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A series of Ba8Ga16Si30 clathrate samples were prepared by arc melting, ball milling, acid washing, and spark plasma sintering (SPS). X-ray diffraction analysis revealed that the lattice of the Ba8Ga16Si30 samples expanded as the SPS temperature was increased from 400 to 750°C. Lattice contraction recurred when the SPS temperature was further increased in the range of 750–1000°C. This phenomenon can be explained by the variation of Ga content in the lattice. The thermoelectric figure of the merit ZT value of clathrates increased with the increase in SPS temperature and reached a maximum when the sample was subjected to SPS at 800°C. A further increase in SPS temperature did not contribute to the improvement of ZT. The variation of the lattice parameter a vs. SPS temperature T was similar to the variation observed in the ZT-T curve.

A series of samples with the chemical formula Ba8Si46−xAlx (x = 2, 3, 5, 6, 7 and 8) were prepared by arc melting, ball milling and washing with diluted HCl. The lattice parameter of Ba8Si46−xAlx increases linearly with the increase of nominal Al content x. The composition analysis by energy-dispersive X-ray spectroscopy (EDS) shown that the actual Al contents in clathrates are lager than the nominal compositions because the dilute Al-contained impurity phases were washed out. The experimental results show that the minimum incorporation of Al into clathrate structure is expected to be about 3 at ambient pressure, which is in agreement with a first-principle simulation. The Al substitution for Si results in the decrease of superconducting transition temperature TC, which can be explained on the BCS theoretical frame. The electron density of state at Fermi level N(EF) decreases with the increment of x except for an abnormal increase for the sample x = 6. Such sample has a higher spatial symmetry of the structure in which all the six Si atoms at 6c sites were substituted by Al atoms. Its higher N(EF) causes to a higher TC. In addition, we calculated the phonon-dispersion relations and vibrational density of states for Al-doped silicon clathrates. The high frequency acoustic branch has a red shift from 430 cm−1 to 420 cm−1 with the doping of Al. The decreased frequency of bond-stretching vibration modes is another reason for the suppression of TC induced by Al substitution.

The potential thermoelectric and magnetic application of clathrate materials with rare-earth doping is the focus of much of the recent research activity in the synthetic material physics and chemistry. A series of clathrate samples with the chemical formula Ba 8− x Yb x Si 30 Ga 16 ( x = 0, 0.5, 0.7, 1 and 1.5) were prepared by combining arc melting, ball milling and spark plasma sintering (SPS) techniques. X-ray diffraction and scanning electronic microscopy combined with energy-dispersive X-ray spectroscopy (EDS) analysis showed the dominant phase to be the type-I clathrate. Whereas, X-ray structural refinement and EDS analysis indicated that some Yb atoms enter the clathrate lattice to replace Ba at 2 a sites, while other Yb atoms are oxidized as Yb 2 O 3 precipitated around grain boundaries. The solid solubility of Yb into clathrate lattice yielded x ∼ 0.3. Comparative analysis between Yb-doped and Yb-free clathrates showed that the thermal conductivity decreases with Yb-doping. Consequently, thermoelectric figure of merit ZT significantly increased.

A series of single crystals with starting composition Y1−xEuxBa2Cu3O7−δ (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 1.0) were prepared in Al2O3 crucibles in the self-flux method. A home-made setup for crystal growth and a new flux separation method are introduced. Eu-doped Y-123 single crystals were characterized by means of X-ray diffraction, energy dispersive X-ray spectrometry, scanning electron microscopy (SEM), atomic force microscopy (AFM) and AC susceptibility. All single crystal samples maintain the crystallographical structure of the 123-phase. The SEM and AFM measurements indicate a growth anisotropy. The growth of the (0 0 1) surface is related to different growth mechanisms, and the spiral growth mechanism is the predominant one. For the first time, we observed a regular array of groove on (0 0 1) surface. A drastic suppression of the superconducting transition temperature with Eu concentration was observed. Al contamination from crucible corrosion and Eu substitution for the Ba site result in a heavy reduction of Tc in single crystals.

