One-dimensional thermal expansivity in natural rhodochrosite ( MnCO 3 )

Abstract

Single crystals of natural rhodochrosite show a large phonon-driven thermal expansivity along the trigonal c axis (α = 1 c ( ∂c ∂T )p = 1.53 × 10−5 K−1 at T = 120 K) and a negligible expansivity perpendicular to it (α⊥ = 1 a ( ∂a ∂T )p = 0.057 × 10−5 K−1 at T = 120 K). Impurities in minor concentrations, particularly calcium, seem to be responsible for this one-dimensional thermal expansivity. Interestingly, at lower temperatures (T 50 K) where the magnetic interactions switch on, α⊥ awakens and gives rise to an anisotropic three-dimensional magnetic thermal expansivity, most probably associated with a prominent isotropic magnetoelastic coupling. This is confirmed by magnetostriction experiments. Our results show that expansivity can be easily fine-tuned (from positive to negative) by small amounts of impurities naturally present in carbonate minerals, in contrast with much more sophisticated methods used in functional materials.