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In this work, praseodymium(III) borohydride, Pr(BH4)3, and the isotopically enriched analogue, Pr(11BD4)3, are prepared by two routes. The first approach started by mechanochemical synthesis of PrCl3 and LiBH4, and extraction of LiCl [1], while the second approach, shows a new route and proceeded via a solvate complex, Pr(11BD4)3S(CH3)2 starting from the metal Pr itself. α-Pr(BH4)3 is isostructural with cubic unit cells (Pa-3) stable at room temperature (RT) and unit cell volume per formula unit (V/Z) of 180.1 and 175.8 Å3, respectively. Heating α-Pr(BH4)3 to T = 190 °C, p(Ar) = 1 bar, introduces a transition to a rhombohedral polymorph, r-Pr(BH4)3 (R3 ̅c) with smaller unit cell volume and denser structure, V/Z = 156.06 Å3. However, heat treatment of α-Pr(BH4)3, at T = 190 °C, p(H2) = 40 bar facilitates reversible formation of another three cubic polymorph, denoted β, β’-Pr(BH4)3 and β’’-Pr(BH4)3 (Fm3 ̅c). Moreover, the transition β- to β’- to β’’- is considered a rare example of stepwise negative thermal expansion, which we pursue to validate via PDF analysis. For Pr(BH4)3, 2/3 of the sample take this route of transformation whereas in argon only 5 wt%, and the remaining transforms directly from alpha- to r-Pr(BH4)3. The β-polymorphs are porous with V/Z = 172.4 and 172.7 Å3 for β’’-Pr(BH4)3, and are stabilized by the elevated hydrogen pressures. The polymorphic transitions occur due to rotation of RE(BH4)6 octahedra without breaking or forming chemical bonds. Structural DFT optimization reveals decreasing stability of α-Pr(BH4)3 > β-Pr(BH4)3 > r-Pr(BH4)3.
