Lattice effects in the quasi-two-dimensional valence-bond-solid Mott insulator EtMe${}_3$P[Pd(dmit)${}_2$]${}_2$

The organic charge-transfer salt EtMe${}_3$P[Pd(dmit)${}_2$]${}_2$ is a quasi-two-dimensional Mott insulator with localized spins $S$ = 1/2 residing on a distorted triangular lattice. Here we report measurements of the uniaxial thermal expansion coefficients ${\alpha }_i$ along the in-plane $i$ = $a$- and $c$-axis as well as along the out-of-plane $b$-axis for temperatures 1.4,K $\le$T$\le$ 200,K. Particular attention is paid to the lattice effects around the phase transition at $T_{VBS}$ = 25,K into a low-temperature valence-bond-solid phase and the paramagnetic regime above where effects of short-range antiferromagnetic correlations can be expected. The salient results of our study include (i) the observation of strongly anisotropic lattice distortions accompanying the formation of the valence-bond-solid, and (ii) a distinct maximum in the thermal expansion coefficients in the paramagnetic regime around 40,K. Our results demonstrate that upon cooling through $T_{VBS}$ the in-plane $c$-axis, along which the valence bonds form, contracts while the second in-plane $a$-axis elongates by the same relative amount. Surprisingly, the dominant effect is observed for the out-of-plane $b$-axis which shrinks significantly upon cooling through $T_{VBS}$. The pronounced anomaly in ${\alpha }_i$ around 40,K is attributed to short-range magnetic correlations. It is argued that the position of this maximum, relative to that in the magnetic susceptibility around 70,K, speaks in favor of a more anisotropic triangular-lattice scenario for this compound than previously thought.