B.J. Gibson

Specific heat of MgB2 in a one- and a two-band model from first-principles calculations

The heat capacity anomaly at the transition to superconductivity of the layered superconductor MgB2 is compared to first-principles calculations with the Coulomb repulsion, µ*, as the only parameter which is fixed to give the measured Tc. We solve the Eliashberg equations for both an isotropic one-band model and a two-band model with different superconducting gaps on the π-band and σ-band Fermi surfaces. The agreement with experiments is considerably better for the two-band model than for the one-band model.

Ternary germanides LnAgGe (Ln = Y, Sm, GdLu) with ordered Fe2P-type structure

Abstract The title compounds were prepared from the elements by arc-melting and subsequent annealing at 970 K. They crystallize with the hexagonal ZrNiAl-type structure, a ternary ordered variant of the well known Fe2P type. The structures of GdAgGe, DyAgGe and ErAgGe (all space group P 6 2m ) have been refined from single-crystal X-ray diffractometer data: a = 715.4(2) pm, c = 423.7(1) pm, wR2 = 0.0467, 249 F2 values and 14 variables for GdAgGe; a = 710.67(7) pm, c = 419.73(5) pm, wR2 = 0.0463, 245 F2 values and 14 variables for DyAgGe: a = 706.34(3) pm, c = 416.87(2) pm, wR2 = 0.0514, 279 F2 values and 14 variables for ErAgGe respectively. The structures contain two crystallographically different germanium positions. Both of them are located in tricapped trigonal prisms: [Ge1Ln6Ag3] and [Ge2Ag6Ln3]. Magnetic susceptibility measurements show antiferromagnetic ordering at Neel temperatures of 13.0(2) and 3.3(1) K for GdAgGe and ErAgGe respectively. DyAgGe shows ferromagnetic ordering at TC = 14.0(2) K. The experimentally determined magnetic moments at high temperature compare well with those expected for the free Ln3+ ions. The latter three germanides are metallic conductors.

Synthesis, structure, and magnetic properties of LaTMg and CeTMg (T = Pd, Pt, Au)

The title compounds were prepared from the elements by reactions in sealed tantalum tubes in a water-cooled sample chamber of a high-frequency furnace. They crystallize with the ZrNiAl-type structure, space group P2m. The structures of the cerium compounds were refined from single-crystal x-ray diffraction data: a = 767.3(1) pm, c = 410.37(4) pm, wR2 = 0.0324, 521 F2-values for CePdMg; a = 755.02(7) pm, c = 413.82(4) pm, wR2 = 0.0393, 514 F2-values for CePtMg; and a = 774.1(3) pm, c = 421.6(1) pm, wR2 = 0.0355, 395 F2-values for CeAuMg, with 14 variables for each refinement. The palladium compound shows a small homogeneity range: CePd1+xMg1-x. The structures contain two crystallographically different transition metal sites T1 and T2 which are located in tri-capped trigonal prisms: [T1 Mg6Ce3] and [T2 Ce6Mg3]. Magnetic susceptibility and heat capacity measurements reveal long-range magnetic ordering at 2.1(2) K for CePdMg, 3.6(2) K for CePtMg, and 2.0(2) K for CeAuMg. Curie-Weiss behaviour at higher temperatures shows that the cerium ions are in the 3+ oxidation state. The isotypic LaTMg compounds are Pauli paramagnetic down to lowest temperatures (T = 0.3 K). All the compounds, RETMg (RE = La, Ce; T = Pd, Pt, Au) show metallic behaviour.

Magnetic ordering within the layered terbium carbide iodide, Tb2C2I2

Abstract The compound Tb 2 C 2 I 2 crystallizes with a layered structure (space group= C2/m ), which has a close-packed arrangement of I–Tb–Tb–I slabs. C 2 units are located within the Tb octahedral voids. Powder magnetic susceptibility measurements suggest two antiferromagnetic ordering transitions below 90 K. From the refinement of neutron powder diffraction patterns, the Tb moments order with a propagation vector k =(0,0,0). The refined Tb moments are 8.0(1) μ B at 10 K, and they are aligned antiferromagnetically in the ac plane.

