THE ELASTIC BEHAVIOR OF NATURAL SINGLE-CRYSTALS OF THE HIGH-SPIN COMPOUND MNS2

The hydrostatic pressure and temperature dependences of velocities of ultrasonic modes propagated in natural single crystals of MnS2 have been measured. The elastic stiffness tensor components and the bulk modulus of this high-spin pyrite structure compound are much smaller than those of the low-spin isostructural crystal FeS2. This is consistent with MnS 2 having substantially weaker attractive binding forces than those in FeS2 as would be expected from its larger lattice parameter due in turn to the high-spin configuration of the manganese 3d electrons. The temperature dependences of the elastic stiffnesses measured between 4.2 K and 293 K show that the crystal softens elastically near the Neel temperature but other than this the elastic properties are very similar in the antiferromagnetic and paramagnetic phases. The hydrostatic pressure derivatives of the elastic stiffness tensor components are all positive at room temperature: there is no acoustic mode softening under pressure. The results are used to describe the anharmonicity of the long wavelength acoustic modes in terms of mode Gruneisen parameters in the elastic continuum approximation. The mean long wavelength acoustic mode Gruneisen parameter gamma el (=2.4) is much larger than the thermal Gruneisen parameter gamma th (=0.98) which implies that the mode Gruneisen gammas for optic modes must on average be much smaller than those for the long wavelength acoustic phonons.