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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.

We present a joint experimental and theoretical study of the superconductivity and electronic structures in type-I Cu-doped silicon clathrates and germanium clathrates. The superconducting critical temperature in Ba8Si46–xCux is shown to decrease strongly with copper content increasing. These results are corroborated by CASTEP approach, first-principles simulations calculated from the density-functional theory with plane waves and pseudopotentials. The simulations show that Cu-doping results in a large decrease of electronic density of states in Fermi level, which can explain the superconducting critical temperature decrease with Cu-doping in the BCS theoretical frame. Further, comparison of Ba8Ge46 and Ba8Si46 within the CASTEP approach shows that the superconductivity is an intrinsic property of the s p 3 silicon and germanium clathrates without vacancy in the cage framework. By analysis of the density of states (DOS) and reported experimental results of the Zintl-like Ba8Ge43, a new mechanism of vacancy defect is suggested to explain the absence of superconductivity in Ge clathrates, which is of benefit to eliminating the divarication between theoretical prediction and the experimental observation for superconductivity in Ge clathrates. Keeping an entire Si and Ge cage structure without vacancy is the prerequisite for occurrence of superconductivity in clathrates.

The effects of Eu substituting for Ba on structural and superconducting properties were investigated for a series of superconductors Eu 1+ x Ba 2− x Cu 3 O 7− δ ( x =0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, and 0.2). With Eu content increasing, the random substitution of excessive Eu for Ba ion forms a Eu–Ba non-stoichiometric distribution on Ba site, which deviates from the ideal 123 phase stoichiometry. The analysis of crystallographic structure shows that the solid solubility x of Eu on Ba site is over 0.2. Eu 1+ x Ba 2− x Cu 3 O 7− δ take on an orthorhombic–tetragonal (O–T) phase transition from x =0.06, and end in x =0.18. Corresponding to the O–T phase transition, superconducting critical temperature T c rapidly decreases with Eu content increasing, which is attributed to the decline of carrier density and the proximity effects of non-superconducting clusters. The positron annihilation measurements show anomalous changes for lifetime spectra and Doppler-broadened radiation spectra when the O–T phase transition occurs. By the model of excessive oxygen defects in Cu–O chain regions, we discuss effects of Eu–Ba non-stoichiometric distribution on structural and superconducting properties.

The effect of Sn-doping on (La 1− x Sr x ) 2 Cu 1− x Sn x O 4 ( x =0.075, 0.090 and 0.110) has been studied. Lattice parameters monotonically decrease with Sn dopant increasing. Sn atoms dominantly occupy Cu sites and form Sn–O clusters as point-like defects. Flux pinning properties of ceramic samples of (La 1− x Sr x ) 2 Cu 1− x Sn x O 4 have been investigated by magnetic measurements from 6 to 30 K up to 5 T. The current carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the more richly Sn-doped sample exhibit stronger pinning behavior, especially in the higher temperature range. The intra-granular critical current J c in the rich Sn-doping sample has a significant increase. We discuss the experimental results within a pinning model of point-like defects. From our data we infer that the Sn–O cluster near Cu–O layers provides an effective pinning center.

The magnetic properties of ceramic samples of Bi 2−2 x (PbSn) x Sr 2 CaCu 2 O y ( x =0,0.1,0.2,0.3 and 0.4) superconductors have been investigated using a vibrating sample magnetometer from 20 to 50 K and up to 8 T. Phase analysis shows that most of the (Pb,Sn) dopant enters the Bi-2212 lattice and occupies Bi sites. The current-carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the (PbSn)-doped sample exhibit strong pinning behaviour in the higher temperature range. The effect of (Pb,Sn) doping on intragranular critical current J c was found to be enhanced by one order of magnitude at 50 K. We discuss the experimental results within a model of (PbSn) cluster pinning in blocking layers. Doping by (PbSn) clusters into the Bi–O layers provides effective pinning centres, as well as a large reduction of the resistivity anisotropy.

The magnetic properties of a series of superconductors (Eu1-xYx)Ba2Cu3O7-y (x = 0, 0.2, 0.3, 0.5, 0.7 and 0.8) have been investigated by magnetic measurements from 20 to 90 K. Magnetization hysteresis loops of the Y-doped sample exhibit a strongly developed second peak in the higher temperature range. We discuss the experimental results in the framework of a stress-field pinning model. The stress field induced by the lattice mismatch is also an effective pinning centre, in addition to pinning by 211 phase precipitates and oxygen vacancies. We further suggest that in the (Eu1-xYx)Ba2Cu3O7-y samples the oxygen deficient regions induced by the stress field in Cu-O chains also act as pinning centres, thus causing the second peak in the Jc-B curves of (Eu1-xYx)Ba2Cu3O7-y.

