A new low-temperature Tcr=101 K phase transition in (NH4)3H(SeO4)2 evidenced by single crystal ESR

Temperature ESR measurements on SeO₃ - radicals in (NH₄)₃H(SeO₄)₂ single crystals disclosed new ESR anomalies at T = 101 K. Below this temperature the hyperfine splitting becomes to be temperature averaged, ESR linewidth shows motional narrowing and an anomaly in forbidden transition intensity is observed. Also the dielectric measurements show an anomaly of the ε at this temperature. We suggest an existence of a new crystal phase VI below T_cr = 101 K with NH₄-groups ordering and slow SeO₄-group dynamics.     

Dilatometric studies of phase transition in NaNH4SeO4·2H2O single crystal

Thermal expansion of NaNH₄SeO₄SeO₄2H₂O crystal was measured by means of dilatometric method along three crystallographic axes in the temperature range of 300-140 K. Anomalies of relative expansion and thermal expansion coefficients related to ferroelectric phase transition were observed and confirmed its continuous character.     

NMR Study on (CH 3 ) 2 NH 2 H 2 AsO 4

Proton NMR line width and spin-lattice relaxation time T 1 for the ferroelectric (CH 3 ) 2 NH 2 H 2 AsO 4 have been measured at the temperature range between 77 K and 300 K. The temperature dependence of T 1 is well described by the expressions based on rotation of the methyl group using the value of activation energy E a = 99 meV and elementary correlation time τ0 = 1.5 ×10 -13 sec. Moreover, a sharp dip of T 1 near ferrroelectric phase transition temperature ( T c = 274.5 K) results from the ordering of protons. In addition, the temperature dependence of T 1 deviates from the expression for molecular rotation above around 200 K. It is deduced that the deviation is caused by the suppression of some kind of proton motion.     

Thermal conductivity of systems with incommensurate phase: K2SeO4 and (NH4)2BeF4

The thermal conductivity of systems with incommensurate phase was measured in the temperature range from 300 K to 4 K. The resulting anomalous temperature dependence, in comparison to other ferroelectric systems is discussed in terms of the consequence of incommensurate phase and of the isotopic composition of the substances.     

The peculiarities of isotopic substitution in the RbD3(SeO3)2 crystal

The weakest ferroelectric RbD₃(SeO₃) as distinguished from RbH₃(SeO₃)₂ crystals after lock-in phase transition from incommensurate to commensurate phase has triclinic symmetry has been established. Theoretical-group analysis of this effect has been given.     

Phase transitions in R2 SnCl6-type crystals (R = NH3 C2H3, NH3C2H5 and N(CH3)4)

The thermal dilatation in (NH₃ ·CH₃) SnCl₆, (NH₃ · C₂H₅) SnCl₆ and [N(CH₃)] SnCl₆ was measured, and as the results it has turned out that (NH₃ ⁶·C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnCl₆ undergo the first order transitions at 128 K and 158 K, respectively. The low temperature phases of (NH · C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnC1₆ are found to be monoclinic and tetragonal, respectively, No phase transition was observed in (NH₃ ·CH₃)₂ SnCl₆ down to 77 K.     

Crystallographic phase transition of NH4ZnF3 by single crystal XRD

Below T_c=115K, the Pm3m perovskite structure of NH₄ZnF₃ is deformed by two tilts of the coupled octahedra ZnF₆, about two of its tetrad axes. The first tilt of 2.8° is due to the condensation of a M₃ soft mode, while the second one of 1.4° arise from a R₂₅ mode.     

Dielectric dispersion in [N(C2H5)4]2ZnCl4 single crystal

Temperature dependence of the complex dielectric permittivity of [N(C₂H₅)₄]₂ZnCl₄ crystal was measured in the frequency range 300 Hz / MHz. Jump-wise changes in the real and imaginary components of ε* were observed for the three crystallographic directions at the phase transition temperature. The dielectric dispersion clearly apparent in the high-temperature phase, rapidly disappears at the phase transition. The anomalous changes in the dielectric permittivity are related to vibrations of the tetraethylammonium ion. The activation energy of these vibrations amounts to 40 kJ/mol.     

Synthesis and crystal structure of mer-nitroaquatriamminenitrosylruthenium(II) nitrate [RuNO(NH3)3(NO2)(H2O)](NO3)2

The reaction between mer-[RuNO(NH₃)₃(NO₂)(OH)]Cl·0.5H₂O and an excess of 16 M HNO₃ leads to the protonation of the starting complex and crystallization of the complex with a coordinated water molecule and nitrite anion. The crystal structure of the product has been determined. The coordinated water molecule tends to be a weak acid with a measured pK_a of 2.4.     

Optical second harmonic study of the ieicommensuirate phase in K2SeO4 and (NH4)2BeF4 crystals

Second harmonic generation was studied in the region of phase transition in K₂SeO₄ and (NH₄)₂BeF₄. Symmetry-forbidden harmonic generation was observed in the incommensurate phases near T_c. The experimental results are explained under assumption that the unipolar domain-like structure with C_2v symmetry appears in the incommensurate phase.     

