Optical properties of natural quantum-well compounds (C6H5-CnH2n-NH3)2PbBr4 (n=1-4)

This paper describes the synthesis and characterization of self-assembled organic-inorganic layered perovskite compounds, (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ (n=1-4). the effect of the number of carbon atoms of the alkyl chain length (n) on optical properties has been studied. (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ films fabricated by spin-coating are microcrystalline form, single phase and oriented with the c-axis. Crystallinity, the maximum PL intensity and the lifetime of exciton emissions varied with the number of carbon atoms. the lowest-energy exciton splits into a few fine-structure levels at low temperatures. Time-resolved photoluminescence spectra reveal that (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ shows both singlet and triplet excitons. with decreasing temperature, triplet exciton emissions become dominant for (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ (n=1-3), while (C₆H₅-C₄H₈-NH₃)₂PbBr₄ shows mainly singlet exciton emissions. The intersystem crossing from excited singlet state to triplet state plays an important role in the relaxation process of excitons.     

Quasi-two-dimensional magnetism in (CnH2n+1NH3)2CuCl4 studied by electron paramagnetic resonance

The quasi-two-dimensional magnetism in the layered transition metal compound (C_nH_2n+1NH₃)₂CuCl₄ (n=10, 14) was investigated by means of electron paramagnetic resonance (EPR) and superconducting quantum interference device measurements. As a result, the high temperature magnetic phase transitions were reflected in the EPR parameters in a sensitive manner.     

Chloride perovskite layer compounds of [NH3-(CH2)n-NH3]MnCl4 formula

[NH₃-(CH₂)_n-NH₃]MnCl₄ compounds with n = 2, …, 5 are chloride perovskite layer structures. The room temperature phase of members with odd numbers of carbon atoms is orthorhombic, whereas even numbers lead to monoclinic structures. Structural phase transitions were found in all compounds with n > 2. The magnetic behaviour is similar to the (C_nH_2n+1NH₃)₂MnCl₄-family.     

Electron impact ionization of Ar n , (H2O) n , Ar n (H2O) m clusters

Signals from ions forming in a supersonic molecular beam consisting of an argon-water vapor mixture are measured as functions of the exciting electron energy in the range to 120 eV. The thresholds of electron impact excitation of (H₂O) _{n − 1}H⁺ and Ar _n (H₂O _m ⁺ clusters are determined for the first time. It is found that the proton-hydroxyl group binding energy decreases considerably both in the case of water molecule clustering and when mixed Ar _n (H₂O) _m clusters arise.     

Synthesis and characterization of a new monophosphate [2,5-(CH3)2C6H3NH3]H2PO4

Chemical preparation, calorimetric studies, crystal structure and spectroscopic investigations are given for a new noncentrosymmetric organic cation monophosphate [2,5-(CH₃)₂C₆H₃NH₃]H₂PO₄. This compound is orthorhombic P2₁2₁2₁ with the following unit-cell parameters: a=5.872(4), b=20.984(3), c=8.465(1) Å, Z=4, V=1043.0(5) Å³ and D_x=1.396 g cm⁻³. Crystal structure has been solved and refined to R=0.048 using 2526 independent reflections. Structure can be described as an inorganic layer parallel to (a,b) planes between which organic groups [2,5-(CH₃)₂C₆H₃NH₃]⁺ are located. Multiple hydrogen bonds connecting the different entities of compound thrust upon three-dimensional network a noncentrosymmetric configuration.     

Structural mechanisms of proton conduction in MemHn(XO4)(m + n)/2 crystals

Investigations of K₃H(SO₄)₂ samples, which belong to the family of Me _m H _n (XO₄)_{(m + n)/2} (Me = K, Cs, Rb, NH₄; X = S, Se, As) crystals, revealed temperature anomalies in the dielectric and optical properties and in the heat capacity upon phase transitions. X-ray diffraction studies confirmed that K₃H(SO₄)₂ crystals undergo a structural phase transition from the monoclinic phase to the trigonal phase C2/c → R m and that the high proton conductivity in the high-temperature phases of crystals belonging to this family is due to the formation of a qualitatively new system of hydrogen bonds. Original Russian Text ? A.A. Simonov, I.P. Makarova, V.V. Grebenev, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 8, pp. 1477–1479.     

