Variations of phase transition temperature and enthalpy in the systems Na2Mo1− x W x O4 and Na2Mo1− y S y O4

The samples in two material systems, Na₂Mo_{1? x} W _x O₄ and Na₂Mo_{1? y} S _y O₄, were prepared by using conventional solid reactions and characterized by X-ray diffractometer (XRD), energy dispersive X-ray spectroscopy (EDX), and difference scanning calorimetry (DSC). The XRD and EDX data indicated that all the samples in both systems were in the solid solution range. The DSC data indicated that in the system Na₂Mo_{1? x} W _x O₄, the solid–solid transition temperatures increased and in the system Na₂Mo_{1? y} S _y O₄, the solid–solid transition temperatures decreased. The total enthalpy (ΔH _total) of the solid–solid transitions in the system Na₂Mo_{1? x} W _x O₄, decreased much less than that in the system Na₂Mo_{1? y} S _y O₄. This is probably because similar to Na₂MoO₄, the solid–solid transition of Na₂WO₄ has relatively large ΔH _total, but Na₂SO₄ has much smaller solid–solid transition enthalpy. In order to modify the transition temperatures of Na₂MoO₄ and also to keep its relatively large ΔH _total, it is necessary to choose a doping material with large ΔH _total.     

Magnetic behaviour of Eu x (Rh1−y Fe y )3B2;x=0.6, 0.75, 1.04 andy=0.02

⁵⁷Fe ME spectra taken for Eu_x(Rh_1–yFe_y)₃B₂ compounds (x=0.6, 0.75, 1.04 and y=0.02) show quadrupole doublet and a magnetically split pattern for T < T_c ^*. Above T_c ^* the spectra merge into a single quadrupole doublet. It is seen that the addition of Fe shifts the T_c corresponding to undoped compound to T_c ^*, T_c ^*>300 K for all the three samples.     

Features of the absorption spectra of thin films of M 2Ag1 − x CuxI3 solid solutions (M = Rb, Cs)

The absorption spectra of thin films of (MI)_{1 ? y} (Ag_{1 ? x} Cu_xI)_y solid solutions (M = Rb, Cs) with the initial molar concentration y = 0.33 have been investigated. It is established that, at low concentrations x, a local exciton band due to Cu⁺ ions is split off from the main long-wavelength exciton bands. In Rb₂Ag_{1 ? x} Cu_xI₃ solutions, the concentration shift of exciton bands indicates the formation of a persistent-type exciton spectrum. However, in Rb₂Ag_{1 ? x} Cu_xI₃ with x ≥ 0.5 and in Cs₂Ag_{1 ? x} Cu_xI₃ with x > 0.2, exciton spectra of amalgamation type are observed, which are related to the formation of more stable M ₃Ag_{2 ? 2x} Cu_2x I₅ solid solutions. The formation of these solutions leads to broadening of the exciton bands and to the concentration transition from persistent-to amalgamation-type exciton spectra.     

Elastic properties of MBH4 (M=Na, K, Rb, Cs)

The elastic properties of MBH₄ (M=Na, K, Rb, Cs) are studied by first-principles calculations using a projected augmented plane-wave method and finite strain method. KBH₄ is found to be the most isotropic material among MBH₄, which can be attributed to the contribution of C₄₄. The Debye temperatures are also calculated using theoretical elastic constants and the change along the series is explained.     

Effect of copper impurities on the absorption spectrum of thin films of superionic conductors MAg4I5 (M = K, Rb)

The absorption spectra of thin films of the solid electrolytes MAg₄I₅ (M = K, Rb) doped with copper (0 ≤ x ≤ 0.15) are studied in the spectral range 2–6 eV at temperatures of 90 and 290 K. It is established that the critical Cu content reaches x _crit = 0.05. A decrease in the Cu content to x ≤ 0.05 leads to the formation of MAg_{4 ? 4x} Cu_4x I₅ solid solutions, and the films remain spectrally stable. At x > 0.05, the films segregate into different phases: MAg_{4 ? 4x} Cu_4x I₅, Ag_{1 ? x} Cu_xI, and M ₂AgI₃.     

