C. Schlenker

Structure cristalline d'un conducteur métallique bidimensionnel: Le bronze violet de potassium et molybdene K0.9Mo6O17

Abstract Single crystals of the purple bronze K0.9Mo6O17 have been grown by a electrolytic reduction technique. Measurements of the electrical resistivity show that this compound is a quasi two-dimensional metal. A crystal structure determination is used to explain the strongly anisotropic resistivity. K0.9Mo6O17 is found to be trigonal with lattice parameters: a = 5.538 A and c = 13.656 A; Z = 1. The space group is P 3 . The K cations are surrounded by 12 oxygen anions which form icosahedral sites. One third of the Mo cations (Mo(1)) occupy tetrahedral sites, while the Mo(2) and Mo(3) cations have octahedral surroundings. The structure can be described as slabs of ReO3-type connected by KO12 icosahedra. Each slab is built up of 4 layers, parallel to the (001) plane, of Mo(2) and Mo(3) octahedra sharing corners. Two slabs are connected by K icosahedra which share 2 faces with the upper and lower neighboring Mo(3) octahedra. Each K icosahedron is edge linked to 3 upper and 3 lower Mo(1) tetrahedra. Each Mo(1) tetrahedron is itself corner linked to 3 Mo(3) octahedra of the neighboring layer. The deformations of the occupied polyhedra are a good illustration of the Paulings third rule. There is no Mo(1)OMo(1) bonding, so that the MoOMo bonding, infinite in the a and b directions, is disrupted in the c direction. The Zachariasens bond length-bond strength relation has been applied to the MoO bonds. The computed effective mean Mo valences are +6 for Mo(1) on tetrahedral sites and about 5.1 and 5.8, respectively, for Mo(2) and Mo(3) on octahedral sites. The 4d electrons of Mo atoms are so located in the two-dimensional infinite slabs of octahedra. The conduction band is expected to be an antibonding π∗ band resulting from the hybridization of the Mo 4d t2g and oxygen pπ states. Thus, the structural properties should lead to a very anisotropic Fermi surface and thus, to a quasi two-dimensional electrical conductivity.

Metal-insulator transitions and phase diagram of (Ti1-xVx)4O7: electrical, calorimetric, magnetic and EPR studies

The electrical resistivity, magnetic susceptibility and EPR of (Ti1-xVx)4O7 single crystals (0 0.35 at.%, only the disordered pairs and metallic phases are found. The complete phase diagram has been obtained by electrical resistivity studies and corroborated by all other data. The electrical properties of the disordered phase are attributed to bipolaron mobility; the bipolaron binding energy Eb is estimated from these data and it is shown that vanadium induces pairs with a smaller Eb in its vicinity.

Weak magnetism in the two dimensional purple bronze K0.9Mo6O17

Abstract Magnetoresistance, magnetization and 57 Fe Mossbauer studies in K 0.9 Mo 6 O 17 single crystals are reported. A large and positive magnetoresistance, sigmoidal magnetization curves and a quadrupole splitting of the Mossbauer spectrum are found at low temperatures. The results are discussed in relation with the onset of a charge density wave below 120 K.

Crystal structure of Mn3Si2Te6

The synthesis, magnetic, and electrical properties of a new silicon manganese telluride, semiconducting and ferrimagnetic, has been previously reported. The X-ray determination of the crystal structure of this compound was achieved by means of direct methods and three-dimensional Fourier syntheses. The final least-squares Rw factor of the refinement is Rw = 0.035. According to this study the chemical formula is Mn3Si2Te6 with Z = 2, the trigonal unit cell is a = 7.029(2), c = 14.255(3) A, the space group is P31c. The crystal structure can be described as one of a layer-type compound. Along the c-axis direction, we observe a fully occupied layer of Mn or Si pairs octahedra, then a partially occupied layer of ordered Mn octahedra, and so on. The surroundings of Si atoms are similar to the one found in Si2Te3, and the fully occupied layer of octahedra may be compared to the one found in Fe2P2Se6.

A new semiconducting ferrimagnet: A silicon manganese telluride

Abstract MnSiTe3 has been synthetized for the first time; single crystals have been grown by the vapor phase transport. X-ray studies show that the crystal structure is hexagonal with a=7.02 A and c=14.26 A. The samples are found ferrimagnetic with an ordering Curie temperature Tc of 82 K. In the paramagnetic state, the susceptibility follows a Curie-Weiss law, with a Curie-Weiss temperature θ of (-75±5) K and an effective magnetic moment of 5.5 μB per Mn, roughly consistent with Mn2+, in a high spin state. EPR spectra show a single line, with a g-value of 2.00, also consistent with Mn2+ ions. In the ferrimagnetic regime, the magnetization curve for H  c and the hysteresis loop for H  a, have been measured; they show that the plane ⊥c is an easy magnetic plane. Saturated and remanent magnetizations, as well as the magnetization obtained by cooling the sample without an applied magnetic field are reported as a function of temperature. The saturated magnetization extrapolated to 0 K is found to be ∼13000 emu/mol. The crystals are semiconducting, with an anomaly in the resistivity just below Tc and different electrical regimes in the paramagnetic and ferrimagnetic states. These results are discussed.

From the spin glass to the antiferromagnetic regime in (Ti1-xVx)2O3

(Ti1-xVx)2O3 single crystals with 0.10 <x<0.50 are shown to be mictomagnetic with a peak in the initial susceptibility at a temperature TB and remanent magnetizations for T < TB. For x 0.50, susceptibility and electrical resistibility and electrical resistivity data give experimental evidence for an antiferromagnetic metallic phase.

Landau diamagnetism in the charge density wave state of γ-Mo4O11

Abstract The quasi-two-dimensional oxide γ-Mo 4 O 11 shows a charge density wave (CDW) transition at T c = 100 K . The magnetic susceptibility χ, measured between 300 and 4.2 K, shows a decrease and a large anisotropy below 100 K. This is due to an anisotropic Landau diamagnetism, associated to small closed orbits induced by the CDW gaps.

Magnetic properties of vanadium-doped titanium diselenide

We have performed magnetic susceptibility measurements of Ti1−xVxSe2 for 0.1% < χ < 30% in the temperature range 4.2–300 K in order to elucidate the nature of the strong effect of V on the transport properties near x = 1%. Local moments on V are found with μeff anomalously large for x < 1% and decreasing smoothly with increasing x.

Magnetic susceptibility and EPR of europium compounds: EuGa2S4, EuGa2Se4 and EuAl2S4

Abstract The magnetic susceptibility of EuGa2S4, EuGa2Se4 and EuAl2S4, measured between 4.2 and 300 K, is found to obey Curie-Weiss laws. EPR spectra show a single broad line at g = 2.00, characteristic of Eu2+; the linewidth (in the range 900–1500 G at 200 K) is found to increase steeply below ∽ 50 K.

Neutron investigations of the antiferromagnetic structure of metallic (Ti1−xVx)2O3

Abstract Single crystal neutron studies at 4.2 K of (Ti 1−x V x ) 2 O 3 ( x = 0.80) show that the extra magnetic peaks can be indexed on the multiple hexagonal cell. The temperature dependence of their intensity is in agreement with the value of the Neel temperature (≈ 42 K). None of the known magnetic structures for corundum-type compounds accounts for the data.