Microstructure And Magnetic Properties Of （MnZFe）₂P_0.77Ge_0.23（Z:Cr,V） Compounds

As a new refrigeration technology with high efficiency,environmental protection,energy saving and simple preparation process,magnetic refrigeration has a very broad application prospect.At present,Mn FePGe series magnetic refrigeration materials have developed rapidly due to their excellent magnetocaloric effect,low cost and non-toxicity.In this paper,Mn_1-xCr_xFeP_0.77Ge_0.23（x=0.02,0.03,0.04,0.05,0.06,0.08）and Mn_1-xV_xFeP_0.77Ge_0.23（x=0.02,0.03,0.04,0.05）compounds were prepared by high-energy ball milling and solid-state-reaction sintering method.The effects of Cr and V doping on the magnetic properties and crystal structure of Mn FeP_0.77Ge_0.23compound was studied by magnetic measurements,X-ray diffraction measurements and X-ray absorption fine structure measurements.The results are as follows:The Curie temperatures of the Mn_1-xCr_xFeP_0.77Ge_0.23（x=0.02,0.03,0.04,0.05,0.06,0.08）and Mn_1-xV_xFeP_0.77Ge_0.23（x=0.02,0.03,0.04,0.05）compounds decrease from 301 K to 157 K（Cr doping）and from 310 K to290 K（V doping）,respectively,with the increase of Cr and V doping content.When the applied magnetic field changes to 0-3 T,the isothermal magnetic entropy change decreases from 22.5 J·kg^-1·K^-1 to 4.0 J·kg^-1·K^-1（Cr doping）and from 11.5 J·kg^-1·K^-1 to 4.1 J·kg^-1·K^-1（V doping）,respectively.Mn_1-xCr_xFeP_0.77Ge_0.23（x=0.02,0.03,0.04,0.05,0.06,0.08）and Mn_1-xV_xFeP_0.77Ge_0.23（x=0.02,0.03,0.04,0.05）compounds are Fe₂P-type hexagonal structures with space group P-62m.The Rietveld full-spectrum fitting results show that with the increase of Cr and V doping elements content,the lattice parameters a and b gradually decrease（Cr doping:from6.09285?to 6.05763?,V doping:from 6.15810?to 6.09056?）,and the lattice parameter c gradually increases（Cr doping:from 3.4400?to3.46437?,V doping:from 3.36140?to 3.44666?）.The increase of c/a value caused by doping in the two groups of compounds weakens the ferromagnetic interaction between the interlayer metals of the compounds,resulting in a decrease in Curie temperature.This is consistent with the magnetic measurement results.The X-ray absorption fine structure measurements results of Mn K,Fe K,Ge K edges show that the Mn-Fe,Mn-Ge,Mn-P,Fe-Ge,Fe-P interatomic distances and X-ray diffraction results of Mn_1-xCr_xFeP_0.77Ge_0.23（x=0.03,0.05,0.08）and Mn_1-xV_xFeP_0.77Ge_0.23（x=0.03,0.05）compounds are consistent within the error range.The Mn atom occupies the 3g sites,the Fe atom occupies the 3f sites,the P atom occupies the 2c and 1b sites,and the Ge atom occupies the 2c sites.The Cr,V doping elements and Mn element in the two groups of compounds occupies the 3g sites together.