Redetermination of the crystal structure of Cr12P7

The novel high-pressure/high-temperature compound Co12P7 determined from synchrotron data

Co₁₂P₇, synthesized at high pressure/temperature conditions, crystallizes isotypically with ordered Cr₁₂P₇ in space-group type P.

The structural properties of cobalt phosphides were investigated at high pressures and temperatures to better understand the behavior of metal-rich phosphides in Earth and planetary inter­iors. Using single-crystal X-ray diffraction synchrotron data and a laser-heated diamond anvil cell, we discovered a new high pressure–temperature (HP–HT) cobalt phosphide, Co₁₂P₇, dodeca­cobalt hepta­phosphide, synthesized at 27 GPa and 1740 K, and at 48 GPa and 1790 K. Co₁₂P₇ adopts a structure initially proposed for Cr₁₂P₇ (space-group type P, Z =1), consisting of chains of edge-sharing CoP₅ square pyramids and chains of corner-sharing CoP₄ tetra­hedra. This arrangement leaves space for trigonal–prismatic channels running parallel to the c axis. Coupled disordering of metal and phospho­rus atoms has been observed in this structure for related M₁₂P₇ (M = Cr, V) compounds, but all Co and P sites are ordered in Co₁₂P₇. All atomic sites in this crystal structure are situated on special positions. Upon decompression to ambient conditions, peak broadening and loss of reflections at high angles was observed, suggesting phase instability.

The crystal structure refinement of a natural mordenite

The crystal structure of natural mordenite from Elba (Italy) with chemical formula (Na1.49K2.80Mg0.04Ca2.05Sr0.05)(Al8.98Si39.13)O96 · 29.07 H2O was refined in its hydrated form up to a wR value of 5.2%. Systematic absences were consistent with the Cmcm space group but the asymmetry of the extraframework ions indicated as more probable the acentric subgroup Cmc2 1. Consequently the atom O8 is no longer set on an inversion center and the T – O8 – T angle is not constrained to 180°. The Si/Al distribution is partially ordered, with an enrichment of Al in the tetrahedra of the four-membered ring. All Ca ions are localized in a eight-coordinated site; K ions alternate with water molecules in a six-coordinated site.

Crystal structure of the rare earth cobalt phosphides Ln6Co30P19 (Ln = Er-Lu)

The new compounds Ln6Co30P19 (Ln = Er, Tm, Yb and Lu) were prepared by reaction of the elemental components in a tin flux. They crystallize with a new structure type, which was determined from four-circle X-ray diffractometer data of Yb6Co30P19: space group P[unk] (No. 174), a = 1470.3(4) pm, c = 357.4(1)pm,Z = 1, R = 0.031 for 669 structure factors and 47 variable parameters. The structure is closely related to that of UCo5Si3 (U6Co30Si18) from which it can be derived by filling an additional atomic site on one six-fold axis with phosphorus atoms. This lowers the symmetry from the centric space group P63/m to the noncentric group P[unk] and doubles the number of positional parameters, all of which could be refined independently. Other examples with this space group ambiguity are given. Like Yb6Co30P19, many of these belong to a large family of structures with a metal to metalloid ratio close to 2:1.

Nb(9)PdAs(7): a unique arrangement in the M(n2+3n+2)X(n2+n)Y family of hexagonal structures

The ternary transition-metal arsenide Nb(9)PdAs(7) has been prepared through reaction of the elements, and its structure has been determined by single-crystal X-ray diffraction methods. It adopts a new structure type (Pearson symbol hP51, hexagonal, space group P6, Z = 3), with unit cell parameters a = 16.6955(6) and c = 3.5582(1) A. The structure contains assemblies of As-centered trigonal prisms that extend as triangular columns through sharing of the triangular faces. Not only does Nb(9)PdAs(7) extend a family of hexagonal structures with general formula M(n2+3n+2)X(n2+n)Y to n = 4, the highest member known thus far, but it also displays the unique feature in which there are two distinct types of triangular columns, one having corner atoms (Pd) different from the other atoms (Nb). Structural relationships between members of the M(n2+3n+2)X(n2+n)Y family are presented. The chemical bonding in Nb(9)PdAs(7) was analyzed through an extended Hückel band structure calculation.

Lanthanum Nickel Silicides with the General Formula La(()(n)()(+1)()(n)()(+2))Ni(n)()(()(n)()(-)(1)+2)Si(n)()(()(n)()(+1)) and Other Series of Hexagonal Structures with Metal:Metalloid Ratios Close to 2:1

The closely related crystal structures of the compounds La(6)Ni(2)Si(3) (La(12)Ni(4)Si(6), n = 2) and La(5)Ni(2)Si(3) (La(20)Ni(8)Si(12), n = 3) as well as those of the new compounds La(15)Ni(7)Si(10) (La(30)Ni(14)Si(20), n = 4) and La(21)Ni(11)Si(15) (La(42)Ni(22)Si(30), n = 5) were determined from single-crystal X-ray diffractometer data. In these hexagonal silicides (space group P6(3)/m) almost all atoms are situated on mirror planes, which extend perpendicular to the short c axes. The only exceptions are nickel atoms located on the 6(3) axes, which show some disorder resulting in the exact compositions La(6)Ni(1.54(2))Si(3) [a = 1224.3(1) pm, c = 438.3(1) pm, Z = 2, Ce(6)Ni(2)Si(3) type structure], La(5)Ni(1.75(1))Si(3) [a = 1624.4(3) pm, c = 434.2(2) pm, Z = 4, Ce(5)Ni(2)Si(3) type], La(15)Ni(6.62(2))Si(10) [a = 2021.2(3) pm, c = 435.1(1) pm, Z = 2, Pr(15)Ni(7)Si(10) type], and La(21)Ni(10.49(3))Si(15) [a = 2427.7(3) pm, c = 435.2(1) pm, Z = 2, a new structure type]. All of these structures have building elements which resemble the AlB(2) type structure. In these the lanthanum atoms correspond to the aluminum atoms of AlB(2), while the nickel and silicon atoms occupy the boron positions in an ordered manner. Other series of ternary hexagonal lanthanoid and actinoid transition metal silicides and phosphides, e.g., the series with the general formula R(n)()(()(n)()(-)(1))Ni(()(n)()(+1)()(n)()(+2))P(n)()(()(n)()(+1)+1) (R = Th, U), are briefly reviewed, and the systematics in the space groups and the diffraction patterns of these compounds are pointed out.