Structural investigation of the phase transitions of Tribromo ammonium mercurate (II) monohydrate, NH4HgBr3·H2O

Single crystal diffraction, Infrared spectroscopy and differential scanning calorimetry DSC techniques have been used to investigate the different phases of NH₄HgBr₃·H₂O, Tribromo ammonium mercurate (II) monohydrate, from room temperature to 120 K. Two anomalies in thermal behaviour were detected for this compound at 198 and 340 K, by DSC experiment. X-ray diffraction measurements confirm the presence of the first anomaly. At room temperature NH₄HgBr₃·H₂O, crystallizes in the orthorhombic space group Cmc2₁, with the lattice constants a = 4.475(1) Å, b = 17.226(2) Å, c = 10.240(2) Å and Z = 4. Below 200 K the structure is monoclinic P2₁ with: a = 4.379(4) Å, b = 17.220(10) Å, c = 10.103(2) Å, β = 90.023(9)° and Z = 2 (T = 120 K). The mercury atom is surrounded by four bromine atoms in an irregular tetrahedron. The tetrahedra are linked at two corners, resulting in infinite chains along the “a” axis. The ammonium groups are located between the chains ensuring the stability of the structure by hydrogen bonding contacts: NHBr, OHBr, NHO. The structural phase transformation was attributed to an orientational disorder of ammonium groups.     

Parallelogram-like microparticles of calcium dihydrogen phosphate monohydrate (Ca(H2PO4)2·H2O) obtained by a rapid precipitation route in aqueous and acetone media

Calcium dihydrogenphosphate monohydrate (Ca(H₂PO₄)₂·H₂O) was prepared by a rapid and simple precipitation method using CaCO₃ and H₃PO₄ in aqueous and acetone media at ambient temperature. The thermal transformation of the synthesized Ca(H₂PO₄)₂·H₂O at 500 °C was obtained to be CaP₂O₆ occurred through the dehydration and the phosphate condensation reactions, as revealed by thermoanalytical techniques (TG/DTG/DTA). The synthesized Ca(H₂PO₄)₂·H₂O and its decomposition product CaP₂O₆ were characterized by X-ray powder diffraction (XRD), Fourier transfer infrared (FTIR) spectroscopy and scanning electron microscope (SEM). Thermal behavior and the morphology of the synthesized Ca(H₂PO₄)₂·H₂O in aqueous and acetone media are compared with those of other works. The SEM micrograph of Ca(H₂PO₄)₂·H₂O show parallelogram-like microparticles containing small and large grain sizes. The aqueous and acetone media are proposed to play an important role in the synthetic process of calcium phosphates in exhibiting different physical properties, which are important for specific applications.     

Spectroscopy, magnetism and non-radiative processes in heteronuclear Cu:Pr squarate; [Pr2Cu(C4O4)4(H2O)16]·2H2O

In the present work, the spectroscopic and magnetic properties of heteronuclear Cu:Pr squarate are reported. Single crystals of [Pr₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O were obtained by reaction of squaric acid, praseodymium chloride and copper chloride in water solution according to the procedure described earlier. The crystals of title compound are isomorphic with [La₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O crystal, where squarate anions participate as bridging ligands between metal ions.

The UV region of absorption spectra of the title compound is dominated by C–T band of Cu(II), f–d transition of Pr(III) and internal π–π*(A_1g→E_u) and π–π*(A_1g→E_g) ligand transitions. In visible and IR regions, t_2g–e_g of copper Cu(II) as well as ³H₄→³P_J, ¹D₂, ¹G₄, ³F_J, ³H₆ Pr(III) transitions at 293 and 4 K were recorded. At low temperature splitting given by Jahn–Teller effect can be observed. Significant anisotropy of d–d transitions intensities confirms well the Jahn–Teller effect, too. Unexpectedly high intensity of ³H₄→¹G₄ transition is probably due to the intensity borrowing from the Cu (II) d–d transition.

The ³P₀ and ¹D₂ emission of Pr(III) in the [Pr₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O crystals is quenched even at 77 K. Whereas emission of appropriate polynuclear europium squarate was detected. The pathways of excited state quenching by e_g levels of Cu(II), multhiphonon relaxation and concentration quenching can be considered in the system under studies. Magnetic susceptibility measurements were carried out in 300–1.7 K temperature range and are discussed in relation to the structure.

