  Hexagonal RMnO3 is a multiferroic compound with a giant spin-lattice coupling
at an antiferromagnetic transition temperature [1]. Despite extensive studies
over the past two decades, however, the origin and underlying microscopic
mechanism of the strong spin-lattice coupling remain still very much elusive.
In this study, we have tried to address this problem by measuring the thermal
expansion and dielectric constant of doped single crystals Y1-xLuxMnO3 with x =
0, 0.25, 0.5, 0.75, and 1.0. From these measurements, we confirm that there is
a progressive change in the physical properties with doping. At the same time,
all our samples exhibit clear anomalies at TN, even in the samples with x = 0.5
and 0.75 as opposed to some earlier ideas, which suggests an unusual doping
dependence of the anomaly. Our work reveals yet another interesting facet of
the spin lattice coupling issue in hexagonal RMnO3.
