  We study magnetoelastic properties of a spin-1/2 Ising-Heisenberg diamond
chain, whose elementary unit cell consists of two decorating Heisenberg spins
and one nodal Ising spin. It is assumed that each couple of the decorating
atoms including the Heisenberg spins harmonically vibrates perpendicularly to
the chain axis, while the nodal atoms involving the Ising spins are placed at
rigid positions when ignoring their lattice vibrations. An effect of the
magnetoelastic coupling on a ground state and finite-temperature properties is
particularly investigated close to a triple coexistence point depending on a
spring-stiffness constant ascribed to the Heisenberg interaction. The
magnetoelastic nature of the Heisenberg dimers is reflected through a non-null
plateau of the entropy emergent in a low-temperature region, whereas the
specific heat displays an anomalous peak slightly below the temperature region
corresponding to the entropy plateau. The magnetization also exhibits a plateau
in the same temperature region at almost saturated value before it gradually
tends to zero upon increasing of temperature. The magnetic susceptibility
displays within the plateau region an inverse temperature dependence, which
slightly drops above this plateau, whereas an inverse temperature dependence is
repeatedly recovered at high enough temperatures.
