From the Nonexistent Polar Intermetallic Pt₃Pr₄ via Pt_2–<i>x</i>Pr₃ to Pt/Sn/Pr Ternaries

Although the Pt–Pr phase diagram has been explored well, recent work on rare-earth metal cluster halides with endohedral transition metal atoms has provided a new binary intermetallic that is nonexistent in the known phase diagram: The binary Pt₃Pr₄ (<b>1</b>) crystallizes in a new structure type (<i>mP</i>56, <i>P</i>2₁/<i>c</i>, <i>a</i> = 12.353(2) Å, <i>b</i> = 7.4837(9) Å, <i>c</i> = 17.279(2) Å, β = 118.003(7)°, <i>Z</i> = 8) with six crystallographically independent Pt as well as eight Pr positions. The subsequent detailed investigation has led to another previously unreported, binary phase with the Ga₂Gd₃ structure type, Pt_2–<i>x</i>Pr₃ (<b>2</b>, <i>tI</i>80, <i>I</i>4/<i>mcm</i>, <i>a</i> = 11.931(9) Å, <i>c</i> = 14.45(1) Å, <i>Z</i> = 16), that is practically overlapping with the rhombohedral Pt₂Pr₃ existing in the phase diagram. Application of different tin containing fluxes to reproduce the newly detected phases brought about two almost iso-compositional ternary compounds with Sn, Pt₄Sn₆Pr_2.91 (<b>3</b>), and Pt₄Sn₆Pr₃ (<b>4</b>), as well as Pt₁₂Sn₂₄Pr_4.84 (<b>5</b>). <b>3</b> is a representative of the Pt₄Ge₆Ce₃ type (<i>oP</i>52, <i>Pnma</i>, <i>a</i> = 7.2863(3) Å, <i>b</i> = 4.4909(2) Å, <i>c</i> = 35.114(2) Å), while <b>4</b> represents a new variant of the prolific <i>T</i>₄<i>E</i>₆<i>R</i>₃ family (<i>T</i> = transition metal, <i>E</i> = main group (semi)­metal, <i>R</i> = rare-earth metal; Pt₄Sn₆Pr₃: <i>oP</i>52, <i>Pnma</i>, <i>a</i> = 27.623(1) Å, <i>b</i> = 4.5958(2) Å, <i>c</i> = 9.3499(5) Å). Pt₁₂Sn₂₄Pr_5–<i>x</i> (<b>5</b>) crystallizes as a variant of the Ni₈Sn₁₆Gd₃ type (<i>cI</i>82, <i>Im</i>3̅, <i>a</i> = 12.274(1) Å, <i>Z</i> = 2). Electronic structure calculations provide hints on the origin of the structural changes (<i>pseudo</i>-polymorphism) for Pt<i>_<i>x</i></i>Pr₃ with <i>x</i> = 1.97 and 2.00, respectively, and reveal that heteroatomic Pt–Pr bonding strongly dominates in both binaries while the addition of the reactive metal tin leads to dominating Pt–Sn bonding interactions in the ternaries; Pt–Pt bonding interactions are strong but represent a minority in the binaries and are not present at all in the ternaries