Semiconducting La2AuP3, the metallic conductor Ce2AuP3, and other rare-earth gold phosphides Ln(2)AuP(3) with two closely related crystal structures

The compounds Ln(2)AuP(3) were synthesized by reaction of the elemental com ponents in evacuated silica tubes. Their crystal structures were determined from single-crystal diffractometer data. The compounds with Ln = La, Ce, a nd Pr crystallize with an orthorhombic U2NiC3 type structure (Pnma, Z = 4). The structure refinement for Ce2AuP3 resulted in a = 774.14(6) pm, b = 421 .11(4) pm, c= 1612.3(1) pm, R = 0.019 for 1410 structure factors and 38 var iable parameters. For Pr2AuP3 a residual of R = 0.024 was obtained. Nd2AuP3 crystallizes with a monoclinic distortion of this structure: P2(1)/c, Z=4, a=416.14(4)pm, b= 768.87(6) pm, c = 1647.1(2) pm, beta =104.06(1)degrees, R = 0.022 for 1361 F values and 56 variables. The near-neighbor coordinatio ns of the two structures are nearly the same. In both structures the gold a nd phosphorus atoms form two-dimensionally infinite nets, where the gold at oms are tetrahedrally coordinated by phosphorus atoms with Au-P distances v arying between 245.8 and 284.2pm. Two thirds of the phosphorus atoms form p airs with single-bond distances varying between 217.7 and 218.9 pm. Thus, u sing oxidation numbers the structures can be rationalized with the formulas (Ln(+3))(2)[AuP3](-6) and (Ln(+3))(2)Au+1 (P-2)P--4(-3). Accordingly, La2A uP3 is a diamagnetic semiconductor. Pr2AuP3 is semiconducting with an antif erromagnetic ground state, showing metamagnetism with a critical field of B -c = 0.5(+0.1) T. In contrast, the cerium compound is a metallic conductor, even though its cell volume indicates that the cerium atoms are essentiall y trivalent, as is also suggested by the ferro- or ferrimagnetic behavior o f the compound.