We report resistivity, magnetic, thermodynamic, and neutron-diffraction data for the heavy-fermion compound Ce5Sn3. These data indicate an antiferromagnetic phase transition at TN=17.5 K, but at lower temperatures the linear coefficient of specific heat (T)=C(T)/T rises to 1.6 J/f.u. mol K2, characteristic of a moderately-heavy-fermion compound. Only a small amount of entropy is liberated by the phase transition (R ln2), whereas most of the entropy [(34)R ln2] is liberated by heavy-fermion processes. This implies a small ordered moment. Further, neutron diffraction implies a rather complicated magnetic order. We discuss two alternatives: (a) that ordering occurs on the Ce site of higher symmetry, while heavy-fermion behavior occurs on the lower-symmetry Ce site; and (b) that itinerant-fermion antiferromagnetism occurs. Due to the overall low site symmetries, crystal-field effects are strong; in particular, the susceptibility is highly anisotropic (by a factor of at least 20 at low temperatures) and enhanced by the presence of strong ferromagnetic correlations in the heavy-fermion state. © 1991 The American Physical Society.

Ce3Bi4Pt3 is a cerium compound that exhibits a hybridization gap and concomitant semiconducting behavior in the coherent ground state. Using neutron powder diffraction, we have determined the variation of the lattice constant with temperature at two pressures (P=10-3 and 17.7 kbar), and determined the bulk modulus and atomic mean-square displacements for Ce3Bi4Pt3 and its normal analog, La3Bi4Pt3. The thermal expansion Δβ in Ce3Bi4Pt3 exhibits a maximum at Tmax=50 K at ambient pressure. Below 50 K, the bulk modulus ΔB is proportional to TΔβ. We apply a Grüneisen analysis, under the assumption that the free energy exhibits T/T0(V) scaling. The Grüneisen parameter deduced from the ratio ΔB/TΔβ is consistent with the value Ω=36 deduced from the pressure variation of Tmax. The analysis allows us to predict the temperature dependence of the 4f specific heat (ΔCΔβ/Ω), which thus has a maximum at 50 K and a high-temperature entropy nearly equal the expected value R ln6. We argue that the maximum at 50 K reflects an (indirect) hybridization gap of order 100 K. We show further that the lattice constant anomaly Δa0 is proportional to the effective moment Tχ; this suggests that a relationship known to be valid for antiferromagnets, namely (Tχ)/TΔC where ΔC is the specific heat, may be valid for Ce3Bi4Pt3. Finally, we show that the temperature dependence of the mean-square atomic displacements is nearly identical for Ce3Bi4Pt3 and La3Bi4Pt3 and can be fitted by a simple Debye-Waller model; hence the expansion anomaly does not affect the average lattice dynamics. © 1992 The American Physical Society.

The T{star, open}-phase is a mixed rare earth oxide phase forming at the composition R2CuO4. Its crystal structure is intermediate between the La2CuO4 and Nd2CuO4 structures. Sr-doped samples have Tc onsets as high as 37 K. We discuss the systematics of Tc of T{star, open}-phases as a function of Sr doping, rare earth mixture and oxygenation. Structural and transport data will also be presented. © 1989.

The structure of the newly discovered "1201" lead cuprate superconductor (Pb, Cu) (Sr, La)2CuO5-δ with Tc=27.5 K at onset and a shielding fraction 38% at 5 K and 10 Oe, has been determined using neutron powder diffraction. The structure is similar to the other 1201 materials TlBa1.2La0.8CuO5 and Tl0.5Pb0.5Sr2CuO5 (where the former superconductors with a Tc of K and the latter is not superconducting), belonging to the space group P4/mmm. The doping scheme in (Pb, Cu)(Sr, La)2CuO5-δ combines the doping scheme used in these two compounds, in that both the TI and Sr sites are doped. The starting stoichiometry, the refined scale factors for the impurity phases and the refined site occupancies for oxygen suggests that the stoichiometry (relative to Cu) of the superconducting phase is Pb0.60Cu0.40Sr1.08La0.92Cu04.96. Calculation of the average hole concentration in these compounds from charge summation is difficult with these compounds because the TI/PbO layers provide polarizable charge reservoirs that can participate in substantial covalent bonding and because of the probable mixed-valent nature of Tl and Pb. Nevertheless, bond valence sums calculated for the Cu ions in the CuO2 layers for the three 1201 cuprates do provide a correlation with the values of Tc or the absence of superconductivity. © 1991.

We present neutron diffraction results for YbIn(Formula presented), and Rietveld refinements of the structure. The ground state has the same structure (C15B) as the high temperature state, so that the first-order phase transition at (Formula presented) K is indeed isomorphic, i.e., the lattice constant changes without a change of crystal symmetry. The site disorder and the diffraction linewidths decrease systematically on going from polycrystal samples with two transitions (at 40 and 70 K) to polycrystals with a single (40 K) transition to flux-grown single crystals with a sharp transition at 40 K. We argue that in site-disordered samples, the effect of doping Yb onto the In site is to increase (Formula presented) and cause the transition to become continuous. © 1996 The American Physical Society.