119Sn Mossbauer research is carried out on (La1-xSrx),Cu1-xSnxO4(x = 0.075 and 0.110) superconductors which are designed under a new concept. The Mossbauer spectra results show that Sn occupies Cu position in Sn4+ state, and there is no Sn2+ ion occupying La position. The local lattice deformation near Sn4+ site is small, but displays an increasing tendency with Sn doping. For La2CuO4 matrix, the simultaneous dopings of Sr/Sn induce holes and electrons on CuO2 layer in a new mechanism which influences superconductivity. Under a new mechanism of extra oxygen, the extra oxygen effect of Sn-doping on superconductivity is discussed

The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra have been performed on (Y 1− x Eu x )Ba 2 Cu 3 O 7− y ( x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1) superconductors. The mean lifetime and S -parameter, respectively show a characteristic significantly symmetric to Eu-doping concentration. Since lifetime and Doppler broadening spectroscopies are rather sensitive to the variation of the lattice distortion and defects induced by the stress-field of lattice mismatch, such symmetric behaviors of the S -parameter and the mean lifetime suggest a striking characteristic of stress in Y 1− x Eu x -123 superconductor. When the doping concentration x reaches 30%, the density of defects corresponding to stress-field appears the maximum, and there is an optimum distribution of the stress-field so as to increase critical current J c in Eu-doped Y-123 superconductors. Our results of the critical current density obtained by means of measurements of susceptibility χ ′( T , H AC ) and χ ″( T , H AC ) also suggest that the enhancement of the critical current density to reach an optimum when Eu doping x is 30%. We also qualitatively discuss experimental results in the framework of the model of stress-field pinning. The model calculating results also show the stress-field induced by lattice mismatch is an effective pinning center, and further conform that the stress pinning mechanism is dominant in RE-doped Y-123 superconductors.

The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra were performed by positron annihilation analysis on Eu-doped Y-123 superconductors. When the doping concentration reaches 30% and 70%, the density of defeat and its stress-field reach the maximum for (Y1-x Eu x )Ba2CU3O7-y The pinning mechanism of such superconductors is discussed, and the results support the stress pinning mechanism.

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.

We attempt to provide a model calculation of the electric field gradient. With reference to the observed 57Fe Mossbauer hyperfine interaction parameters, the effective valence and electric field gradient on different Fe lattice sites have been computed for the Fe‐doped Y‐124 superconductor by means of the point‐charge‐effective‐valence model. According to the present calculations, the sign of electric‐field gradient for the pyramid Fe(B’) site and the quasioctahedral Fe(B) is estimated to be positive and negative, respectively, and furthermore Fe is found to be displaced relative to the CuO2 plane in Y‐124 superconductors.

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.

57Fe Mossbauer measurement have been performed in the Fe-doped high-Tc phase of Bi-system superconductor. Isomer shifts display presence of Fe3+ ions in square and pyramidal coordinations. Regarding the high-Tc superconductors as primarily ionic crystals and using effective valence and point charge model, we have calculated the electric field gradients for Fe as a replacement for Cu in the high-Tc phase. Good agreement between theory and experiment is obtained.

Mossbauer measurements were performed in the temperature range from 77 to 300K. Temperature-dependent57Fe Mossbauer recoil-free fraction in high-Tc superconducting phase of Bi-system display cvidence of a softening in lattice vibrations, which appears at T

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.

We report a joint experimental and theoretical investigation of superconductivity in Ga-substituted type-I silicon clathrates. We prepared samples of the general formula Ba8Si46�xGax, with different values of x .W e show that Ba8Si40Ga6 is a bulk superconductor, with an onset at TC 3.3 K. For x = 10 and higher, no superconductivity was observed down to T = 1.8 K. This represents a strong suppression of superconductivity with increasing Ga content, compared to Ba8Si46 with TC 8 K. Suppression of superconductivity can be attributed primarily to a decrease in the density of states at the Fermi level, caused by a reduced integrity of the sp 3 -hybridized networks as well as the lowering of carrier concentration. These results are corroborated by first-principles calculations, which show that Ga substitution results in a large decrease of the electronic density of states at the Fermi level, which explains the decreased superconducting critical temperature within the BCS framework. To further characterize the superconducting state, we carried out magnetic measurements showing Ba8Si40Ga6 to be a type-II superconductor. The critical magnetic fields were measured to be HC1 35 Oe and HC2 8.5 kOe. We deduce the London penetration depth 3700 A and the coherence length c 200 A. Our estimate of the electron-phonon coupling reveals that Ba8Si40Ga6 is a moderate phonon-mediated BCS superconductor.

Temperature dependent X-ray diffraction is carried out in order to study lattice vibrational behavior in Bi1.6Pb0.4Sr2Ca2Cu3−xFexOy (x=0 and 0.08 respectively). The study focuses attention on the (002) diffraction peak. The lattice parameters and the anomalous behavior of the intensity of the (002) peak show the existence of a lattice softening effect at approximately 130 and 200 K. The samples with different values of Tc show different vibrational behavior.

Anomalous behavior of the Mossbauer factor for tin impurity atoms in Bi-system Superconductors is found by Mossbauer spectroscopy. This indicates the occurrence of lattice softening above Tc.

X-ray diffraction study was conducted over the range from 77K to room temperature about high-Tc superconducting phase Bi-system. Temperature dependent intensity of (002) peak and lattice parameter show evidence of lattice vibrations softening, which occurs at about 130K and 210K.

