Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$-point depend critically on the Fe magnetic moment and atomic volume. Analyzing these non vibrational modes leads to the discovery of a stable monoclinic ground state structure which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice degrees of freedom contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as the electronic and magnetic subsystems and therefore needs to be included in thermodynamic modeling.Comment: 15 pages, 12 figure

The system Ce–Zn–Si for <33.3 at.% Ce: phase relations, crystal structures and physical properties

Phase equilibria of the system Ce–Zn–Si have been determined for the isothermal section at 600 °C for <33.3 at.% Ce by XRPD and EPMA. This partial section is characterized by the formation of five ternary compounds with homogeneity regions at constant Ce-content and partial substitution of Zn/Si: τ1-Ce7Zn21(Zn1−xSix)2 (unique type; 0.45 ≤ x ≤ 0.99), τ2-Ce(Si1−xZnx)2 (AlB2-type; 0.36 ≤ x ≤ 0.73), τ5-CeZn(Zn1−xSix)2 (CeNiSi2-type; 0.68 ≤ x ≤ 0.76), τ6-CeZn2(Si1−xZnx)2 (ThCr2Si2-type; 0.25 ≤ x ≤ 0.30) and τ7-Ce37Zn48Si15 (structure unknown). Whereas τ1, τ2 and τ5 are stable at 600 and 800 °C, the phases τ6, τ7 are unstable at 800 °C. Atom site distribution in the crystal structures of τ5, τ6 and {La,Ce}7Zn21(Zn1−xGex)2 have been elucidated from X-ray intensity refinements on single crystals. The small amounts of the stabilizing tetrel element in {La,Ce}7Zn21[Zn1−xSi(Ge)x]2 suggest a hypothetical binary phase “{La,Ce}7Zn23”. The stabilizing effect of Ge in Ce7Zn23−xGex has been elucidated from density functional theory (DFT) calculations discussing the electronic structure in terms of the density of states (DOS) and defining enthalpies of formation for Ce7Zn23−xGex (x = 0, 0.5, 2) as well as for several neighbouring binary Ce–Zn phases. A Schultz–Scheil diagram for the solidification behaviour in the (Zn,Si)-rich part of the diagram was constructed from DTA measurements in closed silica crucibles along with partial isothermal sections determined in the temperature range from 400 to 900 °C. The phases τ5 and τ6 both form in degenerate ternary peritectic reactions: L + CeSi2,β-Ce2Zn17 ⇔ τ5 at 865 ± 5 °C and L + τ5,CeZn11 ⇔ τ6 at 695 ± 5 °C, respectively. Magnetic susceptibility, specific heat and resistivity measurements of τ5-CeZn(Zn1−xSix)2 revealed Kondo lattice behavior with ferromagnetic ordering below TC = 4.4 K, whereas susceptibility and specific heat studies of τ6-CeZn2(Zn0.28Si0.72)2 revealed Curie–Weiss paramagnetic behaviour down to 3 K. The effective paramagnetic moments of Ce obtained from Curie–Weiss fits of τ5 (2.50μB) and τ6 (2.34μB) reveal a ground state close to trivalent Ce

Superconductivity and magnetism in MPt4Ge12, M = Ca, Ba, Sr, Eu

X-ray powder data for Ba0.8Ca0.2Pt4Ge 12 and X-ray single crystal data for EuPt4Ge12 define cubic body-centered symmetry consistent with novel Ge-based skutterudites SrPt4Ge12 and BaPt4Ge12 (space group Im3). Structural and electron microprobe analysis investigations evidence a complete filling of the icosahedral cages without large atomic displacement parameters. Ba0.8Ca0.2Pt4Ge12 exhibits phonon-mediated superconductivity at Tc = 5.2 K. Density functional theory (DFT) calculations (LDA+U) carried out for EuPt 4Ge12 proved that Eu guest atoms strongly stabilize the compound in which the calculated density of states around the Fermi energy essentially consists of hybridized Ge 4p-like and Pt 5d-like states. Low temperature resistivity studies evidence magnetic ordering at Tm ? 1.7 K. Susceptibility measurements reveal a divalent state for europium, in excellent agreement with the DFT calculations