PrCo2Al8 and Pr2Co6Al19: Crystal structure and electronic properties

Abstract

The praseodymium cobalt aluminides, PrCo2Al8 and Pr2Co6Al19, were prepared by reaction of the elemental components in an arc-melting furnace, followed by heat treatment at 900°C for several days. Their chemical composition was checked by scanning electron microscopy and energy dispersive spectroscopy, and their crystal structure was refined from single crystal X-ray diffraction data. PrCo2Al8 adopts the CaCo2Al8 type of structure, crystallizing with the orthorhombic space group Pbam, with four formula units in a cell of dimensions at room temperature: a=12.4623(3)Å, b=14.3700(4)Å, c=4.0117(1)Å. Pr2Co6Al19 crystallizes in the monoclinic space group C2/m, with four formula units in a cell of dimensions at room temperature: a=17.6031(4)Å, b=12.1052(4)Å, c=8.2399(2)Å and β=103.903(1)°. Its structure belongs to the U2Co6Al19 type. The crystal structures of both compounds studied can be viewed as three-dimensional structures resulting from the packing of Al polyhedra centred by the transition elements. Along the c-axis, the coordination polyhedra around the Pr atoms pack by face sharing to form strands, which are separated one from another by an extended Co–Al network. Magnetic measurements have revealed that PrCo2Al8 orders antiferromagnetically at TN=5K, with a clear metamagnetic transition occurring at a critical field Hc=0.9(1)T. The temperature dependence of the susceptibility of Pr2Co6Al19 does not provide any evidence for long-range magnetic ordering in the temperature domain 1.7–300K. At low temperatures (T<10K), the susceptibility saturates in a manner characteristic of a non-magnetic singlet ground state. At high temperatures, the magnetic susceptibility of each compound follows a Curie–Weiss law, with the effective magnetic moment per Pr atom of 3.48(5)μB and 3.41(2)μB for PrCo2Al8 and Pr2Co6Al19, respectively. These values are close to the theoretical value of 3.58μB expected for a free Pr3+ ion and exclude any contribution due to the Co atoms. Both compounds exhibit in the temperature range 5–300K metallic-like electrical conductivity, and their Seebeck coefficient is of the order of several μV/K.