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We present the detailed inelastic neutron scattering measurements of the noncollinear antiferromagnet Mn3Ge. Time-of-flight and triple-axis spectroscopy experiments were conducted at the temperature of 6 K, well below the high magnetic ordering temperature of 370 K. The magnetic excitations have a 5-meV gap and display an anisotropic dispersive mode reaching 90 meV at the boundaries of the magnetic Brillouin zone. The spectrum at the zone center shows two additional excitations that demonstrate characteristics of both magnons and phonons. The ab initio latticedynamics calculations show that these can be associated with the magnon-polaron modes resulting from the hybridization of the spin fluctuations and the low-energy optical phonons. The observed magnetoelastic coupling agrees with the previously found negative thermal expansion in this compound and resembles the features reported in the spectroscopic studies of other antiferromagnets with the similar noncollinear spin structures. arXiv:1906.07640v1 [cond-mat.str-el]

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Within the Gordon-Kim generalized model with regard to the polarizabilities of ions, the lattice constants, the high frequency permittivity, the Born dynamic charges, and the vibration constants of the crystal lattice are calculated for cation ordered double perovskites Me 1+ Bi 3+ Me 3+ Nb 5+ O 6 . The vibration spectra of all the compounds exhibit two types of instabilities: instability associated with the rotation of the oxygen octahedron and ferroelectric instability. Various combinations of distortions with respect to the rota tion mode yield five energetically most favorable distorted phases. The symmetry and the energy characteris tics of these phases are discussed. In four of the five phases, the distortions associated with the oxygen octa hedron rotation lead to polar phases, thus allowing one to speak of improper ferroelectricity in these com pounds. One phase turns out to be nonpolar; however, it contains unstable polar modes such that a displacement along the eigenvectors of these modes gives rise to polarization in the crystal.

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