A joint experimental and theoretical investigation is performed for Ba–Sn–Ga type-I and type-VIII clathrates. The type-I sample shows a semiconductor characteristic which is accords with the first principle simulation. The type-I clathrate follows Zintl-like behaviour and the semiconducting gap is about 0.6 eV which is agreement with measurement. For the type-VIII clathrate, the simulation shows that a complicated band structure around Fermi level. For the density of states, a peak appears in the fundamental gap, which corresponds to hybridization of Ba bonding orbitals and Sn(Ga) network. A superconducting transition at T C =4 K is observed, which is close to the T C of Sn. An alternative explanation is superconductivity in type-VIII sample arises from increase of electronic density of states around Fermi level E F in the BCS theoretical frame.

The dynamic studies of the hyperfine interactions and the lattice vibrational behavior are performed by means of temperature-dependent Mossbauer spectroscopy and X-ray diffraction on Bi-2223 superconducting phases. The Mossbauer studies show the anomalous lattice effect taking place above, but relatively close to, the transition temperature. The anomalous decrease of the isomer shift reveal the delocalization effect of d-electrons.

There is a great deal of interest vested in the superconductivity of Si clathrate compounds with sp3 network, in which the structure is dominated by strong covalent bonds among silicon atoms, rather than the metallic bonding that is more typical of traditional superconductors. A joint experimental and theoretical investigation of superconductivity in Al-substituted type-I silicon clathrates is reported. Samples of the general formula Ba8Si46−xAlx, with different values of x were prepared. With an increase in the Al composition, the superconducting transition temperature TC was observed to decrease systematically. The resistivity measurement revealed that Ba8Si42Al4 is superconductive with transition temperature at TC = 5.5 K. The magnetic measurements showed that the bulk superconducting Ba8Si42Al4 is a type II superconductor. For x = 6 sample Ba8Si40Al6, the superconducting transition was observed down to TC = 4.7 K which pointed to a strong suppression of superconductivity with increasing Al content as ...

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 magnetic properties of (Eu 1-x Y x )Ba 2 Cu 3 O 7−y (x=0 and 0.3) superconductors have been investigated by vibrating sample magnetometry (VSM) from 20 to 90 K and up to 8 T. Hysteresis loops of the Y-doped sample exhibit a strongly developed second peak at higher temperatures. The critical current densities derived from these data were nonmalised to the values at the peak position, J Cp , and analysed as a function of applied field normalised to the peak field. We discuss the experimental results in terms of a stress-field pinning model: The stress-field induced by lattice mismatch provides an effective pinning centre, in addition to the 211 phase precipitates and oxygen vacancies. We further suggest that in the Eu 0.7 Y 0.3 -123 sample, the oxygen-deficient region induced by the stress-field also adds to the pinning, and accounts for the enhancement of the peak effect in the J C -H curves of the Eu 0.7 Y 0.3 -123 superconductor.

In order to investigate the reason of Jc increment in (RE,Y)-123 superconductors, we prepare Eu-doped (Eu x Y 1−x )-123 ceramic samples with different Eu concentration. Positron lifetime and Doppler-broadening spectrum are simultaneously used to investigated the defects (stress field) verse different Eu-substitution. The experimental results support the stress-field pinning mechanism.

The dynamic studies of the hyperfine interactions and the lattice vibrational behavior are performed by means of temperature-dependent Mossbauer spectroscopy on the Bi-, Tl-, and Y-based superconductors, respectively. The temperature-dependent Mossbauer recoil-free factor f show that, above, but relatively close to, the transition temperature, there is an anomalous lattice effect which is related to the softening of phonon vibrational modes for Bi-2223, Tl-1223, and Y-123 superconductors. Moreover, for such three systems, the anomalous decrease of the Mossbauer isomer shift above Tc reveal the delocalization effect of d-electrons. Since the lattice effect is accompanied by d-electron delocalization effect at the almost same temperature region, it is believed that the structural instability involved in phonon softening is strongly interrelated to the change of the electronic states, and the phase separation behavior.

The phonon softening near T c on iron-doped Tl - 1223 phase samples were observed by 57 Fe Mo¨ssbauer measurements at temperatures ranging from 77K to 250K. Simultaneously, the temperature dependence of positron annihilation parameters on an iron-free sample show the anomaly of the electronic behavior in the temperature range from T c to T c +30K.

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.

The photoemission spectrum was measured on YNi2B2C and was found to be well coincident with calculation except in the vacinity of EF. The DOS peak at EF predicted by band theory has not been detected. The author suggests that the low energy excitation which is responsible for the suppression of DOS at EF, is responsible for the pairing of superconductivity as well.

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.

The occupied position of Fe in the superconducting phase of Fe-doped YBa2Cu2.8807-x was estimated by point charge model. The Mossbauer spectra were measured in the temperature range from 78 K to 273 K. Lattice softenings were found near 110 and 220 K. The observed variations of hyperfine parameters can be considered tobe related to the lattice softening near 110 K. It is suggested that the lattice softening near 110 K is related to the Peierls instability of the one-dimensional Cu(1)−0 chains.