Structure and chemical bonding of UAuGe

UAuGe was prepared from the elements by reaction in an arc-melting furnace and subsequent annealing at about 1200 K in a water-cooled silica tube in a high-frequency furnace. UAuGe crystallizes from the melt and is also stable at 920 K. It has the hexagonal YPtAs-type structure: P63/mmc, with a = 435.26(4) pm, c = 1547.4(1) pm, V = 0.2539(1) nm3, wR2 = 0.0785, 144 F2-values, and 12 variables. The structure of UAuGe may be considered as a superstructure with a quadrupled c-axis of the well known AlB2 type. The gold and germanium atoms order on the boron positions and form two-dimensionally infinite puckered layers of Au3Ge3 hexagons with intralayer Au-Ge distances of 257 pm. Between adjacent layers the gold atoms have weak secondary Au-Au interactions with Au-Au distances of 327 pm. Ab initio calculations of the electronic band structure using the tight-binding linear muffin-tin orbital method are presented. The bonding is illustrated by valence charge density and crystal orbital Hamiltonian population plots which are compared with those of ScAuSi which has a similar structure with Au-Au interactions between the layers. The Au-Au bonding is however much weaker in UAuGe than in ScAuSi. Resistivity measurements exhibit a non-metallic temperature dependence. The increase in resistivity towards lower temperatures is uncharacteristic of intermetallic compounds, and may be fitted to a Curie-Weiss-type formula, suggesting a direct correlation to the magnetic ordering. A maximum in the resistivity is observed at T = 26(1) K.

Crystal and magnetic structure of antiferromagnetic HoAuGe

HoAuGe was prepared from the elements by arc melting and subsequent annealing at 1070 K. Its crystal structure was refined from single-crystal x-ray diffractometer data: a = 440.10(5) pm, c = 723.26(9) pm, V = 0.1213(1) nm3, wR2 = 0.0640, 225 F2-values and ten variables. The nuclear structure was confirmed by neutron powder diffraction. HoAuGe adopts the NdPtSb-type structure. The gold and germanium atoms form two-dimensional infinite [AuGe] polyanions with intralayer Au-Ge distances of 260.5 pm, while the interlayer Au-Ge distances of 304.1 pm are much longer. The polyanions are separated by the holmium ions. Magnetic susceptibility, specific heat capacity and neutron diffraction measurements reveal antiferromagnetic ordering at TN = 5.6(2) K with further magnetic reordering at T1 = 3.5(2) K and T2 = 2.4(2) K. At higher temperatures Curie-Weiss behaviour is observed with µeffexp = 10.2(2) µB and Θp = 0(1) K. From the magnetic specific heat a fourfold-degenerate ground state is found. The magnetic structure at 1.3 K is described by the propagation vector = (1/2,0,0) with the Ho spins oriented within the bc-plane. As the temperature increases towards TN, the magnetic structure becomes incommensurate, which accounts for the observed additional transitions below TN.

Magnetic structure determination of CeAuGe and CeAgGe

Abstract The contrasting magnetic structures of CeAuGe and CeAgGe are determined by neutron diffraction on powdered samples. In the case of CeAuGe, the magnetic peaks are found at the same positions in 2θ as the nuclear peaks, confirming ferromagnetic ordering. The Ce moments lie within the ab-plane, and have a magnitude of μCe≈1.1μB. In the case of CeAgGe, the magnetic peaks can be indexed on the basis of a k=[ 1 3 , 0, 0] propagation vector. Again the Ce moments lie within the ab-plane but with an increased magnetic moment of μCe≈1.6μB.

Susceptibility, specific heat, and transport properties of CeAuGe and GdAuGe

The intermetallic compounds CeAuGe and GdAuGe (ordered CaIn2-structure type) were prepared by arc melting the elements. The specific heatscp(T) were measured between 1.5 and 50 K, the thermal conductivity λ(T) and the electrical resistivity ρ(T) from 1.5 to 300 K. GdAuGe shows antiferro-, CeAuGe a ferromagnetic ordering atTN-16.9 K andTC=10.0 K, respectively. The magnetic, electronic, phononic and crystal field contributions tocp(T) are separated and discussed in connection with measurements of the susceptibility χ(T). The contributions to λ(T) and ρ(T) are discussed in relation to the magnetic properties.

Antiferromagnetic Ordering of HoAuGe

The magnetic properties of the ternary rare earth compound HoAuGe have been extensively studied as part of recent work carried out on the series REAuGe. The nuclear stucture has been shown to be an ordered variant of the hexagonal CaIn2 type. The magnetic and electrical properties were investigated using several techniques. a.c and d.c susceptibility measurements suggest antiferromagnetic ordering atTN=6K. At higher temperatures Curie-Weiss behaviour is observed, with μexp=10.2μB. The heat capacity also confirms a magnetic transition atTN=6K, with two additional anomalies below. Powder neutron diffraction reveals additional peaks which can be indexed with a propagation vector (1/2,0,0).