHg(SCN)4(H2O)2]·2C4H9NO(MMTWD)晶体的光谱分析

根据商群对称性分析法对MMTWD晶体的振动模作了理论计算.计算结果显示,通过实验可观察到的拉曼散射峰和红外反射带应分别不超过297和148个.运用实验的手段,分别测定晶体的拉曼光谱和红外光谱,给出了MMTWD晶体在50～3 000 cm-1范围内的拉曼光谱图和400～4 000 cm-1范围里的红外光谱图,并对光谱中的谱线作了指认.给出了MMTWD晶体的分子结构示意图,最终确定MMTWD晶体结构为三维网状结构,说明在众多新型光功能材料当中MMTWD晶体是一种更适合生长成较大尺寸且稳定性较好的非线性光学晶体,在光电子领域中具有良好的应用前景.     

E.P.R. study of γ-irradiated single crystals of K2C2O4.H2O and (NH4)2C2O4.H2O

The electron paramagnetic resonance of γ-irradiated single crystals of K₂C₂O₄.H₂O and (NH₄)₂C₂O₄.H₂O has been studied. The spectra show interesting microwave power saturation effects. The singlet spectrum is attributed to the C₂O₄ ^- radical derived from the C₂O₄ ^-- ion. The principal g-values are determined to be 1·998, 2·0028 and 2·0004. Certain weak lines are observed with different power saturation and interpreted as due to OH radicals derived from water molecules in the crystal lattice.     

多金属氧酸盐α-Na6H[GeW9Fe3(H2O)3O37]·16H2O的合成和光谱

合成了多金属氧酸盐α-Na6H[GeW9Fe3(H2O)3O37]16H2O，通过元素分析、红外、紫外、光电子能谱、极谱和穆斯堡尔谱等手段进行了表征，并讨论和研究了该配合物的谱学性质。     

Dielectric relaxation in ferroelectric (CH3)2NH2AL(so4)2 · 6H2O crystal

Abstract

This paper presents the results of the investigation of dielectric dispersion and ultrasonic velocity in the ferroelectric (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal. The crystal shows a critical slowing down process of polarization with an extremely long relaxation time of the dipole system (τ = 1.6 · 10^?7s at the phase transition point). The dielectric response over the frequency range up to 56 GHz in the paraelectric phase can be well described in terms of a monodispersive Debye-type formula. The activation energy of dipoles in the paraelectric phase is 0.11 eV = 8.5 kT_c . The results show that the proper ferroelectric phase transition is nearly critical and of the order-disorder type.     

Electron paramagnetic resonance of quasi-one-dimensional crystal [Nd2(Cl3COO)6(H2O)3]n · nH2O

A quasi-one-dimensional single crystal of [Nd₂(Cl₃COO)₆(H₂O)₃]_n · nH₂O in which chains are built up of two alternating neodymium-ion dimer fragments is studied by the EPR technique. It is found that anisotropic interactions between neodymium ions in a chain are responsible for the complex shape of the EPR spectrum. Two groups of EPR signals are distinguished in the spectrum. Each group corresponds to one of the chain dimers disturbed by the interaction with neighbors in the chain. The shape of the EPR spectra is interpreted as a superposition of the spectra of chain fragments which have different lengths and are formed by the alternating magnetic triplet and nonmagnetic singlet states in the chain. Consideration is given to two cases when two alternating dimer fragments are either equivalent or nonequivalent to each other. It is shown that the spectral shape is primarily determined by the superposition of the spectra of an isolated triplet state (S=1), two interacting triplet states, and three interacting triplet states whose weighting contributions differ for the above two cases. The tensors of the anisotropic spin-spin interaction are determined, and the contribution from the isotropic component of the interaction is estimated.     

Dielectric relaxation of α-cyclodextrin-polyiodide complexes (α-cyclodextrin)2 · BaI2 · I2 · 8H2O and (α-cyclodextrin)2 · KI3 · I2 · 8H2O

