Ultrasound velocity measurements of magnesium chromite spinel MgCr2O4 reveal elastic anomalies in the paramagnetic phase that are characterized as due to geometrical frustration. The temperature dependence of the tetragonal shear modulus (C11 − C12)/2 exhibits huge Curie-type softening, which should be the precursor to spin Jahn-Teller distortion in the antiferromagnetic phase. The trigonal shear modulus C44 exhibits nonmonotonic temperature dependence with a characteristic minimum at ∼50 K, indicating a coupling of the lattice to dynamical molecular spin state. These results strongly suggest the coexistence of dynamical spin Jahn-Teller effect and dynamical molecular spin state in the paramagnetic phase, which is compatible with the coexistence of magnetostructural order and dynamical molecular spin state in the antiferromagnetic phase.

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