Pressure-induced phase transformations and magnetic ordering in some intra-rare-earth alloys and samarium

Abstract

Intra-rare-earth alloys in the system La-Y, La-Gd, Ce-Y, Gd-Y, and also the Sm-type PrY, NdY, NdTm, NdGd, Ce_0.30Gd_0.70 and Ce_0.45Ho_0.55, have been investigated at 40 kbar and 450°C. Pressure-induced phase transformation in the sequence hcp → Sm-type → dhcp → fcc seems to be a rule in these alloy systems, with the exception of Ce-Y. In all cases the high-pressure phase is retained metastably on release of pressure and temperature to the ambient. The similarity between the effect of alloying a hcp rare-earth metal from Gd to Lu or Y, with a lighter rare earth, and the effect of pressure on the former, is pointed out, and certain consequences arising therefrom are discussed. In the Ce-Y system decomposition to γ-Ce and α-Y was observed for two compositions, CeY and Ce_0.75Y_0.25 at the pressure and temperature conditions stated. This decomposition is interpreted as the result of 4f-5d electron promotion in Ce and the consequent size incompatibility preventing the alloy structure from being stable. Magnetic behavior of LaGd₃ and NdTm in their normal Sm-type structure and the pressure-induced dhcp structure, and also of the La-Gd alloys in their hcp and pressure-induced Sm-type structure, are reported. In the former two alloys the high-temperature susceptibility anomalies present in the Sm-type phase appear to be absent in the dhcp phase. Both forms however seem to be ordered at low temperatures (1-4°K) with incomplete cancellation of the moments. The La-Gd alloys are all ferromagnetic when in hcp form. In analogy with the behavior under pressure of the Sm form of Gd, it is presumed that the pressure-induced Sm form of La-Gd alloys would order antiferromagnetical. An unambiguous determination of the magnetic behavior of the Sm form is complicated by reversion to the hcp structure on cooling. Some evidence is presented to show that this somewhat unusual reversion of a metastably retained phase on cooling may be connected with magnetic ordering, and it is suggested that the exchange magneto-elastic strain energy associated with a magnetic transition could provide the driving force for the Sm-type → hcp transition. It is suggested that the magnetic-susceptibility anomalies observed in the normal as well as in the pressure-induced dhcp forms of Sm metal are connected with the ordering of the cubic-site layers (c) and the hexagonal-site layers (h) at different temperatures.