Rare earth borohydrides RE (BH₄)₃ are studied in the context of energy storage, luminescence and magnetic applications. We have investigated the structural behavior of praseodymium borohydride Pr (¹¹BH₄)₃ containing ¹¹B isotope because of the previously reported negative thermal expansion. Differential scanning calorimetry (DSC), in-situ variable temperature synchrotron radiation powder X-ray diffraction (SR-PXD) and infrared studies reveal that Pr (¹¹BH₄)₃ undergoes to a volume contraction during the phase transition from alpha α-Pr (¹¹BH₄)₃ to rhombohedral r-Pr (¹¹BH₄)₃ phase upon heating to 493 K. Surprisingly, the phase transition persists upon cooling at room temperature. Vibrational analysis also shows that the stretching frequency of BH₄⁻ anion does not change upon heating which indicates that the B–H bond length remains constant during the structural phase transition from α-Pr (¹¹BH₄)₃ to r-Pr (¹¹BH₄)₃ phase. Additionally, the energy barrier of reorientation motion of the BH₄⁻ anion in the α-phase was estimated to be ca 23 kJ/mol by quasi-elastic neutron scattering (QENS) and Raman spectroscopy.