We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of "FeAs" based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonalorthorhombic crystallographic phase transition at ϳ160 K followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder x-ray and neutrondiffraction measurements is presented. A magnetic moment of ϳ0.35 B per iron is derived from Mössbauer spectra in the low-temperature phase. Evidence of the structural transition is observed at temperatures well above the transition temperature ͑up to near 200 K͒ in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties ͑resistivity, thermal conductivity, Seebeck coefficient, and Hall coefficient͒, heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed.

We have measured the specific heat of crystals of (Ca 1-x Sr x ) 3 Ru 2 O 7 using ac-and relaxation-time calorimetry. Special emphasis was placed on the characterization of the Néel (T N =56 K) and structural (T c = 48 K) phase transitions in the pure, x=0 material.While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L ~ 0.3 R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-fieldlike step at T N , but its magntitude (∆c P ~ R) is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with full ordering of localized spins. The T N transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the T c transition decreases rapidly with Sr substitution and is not observable for our x=0.58 sample. PACS: 71.27.+a, 75.40.Cx, 65.40 The spin-structure of the double-layer, n =2, compounds have been especially unusual. In the pure strontium (i.e. x=1) salt, there is no resulting long range spin order at ambient pressure, but longitudinal strain can induce a ferromagnetic state [5,10]. In the pure calcium (x=0) salt, the spins order ferromagnetically within the bilayers, but the interlayer (c-axis) interactions are predominantly antiferromagnetic, leading to a Néel transition at T N = 56 K with spin polarization along the b-axis [6,[11][12][13]. With further cooling, the spin polarization rotates within the plane toward the a-axis [13,14]. There is a second, more unusual, phase transition at T c = 48 K; here all components of the susceptibility drop abruptly upon cooling through T c [11,14], which has been suggested to be due to formation of a charge-density-wave [15], while the lattice contracts along c by [3,11,20]. All crystals studied were characterized by single crystal or powder x-ray diffraction, EDS and TEM,indicating good crystal quality with no impurities or intergrowths or significant clustering of strontium ions. Magnetic and transport properties were measured using a Quantum Design MPMS XL 7T magnetometer. (Since the shape of all crystals studied in this work is essentially cubic, the demagnetizing factor for all samples and all three principal crystallographic axes is expected to be the same, and is not included in our the susceptibility is almost isotropic in the ab-plane (suggesting that the RuO 6 octahedra are almost "untilted", as in the pure Sr compound) with peaks in M at T p ~ 50 K followed by weak minima (T~ 40 K) and relatively high susceptibilities at lower temperatures.Only small anomalies are observed in the resistance at these temperatures, and the shape of the magnetic anomalies [4] suggests that spin ordering is complex but incomplete. A more complete discussion of the substitution-dependence of the resistance and magnetization, including their field dependence...