Stability of magnetocaloric La(FexCoySi1-x-y)13 in water and air

Stability of cobalt-doped lanthanum iron silicide, La(FexCoySi1-x-y)13 have been investigated under conditions required for magnetocaloric refrigeration. The XRD analysis revealed that both milled and non-milled samples stored in water loose a few Bragg peaks corresponding to the NaZn13 phase of La(FexCoySi1-x-y)13. Samples stored in air show well-defined Bragg peaks similar to that of pristine material. The SEM-EDS of the milled and non-milled samples stored in water and air show an increased concentration of oxygen in the samples, particularly those treated with water. The course non-milled powders stored in air and water show sharp transitions at the Curie temperature TC = 300K without large magnetization above the TC. The milled, fine-particulate sample stored in air shows a slightly broadened transition at TC, and that stored in deionized water for 14 days shows significantly broadened transition from 300K and retains large magnetizations above 400 K. This is indicative of relatively fast hydrolysis and removal of some or all of La, likely as hydroxide, from fine powders, leaving behind La-poor or, potentially, La-free Fe-Co-Si containing ferromagnetic residue with much higher Curie temperature. The non-milled course sample stored in water has sharper magnetic transition and higher magnetization hence it shows the highest entropy change among all 4 type of samples.