EPR lineshape study of the incommensurate phase in γ-irradiated K2SeO4

The temperature dependence of the SeO^4-₄ EPR frequencies and the asymmetric broadening of the EPR lines in the incommensurate phase of K₂SeO₄ can be explained by an incommensurate spatial modulation of the g tensors which corresponds to the “broad” phase soliton limit. A comparison between the experimental and calculated lineshape shows a ≈1% volume fraction of commensurate regions in the middle of the incommensurate phase at 110 K.     

Transferred hyperfine field of Rb2CoCl4 single crystals in the ferroelectric-incommensurate-normal phase by Rb NMR

The molecular susceptibility and paramagnetic shift of Rb₂CoCl₄ single crystals grown using the slow evaporation method were measured, and from these experimental results we obtained the transferred hyperfine interaction due to the transfer of spin density from Co²⁺ ions to Rb⁺ ions. The transferred hyperfine field was obtained for the ferroelectric, incommensurate, and normal phases. In the case of Rb(I), the transferred hyperfine interaction decreases with increasing temperature in the incommensurate phase, and increases with increasing temperature in the normal phase. The value of H_hf in the incommensurate and normal phases increases abruptly with increasing temperature in the case of Rb(II). These results indicate that the effects due to the transfer of spin density from Co²⁺ ions to the Rb(I) and Rb(II) ions are large above T_i. In particular, the effect due to the transfer of spin density to Rb(II) ions in the normal phase is very large; the variations with temperature of the transferred hyperfine interactions of the Rb(I) and Rb(II) nuclei are more or less continuous in T_c1 and T_i, and are not affected by the ferroelectric-incommensurate-normal phase transitions.     

87Rb NMR lineshape study of the incommensurate phase in Rb2ZnBr4

On going from the paraelectric (P) to the incommensurate (I) phase the ⁸⁷Rb $\text{1}\text{2}$ → $\text{1}\text{2}$ NMR lines in Rb₂ZnBr₄ broaden into a continuum which is limited by two edge singularities. The observed line shape and its temperature dependence are well described by the frequency distribution function predicted by the “plane wave” modulation model of the I phase. The anomalous temperature dependence of T₁ shows the presence of the phason branch in the I phase.     

Brillouin scattering of N(CH3)4 ZnCl4

The elastic behaviour with temperature near the commensurate incommensurate phase transition of [N (CH₃)₄ Zn Cl₄] have been investigated by Brillouin scattering and compared to ultrasonic results. Elastic and attenuation anomalies observed at 8 MHz are not found at Brillouin frequency. Photoelastic coefficients have been measured.     

Thermal unpinning of the modulation wave near TI in incommensurate Rb2ZnCl4 detected by 35Cl NQR

Thermal unpinning and floating of the incommensurate modulation wave produces in Rb₂ZnCl₄ close to the incommensurate-paraelectric transition T_I a partial motional averaging of the splitting between the two edge singularities in the ³⁵Cl nuclear quadrupole resonance spectrum. The mean square phase fluctuations are close to T_I proportional to (T_I ? T)^?2β with β = 0.39 ± 0.04.     

The effect of pressure on the spin density wave transition in the layered cobaltites [Ca2CoO3]0.62[CoO2] and [Ca2Co4/3Cu2/3O4]0.62[CoO2]

In order to investigate the pressure effect on the magnetism in the layered cobaltites, positive muon spin rotation and relaxation μ⁺SR experiments have been carried out up to 1.3 GPa using c-aligned polycrystalline samples of [Ca₂CoO₃]_0.62[CoO₂] and [Ca₂Co_4/3Cu_2/3O₄]_0.62[CoO₂]. A transverse field μ⁺SR experiment indicates that the transition temperature to an incommensurate spin density wave IC-SDW state is independent of hydrostatic pressure up to 1.3 GPa for the both compounds. Furthermore, there are no changes in the spontanious muon precession frequency in zero field at 5 K even under 1.3 GPa. These results strongly suggest that the IC-SDW exists not in the rocksalt-type block ([Ca₂CoO₃] and/or [Ca₂Co_4/3Cu_2/3O₄]) but in the CoO₂ plane.     

Soft modes below the incommensurate transition in K2SeO4

A coordinated series of Raman and Brillouin scattering measurements were carried out in K₂SeO₄ with emphasis on temperatures near and below the incommensurate structural phase transition (T_i=129 K). Below the lock-in phase transition (T_c=99 K) Raman scattering from both the amplitude mode and the phase mode analog was observed. However, only the amplitude mode could be followed into the incommensurate phase. Details of the amplitude mode frequency and damping are reported for temperatures considerably closer to T_i than previously possible. Our results do not confirm the diverging linewidth extrapolated from earlier Raman work. We also report here the first Brillouin scattering results in K₂SeO₄. Although substancial interaction effects between the acoustic phonons and the soft amplitude mode are evident near T_i, the coupled mode lineshapes can be well described without the introduction of a relaxing self energy for the soft mode. Finally, limits on the characteristics of the phason are drawn from our failure to observe it directly in the incommensurate phase.     

