A. V. Krechetov

Superhard superconducting materials based on diamond and cubic boron nitride

Superhard superconducting samples with a critical temperature of Tc = 12.6 K are obtained when synthetic diamond powders that were preliminarily coated with a niobium film are sintered at a pressure of 7.7 GPa and a temperature of 1973 K. Superhard superconductors with Tc = 9.3 K are obtained when diamond and molybdenum powders are sintered at a pressure of 7.7 GPa and a temperature of 2173 K. Superconducting samples with Tc = 36.1–37.5 K have been obtained in the systems diamond-MgB2 and cubic boron nitride-MgB2.

Electronic properties of single-crystal diamonds heavily doped with boron

Single-crystal diamonds with characteristic sizes of 2–7 mm doped with boron in the concentration range 1019–1020 cm−3 have been grown by the temperature gradient method at high static pressures. The temperature dependence of the resistance R of the synthesized single crystals has been measured in the range 0.5 K < T < 297 K. An activated dependence R(T) with an activation energy of about 50 meV is observed in the range from room temperature to T ≈ 200 K. At temperatures below approximately 50 K, the temperature dependence of the conductivity for heavily doped crystals is proportional to T1/2, which is characteristic of degenerate semiconductors with a high number of defects.

Study of the paramagnetic centers in heterofullerides MnM′3-nC60 (M = K, Rb, Cs; M′ = Be, Mg, Ca, Ba; n = 1, 2)

New heterofullerides Cs2MC60, CsM2C60, Rb2MC60, K2MC60, and KM2C60 (M = Be, Mg, Ca, Ba) have been synthesized; the temperature dependences of the magnetic susceptibilities of these compounds in the temperature range from 4.2 to 297 K have been measured. Among these heterofullerides, K2MgC60, KMg2C60, K2CaC60, K2BeC60, and Rb2BeC60 pass to the superconducting state at temperatures Tc = 13–24.3 K. The paramagnetic electronic states of the compounds have been studied by EPR at temperatures of 105–300 K, which shows the existence of two types of paramagnetic centers, related to oxygen defects and conduction electrons.

Superconductivity, Electron Paramagnetic Resonance, and Raman Scattering Studies of Heterofullerides with Cs and Mg

In the present study, the results of investigation of physical properties of heterofullerides (A=K, Rb, Cs, M=Be, Mg, Ca, Al, Fe, Tl, ); as well as , , and are described. All of the fullerides were synthesized by the exchange reactions of alkaline fullerides with anhydrous metal halides. Superconductivity was found in and .

Magnetic and structural anomalies of NanC60 (n = 2, 3)

Sodium fullerides NanC60 (n = 2, 3) have been synthesized by a liquid phase reaction and investigated with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), electron paramagnetic resonance, and differential thermal analysis. XRD data indicate that the crystal structure of Na2C60 at 300 K is face centered cubic (FCC). A phase transition from primitive cubic to FCC crystal structure has been observed in this work in Na2C60 fulleride at 290 K. The transition is accompanied by the step-like change of paramagnetic susceptibility. The crystal structure of Na3C60 is more complicated than, and different from, what has been reported in the literature. A nearly seven-fold increase of paramagnetic susceptibility with increasing temperature has been observed in the Na3C60 fulleride at 240–260 K. In the same temperature range, a new line at about 255 ppm appears in the 23Na NMR spectrum, indicating a significant increase of electron density near the Na nucleus. The observed effect can be explained by a metal-insulator transition caused by a structural transition.

Features of structure and properties of NanC60 (n = 2, 3) fullerides synthesized in toluene

The structure and electron properties of NanC60 (n = 2, 3) sodium fullerides synthesized from simple compounds in toluene were studied. It was shown that Na2C60 fulleride forms a face-centered cubic lattice at temperatures above 300 K. As the temperature is lowered, the phase transition to a structure with a simple cubic lattice takes place. The temperature dependences of the properties of Na3C60 with a more complex structure exhibit features that are presumably due to sodium atom redistribution in the Na3C60 fulleride lattice and the formation of sodium ion clusters.

Superconductivity and spectroscopy of heterofullerides Rb2MC60, K2MC60, and KM2C60 (M = Mg, Be)

A series of new heterofullerides with compositions Rb2MC60, K2MC60, and KM2C60 (M = Mg, Be) have been synthesized. Measurements of the temperature dependences of the magnetic susceptibility in the temperature interval from 4.2 to 300 K reveal a superconducting transition in heterofullerides K2MgC60, KMg2C60, K2BeC60, and Rb2BeC60 at temperatures Tc = 13–22 K. The electron states with uncompensated spin are studied by the electron paramagnetic resonance technique. The contributions of conduction electrons and localized electrons to the paramagnetic susceptibility are extracted.

Heterometallic Fullerides of Transition Metals with the Composition K2MC60

Heterometallic fullerides with composition K2C60, synthesized by exchange chemical reaction of K5C60 or K4C60 with chlorides of metals Fe and Cu groups have been investigated by X-ray diffraction, magnetic resonance, Raman and Mossbauer spectroscopy. Magnetization and susceptibility measurements also have been carried out. Metal chlorides from Fe and Cu groups enable to cover the whole range of electronic configuration of metal from d1 to d10. Heterometallic fullerides with M=Cu+2, Fe+2, Fe+3, V+2, Cr+3 and Ni+2 appeared to be superconductors with Tc=13.9–16.5K.