K7[P2Mo4W13M(H2O)O61]及其有机复合材料的合成及光谱研究

用降解法制得了Dawson结构铬、铁取代的磷钼钨酸钾,并将其与溴化(E)-N-丁基-4-(2-(4-二甲氨基苯基)乙烯基)吡啶反应制备了有机复合材料.通过元素分析和TG-DTA确定了配合物的组成分别为K7[P2Mo4W13M(H2O)O61](M=Cr(1),Fe(2))和(C19H25N2)6K3[P2Mo4W13MO62](M=Cr(3),Fe(4)).利用红外光谱、紫外-可见光谱、固体漫反射紫外-可见-近红外光谱、X射线光电子能谱以及荧光光谱对上述化合物进行了表征,并研究了复合材料中无机与有机组分间的相互作用及其荧光性质.     

Thermoelectric properties of homogeneously and non-homogeneously doped CdTe15/16M1/16 (M=N,P, As,Sb) and Cd15/16TeM1/16 (M=Na,K, Rb,Cs)

The electrical transport properties of p-doped semiconductors CdTe_15/16M_1/16 (M=N, P, As, Sb) and Cd_15/16TeM_1/16 (M=Na, K, Rb, Cs) with two configurations are investigated through first-principles calculations combined with Boltzmann transport theory under the relaxation time approximation. It is found that N and Cs atoms in the homogeneous structure induce much sharper electron densities of states (DOSs) and flatter energy bands at the valence band edges than the rest of doped elements, resulting in much larger Seebeck coefficients. The calculations reveal that most of the Seebeck coefficients and electrical conductivities are impacted unfavorably by the conglomeration of impurity atoms considered. Though the power factors for homogeneous doping of N and Cs are comparatively smaller, the electronic figures of merit are much larger at 800–1000 K than the rest ones due to much smaller electronic thermal conductivities, therefore probably enhancing the thermoelectric figures of merit. The results show that doping the elements with electronegativities distinct from the host atoms can enhance the Seebeck coefficients and the thermoelectric performances of bulk semiconductors efficiently if the energy levels of doped atoms resonate with those of host atoms and the arrangement of doped atoms is modulated appropriately to avoid deteriorating the sharpness of the DOS (or transport distribution).     

Magnetic and57Fe Mössbauer studies of intermetallic compounds R2 (Fe1−x−y Ni x Co y )17 (R=Y,Gd,Tb,x=0,0.10, 0.20,y=0,0.05, 0.10)

Mössbauer studies of R₂(Fe_1?x?y Ni _x Co _y )₁₇ showed that the transferred hyperfine field at Fe nuclei due to magnetic rare earth (R) atoms is about one Tesla. Magnetic moments of the R atoms were determined from magnetic measurements as μTb=8.52μB, μGd=6.22μB. The mixed substitution of Ni and Co for Fe leads to an increase of the ordering temperature. A slight preference occupancy for Fe was observed involving the dumbbell shaped f or c site. The substitution effects of Ni and Co on the hyperfine field of f or c site, the average hyperfine field and the average isomer shift were also discussed.     

Transport properties of Mg2 X 0.4Sn0.6 solid solutions (X = Si, Ge) with p-type conductivity

The transport properties of Mg₂ X _0.4Sn_0.6 (X = Si, Ge) solid solutions are investigated. It is shown that these materials can be rendered p-type with a hole concentration of up to 4 × 10¹⁹ cm^?3. The Hall coefficient, thermopower, and electrical conductivity are measured over a wide temperature range. The mobility of holes in these solid solutions is less than that of electrons by a factor of 2 for Mg₂Si_0.4Sn_0.6 and by a factor of 1.5 for Mg₂Ge_0.4Sn_0.6. Solid solutions in the Mg₂Ge-Mg₂Sn system appear more promising for thermoelectric applications.     