We have determined the structure of the newly discovered electron-doped superconductor Nd1.85Ce0.15CuO4- at 11 and 300 K using high-resolution neutron-powder-diffraction techniques. Both nonsuperconducting oxygenated and superconducting Zr-gettered (Tc=24 K with shielding and Meissner fractions of 30 and 16% at 5 K, respectively) samples were investigated to study the effect of Zr gettering on the crystal structure and oxygen stoichiometry. We find that the structure remains tetragonal (I4/mmm), with no evidence for Nd/Ce ordering, for both samples at both temperatures. Structure and oxygen stoichiometry of the oxygenated and Zr-gettered samples remain remarkably similar at both temperatures, the only difference being a slight decrease (but greater than three standard deviations) in the thermal parameters of the rare-earth ions along the c axis for the superconducting sample. Thermogravimetric analyses confirm the oxygen stoichiometry determined from the structural refinements. © 1989 The American Physical Society.

Measurements of both the average crystal structure from Rietveld refinement of neutron powder diffraction data and the local structure from La LIII-edge x-ray-absorption fine structure (XAFS) are presented for a La11B6 sample as a function of temperature (∼10-320 K). These data are compared to XAFS results on a EuB6 sample. The single-site La and B positional distribution widths and the La-B and La-La bond length distribution widths and their temperature dependence are compared. This comparison allows an estimate of the La and B site displacements, and we find that these sublattices are only slightly correlated with each other. Moreover, while the temperature dependence of the displacement parameters of the average sites obtained from diffraction fit an Einstein model well, the temperature dependence of the La-B bond length distribution width requires at least two vibrational frequencies, corresponding to the La and B frequencies of the individual sites. XAFS data on EuB6 indicate that the situation is the same in the Eu compound. In addition, comparisons between data taken below and above the ferromagnetic transition temperature for EuB6 place stringent limits on the lattice involvement in the associated metal-insulator transition and the ensuing large magnetoresistance effect. This lack of lattice involvement in the magnetoresistance transition is in sharp contrast to the strong lattice involvement observed in the colossal magnetoresistance lanthanum manganese perovskites.

We report measurements of the low-temperature specific-heat coefficient (Formula presented) cell volume (Formula presented) Hall coefficient (Formula presented) and valence (Formula presented) [where the Yb hole occupation (Formula presented) was determined from (Formula presented) x-ray absorption] of single crystals of (Formula presented) Alloying (Formula presented) with Ag increases the temperature (Formula presented) of the first-order isomorphic phase transition and causes it to terminate at a critical point at (Formula presented) and (Formula presented) The variation of (Formula presented) near the critical point is well described by a mean-field equation of state. The phase transition involves a large change in the Kondo temperature, and the transition temperatures (Formula presented) are of order of the Kondo temperatures (Formula presented) of the high-temperature state. The cell volume is found to vary proportionally to (Formula presented) At low temperatures, well away from the transition, the Wilson ratio of the susceptibility χ(0) and specific heat coefficient γ falls within 20% of the value predicted for a Kondo impurity, and (Formula presented) and χ(0) are roughly proportional as predicted from the Anderson model. The temperature dependence (Formula presented) for temperatures away from the phase transition also fits the predictions of the Kondo model. The small volume discontinuity (Formula presented) observed at (Formula presented) suggests that the phase transition is not due to a Kondo volume collapse. The large Hall coefficients (Formula presented) observed for (Formula presented) and (Formula presented) suggest instead that a low carrier density in the high-temperature state plays a key role in the phase transition. © 1997 The American Physical Society.

Properties of the hole-type, single-layer cuprate T*-phase superconductors are discussed. There is a strong variation of Tc with rare-earth constituent and oxygenation pressure. The magnetic properties of these materials have not yet been well investigated, but some aspects of the rare-earth ordering appear to be different from other known cuprates. © 1991.

We have discovered a new family of rare earth compounds that we identify as R2-xPt4Ga8+y. Single crystals of this family have been grown for RLa, Ce, Pr, Nd, Sm, Gd, Er, Yb and Y. Many members (cerium, praseodymium, neodymium, samarium and erbium) show large anisotropies in their magnetic susceptibility; further, electrical resistance, magnetic susceptibility and specific heat measurements on the cerium member indicate that it is very possibly a heavy-electron compound with an electronic specific heat γ ≈ 1 J K-2 mol-1. Additionally, other members of the family have a magnetic transition at low temperatures. The electrical resistance, magnetic susceptibility and specific heat in fields will be presented for the praseodymium-based compound in which there is a field-induced transition. © 1992.