New phase transition in (CnH2n+1NH3)2MnCl4

(C_nH_2n+1NH₃)₂MnCl₄ consist of perovskite-type layers sandwiched between hydrocarbon radical layers. New phase transitions were found between liquid nitrogen and decomposition temperature in these compounds by differential thermal analysis and hot stage microscopy. A systematic research of the lattice dynamics, the transition mechanisms, their influence on physical properties and their dependence on the various possibilities of molecular engineering offered in these quasi two-dimensional perovskites was started.     

工作压强对射频辉光放电H2/C4H8等离子状态的影响

利用辉光放电技术采用等离子体质谱诊断的方法研究了不同工作 压强下H₂/C₄H₈混合气体等离子体中 主要正离子成分及其能量的变化规律, 并分析了压强对H₂/C₄H₈混合气体的离解机理以及主要正离子形成过程的影响. 结果表明: 随着工作压强的增加, 碳氢碎片离子的浓度和能量均逐渐减小. 当工作压强为5 Pa时, H₂/C₄H₈混合气体等离子体中C₃H₅⁺相对浓度最大; 压强为10 Pa时, C₃H₃⁺相对浓度最大; 压强为15, 20 Pa时, C₂H₅⁺相对浓度最大; 压强为25 Pa时, C₄H₉⁺相对浓度最大. 对H₂/C₄H₈等离子体中的主要组分及其能量分布所进行的定性分析, 将为H₂/C₄H₈混合气体辉光放电聚合物涂层的工艺参数优化提供参考技术基础. 关键词： 辉光放电技术 等离子体质谱诊断 工作压强     

Conductivity and water uptake of Sr4(Sr2Nb2)O11·nH2O and Sr4(Sr2Ta2)O11·nH2O

The hydrated oxygen deficient complex perovskite-related materials Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O were studied at high water vapour pressures over a large temperature range by electrical conductivity measurements, thermogravimetry (TG), and X-ray powder diffraction (XRPD). In humid atmospheres both materials are known to exhibit protonic conductivity below dehydration temperatures, with peak-shaped maxima at about 500 °C. In this work we show that the peaks expand to plateaus of high conductivity from 500 to 700 °C at a water vapour pressure of 1 atm. However, in situ synchrotron XRPD of Sr₄(Sr₂Nb₂)O₁₁·nH₂O as a function of temperature shows that these observations are in fact coincident with melting and dehydration of a secondary phase Sr(OH)₂. The stability of Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O in humid atmospheres is thus insufficient, causing decomposition into perovskites with lower Sr content and SrO/Sr(OH)₂ secondary phases. This, in turn, rationalizes the observation of peaks and plateaus in the conductivity of these materials.     

Crystal structure and thermal behavior of two new organic monosulfates NH3(CH2)5NH3SO4 1.5H2O and NH3(CH2)9NH3SO4 H2O

The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH₃(CH₂)₅NH₃SO₄ 1.5H₂O (DAP-S) and NH₃(CH₂)₉NH₃SO₄·H₂O (DAN-S). DAP-S is monoclinic P2₁/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å³; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P2₁/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å³ and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal.     

Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4)

A new compound, K₄(SO₄)(HSO₄)₂(H₃AsO₄) was synthesized from water solution of KHSO₄/K₃H(SO₄)₂/H₃AsO₄. This compound crystallizes in the triclinic system with space group P1¯ and cell parameters: a=8.9076(2) Å, b=10.1258(2) Å, c=10.6785(3) Å; α=72.5250(14)°, β=66.3990(13)°, γ=65.5159(13)°, V=792.74(3) Å³, Z=2 and ρ_cal=2.466 g cm⁻³. The refinement of 3760 observed reflections (I>2σ(I)) leads to R₁=0.0394 and wR₂=0.0755. The structure is characterized by SO₄²⁻, HSO₄⁻ and H₃AsO₄ tetrahedra connected by hydrogen bridge to form two types of dimer (H(16)S(3)O₄⁻?S(1)O₄²⁻ and H(12)S(2)O₄⁻?H₃AsO₄). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)-H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H₃AsO₄ group to build the chain S(1)O₄?H₃AsO₄ which are parallel to the “a” direction. The potassium cations are coordinated by eight oxygen atoms with K-O distance ranging from 2.678(2) to 3.354(2) Å.Crystals of K₄(SO₄)(HSO₄)₂(H₃AsO₄) undergo one endothermic peak at 436 K. This transition detected by differential scanning calorimetry (DSC) is also analyzed by dielectric and conductivity measurements using the impedance spectroscopy techniques. The obtained results show that this transition is protonic by nature.     