The frequency and temperature dependence of the real (ε′) and imaginary (ε″) parts of the dielectric constant of the polycrystalline complexes (α-CD)₂ · Bal₂ · I₂ · 8H₂O and (α-CD)₂ · KI₃ · I₂ · 8H₂O (α-CD = α-cyclodetrin) have been investigated over the frequency and temperature ranges 0–100 kHz and 120–300 K, respectively. The temperature dependences of ε′, ε″ and the phase shift φ show two steps, two peaks and two minima, respectively, revealing the existence of two kinds of water molecule, the tightly bound and the easily movable water molecules, in both complexes. The first peak of (T) or the first minimum of φ(T) presents the transformation of flip-flop hydrogen bonds to the normal state. The second ε″ (T) peak or φ(T) minimum corresponds to the easily movable water molecules or to a partial transformation of tightly bound to easily movable water molecules. For T > 270K both samples show semiconductive behaviour with energy gaps of 1.84eV for the (α-CD)₂ · BaI₂ · I₂ · 8H₂O complex and 1.36eV for the (α-CD)₂ · KI₃ · I₂ · 8H₂O complex. The conductivity at room temperature decreases in the order: (α-CD)₂ · BaI₂ · I₂ · 8H₂O > (α-CD)₂ · LiI₃ · I₂ · 8H₂O > (α-CD)₂ · KI₃ · I₂ · 8H₂O > (α-CD)₂ · Cd_0.5 · I₅ · 26H₂O. The relaxation time varies in a Λ-like curve (from 120 to 250 K) and rises rapidly for temperatures greater than 250 K, indicating the process of ionic movements. The activation energies around the transition temperature 0.98–1.09 k _B T _trans for (α-CD)₂ · BaI₂ · I₂ · 8H₂O and 1.06-1.55 k _B T _trans for (α-CD)₂ · KI₃ · I₂ · 8H₂O reveal the greater stability of the α-K complex against that of the α-Ba complex.     

The Application of Raman Spectroscopy to the Study of the Uranyl Mineral Coconinoite Fe2Al2(UO2)2(PO4)4(SO4)(OH)2 · 20H2O

ABSTRACT

Raman spectra of the uranyl-containing mineral coconinoite, Fe₂Al₂(UO₂)₂(PO₄)₄(SO₄)(OH)₂ · 20H₂O, are presented and compared with the mineral's infrared spectra. Bands connected with (UO₂)²⁺, (PO₄)^3?, (SO₄)^2?, (OH)^?, and H₂O stretching and bending vibrations are assigned. Approximate U?O bond lengths in uranyl, (UO₂)²⁺, and O?H…O hydrogen bond lengths are calculated from the wavenumbers of the U?O stretching vibrations and (OH)^? and H₂O stretching vibrations, respectively, and compared with published data for similar natural and synthetic compounds.     

Phase transition studies in CoSiF6·6H2O and Co1−xZnxSiF6·6H2O by Mn(II) EPR

Phase transition studies on single crystals of CoSiF₆·6H₂O and Co_1−xZn_xSiF₆·6H₂O (x = 0.082, 0.182, 0.248) using Mn(II) EPR at X-band were carried out in the temperature range 10–300 K. Phase transitions with considerable thermal hysteresis have been detected in CoSiF₆·6H₂O and Co_1−xZn_xSiF₆·OH₂O (x = 0.082, 0.182). In CoSiF₆·6H₂O, the transition temperatures during cooling (T^c_c) and during heating (T^h_c) were found to be considerably less than those obtained previously from magnetic anisotropy experiments, i.e. 230 and 246 K as against 246 and 259 K. These characteristic temperatures for the dilute crystals having x = 0.082 and 0.182 were 205, 218 K and 175, 185 K, respectively. It is thus evident that magnetic dilution with zinc causes a progressive reduction in the transition temperatures as well as in the thermal hysteresis. The phase transition manifests itself in a large discontinuous increase in the Mn(II) zero-field splitting parameter (D), i.e. from −187 ± 1 G to −290 ± 1 G, in these crystals. The hyperfine coefficient (A) also increases discontinuously from 99 ± 1 G to 102 ± 1 G at T_c. In the dilute crystal with x = 0.248 a very slow continuous decrease in D and practically no change in A have been detected with a lowering of the temperature. A small temperature independent positive g-shift (g = 2.020 ± 0.005) has been observed in all the crystals studied. The spin-lattice relaxation time (T₁) of the Co(II) ion in CoSiF₆·6H₂O at different temperatures, both above and below T_c, has been estimated from the observed Mn(II) hyperfine linewidth. A discontinuous change in T₁ at T_c has been observed. Analysis of the temperature dependence of the linewidth has further revealed that a two-level Orbach process describes well the thermal behaviour of the spin-lattice relaxation of the Co(II) ions in these crystals. The location of the first excited level in CoSiF₆·6H₂O determined from such studies at temperatures both above and below T_c has been examined in the light of the ligand field energy scheme of the Co(II) ion in an octahedral field. It is concluded that the phase transition in this crystal is probably accompanied by a change in sign of the axial field parameter (Δ).