Nonlinear refractive indices of disordered NaT(XO4)2 T=Y, La, Gd, Lu and Bi, X=Mo, W femtosecond laser crystals

The nonlinear refractive index (n₂) of several NaT(WO₄)₂ (T?=?Y, La, Gd, Lu and Bi) and NaY(MoO₄)₂ femtosecond laser crystals has been measured using the z-scan technique. These tetragonal crystals with structural local disorder present significant n₂ values, which should allow their efficient laser pulsed operation by Kerr-lens mode locking, especially NaBi(WO₄)₂, 68×10^-16 cm²/W (for σ light), which is about twice than for the others. This difference is attributed to the high polarizability associated with the lone electron pair of Bi³⁺.     

含ME4(M＝Mo,W;E＝S,Se)单元簇合物的合成和红外谱表征

合成了一系列单立方烷Ｍ－Ｆｅ－Ｓｅ和Ｍ－Ｃｕ（Ａｇ）－Ｓ（Ｓｅ）簇合物,并用ＵＶ,ＩＲ和Ｘ－光衍射等方法对上述簇合物进行了分析。     

Interfacial defect engineering and photocatalysis properties of hBN/MX2 (M = Mo,W, and X = S,Se heterostructures

Van der Waals (VDW) heterostructures have attracted significant research interest due to their tunable interfacial properties and potential applications in many areas such as electronics, optoelectronic, and heterocatalysis. In this work, the influences of interfacial defects on the electronic structures and photocatalytic properties of hBN/MX₂ (M = Mo, W, and X=S, Se) are studied using density functional theory calculations. The results reveal that the band alignment of hBN/MX₂ can be adjusted by introducing vacancies and atomic doping. The type-I band alignment of the host structure is maintained in the heterostructure with n-type doping in the hBN sublayer. Interestingly, the band alignment changed into the type-II heterostructrue due to V_B defect and p-type doping is introduced into the hBN sublayer. This can conduce to the separation of photo-generated electron-hole pairs at the interfaces, which is highly desired for heterostructure photocatalysis. In addition, two Z-type heterostructures including hBN(Be_B)/MoS₂, hBN(Be_B)/MoSe₂, and hBN(V_N)/MoSe₂ are achieved, showing the decreasing of band gap and ideal redox potential for water splitting. Our results reveal the possibility of engineering the interfacial and photocatalysis properties of hBN/MX₂ heterostructures via interfacial defects.     

Electronic and geometric structures of MoxSy and WxSy (x =1, 2, 4; y =1–12) clusters

Electronic and geometric structures of M_xS_y (M = W, Mo; x=1,2,4;y=1–12) clusters have been studied using density functional theory calculations, and compared to experimental photoelectron spectra. For the metal atoms, an uptake of up to six sulfur atoms has been observed, which can be explained by the bonding of S₃ ^- chains. A structural difference to the corresponding oxides is the preference of bridging sites for S, which might be the origin of the differences between the structures of bulk MO₃ and MS₂. For x=1,2 the HOMO–LUMO gaps vary irregularly. For x=4, a large HOMO–LUMO gap has been found for y=6,7, and 8 and the W₄S₆ and Mo₄S₆ clusters have been found to be magic with an extraordinarily high stability. PACS 73.22.-f; 61.46.+w     

Structural investigations of non-stoichiometric solid solutions (Bi2O3)1−x(M2O3)x (M=Y, La, Pr, Nd, Gd and Dy)

The structural investigations of the non-stoichiometric solid solutions (Bi₂O₃)_1−x(M₂O₃)_x (M=Y, La, Pr, Nd, Gd and Dy) have been carried out. Polycrystalline samples have been synthesized using ceramic technique and the synthesis parameters have been optimized for each dopant concentration. The structural phase evolution studies with dopant concentrations have been performed using X-ray powder diffraction data over a wide range of compositions 0.10≤x≤0.40. The unit cell parameters have been obtained from the analysis of the data. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Properties of a new ferroelectric crystal family: MCd(NO2)3 [M=NH4, K,Rb, Cs,Tl]