磁场对(C4H9NH3)2(CH3NH3)Pb2I7钙钛矿发光特性调控的研究

(C₄H₉NH₃)₂(CH₃NH₃)Pb₂I₇是一种Ruddlesden-Popper相钙钛矿材料,具有层状结构,层间通过van der Waals力结合,因此可以通过机械剥离的方法,获得其二维薄层。二维(C₄H₉NH₃)₂(CH₃NH₃)Pb₂I₇内束缚激子会自发复合从而表现出强的荧光,其圆极化率、峰位均受磁场调控。在外磁场中,荧光圆极化率与磁场呈线性关系,激子中电子和空穴之间的朗德因子差Δg约为0.43。束缚激子在磁场中受抗磁效应影响,致使其发光峰随磁场增加而红移。     

Synthesis and crystal structure of trans-nitrosoaquatetraammineruthenium(II) sulphate hydrosulphate, [Ru(NO)(NH3)4(H2O)](SO4)(HSO4)

The reaction between [RuNO(NH₃)₄OH]Cl₂ and an excess of 2 M H₂SO₄ leads to the protonation of the starting complex and crystallisation of the title complex with sulphate and hydrosulphate anions. The crystal structure of the product has been determined. The linear nitroso group and water molecule are coordinated in trans positions.     

Raman scattering investigation of the phase transitions in (NH4)2ZnBr4

Results of Raman spectroscopic studies of (NH₄)₂ZnBr₄ crystal in the spectral range from 20-250 cm^-1 and over a range of temperature from 90K to 440K covering the low temperature ferroelectric and high temperature incommensurate phases are presented. The plots of the integrated areas and peak heights of the strong Raman lines versus temperature show anomalous behaviour near the two phase transitions.     

Raman scattering in CsTiOAsO4, single crystal

The longitudinal (LO) and transverse (TO) A₁ vibrational modes have been measured between 30-1200 cm^-1 as a function of temperature (30-1240 K) for CsTiOAsO₄ (CTA). The frequencies for all corresponding Raman components shifted to lower frequencies on increasing the temperature, however, there is no typical soft-mode like behavior observed in the measured frequency range. The relative intensities of the low frequency bands increase substantially with increasing temperature due to high mobility of Cs⁺ ion. Transition to higher symmetry structure was noticed above 940K and the corresponding changes in the phonon spectra were observed.     

X-ray single crystal analysis of the crystallographic phase transition in NH4 MnF3

The perovskite structure of NH₄ MnF₃ undergoes a crystallographic phase transition at Tc=181K. This consists of rotations of the MnF₆ octahedra around two own 4-fold axes of 4.6 and 1.8° associated to condensation of the M₃ and R₂₅ soft modes, respectively. Analogous results as found previously for NH₄MnCl₃.     

Optical absorption spectrum of Co(NH4)2 (BeF4)2.6H2O complex

Optical absorption spectrum of ammonium cobalt orthofluoroberyllate hexahydrate has been investigated at the laboratory and liquid nitrogen temperatures. The gross features of the observed spectrum are characteristic of divalent cobalt ion in an octahedral field. Splittings of some of the bands at liquid nitrogen temperature are explained as due to an orthorhombic distortion.     

非线性光学晶体3BiCl3·7SC(NH2)2的合成及其转化过程

针对金属有机配合物非线性光学晶体3BiCl3?7SC(NH2)2 (DCBPB)合成过程中伴生BiCl3?3SC(NH2)2 (β-BTC)的问题,以硫脲与氯化铋为原料,在甲酸?水体系中研究了溶剂配比、反应温度、反应物配比等因素对DCBPB纯度的影响,对反应条件进行了优化,并基于XRD特征峰面积标准曲线法研究了β-BTC与DCBPB之间的转化过程。结果表明,甲酸抑制氯化铋水解且选择性生成DCBPB,氯化铋与硫脲摩尔配比大于1:3时可选择性生成DCBPB。合成过程先生成β-BTC,再由β-BTC转化为DCBPB,甲酸?水体系中氯化铋水解发生在β-BTC生成过程中,且不可避免,导致基于氯化铋直接合成只能获得最高含量为89.91wt%的DCBPB晶体。β-BTC向DCBPB转化率极高且无水解,可获得DCBPB含量大于98wt%的晶体。     

A new binuclear Cr(III) citrate anion: Synthesis,characterization and crystal structure of [Cr(CO(NH2)2)6]2[Cr2(CO(NH2)2)2(C6H4O7)2]3

The new anion [Cr₂(CO(NH₂)₂)₂(C₆H₄O₇)₂]^2 − has been isolated in the complex [Cr(CO(NH₂)₂)₆]₂[Cr₂(CO(NH₂)₂)₂(C₆H₄O₇)₂]₃. The crystals of this complex were obtained by a one-pot synthetic method using 1:1 molar ratio of hexaureachromium(III) chloride and trisodium citrate in aqueous medium. It was characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction analysis. The compound crystallizes in space group R‾3 of the trigonal system with three formula units in a cell of dimensions a = b = 24.7461(4) Å, c = 13.7204(3) Å and γ = 120°. The crystal structure comprises the columns consisting of [Cr(CO(NH₂)₂)₆]^3 + cations surrounded by a cylinder of complex anions, [Cr₂(CO(NH₂)₂)₂(C₆H₄O₇)₂]^2 − which are packed in a honeycomb-like manner. This supramolecular architecture is formed and stabilized by inter- and intramolecular NH⋯O hydrogen bonds.     

On some properties of γ-irradiated K2SeO4 single crystals

Experimental study is presented of some effects of γ-irradiation on the temperature dependence of permittivity.