Excitonic Instability Revealed by the Elastocaloric Effect in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>Ta</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:

On cooling through a temperature T_{S} of around 324 K, Ta_{2}NiSe_{5} undergoes a transition from a semimetallic state to one with a gapped electronic spectrum that is suspected to be an excitonic insulator. However, at this transition the structure also changes, from orthorhombic to monoclinic, leaving open the question of whether it is driven primarily by excitonic ordering or by a lattice instability. A lattice instability of this symmetry would correspond to softening of a B_{2g} optical or acoustic phonon mode. Here, we report that elastocaloric measurements of Ta_{2}NiSe_{5} with induced B_{2g} strain reveal a thermodynamic susceptibility described by a Curie-Weiss law with a Curie temperature T^{*} of 298 K. The fact that T^{*} is close to T_{S} rules out the possibility that the B_{2g} acoustic mode is responsible for the transition. Since prior Raman measurements have shown minimal softening of the B_{2g} optical mode as well, our finding strengthens the case that the transition is largely excitonic in nature. Our work underscores the potential of using strain as a tool for separating electronic and lattice contributions in phase transitions.