Selection rules for superstructures in thiourea,Rb2ZnBr4 and Rb2ZnCl4

This paper concerns the superstructures to which a modulated structure locks in. For thiourea, all observed rational values of $\text{γ (}\text{q}\text{= γ}\text{c}{}^1\text{)}$ belong to a certain type of fractions, namely - if we put $\text{γ =}\text{e}\text{v}$ - those with both u and v odd. In Rb₂ZnBr₄ and Rb₂ZnCl₄, published data point strongly to the existence of superstructures which so far had been regarded as incommensurate, but which turn out to yield values $\text{u}\text{v}$ also obeying the above parity conditions. Such conditions can be explained by a new hypothesis, viz. that the point group of the lock-in phase is identical to the three-dimensional part of the point group of the adjoining incommensurate phase.     

The splitting of the 4A2 ground state of Re4+ ions in (NH4)2PtCl6 by trigonal distortions

Recent EPR measurements on Re⁴⁺ ions in (NH₄)₂PtCl₆ by Bufaical and Harris have indicated the presence of non-octahedral centres. An estimation of the degree of distortion of the ReCl₆ complex required to account for these results is made.     

New Li+ ion conductors in the system Li4SiO4−Li3AsO4

In the system Li₄SiO₄-Li₃AsO₄, Li₄SiO₄ forms a short range of solid solutions containing up to 14 to 20% Li₃AsO ₄, depending on temperature, and γ-Li₃AsO₄ forms a more extensive range of solid solutions containing up to ≈55% Li₄SiO₄. The Li₄SiO₄-Li₃AsO₄ phase diagram has been determined and is of binary eutectic character. The ac conductivity of polycrystalline samples was measured over the range 0 to at least 300°C for nine different compositions. The two solid solution series have much higher conductivity than the pure end-members; maximum conductivity was observed in the γ-Li₃AsO₄ solid solutions containing ≈40 to 55% Li₄SiO₄, with values of $\text{≈2×10}{}^{\mathrm{?6}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$ at 20°C rising to ≈0.02 Ω^?1 cm^?1 at 300°C. These values are comparable to those found in the system Li₄SiO₄-Li₃PO₄. The variation with composition of the Arrhenius prefactor and activation energy has been interpreted in terms of the mechanisms of conduction. Li₃AsO₄ is a poor conductor essentially because the number of mobile Li⁺ ions is very small. This number, and hence the conductivity, increases dramatically on forming solid solutions with Li₄SiO₄, by the creation of interstitial Li⁺ ions. At ≈40 to 55% Li₄SiO₄, the number of mobile Li⁺ ions appears to be optimised. An explanation for the change in activation energy of conduction at ≈290°C in Li₄SiO₄ and at higher temperatures in Li₄SiO₄ solid solutions is given in terms of order-disorder of the Li⁺ ions.     

Properties of the secondary order parameter in Rb2ZnCl4

The dielectric constants and the spontaneous polarization of Rb₂ZnCl₄-crystals have been investigated in order to characterize the nature of the transitions at 303, 195 and 74.5 K. The dielectric anomalies around 303 K hint at a critical exponent β = 0.36 ± 0.03 in the incommensurate phase. Close to 195 K the commensurate state can be induced by electric fields in agreement with an appropriate Clausius-Clapeyron relation. The polarization measurements further show that Rb₂ZnCl₄ has a (second) ferroelectric transformation at 74.5 K.     

Ferroelectricity and phase transition from incommensurate phase into commensurate one in (NH4)2ZnCl4

Phase transition has been found in (NH₄)₂ZnCl₄ at T = 266 ± 0.5 K by NQR method. There is a ferroelectric phase below T_c with a space group P2₁cn and with the trebling of the elementary lattice parameter along the axis c. Above the phase transition temperature in the crystal (NH₄)₂ZnCl₄ an incommensurate phase is realized.     

Inelastic light scattering and photoluminescence in the mixed valence systems Eu3O4, Eu3S4 and Sm3S4

The thermally activated valence fluctuations of Eu₃S₄ and Sm₃S₄ have been studied using light scattering and photoluminescence techniques and are compared with measurements on the non-fluctuating compound Eu₃O₄, Eu₃S₄ exhibits an anomalous vibrational mode which is associated with the frequency factor of hopping, ν₂=1.3×10¹³ sec^?1. In Sm₃S₄ electronic Raman scattering is observed within the ⁷F₉ multiplet of the Sm²⁺ ion. An anomalous frequency shift of the5d-4f photoluminescence emission band in Eu₃S₄ is related to the temperature dependent fluctuation rate which passes through the reference time scale of the photoluminescence. Intra-4f photoluminescence has also been observed in Eu₃S₄ and Sm₃S₄.     

Symmetry determination of commensurate and incommensurate phases of K2SeO4 by EPR

From a reinterpretation of EPR results on γ-irradiated K₂SeO₄ crystals, previously reported by Aiki et al., the space group symmetry for commensurate (ferroelectric) phase of K₂SeO₄ is determined to be Pna2₁. The EPR results support the modulation of polarization as the reason for disappearance of ferroelectricity in incommensurate phase.     

