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Lecture notes in Chemistry

The lattice dynamics in the polycrystalline shape-memory Heusler alloy Ni50Mn35In15 have been studied by means of resonant ultrasound spectroscopy (RUS). RUS spectra were collected in a frequency range 100-1200 kHz between 10 and 350 K. Ni50Mn35In15 exhibits a ferromagnetic transition at 313 K in the austenite phase and a martensitic transition at 248 K accompanied by a change of the magnetic state. Furthermore it displays a paramagnetic to ferrimagnetic transition within the martensitic phase. We determined the temperature dependence of the shear modulus and the acoustic attenuation of Ni50Mn35In15 and compared it with magnetization data. Following the structural softening, which accompanies the martensitic transition as a pretransitional phenomenon, a strong stiffening of the lattice is observed at the martensitic magneto-structural transition. Only a weak magnetoelastic coupling is evidenced at the Curie temperatures both in austenite and martensite phases. The large acoustic damping in the martensitic phase compared with the austenitic phase reflects the motion of the twin walls, which freezes out in the low temperature region.

https://iopscience.iop.org/article/10.1088/0953-8984/27/41/415402/pdf

Strain behavior and lattice dynamics in Ni50Mn35In15.

Tight-binding band structure calculations have been performed on both the NiAs (hexagonal) and MnP (orthorhombic) structures of vanadium sulfide VS. A band folding procedure is used to generate the electronic structure of the hexagonal phase in orthorhombic symmetry and allows one to understand the reasons for the NiAs to MnP structural distortion as vanadium vacancies in VS are filled. When a critical electron filling of the metal d bands is reached, a second-order Jahn-Teller mixing operates to stabilize the MnP structure and induces the concomitant formation of metal-metal zig zag chains. The resulting “dip” in the density of states at the Fermi level is analyzed in conjunction with the experimentally observed changes in conductivity, and Knight shift measurements. Finally, the computed relative stability of the two structures for different metal monosulfides is shown to agree qualitatively with the experimental data available.

A second-order Jahn-Teller distortion in the solid: the phase transition in VSx

An ordered form of niobium intercalated NbS2 has been prepared and its crystal structure determined by single-crystal X-ray diffraction techniques. The new ordered structure is a 3 1 2 × 3 1 2 superstructure of the 2H-NbS2 structure. The restraints placed on the ordering process by symmetry are analyzed using the theory of symmetry and phase transitions due to Landau, and it is shown that the ordering occurs as a weakly first-order phase transition.

Nb1.25S2: an ordered modification of 2H-Nb1+xS2

The lattice dynamics in the polycrystalline shape-memory Heusler alloy Ni$_{50}$Mn$_{35}$In$_{15}$ has been studied by means of resonant ultrasound spectroscopy (RUS). RUS spectra were collected in a frequency range $100-1200$ kHz between 10 and 350 K. Ni$_{50}$Mn$_{35}$In$_{15}$ exhibits a ferromagnetic transition at 313 K in the austenite and a martensitic transition at 248 K accompanied by a change of the magnetic state. Furthermore it displays a antiferromagnetic to ferromagnetic transition within the martensitic phase. We determined the temperature dependence of the shear modulus and the acoustic attenuation of Ni$_{50}$Mn$_{35}$In$_{15}$ and compared it with magnetization data. Following the structural softening, which accompanies the martensitic transition as a pretransitional phenomenon, a strong stiffening of the lattice is observed at the martensitic magneto-structural transition. Only a weak magnetoelastic coupling is evidenced at the Curie temperatures both in austenite and martensite phase. The large acoustic damping in the martensitic phase compared with the austenitic phase reflects the motion of the twin walls, which freezes out in the low temperature region.

Strain behavior and lattice dynamics in Ni50Mn35In15

Space Lattice Symmetry- Translational Periodicity- Proper Rotational Symmetry- Symmetries of Plane Lattices- Space Lattices- The Bravais Lattices- Space Group Symmetry- Proper and Improper Rotations- Combination of Rotations and Translations- Seitz Operators- Screw Axes and Glide Planes- Combination of Symmetry Operations- Reciprocal Space and Irreducible Representations of Space Groups- Reciprocal Lattice- Reciprocal Space- Irreducible Representations of Space Groups- Second-Order Phase Transitions- Thermodynamics of Second-Order Phase Transitions- Landau Theory (without symmetry)- Landau Theory, with Consideration of Symmetry, Applied to the NiAs-type to MnP-type Phase Transition- General Development of the Landau Theory with Consideration of Symmetry- Landau's 4th Condition- Another Example of the Application of Landau Theory: $$ Fm3m - R\bar{3}m $$ Order-Disorder Transition in Sc1?xS- Application of Landau Theory to a Transition to a Structure with a Mixed Order-Disorder and Distorted Structure

Hugo F. Franzen

Papers

Second-Order Phase Transitions and the Irreducible Representation of Space Groups