Single crystals of MCd(NO₂)₃, with M=NH₄, K, Rb, Cs, and Tl, respectively, were grown and investigated by means of optical and dielectric methods. The crystals show rhombohedral symmetry (possible space group R 3) and multi-domain patterns at ambient temperature. The compounds exhibit phase transitions above room-temperature of first (M=K) and second order (M=Rb, Cs, Tl), respectively, into cubic high-temperature phases. Observed dielectric anomalies and hysteresis loops point to ferroelectric properties and suggest a relaxational order-disorder mechanism of the nitrite ions to be responsible for the transitions. The MCd-(NO₂)₃-compounds are seen to represent a new family of ferroelectrics with a cubic prototypic phase closely related to the structural type of the perovskites. The transions are classified by symmetry considerations, and a possible structure of the cubic high-temperature phase with space group Pm3 is proposed.     

Anomalous magnetoresistance in [ Sr2Bi2-xPbxO4] RS[ CoO2] y (x = 0, 0.3, and 0.4; y ≈ 1.85) single crystals

Magnetoresistance (MR) of the [ Sr₂Bi_2-xPb_xO₄] ^RS[ CoO₂] _y (x = 0, 0.3, and 0.4; y ≈ 1.85; RS is referred to rocksalt block) single crystals is investigated systematically. A nonmonotonic variation of the isothermal in-plane and out-of-plane MR with the field is observed. The out-of-plane MR is positive at high temperatures and increases with decreasing T, and exhibits a pronounced hump, then changes the sign from positive to negative at a certain temperature. These results strongly suggest that the observed MR consists of two contributions: one negative and one positive component. The isothermal MR in high magnetic fields follows a H² law. The negative contribution comes from spin scattering of carriers by localized-magnetic-moments based on the Khosla-Fischer model.     

Anomalous states of the crystal structure of (Rb0.1(NH4)0.9)2SO4 solid solutions in the temperature range 4.2–300 K

Crystals of (Rb_0.1(NH₄)_0.9)₂SO₄ solid solutions are studied using x-ray diffractometry. It is revealed that the temperature dependence of the lattice parameter a exhibits an anomalous behavior, namely, the “invar effect” at temperatures above the ferroelectric phase transition point T _c and an anomalous increase in the temperature range from T _c to the liquid-helium temperature. An anomalous increase in the lattice parameter a and an increase in the intensity of Bragg reflections with a decrease in the temperature are interpreted within the model of the coexistence of two sublattices hypothetically responsible for the ferroelectric phase transition. A series of superstructure reflections observed along the basis axes corresponds to a sublattice formed in the matrix of the host structure. Analysis of the ratio between the lattice parameters of these structures allows the inference that, in the temperature range 4.2–300 K, the structure of the crystal under investigation can be considered an incommensurate single-crystal composite.     

Theoretical study of local lattice structure of Mn in perovskite fluorides A2MF4 (A=K, Rb; M=Mg, Zn, Cd) series

A theoretical method for investigating the inter-relation between the molecular structure and electronic structure has been established on the basis of the 252×252 complete energy matrices for a 3d⁵ configuration ion in a tetragonal ligand field. By means of this method, which is independent of the X-ray diffraction, the local structure of the paramagnetic Mn²⁺ ion in perovskite fluorides A₂MF₄ (A=K, Rb; M=Zn, Mg, Cd) are determined directly by analyzing the EPR spectrum of octahedral Mn²⁺ center in A₂MF₄ crystals and the optical absorption spectrum of the (MnF₆)⁴⁻ cluster. It is shown that, comparing with the octahedral cubic structure, the local micro-structure in the vicinity of Mn²⁺ displays an elongated distortion when and a compressed distortion when , and ΔR vs. as well as ΔR vs. in the distortion region is, respectively, approximately linear. Simultaneously, the theoretical zero-field-splitting parameters , and are in good agreement with the experimental values.