Jahn-Teller induced nematic orbital order in tetragonal Sr$_2$VO$_4$

Using high resolution X-Ray diffraction (XRD) on high purity powders, we resolved the structure and $ab$ symmetry of the intriguing compound Sr$_2$VO$_4$ from room temperature down to 10K to an unprecedented level of accuracy. Upon cooling, this new set of data unambiguously reveals a second order phase transition lowering the symmetry from tetragonal to orthorhombic at a temperature $T_{2}=140$ K. The observation of an orthorhombic distortion of the $ab$-plane is attributed to nematic phase formation supported by local Jahn-Teller (JT) dynamical instability. At $T_{1}=97$ K, spins order and the tetragonal structure is recovered with an elongated c-axis through a first-order-like phase transition.