This thesis presents a study of the crystal and magnetic structure, the magnetocaloric effect and related physical properties in Mn-Fe-P-X compounds. The influences of boron addition in (Mn,Fe)2(P,As) compounds have been studied. It is found that boron atoms occupy interstitial sites within the basal plane. First order magnetoelastic phase transitions with small thermal and magnetic hysteresis are observed in all these compounds. The ferromagnetic ordering temperatures increase by boron addition. The optimal working temperatures can be finely adjusted by varying the boron content without losing the good magnetocaloric properties. Both the maximal magnetic entropy changes and the Relative Cooling Power (RCP) are slightly enhanced. All these features make boron addition a good tool to tune and improve magnetic and magnetocaloric properties in (Mn,Fe)2(P,As) compounds. (Chapter 4) The effect of transition metal substitution on TC and ?Thys has been studied in (Mn,Fe,T)1.95P0.50Si0.50 (T = Co, Ni and Cu) compounds. X-ray diffraction patterns imply that all the compounds crystallize in the hexagonal Fe2P type of structure. It is found that all these transition metal substitutions for Mn(3g)/Fe(3f) weaken the ferromagnetic ordering. Ni substitutions reduce the thermal hysteresis, while the Cu substitutions enhance the thermal hysteresis. Moreover, the Co substitutions for Mn(3g) reduce thermal hysteresis, while the Co substitutions for Fe(3f) result in hardly any change in thermal hysteresis. (Chapter 5) Single phase compounds Mn0.66Fe1.29P1-xSix (0 ? x ? 0.42) have been synthesized using the melt-spinning (rapid solidification) technique. All the compounds form in the Fe2P type hexagonal structure, except a Co2P type orthorhombic structure of the Si free Mn0.66Fe1.29P compound. The compounds with 0.24 ? x ? 0.42 present a FM-PM phase transition, while the compounds with lower Si content show an AFM-PM phase transition. By increasing the Si content from x = 0.24 to 0.42, TC increases from 195 to 451 K and ?Thys is strongly reduced from ~61 to ~1 K. TC increases and ?Thys decreases with increasing magnetic field. It is also found that TC and ?Thys are not only Si content dependent, but also magnetic field dependent. Mn0.66Fe1.29P1-xSix compounds show large spontaneous magnetic moments with values up to 4.57 ?B/f.u.. A large MCE with a small thermal hysteresis is obtained simultaneously in Fe-rich Mn0.66Fe1.29P1-xSix melt-spun ribbons. The compounds with a high working temperature may also be useful for other applications, e.g. thermomagnetic generators and heat pumps. (Chapter 6) Single phase Mn1.95 xFexP2/3Si1/3 compounds with 1.0 ? x ? 1.95 have been synthesized using ball-milling technology and solid state reactions. All the compounds show a FM PM phase transition. The compounds with x ? 1.6 crystallize in Fe2P based hexagonal structure, for the higher Fe content compounds, a bco hex structural transition is observed. In contrast to the Mn rich Mn Fe P-Si system, no coupling between the magnetic and structural transition is found. The TC and ?Thys in the Mn1.95 xFexP2/3Si1/3 compounds can be easily tuned by adjusting the Fe/Mn ratio. By increasing the Fe content from x = 1.0 to 1.95, TC increases from ~269 to ~647 K and the ?Thys strongly reduces from ~65 to ~1 K. The reduction in the magnetic moment of the Fe-rich Mn1.95 xFexP2/3Si1/3 compounds, suggests that the dominant effect on the size of the moment is the change in local electron configuration rather than the interlayer exchange coupling, thus confirming a more localized magnetism on the 3g sites. (Chapter 7) High resolution neutron diffraction has been employed to determine the crystal and magnetic structure, the magnetic moment and the interatomic distances in the melt-spun ribbons Mn0.66Fe1.29P1-xSix. Introducing site disorder at the 3g site by replacing 1/3 of Fe with Mn appears to enhance the magnetic interaction, while the strong magnetoelastic coupling is maintained. The Mn substitution also shows a stabilizing effect on the hexagonal crystal structure, which is maintained to a high Si content. The moment alignment within the crystallographic unit cell is affected when the Si content increases from x = 0.34 to 0.42 in Mn0.66Fe1.29P1 xSix compounds, as the canting angle with respect to the c direction increases. The canted magnetic moment alignment confirms a low magnetic anisotropy, which ensures soft magnetic properties of Fe rich Mn Fe P Si compounds. (Chapter 8) The effect of varying the Mn/Fe ratio in the Mn1.95-xFexP2/3Si1/3 compounds has been studied using high resolution neutron diffraction. The alignment of the magnetic moment is canted from nearly the a b plane towards c axis with increasing Fe content in the Fe rich Mn1.95 xFexP2/3Si1/3 compounds with 1.0 ? x ? 1.4. The canted magnetic moment alignment confirms a low magnetic anisotropy of the Mn Fe P Si compounds, which display properties close to that of soft magnets. The magnetic moments are enhanced on both the 3f and 3g sites by introducing Mn atoms into the 3g sites. Decrease of the Mn/Fe ratio on the 3g sites does not affect the Mn/Fe(3g) magnetic moment, but results in a gradual decrease in the magnitude of the Fe(3f) magnetic moment, supporting the description of the 3f sites as weakly magnetic. Microstrains develop with decreasing Mn content, and are proposed to be caused by the inconsistent change of the intralayer and interlayer interatomic distances with respect to the changes in a and c lattice constants, respectively. Hence, the Mn favors not only the high magnetic moment, but also the hexagonal crystal structure of the Fe-rich Mn Fe P Si compounds. (Chapter 9)