We present results of the electronic structure study for the solid solution (Fe_{1-x}Ni_{x})_{2}P (space group P6̅2m). The samples were synthesized for x=0.0 up to 0.15. From the electrical conductivity measurements on these materials, it was verified that resistivity increases with increasing temperature up to the Curie point T_{C} and changes the slope at T_{C} decreasing when temperature is raised above T_{C}. To get insight into the possible origin of such observations, we carried out the Korringa-Kohn-Rostoker-coherent potential approximation electronic structure calculations for (Fe_{1-x}Ni_{x})_{2}P in the range of concentration x=0.0 up to 1.0. It was found that alloying with Ni in Fe_{2} P causes vanishing of spin band splitting and non-rigid band behaving of electronic valence states near the Fermi level. The polarization factor for electrons at E_{F} changes in (Fe_{1-x}Ni_{x})_{2}P with increasing x, and interestingly, it reaches maximum at x=0.1, closely to the concentration (x=0.08), when the largest value of T_{C} and a kink in temperature dependent resistivity were detected.
The magnetoelastic phase transitions were observed in MnRhAs_{1-x}P_{x} system (space group P6̅2m) by X-ray measurements (80 K < T < 300 K). For x = 0.33 and x = 0.4 contents an abrupt change of both lattice parameters and of the elementary cell volume at the metamagnetic transition was detected. The Korringa-Kohn-Rostoker electronic structure calculations performed for MnRhP compound show significant variations of densities of states at the Fermi level, while changing lattice data. The Korringa-Kohn- Rostoker-coherent potential approximation studies of disordered MnRhAs_{1-x}P_{x} system (x=0.6, 0.8, 1.0) were done at the ferromagnetic region. From our calculations we observe moving of bands near E_{F}, while substituting with isoelectronic metalloid, likely as while changing the crystal data in MnRhP.
Magnetic structures of the MnFeAs_{y}P_{1-y} system were examined by means of the neutron diffraction technique in the 1.5-300 K temperature range. Atomic ordering in the metal sublattices, temperature dependence of the incommensurate (y = 0.2 and y = 0.275) and of the ferromagnetic (y = 0.275, y = 0.3, y = 0.5) ordering is discussed.
This report focuses on magnetoelastic properties and critical behaviour of (Fe_{1-x}Ni_{x})_{2}P system. For low Ni content (x=0.01) an isolated critical point was found. Analysis of this critical behaviour in the frame of molecular field approximation was carried out. Moreover, thermal variations of the lattice parameters, as measured by X-ray diffraction techniques, were studied. The (P,T) phase diagrams obtained for several Ni contents show the evidence that the Curie temperature decreases with pressure. The Mössbauer spectra collected for x = 0.025, 0.1, 0.2, 0.25 at different temperatures are also analysed. The existence of two nonequivalent crystallographic sites occupied by iron atoms as well as local magnetic structure is discussed.
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