Micrometre-sized MgB2 crystals of varying quality, synthesized at low temperature and autogenous pressure, are compared using a combination of Raman and infra-red (IR) spectroscopy. These data, which include new peak positions in both spectroscopies for high quality MgB2, are interpreted using DFT calculations on phonon behaviour for symmetry-related structures. Raman and IR activity additional to that predicted by point group analyses of the P6/mmm symmetry are detected. These additional peaks, as well as the overall shapes of calculated phonon dispersion (PD) models are explained by assuming a double super-lattice, consistent with a lower symmetry structure for MgB2. A 2× super-lattice in the c-direction allows a simple correlation of the pair breaking energy and the superconducting gap by activation of corresponding acoustic frequencies. A consistent physical interpretation of these spectra is obtained when the position of a phonon anomaly defines a super-lattice modulation in the a-b plane.

The surface Li depletion affects the determination of optical gap for LiFePO4, which was previously used for validation of DFT calculations.

We show that the well-known Kohn anomaly predicts Tc for ordered AlB2-type structures. We use ab initio density functional theory to calculate phonon dispersions for Mg1-xAlxB2 compositions and identify a phonon anomaly with magnitude that predicts experimental values of Tc for all x. Key features of these anomalies correlate with the electronic structure of Mg1-xAlxB2. This approach predicts Tc for other known AlB2-type structures as well as new compositions. We predict that Mg0.5Ba0.5B2 will show Tc = 63.6 ± 6.6 K. Other forms of the Mg1-xBaxB2 series will also be superconductors when successfully synthesised. Our calculations predict that the end-member composition, BaB2, is likely to show a Tc significantly higher than currently achieved by other diborides although an applied pressure ∼16 GPa may be required to stabilise the structure.

We have studied weak links and dc–SQUIDs made from pulsed laser deposited YBa2Cu3O7−δ thin films on Y–ZrO2 bicrystal substrates. The transport properties of the weak links were studied as a function of the misorientation angle (θ) between the two halves of the bicrystal and an exponential dependence of the weak link critical current density was observed for angles up to 40° at 77 K. Josephson effects with clear microwave and magnetic field responses were observed. An optimum dc–SQUID performance at 77 K was obtained for θ=32°. At this temperature, we achieved a periodic magnetic field response with a modulation depth of 12 μV.

The ground state physical properties of the newly synthesized 312 MAX compound, Hf 3 AlC 2 have been investigated using the first-principles density functional theory (DFT). The optimized unit cell parameters show good agreement with the experimental values. The calculated elastic constants and phonon dispersion confirm the mechanical and dynamical stabilities of this new compound. High bulk modulus, combined with low shear resistance and low Vickers hardness, indicates good machinability of Hf 3 AlC 2 , as expected for a metallic compound. On the other hand, significant stiffness due to large Young's modulus as well as the brittle nature according to the calculated Pugh's and Poison's ratios and Cauchy pressure are comparable to that of a ceramic. The present calculations show that Hf 3 AlC 2 is elastically and optically anisotropic. The chemical bonding in Hf 3 AlC 2 consists of a mixture of metallic, covalent and ionic contributions. The calculated Fermi surface contains quasi-twodimensional topology, which indicates possible superconductivity of Hf 3 AlC 2 . The new phase Hf 3 AlC 2 may also be a promising thermal barrier coating (TBC) material. The calculated enthalpy and entropy are found to increase with temperature above 100 K though a decrease is observed for the free energy.

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High quality, micron-sized interpenetrating grains of MgB2, with high density, are produced at low temperatures (~420 °C < T < ~500 °C) under autogenous pressure by pre-mixing Mg powder and NaBH4 and heating in an Inconel 601 alloy reactor for 5–15 h. Optimum production of MgB2, with yields greater than 75%, occurs for autogenous pressure in the range 1.0 MPa to 2.0 MPa, with the reactor at ~500 °C. Autogenous pressure is induced by the decomposition of NaBH4 in the presence of Mg and/or other Mg-based compounds. The morphology, transition temperature and magnetic properties of MgB2 are dependent on the heating regime. Significant improvement in physical properties accrues when the reactor temperature is held at 250 °C for >20 min prior to a hold at 500 °C.