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Synthesis and Structure of Ca14GaP11 with the New Hypervalent P37- Anion. Matrix Effects within the Family of Isostructural Alkaline-Earth Metal Pnictides1

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posted on 14.03.1996, 00:00 by J. T. Vaughey, John D. Corbett
The new Zintl phase Ca14GaP11 has been synthesized and studied by single-crystal X-ray diffraction. The compound is isostructural with Ca14AlSb11 and contains as separate anions a GaP49- tetrahedron, the novel hypervalent linear P37-, and four isolated P3-. The phase is tetragonal, space group I41/acd (No. 142) with Z = 8, a = 15.347 (4), c = 20.762 (8) Å (296 K), R(F)/Rw = 3.6/5.0% for 759 reflection (I/σ(I) > 3) and 63 variables. The central P4 atom in P37- refines with its position split by 0.82 Å which in the limit corresponds to disordered P3- and P24-. Matrix effects in all 20 refined pnictide (Pn = P−Bi) examples of this structure are reflected by the cation environment about the Pn73- anions. This approach provides complete correlation of the splittings of the central Pn4 atoms (or their displacement ellipsoid asymmetry) and the average distances in Pn73-. Increased asymmetry of and bond lengths in Pn37- and, by implication, lesser phase stability are logically associated with smaller Pn, larger cations, and, to a smaller degree, increased size of the metal that centers the MPn4 tetrahedra. Angular distortions of the last are similarly based.