Effect of the polymeric structure on spectroscopic behaviour is presented. Selectivity of polymeric europium squarate in vitro test for different tumor cells is shown.     

Transport anisotropy and pseudo-gap state in oxygen deficient ReBa2Cu3O7−δ (Re = Y, Ho) single crystals

The temperature dependence of the transport anisotropy of ReBa₂Cu₃O_7−δ (Re = Y, Ho) single crystals is investigated. We experimentally determine the parallel and perpendicular conductivity in oxygen deficient single crystals. It is found that, in these single crystals, oxygen deficiency results to uneven oxygen distribution within their volume that leads to the formation of superconducting phases with different critical temperatures. We present an analysis regarding the agreement of the experimental data with the predictions of different theoretical models. It is determined that, the absolute values of the energy gaps along and perpendicular to the basis plane are changed, with different signs of their derivatives. When the value of Δ_c increases, the value of the pseudo-gap decreases and vice versa, testifying that the PG regime is suppressed, with a synchronous strengthening of the localization effects. In distinction to YBa₂Cu₃O_7−δ, the temperature dependence of the anisotropy of the resistivity ρ_c/ρ_ab(T) for the HoBa₂Cu₃O_7−δ samples is well described, by the universal “law of 1/2” for the thermally activated hopping conductivity.     

Optical absorption and emission spectra of Tb in hexagonal crystals of Na3[Tb(pyridine-2,6-dicarboxylate)3] · NaClO4 · 10H2O

We have measured and analyzed the polarized orthoaxial luminescence and absorption spectra of Tb³⁺ and Eu³⁺ in single crystals of trigonal Na₃[Ln(dpa)₃]·NaClO₄·10H₂O (dpa = 2,6-pyridinedicarboxylate, the dianion of dipicolinic acid). Differential linear polarization (σvs. π) is measured in the orthoaxial luminescence and absorption spectra. A semiempirically derived energy level scheme is developed for the crystal field components of the Tb³⁺ (⁷F_j (J = 0–6), ⁶D₄, ⁶D₃, ⁵G₈, ⁶L₁₀, ⁶D₂) ⁶G₅) and the Eu³⁺ (⁷F_j (J = 0–6), ⁶D_j (J = 0–3) multiplets.     

A mass-spectroscopical study of the decomposition of Ti(NMe2)4 in a mixed Ar–H2–N2 pulsed d.c. plasma

The favourable gas-phase conditions for deposition of TiN have been determined by a mass-spectroscopic investigation of the gaseous species in an ambient of tetrakis(dimethyl-amino)titanium (TDMAT) molecules during pulsed d.c. plasma-enhanced deposition processes. The gas-phase composition was varied, at a pressure of 0.4 Torr, a temperature of 350 °C and a bias voltage of 500 V, based on an Ar, H₂, N₂ ternary diagram. The results reveal that hydrogen plays a key role in the cleavage of –NMe₂ from the central Ti atom. The addition of N₂ to the hydrogen plasma opens up the possibility for transamination reactions by NH_x formation (1<x<3), known by thermal CVD using NH₃. This addition also leads to powder formation, which seems to reach a maximum within a 100% N₂ plasma. In a nitrogen plasma, only relatively small amounts of gaseous species, like HCN, NH₂CN, and CH₃CN, are detected, which indicates that residual hydrocarbon fragments of TDMAT must be incorporated into the powder and coating. Even small amounts of Ar addition to a hydrogen plasma convert TDMAT to powder particles, which is the opposite of the densification purpose of Ar bombarment. No gaseous species, apart from small amounts of HCN, are detected, suggesting hydrocarbon-containing coatings. If Ar:H₂:N₂=1:1:1, no specific mechanism is dominant under the conditions used here. Decreasing the deposition temperature and pressure and increasing the bias voltage seem to favour the cleavage of –NMe₂ ligands.     

6,7LiX(X=H, D, T)_H2O潮解反应过程中氢同位素效应：量子化学研究

采用MP2方法对LiD/LiH/LiT+H2O反应机制进行了理论研究。计算获得的焓变化和反应速率常数与实验数据和其他理论值一致。研究结果表明LiD+H2O反应存在两个反应通道,通道1是主要的反应通道。虽然反应中同位素键发生破裂,但是动力学同位素效应不是非常明显,这与主级同位素效应的理论极大值不同。在动力学同位素效应的贡献中ηvib>ηrot>ηtun>ηpot>ηtrans,这表明同位素效应的主要贡献来自振动因子。