Extremely large binding energy of biexcitons in an organic-inorganic quantum-well material (C4H9NH3)2PbBr4

We have confirmed biexciton formation in an organic-inorganic hybrid quantum-well material (C₄H₉NH₃)₂PbBr₄ by photoluminescence and two-photon absorption measurements. The biexciton has extremely large binding energy, 60 meV, which to our knowledge is the largest value ever reported for a semiconductor. By analyzing the spectrum of biexciton luminescence, the biexciton gas temperature was found to be much higher than the bath temperature due to a higher local temperature arising from the large biexciton binding energy.     

Electronic photodissociation spectra of the Arn-C4H2 (n=1-4) weakly bound cationic complexes

Electronic spectra of a series of weakly bound clusters consisting of argon (Ar_n, n=1-4) bound to the butadiyne cation, C₄H₂⁺, have been recorded in the visible range from 440 to 520 nm by photodissociation. The C₄H₂⁺ fragment signal was recorded as a function of the laser wavelength during excitation of the A←X electronic transition. The observed transitions were assigned to the band origin of the cationic complexes and to vibronic bands involving excitation of the ν₃ and ν₇ vibrational modes of the C₄H₂⁺ moiety, as well as combination bands of these modes. Comparison of the photodissociation spectra of the various clusters reveals a small blue shift, 25 cm⁻¹ of the band maxima relative to the corresponding transitions reported from gas phase spectra of the bare C₄H₂⁺ cation. The magnitude of the blue shift of each band increases with successive Ar solvation up to n=3. Furthermore, each band becomes increasingly broadened towards the red with the addition of Ar atoms due to an increasing number of unresolved transitions involving excited intermolecular modes.     

Flame structure studies of rich ethylene-oxygen-argon mixtures doped with CO2, or with NH3, or with H2O

Structures of several premixed ethylene-oxygen-argon rich flat flames burning at 50 mbar have been established by using molecular beam mass spectrometry in order to investigate the effect of CO₂, or NH₃, or H₂O addition on species concentration profiles. The aim of this study is to examine the eventual changes of profiles of detected hydrocarbon intermediates which could be considered as soot precursors (C₂H₂, C₄H₂, C₅H₄, C₅H₆, C₆H₂, C₆H₄, C₆H₆, C₇H₈, C₆H₆O, C₈H₆, C₈H₈, C₉H₈ and C₁₀H₈). The comparative study has been achieved on four flames with an equivalence ratio (f) of 2.50: one without any additive (F2.50), one with 15% of CO₂ replacing the same quantity of argon (F2.50C), one with 3.3% of NH₃ in partial replacement of argon (F2.50N) and one with 13% of H₂O in replacement of the same quantity of argon (F2.50H). The four flat flames have similar final flame temperatures (1800 K).CO₂, or NH₃, or H₂O addition to the fresh gas inlet causes a shift downstream of the flame front and thus flame inhibition. Endothermic processes CO₂ + H = CO + OH and H₂O + H = H₂ + OH are responsible of the reduction of the hydrocarbon intermediates in the CO₂ and H₂O added flames through the supplementary formation of hydroxyl radicals. It has been demonstrated that such processes begin to play at the end of the flame front and becomes more efficient in the burnt gases region.The replacement of some Ar by NH₃ is responsible only for a slight decrease of the maximum mole fraction of C₂H₂, but NH₃ becomes much more efficient for C₄H₂ and C₅ to C₁₀ species. Moreover, the efficiency of NH₃ as a reducing agent of C₅ to C₁₀ intermediates is larger than that of CO₂ and H₂O for equal quantities added.     

Structure and phase transitions in chloroantimonate(V) crystals: [(C2H5)3NH]SbCl6 and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl

New triethylammonium salts: [(C₂H₅)₃NH]SbCl₆ (TCA) and [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl (TCAT) have been synthesized. The compounds crystallise in monoclinic symmetry: space groups P2₁/n and P2₁/c, for TCA at 293 K and TCAT at 100 K, respectively. The crystal structure of [(C₂H₅)₃NH]SbCl₆ consists of discrete ionic pairs—triethylammonium cations and hexachloroantimonate anions—linked via the bifurcated N-H?Cl hydrogen bonds. The crystal structure of [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl is composed of three symmetrically independent triethylammonium cations, chlorine anion and two symmetrically independent hexachloroantimonate anions. TCA undergoes a structural phase transition at 336 K (on heating) into the orthorhombic C222 space group, whereas TCAT reveals a structural phase transition at 332 K. The phase transitions are of the first order type. TCA shows a ferroelastic domain structure below 336 K. Differential scanning calorimetry, dilatometric, dielectric dispersion and Raman scattering measurements have been used to study the phase transition mechanisms in these triethylammonium salts.     