The Fe-doping effects on superconductivity, transport and microstructure are investigated by means of Hall coefficient measurements, thermogravimetric analysis, Mossbauer spectroscopy and the calculation of the point-charge-effective-valence model (PCEV model) in Fe-doped Tl 0.5 Pb 0.5 (Sr 0.8 Ba 0.2 ) 2 Ca 2 (Cu 1− x Fe x ) 3 O y (Tl-1223) superconductors. In the doping level of Fe ( x =0–0.05), both zero-resistance temperature T co and carrier concentration n H decrease linearly with Fe dopants increasing. The striking linear behavior of T co and n H suggests the potential effects of charge localization on the transport properties and also shows that Fe dopants occupy different Cu sites in a stable manner with Fe content increasing. The thermogravimetric analysis shows the Fe 3+ ions substituting for Cu 2+ ions can bring excess oxygen atoms into lattice and form some Fe–O defect clusters. Mossbauer measurements are performed to investigate the microstructure of Fe–O defects. In addition, the atomic position of Fe dopant and the electric field gradient (EFG) on Fe sites are investigated under the PCEV model. We found that the signs of the EFG on Fe sites are positive. According to quadrupole splittings deduced from Mossbauer spectroscopies and results of model calculation, it is further confirmed that there is a small shift of Fe ion relative to the original center of the CuO 2 network, and in some different kinds of Fe–O defect clusters, Fe dopants can lead to excess oxygen atoms entering lattice. These results provide a further support for the interpretation about the displacement of Fe dopant and the assignment of Fe site. Our discussion suggests that the microstructural distortion and the excess oxygen defect induced by Fe doping encourage in the localization of cruising carrier on CuO 2 planes, which is one of the reasons to explain the decrease of T c and carrier concentration.

The HgBa 2 CaCu 2 O 6+ x high temperature superconducting materials are synthesized under a high pressure of 30 kbar at 850 °C for 4 h. The zero resistance temperature of the materials measured by a four-probe method is 123 K. Using XRD, SEM, TEM and EDS the morphologies and features of the non-superconducting phases in the materials, such as phase stabilities, structure uniformities and transformations, were investigated. The decomposition of the CaHgO 2 phase under electron beam heating and bombardment was observed in-situ and an orientation evolution of the CaHgO 2 during decomposition was analysed. The special structures in some non-superconducting phases, such as the mosaic structure and the tweed structure in CaO and Ba 2 Cu 3 O 5+ x grains respectively, were investigated.

The superconductors HgBa2CaCu2O6+ delta (Hg-1212) and HgBa2Ca2Cu3O8+ delta (Hg-1223) have been synthesized by solid state reaction, carried out at 850 degrees C under 25 kbar, using the nominal composition of HgBa2Ca2Cu3Ox. The results indicate that the optimum sintering time for the Hg-1223 phase is about 0.5 h. With increasing sintering time, the Hg-1223 phase decomposes, and the Hg-1212 phase increases gradually. When the sintering time increased to 3.8 h, we obtained a sample with nearly a single phase of Hg-1212. We synthesized the HgBa2CaCu2O6+ delta superconductor under 27 kbar at various sintering temperatures using the nominal composition of HgBa2CaCu2Ox. The results indicate that the optimum sintering temperature for the Hg-1212 phase is 820-830 degrees C. The effect of annealing at various temperatures in flowing O2 on the superconductivity of HgBa2Ca2Cu3O8+ delta has been investigated.

A significant drop of the resistance around 235 K was observed in our experiment of mercury-based samples synthesized under high pressure and high temperature. The drop of the resistance alwaysappeared in samples containing tiny mercury droplets or HgAu alloys; it was sometimes also observed in samples with grains of Hg-1223 and grain boundaries containing an excess of Hg compared with the composition ratio of the superconducting phase of Hg-12(n-1)n. The measurements of DC magnetic susceptibility by a SQUID magnetometer did not demonstrate any diamagnetic behavior around 235 K although a very small drop in the AC magnetic susceptibility was observed at this temperature with a Lakeshore magnetometer. The paper will discuss if the drop in the resistance for the mercury-based compounds at 235 K is due to superconductivity; we rather believe that it is due to the presence of liquid mercury of its alloys in the sample.

High-temperature superconductors of the mercury-based system were synthesized under high temperature and high pressure. The measurements of resistance and DC magnetic susceptibility for the samples prepared at 20-35 kbar and 850-degrees-C for 1-3.5 h show that the as-synthesized sample (1) prepared at 20 kbar, 850-degrees-C for 1.5 h was a multiphase of Hg-1212 and Hg-1223 phases and showed superconducting transitions of both 132 and 120 K. The sample (2) prepared at 30 kbar, 850-degrees-C for 3.5 h has Hg-1212 as the major phase with only a small amount of impurity phase. The intergrowth of Hg-1212/Hg-1223 was found in the sample (1). The dependence of pressure on T(c) for the Hg-1212 and Hg-1223 phases, measured under hydrostatic pressure, shows that dT(c)/dp is 0.17 and 0.23 K/kbar, respectively, in the pressure range below 10 kbar.

