Synthesis and study of the properties of synthetic analogues of the mineral naffildite with the participation of rare earth elements

Abstract

In recent years, there has been a significant increase in interest in ternary and quaternary compounds with the participation of rare earth elements (REE), as well as copper, antimony, and bismuth, which have a wider range of physical properties. Due to their valuable physical properties, they have become promising targets of research for modern materials science. Therefore, the purpose of this work is to study the properties of synthetic analogues of naffildite with the participation of rare earth elements. The samples were synthesised from ternary sulphides (in the case of obtaining Cu2LaSb3S7, Cu2CeSb3S7, Cu2PrSb3S7 and Cu2NdSb3S7) by fusion in sealed quartz ampoules, preliminarily evacuated to a residual pressure of 10-3 mmHg at 950–1100 K for 8 hours. The remaining antimony and bismuth-containing analogues, due to the peritectic nature of formation, were synthesised through batches (2Cu + Ln + 3Sb(Bi) + 7S) from especially pure elements. After completion of the reaction, homogenizing annealing was carried out for a month at 600–700 K, depending on the composition. The obtained samples were investigated by differential thermal (DTA), X-ray phase (XRD) and microstructural (MSA) analyzes, as well as by measuring the microhardness and determining the density. Compounds of the Cu2LnSb3S7 or Cu2LnBi3S7 (Ln – REE) type have been synthesised by direct synthesis from elements or from the ligature CuSbS2(CuBiS2) and LnSbS3(LnBiS3). It was found that they are sostructural and crystallize in the orthorhombic system (Cu2LnSb3S7 – a = 14.490÷14.264, b = 21.422÷21.206, c = 3.902–3.804 Å; Cu2LnBi3S7 – a = 14.722÷14.506, b = 21.864÷21.564, c = 4.142÷3.464 Å; Z = 4, simple group Pbnm or Pbn21) and belong to the structure type of naffildite Pb2Cu (Pb, Bi) Bi2S7. Cu2LaSb3S7, Cu2PrSb3S7 and Cu2NdSb3S7 melt congruently at 975, 985, and 1015 K, respectively, and other compounds are formed by a peritectic reaction.

Keywords

• compound
• crystal structure
• naffildite
• lattice parameters
• electrical conductivity
• thermal electromotive force