Li4SiO4-Li3VO4赝二元系和Li4GeO4-Li4SiO4-Li3VO4赝三元系中新的锂离子导体

本文在室温到300℃的温度范围内研究了Li₄SiO₄-Li₃VO₄和Li₄GeO₄-Li₄SiO₄-Li₃VO₄体系中的离子导电性,发现γ_II相固溶体Li_3+xV_1-xSi_xO₄是好的锂离子导体。所研究的成分中Li_3.3V_0.7Si_0.3O₄的离子电导率最高,室温下为1×10^-5Ω^-1·cm^-1,在42—192℃的电导激活能为0.36eV,电子电导率可以忽略,因而这是迄今所发现的最好的锂离子导体之一。粗略确定了Li₄GeO₄-Li₄SiO₄-Li₃VO₄三元系中电导率高的范围,发现在Li_3.5V_0.5Ge_0.5O₄中Si部分取代Ge可以使电导率进一步提高,Li_3.5V_0.5Ge_0.4Si_0.1O₄的室温电导率可达1.3×10^-5Ω^-1·cm^-1,电导激活能为0.40eV。 关键词：     

Nd5Fe18B18 (Nd1.11Fe4B4), a new nowotny-like phase. structural and magnetic properties

The crystal structure of a ternary compound, whose chemical composition is Nd₅Fe₁₈B₁₈, has been determined. Nd atoms on the one hand, Fe and B atoms on the other hand, form 2 different substructures, both of tetragonal symmetry. In a previous investigation¹ only incommensurate or very long period ordering along the $\overrightarrow{c}$ directions between the 2 substructures was proposed, with non-definite composition. The whole structure is now described in the Pccn orthorhombic space group (a=b=7.117Å ; c = 35.07Å). This compound appears then as a new Nowotny-like phase. Fe atoms are essentially non magnetic. Ferromagnetic order between Nd atoms occurs only below 14 K. 2^nd order Crystalline Electric Fiel (CEF) terms are deduced from the anisotropy of the paramagnetic susceptibility. Higher order CEF terms must also be considered.     

Li3VO4-Li4SiO4-Li4GeO4赝三元系相图的研究

本文用差热分析法和高温、室温X射线衍射法对Li₃VO₄,Li₄SiO₄的相变过程,Li₃VO₄-Li₄SiO₄,Li₃O₄-Li-4GeO₄赝二元系相图以及Li₃VO₄-Li₄SiO₄-Li₄GeO₄赝三元系相图室温截面进行了研究。发现在Li₃VO₄-Li₄SiO₄,Li₃VO₄-Li₄GeO₄赝二元系中,由于Li₄SiO₄或Li₄GeO₄的加入而使Li₃VO₄的高温γ_II相稳定存在于室温,从而得到一种新的具有高电导率的锂离子导体。作者认为探寻使高温态稳定存在于室温的方法是探索新的离子导体研究中有效途径之一。 关键词：     

Statics and dynamics of the modulation in incommensurate Rb2ZnBr4 as detected by NMR

The dynamics of the incommensurate modulation of Rb₂ZnBr₄ is investigated near the transition to the normal high-temperature phase using first-order quadrupole effects in nuclear magnetic resonance (NMR).⁸⁷Rb NMR spectra and two-dimensional⁸⁷Rb NMR exchange spectra are reported. All results can be described consistently in terms of a static modulation in the incommensurate phase without any indication for “floating” or large-scale fluctuations of the modulation wave. The spectra taken about 135 K below T_i in the lower incommensurate phase well above the soliton regime show no indication for the existence of a higher-order commensurate modulation in Rb₂ZnBr₄.     

Synthesis, vibrational and optical properties of a new three-layered organic-inorganic perovskite (C4H9NH3)4Pb3I4Br6

An organic-inorganic hybrid perovskite (C₄H₉NH₃)₄Pb₃I₄Br₆ was synthesized and studied by X-ray diffraction, Raman and infrared spectroscopies, optical transmission and photoluminescence. The title compound, abbreviated (C₄)₄Pb₃I₄Br₆, crystallises in a periodic two-dimensional multilayer structure with P2₁/a space group. The structure is built up from alternating inorganic and organic layers. Each inorganic layer consists of three sheets of PbX₆ (X=I, Br) octahedra. Raman and infrared spectra of the title compound were recorded in the 100-3500 and 400-4000 cm⁻¹ frequency ranges, respectively. An assignment of the observed vibration modes is reported. Optical transmission measurements, performed on thin films of (C₄)₄Pb₃I₄Br₆, revealed two absorption bands at 474 and 508 nm. Photoluminescence measurements have shown a green emission peak at 519 nm.     

LiKSO4低温相变的核磁共振研究

研究了LiKSO₄中⁷Li的核磁共振谱与温度及晶体取向的关系。证实了在T_c=195K存在一个结构相变,并由实验数据推导出在这一温度以下,晶体的对称性低于P31c,每单胞至少含有六个分子式。没有证据说明有一个无公约相存在。测得181K的四极耦合常数为38.9kHz,不对称因子η=0.46,电场梯度主轴方向z与原c轴成11.6°角。 关键词：