The electronic structures of FeO 4 Ca 8 and FeO(1) 4 O(2)Ca 4 Sr 4 clusters in the Fe-doped Bi-systemhigh- T c phase are calculated using the SCF-Xα-SW method. From the calculated result, we find that the Fe atom does not contribute to the density of states near the Fermi energy in both clusters. Fe substitution for Cu sites of Bi-2233 phase is highly deleterious to superconducting behavior. The energy gap of FeO(1) 4 O(2)Ca 4 Sr 4 on the Fermi energy is about seven times larger than that of FeO 4 Ca 8 . Moreover, for the FeO(1) 4 O(2)Ca 4 Sr 4 cluster, the peak of the density of states (DOS) in the high energy range shifts to the low energy range and forms new DOS peaks, in contrast to the density of states in the FeO 4 Ca 8 cluster. The bonding of the hybridized orbital in FeO(1) 4 O(2)Ca 4 Sr 4 is stronger than in FeO 4 Ca 8 , namely the FeO(1) 4 O(2)Ca 4 Sr 4 is even stabler than the FeO 4 Ca 8 cluster. Such calculated result provides satisfactory agreement with Fe-doped experiments. It is observed that T c decreases with increasing iron content and Fe dopant gives rise to enhancement of the 2212 phase at expense of the 2223 in Bi-system superconductors. The calculated results can help us to understand the nature of Fe-doping for Cu on quantum mechanics background. Furthermore the theoretical value of the electric field gradient given by the SCF-Xα-SW method is agreement with the value deduced from quadrupole splittings by Mossbauer spectroscopy. This calculation can provide theoretical support for assignment of Fe species to Cu sites in high- T c superconductors.

57 Fe Mossbauer measurements have been performed on Fe-doped samples of Bi 1.6 Pb 0.4 Sr 2 Ca 3 Cu 4- x Fe x O y ( x =0.06 and 0.08) at room temperature. The Mossbauer spectra show three quadrupole doublets that can be related to different Cu sites. Two doublets (B and C) with larger quadrupole splitting (QS) are assigned with a 57 Fe probe atom on the Cu-O square planar sites of the compound with different local charge environments. Another doublet A with smallest quadrupole splitting is associated with a Fe atom in a planar pyramid site. the doublet with smallest QS possesses relatively the smaller area indicating that the Fe atom preferentially occupies a square planar site rather than a pyramid site. To assure our assignment, we calculated the EFG (electric field gradient) on different Cu sites with the use of the point charge model and of the calculation of an effective ions valence.

Samples with different fractions of structural modulation were synthesized in order to study the effect of a defective modulated structure on the critical temperature T c of Hg-1234. Electron microscopy revealed the existence of modulated and unmodulated Hg-1234 structures in these samples. Measurements of critical temperature T c on these samples revealed that the structural modulation significantly suppresses the T c . For the Hg-1234 sample without the structural modulation, the T c was found to be close to the highest T c for the Hg-1223 phase.

The variety of intergrowth in the Hg Ba Ca Cu O oxide system was found by the high resolution electron microscope(HREM). A new phase of Hg-2435 was synthesized under high pressure for long time. Another longer period structure of -2223- was also found in such prepared sample. However, these complex intergrowth structures only contain the Hg-1212 and Hg-1223 phases.

A excess oxygen model that explains the Fe-doping mechanism in 1223 Tl Ba Ca Cu O superconducting phase is presented. So far the details of the coordination of Fe dopant is not understood completely yet, for Fe dopant substitution in Tl-system cuprate oxide. The similarity in the ionic radii and electronegativity of Cu and Fe lead to the Fe occupying Cu lattice site. However, the difference between the oxidation states of Fe and Cu means that Fe will bring the excess oxygen into lattice to keep the charge balance. We show that, for the evidence of the excess oxygen., the bonding energy of Fe-cluster in Cu O layer reach its minimum when Fe make a shift from the center of Cu O square network. Moreover, under the scheme of effective valence, the calculated value of Fe effective valence is relatively close to that of Mossbauer measurement.