Crystal structure and ferromagnetism of (n-C3H7)4N[CoFe(dto)3] (dto=C2O2S2)

Recently, we have discovered a new type of first order phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto=C₂O₂S₂), where the charge transfer transition between Fe^II and Fe^III occurs reversibly. In order to elucidate the origin of this peculiar first order phase transition. Detailed information about the crystal structure is indispensable. We have synthesized the single crystal of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃] whose crystal structure is isomorphous to that of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and determined its detailed crystal structure. Crystal data: space group P6₃, a=b=10.044(2) Å, c=15.960(6) Å, α=β=90°, γ=120°, Z=2 (C₁₈H₂₈NS₆O₆FeCo). In this complex, we found a ferromagnetic transition at T_c=3.5 K. Moreover, on the basis of the crystal data of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃], we determined the crystal structure of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] by simulation of powder X-ray diffraction results.     

Preparation and characterization of the layered hybrids (CnH2n+1NH3)2FeCl4

The series of the hybrids (C_nH_2n+1NH₃)₂FeCl₄, where n=2, 4, 6, 8, 10 and 12, were synthesized in HCl solutions. The optimum conditions of synthesis were investigated including the sequence of adding reactants, the stoichiometric ratio of the reactants, the reaction time and the drying procedures. The results demonstrate that the order of adding reactants, the drying temperature and drying time have little effect on the structures of the products. But for the hybrids of n?10, longer reaction time is needed to form the layered structures. The X-ray diffraction (XRD) patterns and the images of scanning electron microscopy of the products demonstrate that the hybrids crystallize well with typical layered structures. Chemical analysis of C, H and N confirms that the hybrids consist of the elements in the ratio of the molecular formulas. The d values, the interlayer distance between the two adjacent inorganic sheets of each hybrid, are obtained from the XRD patterns and they suggest that the organic chains interdigitate between the inorganic sheets. The longer the organic chains are, the more interdigitations there are. This can be explained by the different interaction strengths between the organic species when the chain lengths change. These different interactions can also explain the phenomena observed in the synthesis and the decomposing temperatures of the hybrids.     

H nuclear magnetic resonance study of the ferroelastic and superionic phase transitions of (NH4)4LiH3(SO4)4 single crystals

We investigated the temperature dependences of the line shape, spin-lattice relaxation time, T₁, and spin-spin relaxation time, T₂, of the ¹H nuclei in (NH₄)₄LiH₃(SO₄)₄ single crystals. On the basis of the data obtained, we were able to distinguish the “ammonium” and “hydrogen-bond” protons in the crystals. For both the ammonium and hydrogen-bond protons in (NH₄)₄LiH₃(SO₄)₄, the curves of T₁ and T₂ versus temperature changed significantly near the ferroelastic and superionic phase transitions at T_C (=232 K) and T_S (=405 K), respectively. In particular, near T_S, the ¹H signal due to the hydrogen-bond protons abruptly narrowed and the T₂ value for these protons abruptly increased, indicating that these protons play an important role in this superionic phase transition. The marked increase in the T₂ of the hydrogen-bond protons above T_S indicates that the breaking of O-H?O bonds and the formation of new H-bonds with HSO₄^- contribute significantly to the high-temperature conductivity of (NH₄)₄LiH₃(SO₄)₄ crystals.     

Local order and structural transitions in the molecule-based layer ferrimagnet (n-C4H9)4N FeFe(C2O4)3

Local structural order and temperature-dependent structural variation have been studied in the molecular-based layer ferrimagnet (n-C₄H₉)₄N Fe^IIFe^III(C₂O₄)₃ by EXAFS and high resolution X-ray powder diffraction. The EXAFS spectra measured at the Fe K-edge are successfully modelled by successive O, C, O and metal shells, showing that even when there is extensive structural disorder due to stacking faults, the local structural order in this class of ferrimagnets is fully retained. In this salt, which shows remarkable negative magnetisation at low temperature (Néel class Q), the EXAFS Debye-Waller factor has a discontinuity at 40 K, corresponding to one found in the magnetisation. At the same temperature there is also a change in the expansion of the lattice as evidenced by the high resolution X-ray powder diffraction.     

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.