Structural transitions in RNi₁₀Si₂intermetallics

Abstract

Intermetallic compounds of the type RFe(10)Si(2) and RCo(10)Si(2) crystallize in the ThMn(12) structure (space group I4/mmm) whilst the heavy rare earth series RNi(10)Si(2) crystallize in a maximal subgroup of I4/mmm, P4/nmm. Reported here are neutron powder diffraction investigations for TbNi(10)Si(2) and ErNi(10)Si(2) which show that the P4/nmm structure undergoes a high temperature order-disorder phase transition at approximately 930 °C above which the ordered Ni and Si fractions revert to a random distribution on 4d and 4e sites. The volume expansion has been tracked in detail via the temperature dependence of the lattice parameters, whilst the temperature dependence of the thermal expansion coefficients α(11), α(33) and α(volume) has been determined from the lattice parameters. Associated with the order-disorder transition is a transition associated with a displacement of the R ion along the c-axis. Both transitions are of second order and the critical exponent associated with the order-disorder and displacive transitions, β = 0.31, is in excellent agreement with the exponent determined for the three-dimensional Ising model.

Magnetic behavior of bulk and fine particles of RCr2Si2C (R = La, Ce) compounds: possible magnetic ordering from Cr

“…This assumption was based on the absence of knowledge on the magnetic behavior of Cr-based systems for a long time, as these compounds were surprisingly subjected to extensive magnetic investigations relatively recently. The ThCr 2 Si 2 -type structure was found to be stable for rare earth ions from Nd to Lu only [2][3][4][5] in this Cr-series. The antiferromagnetic ordering (T N > 600 K) of Cr sublattice has been reported for TbCr 2 Si 2 , HoCr 2 Si 2 and ErCr 2 Si 2 [5].…”

Quenching of the magnetic moment of Cr inRCr2Si2compounds upon filling with carbon

“…Remarkably, these materials exhibit strong antiferromagnetic properties ͑T N Ն 600 K͒ due to a large magnetic moment on the Cr sublattice ͑ϳ1.96 B ͒, [1][2][3][4] whereas the rare-earth sublattice orders antiferromagnetically at very low temperature ͑T Ͻ 5 K͒. The origin of the magnetic behavior in these Cr-containing RT 2 X 2 compounds remains a subtle subject for band theory analysis, since, for example, the iron sublattice never orders magnetically in the isotype RFe 2 X 2 silicides and germanides.…”

Crystal field interaction at the Tm3+ site in TmCr2Si2

“…6, the CF parameters determined in this work for TmCr 2 Si 2 are compared with those determined elsewhere for RT 2 Si 2 (R = Gd, Ho, Er, Tm; T = Cr, Fe, Co, Ni, Cu) [1,4,10,12,13]. The experimental techniques involved are 155 Gd-and 169 Tm-Mössbauer spectroscopy and Ho, Er INS.…”

“…Members of the series with T = Fe, Co, Ni and Cu have been classified as Pauli paramagnets where only the R sub-lattice orders magnetically at low cryogenic temperatures. However, for T = Cr and Mn, the transition metal sub-lattices have been demonstrated to order antiferromagnetically at T N ≈ 700 K and ≈500 K, respectively [1,2]. Despite this onset of magnetic order at temperatures well above room temperature, the R sub-lattices still order close to liquid helium temperature (e.g.…”

“…To a large extent, this suppression of rare earth magnetic order is linked with the fact that the R and T sub-lattices reside in separate layers. However, it has also been proposed that the crystal field (CF) interaction at the rare earth site favours low-lying ground states (non-magnetic singlet states in the case of the non-Kramers ions) that frustrate the R-R magnetic exchange process [1]. Although the T and R sub-lattices appear to be independent of one another in terms of magnetic interactions, it is well known that the transition metal influences both the sign and the magnitude of the CF coefficients ( [4] and references therein).…”